Process for removing aromatics from hydrocarbon vapour streams

Abstract

 Process for the removal of aromatic hydrocarbons from a hydrocarbon vapour stream containing such aromatic hydrocarbons, which process comprises the steps of:

(1) contacting in an extractive distillation column the hydrocarbon vapour stream with an extracting solvent capable of selectively dissolving aromatic hydrocarbons,

(2) withdrawing at the upper portion of the extractive distillation column a vaporous raffinate having a reduced content of aromatic hydrocarbons,

(3) withdrawing from the bottom part of the extractive distillation column a liquid solvent stream, which has been enriched with aromatic hydrocarbons,

whereby at least one liquid comprising extracting solvent and aromatic hydrocarbons is withdrawn from the lower portion of the extractive distillation column and subsequently at least part of the liquid withdrawn is heated under such conditions that at least part of the aromatic hydrocarbons present in said liquid evaporates without substantial evaporation of the extracting solvent, after which the heated liquid/vapour stream is returned to the extractive distillation column.

Description

[0001] The present invention relates to a process for removing aromatic hydrocarbons from hydrocarbon vapour streams and in particular to a process for removing monoaromatics, suitably benzene, from catalytic reformate streams suitable as gasoline blending components.

[0002] Three general types of hydrocarbon components are normally used to blend a finished gasoline. These hydrocarbon components are paraffins, olefins and aromatics, each having its own characteristics and contribution to the properties of the final gasoline. The paraffins include inter alia butane, isopentane, alkylate, isomerate, straight-run naphtha and light hydrocrackate and accordingly can be derived from a number of different refinery processes, such as crude oil distillation, isomerization, alkylation and hydrocracking. The olefinic components include monomers, oligomers and polymers and are usually obtained from processes such as catalytic cracking, steam cracking and polymerisation. The aromatic components, finally, are usually obtained via catalytic reforming, which is a combination of various reactions (dehydrogenation, isomerization, dehydrocyclization and hydrocracking) and which has the aim of increasing the octane number of the gasoline. A commonly used feedstock for catalytic reforming processes is heavy naphtha which contains a relatively high amount of paraffinic hydrocarbons, including straight-chain, branched and cyclic paraffins.

[0003] Catalytic reforming processes are well known in the art. In general, the catalytic reforming results in the boiling range of the reformer feed not being significantly changed, whereas the chemical composition of the feed is significantly changed by the conversion of paraffinic hydrocarbons into aromatic hydrocarbons with hydrogen being formed. As is generally recognised, the aromatics have a positive effect on the octane number and therefore the products from the catalytic reformer are very suitable gasoline blending components. Catalytic reforming processes usually involve passing a naphtha type feedstock over a suitable reforming catalyst under reforming conditions, recovering the hydrogen formed and separating the product into two or more reformates. Usually, two or three reformates are formed by using a two- or three-cut splitter, respectively. The composition of each of the reformates obtained can be controlled to a certain extent by the cutting temperature(s) and pressure applied in the splitter. The top fraction or light reformate recovered from the splitter usually comprises a major amount of non-aromatic hydrocarbons and a minor amount of monoaromatics, particularly benzene and to a lesser extent toluene. The concentration in the light reformate of monoaromatics in general and of benzene and toluene in particular, can be regulated by variation of the cutting temperature. Beside a light reformate one or more heavier reformates are obtained, which comprise the heavier aromatic hydrocarbons and which may suitably be used as motor gasoline blending components. The light reformate containing the benzene is also suitably used as a blending component for gasoline. However, over the past years the presence of benzene in motor gasoline has become more and more undesired for environmental reasons and it is expected that environmental limits on benzene content of motor gasoline in the future will be below current benzene levels in most motor gasolines. Accordingly, there is an incentive to lower the benzene concentration in motor gasolines.

[0004] Benzene can be removed from a reformate stream by ways known in the art. One way is fractional distillation, another way is via an extraction process, whereby use is made of an extracting solvent capable of selectively dissolving monoaromatics and in particular benzene. Ethylene glycols and N-substituted morpholines are known to be suitable solvents for this purpose, but a particular useful solvent is sulfolane. However, although these methods remove benzene from the reformate, the resulting benzene concentrate still contains substantial amounts of other components beside benzene. Such components typically include valuable raffinate components which could suitably be used as gasoline blending component. It would accordingly be advantageous if these valuable raffinate components could be recovered to add to the purified raffinate stream which is to be used as gasoline blending component. Moreover, since benzene is a valuable product, it would be advantageous to provide a benzene concentrate having a higher benzene concentration, thus making the concentrate an economically attractive source for the production of benzene.

[0005] In International Patent Application WO-A-94/19426 a process for removing aromatics from a hydrocarbon vapour stream is disclosed, which process involves contacting the hydrocarbon vapour stream with a liquid absorbent solvent which selectively absorbs the aromatics and withdrawing at the upper portion of the absorption zone a raffinate vapour stream which predominantly comprises non-aromatic hydrocarbons, whilst withdrawing at the bottom part of the absorption zone an aromatics-rich liquid solvent. This aromatics-rich solvent is suitably treated in a subsequent distillation step to produce a distillate stream predominantly comprising aromatic hydrocarbons and a bottom stream predominantly comprising liquid absorbent solvent, which can be recycled to the absorption zone. The process is particularly suitable for the removal of benzene from gasoline blending stocks, thereby producing a benzene concentrate having a relatively high benzene concentration. However, this concentrate still contains a significant amount of components other than benzene, mainly non-aromatic hydrocarbons which would be very useful components in gasoline. Accordingly, a further separation treatment of the benzene concentrate to produce benzene meeting the specifications to be sold as such is absolutely required, whilst in transporting the concentrate to benzene manufacture facilities the volume of components other than benzene increases the transport costs per volume unit of benzene which has its impact on the cost price of the final benzene product.

[0006] The present invention aims to provide a process not having the disadvantages of the prior art processes. More specifically, the present invention aims to provide a more effective process for removing aromatic hydrocarbons from hydrocarbon vapour streams, whereby the aromatics-concentrate obtained contains a reduced amount of non-aromatic hydrocarbons and has an increased aromatics content. With respect to the removal of monoaromatics, particularly benzene, from reformates suitable as gasoline blending components, the present invention aims to provide an economically attractive process wherein the benzene concentrate obtained has a reduced content of non-aromatic hydrocarbons and an increased benzene content, thus making it very useful as a source for benzene manufacture at reduced cost price, whilst at the same time a raffinate is produced which is essentially free of benzene, i.e. which contains less than 1% by weight of benzene. As an ultimate object, the present invention aims to provide a process wherein under optimum conditions benzene can be obtained directly in highly pure form, i.e. at concentrations above 95% by weight and even above 99% by weight, thus almost or entirely meeting the benzene selling specifications.

[0007] These objects have been met by applying a specific heating step in the lower portion of the absorption zone.

[0008] Accordingly, the present invention relates to a process for the removal of aromatic hydrocarbons from a hydrocarbon vapour stream containing such aromatic hydrocarbons, which process comprises the steps of: (1) contacting in an extractive distillation column the hydrocarbon vapour stream with an extracting solvent capable of selectively dissolving aromatic hydrocarbons, (2) withdrawing at the upper portion of the extractive distillation column a vaporous raffinate having a reduced content of aromatic hydrocarbons, (3) withdrawing from the bottom part of the extractive distillation column a liquid solvent stream, which has been enriched with aromatic hydrocarbons, whereby at least one liquid comprising extracting solvent and aromatic hydrocarbons is withdrawn from the lower portion of the extractive distillation column and subsequently at least part of the liquid withdrawn is heated under such conditions that at least part of the aromatic hydrocarbons present in said liquid evaporates without substantial evaporation of the extracting solvent, after which the heated liquid/vapour stream is returned to the extractive distillation column.

[0009] In general, extractive distillation processes using an extractive distillation column are known in the art. Examples of such processes are e.g. disclosed in US-A-3,723,256; EP-A-0,073,945 and EP-A-0,155,992. An extractive distillation column generally comprises a tray column containing sieve or valve trays or other fractionating internals, and means for introducing extracting solvent and feed as well as means for withdrawing at least a vaporous top fraction and a liquid bottom fraction. For the purpose of the present invention the extractive distillation column additionally comprises means for withdrawing, heating and reintroducing a heated liquid/vapour stream in its lower zone. Such means suitably consists of one or more reboilers.

[0010] The extracting solvent is introduced at a point in the upper zone of the column and is allowed to flow downwardly through the column. The vaporous feed is introduced into the column at a point below the point where said solvent is introduced and flows upwardly through the column, thereby attaining an effective contact between feed and solvent as they pass countercurrently through the column. At the top of the column a vapour fraction is recovered (the raffinate), whilst at the bottom of the column a liquid solvent stream enriched with aromatics is withdrawn. The number of theoretical stages of the extractive distillation column may vary within wide limits. It has, however, been found advantageous, also taking into account technical and economic considerations, that the number of theoretical stages is in the range of from 2 to 25, suitably 3 to 15. The distribution of the theoretical stages above and below the feed inlet is also not particularly critical. However, usually the number of theoretical stages above the feed inlet will be equal to or higher than the number of theoretical stages below the feed inlet, as in the zone above the feed inlet the major part of the extraction of aromatics from the feed takes place. Accordingly, the ratio between the numbers of theoretical stages above and below the feed inlet is suitably at least 1, more suitably between 1.5 and 20 and most suitably between 2 and 15.

[0011] The hydrocarbon vapour stream used as the feed to the extractive distillation column comprises both aromatic and non-aromatic hydrocarbons, whereby the total content of aromatic hydrocarbons is preferably less than 50% by weight. Suitably, however, the hydrocarbon vapour stream comprises less than 35% by weight, more suitably less than 25% by weight, of aromatic hydrocarbons. In principle, the process of the present invention can be applied to remove monoaromatic hydrocarbons as well as aromatic hydrocarbons comprising two or more aromatic rings, but the process is particularly suitable for the removal of monoaromatic hydrocarbons from a hydrocarbon vapour stream. Monoaromatic hydrocarbons which can be suitably removed include benzene, toluene and xylenes. For the purpose of the present invention, however, it is preferred that at least 50% by weight, but preferably at least 90% by weight, of the aromatic hydrocarbons present is benzene. If at least 95% by weight, and more preferably at least 99% by weight, of the aromatic hydrocarbons present is benzene, the process according to the present invention enables the recovery of benzene of a very high purity. The hydrocarbon vapour fraction should preferably have a boiling range between 50 and 200 °C, more preferably between 60 and 175 °C. Although various mixed hydrocarbon vapour streams meeting one or more of the above requirements as to the aromatics content, benzene content and boiling range may be treated in accordance with the present invention, it is preferred to use a hydrocarbon vapour stream obtained as a light reformate from a catalytic reforming process. Such light reformate, namely, usually has a relatively low aromatics content (usually below 25% by weight), whereby essentially all the aromatics present are monoaromatic hydrocarbons. A high percentage of these monoaromatics (usually more than 90% by weight) is benzene, the exact percentage of (mono)aromatics other than benzene -particularly toluene- being determined by the cutting temperature and pressure applied in the reformate splitter. In order to directly obtain benzene in highly pure form using the process of the present invention, the cutting temperature at the given pressure applied in the reformate splitter should be such that at least 99% by weight of the aromatics present in the light reformate is benzene. An additional advantage of using a light reformate as the hydrocarbon vapour feed is that the catalytic reforming process can be very well integrated with the process of the present invention, as the light reformate recovered from the reformate splitter can be passed directly, i.e. without any intermediate condensation and/or pressure release step, into the extractive distillation column. It will be understood that this is very attractive with respect to the operating efficiency.

[0012] The conditions under which the extractive distillation column is operated may vary within the conventional operating windows for this type of devices. Important operating parameters are temperature, pressure and solvent to feed ratio. It will be understood that all these parameters are closely correlated and it is within the common skills of the person skilled in the art to adjust the individual parameters to one another. Accordingly, the temperature in the extractive distillation column may range 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 window for the temperature of the extractive distillation column is from 40 °C (top) to 175 °C (bottom). The pressure in the extractive distillation column is suitably within in the range of from 1 to 5 bar, preferably 1 to 3 bar, and the solvent to feed weight ratio is suitably in the range of from 1 to 10, preferably 1.5 to 5. The temperature of the feed upon entry in the extractive distillation column should be such that it is in the vapour phase at the pressure at which it enters the column. Usually, this pressure will be equal to or slightly higher than the pressure inside the extractive distillation column and accordingly is in the same order of magnitude as the pressure in the column. The temperature of the extracting solvent upon entry in the upper zone of the extractive distillation column will suitably be in the range of from 30 to 120 °C, more suitably 35 to 100 °C.

[0013] As the extracting solvent any solvent which is capable of selectively dissolving aromatic hydrocarbons and in particular monoaromatic hydrocarbons and which has a boiling point higher than those of the aromatics to be dissolved in it, may be used. If the main incentive is to remove benzene, the solvent accordingly should be capable of dissolving benzene and should have a boiling point higher than that of benzene. Such solvents are known in the art and include N-substituted morpholines, such as N-formyl-morpholin and several ethylene glycols. The preferred extracting solvent, however, is sulfolane. Sulfolane can be applied as such or in admixture with any of the aforementioned solvents. Small amounts of water may also be present.

[0014] In accordance with the present invention, at least one liquid comprising extracting solvent and aromatic hydrocarbons is withdrawn from the lower portion of the extractive distillation column and subsequently at least part of the liquid withdrawn is heated under such conditions that at least part of the aromatic hydrocarbons present in said liquid evaporates without substantial evaporation of the extracting solvent, after which the heated liquid/vapour stream is returned to the extractive distillation column. This is suitably achieved by using a reboiler. The use of reboilers is known in separation technology and reboilers suitably applied for the purpose of the present invention may in principle be any reboiler commonly applied. In general, a reboiler can be defined as a special type of heat exchanger for the supply of heat to the bottom of a fractionating column, in this case an extractive distillation column. The liquid comprising extracting solvent and aromatics which is withdrawn may be identical to the bottom fraction or may have a different composition. In the latter case, said liquid is withdrawn from the column at a point above the point where the bottom fraction leaves the column. After being withdrawn from the column, part of the liquid may be routed to another destination and accordingly is withdrawn as a bleedstream before being heated. In that case, only a part of the liquid withdrawn from the column is heated in the reboiler and reintroduced into the column. Alternatively, the entire stream withdrawn from the column is heated in the reboiler, and the resulting liquid/vapour stream is led back into the column. It has been found very advantageous to use at least one reboiler in the lower portion of the extractive distillation column. It enables the recovery of an aromatics concentrate having an increased aromatics concentration and in the case of a light reformate feed comprising a minor amount of aromatics, whereby more than 99% by weight of the aromatics is benzene, even enables the recovery of benzene in a highly pure form.

[0015] The amount of heat to be supplied by the reboiler to the bottom of the extractive distillation column, conveniently referred to as the reboiler duty, is determined by the operating conditions applied in the extractive distillation column, the desired purity of the aromatics concentrate and the desired quality of the raffinate used as the gasoline blending component.

[0016] The liquid, aromatics-rich solvent stream obtained in step (3) of the process according to the present invention is suitably subjected to a subsequent solvent recovery step in order to separate the solvent from the aromatics. Also some water is usually present which also need to be separate from the solvent. The solvent recovery can be attained by ways known in the art. A particularly suitable solvent recovery step involves passing the liquid solvent stream to a solvent recovery column which is usually operated at subatmospheric pressure and temperatures between 70 °C (top of column) and 200 °C (bottom of column), withdrawing a vapour fraction at the top of the solvent recovery column and a liquid solvent fraction at the bottom of said column, and condensing the top fraction to yield a water phase and a hydrocarbon phase comprising the aromatic hydrocarbons. Suitably, said hydrocarbon phase consists for at least 90% by weight, preferably at least 95% by weight, of aromatic hydrocarbons, which preferably consist for more than 90% by weight of benzene. In case an appropriate light reformate feed is used, the hydrocarbon phase can be highly pure benzene, i.e. it may consist for more than 99% by weight of benzene. In order to further increase the yield and purity of the benzene, part of the hydrocarbon phase may be reintroduced into the solvent recovery column. The liquid solvent fraction withdrawn at the bottom of the solvent recovery column is suitably recycled -in total or in part- to the extractive distillation column for reuse.

[0017] In Figure 1 a suitable embodiment of the present invention is illustrated. Heat exchangers have been indicated with H. Hydrocarbon vapour feed (1) enters extractive distillation column (2) at point F, whilst the liquid extracting solvent enters the column at point S. A vaporous raffinate (3) is withdrawn from the top of the column at point R and is passed to condenser (5) via a heat exchanger (H). Here, water (6) is separated from aromatics-poor raffinate (7), which is used as a motor gasoline blending component. Part of the aromatics-poor raffinate (7) may be refluxed back to the extractive distillation column (2) in order to improve the purity of the stream (7) with respect to liquid extracting solvent (not shown). A liquid aromatics-rich solvent (8) is withdrawn from the lower portion of the extractive distillation column (2) at point A and heated in reboiler (9), after which the resulting liquid/vapour mixture (10) re-enters the column at point B, which is higher in the column than point A. A liquid bottom stream (11), which is a further aromatics-enriched solvent, is withdrawn from the bottom part of the column at point C and is passed to solvent recovery column (12). Solvent (13) leaves the solvent recovery column (12) at the bottom and is passed to extractive distillation column (2) via a heat exchanger H, where it enters said column (2) at point S. The extract (14) leaves the solvent recovery column (12) at the top and is passed to condenser (15) via a heat exchanger H. Water (17) and aromatics concentrate (16) are recovered from the condenser (15). Part of the aromatics concentrate (16) may be refluxed back to solvent recovery column (12) to enhance further the efficiency of the solvent recovery.

[0018] The invention is further illustrated by the following examples.

Example 1

[0019] A light reformate having a composition as listed in Table I, is introduced into an extractive distillation column having 10 theoretical stages and having a reboiler attached to its lower portion. The reboiler duty is 3472 kW. Operating pressure in the extractive distillation column is 1.5 bar, the solvent to feed ratio is 3.0 (on a weight basis) and the feed is introduced into the extractive distillation column at a temperature of 78 °C, whilst the sulfolane is introduced into the top of the column at a temperature of 75 °C.

Table I

Composition of light reformate
Component	% by weight	Component	% by weight
i-pentane	16.7	3-methylhexane	3.0
n-pentane	13.1	2,3-dimethylbutane	2.6
2-methylpentane	12.0	benzene	17.2
n-hexane	11.4	toluene	0.2
2-methylhexane	3.2	other non-aromatics	20.6

[0020] A vaporous fraction is withdrawn from the top of the extractive distillation column and passed to a condenser, where it is condensed and separated in a water phase and a raffinate phase. The raffinate phase contains only 1.0% by weight of benzene.

[0021] The liquid benzene-rich solvent is withdrawn from the bottom of the extractive distillation column and passed into a solvent recovery column which is operated at a pressure of 0.4 bar and which has a top temperature of 94 °C and a bottom temperature of 173 °C. The vapour fraction withdrawn from the top of the solvent recovery column is passed to a condenser where it is condensed and separated into a benzene-concentrate and a water phase.

[0022] The benzene-concentrate contains 97.7% by weight of benzene and 0.9% by weight of toluene and total benzene recovery, i.e. the weight percentage of benzene present in the light reformate which is recovered, is found to be 95% by weight.

Comparative Example 1

[0023] A light reformate as described in Example 1 is introduced into an extractive distillation column having no reboiler attached to its lower portion. The same procedure as in Example 1 is followed. Total benzene recovery is 94% by weight, but the benzene-concentrate contains only 66.3% by weight of benzene.

Example 2

[0024] A light reformate having essentially the same composition as the one used in Example 1 except in that it has a toluene content of 0.1% by weight, is introduced into an extractive distillation column having 10 theoretical stages and having a reboiler attached to its lower portion. The reboiler duty is 4630 kW. Operating pressure in the extractive distillation column is 1.05 bar, the solvent to feed ratio is 4.0 (on a weight basis) and the feed is introduced into the extractive distillation column at a temperature of 65 °C, whilst the sulfolane is introduced into the top of the column at a temperature of 44 °C. The same procedure as in example 1 is followed further.

[0025] Accordingly, a vaporous fraction is withdrawn from the top of the extractive distillation column and passed to a condenser, where it is condensed and separated in a water phase and a raffinate phase. The raffinate phase is found to contain less than 1.0% by weight of benzene.

[0026] The liquid benzene-rich solvent is withdrawn from the bottom of the extractive distillation column and passed into a solvent recovery column which is operated at the same conditions as in Example 1. The vapour fraction withdrawn from the top of the solvent recovery column is passed to a condenser where it is condensed and separated into a benzene-concentrate and a water phase.

[0027] The benzene-concentrate obtained contains 99.8% by weight of benzene and accordingly can be regarded as high purity benzene. Other components present are toluene (0.1% by weight), water (0.1% by weight), sulphur (1 part per million on a weight basis, ppmw) and sulfolane (3 ppmw). Total benzene recovery is again 95% by weight.

Claims

1. Process for the removal of aromatic hydrocarbons from a hydrocarbon vapour stream containing such aromatic hydrocarbons, which process comprises the steps of:

(1) contacting in an extractive distillation column the hydrocarbon vapour stream with an extracting solvent capable of selectively dissolving aromatic hydrocarbons,

(2) withdrawing at the upper portion of the extractive distillation column a vaporous raffinate having a reduced content of aromatic hydrocarbons,

(3) withdrawing from the bottom part of the extractive distillation column a liquid solvent stream, which has been enriched with aromatic hydrocarbons, whereby at least one liquid comprising extracting solvent and aromatic hydrocarbons is withdrawn from the lower portion of the extractive distillation column and subsequently at least part of the liquid withdrawn is heated under such conditions that at least part of the aromatic hydrocarbons present in said liquid evaporates without substantial evaporation of the extracting solvent, after which the heated liquid/vapour stream is returned to the extractive distillation column.

2. Process according to claim 1, wherein the hydrocarbon vapour stream comprises less than 50% by weight, preferably less than 35% by weight and more preferably less than 25% by weight, of aromatic hydrocarbons.

3. Process according to claim 1 or 2, wherein at least 50% by weight, preferably at least 90% by weight, of the aromatic hydrocarbons present is benzene.

4. Process according to claim 3, wherein at least 99% by weight of the aromatic hydrocarbons present is benzene.

5. Process according to any one of the preceding claims, wherein the hydrocarbon vapour stream is a light reformate stream.

6. Process according to any one of the preceding claims, wherein the extracting solvent is sulfolane.

7. Process according to any one of the preceding claims, wherein the temperature in the extractive distillation column ranges 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 in the range of from 1 to 5 bar and the solvent to feed weight ratio is in the range of from 1 to 10.

8. Process according to any one of the preceding claims, wherein the liquid solvent stream obtained in step (3) is subjected to a solvent recovery step.

9. Process according to claim 8, wherein the solvent recovery step involves passing the liquid solvent stream to a solvent recovery column, withdrawing a vapour fraction at the top of the solvent recovery column and a liquid solvent fraction at the bottom of said column, and condensing the top fraction to yield a water phase and a hydrocarbon phase comprising the aromatic hydrocarbons.

Drawing