CA1225338A - N-paraffin-isoparaffin separation process using wash of recycle purge gas - Google Patents

Abstract

IMPROVED N-PARAFFIN-ISOPARAFFIN SEPARATION
PROCESS USING WASH OF RECYCLE PURGE GAS
ABSTRACT
An invention is described in which a portion of one of the product streams from an adsorption system is used to wash a recycle gas stream in order to improve the quality of one of the products.

Description

33~3 IMPROVED N-PARAFPIN-ISOP~RAFFIN SEPARATION
PROCESS USING ASSAY OF RECYCLE PURGE GAS
.
he present invention relates in general to the separation of mixtures of normal and non-normal paraffins. More particularly the present invention relates to lowering the amount of undesired residuals in the recycle purge gas utilized in such separation.
The separation of mixtures of chemical compounds unto two or more frequency by selective adsorption based on molecular size is a conventional procedure which takes advantage of the uniform diameters of pores of a given zeolitic molecular sieve adsorbent. The separation of normal paraffins from isoparaffins has been wound to be particularly adapted to this technique and a number of processes have been proposed for this purpose. Most of them have been based on contacting the mixed hydrocarbon feed in the vapor foe with a 5 Angstrom molecular sieve to adsorb the straight chain hydrocarbon compounds followed by resorption of the straight chain compounds at a lower pressure or higher temperature usually with the aid of a purge gas.
Some have been done with little or no change in temperature of pressure by employing a purge which it sufficiently strongly adsorbed to exert a displacing action on the adsorbed straight chain compound. Dyne process ox particular interest operates under essentially i~obaric end isothermal conditions even though resorption is accomplished using a non-~rbable purge gas instead of a strongly sorbable purge ~ateri~l. That process is refined in detail in US. Put. No. 3,700,589, issued act. 24, ., ~-13685 1972. An improvement of the aforesaid patent is described in US. Patent 4,176,053 issued November 27, 1979.
The present invention is an improvement of the foregoing processes which enables the lowering of the undesired residuals in recycled purge gas utilized in said processes. Also the present in-mention is similarly an improvement in the TIP
process referenced to and disclosed in US. Patent 4,210,771 issued July 1, 1980.
Brief Description of the Drowsy Figure 1 is a flow diagram of the present in-mention showing a four stage cycle process using a seven bed adsorbent system.
Figure 2 shows a simplified prior art flow scheme designed to treat a primarily hexane feed-stock and produce a high purity normal hexane product.
Figure 3 shows the same prior art processing unit as in Figure 2, with the addition of normals product washing in accordance with the present invention.
Figure 4 illustrates a flow scheme for the playacts a'rotal Isomerization Process which incur-prorates the washing step of the present invention.
Figure 5 shows a simplified flow scheme of a prior art Total Isomerization Process unit designed to treat a pontoon/ hexane feed stock and produce one high purity non-normal product.

2S~3~
- pa -Figure 6 shows the same prior art processing unit as in Figure Wyeth the addition of a non-normals product washing step in accordance with the present invention.
In accordance with a particular embodiment of a process, of which the present invention is an improvement, for separating normal paraffins from a mixture with non-normal paraffins; e.g. the pro-cuss of US. Pat. 4,176,053, a feed stock mixture of same in the vapor state and at super atmospheric pressure is passed periodically in sequence through each of at least four fixed beds of a system contain-in a zeolitic molecular sieve adsorbent having effective pore diameters of substantially 5 angstroms, each of said beds cyclically undergoing the states of:
(a) adsorption-fill, wherein the vapor in the bed void space consists principally of a non-sorbable purge gas and the incoming feed sock forces the said non-sorbable purge gas from the bed void space out of the bed without substantial intermixing thereof with non-adsorbed feed stock fraction;
(by adsorption, wherein the feed stock is co-currently passed through said bed and the normal con-stituents of the feed stock are selectively ad-sorbed into the internal cavities of the D-136~5 I

crystalline zeolitic adsorbent and the non adsorbed constituents of the feed stock are removed from the bed as an effluent having a greatly reduced content of non-normal constituents;
) void space purging, wherein the bed loaded with normals adsorb ate to the extent that the stoichiometric point of the mass transfer zone thereof has passed between 85 and 97 percent of the length of the bed and containing in the bed void space a mixture of normals and non-normals in essentially feed stock proportions, is purged counter currently, with respect to the direction of adsorption stage (b), by passing through the bed a stream of non-~orbable purge gas in sufficient quantity Jo remove said void space feed sock vapors but not more than that which produces about 50 mole percent, preferably not more than 40 mole percent, of absorbed feed stock normals in the bed effluent;
and (d) purge resorption, wherein the selectivity adsorbed feed stock normals ore recovered as product stream by passing a non-~orbable purge gas counter currently with respect to adsorption stage lo through the bed until the major proportion of adsorbed normal has been resorbed and the bed void space vapors consist principally of non-sorbable purge gas.
he above-noted process further comprises the recycling directly in the vapor phase, in combination with feed stock the mixture of normals and non-normals purged from each bed of the system during stage (c) to another bed of the system undergoing stage (b) adsorption.

533~3 In conventional practice the effluents from the bed during void space purging, stage (c), have been cooled to condense the higher boiling paraffin keenest tents, flashed to remove and recover any The molecular sieve adsorbent employed in the above-noted process and the present invention can be any of the naturally occurring or synthetically produced three-dimensional crystalline zeolitic aluminosilicates from which the water of hydration can be removed without collapse of the crystal lattice and which will selectively on the -basis of molecular size, adsorb normal paraffins from the mixture thereof with branched chain and/or cyclic paraffins which comprises the weed stream.
Since normal paraffins have a nominal cross-sectional diameter of about 5 angstroms, molecular sieves having pore diameters of about 5 Angstroms are preferred for the practice of the present invention. Especially suitable are the cation forms of zealot A which have a pore diameters of about 5 Angstroms. Zealot A is well known in the art as a synthesized zealot having a very large adsorption capacity and, depending on the cation species present, exhibit apparent pore diameters ranging from about 3 to about 5 Angstroms. As prepared in the sodium cation form, zealot A has pore diameters of about 4 Angstroms.
When 25 percent, preferably at least 40 percent, of sodium cations are exchanged by calcium and/or magnesium cations, the effective pore diameter increases to about 5 Angstroms. Zealot A as used herein in toe Specification sod claims is intended to the denote the zealot described and defined in US. Pat. No. 2,8B2,243. Other zealot molecular .
...2~33 sieves which, in appropriate cation forms have pore diameters of about 5 Angstroms and which, although having less adsorption capacity than zealot A, are suitably employed include zealot T, USE Pat. No.

2,950,9~2 an the minerals shabbiest and errant The hydrocarbon streams treated in accordance with the present invention consist essentially of mixture of branched chain paraffins and normal paraffins boiling in the gasoline and kerosene Ganges. Such mixtures occur as petroleum naphthas, both light and heavy, natural gasolines and natural gas condensates, but can be the products of processed outside the petroleum production and refining industry In general, the hydrocarbons of these streams contain from about 4 to about 13 carbon atoms and preferably are substantially free of olefinically and acetylenically unsaturated species. It as also advantageous that sulfur compound impurities, if present, be present in a concentration less than about 400 parts per million, and the water impurity levels are below saturation. Although the process is operable regardless of the relative molar proportions of normals end non-normals present on the feed, the greatest benefit is afforded when the process is operated as one o bulk separation wherein both non-normals and normal paraffins each constitute at least 10 mole percent of the feed stock.
The entire process is operated at a substantially uniform temperature selected from the range Of about 350F to 750F. At temperatures below abut 3~0~F, the efficiency of the non sorbable purge gas is decreased to the point where undesirably large quantities are required ~53~3 .

adequately to purge the normals from the bed. Above about 750~F, the rate of coke deposition increases rapidly and the need for more f sequent oxidation regenerations of the adsorbent arises It it to be understood that the denomination of the process as being isothermal" is done so for the reason that the temperature of the feed and purge gas stream are essentially the same, Leo within about 30~F, when entering the bed. In this, as in any ~dsorption-desorption cycle it is possible for thermal gradients to develop in the bed due to heats of adsorption and resorption.
The pressure range suitable for the present process is from about 50 Asia TV bout 400 Asia. In general, the preferred pressure is dependent on the particular feed stock involved, with the higher pressures being used for the more volatile feed stocks to enhance the separation obtained and to facilitate the condensation of the product effluents. it is important that none of the feed stock components condense in the void space of the bed since such liquid phase material cannot be removed by the limited quantity of non sorbable purge gas allotted for this purpose Accordingly, the pressure is to be maintained at less than 80 percent of the critical pressure of the highest boiling key component of the feed or less than about 60 percent of the dew point pressure of the feed at the process temperature, whichever is the lower value Similarly, as in the case of the term isothermal swooper, the process is termed isobar because the pressure of the Ddsoxber feed and purge gas streams are within conventional limits toe tame at their respective ~2~5~3 in fluent ends of the bed. The term iceberg: is thus used in its accepted sense in the art to indicate that the present process does not utilize a pressure swing type ox resorption, By the term key component" used herein in conjunction with the delineation of pressure requirements it meant any paraffinic constituent of the feed mixture which is present in significant amount. As is well understood in the art, what constitutes a significant quantity of a particular component of a mixture depends somewhat on the other components present and the nature of the treatment the feed is undergoing. Generally however in the present process, a key component will be present in an amount of about 10 mole percent or greater.
When the pressure conditions are dictated by the dew point criterion, the dew point of the hydrocarbon mixture involved can be determined by the procedure set forth in process Heat Transfer earn Donald Q., ~cGraw-Hill Book Company New York, NY. (USE.), at pages 319 to 325 inclusive. Other procedures are well known in the art to make these calculations. Routine experimentation can, of course, be resorted to; instead of calculating the dew point.
The non-sorbable purge gas used to flush the bed void space vapors and carry from the bed resorbed normal paraffins in this process is any permanent gas or mixture of such gases which have molecular dimensions sufficiently small to enter the intracrystalline cavities of the molecular sieve, but Are not themselves strongly enough adsorbed to displace the normal hydrocarbons adsorbed thereon to any significant degree. Nitrogen, hydrogen, helium .

~Z~5338 and methane are such materials and ore preferred in the practice of this invention. Other permanent gases are known in the art, but lack of commercial availability at reasonable cost renders the impractical although operable.
Bed void space for purposes of this invention is intended to mean any space in the bed not occupied by solid material except the intracrystalline cavities of the Zulu crystals.
The pores within any binder material which may be used to form agglomerates of the zealot crystals is considered to be bed void space.
As stated herein before, the adsorption stroke wherein the normal paraffins are selectivity -adsorbed on the bed is continued for a period ugh that the stoichiometric point of the normal hydrocarbons mass transfer zone has moved through between 85 and 97 percent of he bed length. The term mass transfer zone as used herein has the same meaning as is generally accepted in the art, i.e., it denotes that section of the adsorbent bed in which the adsorb ate loading of the adsorbent bed and the concentration of the adsorb ate fraction in the fluid stream are both increasing with time. The ~stoichiometric point lies within the mass transfer zone end is that point at which the expended capacity of the leading section of the mass transfer zone it equal to the unexpended capacity of the transfer zone.
In order to optimize the four stage cycle of the process and to give a substantially constant flow of both normal and non-normal hydrocarbon product stream it it preferred to use at least four duration beds of essentially equal capacity . .

3l~Z5~3~

g in an integrated time-controlled sequence. This system provides optimum conditions or heat exchange and purge gay recovery, as well as favorable mass transfer and pressure drop characteristics during the adsorption stage tub). All process valves can be automatically controlled from a timer control system. For purposes of illustrating the exemplary flows for the cycling of absorbers with a feed stock as set forth in Table I, the following description is provided in conjunction with Figure 1 of the drawing which is a flow diagram of such a four stage cycle prowess using a seven bed adsorbent system.
This description is applicable to the naptha range, n-paraffin/isoparaf~in separation process of the present invention regardless of the quantity or composition of the mixed pentanes/hexanes feed stocks used in the present invention, examples of which are provided hereinbelow. For purposes of the exemplification, it is presumed that the system has already attained a steady state. The temperature of all primary adsorption beds and awl feed streams to end effluents therefrom is 700~F. The beds ore under a pressure of about 25D Asia. The composition of the primary feed stock I shown in tabular form below:

TABLE I

Non normals Saturatedwt 4 of Come Wit Normals Hydrocarbonponent in Feed in Feed~tock I Trace C4 0.~0 1.60 C5 33,37 12~52 C6 44.66 5.92 C7 1.33 Trace CUB+ Trace Trace With reference to Fig 1 of the drawing, the feed stream is fed through line 10 to accumulator tank 12 from which i is drawn by pump 14 through a feed rate controller 6 and thereafter heft exchanger 18 where it is heated to about 450-500F by heat exchange with effluent from an adsorption bed undergoing second stage adsorp~ionO
The partially heated feed stream is raised to pull 700~F operating conditions in a gas fired heater 20. The flow rate of the feed stream from heater 20 into line 22 is 356,578 pounds per hour and has a normal paraffin content ox about 20~0 wit I, The stream from lone 22 it directed partially to line 24 by way of pressure controller 23 the use of which will be described later) and partially to line 26 by means of Plow rate controller 28 in line 26.
Through line 26 the minor portion of the feed from line 22, namely 99,026 pounds per hours, is directed through manifold 30 end valve 3Z to adsorption bed 34. Each ox the seven adsorptive beds in the D~136B5 , S33~

system, namely beds 34, 36, 37, 38~ 40, 41 and 42 Chinatown 93~000 pourlds of coulomb zec:lite A in the form of 1~16 inch cylindrical pellets. Each bed is 17.5 feet long and 12.5 feet in diameter. Bed 34, at the time that feed passing through valve 32 enters, contains residual hydrc>g~n purge gas from the preceding resorption stroke. The rate of flow of the feed through line 26, manifold 30 and valve 32 is controlled such that bed 34 it flushed of residual hydrogen uniformly suer a period of about one minute i.e., the effluent from bed 34 exists at a rate of about 3~B4~ pounds per hour. During this first stage of the adsorption stroke in bed 34, the hydrogen effluent passes prom the bed through valve 45 into manifold 104. During the one minute period when the hydrogen was being flushed from bed 34, feed passes from valve 23 through line 24, through manifold 62, and valves 64, 65 and 66 to beds 36, 37 and 38 respectively at the rate of 79,184 pounds per hour. The normal paraffins in the feed are adsorbed by each of beds 36, 37~ 38 and the non-adsorbed non-normal and residual hydrogen purge gas emerge from the beds through valves 68, 69, 70 respectively and fed to manifold 460 The non-normals and residual hydrogen purge gas flow through line 48, heat exchanger lo, line 50, water cooler 52, separator 54 an the condensed product it accumulated on accumulator 56. The residual hydrogen purge gas in the non-normals effluent leaves separator 54 through line 57 and ultimately to purge recycle compressor 60 via line 203.
Since the recycle purge gas separated form the non-normals effluent inevitably contains at least a small amount of non-normals, the direct D-1368~

So introduction of the recycle purge gas to recycle compressor 60 would result, an the course of second stage resorption, in non-normals passing through with resorbed normals in manifold 76 to normal prvbuct accumulator 114, thus detrimentally affecting the purity of the normals product.
In accordance with the present invention the recycle purge gas in line 57 containing residual non-normals is introduced to a wash vessel 200, which can be a traded or packed column, wherein the recycle purge gas is washed with a small portion of the normal product introduced into the wash vessel 200 via line 204. In the Nash vessel 200 the residual non-normals in the recycle purge gas are essentially displaced by normals product and recycle purge gas with lowered non-normal content is returned to recycle compressor 60 via line 203~ The stream exiting Nash vessel 200 via line 206 contains "wash normals and non-normals removed from the recycle purge gas and this stream is recycled to the feed.
During the one minute period when the residual hydrogen is being slushed from bed 34 i ye O
footage (a), bed 40 is undergoing the first stage of purging with hydrogen wherein the hydrocarbons in the bed void space are being flushed from the bed, isle, stage (c). During the same one minute interval, beds I and 42 are undergoing the second stage of resorption, i.e., stage Ed), in which the normal hydrocarbons are descried from the molecular sieve adsorbent using hydrogen and removed prom the bed From compressor 60, the hydrogen gas stream is passed through lone 72 and through heat exchanger 74 wherein it it heated to bout 55D-600F by heat .
.. .

53;~8

3 -from hot disrobed normals from any of the adsorption beds flowing through manifold 76. From the heat exchanger 74 the hydrogen gas stream passes through line 7B to gas fired heater 80 where it it heated to 700~F and hence through line 82. By means of flow controllers 84 and By the gas flow from line By is divided into two streams, the lesser stream being passed at the rate of 7,709 pounds per hour through Lowry By, manifold 88, and valve 90 counter currently (with respect to the previous adsorption stroke) through bed 40. The low controlled flow rate employed for the one minute first stage resorption is or the purpose of flushing non-adsorbed hydrocarbon from the bed voids without causing excessive desorpti~n of the normals from the adsorbent. The effluent from bed 40, consisting of 124,693 pounds per hour hydrocarbon and 2,038 pounds per your hydrogen passes through valve 92 and into manifold 62. The effluent from the first stage of desorpti~n containing the void space loading from the previous adsorption stroke plus any resorbed normal is recycled directly to the feed used during the second stage of adsorption without intermediate cowling, phase separation and reheating. In fact, valve 92 it used as the second stage adsorption feed valve when bed I it on that step in the cycle. The major portion ox the hydrogen stream from line 82, namely 37,659 pounds per hour is passed through control valve 85, line lQ2, to manifold 104 where it it mixed with the previously mentioned first stage adsorption e fluent from valve 45 and thence equally through valves 105 and 106 through beds 41 and 42.
During this purred selectively adsorbed normal paraffins are resorbed from the zeolitic molecular .

~Z~33~

sieve and flushed prom the bed. The effluent from beds 41 end 42 consisting of B5,000 pound per hour normal paraffins and 209654 pounds per hour hydrogen are fed through valves 107 and 108 to manifold 76 and thereafter through heat exchanger 74. the cooled normal purify f ins and hydrogen leaving heat exchanger 74 are fed to separator 110 through line 112 wherein the normals are fed to normals accumulator 114 and the hydrogen recycled to purge recycle compressor 60 through line 116~
The foregoing description is or a single one minute period of seven minute cycle of the system. For the next one minute period, appropriate valves are operated so that bed 34 begins a second stage adsorption stroke beds 37 and 38 remain on second stage adsorption, bed 36 begins a first stage resorption, bed 40 enter a second stage resorption, bed 41 remains on resorption and bed 42 begins a first stage adsorption stroke. similarly a new cycle begins after each one minute period and at the end of a seven minute period, the beds have all gone through all stages of adsorption and resorption.
The following charts indicate the functioning of each of the seven beds for each one minute period.
In the chart, A-l denotes the stage (a adsorption in which a bed is flushed of residual hydrogen using a feedsto~k stream at low weed rites. A-2 denotes a conventional adsorption stroke, i.e., the stage (b) adsorption herein, in which the rate of feed of the hydrocarbon mixture it commensurate with efficient use of the bed. D-l denotes the stage I
resorption in which purge gas is used in on mount sufficient to remove hydrocarbon vapor from the bed void spaces, and D-2 denotes stage (do, i.e., DBASE
.

:12~533~3 hydrogen purge using flow rates sufficient to resorb normals from the bed TIME SEQUENCE
Time, mix D 1 2 3 4 5 6 Bed 34 A-l A-2 A-2 A-2 D-l D-2 D-2 Bed 36 A 2 Do 2 D-2 Awl A-2 A-2 Bed 37 A-2 A-2 D-l D-2 D-2 A-l A-2 Bed 38 A-2 AYE D-l D 2 D-2 A-l Bed 40 D-l D-2 D-2 I A-2 A-2 A-2 Bed 41 Do D-2 . A-l A-2 A-2 A-2 D-l Bed 42 D-2 A-l A-2 A-2 A-2 D-l D-2 he purpose of valve 23 in the A-l feed line 24 is best shown by the following example.
Assume a feed pressure in line 22 of 250 Asia and also assume for purposes of this example what flow control valves 28, 34 and 85 and all line have negligible pressure drop. Then A-l feed lines 26 and 30 are at 250 Asia. A-l effluent line 104 reflecting the pressure drop through the adsorbed vessel is a 249 Shea This then must be the pressure of lines 102, 82, 86 and the D-l feed line 88. The D-l effluent line 62 reflecting the pressure drop through the ads~rber vessel is at 246 Asia. Since line 62 and line 24 are also the A-2 feed lines, their pressure is about 246 Asia. In this example, therefore, line 24 must be controlled by valve 23 to 4 psi lower than line 22 to ensure the mixing of Do effluent and the A-2 feed.
As a bed is cycled at the 700F operating temperature, a carbsnace~us deposit gradually ~ccumulatesO This deposit reduces the capacity of the adsorbent, which results in a breakthrough of normal paraffins into the isomer product stream and 331~ .

decreased normal paraffin recovery. The rate at which this deposit accumulates depends on factors such as temperature, feed impurities, feed properties, cycle time, and residual paraffin loadings. This type of adsorbent deactivation is temporary as that original bed capacity can be restored by burning off this deposit under controlled cond~ti~n~.
Oxidative regeneration is a blocked operation with turnoff of the five adsorbent beds in sequence, and is required to maintain the working capacity of the molecular sieve. The basis for this procedure is a three-day shutdown consisting of the following stages:

Stage: Time, his.
(1) System preparation or regeneration 2 (2) Oxidative regeneration 68 (3) System preparation or adsorption step 2 Total time 72 When the beds have been cycled to the point that oxidative regeneration is required, the normal process cycle is shut down, and the beds undergo an additional long resorption purge to remove as much of the residual normal paraffins as possible.
Countercurrent circulation of nitrogen is established by means of the purge gas compressor 60 at 100 Asia and 750~F. The circulation of the hot nitrogen has two purposes, namely to sweep the purge gas from the bed if it is combustible (i.e., fuel 5~338 gas, hydrogen, etc.), and to raise the temperature of the bed to above the coke ignition point prior to introduction of oxygen into the system the effluent gas from the beds manifold 76 is cooled Jo condense the hydrocarbons and water that are resorbed. When the bed is up tug temperature, air is introduced into the circulating stream at a rate such that the oxygen content of the gas entering the bed is between 0 and 1 percent by volume. the oxygen in the gas combust~ with coke in the top of the bed. The heat released from combustion is carried out of the burning zone as a preheat front traveling ahead of the burning front. This preheat front raises the bed temperature to about 950F.
This temperature is controlled by regulating the amount of oxygen in the entering gas. Internal pellet temperatures in excess of 130~F will permanently destroy the molecular sieve crystal so the was phase temperature i held to a maximum of 1000F. As the burning front passes through the bed, the temperature will drop back to the gas inlet temperature of 750F. Since the coke deposit contains hydrogen, water is formed during combustion in addition to carbon oxides. this water must be removed from the system because the molecular sieve it permanently damaged by repeated exposure to water at high temperatures. A refrigeration unit is used to remove Yost of the water, thereby minimizing this damage Figure 2 depicts a simplified prior art flow scheme designed to treat a primarily hexane ~eedstock end produce a high purity normal hexane product using conventional technology of cooling the streams to recover the products from the recycle gas D-136~5 .

~53 instead of the wisher' I f the present invention .
Table A shows a material balance round the normal product recovery pectin of the unit.
In the conventional prior art unit with reference to figure 2, normals product purity is controlled in two ways. First a level of cooling is maintained on the recycle vent gas (Sir. 1, Fig 2), in order to minimize its concentration so non-normal hydrocarbon before the stream enters the normal product recovery section. The second way is to counter-currently purge an adsorbed, which has just completed an adsorption step, with hydrogen (D-l step, Sir. 6, Fig. 2), to remove feed non-normals from the void space inside the vessel. Non-normals which remain in the void space after this step will enter the normal product during the resorption Taipei It should be noted that any hydrogen which leaves the bottom of an adsvrber during the D-l step, will enter an adsorbed on a hydrocarbon feed step (A-2) and will ultimately end up as recycle vent gas (Sir. 1, Fig. 2), The material balance in table A shows a C5~ normal hexane product purity of 96.3 mole percent for the conventional unit of Figure 2. Of the 2.00 moles/hr of non-normals in the normal product, it can be calculated Prom the material balance that 1.41 moles per hr. (47~) of these come from the void spaces at the end ox the D-l step, 1.56 moles/hr (52~) come from the recycle vent gas and 0.02 moles per hour ~14) comes from the hydrogen make up.
Increasing the normal product purity beyond the 96.3 mole percent with the prior art 10~ scheme in Fig. 2 it costly and impractical. Lowering the D-136B~

33~3 temperature on the product recovery sections is expensive from the investment, utilities end Montanans standpoint and would require Peed drying as a pretreatment step. Increasing the hydrogen feed Turing the Do purge step will not work. Yost of the non-normals are removed from the vessel near the beginning of the D-l step and a the step continues, decreasing amounts of non-normals are removed. A this D-l purge rate is increased, more purge gas leaves the bottom of a vessel on the D-l step and ultimately carries non-normals into the normal product recovery section via the recycle vent gas. When the quantity of non normals removed per mole of purge gas during the D-l step becomes less that the quantity of non-normals carried back to the normal product recovery section by that same mole of gas, increasing the flow to the D-l atop will lower the normal product purity instead of increasing it.
over incorporating the normals product wash into the prior art unit flow scheme, in accordance aye the present invention, can increase the normal product purity with only a small ( I increase in adsorbent inventory and about I increase in compressor horsepower.
Figure 3 shows the tame prior art processing unit as in Figure 2, with the addition of normals product washing in accordance with the present invention used to increase the C5+ normal hexane product purity from 96.3 to 97.6 mole percent. In Figure 3, in accordance with the present invention, the recycle vent gas is washed counter currently with a small quantity about (5.5%) of the normals precut in a traded or packed column The washed recycle vent was leaving the top 33~

of the wash column carries considerably less non-norm~ls back to normal product recovery section than in the prior art scheme (jig. 2). The recycle wash normals (Starr 3, Fig. 3) flow back to the feed print carrying with it the recovered non-normals.
The wash vessel in this arrangement was simulated with one theoretical stage. Increasing the number of theoretical stages to two or three will reduce the quantity of wash normals needed to obtain the tame increase in product purity.
Table B shows a material balance for the normal product washing and normal product recovery section of Figure 3 in accordance with the present invention. Of the 1.72 moles per hr. of non-normals in the C5~ normals product (Sir. 8, it 3), it can be calculated from the material balance that 1.28 moles per hr. ~74~) of these come from the void spaces it the end of a D-l step, 0.42 mole per hr.
~25%) oozes from washed recycle vent gas and 0002 mole per hr. I comes from hydrogen makeup.
The above mentioned material balance information shows that the contribution of the recycle vent gas non-normals, to that of the total C5~ impurities in the normals product, has been reduced to 27 percent ox the quantity found in the product when not using normals product washing. An added advantage of the normals product wash is that the D-l step feed rate can now be increased to a higher quantity without having a negative impact on the normal product purity.
In place of providing a higher normal product purity, the produce wash embodiment Do the present invention could also be used to reduce the investment end utilities of the unit for the tame it product purity (96.3 mole I, By reducing the recycle vent gas non-normals concentration, the quantity of non-normals let in the Bessel at the end of a D-l step can almost be doubled, thereby requiring less D-l step purging. Lowering the D-l purge rate reduces the quantity of normals recycled back to the adsorption feed point (try. 6, jig 2) and therefore reduces the required adsorbent inventory. In this case the reduction in adsorbent inventory would be around 10 percent.
The present invention is also applicable to what is known as the TIP process described in US.
Patent 4,210,771 which is a process for improving the octane rating of certain petroleum fractions by virtually complete isomerization of the normal paraffin hydrocarbons contained in feed stream essentially of mixed normal and non-normal hydrocarbons, more particularly, to virtually complete isomerization of normal pontoons and normal hexanes contained in a feed stream containing normal pontoons and normal hexanes, as well as nsn-normal hydrocarbons to drum branched chain iso/pentanes and iso/hexanes~
Essentially, the TIP process comprises passing a stream containing a mixture of normal and non-normal hydrocarbons into an isomerization reactor to catalytically isomers at least a portion of the normal in the presence of hydrogen by contact in the reactor with a catalyst composition, which preferably is a zeolitic molecular sieve with a hydrogenation component.
Other catalyst compositions such as alumina-base catalysts may be used as well. The temperature of the reactor is dependent in part on the particular - I
catalyst employed, but preferably is within the range of 200C to 390~C end the pressure in the reactor ranges between 175 Asia and 600 Asia when a molecular sieve catalyst it employed. The effluent from toe reactor till contains approximately 20 30 White normals. The hydrocarbon fraction of the reactor effluent stream it circulated to a zeolitic molecular sieve adsorbent bed where the normals are selectively adsorbed and the non-normals are passed out to the adsorbed as an adsorbed effluent and eventually an isomer ate product The normals are then resorbed from the bed using a hydrogen purge stream. Figure 4 illustrates an arrangement for the practice of the TIP process which incorporates the wish ox the present invention. with reference to Figure 4 an is~merization reactor, such as described in Us Patent 4,210,771, is provided at 300 and receives via line 112 normal paraffins and recycle hydrogen gas together with feed I mixed pontoons and hexanes. the i~o~erized product and recycle gas, which in this case contains residual normals, are transferred to separator 110 from which the i~omerization product is transferred via line 301 to the absorption bed for processing as hereinabove described in connection with Figure 1, with the product in this case being non-normals, which are recovered it 56. The recycle purge gas from separator 110 contains residual normals which, if not removed, decrease the purity of the non-normals product recovered it 56. Consequently; in accordance with the present invention, the recycle purge gas from separator 110 is fed via lone 302 into wash vessel 200 e.g., traded or packed column, wherein the recycle purge gas it washed with ~5331~

a small portion of the non-normals product introduced into the wash vessel 200 via line 304.
In the wash vessel 200 the residual normals in the recycle purge gas are essentially displaced by non-normal product and the recycle purge was with lowered normal content it returned to recycle compressor 60 via line 303. the stream exiting wash vessel 200 via line 306 contains wish non-normals and normals removed from the recycle purge gas and this stream is recycled to the absorption bed with normals from accumulator 114.
Figure 5 depicts a simplified flow scheme of a prior art Total Isomerization Process (TIP) unit designed to treat a pentane/hexane feed stock end produce one high purity non-normal product which uses conventional cooling to recover hydrocarbons from the recycle hydrogen instead of the wish of the present invention. Normal paraffin in the recycle hydrogen in the TIP process contribute to inefficiencies and product impurities in the TIP
process, in which the product is normal paraffins. Normal paraffin are adsorbed/desorbed from molecular sieve adsorbent due to a swing in their partial pressure over the adsorbent. A higher concentration of normal paraffin in the recycle hydrogen (Starr 2, ~i99 5) will reduce this swing of partial pressure and thereby cause a larger quantity of adsorbent to be used for the separation of the normal from the non-normals. During the resorption step, normal paraffin will load unto the top of an adsorbent bed in equilibrium with their concentration in the recycle gas. During a subsequent absorption step, this residual concentration of normals sets the limit as to how opals - I -low the feed normals eoncen~ration can be reduced.
Because of this the adsorption effluent Starr. 3 jig.
5) leaving the top of a vessel during on adsorption AYE) step, will always have a normal paraffin concentration higher than that of the recycle hydrogen. Valve leakage and normals front leakage will also contribute Rome impurities to the adsorption effluent. They represent about 50~ of design impurities in this case. Therefore, it can be understood that TV maintain a high adsorption effluent purity (or ultimately non-normals product purity), the normal paraffin content of the recycle hydrogen must be maintained at a low Lyle The conventional way to Monet n a low level of normals in the recycle hydrogen is to chill the isomeri~ation reactor effluent stroll, Fig S) to 60F. the resulting normal paraffin content of the recycle hydrogen and the C5~ non-normals products, aster chilling to 60~F~ can be teen in columns 1 and 2 ox Table I
By reducing the severity of cooling during product recovery prom 60F to 85~F~ the refrigeration package associated with conventional TIP units can be eliminated, thus saving the investment and the operating utilities of the chilling package. There are, however, penalties for doing this which will offset the above savings. As can be seen in columns 2 and 3 of Table C, the nC5 nC6 content of the recycle hydrogen has increased, relative to the 60~F case, from 1.0 to 1.5 mole percent and the normals content of the C5+ product has increased from 2.4 to 3.4 mole percent. the higher normals concentration in recycle hydrogen will also cause the normal paraffin delta loading in the absorbers to drop, thereby requiring bout 10 percent more ~dsorbentO This adsorbent increase will in turn Allah require a higher recycle hydrogen rate by about 10 percent.
In addition to the above penalties, the quality of the product Spas dropped, as can be seen by a decrease in the produce RON-O, from 8B.5 to Braille.
The penalties for increasing the product recovery temperature prom 60F to 85F can be reduced by using non-normals product washing in accordance with the present invention as illustrated in Figure 5. Figure 6 depicts the TIP unit using non-normals product washing, in accordance with the present invention, to improve the recovery of normals from the recycle gas. In the arrangement of Figure 6, the recycle hydrogen leaving the isomerization product recovery section enters the bottom of a traded or packed column. In the column it is counter-currently washed walk about 11 percent of the non-n~rmals product. The recycle wash non-normals, which contain the normals removed from he recycle hydrogen, ore recycled back to the adsorbed feed to be reprocessed in the adsorption section. the washed recycle hydrogen leaves from the top of the vessel to be recycled back to the adsorption section. Table D shows a material balance for the non-normals product washing which at o used 85~ cooling in the product recovery section of the unit. This material balance was simulated using 5 theoretical stages in the wash column .
It can be teen from the washed recycle hydrogen composition end the I non-normals product composition (Sir. 5 and 6) in table Cud that D~13685 ~5~3~

the efficiency of the washing step in accordance with the present invention approaches that ox chilling to 60F) in the product recovery section.
Non-normals wishing of the recycle hydrogen has reduced the normals content of both the recycle hydrogen and the nonformal product back to the low level experienced before by the use of a chiller.
The recycled wash non-normals stream will cause an increase in the adsorbent inventory and recycle hydrogen requirement of about 10 percent, and will also lead to proportionally higher fuel cost.
Because of this, the wash non-normals rate will have to be carefully optimized during a design. However, due to the elimination ox the refrigeration package the overall investment cost of the unit, relative to the conventional scheme, would be reduced by about 6 percent and the utilities due to compression equipment would be reduced by about 30 percent.

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S U N Pi

Claims

WHAT IS CLAIMED IS:

1. In a process for separating normal paraffins and non-normal paraffins from a vapor state mixture containing the same wherein (i) said mixture is passed through in sequence a plurality of fixed beds of a system containing a zeolitic molecular sieve absorbent which selectively absorbs normal paraffins, (ii) a non sorbable purge gas is used to (a) desorb normals from each bed to provide an effluent containing desorbed normals in mixture with purge gas and (b) transfer non absorbed non-normal paraffins from each bed to provide separate effluent containing non-normal paraffins in mixture with purge gas (iii) purge gas containing residuals of normals and a normal product stream is separated from the effluent of (ii)(a) and said purge gas is recycled and (iv) purge gas containing residual of non-normals and a non-normal product stream is separated from the effluent of (ii)(b) and said purge gas is recycled, the improvement for lowering the residual content of recycle purge gas which comprises washing the purge gas from a selected effluent of (ii)(a) or (ii)(b) prior to recycle with a portion of the product stream from the non-selected effluent.