CA2009332A1 - Cyclic adsorption/desorption purification - Google Patents

Abstract

"CYCLIC ADSORPTION/DESORPTION PURIFICATION "

ABSTRACT

Organic feedstocks containing an impurity are purified in a cyclic adsorption process by passage in the liquid phase through a fixed bed of an impurity selective adsorbent whereby the impurity is selectively adsorbed and a purified feedstock recovered in unusually high yield by an adsorbent bed regeneration procedure involving the use of a portion of the purified feedstock in the vapor phase to countercur-rently remove the liquid held in the bed void space and to displace a non-sorbable purge gas which is used to regener-ate the adsorbent bed.

Description

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"CYCLIC ADSORPTION~DESORPTION PURIFICATION "

FIELD OF THE INVENTION

The present invention relates in general to the purifi-cation of organic feedstocks and more particularly to the purification of such feedstocks which contain at least one impurity component more strongly adsorbed on a molecular sieve adsorbent than the purified feed. The process is advantageously employed where it is desired to reduce the loss of purified product caused by coadsorption and "sponging" on the adsorbent. The process, which is carried out in the liquid phase during the adsorption-purification step and in the gas phase during the adsorbent regeneration step, makes use of purified product vapor to increase the recovery of purified product.

DESCRIPTION OF THE PRIOR ART

The purification in the liquid phase of organic feed-stocks such as hydrocarbons containing sulfur compound impu-rities by the selective adsorption of the impurity compounds on molecular sieve adsorbents is well known in the art. For example, the liquid phase purification of petroleum-derived full range hydrocarbon feedstocks containing carbonyl sul-fide is disclosed in U.S. Patent 3,654,144. Therein the process comprises passing the sour hydrocarbon feed in the liquid phase through a fixed bed of a bivalent metal cation form of zeolite A to selectively adsorb the COS. The peri-odic regeneration of the adsorbent bed is accomplished in the vapor phase using a non-sorbable purge gas such as nitrogen for displacement of the void space liquid as well as desorption of the COS impurity. This procedure is typi-cal of the prior art processes insofar as regeneration isconcerned. The entire volume of the gas stream used for -2- 2~ 33~
displacement and purge desorption is isolated from the sys-tem and is largely a waste material. Equally disadvanta-geous economically is the fact that the full bed void space volume of ~vapor phase hydrocarbon re~aining in the bed along with a significant quantity of "sponged" liquid hydrocarbon and any coadsorbent hydrocarbon becomes admixed with the purge gas and/or the desorbed impurity constituent and is either not recovered as product or must be reclaimed by a further purification procedure.

SUMMARY QF THE INVENTION

There has now been discovered a novel cyclic adsorp-tion/desorption process for purifying organic materials con-taining undesired amounts of an impurity which permits the recovery as purified product of an appreciably greater pro-portion of the feedsto~k being treated. The process com-prises the sequential steps of:

(a) passing the impure organic feedstock in the liquid phase through a fixed bed containing a molecular sieve adsorbent capable of selectively adsorbing the impurity to produce a purified product and terminating such passage prior to the time of breakthrough of an undesirable concen-tration of the impurity into the product stream;

tb) draining the bed of its void space held liquid by displacing said liquid with vapor containing one or more of the constituents of the purified product in an amount and under temperature and pressure conditions such that substan-tially all of the adsorbed impurity remains adsorbed on the molecular sieve;

tc) countercurrently to the direction of flow of the feedstock in step (a), introducing a substantially non-~3~ 2 ~ 3~3 2 sorbable purge gas into the bed at a rate which displacesthe void space vapor from the bed without significant inter-mixing of the two gas phases and without d sorbing the adsorbed impurity, and collecting the displaced vapor as product;

(d) continuing the flow of the non-sorbable purge gas through the bed, preferably at an elevated temperature, to desorb the adsorbed impurity and flush same from the bed and out of the adsorption-purification system;

lo (e) displacing the non-sorbable purge gas remaining in the bed at the termination of step (d) by the cocurrent flow, with respect to the direction of flow in step (d), of vapor containing one or more constituents of the purified product and thereafter;

lS (f) countercurrently with respect to the direction of flow in step (e) displacing the void space vapor remaining in the bed at the termination of step (e) by introducing in the liquid phase a portion of the purified product from step (a) in an amount sufficient to fill the bed void space and optionally to cool the bed to the desired temperature for performing step (a) and thereby establish the bed in condi-tion to begin a new purification and regeneration cycle.

DETAILED DESCRIPTION OF THE INVENTION

As will readily be apparent to those skilled in the 2S art, the enhanced recovery of purified product attributable to the present process is due in large part to the use of purified product, or one or more fractions thereof if the purified product is a mixture of two or more organic species, in the vapor phase to perform the displacement functions conventionally performed using a non-sorbable purge gas which is extraneous to the feedstocks belng 3 treated. During the bed draining and liquid displacement steps at the beginning of bed regeneration, the bed effluent is at no time contaminated with any materials not acceptable as a part of the purified product, thus greatly simplifying subsequent puri~ication on recycle. At the end of the liq-uid displacement step the bed void space contains either purified product vapor or one or more constituents thereof which is supplemented by any constituents of the purified product which are coadsorbed with the impurity constituent during the initial adsorption-purification step. Had an extraneous displacement gas been used in the draining and liquid displacement steps, the desorbed constituents of the purified product would be effectively lost as product by virtue of admixture with a non-product material. The use of the extraneous purge material is limited to plug flow dis-placement of product-compatible material, which can be col-lected as product, and desorbing the impurity constituent of the feedstock to regenerate the adsorption capacity of the bed. As used herein, the term "plug flow" is intended to have its usual meaning in the art that displaced and dis-placing fluids in the bed are maintained with only a minimal intermixture at their interface in the bed. In addition to significantly increasing the purified product yield, the amount of extraneous purge gas required is also reduced, usually to economic advantage.

The molecular sieve adsorbent used in the process is not a narrowly critical aspect of the invention and is selected with regard to the molecular dimensions, polarity, volatility and the like of the impurity constituent to be removed. Zeolitic molecular sieves such as 13X, 4A and 5A
widely used for adsorption-purification processes are pre-ferred adsorbents. Non-zeolitic adsorbents such as the AlP04's, SAPO's and MeAPO's more recently discovered and -5- ~ ~3~2 made available commercially are also suitably employed.
Such materials are described in detail in the patent litera-ture, for instance, U.S. Patents 4,567,029; 4,310,440 and 4,~40,871.

The organic feedstocks are also not a narrowly critical factor. Most commonly the hydrocarbon feedstocks derived from the various petroleum refining operations require adsorption purification since sulfur compounds are almost universal contaminants of the petroleum crudes and must be removed somewhere along the processing chain to avoid atmo-spheric contamination when the final products are burned as fuel. The process is especially useful in the removal of carbonyl sulfide (COS) from propane, but other feedstocks such as ethane, butane, natural gasoline, propylene, buty-lene, C2 through C4 paraffins, olefins and diolefins gener-ally are suitably treated. The major requirements of the feedstocks are (a) that they can exist in both the liquid and the vapor phase under the temperature and pressure con-ditions which can reasonably be imposed on the adsorption system, and (b~ that the impurity to be removed is more strongly adsorbed on the adsorbent under the imposed treat-ment conditions than the purified product constituents.

The temperature and pressure conditions, flow rates, adsorption bed sizes and configurations will vary depending 2~ upon the feedstock being treated, but their selection is well within the routine skill of the art.

THE DRAWINGS

The sole figure of the drawings is a schematic flow diagram of one embodiment of the present process.

-6- ~,?~ ~32 ILLUSTRATIVE EM~ODIMENT

The various embodiments of the present invention are illustrated by the following specific process system which concerns the removal of carbonyl sulfide from propane. The source of the propane is a natural gas stream which has heen treated in a depropanizer column and a deethanizer column to remove ethane and propane. In this embodiment, the overhead from the deethanizer column (hereinafter "DEO"~ is conve-niently employed as the extraneous non-sorbable purge gas.
With reference to the drawing, the bottoms effluent from a natural gas deethanizer column is fed through line 10 and valve 12 to depropanizer column 14. The overhead from column 14 which comprises propane and COS, as an impurity constituent, passes through lines 16, ~8 and 20, condenser 22, line 24 and 26, valve 28 and line 30 into the bottom of adsorbent bed 32 which contains zeolite 5A. Bed 32 has, at the beginning of the adsorption purification step, been regenerated, cooled to 90F and filled with feedstock. The flow rate of feedstock into bed 32 through line 30 is from about 56,000 to 66,000 pounds per hour. Purified propane is recovered through line 34, valve 36 and line 38, while COS
is retained in bed 32 as an adsorbate. The bed capacity is such that the adsorption-purification step proceeds for 8 hours, after which time feedstock flow is diverted via valve 28 through line 42. During the 8 hour period, in which bed 32 is engaged in the adsorption-purification step, adsorbent bed 42 is undergoing regeneration following a similar adsorption-purification step therein. As the first step of bed 42 regeneration, the void space liquid is drained through line 44, valve 46 and the line connecting valve 46 to valve 28 which is then passed via line ~0 as a portion of the feedstock to bed 32. Draining and displacement of the liquid from bed 42 is aided by the introduction of a portion of the overhead from depropanizer 14 which has been con-verted from liquid phase to the vapor phase by heating in furnace 50. The depropanizer overhead contains COS, but in terms of absolute quantities the vapor phase stream entering bed 42 from furnace 50 through line 52, valve 54 and line 56 imparts very little COS to the adsorbent of bed 42. At the beginning of the draining step, the temperature of the vapor stream entering bed 42 is initially about 150F (65.6C) and flows at the rate of 13,600 lb/h (6169kg/h) at 325 psig 10 (2342 kPa) for a period of 0.5 hours. Over this period the temperature of the vapor stream is increased to 250F
(121C) which aids in desorbing coadsorbed-propane from the adsorbent mass without appreciably desorbing COS impurity.
Thereafter, over the period of the next 15 minutes, the vapor phase propane in the bed void space is displaced by the plug flow introduction of the vapor phase overhead from the deethanizer ~DEO) through line 58 to furnace 50, wherein the gas is heated to about 150F (65.6C), line 60, valves 62 and 54, and line 56. The DEO stream is at a pressure of 400 psig (2859 kPa) and flows at the rate of 33,000 lb/h (1469 kg/h). The propane vapor leaving bed 42 is fed through line 44, valve 46, line 66 and valve 72 to condenser 68 and then recycled to the depropanizer column 14 through line 70 and valve 12. The flow of DEO into bed 42 is con-tinued for an additional 1.25 hours, optionally withincreasing temperature of DEO up to about 550F (288C). It is possible during this period to recover additional propane by recycling the bed effluent to the deethanizer unit, or the effluent can be vented from the system. In the next step of the regeneration the adsorbed COS i5 desorbed by the continued flow of DEO at 550F (288C) and at the same flow rate and pressure as used to displace the void space propane vapor. The desorbed COS and DEO purge gas is vented from the system through valve 72 and line 74. The COS desorption period is 4 hours and at the end of that period the flow of -8- 2~ 33 2 DEO is terminated and the DEO in the bed void space is dis-placed using the propane vapor stream from depropanizer column 14 through lines 16 and 18, furnace 50, line 52, valve 54 and line 56, at a flow rate Gf 13,600 lb/h (6169 kg/h), a temperature of 150 to 250F ~55.6 to 121.1C) and a pressure of 335 psig (2411 kPa). The DEO vapor is passed from bed 42 through line 44, valve 46, line 66, valve 72 and line 74. This DEO displacement step requires 15 minutes.
As the final stage of regeneration, bed 42 is refilled with liquid phase product propane introduced into bed 42 via lines 76 and 40, at the rate of 56,000 lb/h (25,400 kg/h) at 300 psig (2515 kPa) and at a temperature of ~0F (32C). As the bed is filled with purified propane and cooled the propane vapor in the void space is either recycled to the depropanizer column 14 (conduits not shown) or recovered as product. Once bed 42 has been filled with liquid phase product, the bed is in condition to begin a new purification cycle while bed 32 is subjected to regeneration in the manner described.

Claims (4)

1. Process for purifying an organic feedstock contain-ing undesired amounts of an impurity which comprises the sequential steps of:
(a) passing the impure organic feedstock in the liquid phase through a fixed bed containing a molecular sieve adsorbent capable of selectively adsorbing the impurity to produce a purified product stream and terminating such pas-sage prior to the time of breakthrough of an undesirable concentration of the impurity into the product stream;
(b) draining the bed of at least a substantial portion of its void space held liquid by displacing said liquid with vapor containing one or more constituents of the purified product;
(c) countercurrently to the direction of flow of the feedstock in step (a), introducing a substantially non-sorbable purge gas into the bed at a rate which displaces the void space vapor from the bed without significant inter-mixing of the two gas phases and without desorbing the adsorbed impurity and collecting the displaced vapor as product;
(d) continuing the flow of the non-sorbable purge gas through the bed to desorb the adsorbed impurity and flush same from the bed;
(e) displacing the non-sorbable purge gas remaining in the bed at the termination of step (d) by the cocurrent flow, with respect to the direction of flow in step (d), thereinto of vapor containing one or more constituents of the purified product; and thereafter, (f) countercurrently with respect to the direction of flow in step (e) displacing the void space vapor remaining in the bed at the termination of step (e) by introducing in the liquid phase a portion of the purified product from step (a) in an amount sufficient to fill the bed void space and thereby establish the bed in condition to begin a new purification and regeneration cycle.

2. Process according to claim 1 wherein the organic feedstock is a hydrocarbon containing from 3 to 6 carbon atoms and the impurity to be removed is a sulfur compound.

3. Process according to claim 3 wherein the sulfur compound is carbonyl sulfide.

4. Process according to claim 1 wherein the molecular sieve adsorbent is a crystalline zeolite molecular sieve.