Thermal Cracking Process for Producing Petrochemical Products From Hydrocarbons

1. Field of the Invention

This invention relates to a process for producing petrochemical products such as olefins, aromatic hydrocarbons (hereinafter abbreviated as BTX), synthetic gas (for methanol, synthetic gasoline and C.sub.1 chemistry) and the like by thermal cracking of hydrocarbons. More particularly, it relates to a process for producing petrochemical products in high yield and high selectivity which comprises the steps of burning hydrocarbons with oxygen in the presence of steam to generate a hot gas comprising steam, feeding, to the hot gas comprising steam and serving as a heat source for thermal cracking, a mixture of methane and hydrogen so that a methane/hydrogen molar ratio is over 0.05, and further feeding to the hot gas comprising the methane, hydrogen and steam, hydrocarbons in such a way that hydrocarbons comprising higher boiling point hydrocarbon components are fed to and cracked at higher temperature zones.

2. Description of the Prior Art

As is well known, the tube-type thermal cracking process called steam cracking has heretofore been used to convert, into olefins, light gaseous hydrocarbons such as ethane and propane as well as liquid hydrocarbons such as naphtha and kerosine. According to this process, heat is supplied from outside through tube walls, thus placing limits on the heat transmission speed and the reaction temperature. Ordinary conditions adopted for the process include a temperature below 850.degree. C. and a residence time ranging from 0.1 to 0.5 second. Another process has been proposed in which use is made of small-diameter tubes so that the cracking severity is increased in order to effect the cracking within a short residence time. In this process, however, because of the small inner diameter, the effective inner diameter is reduced within a short time owing to coking on the inner walls. As a consequence, the pressure loss in the reaction tubes increases with an increasing partial pressure of hydrocarbons, thus worsening the selectivity to ethylene. This, in turn, requires short time intervals of decoking, leading to the vital disadvantage that because of the lowering in working ratio of the cracking furnace and the increase of heat cycle due to the decoking, the apparatus is apt to damage. In the event that the super high temperature and short time cracking would become possible, it would be difficult to stop the reaction, by quenching, within a short time corresponding to the cracking severity. This would result in the fact that the selectivity to ethylene which has once been established in a reactor unit considerably lowers by shortage of the quenching capability of a quencher.

In view of these limitations on the apparatus and reaction conditions, starting materials usable in the process will be limited to at most gas oils. Application to heavy hydrocarbons cannot be expected. This is because high temperature and long time reactions involve side reactions of polycondensation with coking occurring vigorously and a desired gasification rate (ratio by weight of a value obtained by subtracting an amount of C.sub.5 and heavier hydrocarbons except for BTX from an amount of hydrocarbons fed to a reaction zone, to an amount of starting hydrocarbon feed) cannot be achieved. Consequently, the yield of useful components lowers. Once a starting material is selected, specific cracking conditions and a specific type of apparatus are essentially required for the single starting material and a product derived therefrom. This is disadvantageously unadaptable to the type of starting material and the selectivity to product.

For instance, a currently used typical tube-type cracking furnace has for its primary aim the production of ethylene. Thus, it is difficult to arbitrarily vary yields of other fundamental chemical products such as propylene, C.sub.4 fractions and BTX in accordance with a demand and supply balance. This means that since it is intended to secure the production of ethylene from naphtha as will otherwise be achieved in high yield by high severity cracking of other substitute materials, great potentialities of naphtha itself for formation of propylene, C.sub.4 fractions such as butadiene, and BTX products are sacrificed. The thermal cracking reaction has usually such a balance sheet that an increase in yield of ethylene results in an inevitable reduction in yield of propylene and C.sub.4 fractions.

Several processes have been proposed in order to mitigate the limitations on both starting materials and products. In one such process, liquid hydrocarbons such as crude oil are used as a fuel and burnt to give a hot gas. The hot gas is used to thermally crack hydrocarbons under a pressure of from 5 to 70 bars at a reaction temperature of from 1,315.degree. to 1,375.degree. C. for a residence time of from 3 to 10 milliseconds. In the process, an inert gas such as CO.sub.2 or N.sub.2 is fed in the form of a film from the burning zone of the hot gas toward the reaction zone so as to suppress coking and make it possible to crack heavy oils such as residual oils.

Another process comprises the steps of partially burning hydrogen to give a hot hydrogen gas, and thermally cracking various hydrocarbons such as heavy oils in an atmosphere of hydrogen under conditions of a reaction temperature of from 800.degree. to 1800.degree. C., a residence time of from 1 to 10 milliseconds and a pressure of from 7 to 70 bars thereby producing olefins. In this process, the thermal cracking is carried out in an atmosphere of great excess hydrogen, enabling one to heat and crack hydrocarbons rapidly within a super-short residence time while suppressing coking with the possibility of thermally cracking even heavy oils. However, power consumptions for recycle and separation of hydrogen, make-up, and pre-heating energy place an excessive economical burden on the process.

These processes all require very severe reaction conditions in order to obtain olefins in high yield from heavy hydrocarbons. As a result, olefinic products obtained are predominantly composed of C.sub.2 products such as ethylene, acetylene and the like, with an attendant problem that it is difficult to operate the processes so that propylene, C.sub.4 fractions, and BTX are obtained at the same time in high yields.

A further process comprises separating a reactor into two sections, feeding a paraffinic hydrocarbon of a relatively small molecular weight to an upstream, higher temperature section so that it is thermally cracked at a relatively high severity e.g. a cracking temperature exceeding 815.degree. C., a residence time of from 20 to 150 milliseconds, thereby improving the selectivity to ethylene, and feeding gas oil fractions to a downstream, low temperature section so as to thermally crack them at a low severity for a long residence time, e.g. a cracking temperature below 815.degree. C. and a residence time of from 150 to 2,000 milliseconds whereby coking is suppressed. Instead, the gasification rate is sacrificed. Similar to the high temperature section, the purposes at the low temperature side are to improve the selectivity to ethylene.

In the above process, the starting materials are so selected as to improve the selectivity to ethylene: paraffinic materials which are relatively easy to crack are fed to the high temperature zone and starting materials abundant with aromatic materials which are relatively difficult to crack are fed to the low temperature zone.

However, starting materials containing aromatic components are cracked in the low temperature reaction zone at such a low severity, so that components which can be evaluated as valuable products after gasification are utilized only as fuel. Thus, this process is designed to place limitations on the types of starting materials and products, thus presenting the problem that free selection of starting materials and production of intended products are not possible.

We made intensive studies to develop a thermal cracking process of hydrocarbons to selectively obtain desired types of olefins and BTX in high yields from a wide variety of hydrocarbons ranging from light to heavy hydrocarbons in one reactor while suppressing the coking. As a result, it was found that thermal cracking of hydrocarbons effectively proceeds by a procedure which comprises the steps of burning hydrocarbons with oxygen in the presence of steam to produce a hot gas stream containing steam, and feeding arbitrary starting materials to different cracking positions in consideration of the selectivity to desired products and the characteristics of the starting hydrocarbons. By the thermal cracking, a variety of hydrocarbons ranging from gas oils such as light gas and naphtha to heavy oils such as asphalt can be treated simultaneously in one reactor. Moreover, olefins and BTX can be produced in higher yields and higher selectivities than in the case where individual hydrocarbons are thermally cracked singly as in a conventional manner. The present invention is accomplished based on the above finding.

It is accordingly an object of the invention to provide a thermal cracking process for producing petrochemical products such as olefins, BTX and synthetic gas in high yields and high selectivities in one reactor while suppressing coking.

It is another object of the invention to provide a thermally cracking process in which the petrochemical products are obtained from a wide variety of starting hydrocarbons including light and heavy hydrocarbons by cracking different types of starting hydrocarbons under different cracking conditions.

The above objects can be achieved, according to the invention, by a thermal cracking process for selectively producing petrochemical products from hydrocarbons, the process comprising the steps of: (a) burning hydrocarbons with oxygen in the presence of steam to produce a hot gas of from 1300.degree. to 3000.degree. C. comprising steam; (b) feeding a mixture of methane and hydrogen to the hot gas in such a way that a methane/hydrogen molar ratio is over 0.05; (c) further feeding starting hydrocarbons to the hot gas comprising the methane, hydrogen and steam so that starting hydrocarbons containing hydrocarbon components of higher boiling points are fed to higher temperature zones; (d) subjecting the starting hydrocarbons to thermal cracking while keeping the cracking temperature at 650.degree. to 1500.degree. C., the total residence time at 5 to 1000 milliseconds, the pressure at 2 to 100 bars, and the partial pressure of hydrogen, after thermal cracking of a hydrocarbon comprising hydrocarbon components whose boiling point exceeds 200.degree. C., at at least 0.1 bar; and (e) quenching the resulting reaction product.

FIG. 1 is a flowchart of a process according to the invention;

FIG. 2 is a graph showing the relation between yield of coke and partial pressure of hydrogen; and

FIG. 3 is a graph showing the relation between yield of C.sub.2 -C.sub.4 olefins+ethane and residence time for different CH.sub.4 /H.sub.2 ratios.

According to the present invention, heat energy necessary for the thermal cracking reactions is supplied from a hot gas comprising steam which is obtained by burning hydrocarbons with oxygen in the presence of steam. The heat is supplied by internal combustion and such high temperatures as will not be achieved by external heating are readily obtained with the heat generated being utilized without a loss.

The heating by the internal combustion of hydrocarbons has been heretofore proposed. In general, gaseous hydrocarbons and clean oils such as kerosine are mainly used for these purposes. Use of heavy oils as fuel has also been proposed. However, burning of these oils will cause coking and sooting, which requires circulation of an inert gas such as CO.sub.2, N.sub.2 or the like in large amounts as described before.

In the practice of the invention, burning is effected in the presence of steam, including such steam as required in a downstream reaction zone, in amounts of 1 to 20 times (by weight) as large as an amount of a fuel hydrocarbon. By this, coking and sooting can be suppressed by mitigation of the burning conditions and the effect of reforming solid carbon with steam. Accordingly, arbitrary hydrocarbons ranging from light hydrocarbons such as light gas and naphtha to heavy hydrocarbons such as cracked distillates and asphalt may be used as the fuel. Alternatively, hydrogen and carbon monoxide may also be used as the fuel.

The amount of oxygen necessary for the burning may be either below or over the theoretical. However, if the amount of oxygen is excessive, effective components and hydrogen for the reaction are unfavorably lost in a reaction zone at a downstream position. On the other hand, when the amount of oxygen is less than the theoretical, it is advantageous in that hydrogen and carbon monoxide are produced by partial burning and thus an amount of hydrogen being recycled to the reaction system can be reduced. The produced carbon monoxide can be readily converted to hydrogen by the shift reaction in a high temperature zone prior to or after the reaction zone or during the recycling process. Thus, the hydrogen consumed by the reaction can be made up by the converted hydrogen. The hydrogen and carbon monoxide generated by the partial burning both serve as a feed source of hydrogen which is important as a fundamental constituent of the invention.

By the supplement of the hydrogen, hydrogen relatively deficient in heavy hydrocarbons is made up, increasing the gasification rate and the yield of olefins with a remarkable improvement in control of selectivity to a desired product upon thermal cracking of arbitrary starting materials. Additionally, coking is advantageously further suppressed.

In some cases, the partial oxidation of fuel may be advantageous because synthetic gas useful for the manufacture of methanol is obtained as a main product or byproduct. In this case, the make-up or recycle of hydrogen for the reaction becomes unnecessary. This is particularly described in our Japanese Patent Application No. 041932/1983 which is incorporated herein by reference.

Different from CO.sub.2, N.sub.2 and other gases, steam added to the reaction system is readily condensed and recovered in a separation and purification procedure of the cracked gas, with an advantage that little or no additional burden is imposed on the purification system.

Oxygen necessary for the process of the invention is usually enriched oxygen which is obtained from air by low temperature gas separation, membrane separation or adsorption separation. If air is effectively used by combination with, for example, an ammonia production plant, such air may be used.

It is thermally advantageous that the hot gas from a burner (the combustion gas from the burner) is maintained at high temperatures while reducing the feed of steam from outside and is fed to a reactor as it is. However, when the temperature of the combustion gas exceeds 2400.degree. C., a concentration of oxygen-containing radicals such as O, OH and the like increases, so that valuable products are lost considerably in a downstream reaction zone with an increase of acetylene, CO and the like in amounts. This makes it difficult to uniformly heat starting materials. In view of the stability of the burner construction, the gas temperature has a certain upper limit.

The invention is characterized by feeding a mixture of methane and hydrogen to the hot gas of 1300.degree. to 3000.degree. C. comprising steam which is produce in the burner and then thermally cracking a high boiling hydrocarbon in the presence of the hydrogen, methane and steam.

In the thermal cracking of a high boiling heavy hydrocarbon, it is important that the starting hydrocarbon be rapidly heated and evaporated for gasification and thermally cracked in the gas phase diluted such as with steam into low molecular weight olefins such as ethylene, propylene, butadiene and the like. By this, it becomes possible to attain a high gasification rate and produce olefins, BTX and the like in high yields. In contrast, if a satisfactory high heating rate is not attained, polycondensation in liquid phase takes place, with the result that the gasification rate and the yields of olefins and BTX become very unsatisfactory. In the practice of the invention, to a hot gas of from 1,300.degree. to 3,000.degree. C., preferably from 1,400.degree. to 2,400.degree. C., comprising steam are further fed hydrogen and methane. Subsequently, the hot gas comprising the steam, hydrogen and methane is directly contacted with the high boiling hydrocarbon. This direct contact enables one to achieve the rapid heating necessary for thermal cracking of the heavy hydrocarbon.

In practice, starting materials having higher boiling points and higher contents of polycyclic aromatic components such as asphaltene which are difficult to crack should be fundamentally fed to higher temperature zones of the reactor in which hydrogen and methane coexist in higher contents. This permits accelerated thermal cracking of the heavy hydrocarbon thereby producing petrochemical products at a higher gasification rate in a higher yield and selectivity.

The existence of hydrogen and methane in the thermal cracking atmosphere has the following advantages.

Firstly, hydrogen has a thermal conductivity higher than other substances, so that even heavy hydrocarbons can be rapidly heated to a desired high temperature in an atmosphere comprising hydrogen. This is important in the thermal cracking of heavy hydrocarbons as described before.

Secondly, the polycondensation reaction in the liquid phase as described above is suitably suppressed by the hydrogenation reaction. With heavy hydrocarbons, hydrogen is deficient relative to the high content of carbon atoms in the heavy hydrocarbon. The gasification of heavy hydrocarbons is promoted by making up hydrogen from outside, resulting in an increased amount of light gases. With regard to formation of coke from the gas phase, it is possible to reduce an amount of acetylene which is a precursor necessary for the coking reaction.

Thirdly, hydrogen has the effect of increasing a concentration of radicals in the reaction system, leading to a high cracking speed and a high gasification rate.

The above three effects of hydrogen are more pronounced at higher temperatures under a higher partial pressure of hydrogen. Hence, use of hydrogen in the reaction atmosphere leads to a high gasification rate and a high yield of olefins synergistically with the condition where the heaviest hydrocarbon is thermally cracked in a reaction zone of the highest temperature.

However, use of hydrogen has disadvantages which should not be overlooked. Although it is possible to attain high gasification rate and high yield of olefins by permitting hydrogen to coexist in the reaction system for the cracking of heavy hydrocarbons, high temperature conditions are essential, so that side reactions inevitably take place, making it very difficult to arbitrarily control the yield of desired olefins and BTX. In other words, there is the fear that the selectivity to product is worsened. More particularly, propylene and ethylene produced by the cracking of starting hydrocarbons are hydrogenated in an atmosphere of hydrogen alone according to the following reaction formulas (1) through (3)

As will be seen from the above, amounts of methane and ethane, particularly methane inevitably increase. The reason why all propylene and ethylene disappear is that the speed of formation reaction of these olefins is higher than the speed of the reactions (1) through (3). Additionally, these hydrogenation reactions are greatly exothermic reactions with the reaction temperature increasing by the hydrogenation and have thus the tendency toward the run-away reaction that the further cracking of once formed olefins proceeds. It is very difficult to keep a stably controlled yield.

Such an excess hydrogenation can be suppressed without a loss of the advantages attained by addition of hydrogen according to the invention in which methane as well as hydrogen is fed prior to reaction. Simultaneously with the reactions (1) through (3), the following reactions (4) through (6) of converting methane into ethane and ethylene are caused to occur by addition of methane.

Hence, the conversion into methane by the hydrogenation can be prevented. When the reaction temperature, pressure and methane/hydrogen ratio in the reaction atmosphere can be suitably controlled, the cracking of methane can be promoted and the added methane can be converted into more valuable products such as ethylene, ethane and acetylene. For instance, when the reactions (4) and (5) where ethylene is produced from methane are taken as elementary reactions, the following reactions take place. Under high temperature conditions, highly active methyl radicals (CH.sub.3.) are formed from methane and recombined into ethane. Subsequently, the reaction of withdrawing hydrogen and hydrogen radical (H.) takes place, so that the ethane is converted directly or via ethyl radical (C.sub.2 H.sub.5.) into ethylene. This is represented according to the following reaction formula ##STR1##

In coexistence of hydrogen and methane, the formation reaction of the methyl radicals proceeds such that the concentration of hydrogen radicals decreases while increasing a concentration of methyl radicals. Methane serves as an absorbent for the hydrogen, thus preventing hydrogenation reaction of olefins with the hydrogen radical and promoting the dehydrogenation reaction. At the same time, methane is converted into methane and ethylene by recombination of methyl radicals. In the above reaction, hydrogen is produced and is usable, along with the hydrogen initially fed to the reaction system, as makeup hydrogen for heavy hydrocarbons which are deficient with hydrogen.

As will be seen from the above, methane does not act as a diluent, but greatly contributes to increase yields of ethylene and the like according to the proper reaction mechanism.

The thermal cracking of heavy hydrocarbons is an endothermic reaction. The temperature of the reaction fluid after the thermal cracking slightly lowers but is still maintained at a high level. According to the invention, the reaction fluid is successively brought to direct contact with light hydrocarbons of lower boiling points while promoting thermal cracking of heavy hydrocarbons. The initially charged heat energy is thus effectively utilized or recovered and the reaction product obtained from a heavier hydrocarbon can be rapidly quenched by the thermal cracking endothermic reaction of a lighter hydrocarbon.

In this manner, a light hydrocarbon with a lower boiling point is thermally cracked at a lower temperature under a lower partial pressure of hydrogen. It was found that a partial pressure of hydrogen after the cracking of hydrocarbons (including recycled cracked oils) containing hydrocarbon components whose boiling point exceeds 200.degree. C. is essentially at least 0.1 bar in order to produce the effects of hydrogen described before and to attain a high gasification rate and a high yield of olefins.

As described before, the thermal cracking of heavy hydrocarbons is carried out under high severity in order to attain a high gasification rate and a high yield of olefins. Because the thermal cracking is effected in an atmosphere in which hydrogen and methane coexist, the yield of olefins increases remarkably over the case where hydrogen alone is used. The distribution of yield is characterized in that the content of ethylene among various olefins is high by the influence of inherent characteristics of heavy hydrocarbons.

In the process of the invention, relatively light hydrocarbons are fed to and thermally cracked in a downstream, low temperature zone while appropriately controlling the range of boiling point (the type of hydrocarbon, e.g. naphtha fraction, kerosine fraction or the like), the amount, and/or the thermal cracking conditions. The distribution of yield of finally obtained, total olefins, BTX and the like can be arbitrarily controlled to have a desired composition of the final product. In other words, the selectivity to product can be arbitrarily controlled. In particular, the thermal cracking conditions are properly controlled depending on the feed position of starting material, the total pressure, the residence time and the temperature.

In order to optimize cracking conditions of the respective starting hydrocarbons from the standpoint of the flexibility in starting hydrocarbons and products therefrom steam, water, hydrogen, methane, hydrogen sulfide and the like may be fed at a position between feed positions of the starting hydrocarbons or simultaneously with the charge of starting hydrocarbons (in which case coking is suppressed during the course of feed of the starting hydrocarbons). As mentioned, this is also advantageous in suppressing coking. A similar procedure may be taken in order to offset the disadvantage produced by a partial load operation.

High boiling heavy hydrocarbons used in the practice of the invention include, for example, hydrocarbons comprising large amounts of polycyclic aromatic components such as asphaltene which have boiling points not lower than 350.degree. C. and which are difficult to crack, e.g. topped crudes, vacuum residues, heavy oils, shale oil, Orinoko tar, coal liquefied oil, cracked distillates, cracked residues and petroleum pitches; and substances substantially free of asphaltene but containing large amounts of resins and aromatic compounds, e.g. vacuum gas oils, solvent-deasphalted oils, other heavy crude oils, and coal.

On the other hand, the low boiling light hydrocarbons whose boiling points not higher than 350.degree. C. include, for example, various cracked oils and reformed oils such as LPG, light naphtha, naphtha, kerosine, gas oil, cracked gasolines (C.sub.5 and higher fractions up to 200.degree. C. but excluding BTX therefrom). As will be described hereinafter, light paraffin gases such as methane, ethane, propane and the like are different in cracking mechanism and are thermally cracked under different operating conditions.

The above classification depending on the boiling point or cracking characteristics is merely described as a basic principle. For instance, even though starting hydrocarbons contain such hydrocarbons having boiling points not lower than 350.degree. C., those hydrocarbons such as light crude oil which contain substantial amounts of light fractions, abound in paraffinic components relatively easy in cracking, and which have a small amount of asphaltene are handled as light hydrocarbons. Likewise, starting hydrocarbons which coantain hydrocarbon components having boiling points over 350.degree. C. but consist predominantly of hydrocarbons having substantially such a cracking characteristic as of hydrocarbons whose boiling point is below 350.degree. C., are handled as light hydrocarbons whose boiling point is below 350.degree. C.

If fuel oil is essential in view of the fuel balance in the system or other specific conditions exist, even hydrocarbons having boiling points over 350.degree. C. may be thermally cracked under conditions similar to those for light hydrocarbons whose boiling point is below 350.degree. C. in order to intentionally suppress the gasification rate.

In the event that a starting hydrocarbon contains hydrocarbon components whose boiling point is below 350.degree. C. but relatively large amounts of hard-to-crack components such as resins, cracking conditions for high boiling hydrocarbons may be adopted in view of the requirement for selectivity to a desired product. In practice, similar types of starting materials which have slight different boiling points are fed from the same position so that the same cracking conditions are applied. As the case may be, starting materials of the same cracking characteristics may be thermally cracked under different conditions in order to satisfy limitations on the starting materials and requirements for final product.

As a principle, it is favorable that a hydrocarbon is thermally cracked under optimum cracking conditions which are determined on the basis of the cracking characteristics of the hydrocarbon. However, in view of limitations on starting hydrocarbons and requirements in composition of a final product, optimum cracking conditions may not always be applied.

In accordance with the process of the invention, starting hydrocarbons are fed to a multistage reactor and can thus satisfy the above requirements without any difficulty.

The cracking characteristics of a starting hydrocarbon are chiefly judged from the boiling point thereof. More particularly and, in fact, preferably, the feed position and cracking conditions should be determined in view of contents of paraffins, aromatic compounds, asphaltene and the like substances in the individual starting hydrocarbons.

Needless to say, even though a hydrocarbon containing components whose boiling points are not lower than 350.degree. C. is not utilized as a starting hydrocarbon, naphtha may be, for example, thermally cracked under high temperature and short time residence time conditions as described with reference to high boiling heavy hydrocarbons in order to carry out the thermal cracking at high selectivity to ethylene. In a subsequent or downstream reaction zone, naphtha, propane or the like is fed and cracked under mild conditions so that selectivities to propylene, C.sub.4 fractions and BTX are increased. Thus, when the system is taken into account as a whole, a desired composition of the product can be arbitrarily obtained.

A further feature of the invention resides in that the light paraffin gases such as ethane, propane and the like, and the cracked oil produced by the thermal cracking are fed to positions of the reactor which are, respectively, determined according to the cracking characteristics thereof so as to increase a gasification rate to a high level (e.g. 65% or more with asphalt and 95% or more with naphtha). The recycling of such a cracked oil to the same reactor has been proposed in some instances, in which the cracked oil is merely fed to the same position and cracked under the same conditions as starting hydrocarbons. Little contribution to an improvement of yield can be expected. This is because when a cracked oil is fed at the same position as a fresh starting material, the starting material which is more likely to crack is preferentially cracked. The cracked oil merely suffers a heat history and is converted to heavy hydrocarbons by polycondensation reaction. In contrast, according to the invention, the cracked oil is fed to a higher temperature zone than the position where a starting virgin hydrocarbon is being fed, by which the cracked oil is further cracked at a higher severity than the initial starting hydrocarbon from which the cracked oil is produced. In this manner, the cracked oil is recycled to the reactor and utilized as a starting material.

The feed position of the cracked oil is determined depending on the cracking characteristics and the desired composition of a final product. Especially, in order to increase selectivities to propylene, C.sub.4 components and BTX, relatively mild cracking conditions of light hydrocarbons are used in the downstream reaction zone. As a consequence, the yield of the cracked oil increases while lowering a gasification rate. However, when this cracked oil is fed to a higher temperature zone upstream of the feed position of the initial starting hydrocarbon from which the cracked oil is mainly produced, it is readily cracked and converted into ethylene, BTX and the like. As a whole, the gasification rate and the total yield of useful components increase. At the same time, high selectivity to a desired product is ensured.

In known naphtha cracking processes, 15 to 20% of cracked oil (exclusive of BTX) is produced. In the practice of the invention, 70 to 80% of the cracked oil used as fuel is recovered as useful components (ethylene, BTX and the like).

Light paraffinic gases such as ethane, propane and the like are fed to a reaction zone of a temperature from 850.degree. to 1,000.degree. C. and cracked to obtain ethylene, propylene and the like in high yields. When heavy hydrocarbons are simultaneously cracked at a high severity, these gases serving also as a hydrogen and methane carrier gas may be fed to a position upstream of or to the same position as the feed position of the heavy hydrocarbon.

On the other hand, hydrogen and methane may be fed to the reaction zone, according to the principle of the present invention, along with the hydrogen and carbon monoxide produced by the partial combustion unless the synthetic gas is not required. Alternatively, they may be fed to a position same as or upstream of the feed position of a starting hydrocarbon predominantly composed of hydrocarbon components having boiling points not lower than 350.degree. C. in order to supplement hydrogen deficient in the heavy hydrocarbon and convert to useful components.

Moreover, when a light hydrocarbon such as naphtha having a high content of hydrogen is fed to a downstream zone of the reactor, a partial pressure of hydrogen increases at the zone. As a result, the thermally cracked oil, cracked residue and the like which contain large amounts of the radicals produced by the cracking of the heavy hydrocarbon in the upstream zone of the reactor are hydrogenated and thus stabilized. Thus, formation of sludge, and coking in the reactor and the quenching heat exchanger are suppressed with the thermally cracked residue being stabilized. However, the stabilization of the thermally cracked residue only by the action of the hydrogen may be unsatisfactory depending on the type of starting hydrocarbon and the cracking conditions. In such case, the residue may be separately treated with hydrogen, or may be stabilized by additional feeding of hydrogen from the required optimum position and recycling of hydrogen and methane from the product separation and purification system via a bypass to a desired position.

A carbonaceous cracked residue which is produced by cracking of a heavy hydrocarbon alone under a super severity was, in some case, hard to handle (or transport) for use as a starting material or fuel or to atomize in burners. However, these problems of the handling and the atomization in burners are readily solved, according to the invention, due to the fact that the thermal cracking is effected in an atmosphere of hydrogen and the cracked oil obtained by mild cracking of a light hydrocarbon at a downstream, low temperature side is mixed with a carbonaceous cracked residue obtained by thermal cracking at an upstream, high temperature side. The cracked oil from the light hydrocarbon abounds in volatile matters and hydrogen-yielding substances, so that the solid cracked residue is stably converted to a slurry by mixing with the oil. In addition, an increase of the volatile matters makes it easier to boil and spray the mixture in burners, thus facilitating atomization. Accordingly, effective components in the cracked residue may be re-utilized as a starting material.

The present invention have further advantages and characteristic features of the described below.

As described before, the feed of a light hydrocarbon comprising low boiling hydrocarbon components which have boiling points below 350.degree. C. and are more likely to crack contributes to more effectively recover heat energy used to thermally crack a heavier hydrocarbon by absorption of heat required for the reaction of the light hydrocarbon. Because the reaction fluid, from the high temperature upstream side, comprising a cracked gas from the heavy hydrocarbon is rapidly cooled by the endothermic reaction of the light hydrocarbon, a loss of valuable products by excessive cracking can be avoided.

In the practice of the invention, the thermal cracking of hydrocarbons is effected by making use of the heat energy supplied for the cracking to a maximum, and thus a consumption of fuel gas per unit amount of product can be markedly reduced, with the advantage that the power consumption required for the separation and purification of the cracked gas can be much more reduced than in known similar techniques. In other words, the utility including fuel, oxygen and the like per unit product considerably lowers.

Once again, the present invention is characterized in that light and heavy hydrocarbons having significant differences in cracking characteristics are, respectively, cracked under optimum conditions required for the respective cracking characteristics in view of the desired type of product. High boiling heavy hydrocarbons such as topped crudes, vacuum residues and the like undergo polycondensation reaction in liquid phase competitively with the formation reaction of olefins. In order to increase the gasification rate and the yield of olefins, it is necessary to shorten the residence time in liquid phase as short as possible and to supplement to the reaction system hydrogen which is relatively deficient in the system. In this sense, it is very important to effect the cracking under high temperature and super-short time conditions in the presence of of hydrogen and methane. However, when cracked at such high temperatures, once formed propylene and C.sub.4 components will be further cracked irrespective of the short residence time, thereby giving ethylene. Thus, a content of ethylene in the final product becomes very high. Because cracking of ethylene and the like with hydrogen takes placed successively and simultaneously, it is necessary to feed methane and hydrogen to the reaction system in order to suppress the cracking of ethylene and the like by hydrogenation and stabilize the reaction yield. On the contrary, if it is intended to increase selectivities to propylene and C.sub.4 components, the gasification rate lowers. Although propylene and C.sub.4 components slightly increase in amounts, the yield of ethylene lowers considerably. Judging from the above, heavy hydrocarbons should preferably be cracked under conditions which permit an increase in selectivity mainly to ethylene.

On the other hand, light hydrocarbons such as naphtha are readily gasified, and either polycondensation of acetylene, ethylene or butadiene in gas phase or cyclization dehydrogenation reaction of starting paraffins gives BTX and cracked oil. As compared with heavy hydrocarbons, the influence of the heating velocity is smaller and a relatively wider range of reaction conditions may be used. For instance, high temperature cracking permits predominant formation of lower olefins by cracking of the paraffin chains. The yield of BTX and cracked oil by the cyclization dehydrogenation reaction lowers. BTX formed by polycondensation of lower olefins and acetylene in gas phase increases with an increase of the residence time. For short residence time, the yield of BTX lowers. The content of propylene and C.sub.4 components in the lower olefins lowers at a higher severity (i.e. under higher temperature and longer residence time conditions) because, under such conditions, they tend to be cracked into ethylene with an increasing selectivity to ethylene. With light hydrocarbons, a high gasification rate may be obtained by cracking even at low temperatures, which is different from the case of heavy hydrocarbons. In addition, the product comprises an increasing ratio of propylene and C.sub.4 fractions with less valuable methane which is formed by cracking of the above olefins being reduced in amounts. The total yield of valuable olefins including C.sub.2 to C.sub.4 increases to the contrary.

The hydrogen existing in the reaction system accelerates conversion of propylene and the like into ethylene at high temperatures as will be experienced under cracking conditions of heavy hydrocarbons. However, under mild reaction conditions of relatively low temperatures, the accelerating effect of hydrogen considerably lower.

In the cracking at low temperatures, the relative yield of BTX and the cracked oil produced by the cyclization dehydrogenation reaction increases. The increase in yield of the cracked oil may bring about a lowering of the gasification rate when the cracked oil is left as it is. In the practice of the invention, the cracked oil is fed to a position of temperature higher than the temperature at which the cracked oil is formed, by which it is converted into ethylene, BTX and the like. As a whole, the gasification rate, yield of useful components and selectivity can be improved over ordinary cases of single stage cracking at high temperatures.

In the process of the invention, light hydrocarbons and heavy hydrocarbons having different cracking characteristics are cracked under different conditions: a heavy hydrocarbon is cracked under high temperature and high severity conditions in the presence of hot steam, hydrogen and methane so as to attain a high gasification rate and a high yield of olefins (mainly composed of ethylene). Subsequently, a light hydrocarbon is cracked under low temperature and long residence time conditions in order to achieve high selectivity to C.sub.3 and C.sub.4 olefins and BTX, thereby preparing a controlled composition of product. The cracking conditions under which high selectivity to C.sub.3 and C.sub.4 olefins and BTX is achieved are relatively low temperature conditions as described before. The excess of heat energy which is thrown into the reactor for thermal cracking of heavy hydrocarbons is effectively utilized for the low temperature cracking. Moreover, the cracked oil produced by cracking of a starting hydrocarbon is further cracked under higher temperature conditions than in the case of the starting hydrocarbon. In this manner, the component which has been hitherto evaluated only as fuel can be converted into valuable BTX components and ethylene. For instance, condensed aromatic ring-bearing substances such as anthracene are cracked at high temperatures for conversion into highly valuable components such as methane, ethylene, BTX and the like. This conversion is more pronounced at a higher partial pressure of hydrogen.

Methane which is fed to the reaction system along with hydrogen can be converted into valuable components such as ethylene by a suitable combination of the methane/hydrogen ratio and the severity of the cracking conditions. As a result, the yield of methane can be controlled to have a desired value, for example, in such a way that the methane balance in the plant is established. In this way, the yield of olefins can be increased.

In the practice of the invention, in order to effectively utilize starting hydrocarbons, the starting hydrocarbons are fed to different positions of a multi-stage reactor depending on the cracking characteristics. In the high temperature zone, cracking under high severity conditions is effected to achieve a high gasification rate and a high yield of ethylene. In a subsequent zone, a hydrocarbon is cracked so that high selectivity to C.sub.3 and C.sub.4 fractions and BTX is achieved. Thus, there are prepared the cracked gas which is obtained under high severity cracking conditions in the high temperature zone and is predominantly made of ethylene, and the cracked gas obtained in the low temperature zone and having high contents of C.sub.3 and C.sub.4 olefins and BTX, making it possible to selectively produce a product of a desired composition as a whole.

As described before, it is not necessarily required that a heavy hydrocarbon having a boiling point not lower than 350.degree. C. be used as a starting virgin material. For instance, naphtha or kerosine may be cracked at high temperatures in the upstream zone, thereby giving a cracked gas enriched with ethylene. In the downstream zone, hydrocarbons which have the high potentiality of conversion into C.sub.3 and C.sub.4 olefins such as LPG, naphtha and the like, and BTX are thermally cracked under conditions permitting high selectivity to the C.sub.3, C.sub.4 olefins and BTX, thereby obtaining a controlled composition.

According to the present invention, one starting material such as naphtha may be divided into halves which are, respectively, subjected to the high temperature and low temperature crackings. Alternatively, all of virgin naphtha may be cracked at low temperatures, followed by subjecting the resulting cracked oil to the high temperature cracking so as to meet the purposes of the invention. The latter procedure is the most favorable embodiment of the invention. On the contrary, with heavy hydrocarbons such as vacuum gas oil made of components with boiling points over 350.degree. C. and having hifh selectivity to C.sub.3, C.sub.4 olefins and BTX, cracking of the heavy hydrocarbon at high and low temperature zones is within the scope of the present invention. The manner of application as described above may be suitably determined depending on the availability of starting hydrocarbon and the composition of final product based on the trend of demand and supply.

Cracking of heavy hydrocarbons involved the problem that in order to attain a high gasification rate, high temperatures or high heat energy is needed and that a composition of product is much inclined toward ethylene, thus being short of flexibility of the product. The practice of the present invention ensures a lowering of heat energy per unit product and a diversity of components obtained as products. Various heavy hydrocarbons can be effectively utilized as starting materials.

The process of the invention is described in detail by way of an embodiment.

Reference is now made to the accompanying drawings and particularly to FIG. 1 which shows one embodiment of the invention where the industrial application of the process of the invention is illustrated but should not be construed as limiting the present invention thereto.

In FIG. 1, a fuel hydrocarbon 1 is pressurized to a predetermined level and fed to a burning zone 2. To the burning zone 2 is fed preheated oxygen 4 from an oxygen generator 3, followed by partially burning the fuel hydrocarbon 1 in the presence of steam fed from line 5 to give a hot combustion gas stream 6 of from 1,300.degree. to 3,000.degree. C. The steam may be fed singly or in the form of a mixture with the oxygen 4 and the fuel 1 or may be fed along walls of the burning zone 2 in order to protect the walls and suppress coking.

The hot combustion gas stream 6 which is charged from the burning zone 2 and comprises hydrogen and steam is passed into a reaction zone 8 after mixing with hydrogen and methane fed from line 30. To the reaction zone 8 is first fed a heavy virgin hydrocarbon 7, e.g. asphalt, chiefly composed of hydrocarbon components with boiling points not lower than 350.degree. C. in which it directly contacts and mixes with the hot combustion gas stream 6, and is rapidly heated and cracked. As a result, there is produced a hot reaction fluid 9 comprising a major proportion of olefins, particularly ethylene.

Subsequently, the hot reaction fluid 9 is brought to contact with a high boiling cracked oil (boiling point: 200.degree. to 530.degree. C.) 10, cracked gasoline 11 (C.sub.5 -200.degree. C.), a light paraffin gas 12 including ethane, propane, butane and the like, and a light virgin hydrocarbon 13 having a boiling point not higher than 350.degree. C., which are successively fed to the reaction zone 8 in which there are thermally cracked. At the same time, the hot reaction fluid 9 is gradually cooled and the heat energy initially thrown into the burning zone 2 is utilized as the heat of reaction for thermally cracking the hydrocarbons.

Next, the reaction fluid 14 discharged from the reaction zone 8 is charged into a quencher 15 in which it is quenched and heat is recovered. The quencher 15 is, for example, an indirect quenching heat exchanger in which two fluids passed through inner and outer tubes are heat exchanged. The reaction fluid 16 discharged from the quencher 15 is then passed into a gasoline distillation tower 17 where it is separated into a mixture 21 of cracked gas and steam and a cracked residue 19 (200.degree. C.+). The separated cracked oil 19 is separated, in a distillation apparatus 32, into a high boiling cracked oil 10 and a fuel oil 20 (530.degree. C.+). The high boiling cracked oil 10 is recycled downstream of the position where the heavy virgin hydrocarbon 7 is fed, and is again cracked. On the other hand, the fuel oil 20 is used as a heat source such as process steam, or as the fuel 1 fed to the burning zone 2.

The mixture 21 of cracked gas and steam is passed into a high temperature separation system 22 where it is separated into cracked gas 26, process water 23, BTX 24, and cracked gasoline 25 obtained after separation of the BTX. The cracked gas 26 is further passed into an acid gas separator 27 in which CO.sub.2 and H.sub.2 S 34 are removed, followed by charging through line 28 into a production separation and purification apparatus 29. In the apparatus 29, the gas 26 is separated into hydrogen and methane 30, olefins 18 such as ethylene, propylene, butadiene and the like, light paraffin gases 12 such as ethane, propane, butane and the like, and C.sub.5 and heavier components 31. Of these, the hydrogen and methane 30 may be withdrawn as fuel 33. Alternatively, it may be mixed with the hot gas 6 comprising steam or fed to either the feed position of the heavy hydrocarbon 7 at an upper portion of the reaction zone 8 or an upper portion of the feed position for further cracking. The light paraffin gases 12 may be fed to a zone of an intermediate temperature ranging from 850.degree. to 1000.degree. C. in order to obtain ethylene, propylene and the like in high yields. Alternatively, they may be recycled by mixing with hydrogen and methane and further cracked in which the mixture has the function of yielding hydrogen to heavy hydrocarbons as well. The C.sub.5 and heavier components 31 is recycled, after separation of the BTX 24, from line 11 to a position intermediate between the feed positions of the high boiling cracked oil 10 and the light hydrocarbon 13 along with the cracked gasoline 25 from the high temperature separation system 22 and is further cracked.

The fuel hydrocarbon 1 is not critically limited. Aside from the cracked residues, there are used a wide variety of materials including light hydrocarbons such as light hydrocarbon gases, naphtha, kerosine and the like, heavy hydrocarbons such as topped oils, vacuum residues, heavy oils, shale oil, bitumen, coal-liquefied oil, coal, and the like, various cracked oils, non-hydrocarbons such as CO and H.sub.2, and the like. These materials are properly used depending on the process and the availability. Fundamentally, materials which are relatively difficult in conversion into valuable products and are low in value are preferentially used as fuel.

Examples of the starting heavy hydrocarbon 7 which has boiling points not lower than 350.degree. C. are petroleum hydrocarbons such as vacuum gas oils, topped crudes, vacuum residues and the like, shale oil, bitumen, coal-liquefied oil, coal and the like, but are not limited thereto. Examples of the light hydrocarbon 13 are LPG, naphtha, kerosine, gas oil, paraffinic crude oils, topped crudes and the like.

The position where the cracked oil is recycled is finally determined in view of the type of starting virgin hydrocarbon, the properties of the cracked oil, and the composition of final product. For instance, when topped crude is used as the starting heavy hydrocarbon 7, it is preferable that the high boiling cracked oil 10 is fed at a position upstream of the heavy virgin hydrocarbon 7. On the other hand, when vacuum residue is used as the heavy virgin hydrocarbon 7, it is preferable to feed the cracked oil at a position particularly shown in FIG. 1.

The high boiling cracked oil may be further separated, for example, into a fraction of 200.degree. to 350.degree. C. and a fraction of 350.degree. to 530.degree. C., after which they are fed.

In FIG. 1, there is shown the embodiment in which there are used as starting materials a heavy hydrocarbon mainly composed of hydrocarbon components whose boiling points are not lower than 350.degree. C. and a light hydrocarbon mainly composed of hydrocarbon components whose boiling points are not higher than 350.degree. C. However, as described before, instead of using the heavy hydrocarbon comprising components having boiling points not lower than 350.degree. C., there may be fed, for example, naphtha alone as the starting material. In this case, feed line 7 of the heavy virgin hydrocarbon may be omitted but nevertheless similar effects are obtained. Naphtha may be fed instead of the starting heavy virgin hydrocarbon 7 and the cracked oil may be recycled to an upstream position of the feed of the naphtha.

Even when three or more starting materials including asphalt, light gas and naphtha are used, the process of the invention is feasible by feeding asphalt from the feed position of the heavy hydrocarbon 7 of FIG. 1, naphtha from the feed position of the light hydrocarbon 13, and the gas oil from the stage intermediate therebetween. In the embodiment of FIG. 1, the makeup of hydrogen consumed by partial combustion of the fuel 1 is balanced with the hydrogen 30 recycled from the separation and purification system in order to keep the partial pressure of hydrogen in the reaction system. The consumption of hydrogen in the entirety of the reaction system is determined depending on the H/C ratio (atomic ratio) of starting heavy and light hydrocarbons. In case where the H/C ratio in the starting materials is fairly high as a whole, makeup hydrogen obtained by partial oxidation of fuel is not necessarily required. This is because when naphtha is used as the light hydrocarbon, its H/C ratio is relatively high, so that hydrogen is produced by the thermal cracking and thus a substantial amount of hydrogen deficient in the heavy hydrocarbon can be made up by the product hydrogen depending on the conditions. For the makeup of hydrogen, it is favorable to resort to the partial oxidation of the fuel 1. Of course, hydrogen may be supplemented from a hydrogen generator based on ordinary hydrogen reforming.

As described in detail, the present invention has a number of features as will not be experienced in prior art techniques. More particularly, a hydrocarbon is burnt with oxygen in the presence of steam to supply a heat energy required for the reaction. To the resulting hot gas are fed hydrogen and methane to obtain a gas comprising hydrogen, methane and steam, to which are successively fed at least two kinds of starting hydrocarbons so that a starting hydrocarbon having a higher boiling point is fed to and thermally cracked in a higher temperature zone.

The above manner of thermal cracking has the following advantages and features.

(1) Arbitrary heavy hydrocarbons, arbitrary light hydrocarbons and cracked oils thereof can be thermally cracked in one reactor but under different conditions which are properly determined depending on the cracking characteristics of the individual starting materials and the selectivity to a desired product. As a result, there can be obtained ethylene, propylene, C.sub.4 fractions, BTX and synthetic gas (methanol, etc.) in arbitrary ratios while achieving high gasification rates, high yields and high heat efficiencies.

(2) In a favorable range of the partial pressure of hydrogen (or a partial pressure of methane) necessary for obtaining high yields of olefins from an arbitrary starting hydrocarbon in the presence of steam, undesirable paraffination of the olefins as will be caused by the hydrogen is suppressed. On the other hand, the yield of useful components such as olefins can be remarkably improved over the yield of known techniques by the useful gasification accelerating performance inherent to hydrogen. For instance, where starting asphalt is thermally cracked in a hydrogen atmosphere according to known techniques, the yield of olefins is about 25%. According to the process of the invention, the yield of olefins reaches 40% or higher.

(3) Paraffination of olefins by addition of hydrogen can be suppressed due to the coexistence of methane. Accordingly, olefins increase in amounts and the consumption of expensive hydrogen proportionally decreases.

(4) Generation of heat accompanied by the hydrogenation of olefins is suppressed, so that the distribution of yield varies only gently relative to the variation of the reaction temperature, residence time and quenching time. This gentle variation is very effective in improving operation and working efficiencies.

(5) For the thermal cracking of heavy hydrocarbons, it is necessary to effect the cracking under high severity conditions of high temperature and short residence time in order to increase a gasification rate to a maximum. As a result, although a high yield of olefins can be expected, there is the problem that a content of ethylene in the total olefins becomes high, with a reduced selectivity to product or a loss of flexibility with regard to the kind of product and the energy cost per unit product increases. According to the invention, in order to remarkably improve the selectivity to product, a light hydrocarbon is thermally cracked in a downstream reaction zone under controlled cracking conditions. This contributes to increase the flexibility of the composition of product as a whole with the energy cost per unit product being reduced considerably.

(6) Cracked oils, cracked residues and secondarily produced gases are fed in different reaction stages and thermally cracked under cracking conditions which are different from the conditions of vargin materials and which are determined according to the cracking characteristics thereof and the selectivity to product. Thus, they are fully used in an efficient manner. The cracked oils which are utilized only as fuel in prior art can be converted into useful components such as BTX, olefins and the like. Thus, effective use of less valuable materials which could not be expected at all in prior art techniques becomes possible.

(7) By the coexistence of hydrogen and methane in a thermal cracking atmosphere for heavy hydrocarbons, hydrogen which is deficeint in the heavy hydrocarbons and cracked oils is made up, so that olefins, BTX and the like are produced in high yields even from the heavy hydrocarbons and cracked oils.

(8) The utility such as fuel, oxygen and the like per unit product is remarkably reduced by the multistage thermal cracking, with the result that the consumption of combustion gas lowers considerably and thus the separation and purification cost for cracked gas can also be reduced. noticeably.

(9) Because the thermal cracking of hydrocarbons is carried out in coexistence of steam, hydrogen and methane, occurrence of coking can be more effectively suppressed than in the case of known processes. Especially, the runaway reaction caused by generation of heat from hydrogenation can be inhibited by the presence of methane, and it is possible to suppress formation of acetylene which will cause coking in gas phase.

On the other hand, because the thermal cracking is effected in an atmosphere of hydrogen, and hydrogen and methane are produced by thermal cracking of light hydrocarbons, radicals produced by thermal cracking of heavy hydrocarbons or cracked oils in an upstream zone are stabilized, thereby suppressing formation of sludge and coking in the reactor and the quenching heat exchanger. There appears an additional effect of diluting coking substances with the cracked gas from the light hydrocarbon. Recovery of heat energy of a high level from the cracked gas of starting heavy hydrocarbon was believed to be very difficult. However, according to the invention, it becomes possible to recover heat in the form of high pressure steam such as, for example, in ordinary indirect quenching heat exchanger even when heavy hydrocarbons such as asphalt are thermally cracked. Thus, the heat economy is remarkably improved.

(10) Upon cracking of light hydrocarbons which are ready for cracking, the hot crcked gas passed from an upstream zone is effectively quenched, preventing a loss of useful products as will be caused by excess cracking.

The present invention is described in more detail by way of examples, which should not be construed as limiting the present invention but for explanation only.

A vacuum residue (specific gravity 1.02, S content 4.3%, pour point 40.degree. C.) from crude oil of the Middle East was used as fuel. The vacuum residue was charged into an ordinary combustor of the burner type located above a reactor where it was burnt with oxygen while blowing steam preheated to over 500.degree. C. from all directions, thereby generating a hot gas comprising steam. At a downstream zone of the combustor, hydrogen and methane which were heated to about 500.degree. C. were injected into a portion just above the reactor and mixed with the hot gas. The hot gas was introduced into the reactor provided beneath the combustor where it was uniformly mixed with a starting hydrocarbon which was fed from a plurality of burner-type atomizers mounted on the side walls of the reactor, thereby thermally cracking the starting hydrocarbon. Thereafter, the reaction product was indirectly cooled with water from outside, followed by analyzing the product to determine a composition thereof. On the side walls of the reactor were provided a number of nozzles along the direction of flow of the reaction fluid in order to set different cracking conditions for different types of starting hydrocarbons. By this, it was possible to make a test in which different types of starting hydrocarbons or cracked oils were fed to different positions of the reactor. In order to suitably control the reaction conditions, it was also possible to fed hot steam from the nozzles. The residence time was calculated from the capacity of the reactor and the reaction conditions.

Table 1 shows the results of the test concerning the relation between cracking conditions and yields of products in which the Middle East naphtha (boiling point 40.degree.-180.degree. C.) was cracked at a pressure of 10 bars.

The reason why the cracking performance of the reaction system where hydrogen and methane coexist is significantly superior to the cracking performance of a reaction system where hydrogen alone coexists. In Table 1, Comparative Example A shows cracking yields attained in the presence of hydrogen, and Comparative Example 1 shows cracking yields attained in the coexistence of hydrogen and methane. With the case using hydrogen alone, the yield of methane is about two times as high as the yield attained by the system using methane and hydrogen. This is believed for the following reason: valuable olefins, particularly propylene, C.sub.4 component, which were once formed were cracked and hydrogenated into less valuable methane. In other words, with the hydrogen and methane system, hydrogen radicals having the function of hydrogenation are stabilized with methane to give methyl radicals. At the same time, methane is cracked in the presence of hydrogen and converted into useful component.

Aside from the advantages based on methane, the present invention have the following advantages produced by the multistage cracking.

Comparative Example 1 shows the results of a test in which naphtha is merely cracked without recycling. Comparative Example 2 shows the results of a test in which the cracked gasoline and the cracked residue produced in Comparative Example 1 were both recycled to substantially the same position as the feed position of the stating naphtha and thermally cracked. On the other hand, Example 1 shows the results of a test in which cracked residue, cracked gasoline and starting naphtha were fed to and cracked in different positions in this order. The temperature at the outlet of the reactor was from 750.degree. to 800.degree. C. in Comparative Example 2 and Example 1. The cracking temperature of the cracked residue and the cracked gasoline in Example 1 were, respectively, 1430.degree. C. and 1400.degree. C., and the residence time from the feed to the reactor till a feed of fresh hydrocarbon was about 5 milliseconds. As will be clear from the results of Example 1, when the cracked residue and the cracked gasoline are further cracked under severer conditions than the starting naphtha, a high gasification rate and selectivities to C.sub.3 and C.sub.4 components and BTX are realized while keeping a high yield of olefins. On the other hand, when the cracked residue and cracked gasoline are recycled and cracked under the same conditions as the starting naphtha (Comparative Example 2), the gasification rate and the yield of BTX slightly increase with an undesirable increase in amount of the cracked residue. As compared with the high cracking rate in Example 1, the results of Comparative Example 2 are very unsatisfactory.

Table 2 shows the results of a test in which the same vacuum residue as used for fuel was employed as a heavy hydrocarbon, and naphtha used in the foregoing examples was used as a light hydrocarbon for cracking.

Comparative Example 3 shows cracking yields using the same methane and hydrogen system as in the present invention. The cracking yields attained in the presence of hydrogen alone are shown in Comparative Example B.

Similar to the case using naphtha, the yield of methane in the system using hydrogen alone is higher than two times the yield of the hydrogen and methane system. Because the cracking of heavy hydrocarbons is effected on the assumption that the gasification rate is high, severer cracking conditions are required than in the case of naphtha. In the cracking using hydrogen alone, propylene and C.sub.4 components such as butadiene are cracked and hydrogenated, and are thus reduced in amounts with ethylene being considerably hydrogenated. As a result, yields of ethane and methane increase greatly. On the contrary, in the hydrogen and methane system, the total field of olefins increases by 50% or more than in the system using hydrogen alone, bringing about a revolution in this field.

The advantages of the invention produced from the inherent multistage cracking other than the effects of methane added to the system are described in Example 2 and 3. Comparative Example 3 shows the results of a test in which a vacuum residue alone was thermally cracked at an initial temperature of 1150.degree. C. In this case, because the temperature at the outlet of the reaction was very high, water was directly injected into the water for quenching to determine a composition of the reaction product. Example 2 shows the results of a test in which instead of injecting water, naphtha was fed and cracked under cracking conditions thereof or under conditions close to those of Comparative Examples 1 and 2 without recycling. In order to control a partial pressure of hydrogen and a temperature in the cracking atmosphere, hot steam was fed in an amount of 1.5 kg/kg of the starting vacuum residue prior to the feed of naphtha. In this way, the hot gas after the thermal cracking of the vacuum residue was utilized in order to crack naphtha in amounts almost equal to the amount of the starting vacuum residue. As a consequence, the composition of a product is desirably improved. On the other hand, where the vacuum residue is singly cracked at an initial temperature of 950.degree. C., the gasification rate was about 45% in spite of the presence of hydrogen, which was much lower than about 70% which was attained by the high temperature cracking described in Comparative Example 3. From the above results, it will be seen that in order to obtain a high gasification rate from heavy hydrocarbons, it is preferable to crack them at high temperatures over 1000.degree. C. This leads to the fact that the gas after the cracking of the heavy hydrocarbons are substantially high. In particular, when hydrogen is caused to exist beforehand in the reaction system, the hydrogenation reaction is likely to proceed. This hydrogenation reaction is suppressed to a substantial extent by addition of methane. However, as compared with a cracking in the absence of hydrogen, the temperature of the atmosphere after the cracking is relatively higher. As particularly shown in Example 2, the hot gas can be used as a heat source, enabling a light hydrocarbon such as naphtha to be readily thermally cracked. This permits the yield of product relative to an amount of fuel to be much more improved over the case of Comparative Example 3. Example 3 shows a thermal cracking process in which the cracked residue produced in Example 2 was separated by distillation and a part of a fraction below 500.degree. C. provided as a high boiling cracked oil was fed to a position corresponding to about 10 milliseconds after the feed of the starting vacuum residue, followed by feeding cracked gasoline to a position corresponding to about 5 milliseconds thereafter and further feeding virgin naphtha to a position corresponding to further about 5 milliseconds after the preceding feed. At this time, similar to Example 2, the same amount of steam was fed to a position just before the feed position of virgin naphtha in order to control the cracking conditions. It will be noted that addition of steam is not essential but steam is used to allow easy comparison between the procedures of Examples and Comparative Examples. The cracked residue from which the high boiling cracked oil was removed was used as fuel instead of the vacuum oil. The cracking temperature of the high boiling cracked oil was about 1150.degree. and the cracking temperature of the cracked gasoline was about 1100.degree. C. The partial pressure of hydrogen after the cracking of the vacuum residue was from about 1.5 to 2.0 bars. On the other hand, the reactor output temperature after the cracking of naphtha was about 800.degree. C. When the cracked gasoline and the high boiling cracked oil were recycled, the yield of C.sub.3 and C.sub.4 components was maintained at a level with an increase in yield of ethylene and BTX. From this, it will be seen that the recycled oils are effectively converted into useful components.

As described in detail above, the effective range of scope of the invention is described as follows.

Hydrocarbons being fed to a reactor may be selected from a wide variety of hydrocarbons including light to heavy hydrocarbons and should be fed to a reactor of at least two or larger stages. The feed positions of individual hydrocarbons are finally determined depending on the cracking characteristics of the individual hydrocarbons and the composition of a required product. Fundamentally, however, it is desirable that a hydrocarbon comprising hydrocarbon components having higher boiling points be fed to a higher temperature zone in which it is cracked. Moreover, a position where the cracked oil is to be recycled should involve at least severer conditions than the conditions for a starting virgin hydrocarbon from which the cracked oil is chiefly produced.

The reaction temperature is determined such that as described above, heavier hydrocarbons are cracked under higher temperature conditions. Especially, where a heavy hydrocarbon comprising components whose boiling points not lower than 350.degree. C. is used, it is preferable that an initial cracking temperature is over 1,000.degree. C. When the initial cracking temperature lower than 1,000.degree. C. is applied to such a heavy hydrocarbon, the gasification rate considerably lowers with an increase in amount of heavy cracked residue. Thus, the merit of the use of heavy hydrocarbons as starting materials is substantially lost. The temperature at the outlet of the reactor should preferably be over 650.degree. C. Lower temperatures involve a considerable lowering of the speed of cracking into gaseous components and permit coking to proceed, making it difficult to attain a high gasification rate.

The residence time can be shorter for a starting material being fed at a higher temperature zone. Where starting heavy hydrocarbons are cracked at temperatures over 1,000.degree. C., hydrogenation by methane is suppressed, so that a longer cracking time is possible as compared with the case of an atmosphere of hydrogen alone. The residence time is generally below 100 milliseconds, preferably below 50 milliseconds. Longer reaction times will bring about a lowering of the yield of olefins by cracking and a lowering of the effective amount of heat energy by heat loss. On the other hand, the residence time required for the thermal cracking of hydrocarbons of relatively low boiling points in a downstream zone of the reactor is preferred to be below 1000 milliseconds. The residence time is determined depending on the reaction type, the pressure, the characteristics of starting materials and the composition of a final product. Residence times longer than 1000 milliseconds will lower a yield of olefins by excessive cracking of once produced olefins.

The reaction pressure is determined in view of the types of starting materials, the reaction conditions, and the conditions of cracked gases being treated in or downstream of the reactor. Higher temperatures result in a larger amount of acetylene. Formation of acetylene is the endothermic reaction which requires a larger amount of heat than in the case of formation of more useful ethylene, thus bringing about an increase in amount of heat per unit amount of desired ethylenic olefin product. In order to suppress the formation of acetylene, it is necessary to increase the reaction pressure. However, an increase of the reaction pressure invites an increase of partial pressure of hydrocarbons, thus acclerating coking. In this sense, it is necessary that coking be suppressed while shortening the residence time as well as increasing the reaction pressure. The reaction pressure has relation with treating conditions of cracked gas. When the process of the invention is operated as an ordinary olefin production plant, the pressure of the separation and purification system ranging from 30 to 40 bars should be taken into account. The reaction pressure should be determined in view of the types of starting materials and the cracking conditions. In case where partial combustion is effected in the combustion zone to obtain synthetic gas as well, the reaction pressure should be determined in consideration of applications of the synthetic gas. In the olefin production plant, the pressure is preferably below 50 bars, and in the case where synthetic gas is also produced, it is preferable to crack at a pressure below 100 bars in view of conditions of preparing methanol which is one of main applications of the synthetic gas. If the reaction pressure is below 2 bars, formation of acetylene in the high temperature cracking zone becomes pronounced. Preferably, the pressure is above 2 bars.

The partial pressure of hydrogen has the relation with the suppression in formation of acetylene as described above and the inhibition of coking and is preferred to be over at least 0.1 bar with regard to a partial pressure of hydrogen after cracking of a hydrocarbon comprising hydrocarbon components having boiling points over 200.degree. C. This atmosphere of hydrogen makes it possible to supplement hydrogen which tends to be deficient in the hydrocarbons, to suppress coking, and to attain a high gasification rate. A higher partial pressure of hydrogen is favorable for a heavier hydrocarbon: wit a very heavy hydrocarbon such as vacuum residue, the partial pressure is preferably in the range over 1.5 bars.

FIG. 2 is a graph showing the relation between partial pressure of hydrogen and yield of coke when a vacuum residue from the Middle East crude oil and naphtha were thermally cracked under conditions of the outlet temperature of a reactor at 1000.degree. to 1200.degree. C., the CH.sub.4 /H.sub.2 molar ratio at 0.5, the total pressure at 30 bars, and the residence time at 20 milliseconds. The curve a indicates the yield of coke in case where the Middle East vacuum residue was thermally cracked, and the curve b indicates the yield of coke in case where naphtha were thermally cracked. As will be seen from the figure, the heavier hydrocarbon needs a higher partial pressure of hydrogen.

FIG. 3 shows the relation between yield of C.sub.2 -C.sub.4 olefins+ethane and residence time in case where the Middle East vacuum residue was provided as a starting material and thermally cracked under conditions of the pressure at 30 bars, the reactor outlet temperature at 1000.degree.-1030.degree. C., and the total pressure at 30 bars for different CH.sub.4 /H.sub.2 molar ratios. The reason why the yield of ethane is evaluated in combination with the yield of C.sub.2 -C.sub.4 olefins is due to the fact that the amount of ethane is relatively large and ethane can be readily converted into ethylene. As will be seen from FIG. 3, when the ratio of methane increases, the yield of C.sub.2 -C.sub.4 +ethane increase and the variation in the yield in relation to the residence time becomes small with the distribution of the yield being thus stabilized. The ratio of C.sub.3 and C.sub.4 olefins to the total of C.sub.2 -C.sub.4 olefins+ethane (yield of ethane being from 5 to 10%), i.e. C.sub.3 -C.sub.4 olefins/C.sub.2 -C.sub.4 +ethane, is higher at a higher ratio of methane and ranges from 10 to 40% when the CH.sub.4 /H.sub.2 molar ratio is at 1 (the ratio becomes smaller at a longer residence time). From the above results, the addition of methane results in a higher yield of olefins than the comparative case using no CH.sub.4 (CH.sub.4 /H.sub.2 =0). In addition, the variation of the yield relative to the residence time is appreciably improved. The effect of the addition of CH.sub.4 is shown even when the CH.sub.4 /H.sub.2 molar ratio is 0.05 and is very significant when the ratio is over 0.1. The residence time may be selected from a wide range from 5 to 300 milliseconds for starting materials used singly.