KR20040075881A - Adsorptive separation product recovery by fractional distillation - Google Patents

Abstract

By carrying out the separation using a dividing wall column, the structure and operating cost of recovering the extraction or extraction residue products of the simulated mobile bed adsorptive separation process are reduced. Extraction debris or extract stream is introduced into the column at an intermediate point on the first side of the separation wall, and the column carries the adsorptive separation product from the opposite side of the separation wall to the side discharge. The co-adsorbed impurity stream is removed to the top stream and the desorbent is recovered to the final bottoms stream.

Description

Adsorption separation product recovery by fractional distillation {ADSORPTIVE SEPARATION PRODUCT RECOVERY BY FRACTIONAL DISTILLATION}

In many commercially important petrochemical and petroleum industry processes, it is desirable to isolate compounds with near boiling points, or to perform separation of compounds by structural grades. Due to the need for a large number of fractionation columns that can consume excessive amounts of energy, it is very difficult or impossible to do this by conventional fractional distillation. The industry has responded to this problem by using other separation methods that can perform separation based on chemical structure or properties. Adsorptive separation is one such method.

In the practice of adsorptive separation, one or more feed mixtures comprising two or more compounds of different skeletal structure selectively adsorb the compounds of one skeletal structure and allow other components of the feed stream to pass through the adsorption zone unchanged. Pass through the adsorbent bed. The flow of feed through the adsorbent bed is stopped and the adsorption zone is washed to remove nonadsorbed material around the adsorbent. The desorbent stream is then passed through an adsorbent bed to desorb the desired compound from the adsorbent. Desorbent materials are also commonly used to clean nonadsorbent materials from voids in and around the adsorbent. This may be done in a single mass adsorbent bed or in several parallel beds on a rotating bed basis. However, it has been found that simulated mobile bed (SMB) adsorptive separation provides some advantages such as high purity and recovery.

Passage of the desorbent through adsorbent selectively removes retained compounds to produce an extract stream. The extract stream contains a mixture of desorbent and the desired compound, and these materials are separated by distillation in a column called extraction column. If two or more compounds are retained in the adsorbent and removed as part of the extract, another fractionation needs to be performed in the finishing column. The present invention aims to improve the economics of fractionation used to recover the final desired compound from the extract stream.

Compared to the batch formula, several economic benefits derive from the continuous operation of large-scale adsorptive separation processes. SMB processes generally use rotary valves and a plurality of lines to facilitate backflow movement of the adsorbent bed through the adsorption and desorption zones. This is described, for example, in US-A-3,205,166 and US-A-3,201,491.

US-A-3,510,423 describes the conventional handling of extract residues and extract streams removed from the SMB process where the desorbent is recovered, combined and recycled to the adsorption zone. US-A-4,036,745 describes the use of double desorbents with a single adsorption zone to provide a higher purity paraffin extract. US-A-4,006,197 extends the above teaching on desorbent recycling to tricomponent desorbent mixtures. US-A-5,177,295 describes the fractionation of "heavy" desorbents used for the recovery of paraxylene from aromatic hydrocarbon mixtures.

A dividing wall or Pettyluk arrangement for the fractionation column was first introduced about 50 years ago by Pettyruk et al. The recent commercialization of fractionation columns using this technique has facilitated more recent studies as described in the paper in a Supplement to The Chemical Engineer, August 27, 1992, page 14.

US-A-2,471,134 describes the use of a dividing wall column in the separation of light hydrocarbons from methane to butane. US-A-4,230,533 describes a control system for a partition wall column and describes the use of the claimed invention for the separation of aromatics, including benzene, toluene and orthoxylene.

Summary of the Invention

The present invention is an improved simulated mobile bed adsorptive separation process characterized by separating a stream comprising two extraction components and a desorbent into three product streams in only a single fractionation column using an integrated fractional distillation column. That is, the three component extraction stream of the adsorptive separation zone is separated into three high purity process streams in a single separation column. Part of the column is divided by vertical walls into parallel fractionation zones, one receiving the extract stream and the other delivering the product stream of the adsorptive separation zone. The desorbent is preferably withdrawn from the bottom of the column. The light product can be recovered at the top. This reduces the separation required, and therefore the capital and operating costs of the adsorption process.

One broad aspect of the present invention provides a feed stream comprising first, second, and third compounds, wherein the first compound is retained on a larger amount of selective adsorbent relative to the second compound, and the third compound is facilitated by fractional distillation. Having a boiling point different from that of the first and second compounds, so that the third compound is introduced into the adsorption zone containing a selective adsorbent layer maintained at adsorption promoting conditions that are less adsorbed on the adsorbent than the first compound, Forming an extract residue stream comprising a second compound and a desorbent already present in the selective adsorbent; Contacting the desorbent stream comprising the desorbent compound with the large amount of selective adsorbent having a first compound under desorption promoting conditions to obtain an extract stream comprising the desorbent compound, the first compound and the third compound; The extract stream is operated in fractionation conditions and is divided into at least first and second parallel fractionation zones by partition walls and the first and second fractionation zones respectively have upper and lower ends located in the fractionation column and have first and second ends. The fractionation zone is introduced at the midpoint of the first fractionation zone into a partitioning fractionation column through its undivided upper portion of the fractionation column at its upper end and through its undivided lower portion of the fractionation column at its lower end; Removing the extract product stream comprising the first compound from the midpoint of the second fractionation zone; Characterized by a simulated mobile bed adsorptive separation process comprising recovering a product stream comprising a third compound from the first end of the fractionation column and removing a process stream comprising the desorbent compound from the second end of the fractionation column. Can lose.

The present invention relates to a continuous adsorptive separation process used to separate compounds such as C ₈ aromatic hydrocarbons.

FIG. 1 is a very simplified process flow diagram showing the extraction stream recovered from the adsorbent chamber 14 is introduced into the left fractionation zone of a single dividing wall product recovery column 6.

Many commercially important petrochemical and petroleum industry processes require the separation of compounds with near boiling points, or the separation of compounds by structural grades. Examples thereof include recovery of standard paraffins from petroleum kerosene fractions for use in the preparation of detergents, and recovery of paraxylene from C ₈ aromatic mixtures in the production of polyesters and other plastics. Separation of high octane hydrocarbons from naphtha boiling range petroleum fractions, and recovery of olefins from mixtures of paraffins and olefins, is a situation in which the close volatility of a compound or overlap of boiling points over a wide boiling range of a compound renders the use of fractional distillation impractical. Another example. For example, for the standard paraffin recovery mentioned above, it is often desirable to recover paraffins having a carbon number range from C ₉ to C ₁₂ .

Adsorptive separation is widely used to carry out the abovementioned separation. In adsorptive separation, the desorbent provides the driving force to release the compound selectively retained on the adsorbent. Thus, the adsorbent must be cycled continuously between exposure to the feed stream and exposure to the desorbent stream. As described below, this comprises two or more effluent streams, i.e., an extract residue stream containing non-adsorbed compounds, and an extract stream containing the desired desorbent compound which may comprise a mixture of several compounds and a desorbent. Form. It is an object of the present invention to provide a more economical method for recovering desorbent compounds from the two streams produced during adsorptive separation. It is a specific object of the present invention to provide an improved simulated mobile bed adsorptive separation process with reduced cost. This object is achieved by reducing the number of fractionation columns required to recover the desorbent from the extract and the desorbent. Instead of individual columns, an integrated single column containing parallel fractionation zones within a single column is used. Each fractionation zone occupies only a portion of the cross section of the column, and both zones are through both ends and the non-divided portion of the larger area of the column. Due to this distribution at the top or bottom end of the fractionation zone, the process may be suitable for desorbents having a lower or higher boiling point than the extract residues and the extract components of the feed.

1 illustrates a portion of a simulated moving bed adsorptive separation process with a single adsorbent chamber 14. Only one of the two columns used in the process is shown. The remaining columns can be of a conventional shape similar to the one shown. For the sake of description, assume that the process is used to separate a feed stream (line 1) comprising a mixture of several C ₈ aromatic hydrocarbons, including small amounts of toluene, and paraxylene. The feed will also generally include methaxylene, orthoxylene and ethylbenzene. Toluene is an obscure upside fraction. The volatility of the C ₈ aromatic compounds is so close that it is impractical to separate them on a commercial scale by fractional distillation.

Feed stream (line 1) is introduced into the rotary valve (2). This rotary valve has a port number corresponding to the number of adsorbent chamber process streams and the number of sub beds of adsorbent located in one or more adsorbent chambers used in the process. Since the adsorbent chamber (s) may contain 8 to 24 adsorbent inner layers, a number of layer lines are involved in the process and each of 12 to 30 lines is generally connected to the rotary valve 2. For simplicity, only the layer lines in use at the moment described are shown in the figures. The flow at other moments is similar, but occurs through other layer lines not shown.

The rotary valve 2 directs the feed stream to the bed line 3, which carries it to the adsorbent chamber 14. The feed stream is introduced into the adsorbent chamber at the boundary between the two inner layers and is distributed over the cross section of the chamber. This then flows downward through several inner layers of adsorbent containing particles. The adsorbent optionally possesses one compound or structural group of compounds, in this case paraxylene. The adsorbent also has a constant toluene. The other C ₈ components of the feed stream continue to flow downward and are removed from the adsorbent chamber with the extract residue stream carried by line 4. In addition, the extract residue stream will comprise various amounts of desorbent compound (s) that have been removed from the interparticle pore volume of the adsorbent by the flowing feed stream and also removed from the surface of the adsorbent. This desorbent is present in the bed before the adsorption step due to the preliminary performance of the desorption step. The extract residue stream is introduced into rotary valve 2 and then directed to line 24 by the valve. Line 24 carries the extract residue stream to the fractionation zone (not shown to simplify the figure). This may be a conventional column or a partition wall column.

Simultaneously with the adsorption procedure, a desorbent stream is introduced continuously through line 12 into adsorbent chamber 14. The desorbent is distributed over the cross section of the column and moves downward in several layers of adsorbent to form the desorption zone. The desorbent removes paraxylene and toluene from the adsorbent. This produces a mixture of paraxylene, toluene and desorbent, flowing through a portion of the adsorbent chamber that functions as a desorption zone. This material is removed from the bottom of the chamber 14 and returned to the top of the chamber via a line (not shown) referred to in the art as a "pump around" line. This stream flows more adsorbent on top of the chamber forming the rest of the desorption zone. This is then removed from the adsorbent chamber 14 as an extraction stream via line 13 and introduced into rotary valve 2.

The rotary valve directs the extraction stream of line 13 to line 15. Line 15 delivers the extract stream to a first vertical fractionation zone, which occupies most of the left side of the middle portion of the fractional distillation column 6. This fractionation zone contains 30 to 50 fractionation trays 21 and is separated from the parallel second fractionation zone occupying the other half of the column cross section by a substantially fluid impermeable vertical wall 19. The vertical walls do not necessarily have to be centered in the column, and the two fractionation zones may differ in cross-sectional area or shape. The vertical wall 19 divides the wide vertical part of the column 6 into two parallel fractionation zones. The two zones are isolated from each other by the height of this wall, but through the top and bottom ends. There is no direct vapor or liquid flow between the two zones through the dividing wall, but the upper end of the fractionation zone containing the extract stream of line 15 contains a column that contains an undivided fractionation zone with an additional tray 21. It is open to the internal volume of (6). The liquid may pass under the separation wall 19 at the bottom of the two fractionation sections, but the vapor flow is preferably limited. Thus, vapor and liquid can move freely around the wall between these two parts of the column. In operation, the extract stream separates in the first fractionation zone and the more volatile toluene moves upward from the left first fractionation zone and exits into the undivided upper portion of column 6. As in the first fractionation zone, the upper end of the right second zone is through the upper portion of the column 6 which contains an additional fractionation tray 21 over the entire column cross section.

Toluene present in the extraction stream of line 15 is conveyed upward in the first fractionation zone and introduced to the top of column 6. The top of the column is a purification zone designed to separate the extraction components from toluene. This purification zone can also be used for the separation of different desorbent components when a multicomponent desorbent stream is used. A toluene-rich vapor stream is removed from the top of column 6 via line 7 and through an upper condenser (not shown) to form a liquid that is delivered to receiver 8. The liquid toluene stream is removed from the receiver and sorted into a first portion which is returned to the top of the fractionation column 6 as countercurrent and from the process via line 25. As used herein, the term “rich” refers to the concentration of the indicated compound or group of compounds above 50 mol%, preferably above 75 mol%.

The bottom of the column also contains an undivided fractionation band. This zone may receive liquid discharged from the first and second fractionation zones. This liquid is fractionally distilled so that the C ₈ aromatic hydrocarbons are carried upwards as a vapor and the less volatile desorbent is concentrated to the bottom liquid which is removed via line 19. This separation is achieved using a postboiler (not shown) that provides steam to the bottom, undivided fractionation zone. The desorbent rich bottoms liquid is combined with the desorbent stream of line 22 obtained from the column containing the extract residues of line 24. The recovered desorbent is introduced via line 23 to the rotary valve 2 for reuse in the process.

The undivided bottom portion of column 6 is shown separated from the two parallel fractionation zones by gas flow control or gas trap out tray 12 located just below the bottom of wall 19. The slight gap at this point allows for the liquid flow of liquid between the parallel fractionation zones. This tray may have a liquid sealing hole that allows normal downward flow of liquid, but its structure allows the upward flow of steam to be at least significantly limited. The tray can completely block the upward flow of steam. The use of this tray is preferred, as this provides a means of clearly controlling the division of the upstream flow of gas between the two fractionation zones, which is the primary means of controlling the performance of the two zones. Thus, the total vapor flow is preferably removed from the column via line 16 and distributed between lines 18 and 20 which supply steam to the bottom of the two fractionation zones. The gas flow can be regulated by one or more flow control valves or by adjusting the relative liquid level at the bottom of the two zones. This is described in detail in US-A-4,230,533, which is already cited for a slightly different arrangement.

Thus, a preferred aspect of the present invention provides a feed stream comprising para-xylene, meta-xylene and toluene, in which para-xylene is selectively retained on the selective adsorbent and toluene is adsorbed on the adsorbent less than on para-xylene. Introducing into an adsorption zone comprising a selective adsorbent layer maintained at promoting conditions to form an extract residue stream comprising meta-xylene; Contacting a desorbent under said desorption promoting conditions with said selective adsorbent layer with para-xylene and toluene to obtain an extract stream comprising desorbent compound, para-xylene and toluene; The extract stream is operated in fractionation conditions and is divided into at least first and second parallel fractionation zones by partition walls, each fractionation zone having upper and lower ends located within the fractionation column and the first and second fractionation zones having their upper ends. Introducing at a midpoint of the first fractionation zone into a partitioning fractionation column through an undivided upper portion of the fractionation column and at its lower end through an undivided lower portion of the fractionation column; Removing the para-xylene rich extraction product stream from the middle point of the second fractionation zone; Simulating the separation of xylene isomers, comprising removing a top vapor stream comprising toluene from the top first end of the fractionation column and removing a bottom stream comprising desorbent from the bottom second end of the fractionation column. Characterized by a bed adsorptive separation process.

Representative comparisons of the separation of FIG. 1 based on engineering design calculations, which have the advantage of comparison with multiple work units in the common case, have shown that a typical two column system (extraction column and finishing column) containing a total of 110 trays has a total of 100 It may be replaced by a single dividing wall column containing two trays. The tray in the basic case contains 50 in the extraction column and 60 in the finishing column. Separation wall columns require a total reboiling duty of 93 MMBTU / hour, while 110 MMBTU / hour for a typical column pair. The partition wall column requires a total condenser capacity of 87 MMBTU / hour, while 109 for a standard two column. Other economics are derived from the reduction of regulating systems, pipes, pumps and placement space.

Working conditions for the adsorption generally include a temperature range of 20 to 250 ° C, with 60 to 200 ° C being preferred. Temperatures between 90 and 160 ° C. are very preferred for the second adsorption zone. In addition, the adsorption conditions preferably include a pressure sufficient to maintain the process fluid in the liquid phase, and may be from atmospheric pressure to 600 psig. Desorption conditions generally include the same temperature and pressure as used for adsorption conditions. The SMB process operates with a wide range of A: F flow rates in the adsorption zone of 1: 1 to 5: 1.0, where A is the volume ratio of the "circulation" of the selective pore volume in the adsorbent, and F is the feed flow rate. to be. The practice of the present invention does not require any significant change in operating conditions, adsorbent or desorbent composition within the adsorbent chamber. That is, the adsorbent is preferably maintained at the same temperature in the process.

While most of the description herein has been set forth in terms of use in the SMB process of the present invention, it is contemplated that the present invention may be applied to other ways of performing adsorptive separation, such as a rotating bed system using one or more separate adsorptive layers. A substantial limitation to the application of the present invention is that the process produces an extract or extract residue stream comprising three compounds which are preferably separated by fractionation. Another variation in the performance of this process is the replacement of rotary valves with composite valve systems. Such systems are described in the art, such as US-A-4,434,051, and become more practical as the number of adsorbent inner layers decreases.

Another variant that deviates from the depiction of FIG. 1 is the case of separation where the boiling point of the desorbent is lower than the extract residues and extract components. In this case, the desorbent is removed from the top of the column. Both parallel fractionation zones will still be through the wider cross-sectional portion of the column from which the desorbent is discharged. The use in the separation of paraxylene of "heavy" desorbents, which are desorbents with a higher boiling point than the extract residues or extract components of the feed, is described in US-A-5,107,062, US-A-5,057,643 and US-A-5,012,038. The fractionation of heavy desorbents from extracts and extract residues is shown in US-A-5,177,295 already cited.

The success of a particular adsorptive separation is determined by many factors. The dominant factor among these is the composition of the adsorbent (still phase) and the desorbent (mobile phase) used in the process. The remaining factors are basically related to the process conditions.

This process is not considered to be limited to use with any particular type of adsorbent. The adsorbent used in the present process preferably comprises a molecular sieve, such as A, X or Y zeolite, or silicalite. Silicalites are well described in the literature. This is described and claimed in US-A-4,061,724. See, eg, "Silicalite, A New Hydrophobic Crystalline Silica Molecular Sieve," Nature , Vol. 271, February 9, 1978, for more details.

The active ingredient of the adsorbent is generally used in the form of particle aggregates having high physical strength and abrasion resistance. The aggregate contains an active adsorbent material dispersed in an amorphous inorganic matrix or binder having channels and cavities so that the fluid is accessible to the adsorbent material. The method of forming the crystalline powder into such aggregates involves adding an inorganic binder, generally clay comprising silicon dioxide and aluminum oxide, to the high purity adsorbent powder in a wet mixture. The binder helps to form or aggregate crystalline particles. The blended clay-adsorbent mixture may be extruded or formed into beads into cylindrical pellets and then calcined to convert the clay into an amorphous binder having significant mechanical strength. The adsorbent may be bound into irregularly shaped particles formed by spray drying or screening after crushing of larger particles. Thus, the adsorbent particles may be in the form of extrudates, tablets, spheres or granules having a desired particle range, preferably 1.9 mm to 250 microns (16 to 60 standard U.S. mesh). Kaolin type clays, water permeable organic polymers or silicas are generally used as binders.

The active molecular sieve component of the adsorbent will normally be in the form of small crystals present in the adsorbent particles in an amount of 75 to 98% by weight of the particles, based on the volatile-free composition. Volatile-free compositions are generally determined after the adsorbent is calcined at 900 ° C. to remove all volatiles. In general, the remainder of the adsorbent will be an inorganic matrix of binder present in an intimate mixture with the small particles of silicalite material. This matrix material may be derived from by-products of the silicalite manufacturing process, for example, the intended incomplete purification of the silicalite during manufacture.

Those skilled in the art will appreciate that the performance of an adsorbent is often greatly influenced by a number of factors independent of its composition, such as operating conditions, feed stream composition and moisture content of the adsorbent. Thus, the optimum adsorbent composition and operating conditions in the process depend on a number of interrelated variables. One such variable is the moisture content of the adsorbent expressed in terms of Loss on Ignition (LOI) testing as approved herein. In the LOI test, the volatile content of the zeolite adsorbent is determined by the weight difference obtained before and after drying the adsorbent sample at 500 ° C. under an inert gas purge such as nitrogen for a time sufficient to achieve a constant weight. For this process, the moisture content of the adsorbent is preferably less than 7.0%, preferably 0 to 4.0% by weight of LOI at 900 ° C. Since the process fluid can dry the adsorbent, the hydration level of the molecular sieve is generally controlled by controlled water injection, such as through a desorbent stream.

As mentioned above, the desorbent may be a mixture of two or more compounds. For example, preferred desorbents for the separation of standard C ₉ -C ₁₁ paraffins from kerosene include mixtures of standard paraffins and cycloparaffins (naphthenes). Very preferred are mixtures in which the standard and cycloparaffins have the same carbon number, and the carbon number of the desorbent compound is in the general range of 5-8. Preferred standard paraffins are n-hexane, and desorbents may range from 0 to 100% standard paraffins. The desorbent may be 100% cycloparaffin. Preferred desorbents for the separation of C ₈ aromatic hydrocarbons differ from those for paraffin separation. The preferred "light" desorbent removed at the top of the process is toluene. Preferred heavy desorbents are para-diethylbenzene, which can be used in admixture with saturated hydrocarbons. Other desorbents for this separation include indane and indane derivatives, diethyltoluene and tetralin derivatives described in US Pat. No. 5,107,062.

A related SMB processing technique is the use of "zone flushes". The zone flush forms a buffer zone between the feed and extract bed lines to keep the desorbent (eg standard pentane) introduced into the adsorption zone. The use of zone flushes requires more complex, and therefore more expensive, rotary valves, but the use of zone flushes in the adsorption zone is preferred when aiming for high purity extraction products. Indeed, a large amount of mixed component desorbent recovered at the top from the extract and / or extracting residue column is introduced into a separate separator column. The high purity stream of the lower intensity component of the mixed component desorbent is recovered and used as the band flush stream. Further information on techniques for improving product purity, such as the use of dual component desorbents and the use of flush streams, is described in US-A-3,201,491, US, which is incorporated herein by reference in the teaching of the SMB technique in this aspect. -A-3,274,099, US-A-3,715,409, US-A-4,006,197 and US-A-4,036,745.

SMB technology has been applied to a variety of chemicals other than those described above. For example, US-A-4,467,126 describes the recovery of double substituted benzenes such as nitrotoluene isomers. The separation of 2,6-dimethyl naphthalene is described in US-A-5,004,853 and the separation of 2,7-diisopropylnaphthalene is described in US-A-5,012,039. SMB technology has been extended to the separation of sugars, the separation of chiral compounds and more complex organics such as fatty acids and triglycerides, as described in US-A-5,225,580. The process can be applied to any of the above SMB processes that require desorbent recovery from the extraction or extract residues, especially when there is a third component separable by fractionation.

In the present invention, various terms used herein are defined as follows. A "feed mixture" is a mixture containing at least one extract component and at least one extract residue component separated by the process. "Feed stream" refers to the stream of feed mixture that comes into contact with the adsorbent used in the process. An "extract component" is a compound or group of compounds that is more selectively adsorbed by an adsorbent, and an "extract component" is a compound or class of compounds that is less selectively adsorbed. "Desorbent material" generally means a material capable of desorbing the extract component from the adsorbent. "Extract residue stream" or "extract residue stream" means a stream from which an extract residue component is removed from an adsorbent bed after adsorption of the extraction component. The composition of the extract residue stream may vary from essentially 100% desorbent material to essentially 100% extract residue. "Extract stream" or "extraction output stream" means a stream from which an extract material adsorbed by a desorbent material is removed from an adsorbent bed. The composition of the extract stream may vary from essentially 100% desorbent material to essentially 100% extract components. "Extract product" and "extract residue product" mean streams produced by the process that contain extract components and extract residue components at higher concentrations than those found in the extract stream and the extract residue stream exiting the adsorbent chamber, respectively. do. The extract stream may be rich in the desired compound or may only contain increased concentrations.

It has become common practice in the art to classify multiple layers into multiple zones in SMB adsorption chamber (s). Typically, the process is described in terms of four or five bands. The first contact between the feed stream and the adsorbent is in zone I, the adsorption zone. In zone I the adsorbent or stationary phase is surrounded by a liquid containing the undesired isomer (s), i.e. extraction residue. This liquid is removed from the adsorbent in zone II, referred to as the purification zone. In the purification zone, undesired extract residues are washed out of the pore volume of the adsorbent bed by materials which are easily separated from the desired components by fractional distillation. In zone III of the adsorbent chamber (s), the desired isomer is liberated from the adsorbent by exposing and washing the adsorbent to the desorbent (mobile phase). The desired desired component and accompanying desorbent are removed from the adsorbent in the form of an extract stream. Zone IV is the portion of the adsorbent located between zones I and III, which is used to sequester Zone I and III. In zone IV, the desorbent is partially removed from the adsorbent by the flowing mixture of undesired components and desorbent of the feed stream. The liquid flow in zone IV prevents contamination of zone III by zone I liquids by a flow consistent with the simulated movement of the adsorbent from zone III to zone I. A more detailed description of the simulated mobile bed process is presented in the Adsorptive Separation section of the Kirk-Othmer Encyclopedia of Chemical Technology, page 563. "Upward" and "downward" are used herein in their general sense and are interpreted based on the overall direction of liquid flow in the adsorbent chamber. That is, if the liquid is generally flowing down the vertical adsorbent chamber, the upwards is equivalent to the upper or higher position in the chamber.

Claims (10)

a) a feed stream comprising the first, second and third compounds is selectively retained on a larger amount of selective adsorbent relative to the second compound and the third compound is easily separated by fractional distillation The second compound and previously have been introduced into the adsorption zone having a boiling point different from that of the first and second compounds and comprising a selective adsorbent layer maintained at adsorption promoting conditions that the third compound is adsorbed less on the adsorbent than the first compound. Forming an extract residue stream comprising desorbent present in the bulk of the selective adsorbent; b) contacting the desorbent stream comprising the desorbent compound with the large amount of selective adsorbent having a first compound under desorption promoting conditions to produce an extract stream comprising the desorbent compound, the first compound and the third compound ; c) The extraction stream is operated in fractionation conditions and is divided into at least first and second parallel fractionation zones by partition walls, each fractionation zone having upper and lower ends located within the fractionation column and the first and second fractionation zones thereof. Introducing at a midpoint of the first fractionation zone into a partitioning fractionation column at an upper end through an undivided upper portion of the fractionation column and at its lower end through an undivided lower portion of the fractionation column; d) removing the extract product stream comprising the first compound from the midpoint of the second fractionation zone; e) recovering the product stream comprising the third compound from the first end of the fractionation column; f) removing the process stream comprising the desorbent compound from the second end of the fractionation column Including, simulated mobile bed adsorptive separation process. The process of claim 1 wherein the undivided top portion of the fractionation column is located in the upper third of the fractionation column and the process stream comprising the desorbent compound is removed from the upper end of the fractionation column. The process of claim 1 wherein the undivided bottom portion of the fractionation column is located in the lower third of the fractionation column and the process stream comprising the desorbent compound is removed from the lower end of the fractionation column. The process of claim 1 wherein the first and second compounds are paraffinic hydrocarbons. The process of claim 1 wherein the first and second compounds are aromatic hydrocarbons. The product stream of claim 1 wherein the third compound has a lower boiling point than the first and second compounds and is less adsorbed onto the adsorbent than the first compound and the product stream comprising the third compound is from the top from the top first end of the fractionation column. Wherein the process stream is removed into the stream and the desorbent compound is removed from the bottom second end of the fractionation column into the bottoms stream. The process of claim 6, wherein the first compound is standard paraffin. The process of claim 6 wherein the first compound is an olefin. The process of claim 1 wherein the simulated moving bed adsorptive separation process separates xylene isomers, the feed stream contains para-xylene, meta-xylene and toluene, the para-xylene is selectively retained on the selective adsorbent and toluene is Less adsorbed onto the adsorbent than para-xylene, the extract residue stream contains meta-xylene and desorbent compounds, the extract stream contains desorbent compounds, para-xylene and toluene, and the extraction product stream is para-xylene A rich, toluene containing top vapor stream is removed from the top first end of the fractionation column to produce an extract product stream comprising toluene and a bottom stream comprising desorbent is removed from the bottom second end of the fractionation column. A process for producing a process stream comprising a desorbent compound. 10. The process of claim 9, wherein an upward flow of vapor from the lower portion of the fractionation column to the first and second fractionation zones is collected and then distributed in a controlled manner between the first and second fractionation zones.