JPH08319486A - Method for removing aromatic hydrocarbon from hydrocarbon vapor stream - Google Patents

Abstract

PROBLEM TO BE SOLVED: To remove arom. hydrocarbons from a hydrocarbon vapor stream contg. arom. hydrocarbons by contacting a hydrocarbon vapor stream with an extracting solvent which selectively dissolves arom. hydrocarbons in an extractive distillation column.

SOLUTION: This method of removing arom. hydrocarbons from a hydrocarbon vapor stream contg. arom. hydrocarbons comprises a step where an extracting solvent which selectively dissolves arom. hydrocarbons is contacted with the hydrocarbon vapor stream in an extractive distillation column (1), a step where a vapor raffinate of low arom. hydrocarbon content is drawn out from the upper part of the column (2), and a step where the liq. solvent stream of increased arom. hydrocarbon content is drawn out from the bottom of the column (3), and a part of the liq. is heated at a condition that, at least a part of the arom. hydrocarbons existing in the liq. withdrawn from the bottom of the column is vaporized, but the extracting solvent is not vaporized, and the heated liq. and vapor stream are returned to the extractive distillation column. It is pref. that the arom. hydrocarbons in the hydrocarbon vapor stream is less than 50 wt.%.

COPYRIGHT: (C)1996,JPO

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a method for removing aromatic hydrocarbons from hydrocarbon vapor streams, particularly monoaromatic hydrocarbons, suitably benzene, from catalytic reformate streams suitable as gasoline blending components. Regarding the method.

[0002]

BACKGROUND OF THE INVENTION Three common hydrocarbon components are commonly used to prepare finished gasoline. These hydrocarbon components are paraffins, olefins and aromatic hydrocarbons, each with its own characteristics and contributing to determining the properties of the final gasoline. Paraffins include among others butane, isopentane, alkylate, isomerate, straight run naphtha and light hydrocrackers and are thus obtained from a number of different refining processes such as crude oil distillation, isomerization, alkylation and hydrocracking. The olefin component includes monomers, oligomers and polymers and is usually obtained from processes such as catalytic cracking, steam cracking and polymerization. Finally, aromatic hydrocarbon components are usually obtained by catalytic reforming, which is a combination of various reactions (dehydrogenation, isomerization, dehydrocyclization and hydrocracking), which increases the octane number of gasoline. The purpose is to Feedstocks commonly used in catalytic reforming processes are heavy naphthas containing relatively large amounts of paraffinic hydrocarbons, including straight chain, branched chain and cyclic paraffins.

Catalytic reforming processes are well known to those skilled in the art. In general, catalytic reforming does not significantly change the boiling point of the modifier feed, but because the paraffinic hydrocarbons are converted to aromatic hydrocarbons to form hydrogen, the chemical composition of the feed changes significantly. As is generally accepted, aromatic hydrocarbons have the effect of increasing the octane number, making the products from catalytic modifiers very suitable as gasoline blending components. Catalytic reforming processes usually involve passing naphtha feedstock over a suitable reforming catalyst under reforming conditions, recovering the hydrogen formed, and separating the product into two or more reformates. Including the step of doing. Two or three cuts, respectively, are typically used to form two or three reformates. The composition of each of the resulting reformates can be adjusted to some extent by the fractionation temperature and pressure applied to the splitter. The top fraction or light reformate recovered from the splitter usually contains major amounts of non-aromatic hydrocarbons and minor amounts of monoaromatic hydrocarbons, especially benzene and lower concentrations of toluene. The concentration of monoaromatic hydrocarbons in general, especially benzene and toluene, in the light reformate can be adjusted by changing the fractionation temperature. In addition to the light reformate, one or more heavy reformates can be obtained. The heavy reformate contains heavy aromatic hydrocarbons and is suitable for use as an automotive gasoline blending component. Light reformate containing benzene is also suitable for use as a blending component in gasoline. However, in recent years, the presence of benzene in automobile gasoline has come to be regarded as inappropriate for environmental reasons, and the environmental standard for the benzene concentration of future automobile gasoline is more than that of most present automobile gasoline. Is also expected to be set to a low value. Therefore, it is necessary to reduce the benzene concentration of automobile gasoline.

Benzene can be removed from the reformate stream by methods known to those skilled in the art. For example, a fractional distillation or an extraction method using an extraction solvent capable of selectively dissolving a monoaromatic hydrocarbon, particularly benzene, is known. Ethylene glycol and N-substituted morpholines are known as suitable solvents for this purpose, but a particularly useful solvent is sulfolane. Although benzene is removed from the reformate by these methods, the benzene concentrate still contains substantial amounts of other components in addition to benzene. A typical example of such a component is a valuable raffinate component that can be suitably used as a gasoline blending component. Therefore, it would be advantageous if these valuable raffinate components could be recovered and added to the refined raffinate stream for use as gasoline blending components. Furthermore, since benzene is a valuable product, it would be advantageous to be able to provide a benzene enriched benzene concentrate that would be an economically attractive source of benzene production.

International patent application WO-A-94 / 19426
Discloses a method for removing aromatic hydrocarbons from a hydrocarbon vapor stream, which method comprises contacting the hydrocarbon vapor stream with a liquid absorbing solvent that selectively absorbs the aromatic hydrocarbons, mainly The step of withdrawing a vapor stream of non-aromatic hydrocarbons from the top of the absorption zone and the liquid solvent rich in aromatic hydrocarbons from the bottom of the absorption zone. It is expedient to treat this aromatic hydrocarbon-rich solvent in a subsequent distillation stage to produce a distillate stream consisting mainly of aromatic hydrocarbons and a bottom stream consisting mainly of liquid absorbing solvent, which is then recycled to the absorption zone. Can be circulated. This method
It is particularly suitable for removing benzene from gasoline blend stocks to produce benzene concentrates with relatively high benzene concentrations. However, in addition to benzene, this concentrate still contains significant amounts of components, mainly non-aromatic hydrocarbons, which make gasoline a very useful component. Therefore, it is imperative that the benzene concentrate be further separated to produce benzene that meets specifications that can be sold as is, while the volume of components other than benzene is significant when transporting the concentrate to a benzene manufacturing facility. In addition, it increases the transportation cost of benzene per volume unit and affects the cost price of the final benzene product.

[0006]

It is an object of the present invention to provide a method which does not have the drawbacks of the prior art methods. More specifically, the object of the present invention is to provide a more effective method for removing aromatic hydrocarbons from a hydrocarbon vapor stream,
The result is to reduce the non-aromatic hydrocarbon concentration and increase the aromatic hydrocarbon concentration of the aromatic hydrocarbon concentrate. It is an object of the present invention to provide an economically attractive method for removing monoaromatic hydrocarbons, especially benzene, from reformates suitable as gasoline blending components, so that benzene concentrates of By reducing the concentration of non-aromatic hydrocarbons and increasing the concentration of benzene, the cost and cost are reduced to provide a very useful source of benzene, while the raffinate or benzene concentration substantially free of benzene is 1% by weight. Produces less than% raffinate. As a final object, the object of the present invention is a highly pure form under optimal conditions, i.e. a concentration of more than 95% by weight, and in some cases more than 99% by weight, and therefore to meet the benzene sales specifications almost or completely. It is to provide a method capable of directly obtaining benzene to be satisfied.

[0007]

These aims have been achieved by applying specific heating steps in the lower part of the absorption zone. Accordingly, the present invention relates to a method of removing aromatic hydrocarbons from a hydrocarbon vapor stream containing aromatic hydrocarbons,
The method comprises the steps of (1) contacting an extraction solvent capable of selectively dissolving aromatic hydrocarbons with a hydrocarbon vapor stream in an extractive distillation column, and (2) vapor raffinate having a low aromatic hydrocarbon concentration. From the top of the extractive distillation column, and (3) removing a liquid solvent stream having an increased concentration of aromatic hydrocarbons from the bottom of the extractive distillation column,
Thus at least one containing the extraction solvent and the aromatic hydrocarbon
After removing the seed liquid from the bottom of the extractive distillation column,
Heating at least a portion of said liquid under conditions such that at least a portion of the aromatic hydrocarbons present in the withdrawn liquid are vaporized and the extraction solvent is not substantially vaporized, followed by a heated liquid / vapor stream. Back into the extractive distillation column. In general, extractive distillation methods using extractive distillation columns are known to those skilled in the art. Examples of such methods are eg US-A-3,
723,256, EP-A-0,073,945 and E
P-A-0,155,992. Extractive distillation columns generally include a tray column containing sieves or valve trays or other fractions, means for introducing the extraction solvent and feed, and means for removing at least the vapor top and liquid bottom fractions. For the purposes of the present invention, the extractive distillation column also includes in its lower zone means for withdrawing, heating and reintroducing the heated liquid / vapor stream. Suitably such means consist of one or more reboilers.

The extraction solvent is introduced from one point in the upper zone of the column and flows downward in the column. The vapor feed is introduced into the column from a point lower than the introduction point of the solvent and flows upward in the column, and thus the feed and the solvent flow countercurrently in the column, so that they can be effectively contacted with each other. At the top of the column a vapor feed is withdrawn (raffinate) and at the bottom of the column a liquid solvent stream enriched in aromatic hydrocarbons is withdrawn. Although the theoretical plate number of the extractive distillation column can be in a wide range, the theoretical plate number is 2 to 25, preferably 3 in consideration of technical and economic problems.
It has been found to be advantageous if it is -15. The distribution of the number of theoretical plates above and below the feed inlet is not particularly limited either, but since the extraction of aromatic hydrocarbons from the feed is mainly performed in the zone higher than the feed inlet, the number of theoretical plates higher than the feed inlet is usually It is equal to or more than the theoretical plate number lower than the feed inlet. Therefore, the ratio of theoretical plate numbers above and below the feed inlet is suitably at least 1, more suitably 1.5-20, optimally 2-15.

Since the hydrocarbon vapor stream used as the feed to the extractive distillation column contains both aromatic and non-aromatic hydrocarbons, the total concentration of aromatic hydrocarbons is preferably less than 50% by weight. However, the hydrocarbon vapor flow should be 35
It contains less than 25% by weight, more suitably less than 25% by weight of aromatic hydrocarbons. In principle, the method of the present invention can be applied to remove monoaromatic hydrocarbons and aromatic hydrocarbons containing more than one aromatic ring, but the monoaromatic hydrocarbons are removed from hydrocarbon vapor streams. It is especially suitable for Monohydrocarbons that can be suitably removed include benzene, toluene and xylene. However, for the purposes of the present invention, it is preferred that at least 50% by weight, preferably at least 90% by weight, of the aromatic hydrocarbons present are benzene. At least 95% by weight of the aromatic hydrocarbons present, more preferably at least 99
When the weight percent is benzene, the process of the present invention can recover very high purity benzene. The hydrocarbon vapor fraction should preferably have a boiling point of 50-200 ° C, more preferably 60-175 ° C. Various mixed hydrocarbon vapor streams satisfying one or more of the above requirements for aromatic hydrocarbon concentration, benzene concentration and boiling range can be treated according to the present invention as light reformate from catalytic reforming processes. Preference is given to using the hydrocarbon vapor stream obtained. Such light reformates typically have relatively low aromatic hydrocarbon concentrations (usually less than 25% by weight),
Almost all of the aromatic hydrocarbons present are monoaromatic hydrocarbons. Of these monoaromatic hydrocarbons, the highest percentage (usually above 90% by weight) is benzene, and the exact percentage of non-benzene (mono) aromatic hydrocarbons, especially toluene, is the reformate. Determined by the fractionation temperature and pressure applied to the splitter. In order to directly obtain the highly pure form of benzene using the process of the present invention, the fractional distillation temperature under a given pressure applied to the reformate splitter depends on the aromatic hydrocarbons present in the light reformate. Should be selected to be at least 99% by weight of benzene. An additional advantage of using a light reformate as a hydrocarbon vapor feed is that the light reformate recovered from the reformate splitter is sent directly to the extractive distillation column without intermediate condensation and / or pressure relief steps. The catalytic reforming method can be very well combined with the method of the present invention. It will be appreciated that this is very attractive in terms of availability.

The operating conditions of the extractive distillation column can be varied within the operating ranges customary for this type of equipment. Important operating parameters are temperature, pressure and solvent to feed ratio. All these parameters are closely correlated and it is within the common knowledge of the person skilled in the art to adjust the individual parameters to each other. Thus, the temperature in the extractive distillation column can be from a minimum of 30 ° C at the top of the column to a maximum of 200 ° C at the bottom of the column. A particularly suitable operating range for the temperature of the extractive distillation column is 40 ° C (top) to 175 ° C (bottom). The pressure in the extractive distillation column is suitably 1-5 bar, preferably 1-3 bar, and the weight ratio of solvent to the feed is suitably 1-10, preferably 1.5-5. The temperature of the feed at the time of introducing the extractive distillation column should be chosen so that the feed is in the vapor phase at the column introduction pressure. Usually, this pressure is equal to or slightly higher than the pressure inside the extractive distillation column, and is therefore about the same as the pressure inside the column. The temperature of the extraction solvent at the time of introduction into the upper zone of the extractive distillation column is suitably 30 to 120 ° C, more suitably 35 to 100 ° C. Any solvent capable of selectively dissolving aromatic hydrocarbons, especially monoaromatic hydrocarbons, and having a higher boiling point than the dissolving aromatic hydrocarbons can be used as the extraction solvent. Therefore, if the main purpose is to remove benzene, the solvent should be capable of dissolving benzene and have a higher boiling point than benzene. Such solvents are known to those skilled in the art, and examples thereof include N-substituted morpholines such as N-formylmorpholine and several ethylene glycols. However, the preferred extraction solvent is sulfolane. The sulfolane may be added as it is or mixed with any of the above solvents. A small amount of water may also be present.

According to the present invention, after at least one liquid containing the extraction solvent and the aromatic hydrocarbon is taken out from the lower part of the extractive distillation column, at least a part of the aromatic hydrocarbons present in the taken-out liquid is evaporated. And heating at least a portion of the liquid under conditions such that the extraction solvent does not substantially evaporate, after which the heated liquid / vapor stream is returned to the extractive distillation column. This is suitably done with a reboiler. The use of reboilers is known in the separation art and suitable reboilers for the purposes of the present invention can in principle be any conventional reboiler. In general, a reboiler can be defined as a special type heat exchanger that supplies heat to the bottom of a fractionation column, in this case an extractive distillation column. The liquid to be extracted and the liquid containing the aromatic hydrocarbon may be the same as or different in composition from the bottom fraction. In the latter case, the liquid is removed from the column at a point higher than the bottom fraction is removed from the column. After removal from the column, a portion of the liquid may be sent to another destination and thus removed as an outlet stream prior to heating. In that case, only a part of the liquid taken out from the column is heated by the reboiler and re-introduced into the column.
Alternatively, the entire liquid stream withdrawn from the column is heated with a reboiler and the heated liquid / vapor stream is returned to the column. It has proved to be very advantageous to use at least one reboiler in the lower part of the extractive distillation column. This makes it possible to recover an aromatic hydrocarbon concentrate having a high aromatic hydrocarbon concentration, and in the case of a light reformate feed containing a secondary amount of aromatic hydrocarbon, that is, 99% by weight of the aromatic hydrocarbon. When the above is benzene, it is also possible to recover highly pure form of benzene.

The amount of heat supplied by the reboiler to the bottom of the extractive distillation column (conveniently referred to as the reboiler capacity) depends on the operating conditions applied to the extractive distillation column, the desired purity of the aromatic hydrocarbon concentrate and the raffinate used as a gasoline blending component. It is determined by the desired quality of. Suitably, the liquid solvent stream enriched in aromatic hydrocarbons obtained in step (3) of the process of the invention is subjected to a subsequent solvent recovery step to separate the solvent from the aromatic hydrocarbons. Usually some water is also present and such water also needs to be separated from the solvent. Solvent recovery can be accomplished by methods known to those skilled in the art. A particularly suitable solvent recovery step is to send the liquid solvent stream to a solvent recovery column that normally operates under reduced pressure at a temperature of 70 ° C (column top) to 200 ° C (column bottom) and vapor at the top of the solvent recovery column. Withdrawing a fraction, withdrawing the liquid solvent fraction at the bottom of the column, and condensing the top fraction to obtain an aqueous phase and a hydrocarbon phase containing aromatic hydrocarbons. Suitably, the hydrocarbon phase consists of at least 90% by weight, preferably at least 95% by weight of aromatic hydrocarbons, which aromatic hydrocarbons preferably consist of> 90% by weight of benzene. If a suitable light reformate feed is used, the hydrocarbon phase may be highly pure benzene, ie> 99% by weight.
May be composed of benzene. A portion of the hydrocarbon phase may be reintroduced into the solvent recovery column to further increase the yield and purity of benzene. Suitably, all or part of the liquid solvent fraction withdrawn at the bottom of the solvent recovery column is recycled to the extractive distillation column for reuse.

[0013]

FIG. 1 illustrates a suitable aspect of the present invention. H indicates a heat exchanger. The hydrocarbon vapor feed (1) is introduced into the extractive distillation column (2) from point F and the liquid extraction solvent is introduced into the column from point S. Vapor raffinate (3) is withdrawn from point R at the top of the column and sent through heat exchanger (H) to condenser (5). Water (6) is separated from the raffinate (7) having a low aromatic hydrocarbon concentration in the condenser, and the raffinate is used as a blending component for automobile gasoline. A portion of the raffinate (7) having a low aromatic hydrocarbon concentration may be refluxed to the extractive distillation column (2) to improve the purity of the raffinate stream (7) with respect to the liquid extraction solvent (not shown). Extractive distillation column (2)
After the liquid solvent (8) having a high aromatic hydrocarbon concentration is taken out from the point A at the lower part of the column and heated by the reboiler (9), it becomes a liquid / vapor mixture (10), which is higher than the point B. To be reintroduced into the column. Point C at the bottom of the column
A liquid bottoms stream (11), which is another solvent fraction with a high aromatic hydrocarbon concentration, is taken from and is sent to a solvent recovery column (12). The solvent (13) is taken out from the bottom of the solvent recovery column (12) and introduced into the extractive distillation column (2) from the point S through the heat exchanger H. The extract (14) is withdrawn from the top of the solvent recovery column (12) and sent through the heat exchanger H to the condenser (15). Water (17) and aromatic hydrocarbon concentrate (16) are recovered from the condenser (15). Aromatic hydrocarbon concentrate (16)
A part of the solvent may be refluxed to the solvent recovery column (12) to further improve the solvent recovery efficiency.

[0014]

EXAMPLES The present invention will be further described below with reference to examples. Example 1 A light reformed oil having a composition as shown in Table 1 is introduced into an extractive distillation column having 10 theoretical plates and a reboiler provided at the bottom. The reboiler capacity is 3472 kW. The working pressure in the extractive distillation column is 1.5 bar, the solvent to feed ratio is 3.0 (by weight), the feed is introduced into the extractive distillation column at a temperature of 78 ° C. and the sulfolane is at 75 ° C. Introduce to the top of the column at temperature.

[0015]

[Table 1]

The vapor fraction is removed from the top of the extractive distillation column and sent to a condenser where it is condensed and separated into an aqueous phase and a raffinate phase. Raffinate phase is benzene 1.0
Contains only wt%. The benzene rich liquid solvent is withdrawn from the bottom of the extractive distillation column and sent to a solvent recovery column operating at a pressure of 0.4 bar and having a top temperature of 94 ° C and a bottom temperature of 173 ° C. The vapor fraction taken from the top of the solvent recovery column is sent to a condenser where it is condensed and separated into a benzene concentrate and an aqueous phase. The benzene concentrate contains 97.7 wt% benzene and 0.9 wt% toluene, and the total benzene recovery, ie the weight percentage of benzene present in the recovered light reformate, is 95 wt%. found.

Comparative Example 1 Example 1 without a reboiler below the extractive distillation column
A light reformate as described in 1. is introduced. Follow the same procedure as in Example 1. The total benzene recovery is 94% by weight, but the benzene concentrate contains only 66.3% by weight of benzene.

Example 2 A light reformer having almost the same composition as that used in the example except that an extractive distillation column having 10 theoretical plates and a reboiler at the bottom has a toluene concentration of 0.1% by weight. Introduce quality oil. The reboiler capacity is 4630 kW.
The working pressure in the extractive distillation column is 1.05 bar,
The solvent to feed ratio is 4.0 (by weight), the feed is introduced into the extractive distillation column at a temperature of 65 ° C and the sulfolane is introduced at the top of the column at a temperature of 44 ° C. Otherwise, follow the same procedure as in Example 1. Therefore, the vapor fraction is removed from the top of the extractive distillation column, sent to a condenser for condensation and separated into an aqueous phase and a raffinate phase. The raffinate phase was found to contain less than 1.0 wt% benzene. The liquid solvent with high benzene concentration is taken out from the bottom of the extractive distillation column and sent to the solvent recovery column operating under the same conditions as in Example 1. The vapor fraction taken from the top of the solvent recovery column is sent to a condenser for condensation and separated into a benzene concentrate and an aqueous phase. The benzene concentrate obtained contains 99.8% by weight of benzene and can therefore be regarded as highly pure. Other components present are toluene (0.1% by weight), water (0.1% by weight).
% By weight), sulfur (1 ppm by weight or ppmw) and sulfolane (3 ppmw). The total benzene recovery is again 95% by weight.

[Brief description of drawings]

FIG. 1 illustrates a suitable aspect of the present invention.

[Explanation of symbols]

 H heat exchanger 1 hydrocarbon vapor feed 2 extractive distillation column 3 vapor raffinate 5,15 condenser 6,17 water 7 raffinate with low aromatic hydrocarbon concentration 8 liquid solvent with high aromatic hydrocarbon concentration 9 reboiler 10 liquid / vapor Mixture 11 Liquid bottom stream 12 Solvent recovery column 13 Solvent 14 Extract 16 Concentrate

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Brahm de Ruiter Holland 2596 H. Earl, The Hague, Karel van Bilantran 30 (72) Inventor Eritsk Seef Holland 2596 H. Earl, The・ The Hague, Karel Wan Billantran 30

Claims (9)

[Claims] 1. A method for removing aromatic hydrocarbons from a hydrocarbon vapor stream containing aromatic hydrocarbons, the method comprising:
Contacting an extraction solvent capable of selectively dissolving aromatic hydrocarbons with a hydrocarbon vapor stream in the extractive distillation column, and (2) vaporizing raffinate having a low aromatic hydrocarbon concentration at the top of the extractive distillation column. And take it out from (3)
Withdrawing a liquid solvent stream having an increased concentration of aromatic hydrocarbons from the bottom of the extractive distillation column, thus removing at least one liquid containing the extractant solvent and the aromatic hydrocarbons from the bottom of the extractive distillation column, Heating at least a portion of said liquid under conditions such that at least a portion of the aromatic hydrocarbons present in the withdrawn liquid are vaporized and the extraction solvent is not substantially vaporized, followed by a heated liquid / vapor stream. To the extractive distillation column. 2. A process according to claim 1 wherein the hydrocarbon vapor stream contains less than 50% by weight, preferably less than 35% by weight and more preferably less than 25% by weight aromatic hydrocarbons. 3. At least 5 of the aromatic hydrocarbons present.
Process according to claim 1 or 2, wherein 0% by weight, preferably at least 90% by weight, is benzene. 4. At least 9 of the aromatic hydrocarbons present.
The method according to claim 3, wherein 9% by weight is benzene. 5. A method according to any one of claims 1 to 4, wherein the hydrocarbon vapor stream is a light reformate stream. 6. The method according to claim 1, wherein the extraction solvent is sulfolane. 7. The temperature in the extractive distillation column is from a minimum of 30 ° C. at the top of the column to a maximum of 200 ° C. at the bottom of the column,
The pressure in the extractive distillation column is 1-5 bar and the weight ratio of solvent to feed is 1-10.
7. The method according to any one of items 6 to 6. 8. A process according to claim 1, wherein the liquid solvent stream obtained in step (3) is subjected to a solvent recovery step. 9. The solvent recovery step directs a liquid solvent stream to a solvent recovery column, withdraws a vapor fraction from the top of the solvent recovery column and withdraws a liquid solvent fraction from the bottom of the column, and condenses the top fraction. 9. The method of claim 8 including the step of obtaining an aqueous phase and a hydrocarbon phase comprising aromatic hydrocarbons.