EP0105066B1

EP0105066B1 - Process for the separation of fatty acids using a solid bed of adsorbent

Info

Publication number: EP0105066B1
Application number: EP82305310A
Authority: EP
Inventors: Michael Terence Cleary; Santi Kulprathipanja
Original assignee: UOP LLC
Current assignee: Honeywell UOP LLC
Priority date: 1982-10-06
Filing date: 1982-10-06
Publication date: 1986-09-17
Also published as: EP0105066A1; ATE22323T1; DE3273366D1

Abstract

Stearic acid and oleic acid are separated from mixtures of stearic acid with palmitic acid and oleic acid with linoleic acid by contacting the mixture at adsorption conditions with an adsorbent comprising a nonionic hydrophobic insoluble crosslinked styrene polymer, thereby selectively adsorbing the stearic or oleic acid. The adsorbed acid may be desorbed using a desorbent. Preferably the process is carried out using a simulated moving bed countercurrent adsorption system.

Description

This invention relates to the separation of fatty acids from mixtures of fatty acids.
The fatty acids are a large group of aliphatic monocarboxylic acids, many of which occur as glycerides (esters of glycerol) in natural fats and oils. Although the term "fatty acids" has been restricted by some to the saturated acids of the acetic acid series, both normal and branched chain, it is now generally used, and is so used herein, to include also related unsaturated acids, certain substituted acids, and even aliphatic acids containing alicyclic substituents. The naturally occurring fatty acids with a few exceptions are higher straight chain unsubstituted acids containing an even number of carbon atoms. The unsaturated fatty acids can be divided, on the basis of the number of double bonds in the hydrocarbon chain, into monoethanoid, diethanoid, triethanoid, etc. (or monoethylenic, etc.). Thus the term "unsaturated fatty acid" is a generic term for a fatty acid having at least one double bond, and the term "polyethanoid fatty acid" means a fatty acid having more than one double bond per molecule. Fatty acids are typically prepared from glyceride fats or oils by one of several "splitting" or hydrolytic processes. In all cases the hydrolysis reaction may be summarized as the reaction of a fat or oil with water to yield fatty acids plus glycerol. In modern fatty acid plants this process is carried out by continuous high pressure, high temperature hydrolysis of the fat. Starting materials most commonly used for the production of fatty acids include coconut oil, palm oil, inedible animal fats, and the commonly used vegetable oils, soybean oil, cottonseed oil and corn oil. The composition of the fatty acids obtained from the "splitter" is dependent on the fat or oil from which they are made. As detailed data for the fatty acid composition of fats have accumulated over a wide range of material, it has become more and more apparent that natural fats tend to align themselves, by their component acids, in groups according to their biological origon. Moreover, it has become clear that the fats of the simplest and most primitive organisms are usually made up from a very complex mixture of fatty acids whereas as biological development has proceeded, the chief component acids of the fats of the higher organisms have become fewer in number. In the animal kingdom this change in type is remarkably consistent and culminates, in the fats of the higher land animals, in fats in which oleic, palmitic and stearic acids are the only major components. All fats of aquatic origin contain a wide range of combined fatty acids, mainly of the unsaturated series. On passing from fats of aquatic to those of land animals there is also a marked simplification in the composition of the mixed fatty acids; most of the unsaturated acids, except oleic acid disappear. The final result is that in most of the higher land animals the major component acids of the fats are restricted to oleic, palmitic and stearic and, moreover, that about 60-65% of the acids belong to the C,₈ series, saturated or unsaturated. Thus the composition of the fatty acids obtained from the "splitter" can vary widely depending upon the fat or oil charged to the "splitter". Rarely will the composition of the fatty acid mixture obtained from the "splitter" be ideal or even satisfactory for most uses. Hence fractionation is used almost universally to prepare products more desirable for specific end uses then the mixtures obtained from the "splitter". Fractionation according to molecular weight is usually accomplished in fractional distillation. There is somewhat of a difference in the volatility of any two fatty acids of different chain length, and in practice, the utility of fractional distillation is enhanced by the absence of odd-membered acids in the natural fats, so that 2 carbon atoms is nearly always the minimum difference in chain length of the fatty acids present in a mixture. Fractionating columns in such operation are sometimes capable of producing fatty acids of 95% purity or better from the viewpoint of chain length depending on the chain length in question. It is not possible, however, to separate unsaturated fatty acids from each other or from saturated fatty acids or to separate chain saturated fatty acids from each other by commercial fractional distillation when all have the same chain length or minimum difference in chain length.
It is known in the separation art that certain crystalline aluminosilicates can be used to separate certain esters of fatty acids from mixtures thereof. For example, U.S.-A-4,048,205, 4,049,688 and 4,066,677, disclose processes for the separation of esters of fatty acids of various degrees of unsaturation from mixtures of esters of saturated and unsaturated fatty acids. These processes use adsorbents comprising an X or a Y zeolite containing a selected cation at the exchangeable cationic sites.
It would be desirable however to provide a method for the separation of fatty acids using a solid adsorbent bed. Substantial uses of fatty acids are in the plasticizer and surface active agent fields. Derivatives of fatty acids are of value in compounding lubricating oil, as lubricants for the textile and molding trade, in special lacquers, as water-proofing agents, in the cosmetic and pharmaceutical fields, and in biodegradable detergents.
We have discovered that absorbents comprising non-ionic hydrophobic insoluble crosslinked styrene polymers exhibit adsorptive selectively for certain fatty acids with respect to other fatty acids, thereby making separation of such fatty acids by solid bed selective adsorption possible. These adsorbents can be used both for separating stearic acid from its admixture with palmitic acid, and for separating oleic acid from its admixture with linoleic acid.
According to the invention a process for separating stearic acid or oleic acid from a mixture of. stearic acid and palmitic acid or a mixture of oleic acid and linoleic acid is characterised in that the mixture is contacted with an adsorbent comprising a nonionic hydrophobic insoluble crosslinked styrene polymer whereby stearic acid or oleic acid is selectively adsorbed.
In one embodiment the process is characterised in that it comprises the steps of: (a) maintaining net fluid flow in a single direction through a column of the adsorbent, which column contains at least three zones having separate operational functions occurring therein and serially interconnected with the terminal zones of the column connected to provide a cyclic connection; (b) maintaining in the column, an adsorption zone defined by the adsorbent located between a feed mixture input at its upstream boundary and a raffinate stream output at its downstream boundary; (c) maintaining in the column, immediately upstream of the adsorption zone, a purification zone defined by the adsorbent located between an extract stream output at its upstream boundary and the feed mixture input at its downstream boundary; (d) maintaining in the column, immediately upstream of the purification zone a desorption zone defined by the adsorbent located between a desorbent input at its upstream boundary and the extract stream output at its downstream boundary; (e) passing the feed mixture into the adsorption zone at adsorption conditions effecting the selective adsorption of stearic acid or oleic acid from the mixture by the adsorbent in the adsorption zone and withdrawing a raffinate stream comprising palmitic acid or linoleic acid from the adsorption zone; (f) passing the desorbent into the desorption zone at desorption conditions effecting the displacement of the adsorbed acid from the adsorbent in the desorption zone; (g) withdrawing an extract stream comprising the displaced acid and the desorbent from the desorption zone; (h) passing at least a portion of the extract stream to a separation means and therein separating at least a portion of the desorbent; and (i) periodically advancing through the column of adsorbent in a downstream direction with respect to fluid flow in the adsorption zone the feed mixture input, the raffinate stream output, the desorbent input, and the extract stream output to effect the shifting of the adsorption, purification and desorption zones through the adsorbent.
In the context of the present invention the feed mixture is a mixture of an extract component and a raffinate component. The "extract component" is the compound that is selectively more adsorbed by the adsorbent while the "raffinate component" is the compound that is selectively less adsorbed. In this process stearic acid or oleic acid is the extract component and palmitic acid or linoleic acid is the raffinate component. The term "desorbent" means a material capable of desorbing the extract component from the adsorbent. The term "raffinate stream" means the stream through which the raffinate component is removed from the adsorbent. The composition of the raffinate stream can vary from essentially 100% desorbent to essentially 100% raffinate component. The term "extract stream" means a stream through which the extract component which has been desorbed by the desorbent is removed from the adsorbent. The composition of the extract stream, likewise, can vary from essentially 100% desorbent to essentially 100% extract component. Preferably, at least a portion of the extract stream, and more preferably also at least a portion of the raffinate stream, is passed to separation means, typically fractionators where at least a portion of the desorbent is separated to produce an extract product and a raffinate product. The terms "extract product" and "raffinate-product" means products produced by-the process containing, respectively, the extract component and the raffinate component in higher concentrations than those found in the extract stream and the raffinate stream. Although it is possible by the process of this invention to produce a high purity, stearic or oleic acid product or a high purity palmitic or linoleic acid product (or both) at high recoveries, it will be appreciated that an extract component is never completely adsorbed by the adsorbent, nor is a raffinate component completely, non-adsorbed by the adsorbent. Therefore, varying amounts of the raffinate component can appear in the extract stream and, likewise, varying amounts of the extract component can apepar in the raffinate stream. The extract and raffinate streams then are further distinguished from each other and from the feed mixture by the ratio of the concentrations of an extract component and a raffinate component appearing in the particular stream. More specifically, the ratio of the concentration of stearic or oleic acid to that of palmitic or linoleic acid will be lowest in the raffinate stream, next highest in the feed mixture, and the highest in the extract stream.
An example of a typical feed mixture for the process of the present invention is known as "tall oil fatty acids" and typically comprises about 1 vol. % of palmitic acid, 2 vol. % stearic acid, 51 vol. % oleic acid, 45 vol. % linoleic acid and 4 vol. % others. Feed mixtures which can be charged to the process may contain, in addition to fatty acids, a diluent material that is not adsorbed by the adsorbent and which is preferably separable from the extract and raffinate output streams by fractional distillation. When a diluent is employed the concentration of diluent in the mixture of diluent and fatty acids may be from a few vol. % up to about 90 vol. %.
Desorbent materials used in adsorptive separation processes vary depending upon such factors as the type of operation employed. In the swing-bed system in which a selectively adsorbed feed component is removed from the adsorbent by a purge stream desorbent selection is not as critical and desorbent materials comprising gaseous hydrocarbons such as methane or ethane or other types of gases such as nitrogen or hydrogen may be used at elevated temperatures or reduced pressures or both to effectively purge the adsorbed feed component from the adsorbent. However, in adsorptive separation processes which are generally operated continuously at substantially constant pressures and temperatures to ensure liquid phase, a desorbent material must be judiciously selected to satisfy many criteria. First, a desorbent material should displace an extract component from an adsorbent with reasonable mass flow rates without itself being so strongly adsorbed as to unduly prevent an extract component from displacing the desorbent material in a following adsorption cycle. Expressed in terms of the selectivity, it is preferred that an adsorbent be more selective for all extract components with respect to a raffinate component than it is for a desorbent material with respect to a raffinate component. Secondly, desorbent materials must be compatible with a particular adsorbent and a particular feed mixture. More specifically, a desorbent material must not reduce or destroy the critical selectivity of an adsorbent for an extract component with respect to a raffinate component. A desorbent material should additionally be a substance which is easily separable from the feed mixture that is passed into the process. A raffinate stream and an extract stream removed from an absorbent both typically contain desorbent material and without a method of separating at least a portion of the desorbent material the purity of an extract product and a raffinate product would not be very high, nor would a desorbent material be available for reuse in the process. It is therefore contemplated that any desorbent material used in this process will preferably have an average boiling point substantially different from that of a feed mixture to allow separation of at least a portion of desorbent material from feed components in an extract and a raffinate stream by simple fractional distillation, thereby permitting reuse of a desorbent material in the process. The term "substantially different" as used herein means that the difference in the average boiling points between a desorbent material and a feed mixture is at least 5°C. The boiling range of a desorbent material may be higher or lower than that of a feed mixture. Finally, a desorbent material should also be a material which is readily available and therefore resonable in cost.
In the process of our invention, we have found that particularly effective desorbents, especially for liquid phase operation, comprise the mixtures acetonitrile and methanol; acetonitrile, tetrahydrofuran and water; acetone and water; dimethyl acetamide and water; methanol and water; dimethyl formamide and water; . quarternary methyl ammonium hydroxide, water and methanol; and quarternary propyl ammonium hydroxide, water and methanol.
The prior art has also recognized that certain characteristics of adsorbents are highly desirable, if not absolutely necessary, to the successful operation of a selective adsorption process. In this connection attention is directed to the passage in GB-A-2049667 from page 5 line 26 to page 6 line 61.
The adsorbents used in the process of this invention comprise nonionic hydrophobic insoluble crosslinked styrene polymers, preferably those manufactured by the Rohm and Haas Company and sold under the Registered Trade Mark "Amberlite". Types of Amberlite polymers known to be effective for use in this invention are those referred to in Rohm and Haas Company literature as Amberlite XAD-2 and Amberlite XAD-4, and described in the literature as "hard, insoluble spheres of high surface, porous polymer". The various types of Amberlite polymeric adsorbents differ in physical properties such as porosity volume, surface area, average pore diameter, skeletal density and nominal mesh sizes. Applications for Amberlite polymeric adsorbents suggested in the Rohm and Haas Company literature include decolorizing pulp mill bleaching effluent, decolorizing dye wastes and pesticide removal from waste effluent. There is, of course, no hint in the literature to our surprising discovery of the effectiveness of Amberlite polymeric adsorbents in the separation of monoethanoid fatty acids (stearic and oleic acids) from diethanoid fatty acids (palmitic and linoleic acids).
A fundamental superiority of the Amberlite polymeric adsorbents over crystalline aluminosilicates is that the former, unlike the latter, may be used for the direct, separation of fatty acids without first converting the fatty acids to their corresponding esters. The processes of the aforementioned prior art patents are applicable only to esters of fatty acids because the free carboxylic group of a fatty acid chemically reacts with the crystalline aluminosilicates used by those processes. The adsorbent used according to this invention exhibits no such reactivity and, therefore, the process of this invention is uniquely suitable for the separation of fatty acids.
A dynamic testing apparatus has been proposed to test various adsorbents with a particular feed mixture and desorbent material to measure the adsorbent characteristics of adsorptive capacity, selectivity and exchange rate. The apparatus consists of an adsorbent chamber comprising a helical column of approximately 70 ml volume having inlet and outlet portions at opposite ends of the chamber. The chamber is contained within a temperature control means and, in addition, pressure control equipment is used to operate the chamber at a constant predetermined pressure. Quantitative and qualitative analytical equipment such as refractometers, polarimeters and chromatographs can be attached to the outlet line of the chamber and used to detect quantitatively or determine qualitatively one or more components in the effluent stream leaving the adsorbent chamber. A pulse test, performed using this apparatus and the following general procedure, is used to determine selectivities and other data for various adsorbent systems. The adsorbent is filled to equilibrium with a particular desorbent material by passing the desorbent material through the adsorbent chamber. At a convenient time, a pulse of feed containing known concentrations of a tracer and of a particular extract component or of a raffinate component or both all diluted in desorbent is injected for a duration of several minutes. Desorbent flow is resumed, and the tracer and the extract component or the raffinate component (or both) are eluted as in a liquid-solid chromatographic operation. The effluent can be analyzed onstream or alternatively effluent samples can be collected periodically and later analyzed separately by analytical equipment and traces of the envelopes of corresponding component peaks developed.
From information derived from the test adsorbent performance can be rated in terms of void volume, retention volume for an extract or a raffinate component, selectively for one component with respect to the other, and the rate of desorption of an extract component by the desorbent. The retention volume of an extract or a raffinate component may be characterized by the distance between the center of the peak envelope of an extract or a raffinate component and the peak envelope of the tracer component or some other known reference point. It is expressed in terms of the volume in cubic centimeters of desorbent pumped during this time interval represented by the distance between the peak envelopes. Selectively, (B), for an extract component with respect to a raffinate component may be characterized by the ratio of the distance between the center of the extract component peak envelope and the tracer peak envelope (or other reference point) to the corresponding distance between the center of the raffinate component peak envelope and the tracer peak envelope. The rate of exchange of an extract component with the desorbent can generally be characterized by the width of the peak envelopes at half intensity. The narrower the peak width the faster the desorption rate. The desorption rate can also be characterized by the distance between the center of the tracer peak envelope and the disappearance of an extract component which has just been desorbed. This distance is again the volume of desorbent pumped during this time interval.
In the process of the present invention the adsorbent may be employed in the form of a dense compact fixed bed which is alternately contacted with the feed mixture and the desorbent. In the simplest embodiment of the invention the adsorbent is employed in the form of a single static bed in which case the process is only semi-continuous. In another embodiment a set of two or more static beds may be employed in fixed bed contacting with appropriate valving so that the feed mixture is passed through one or more adsorbent beds while the desorbent can be passed through one or more of the other beds in the set. The flow of feed mixture and desorbent may be either up or down through the adsorbent. Any of the conventional apparatus employed in static bed fluid-solid contacting may be used.
Countercurrent moving bed or simulated moving bed countercurrent flow systems, however, have a much greater separation efficiency than fixed adsorbent bed systems and are therefore preferred. In the moving bed or simulated moving bed processes the adsorption and desorption operations are continuously taking place, which allows both continuous production of an extract and a raffinate stream and the continual use of feed and desorbent streams. One preferred embodiment of this process utilizes what is known in the art as the simulated moving bed countercurrent flow system. The operating principles and sequence of such a flow system are described in the passage of page 8 line 53-page 10 line 2 of GB-A-2049667.
It is contemplated that at least a portion of the extract stream will preferably pass into a separation means wherein at least a portion of the desorbent can be separated to produce an extract product containing a reduced concentration of desorbent. Preferably, at least a portion of the raffinate stream will also be passed to a separation means wherein at least a portion of the desorbent can be separated to produce a desorbent stream which can be reused in the process and a raffinate product containing a reduced concentration of desorbent. The separation means will typically be a fractionation column, the design and operation of which is well-known to the separation art.
Reference can be made to U.S.-A-2,985,589, and to a paper entitled -"Continuous Adsorptive Processing-A New Separation Technique" by D. B. Broughton presented at the 34 th Annual Meeting of the Society of Chemical Engineers at Tokyo, Japan on April 2, 1969, for further explanation of the simulated moving bed countercurrent process flow scheme.
Although both liquid and vapor phase operations can be used in many adsorptive separation processes, liquid-phase operation is preferred for the process of the present invention because of its lower temperature requirements and because higher yields of extract product can be obtained with liquid-phase operation than with vapor-phase operation. The adsorption is typically carried out at a temperature of from 20 to 200°C, temperatures of 20 to 100°C being preferred, and at a pressure of from atmospheric to 500 psig (3450 kPa gauge), atmospheric to 250 psig (1725 kPa gauge) being preferred to ensure liquid phase. Desorption is typically and preferably carried out in the same ranges of temperatures and pressures as used for adsorption conditions.
The size of the units which can be utilized in the process of this invention can vary anywhere from those of pilot plant scale (see for example U.S.-A-3,706,812) to those of commercial scale and can range in flow rates from as little as a few ml an hour up to many thousands of litres per hour.
The following Examples illustrate the selectivity relationship that makes the process of the present invention possible. The Examples are supplemented by the accompanying drawings of which

Figure 1 is a graphical presentation of the results of the pulse test reported in the Table of Example I, and
Figure 2 is a graphical presentation of the results of the pulse test referred to in Example II.

Example I

This Example presents selectivities for two Amberlite polymeric adsorbents, comprising Amberlite XAD-2 and Amberlite XAD.-4, for oleic acid with respect to a linoleic acid.
The feed mixture comprised the desorbent used in the pulse test in question and tall-oil fatty acids in a ratio of desorbent to tall-oil fatty acids of 90:10. The tall-oil fatty acids had the following composition by volume:
Retention volumes and selectivities were obtained using the pulse test apparatus and procedure previously described. Specifically, the adsorbents were tested in a 70 ml helical coiled column using the following sequence of operations for each pulse test. Desorbent material was continuously run through the column containing the adsorbent at a nominal liquid hourly space velocity (LHSV) of about 1.0. A void volume was determined by observing the volume of desorbent required to fill the packed dry column. At a convenient time the flow of desorbent material was stopped, and a 10 ml sample of feed mixture was injected into the column via a sample loop and the flow of desorbent material was resumed. Samples of the effluent were automatically collected in an automatic sample collector and later analyzed by chromatographic analysis. From the analysis of these samples peak envelope concentrations were developed for the feed mixture components. The retention volume for the fatty acids were calculated by measuring the distances from time zero on the reference point to the respective midpoints of the fatty acids and subtracting the distance representing the void volume of the adsorbent. The selectivities of an adsorbent for oleic acid with respect to linoleic acid in the presence of a desorbent material are in the quotients obtained by dividing the retention volume for the oleic acid by the retention volume for the linoleic acid. The results for these pulse test are shown in the Table and one of them is illustrated in Figure 1, namely the pulse test conducted at 90°C with Amberlite XAD-2 adsorbent and 85 wt.% dimethylformamide-15_ wt.% water desorbent. Linoleic acid was eluted first in each case followed by oleic acid.
The table and Figure 1 show that oleic acid is more strongly adsorbed than linoleic acid, particularly for certain desorbent mixture combinations when used for the separation of fatty acids having 18 carbon atoms per molecule. Furthermore, the separations achieved for many of these combinations are substantial, as exemplified in Figure 1, and clearly of commercial feasibility.

Example II

This example presents the results of using Amberlite XAD-2 for separating stearic acid from about a 50-50 mixture of stearic and palmitic acids diluted in desorbent in a ratio of desorbent to acid mixture of 90:10. The desorbent used was 85 wt.% dimethyl formamide and 15 wt.% water.
Data were obtained using the pulse test apparatus and procedure previously described at a temperature of 90°C. Specifically, the adsorbent was placed in a 70 ml helical coiled column and the following sequence of operations was used. Desorbent material was continuously run upflow through the column containing the adsorbent at a flow rate of 1.2 ml/min. At a convenient time the flow of desorbent material was stopped, and a 10 ml sample of feed mixture was injected into the column via a sample loop and the flow of desorbent material was resumed. Samples of the effluent were automatically collected in an automatic sample collector and later analyzed by chromatographic analysis.
Figure 2 of the drawings shows that stearic acid is more strongly adsorbed than palmitic acid, particularly for the desorbent mixture used. Furthermore the separation achieved for this combination was substantial and clearly of commercial feasibility.

Claims

1. A process for separating stearic acid or oleic acid from a mixture of stearic acid and palmitic acid or a mixture of oleic acid and linoleic acid, characterised in that the mixture is contacted with an adsorbent comprising a nonionic hydrophobic insoluble crosslinked styrene polymer whereby stearic acid or oleic acid is selectively adsorbed.

2. A process as claimed in claim 1, characterised in that the stearic or oleic acid is recovered from the adsorbent by desorption with a desorbent.

3. A process as claimed in claim 2, characterised in that the desorbent comprises one of the mixtures selected from acetonitrile and methanol; acetonitrile, tetrahydrofuran and water; acetone and water; dimethyl acetamide and water; methanol and water; dimethyl formamide and water; quaternary methyl ammonium hydroxide, water and methanol; and quaternary propyl ammonium hydroxide, water and methanol.

4. A process as claimed in any of claims 1 to 3, characterised in that the adsorption and, when it occurs, the desorption are carried out at a temperature of from 20 to 200°C and a pressure of from atmospheric to about 3450 kPa gauge (500 psig).

5. A process as claimed in any of claims 1 to 4, characterised in that it is carried out in the liquid phase.

6. A process as claimed in any of claims 2 to 5, characterised in that it comprises the steps of:

(a) maintaining net fluid flow in a single direction through a column of the adsorbent, which column contains at least three zones having separate operational functions occurring therein and serially interconnected with the terminal zones of the column connected to provide a cyclic connection;

(b) maintaining in the column, an adosrption zone defined by the adsorbent located between a feed mixture input at its upstream boundary and a raffinate stream output at its downstream boundary;

(c) maintaining in the column immediately upstream of the adsorption zone, a purification zone defined by the adsorbent located between an extract stream output at its upstream boundary and the feed mixture input at its downstream boundary;

(d) maintaining in the column immediately upstream of the purification zone a desorption zone defined by the adsorbent located between a desorbent input at its upstream boundary and the extract stream output at its downstream boundary;

(e) passing the feed mixture into the adsorption zone at adsorption conditions effecting the selective adsorption of stearic acid or oleic acid from the mixture by the adsorbent in the adsorption zone and withdrawing a raffinate stream comprising palmitic acid or linoleic acid from the adsorption zone;

(f) passing the desorbent into the desorption zone at desportion conditions effecting the displacement of the adsorbed acid from the adsorbent in the desorption zone;

(g) withdrawing an extract stream comprising the displaced acid and the desorbent from the desporption zone;

(h) passing at least a portion of the extract stream to a separation means and therein separating at least a portion of the desorbent; and

(i) periodically advancing through the column of adsorbent in a downstream direction with respect to fluid flow in the adsorption zone the feed mixture input, the raffinate stream output, the desorbent input, and the extract stream output to effect the shifting of the adsorption, purification and desorption zones through the adsorbent.

7. A process as claimed in claim 6, characterised in that a buffer zone is maintained in the column immediately upstream from said desorption zone, the buffer zone being defined by the adsorbent located between the desorbent input at its downstream boundary and the raffinate stream output at its upstream boundary.