Anhydride Separation

The present invention relates to separation of maleic anhydride or phthalic anhydride from a gaseous mixture containing the anhydride, particularly by the use of an organic absorbent.

Phthalic anhydride and maleic anhydride are both important industrial chemicals which are produced by vapor-phase oxidation of a hydrocarbon feedstock in an oxidation reactor, followed by recovery and then purification of the anhydride. The most common feedstocks for phthalic anhydride plants are naphthalene and orthoxylene; for maleic anhydride plants, the most common feedstock is benzene, although other hydrocarbon feeds have been disclosed, including butene and butane.

Recovery of the anhydride from the gaseous effluent stream from the oxidation reactor can be done by scrubbing the effluent with water, which results in conversion of the anhydride to an acid. The acid then needs to be dehydrated to produce the anhydride product.

In the case of phthalic anhydride (b.p. = 285.degree. C.), condensation of the anhydride present in the reactor gaseous effluent can be done to separate the phthalic anhydride. Frequently maleic anhydride (b.p. = 202.degree. C.) is recovered by the condensation of a part of the maleic anhydride and maleic acid in the gaseous effluent, the remainder being recovered by scrubbing with water, resulting in an aqueous maleic acid solution. Then both the partial condensate and the aqueous acid solution must be dehydrated to obtain product maleic anhydride.

Recovery of maleic anhydride or phthalic anhydride from the oxidation reactor effluent using an organic absorbent as opposed to an aqueous absorbent has also been disclosed. For example, U.S. Pat. No. 2,574,644 discloses the use of dibutylphthalate for the recovery of maleic anhydride or phthalic anhydride from an oxidation reactor effluent stream. According to the process disclosed in U.S. Pat. No. 2,574,644, the oxidation reactor effluent is cooled to first condense a portion of the anhydride vapor. The remaining gaseous stream is contacted with the dibutylphthalate absorbent to remove the remaining uncondensed anhydride by absorption into the absorbent. The resulting rich absorbent is stripped to obtain a product anhydride stream.

British Pat. No. 727,828 discloses the use of dibutylphthalate for simultaneous absorption of both maleic anhydride and phthalic anhydride at absorbent temperatures above 104.degree. F.

U.S. Pat. No. 3,040,059 disclosed the use of molten wax as an absorbent for removing maleic anhydride or phthalic anhydride from an oxidation reactor effluent stream.

U.S. Pat. No. 2,893,924 discloses the use of diphenylpentachloride absorbent as well as tricresyl phosphate as an absorbent for removing maleic anhydride or phthalic anhydride by absorption.

Japanese Pat. No. 35- 7460 discloses the use of dibutylmaleate as an organic absorbent for removing maleic anhydride from gas streams, and Japanese Pat. No. 32- 8408 discloses the use of dimethylterphthalate for removing maleic anhydride from gas streams containing maleic anhydride.

Commonly assigned U.S. application Ser. No. 310,320 discloses the use of certain dialkylphthalates as absorbents for removal of maleic anhydride from gas streams containing maleic anhydride.

Also, commonly assigned U.S. application Ser. No. 209,069 discloses alkyl or alkenyl succinic anhydrides, in general intramolecular carboxylic acid anhydrides, as absorbents for the removal of maleic anhydride from gas streams. The disclosures of U.S. Ser. Nos. 310,320 and 209,069, especially in that they relate to the use of organic absorbents for maleic anhydride removal, are incorporated herein by reference.

The present invention is especially concerned with the manner of carrying out the recovery of maleic anhydride or phthalic anhydride from a hydrocarbon oxidation reactor effluent gas stream containing the anhydride in gaseous or vaporized form. Typical prior art absorption schemes involve the use of what is frequently called a "figure-8 loop". The figure is formed by the absorbent entering the top of the absorber, flowing downwardly therein, and then being transferred as rich absorbent to the absorber stripper, flowing downwardly in the stripper and then being transferred as lean absorbent to the upper part of the absorber to complete the figure-8 loop. In the absorber, the absorbent absorbs the solute and in the stripper the solute is distilled out of the absorbent. Distillation is typically accomplished by generating upflow vapors through heat input to the lower part of the stripper.

For example, the above-mentioned U.S. Pat. No. 2,574,644 shows an anhydride absorption scheme wherein basically a figure-8 loop is used. The process of this patent includes a common modification, namely the inclusion of distillation as a part of the over-all absorbent stripping operation.

According to the present invention, in a process for separating maleic anhydride or phthalic anhydride from a gaseous mixture by absorption into an organic absorbent flowing downwardly and countercurrently to the gaseous mixture in an absorption column having at least 4 stages, the improvement is made which comprises removing heat from a zone of the absorption column above the gaseous mixture feed point and below the top 2 stages of the column in sufficient amount so that more than half of the temperature gradient for the absorbent liquid in the column is across at least 2 stages above the gaseous mixture feed point and below the top 2 stages of the column.

The process of the present invention is preferably applied to maleic anhydride recovery, that is, maleic anhydride separation from a gas stream containing maleic anhydride. The terms "gas" or "gaseous" are used herein to include vapors as well as substances which are normally gaseous at room temperature and pressure. The gaseous effluent from an oxidation reactor of either a phthalic anhydride plant or a maleic anhydride plant contains nitrogen, oxygen, water and carbon dioxide, as well as the product phthalic or maleic anhydride vapors. Maleic anhydride vapors condensed in the presence of water react to form maleic acid, which then partially isomerizes to fumeric acid. It has been found that the process of the present invention is especially advantageously applied to the separation of maleic anhydride from the gaseous effluent of a maleic anhydride plant hydrocarbon oxidation reactor, particularly without the concurrent formation of maleic and fumeric acids.

The process of the present invention is particularly advantageously applied to maleic anhydride separation from the effluent from a butane oxidation process wherein maleic anhydride is produced using a vanadium oxide-phosphorus oxide catalyst, especially those catalysts as disclosed in commonly assigned U.S. application Ser. No. 263,883, filed June 19, 1972.

The above-mentioned heat removal from the absorber is carried out so that there is a substantial temperature gradient in a zone extending above the gaseous mixture feed point to the absorber, but below the top 2 stages of the absorber. The process of the present invention is carried out so that more than half the column temperature gradient occurs in this zone, preferably so that more than 70% of the temperature gradient occurs in this zone, and still more preferably so that more than 90% of the temperature gradient occurs in this zone.

It should be understood that there can be additional heat removal from the absorber column other than the heat removal with which the present invention is especially concerned. Thus, reference to "the heat removal" or the like is, of course, not intended to exclude additional heat removal from the absorber column. The heat removal in accordance with the process of the present invention cannot be attained simply and solely by introducing a cool absorbent to the top of the column, as is done in numerous prior art processes.

Heat removal in accordance with the present invention can be accomplished by various means, for example by the use of cooling coils located in a zone above the gaseous mixture feed point to the column and operated to remove heat from the absorption column in a sufficient amount so that more than half of the temperature gradient for the absorbent liquid in the column is across at least 2 stages above the gaseous mixture feed point and below the top 2 stages of the column.

However, preferably the heat is removed in accordance with the present invention by a cooled recycle loop of absorbent. This is accomplished by withdrawing a stream of absorbent from the lower part of the column, cooling the stream and then recycling the resulting cooled absorbent to the column. Use of the terminology "withdrawing a stream of absorbent from the lower part of the column" connotes that the absorbent is withdrawn below at least the uppermost 2 stages of the column. Preferably the absorbent is withdrawn from a point below about the middle stage of the column. It has been found that this embodiment of the invention can be carried out particularly advantageously by withdrawing absorbent from the bottom of the column, then cooling the absorbent and recycling it to a point between about 2 and 10, preferably between 4 and 7, stages above the gaseous mixture feed point to the column. Preferred absorber columns for use in the process of the present invention have between about 10 and 50 stages.

Among other factors, the present invention is based on our findings that heat removal in accordance with the present invention results in a highly operable maleic anhydride absorption system and an outstandingly efficient absorption system. Solids precipitation or formation is minimized by the process of the present invention. Solids precipitation can, of course, reduce the operability or onstream time of a maleic anhydride absorption process by causing plugging, fouling, and like problems.

Also, in the process of the present invention high loading is achieved for the absorbent. High loading of the absorbent results in the reduction of energy consumption and other benefits, such as reduced absorbent losses and reduced column sizes, all of which go to improve the economic efficiency of the absorption process. Reduction in energy consumption by high loading of the absorbent is achieved because when the absorbent can be more highly loaded with the anhydride solute, it can be more efficiently (less total heat input needed to the stripper) stripped of the anhydride.

The process of the present invention is also based on our finding that to avoid or at least reduce the tendency of solids formation when removing heat from the lower part of the column, the temperature gradient drop imposed by the heat removal should be spread over a zone of 2, preferably 3 or more, stages, rather than partially localized.

The term "stage" is used herein to mean one sieve plate or the equivalent. A theoretical stage might require, for example, from 1 to as many as 3 sieve plates. Bubble cap trays and packing or stage-wise contacting or distillation means can be equated with one another in terms of theoretical stages; see, for example, T. K. Sherwood, "Absorption and Extraction," First Edition, particularly pp. 82-93. However, for purposes of specificness herein, one sieve plate is taken as equal to one bubble cap tray or one foot of packing, all 3 of these alternatives being considered herein as equal to one stage.

According to a preferred embodiment of the present invention wherein heat is removed from the absorber by a cooled recycle stream, a process is provided for separating maleic or phthalic anhydride from a gaseous mixture containing the anhydride which comprises:

a. introducing the gaseous mixture to an absorption column at a mixture feed point near the bottom of the column;

b. introducing lean organic absorbent to the column at an absorbent feed point near the top of the column;

c. countercurrently contacting the ascending gaseous mixture in the absorption column with a downwardly flowing stream of the organic absorbent effective to preferentially absorb the anhydride and obtain rich absorbent;

d. withdrawing a stream of the absorbent from the column;

e. cooling at least a portion of said withdrawn stream to obtain cooled absorbent;

f. recycling the cooled absorbent to the column at a point above the withdrawal point and at least two stages above the gaseous mixture feed point and at least two stages below the lean absorbent feed point;

g. withdrawing the rich absorbent from the bottom of the column; and

h. stripping anhydride from the rich absorbent to obtain a separated anhydride and said lean absorbent.

The temperature of the gaseous feed to the process of the present invention is preferably sufficiently high to maintain all the components of the gas feed in the vapor (gaseous) phase. Thus, in the case of maleic anhydride, the temperature normally should be above 200.degree. F., preferably above 225.degree. F., and usually between about 250.degree. to 350.degree. F. Of course, the exact temperature to maintain vapor phase depends on the pressure as well as the composition of the gas stream fed to the absorption process of the present invention. Preferably the entire product effluent stream from the hydrocarbon oxidation reactor is processed in the absorption step of the present invention.

The term "organic absorbent" is used herein to mean an organic compound which is liquid at atmospheric pressure and a temperature of 20.degree. F. above the normal boiling point of maleic anhydride, and in which maleic anhydride at the aforesaid temperature and pressure is at least 50 times more soluble by weight than water or nitrogen or oxygen or carbon dioxide. Thus, the organic absorbent must be preferential or selective for the absorption of gaseous (vaporous) maleic anhydride compared to the nitrogen, water, oxygen and carbon dioxide which make up nearly all of the balance of the effluent from the reactor wherein maleic anhydride is produced by oxidizing a hydrocarbon.

In the case of applying the process of the present invention to phthalic anhydride recovery, the above definition also applies, with "phthalic anhydride" substituted for "maleic anhydride".

Suitable organic absorbents for the absorption of maleic anhydride include, for example, dibutylphthalate, dihexylphthalate, dioctylphthalate, tricresylphosphate, alkenyl succinic anhydride, alkyl succinic anhydride, etc.

FIG. 1 is a schematic flow diagram illustrating 2 preferred embodiments of the present invention. FIG. 2 is a diagram schematically indicating a preferred temperature gradient for absorption operation in accordance with the present invention.

Referring to FIG. 1, a gaseous mixture containing anhydride is fed via line 1 to absorber 2. This gaseous mixture is generally sufficiently hot to be above the condensation point of any component in the mixture; for example, in the case of maleic anhydride produced by air oxidation of a hydrocarbon, the condensation point is generally between about 200.degree. and 250.degree. F., depending on the pressure and the composition of the gas, particularly the water content. The gaseous mixture is typically generated by air oxidation of a hydrocarbon stream to produce an anhydride as described in various prior art references on production of maleic anhydride or phthalic anhydride by oxidation of a hydrocarbon. The gaseous mixture from the oxidation of the hydrocarbon is usually cooled to a temperature in the range 250.degree. F. to 350.degree. F., preferably 270.degree.-280.degree. F., before it is fed to absorber 2.

In absorber 2, the anhydride is scrubbed out of the gas mixture by an organic absorbent. The absorbent is introduced to the absorber via line 3, and flows downwardly in the absorber in countercurrent contact to the upwardly flowing gaseous mixture. By the time the gaseous mixture reaches the top of the absorber, it is substantially free of anhydride. The gas mixture exits via line 5. The anhydride-rich absorbent leaves the bottom of the absorber via line 4.

In stripper 18, the rich absorbent is stripped of the anhydride solute. The stripped anhydride product leaves the stripper via line 13. The stripping requires a heat input as indicated by heater 6. As indicated in FIG. 1, heat input is accomplished by a hot fluid flowing through heater 6, as indicated by line 7, countercurrent to absorbent material introduced to the heater via line 8. Hot, partially vaporized absorbent is withdrawn from heater 6 and is introduced to stripper 18 via line 9. Alternatively, a hot stripping gas can be used to carry out stripping of the absorbent.

The stripper may be called a distillation column, especially since in anhydride recovery there is typically no injected stripping gas, but rather the stripping is typically effected by the use of a reboiler and distillation column. In any case, it is important that the absorbent be separated from the dissolved anhydride, and this separation operation can be termed either "stripping" or "distillation". The purified absorbent resulting from the stripping can be termed "lean" absorbent. The lean absorbent is withdrawn from the bottom of stripper 18 via line 12.

The lean absorbent can be cooled before being fed to absorber 2. As shown in the drawing, the absorbent is cooled in cooler 10 by heat exchange with countercurrently flowing liquid, as indicated by arrow 11. In prior art processes, the heat removal from the absorber column is typically accomplished by means such as cooler 10, or perhaps also by introducing a cool reflux stream or its equivalent to the top of the absorber column.

In the present invention, removal of heat from a zone below the top 2 stages of the absorber is required. As shown on the drawing, this may be accomplished by mechanical cooling means such as cooling coil 14. However, preferably the heat removal is by means of the recycle loop indicated by lines 17 and 19. In this embodiment, absorbent is withdrawn via line 17 from the bottom of the absorber column, is cooled in cooler 15 and then returned several stages above the bottom of the absorber, for example 2 to 7 stages above the bottom. The cooling effect which will result from the recycle is thus spread over several stages to cause a temperature gradient drop in a zone made up of several stages in the lower part of the absorber column.

FIG. 2 is the profile to be expected with Example 1, and thus exemplifies a temperature gradient resulting from the recycle loop heat removal. The ordinate in FIG. 2 represents position along the length of the absorber. The abscissa in FIG. 2 represents liquid temperature. Thus, it is seen in the drawing that the temperature at the top of the column is the lowest and the temperature at the bottom is the highest. The temperature of the gaseous stream which is directly contacted with the liquid absorbent in the absorber to effect anhydride absorption will substantially parallel that of the liquid absorbent. The feed gas stream to the column heats the liquid absorbent as the absorbent travels downwardly in the column, and the liquid absorbent cools the gaseous mixture as it travels upwardly in the column. As shown by FIG. 2, most of the temperature drop from the bottom to the top of the column is between the bottom tray and tray No. 5, where the cooled absorbent stream from the recycle loop is introduced to the absorber column. FIG. 2 also illustrates the temperature gradient drop spread over several stages along the vertical length of the absorber column.

The following is a calculated example of a process in accordance with a preferred embodiment of the present invention based on experimental data.

Butane is reacted with air in an oxidation reactor using a catalyst as described in the aforementioned Ser. No. 263,883. The oxidation effluent gas stream, after being cooled to about 275.degree. F., is fed via line 1 to absorber 2 at a rate of 522.3 lbs/hr. The gas stream contains 2.1 weight percent (11.1 lbs/hr) of maleic anhydride. The absorber contains 20 sieve trays.

Lean organic absorbent is introduced via line 3 to the top of the absorber at a temperature of 150.degree. F. and a flow rate of 119.9 lbs/hr. The scrubbed effluent gas in the amount of 511.3 lbs/hr and containing 0.02% maleic anhydride is removed via line 5 from the top of the absorber at a temperature of about 160.degree. F. Rich organic absorbent, 130.9 lbs/hr and resulting from the countercurrent contacting of the reactor effluent gases is removed from the bottom of the absorber via line 4, at a temperature of 225.degree. F., and is passed to stripper 18 via line 4. The maleic anhydride content of the organic absorbent is 8.5 weight percent; thus 11.1 lbs/hr of maleic anhydride is removed through line 4.

Another portion of the rich absorbent, 473.1 lbs/hr, is removed from the bottom of the absorber via line 17 at a temperature of 225.degree. F., and is cooled to 165.degree. F. in exchanger 16, and is then recycled to the absorber at tray 5 via line 19. This recycle loop results in sufficient heat removal from the lower part of the absorber column to attain a temperature gradient drop of 60.degree. F. between tray 5 and the first tray. In this regard, the trays are numbered starting with tray 1 for the first tray above the feed gas inlet to the absorber column. As the temperature of the absorbent fed to the top of the column is 160.degree. F., the temperature gradient drop of 60.degree. F. for the 5 trays at the bottom of the column compared to the over-all column temperature gradient drop of 65.degree. F. results in over 92% of the temperature gradient drop occurring from tray 5 to the bottom of the column.

Referring again to the portion of the maleic anhydride-rich absorbent removed from the bottom of the absorber via line 4, this rich absorbent is stripped or fractionally distilled in accordance with known principles in stripper 18 to obtain lean absorbent and product maleic anhydride. The amount of rich absorbent fed to stripper 18 is 130.9 lbs/hr. Stripper 18 contains 10 sieve trays, and is operated at an overhead vacuum of 30 mm Hg absolute pressure. The heat input to the stripper via heater 6 results in a reboiler outlet temperature for the partially vaporized absorbent in line 9 of 435.degree. F. The heating of the absorbent drives the absorbed maleic anhydride out of the absorbent, and stage-wise stripping occurs in the stripper due to the counter-currently flowing vapors and liquid in the stripper column. Upper column cooling is effected by means of the reflux stream. Other cooling means of some sort may be used at the upper part of stripper 18 to minimize losses of absorbent and to keep the stripper in over-all heat balance. Crude maleic anhydride vapor is removed at a temperature of 215.degree. F. from the top of the stripper via line 13, at a rate of 11.0 lbs/hr. Lean absorbent at a rate of 119.9 lbs/hr is removed from the bottom of the stripper via line 12 for recycle to absorber 2. The temperature of the lean absorbent removed from the bottom of the stripper is 435.degree. F., and the maleic anhydride content is 0.1 weight percent (0.1 lbs/hr). The lean absorbent is cooled to 150.degree. F. by heat exchange in cooler 10 and then is introduced to the absorber via line 3.

This example is calculated using an absorption process in accordance with the prior art to give the same recovery of crude maleic anhydride vapor as was obtained in Example 1.

In this example, the same processing is used, except that there is no recycle of organic absorbent through lines 17 and 19. Feed gas is introduced to absorber 2 via line 1 at a rate of 522.3 lbs/hr and a temperature of 275.degree. F. As in Example 1, the feed gas contains 2.1 weight percent maleic anhydride. Lean absorbent is introduced to the top of the absorber at 372 lbs/hr and a temperature of 150.degree. F. via line 5 from the top of the absorber. The temperature of the scrubbed gases is 160.degree. F. and the maleic anhydride content is 0.02 weight percent (0.1 lbs/hr) of maleic anhydride.

Rich absorbent is removed from the bottom of the absorber via line 4 at a rate of 383.0 lbs/hr and a temperature of 250.degree. F. The maleic anhydride content of the absorbent is 2.9 weight percent, or 11.3 lbs/hr.

The rich absorbent is charged to stripper 18, which stripper operates the same as that of Example 1. However, because more absorbent needs to be stripped in accordance with this operation compared to the Example 1 operation, the lower portion of the stripper is about 2 to 4 times larger in cross-sectional area than the stripper used in Example 1, and the heat energy input to heater 6 is much larger than in Example 1. In the stripper, 11 lbs/hr of maleic anhydride are stripped out of the absorbent and removed overhead via line 13. Lean absorbent is removed from the bottom of the stripper at a rate of 372.0 lbs/hr. After cooling the lean absorbent from 435.degree. F. to 150.degree. F., the absorbent is introduced to absorber 2 via line 3. The maleic anhydride content of the lean absorbent recycle stream is 0.1 weight percent, or 0.3 lbs/hr of maleic anhydride.

A comparison of the above examples shows that there is a surprisingly high savings in heat duty achieved by the process of the present invention. This is shown in the following table.

The values in this table show a 50% reduction in total heat duty for the present process over the prior art process. In terms of heat exchanger cost and utility requirements for heating and cooling in a commercial maleic anhydride plant, this reduction in heat duty represents a substantial yearly savings. In addition, the stripper column is less expensive, due to the extra cross-sectional area required in the lower portions of the prior art stripper column.

Prior art processes utilizing the type of flow scheme as in Example 2 include: U.S. Pat. Nos. 2,574,644, 3,040,059, and 2,893,924; and British Pat. Nos. 763,339, 768,551, and 727,828.