Process for Operating a Plant for the Cracking of Hydrocarbons

The invention relates to a process for operating a plant for the cracking of hydrocarbons, whereby the hydrocarbons are indirectly heated in the cracking furnace by heat incurred in the combustion of a heating medium with oxygen containing gas. More specifically, the invention relates to a process where hydrocarbons are cracked to form olefin-gases, such as ethylene (C.sub.2 H.sub.4).

It is known in processes for the generation of high-olefinic cracked gases to feed stock, such as naphtha, ethane, to cracking furnaces, whereupon cracked gases leaving the furnaces are cooled and fed to compressors. After compression, the cracked gases are further processed to obtain, for example, ethylene.

The present invention relates to a process which renders hydrocarbon cracking more economical.

In contrast to the previously known process, cracked gases leaving the furnace are compressed to produce gases high in olefins.

Within a plant for the production of gases high in olefins, many compressors and pumps are used for compressing feed gas, ethylene, and propylene. Conventionally, these compressors are driven by steam turbines.

According to the present invention, a gas turbine and an air preheater are added to increase process efficiency.

In order to produce cracked gases high in olefins in a plant of the kind described above, hydrocarbons are exposed to high temperatures in cracking furnaces. To this end, fuel is subjected to combustion with air in the cracking furnaces. The gas is cooled in quench coolers to maintain reaction equilibrium of the cracked gas at the furnace exhaust. The waste heat of the cracked gases is utilized to generate high pressure steam. Conventionally, the steam thus generated is passed to the steam system consisting of several steam headers. Among other things, the steam is then expanded in steam turbines which drive the main compressors of the plant, as for example, the compressor for crude gas, for the ethylene and for the propylene of the ethylene cycle or propylene cycles, respectively.

As a result of escalating prices for raw materials, operating costs for the processes previously used are very high.

Thus, it is the objective of the invention to introduce a process for operating a plant for the cracking of hydrocarbons which reduces operating costs compared to the process previously used.

This objective is achieved in that the waste gas of a gas turbine is admixed with air, and the gas mixture is passed to the cracking furnace for fuel combustion, whereby the gas turbine drives an electric generator.

It is one of the premises of the inventive process that the waste gas of a gas turbine is available at a high temperature level in comparison with a steam circuit. Utilization of the waste heat of the gas turbine operation greatly improves the total efficiency of the gas turbine process.

On the other hand, selection of a gas turbine is limited by the types offered by suppliers which, at this time, have large jumps in capacity. Adaptation to the output desired, which is available in the case of steam turbines, is not possible. As a result, excess capacity might have to be installed for certain plant sizes, resulting in less advantageous consumption rates due to continuous part load operation.

If speed control of the service turbine is required, the gas turbine must be of a dual shaft design. This type of gas turbine, however, is relatively expensive. In light of the above characteristics of gas turbines, the inventive process offers the following significant advantages.

Due to the preheating of the combustion air for the cracking furnaces, the reuse of gas turbine waste gases and the employment of a gas turbine as a generator drive, a cost reduction in operating olefin plants is achieved, as is further discussed below.

Due to the constant rotational speed in operation, single shaft gas turbines are suitable as a generator drive. Single shaft gas turbines, inclusive of generators, are available as standard components and are essentially lower priced than dual shaft gas turbines for mechanical drives.

The process of the invention uses gas turbines for generating electrical energy. Such energy can, for instance, be utilized within the plant for operating pumps or compressors. Accordingly, the need to install a gas turbine with excess capacity in proportion to plant size is eliminated. Instead, a gas turbine can be selected according to plant size, affording full load operation and thus yielding better consumption rates than a gas turbine with excess capacity for the plant size, operating with only partial loads.

The process of the invention may require that a certain amount of current be imported, which, however will still be less than that required by a plant operating according to the present state of the art. The electrical current to be imported depends upon whether a well-fitted gas turbine can be found for the plant, and upon which mode of drive (steam or electric motors) is selected for the respective plant compressors.

On principle, a failure of a gas turbine does not have an aggravating effect on the plant operation if electrical power can be immediately withdrawn from the net, on a large scale. In such a case, the plant can continue to supply products meeting specifications. Changes in combustion air data have to be detected and balanced by adjustment of the furnace control loops.

In case of a failure by either the cracking furnace or the entire olefin plant, the gas turbine can continue operation. In such a case, the turbine waste gas is passed to a chimney and the electric current generated is exported, if possible.

The process of the invention uses the turbine waste gas which still has a relatively high oxygen content (approximately 16%) as preheated "combustion air" in the cracking furnaces. However, the remaining oxygen, contained in the turbine waste gas, is not sufficient to completely supply all of the furnaces of the olefin plant. The process of the invention, therefore, provides that air is added to the gas turbine waste gas. If the air added to the turbine waste gas has ambient temperature, the relatively high temperature of the waste gas is lowered very much. Unusually high energy losses are incurred. In a preferred embodiment of the inventive process, the air is, therefore, preheated before it is added to the turbine gas. It is particularly beneficial when the air is preheated by heat exchange with steam from the steam system of the plant. Due to the resultant higher theoretical combustion temperature, the effectiveness of the radiation zone is increased so that the fuel requirement for the cracking furnaces is reduced. As a consequence of lower heat supply to the cracking furnaces, the heat potential in the convection zone is reduced and the lower heat supply also reduces steam production of the cracking furnaces. If process pumps and compressors of the plant are not driven by steam turbines but by electric motors, steam export is increased in spite of lower steam production in the process of the invention, as compared to the conventional process.

In another embodiment of the invention, the air added to the waste gas advantageously, is heated to a temperature compensating for temperature fluctuations of the gas turbine waste gas. In this manner, the temperature of the gas mixture consisting of gas turbine waste gas and air is always kept at a constant level. Adverse effects caused by temperature fluctuations of the gas turbine waste gas in operation of the furnace are avoided.

In still another feature of the invention, the air added to the waste gas of the gas turbine is in an amount which compensates for fluctuations in the waste gas quantity. This feature is particularly significant if there is gas turbine breakdown as the cracking furnaces are then supplied with outside air, entirely. In normal operation, the temperature of the turbine waste gas is, for example, approximately 550.degree. C. The air added is preheated to 200.degree. C., for example, in order to attain a temperature of 400.degree. C. for the mixture. The air preheater is appropriately designed for normal operating conditions and is therefore capable of heating the total air quantity to approximately 100.degree. C. in the event of gas turbine failure. This means that the changes at the cracking furnaces regarding radiation zone effectiveness, and, inherently, fuel requirements and steam production, are minimal.

As described above, according to one feature of the invention, it is expedient if compressors and pumps of the plant are driven by electric motors whose electrical energy is supplied by the electric generator powered by the gas turbine. This results in substantial reduction of steam quantity to be condensed, and reduces the cooling water requirements for the plant.

The following two tables compare selected key data for furnace operation and operating means consumption of a conventional plant with the corresponding data of a plant employing the inventive process. The conventional plant used for comparison has a capacity of 250,000 jato ethylene from hydroconverter residue. The main compressors (crude gas, ethylene, propylene) and pumps (like feed water pumps, cooling water, quenching oil, etc.) are powered by steam turbines. The cracking furnaces with integrated waste heat are equipped with self-priming sidewall burners.

The comparison of operating means cost and resultant economy of the two processes is determined by the cost of heat and the relationship between heating costs and steam valuation. Depending upon the value accorded to these two parameters, operating means costs of the inventive process can be lowered in an amount of up to 12% and more in comparison to the conventional process.

A plant operating according to the inventive concept, requires modifications over a conventional plant which result in additional investment costs (e.g. installation of a gas turbine with an electric generator, replacing turbines for cooling medium with electric motors, replacement of small turbines with corresponding motors, installation of fresh air blowers and an air preheater). However, based on the substantial savings in operating costs, the amortization period is short.

The FIGURE is a schematic drawing of a plant operating according to the process of the invention.

Referring to the figure, cracking furnace 1 is supplied with fuel via line 2 and is supplied with oxygen containing gas via line 3 for fuel combustion. In normal operation, the oxygen containing gas comprises two components. One component is the waste gas of a gas turbine 4 whose essential elements are air compressor 5, gas generator 6 and service turbine 7. The waste gas leaves the turbine via line 8 leading to air preheater 9.

The second component is the air which is drawn in via fresh air blower 10 and is passed to air preheater 9. In air preheater 9 the air is indirectly heated by steam which flows in heat exchanger elements 11 and then is subsequently mixed with the waste gas of the gas turbine. This mixture is then fed to line 3 leading to the cracking furnace.

The cracked gas formed in cracking furnace 1 leaves the cracking furnace via line 21. It is cooled in quench cooler 22 in heat exchange with pressurized water from steam pressure vessel 23 and is passed to an oil fractionation process not depicted. Vaporized water from the heat exchange is returned to steam pressure vessel 23. Steam is withdrawn from steam pressure vessel 23 via line 25, then is superheated in heat exchange with flue gases of cracking furnace 1, in heat exchanger 24, and is passed to steam header 26.

Via steam turbines 30, 31, 32 steam is withdrawn from this steam header, expanded, and, depending upon the pressure of the expanded steam, is fed respectively to high-pressure steam header 27, medium-pressure steam header 28 or low-pressure steam header 29. Turbine 30, for example, can drive the crude gas compressor, not depicted. The steam leaving turbine 30, having been expanded to condensation pressure, is condensed in condenser 34 and then is passed to feed water container 33, in which gas is extracted from the condensate.

The invention provides that steam can either be exported from the plant or imported into the plant by high-pressure steam header 27. Steam of the two steam headers 28, 29 is passed to heat sinks 35, such as a process steam generator or a column reheater, and is subsequently condensed. The condensate of these steam headers, as well as the condensate formed in heat exchanger 11 by heat exchange with the air to be preheated, is guided to feed water container 33. Feed water is withdrawn from the feed water container via line 36 and brought to the pressure of steam pressure vessel 23, then is heated within the steam system against condensing steam of various pressure stages (heat exchanger 37). The preheater feed water enters steam pressure vessel 23 via line 38.

The process of the invention uses forced air burners as burners for the cracking furnace or furnaces. In contrast, to the self-priming burners, forced air burners permit measurement of the air quantity which flows towards the burner and, therefore, the ability to keep surplus air to a low level. As there may be a change in the oxygen content of the gas mixture, which consists in the inventive process of gas turbine waste gases and air, the process provides for the measurement and regulation of the amount of oxygen, rather than the amount of air. Of primary consideration is the control of air to prevent a shortage of oxygen, with the heating means being a secondary consideration. To this end, each of the forced air burners is provided with an air supply and equipped with a valve device for quantity regulation.

The furnace exhaust temperature is monitored, and the oxygen quantity necessary to maintain the required heat output (pressure- and temperature-corrected) is preset by means of a flap valve. The amount of fuel possible, for a predetermined amount of excess oxygen, is added by means of a ratio distributor into which the Wobbe Index is entered as correcting value if fluctuations of the heating medium occur.

Line 3 for the gas mixture flowing towards the cracking furnace has pressure regulator 13 which acts upon a torque regulator for fresh air blower 10, if the pressure is too low. If pressure is too high in line 3, a portion of the gas mixture consisting of turbine waste gas and fresh air is passed off to the chimney via flap valve 14.

Moreover, line 3 has built-in temperature control 12, which acts upon regulating valve 39 in the steam line leading to heat exchanger 11. Preheating of the fresh air permits the maintenance of a constant temperature of the gas mixture consisting of gas turbine waste gas and fresh air, thereby avoiding adverse effects upon furnace operation, even in the event of fluctuations in gas turbine exhaust temperature.

An essential feature of the inventive process is that turbine 7 drives electric generator 15. The electric current generated in generator 15 serves to power motors 16, 17, for compressor 18, 19, in which, for example, ethylene and propylene are being compressed. In an identical manner, generator 15 supplies energy to power other compressors or pumps in the inventive plant for cracking hydrocarbons.

In summary, it can be stated that operating costs of an olefin plant can be reduced by employing a turbine for generator drive, and by reutilization of gas turbine waste gas, together with the preheated fresh air.