WO1993002154A1

WO1993002154A1 - Adsorption process for liquid separation

Info

Publication number: WO1993002154A1
Application number: PCT/US1992/006230
Authority: WO
Inventors: John Di-Yi Ou
Original assignee: Exxon Chemical Patents, Inc.
Priority date: 1991-07-25
Filing date: 1992-07-27
Publication date: 1993-02-04
Also published as: AU2422192A

Abstract

The present invention is directed to an adsorption process for liquid separation which involves passing a liquid mixture comprising at least a first component and a second component through an adsorbent material capable of adsorbing at least the first component, flowing a first desorbent capable of desorbing the second component through the adsorbent material to displace the second component from the adsorbent material, and introducing a second desorbent capable of desorbing the first component through the adsorbent material to recover the first component from the adsorbent material.

Description

ADSORPTION PROCESS FOR LIQUID SEPARATION BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to a continuous adsorption process for the separation of hydrocarbon liquid mixtures.

2. Discussion of Background and Material Information

It is generally recognized that bulk separations of liquid mixtures, hydrocarbons or non-hydrocarbons, using fixed bed adsorption processes tend to be inefficient and use more adsorbent and desorbent than necessary.

Techniques developed during the past several decades to overcome these problems include (i) flowing the solid adsorbent; (ii) holding the adsorbent rigidly and moving the equipment; and (iii) using a fixed bed which simulates countercurrent motion by switching valves. Currently, most commercial processes for liquid phase bulk separations belong to the third category. However, these processes are very expensive to build and are not economical for separations targeted for medium and small operating capacity.

In contrast, the present invention is directed to a relatively simple, simulated- countercurrent adsorption process for liquid phase bulk separations. By way of general background, the article entitled "Adsorption Separations for Liquids", Kirk-Othmer Encyclopedia of Chemical Technology, 3rd Edition, Vol. 1, pp. 563-581, describes the ineffectiveness/inefficiency of single-bed adsorption separation, details attempts to approximate continuous, countercurrent operation, and outlines the mechanical shortcomings of the moving bed and segmented bed modes of operation for the use of multiple beds to simulate true continuous countercurrent operation evolved in both gaseous and liquid adsorptive separation processes.

German Patent No. 3,712,291, ^"WESTPHAL, is directed to a gas/vapor adsorption process which includes three or more adsorption beds. In the process disclosed by WESTPHAL, a stream of vapor from an evaporator rectification column, which is formed from a high boiling point liquid, especially water, with more than about 90%, for example, up to about 95% of a lower boiling point, preferably organic, liquid passes through a system with at least three cyclically controlled beds (Al, All, AIII) of solid adsorbent which preferentially retains the higher boiling point liquid. The three beds are periodically charged, regenerated, and taken by indirect heating above the dew point for the mixtures being separated. The separated lower boiling point liquid in pure condition is separately collected by a condenser (Wl) whose collected heat provides some heating for a column operating at reduced pressure which re-processes reflux from the three absorbers.

U.S. Patent No. 3,510,423, UOP, is directed to olefin separation by selective adsorption which involves continuous separation of olefins from a charge containing olefins and paraffins which is effected by selective adsorption on a molecular sieve under isothermal and constant pressure conditions. As disclosed, the charge is fed into the first of four adsorption zones linked in series. Extract, containing at least some of the olefins of the charge, is withdrawn from the second zone. These olefins are desorbed in a third zone immediately above the second zone. At substantially the same time, raffinate containing the less sorbed components is withdrawn from the fourth zone. Periodically, the entry and extract points are advanced simultaneously in a downstream direction. The column is maintained at a constant temperature of 25° to 150"C and a constant manometric pressure of 1 to 34 atmospheres, the conditions being such that the charge is kept liquid. As disclosed, this method is continuous and is run under isothermal and constant pressure conditions. Also, it is disclosed that the olefins in the charge have 10 to 20 carbon atoms and may be straight or branch chained. They should preferably be stable to polymerization under the separation conditions. The desorbate used has a selectivity ratio with respect to the olefins of 0.02 to 1.5 and has a lower boiling range than that of the charge. As disclosed, the desorbate may be branch chained mono-olefin, possibly carrying an alkyl substituent on one of the carbon atoms comprising the double bond. The adsorbent consists of a crystalline alumina silicate with pores from 6 A to 13 A and may be a synthetic faujasite which contains from 1% to 40% wt. of at least one of the following metals: lithium, sodium, potassium, magnesium, calcium, strontium, barium, copper, silver, zinc, cadmium and mercury. U.S. Patent No. 4,512,778, AIR LIQUIDE, is directed to an adsorption-desorption treatment process which uses a multiple adsorber vessel in operating phase sequence. The adsorption process employs a total number (n) of absorbers, of which a number (x) operate simultaneously on their adsorption cycle and has an adsorption-desorption time of T. The absorbers share a common buffer vessel in which heated gas is stored to provide elutriant for the desorption part of the cycle.

Each adsorber cycle is retarded by (T/n) compared to its predecessor in the sequence and each adsorber operates on its adsorption cycle for a period of (xT/n) . Pressure reduction takes place to intermediate pressure more rapidly than the elutration phase, and takes between one-half and one-fifth the time. When elutration is complete, repressurization commences, first from a vessel which has been depressurized to intermediate pressure and finally by vessels which are at full pressure on their adsorption cycle. This patent is directed to gas separations only and uses a heated gas for desorption.

U.S. Patent No. 4,595,950, UNION CARBIDE CORP. , is directed to processing high normal paraffin concentrations of naphtha feedstocks using four beds and mixing drums for feed gas and countercurrent purge effluent to overcome feed discontinuity. In an isobaric process, normal paraffins in high concentrations are separated from a non-normal paraffin and a light naphtha stream using a four or more bed adsorption system with cyclic steps of (A-l) concurrent purge/ dsorption, (A-2) concurrent feed/adsorption, (D-1) countercurrent purge, and (D-2) countercurrent displacement. The four bed system involves using the processing cycle in which the (A-l) and (D-1) steps are continuous and the (A-2) and (D-2) steps are in overlapping sequence so that alternatively one and two beds are on each of the processing steps at given times without the cyclic operation. It is disclosed that all of the hydrocarbon feed gas is passed to a mix drum for mixing with (D-1) effluent with (A- 1) feed gas being withdrawn and passed to the adsorber that is on an (A-l) step at any particular time in the cycle. It is disclosed that by enabling such separations to be performed in a four bed system, the overall technical- economic feasibility of carrying out such a separation is enhanced. Again, this patent is directed solely to gas phase separation of normal paraffins and non-normal paraffins. Also, it uses a single stripping gas, i.e., hydrogen, to displace the non-adsorbed gas in voids and to desorb the adsorbed gas from adsorbent. There is no disclosure of using a relatively non-adsorbing liquid to displace the voids and then a regular liquid desorbent to desorb the adsorbed material. U.S. Patent No. 3,761,533, TORAY INDUSTRIES, INC., is directed to a continuous liquid phase separation process using selective adsorption, especially for xylene isomers separation. The process is directed to the continuous liquid phase separation of a mixture, wherein at least one of its constituents is selectively adsorbed by a solid particulate adsorbent, using a simulated countercurrent flow system in which the liquid flows across three zones in series, connected to form a cycle, namely a desorption zone (1) , a rectification zone (2) , and an adsorption zone (3) . Each zone is divided into a number of sections in series, each filled with stationary adsorbent bed. The desorbent is introduced into the first section of (1) and the desorbed component-containing effluent (A) is removed from the last stage of (1) . The liquid mixture is fed to the first stage of (3) and raffinate containing least-adsorbed constituent and desorbent is removed from the last stage of (3) . The points of interaction and removal are displaced simultaneously in each zone, one section at a time, at selected time intervals. The liquid flows across the three zones which are interrupted at a point between (1) and (2) , the first fraction of (A) , containing little or no desorbent being directly put into circulation while the second fraction, containing selected component of high purity but lower concentration than the first fraction, is fed to a distillation unit, from which a portion of absorbed fraction is used as reflux in the first section of (2) . Commonly owned, copending application

U.S.S.N. 07/638,913 filed January 9, 1991 in the name of OSWALD et al. for "The Preparation of Isoparaffin Lubricant Feedstock from Olefin Paraffin Mixtures" is directed to contacting a mixture of hydrocarbons in the C5 to C^g of range with a zeolite having a silica to alumina ratio of greater than 50, pore diameters greater than about 5 A and which is substantially non-reactive towards olefin isomerization under adsorption and desorption conditions, such as silicalite, whereby linear olefins and paraffins are adsorbed to the substantial exclusion of other hydrocarbons in the mixture followed by desorbing the zeolite to provide a linear olefin and paraffin enriched mixture; and then treating the enriched mixture by a sequence of steps including oligomerization, and separation to obtain in isoolefin feedstock. SUMMARY OF THE INVENTION The present invention is directed to an adsorption process for liquid separation which involves passing a liquid mixture comprising at least two compounds or components which will undergo separation wherein the compounds which will undergo separation include at least a first component, also referred to herein as the more strongly adsorbed component, and a second component, also referred to herein as the less strongly adsorbed component, through an adsorbent material capable of adsorbing at least the first or more strongly adsorbed component, flowing a first desorbent capable of desorbing the second or less strongly adsorbed component through the adsorbent material to displace the second component from the adsorbent material, and introducing a second desorbent capable of desorbing the first or more strongly adsorbed component through the adsorbent material to recover the first or more strongly adsorbed component from the adsorbent material.

The process of the present invention maximizes adsorbent usage by employing multi- columns and multi-desorbents to accomplish the separation. An object of a preferred embodiment of the present invention is to separate the high-value normal olefins and paraffins from other compounds, and the process of the present invention been successfully used for the extraction of normal olefins and paraffins from Flexicoker distillates. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a schematic arrangement of the process in accordance with the present invention to be operated in the continuous mode.

Figure 2 shows a schematic arrangement of an adsorption section in accordance with the present invention.

Figure 3 depicts a graph showing the deficiencies of using a single desorbent. Figure 4 depicts a graph showing an adsorption process wherein a single desorbent is used.

Figure 5 depicts a graph showing the results of a process similar to that of Figure 4 wherein two desorbents are used. DETAILED DESCRIPTION

The adsorption process of the present invention is suitable for separation of components or compounds from various liquid mixtures, such as paraffins/olefins, branched paraffins/normal paraffins, olefins/aromatics, mono-olefins/poly- olefins, and other hydrocarbons mixtures by employing appropriate adsorbents and desorbents. In the discussion which follows, the invention is described using an example of five-column arrangement for the extraction of a liquid mixture linear olefins and paraffins from Flexicoker distillates. The process, however, is by no means limited to this application.

Flexicoker distillate, most preferred for purposes of the present invention, typically contains about 30% normal olefin, 15% normal paraffins, 25% branched olefins and paraffins, and 30% aromatics.

In general, the adsorbent which is particularly suitable for purposes of this separation is silicalite, such as the silica-bound silicalite S-115 manufactured by UOP Inc. Silicalite is a pure silica molecular sieve. Due to the absence of framework aluminum, silicalite has no exchangeable cations and exhibits a strong hydrophobic-organophilic character. It has a pore opening of approximately 5.5 A (see R. W. GROSE et al., U.S. Patent No. 4,061,724 (1977), and E. M. FLANIGEN et al.. Nature, 1978, 271, pp 512) . Due to molecular sieving effect, silicalite adsorbs normal compounds preferentially over branched and aromatic compounds.

Zeolites are preferred, however, particularly for large-scale adsorption separation processes. In selecting the appropriate zeolite, the Si:Al ratio is important because this ratio has been discovered to be related to certain adsorption properties. Another important consideration is pore size. For example, for purposes of the present invention, a zeolite having a pore size average within the range of about 4.5 - 5.5

Angstroms has been discovered to be effective for use in separating mixtures of n-paraffins and isoparaffins as well as n-olefins and isoolefins. For olefin/paraffin separation, zeolites which have a larger pore size greater than about 5.5 Angstroms which preferably contain cations, such as Na, K, Ba, Ca, and Sr, that do not catalyze olefin reactions are preferred. For aromatic separation, large pore zeolites having a pore size of at least about 7 Angstroms are preferred. Porous carbonaceous adsorbents, such as activated carbon and carbon molecular sieves, may also be effective for purposes of the present invention. Commercial activated carbons usually have high surface area ranging from 500 m²/g to 2000 m²/g and a wide distribution of pore openings. Normally, the high surface area and strong surface attraction make activated carbon suitable for separating paraffins/olefins, paraffins/aromatics, or olefins/aromatics mixtures. However, due to the wide pore size distribution, it cannot separate linear and non¬ linear compounds. Carbon molecular sieve is specially treated porous carbonaceous solid. Its pore size distribution may be controlled by selecting a proper carbon precursor and varying the carbonization procedure (see J. R. DACEY et al.. Trans. Faraday Soc. 50, 740 (1954), and R. B. MASON et al., U.S. Patent No. 3,222,412 (1965).

Some carbon molecular sieves with pore opening in the range of 4 - 5 A are good for separating linear and non-linear hydrocarbons.

Amorphous adsorbents, such as silica, alumina, aluminosilicate, and the like, may also be used for the present invention. These adsorbents, through the action of surface hydroxyl groups, are capable of separating molecules based on molecular polarity and acidity/basicity. Amorphous adsorbents, however, possess pores of larger than molecular width and cannot be used for separation of linear and non-linear compounds. However, other adsorbents which have been discovered to be effective for purposes of the present invention include chemically modified adsorbents, and ion exchange resins.

Therefore, for purposes of the present invention, the adsorbent material capable of adsorbing at least the first or more strongly adsorbed component of the liquid mixture may be selected from the group consisting of zeolites, porous carbon, and amorphous adsorbents. Where the adsorbent is zeolite, the preferred zeolites are from the group consisting of zeolites with a pore size within the range of about 4.5 to about 5.5 Angstroms, zeolites with a pore size greater than about 5.5 Angstroms, and zeolites within a pore size greater than about 7 Angstroms. Where the adsorbent is porous carbon, the preferred form of porous carbon is activated carbon and carbon molecular sieve.

The adsorbents suitable for purposes of the present invention may also be selected from the group consisting of amorphous adsorbents, chemically modified adsorbents, and ion exchange resins. Where the amorphous adsorbent is used, it is preferred to select such an adsorbent from the group consisting of silica, alumina, and aluminosilicate.

In accordance with the present invention, the adsorption process for liquid separation involves passing a liquid mixture comprising at least two components undergoing separation, wherein the components include at least a more strongly adsorbed component and at least another less strongly adsorbed component, through an adsorbent material capable of adsorbing at least the more strongly adsorbed component; flowing a first desorbent capable of desorbing the less strongly adsorbed component through the adsorbent material to displace the less strongly adsorbed component from said adsorbent material; and introducing a second desorbent capable of desorbing the more strongly adsorbed component through the adsorbent material to recover the more strongly adsorbed component from the adsorbent material.

For purposes of the present invention, the liquid mixture may be selected from the group consisting of a mixture of normal paraffins and isoparaffins, a mixture of olefins and paraffins, a mixture of normal olefins and isoolefins, and a mixture of aromatics, in which case the adsorbent material capable of adsorbing the more strongly adsorbed component may be selected from the group consisting of zeolites, porous carbonaceous adsorbents, amorphous adsorbents, chemically modified adsorbents, and ion exchange resins. In the embodiment where the liquid mixture is selected from the group consisting of a mixture of normal paraffins and isoparaffins, and a mixture of normal olefins and isoolefins, in which case the first desorbent is selected from the group consisting of isoparaffins, isoolefins, naphthenes, and aromatics having a boiling point substantially different from the boiling point of said second desorbent and the components undergoing separation, and the second desorbent is selected from the group consisting of rtormal paraffins and normal olefins having boiling point substantially different from the boiling point of said first desorbent and the compounds undergoing separation.

In the embodiment where the liquid mixture is a mixture of olefins and paraffins, the first desorbent is a paraffin having a boiling point substantially different from the second desorbent and the compounds undergoing separation; and the second desorbent is selected from the group consisting of an olefin and olefin/paraffin mixture having a boiling point substantially different from the boiling point of said first desorbent and the compounds undergoing separation. In the embodiment where the liquid mixture is selected from the group of aromatic mixtures, said the desorbent is selected from the group of paraffins and naphthenes having a boiling point substantially different from the boiling point of said second desorbent and the compounds undergoing separation, said second desorbent is selected from the group of aromatics and a mixture of aromatics having boiling point substantially different from the boiling point of the first desorbent and the compounds undergoing separation.

In the embodiment where the liquid mixture comprises a mixture of normal paraffins, isoparaffins, normal olefins, isoolefins and aromatics; said first desorbent is selected from the group consisting of isoparaffins, isoolefins, naphthenes, and aromatics having a boiling point substantially different from the boiling point of the second desorbent and the components undergoing separation; and the second desorbent is selected from the group consisting of normal paraffins and n-olefins having boiling point substantially different from the boiling point of the first desorbent and the compounds undergoing separation. In the embodiments of the present invention wherein the liquid mixture undergoing adsorptive separation is selected from the group consisting of a mixture of normal olefins and isoolefins, and a mixture of normal paraffins and isoparaffins, the first desorbent may be selected from the group consisting of isoparaffins, isoolefins, naphthenes, and aromatics having a boiling point substantially different from the boiling point of the second desorbent and the compounds undergoing separation, in which case the second desorbent is preferably selected from the group consisting of n-paraffins and n-olefins having boiling points substantially different from the boiling point of the first desorbent and the compounds undergoing separation.

In the embodiments where the liquid mixture undergoing adsorptive separation is a mixture of olefins and paraffins, the first desorbent is preferably a paraffin having a boiling point substantially different from the boiling point of the second desorbent and the compounds undergoing separation, and the second desorbent is preferably an olefin or olefin/paraffin mixture having a boiling point substantially different form the boiling point of the first desorbent and the compounds undergoing separation.

In yet another embodiment of the present invention wherein the liquid mixture is an aromatic mixture, the first desorbent is preferably selected from the group consisting of paraffins and naphthenes having a boiling point substantially different from the boiling point of the second desorbent and the compounds undergoing separation, and the second desorbent is preferably an aromatic or a mixture of aromatic compounds having a boiling point substantially different from the boiling point of the first desorbent and the compounds undergoing separation.

Figure 1 illustrates schematically the entire process including adsorption, product purification, and desorbent recovery in accordance with the present invention, as described below. Feed is pumped into the adsorption section A (see Figure 2 for details) from feed storage F. The amount of feed introduced should correspond to the capacity of the adsorbent for adsorbing substantially all of the more strongly adsorbed compounds. After feed injection, Desorbent No. 1 is pumped into A from Tank Dl. The material replaced by Desorbent No. 1, i.e., the raffinate R, is sent to a fractionation column Tl which separates the weakly adsorbed compounds from Desorbent Nos. 1 and 2. Desorbent No. 1 is followed by Desorbent No. 2 from Tank D2. Desorbent No. 2 replaces the more strongly adsorbed compounds, i.e., extract, from A. Extract is sent to the second fraction column T2 which separates the strongly adsorbent compounds from Desorbent Nos. 1 and 2. Finally, the mixture of Desorbent Nos. 1 and 2 from Tl and T2 is directed to a desorbent purification column T3 where the two desorbents are separated by distillation. Purified desorbents are sent back to either Dl or D2.

Figure 2 schematically illustrates details of a continuously operating adsorption section in accordance with the present invention, as described below. Referring to Figure 2, Column No. 1 is the adsorption column. Column Nos. 2 and 3 are the rectification columns where the interstitial raffinate is being displaced by Desorbent No. 1. Column Nos. 4 and 5 are the desorption columns where Desorbent No. 2 is being introduced to recover the extract. At the end of a cycle, the regenerated Column No. 5 advances to the position of Column No. 1 to repeat the adsorption step. Column No. 1 moves to the position of Column No. 2 and so forth.

For Flexicoker separation, the cycle time is 7.5 - 10 minutes; in other words, column function changes every 7.5 - 10 minutes. The flow rate for adsorption column is best at 0.9 LHSV and for other columns at 1.8 LHSV.

In the process of the present invention, an appropriate desorbent, designated as Desorbent No. 1, such as 2,3-dimethyl butane (2,3DMB), is introduced after feed injection. Since 2,3DMB cannot be adsorbed by silicalite, it displaces the non-adsorbed aromatic and branched compounds from the interstitial void without affecting the adsorption and desorption of normal compounds. Such a 2,3DMB "buffer zone" separates the non- adsorbed and adsorbed compounds and prevents the overlap of raffinate and extract. After introducing 2,3DMB for a period of time, the desorbent of n-hexane, i.e., Desorbent No. 2, is cut in which desorbs the normal compounds from adsorbent pores. A substantially complete separation is achieved using this two-desorbent approach, as shown for example in Figure 5. The two desorbents have boiling points significantly different from the products which may be easily recovered by distillation.

For purposes of the present invention, it is important that the amount of liquid mixture injected or otherwise passed through the column be calculated relative to the capacity of the adsorbent for adsorbing substantially all of the first component to be removed from the liquid mixture. Thus, it is important that only an amount of liquid mixture be introduced through the column so that substantially all of the first component is adsorbed onto the adsorbent so as to result with only second component of the liquid mixture remaining in the interstitial voids and spaces. To do otherwise would require separation followed by recycle which would be substantially less effective and efficient for purposes of the present invention.

Although the present invention has been discussed above as a continuous operation which employs a plurality of columns, it should be noted that the principles of the present invention are equally suited for use in a semi-continuous adsorption process using only one column cycle. Inasmuch as the interface, particularly in a semi-continuous mode of operation, is not clean cut, there is always some mixing which occurs. Therefore, subsequent to the removal, the desorbents are distilled and then recycled. In some instances, it is also important that this be done in the continuous processes, as well.

For purposes of the present invention, it is important, therefore, that the desorbents selected are easily separated from the feed components, for example, by simple distillation. Furthermore, for each type of liquid mixture used as the feed from which the components are to be separated, the first desorbent (Dl) should have affinity towards the adsorbent slightly higher than the weakly adsorbed feed components (Cl) , but much lower than the strongly adsorbed feed components (C2) . The second desorbent (D2) should be substantially higher than the weakly adsorbed feed components, but not higher than the strongly adsorbed feed components, relative to the adsorbent. This can be represented as follows: Cl < Dl < D2 < C2

In addition, the processes of the present invention, which involve the use of two desorbents does not normally require a sweep gas, but use of such a sweep gas ensures the maximum efficiency of the results of the present invention.

Figure 3 shows the deficiencies of using a single desorbent in a fixed bed liquid phase operation for the separation of linear compounds, such as decene-1 and n-decane, from non-linear compounds, such as methylnonane and trimethylbenzene, using silica-bound silicalite adsorbent and n-hexane desorbent as described in Example II. The data are also needed for calculating capacities of adsorbents, i.e., the difference between extract curves and raffinate curves. Figure 4 depicts a fixed bed liquid phase adsorption process using substantially the same feed, adsorbent, and desorbent as used for Figure 3, particularly with respect to the capacity of the adsorbent for decene-1 and n-decane as described in Example II.

Figure 5 illustrates the results of a run substantially the same as that for Figure 4; however, two desorbent (2,3- dimethylbutane/Desorbent No. 1, and n- hexane/Desorbent No. 2) were used. This Figure shows that the components of the liquid feed mixture were well separated.

EXAMPLES

A process of the present invention was demonstrated on a laboratory scale using a liquid chromatography apparatus which included an HPLC pump, a six-port feed injection valve, a 1/4" x 12" stainless steel adsorbent column, a pressure controller, and an automatic sampler used for periodically collecting samples of column effluent for gas chromatography analysis. The adsorbent was silica-bound silicalite S-115 provided by UOP Inc. It was ground to 40/60 mesh and calcined at 300"C prior to testing. A synthetic feed solution containing 15 wt.% decene-1, 15 wt. % n-decane, 15 wt. % 3-methylnonane, and 55 wt. % 1,3,5,- trimethylbenzene was used to demonstrate the separation. Desorbent No. 1 was 2,3- dimethylhexane and Desorbent No. 2 was n-hexane. Adsorption and desorption were carried out at 140^βC and 200-500 psig. Flow rate ranged from 0.9 to 1.2 LHSV (Liquid Hourly Space Velocity). EXAMPLE I

An experiment was conducted to estimate adsorbent capacity and to demonstrate the difficulty of producing high purity product using a fixed-bed liquid phase adsorption process. The adsorption column was degassed with desorbent n- hexane at the conditions of 140^βC, 280 psig, and 1.2 LHSV. Feed was then introduced into the column for a period of time to effect an equilibrium between feed and adsorbent. The adsorption was followed by n-hexane desorption to displace the feed left in the interstitial voids, and to recover the adsorbed materials. Results were plotted in Figure 3. In the adsorption front, there was a period in which the effluent contained only aromatic and branched compounds indicating a preferential adsorption of linear compounds. Data collected in the adsorption front were used to calculate the adsorption capacity of silicalite for decene-1 and n-decane. In the desorption end, decene-1 and n-decane were the last compounds eluted, but there was only an initial surge in the concentration of linear compounds. Virtually no product recovered was completely free from aromatic and non-linear compounds. The loss of separation and, therefore, product purity was due to mixing in the interstitial voids and the relatively low adsorption capacity of silicalite (4 wt. %) . EXAMPLE II

Figure 4 illustrates the importance of mixing in the interstitial void. The adsorbent, feed and desorbent were the same as in Example I. However, a six-port sample valve was installed which allowed the quantity of decene-1 and n-decane in the feed injected into the adsorption column to be substantially the same as the adsorbent capacity for these two compounds. The test was done at 140°C, 300 psig and 0.9 LHSV. Had there been no interstitial mixing, all normal compounds should have been adsorbed and well separated from other feed components. This, however, did not occur due to mixing in the interstitial void and considerable overlap of elution curves was observed. As a result, more adsorbent, i.e., a long column, and more desorbent were needed to produce high purity products. Thus, Figure 4 illustrates the importance of mixing in the interstitial void. EXAMPLE III

This example demonstrates the capability of the two-desorbent process of the present invention for producing high purity product. The adsorbent, feed, and feed injection device were essentially the same as in Example II. The single desorbent in previous examples was replaced with 2,3- dimethylbutane as Desorbent No. 1 and n-hexane as Desorbent No. 2. The test was conducted at 140"C, 500 psig, and 0.9 LHSV. The results are shown in Figure 5. Desorbent No. 1 was introduced after feed injection. Since 2,3-dimethylbutane could not be adsorbed by silicalite, it displaced the non-adsorbed aromatic and branched compounds from the interstitial voids without affecting the adsorption and desorption of normal compounds. Such a 2,3-dimethylbutane buffer zone separates the non-adsorbed and adsorbed compounds preventing the overlap of raffinate and extract. After introducing Desorbent No. 1 for a period of time, n-hexane was cut in which desorbed the normal compounds from adsorbent pores. As shown in Figure 5, a much better separation and a higher product purity were achieved using this approach.

Although the invention is described with reference to particular means, materials and embodiments, from the foregoing description, one skilled in the art can easily ascertain the essential characteristics of the present invention; and various changes and modifications may be made to various usages and conditions. without departing from the spirit and scope of the invention as described in the claims that follow.

Claims

CLAIMS : 1. An adsorption process for liquid separation comprising: (a) passing a liquid mixture comprising at least two components undergoing separation, said components comprising at least a more strongly adsorbed component and at least another less strongly adsorbed component, through an adsorbent material capable of adsorbing at least said more strongly adsorbed component; (b) flowing a first desorbent capable of desorbing said less strongly adsorbed component through said adsorbent material to displace said less strongly adsorbed component from said adsorbent material; and (c) introducing a second desorbent capable of desorbing said more strongly adsorbed component through said adsorbent material to recover said more strongly adsorbed component from said adsorbent material. 2. The adsorption process of claim 1, wherein said liquid mixture is selected from the group consisting of a mixture of normal paraffins and isoparaffins, a mixture of olefins and paraffins, a mixture of normal olefins and isoolefins, and a mixture of aromatics. 3. The adsorption process of claim 2, wherein said adsorbent material capable of adsorbing said more strongly adsorbed component is selected from the group consisting of zeolites, porous carbonaceous adsorbents, amorphous adsorbents, chemically modified adsorbents, and ion exchange resins. 4. The adsorption process of claim 3, wherein said liquid mixture is selected from the group consisting of a mixture of normal paraffins and isoparaffins, and a mixture of normal olefins and isoolefins; said first desorbent is selected from the group consisting of isoparaffins, isoolefins, naphthenes, and aromatics having a boiling point substantially different from the boiling point of said second desorbent and the components undergoing separation; and said second desorbent is selected from the group consisting of normal paraffins and normal olefins having boiling point substantially different from the boiling point of said first desorbent and the compounds undergoing separation. 5. The adsorption process of claim 3, wherein said liquid mixture is a mixture of olefins and paraffins, said first desorbent is a paraffin having a boiling point substantially different from said second desorbent and the compounds undergoing separation; said second desorbent is selected from the group consisting of an olefin and olefin/paraffin mixture having a boiling point substantially different from the boiling point of said first desorbent and the compounds undergoing separation. 6. The adsorption process of claim 3, wherein said liquid mixture is selected from the group of aromatic mixtures, said first desorbent is selected from the group of paraffins and naphthenes having a boiling point substantially different from the boiling point of said second desorbent and the compounds undergoing separation, said second desorbent is selected from the group of aromatics and a mixture of aromatics having boiling point substantially different from the boiling point of said first desorbent and the compounds undergoing separation. 7. The adsorption process of claim 3, wherein said liquid mixture comprises a mixture of normal paraffins, isoparaffins, normal olefins, isoolefins, and aromatics; said first desorbent is selected from the group consisting of isoparaffins, isoolefins, naphthenes, and aromatics having a boiling point substantially different from the boiling point of said second desorbent and the components undergoing separation; and said second desorbent is selected from the group consisting of normal paraffins and n-olefins having boiling point substantially different from the boiling point of said first desorbent and the compounds undergoing separation. 8. The adsorption process of claim 7, wherein said liquid mixture is Flexicoker distillate compressing about 30% normal olefins, about 15% normal paraffins, about 25% branched olefins and paraffins, and about 30% aromatics. 9. The adsorption process of claim 3 wherein said liquid mixture comprises about 15 wt.% decene-1, about 15 wt.% n-decane, about 15 wt.% 3-methylnonane, and about 55 wt.% 1, 3, 5- trimethylbenzene and said first desorbent is selected from the group consisting of isoparaffins, isoolefins, naphthenes, and aromatics having a boiling point substantially different from the boiling point of said second desorbent and the components undergoing separation; and said second desorbent is selected from the group consisting of normal paraffins and n-olefins having boiling point substantially different from the boiling point of said first desorbent and the compounds undergoing separation. 10. The adsorption process of claim 9, wherein said adsorbent is silicalite.