Abstract:
Molecular sieves are used to remove sulfur compounds (e.g., COS and H 2  S) from liquid hydrocarbon streams, particularly liquid propane. During regeneration of the sieves, some of the liquid hydrocarbons remains on the sieves and is lost be vaporization during the purge step and the liquid fill step. This invention reduces the loss by recovery and recycle of the vaporized hydrocarbons in the liquid recovery process.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention pertains to a method of recovering liquid hydrocarbons from a molecular sieve sorbent bed in a hydrocarbon treating system. This invention is particularly useful in recovering &#34;sponge&#34; or &#34;sponged&#34; propane that has been retained by a bed of zeolitic molecular sieves in a treatment system designed to remove sulfur compounds (e.g., carbonyl sulfide) from liquid propane. 
     2. Description of the Prior Art 
     Various methods have been used to remove sulfur compounds (e.g., carbonyl sulfide (COS), carbon disulfide (CS 2 ), hydrogen sulfide (H 2  S), and the like) from hydrocarbon streams. Such methods have been referred to as &#34;sweetening&#34; or &#34;desulfurization&#34; processes. 
     The reasons for removing sulfur compounds from hydrocarbon streams are almost as diverse as the sweetening methods used. Two of the primary reasons are (a) to protect the environment by eliminating or reducing the amount of noxious/toxic hydrogen sulfide and/or sulfur dioxide formed during combustion, and (b) to protect hydrocarbon process equipment that comes in contact with such hydrocarbon streams. Sulfur compounds, particularly H 2  S, are known to corrode base metals (metal parts) and to foul catalysts used in refineries. 
     The specification limits upon the maximum amount of sulfur compounds that may be present in a particular hydrocarbon product will vary depending upon the product and its conventional use. Increasingly stringent limits have been placed on liquid hydrocarbons conventionally used as fuels or fuel additives. E.g., gasoline and diesel fuels in the transportation area, and liquid propane and butane fuels in combustion heating. 
     One method that has been used to sweeten liquid hydrocarbons involves contacting the liquid hydrocarbon stream with one or more sorbent beds loaded with materials that sorb the sulfur compound(s) as the hydrocarbon stream passes through the bed. Various sorbents have been used, but molecular sieves have been particularly useful in this application. The molecular sieves are typically zeolitic materials, both natural and synthetic, comprising ordered porous crystalline aluminosilicates having a definite crystalline structure, as determined by X-ray diffraction within which there are a large number of small cavities, which are interconnected by a series of still smaller channels or pores having a uniform pore size or pore size distribution in a rigid three-dimensional matrix of SiO 4  and AlO 4 . Such molecular sieves and the methods of making and using them in sweetening hydrocarbon streams are well known, as illustrated by the following nonexhaustive list of patents: 
     
         ______________________________________Assigned to Union Carbide               Others______________________________________USP       USP           USP      USP3,069,362 4,645,516     3,051,646                            4,313,8213,069,363 4,683,217     3,102,855                            4,404,1183,211,644 4,684,511     3,188,293                            4,540,8423,620,969 4,684,617     3,218,250                            4,703,0253,654,144 4,686,029     3,282,8313,660,967 4,686,092     3,450,6294,098,684 4,686,093     3,490,865     4,738,837______________________________________ 
    
     The disclosures of these patents are incorporated herein by reference to the extent that any such patent, or combination of patents, describes a molecular sieve, or a method of making a molecular sieve, that can be used to remove sulfur compounds from a liquid hydrocarbon stream. As these patents indicate, the zeolitic materials can be manufactured with a uniform pore size and with a pore size chosen to selectively adsorb molecules of certain dimensions (e.g., COS) while rejecting molecules of larger dimensions (e.g., propane)--hence, the term &#34;molecular sieve.&#34; 
     Under conditions of use in sweetening operations, a sour stream of liquid hydrocarbons is contacted with the molecular sieves under conditions sufficient to remove appreciable quantities of the sulfur compound(s) from the hydrocarbon stream. Typically, a pressurized hydrocarbon stream is passed through one or more &#34;packed columns&#34; or &#34;beds&#34; of molecular sieves at a flow rate and other process conditions that provides adequate contact time for the sulfur compounds to be removed. As the molecular sieves reach the limit of their capacity to sorb the sulfur compounds (i.e., the molecular sieves become &#34;loaded&#34;) and prior to &#34;breakthrough&#34; when the treated hydrocarbon product passing through the molecular sieves fails to meet specifications with respect to the amount of sulfur compounds in the product, the flow of the hydrocarbon is stopped until the bed of molecular sieves can be regenerated, or more typically, the flow is diverted to another bed of molecular sieves while the first bed is regenerated. 
     The bed of molecular sieves that is loaded with sorbed sulfur species can be regenerated by draining liquid hydrocarbon from the bed and then heating the bed of molecular sieves under conditions sufficient to volatilize and desorb the sulfur species and thereby reactivate the molecular sieves to further sorption. The vapors from the heated bed are typically flared. This technique for regeneration is well known and widely used in the industry. Unfortunately, this technique results in a substantial loss of hydrocarbon product, i.e., material that does not drain from the bed and is lost to the flares when the bed is heated. The loss of product retained by the molecular sieves (sometimes referred to as &#34;sponge&#34; or &#34;sponged&#34; hydrocarbon) is exacerbated when the sulfur content in the hydrocarbon feed stream is high and/or when the specification on the treated product is stringent. In such instances, the bed must be regenerated more frequently. 
     A method has now been discovered that will reduce this loss of hydrocarbon and thereby enhance the economics of the treatment process. 
     SUMMARY OF THE INVENTION 
     The novel sweetening process comprises the steps of: 
     (a) stopping the flow of sour liquid hydrocarbon to a bed of molecular sieves in a hydrocarbon sweetening plant, 
     (b) draining liquid hydrocarbon from the bed of molecular sieves, 
     (c) purging the bed with the gaseous sponged hydrocarbon by heating the bed to a temperature above the boiling point of the sponged hydrocarbon using a first heating means located within said bed until a significant portion of the sponged hydrocarbon has been vaporized and removed from the bed as a gaseous effluent, and combining the gaseous effluent with a hydrocarbon feedstream to the hydrocarbon recovery plant, 
     (d) regenerating the purged bed by contact with a gaseous hydrocarbon heated to a temperature sufficient to volatilize and desorb sulfur species from the molecular sieves, 
     (e) cooling the regenerated bed by contact with hydrocarbon residue gas or sweetened hydrocarbon, 
     (f) refilling the bed with sweetened liquid hydrocarbon, and 
     (g) continuing the sweetening process by starting the flow of sour liquid hydrocarbon through the freshly regenerated bed. 
     The novel process is particularly useful when the liquid hydrocarbon is propane or butane. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 through 6 are schematics of a portion of a hydrocarbon processing plant in which a sour crude hydrocarbon has been separated into different components or &#34;cuts&#34; or &#34;fractions&#34;, and one cut is being sweetened by contact with molecular sieves in columns A and B. The loaded beds are regenerated and sponged hydrocarbon product recovered according to the present invention. FIGS. 1-6 cover the same physical embodiment of the plant, but the valving/gas flow, etc. are changed to represent the various steps of the process. 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     FIG. 1: The sour product flows through line 100 into a heat exchange 101, where it is cooled, and into Column A (117) by way of line 116. Column A and Column B are packed with molecular sieves which sorb sulfur species in the sour product. The sweetened product thus flows from Column A to a collection point (120) through lines 118 and 119. This represents the treatment cycle with Column A on adsorption. 
     FIG. 2: The flow of sour product to Column A is diverted to Column B by changing the valving as shown in FIG. 2. In addition, the liquid product is drained from Column A by the pressure created when the bed is heated (e.g., by heating coil 138) to a temperature above the boiling point of the sponged hydrocarbon; the liquid product is thereby flowed through line 126 into Column B along with sour product for further treatment in Column B. 
     FIG. 3: Once the majority of liquid product has been drained from Column A, the bed of molecular sieves is purged by heating the bed (to drive off residual sponged hydrocarbon) and by flowing the heated gaseous effluent produced during the purge step through line 130 into manifold unit 132, compressor 150, and lines 136/137 where it flows back to the front end of the plant for further process recovery of the vaporous effluent. 
     FIG. 4: In the regeneration cycle, the regeneration fluid (e.g., residue gas) is heated in unit 135 to a temperature sufficient to desorb and to vaporize any sulfur compounds and residual sponged hydrocarbon product remaining on the molecular sieves in Column A. The vaporous effluent passing through Column A is cooled and sent to sales or to the flare or to a sweetening plant through lines 130, 133 and 136. Unit 132 is a manifold to gather the effluent and to help maintain a relatively constant pressure in lines 130, 133 and 136. 
     FIG. 5: After the molecular sieves have been regenerated by vaporizing off the sorbed compounds, the molecular sieves are cooled by passing the hydrocarbon purge gas or sweetened propane through the column. The vaporous effluent is sent to sales or to the flare as shown, or to the front of the hydrocarbon recovery plant. A sweetened hydrocarbon fluid is preferred as the purge gas for this cycle, and sweetened hydrocarbon (e.g., from Column B) is most preferred. This latter embodiment is illustrated in FIG. 5. Note that the vaporous effluent in FIG. 5 can be returned to the front end of the plant for further process recovery. 
     FIG. 6: Column A is then refilled with sweetened liquid hydrocarbon product by diverting the flow of liquid product through line 127 into Column A. Any vaporous effluent is likewise returned to the front end of the plant for further process recovery. 
     After Column A has been refilled with sweetened liquid hydrocarbon product, the unit is again ready to be placed on sorption and Column B is regenerated in a similar process. 
     DETAILED DESCRIPTION OF THE INVENTION 
     As noted above, the molecular sieves that can be used to sorb sulfur species (e.g., COS and H 2  S) from liquid hydrocarbon streams form a known class of compounds with many members, any one of which can be used in the present invention. The molecular sieves of the crystalline aluminosilicate zeolite type, both naturally occurring and synthetic compositions, are presently preferred because of their commercial availability. The crystalline synthetic zeolites having an average pore diameter of about 5 Angstroms are most preferred. The methods of using such materials to sweeten liquid hydrocarbon streams are well known to those skilled in the art. Generally, the method of choice involves flowing the hydrocarbon stream, under pressure, through a packed bed or column of the molecular sieves under conditions (e.g., flow rate) sufficient for sulfur compounds to be removed by the molecular sieves. Other methods, such as a batch process, could be used but they tend to be less cost effective for treating large volumes of sour hydrocarbon. 
     The following experimental field trial is conducted in a hydrocarbon treatment plant where the sour propane stream alone is 7,600 barrels per day (BPD). The generally preferred continuous type of operation illustrated in FIGS. 1-6 is chosen to sweeten the propane stream using a commercial synthetic zeolite, molecular sieve type RK-29 from Union Carbide Corporation, Houston, Tex. According to Union Carbide sales literature, RK-29 molecular sieves are a Type A crystalline zeolite alkali metal aluminosilicate material having: 
     (a) a nominal pore size of 3.6 to 4.4 Angstroms, 
     (b) a bulk density of 41 pounds per cubic foot, 
     (c) a heat capacity of 0.23 British Thermal Units (BTU) per pound at 10° F., 
     (d) a heat adsorption of 1800 maximum BTU per pound of water, 
     (e) a water equilibrium capacity of 21, and 
     (f) a particle size and shape of a 1/16 inch pellet. The sulfur species in the propane is primarily carbonyl sulfide (COS), with trace amounts of hydrogen. The sour propane stream flows through two columns connected in parallel and packed with RK-29 molecular sieves. 
     Each of Column A and B is loaded with the following sequence of materials: 
     3&#34; Layer of 1/2-inch Diameter Inert Supports 
     3&#34; Layer of 1/4-inch Diameter Inert Supports 
     3&#34; Layer of 1/8-inch Diameter Inert Supports 
     9,000 pounds Molecular Sieve Type RK-29 
     Floating 20-mesh Stainless Steel Screen 
     3&#34; Layer of 1/2-inch Diameter Inert Hold Down Balls 
     3&#34; Layer of 1/4-inch Diameter Inert Hold Down Balls 
     The column outage measurement, from top of final layer of 1/4-inch hold down balls to face of top manway flange, is 7.6 feet. The distance from top of the 1/4-inch hold down balls to distributor is 31/2 to 4 feet. 
     The sour propane is then sweetened by flow through Column A, and then through Column B when the molecular sieves in Column A are loaded with sulfur compounds. 
     At inlet conditions of 600 PPM(V) COS, 80° F. and 7,600 BPD, the predicted COS breakthrough time to 2 PPM(V) is about 16 hours. These are understood to be the worst case conditions which can be reasonably expected for the inlet propane. 
     The COS adsorption capacity is consistent with predicted performance. Each bed provides a 20-hour breakthrough time to 2 PPM(V) COS at average inlet conditions of 320 PPM(V) COS, 72° F. and 7,600 BPD. 
     Column A is regenerated in several steps: First, the flow of sour propane is diverted from Column A to Column B. Column A is then drained by heating the bed to a temperature above the boiling point of propane for a time sufficient to vaporize a substantial portion of the liquid propane in the column; the vapor pressure of the vaporized propane forces liquid propane from the column and into Column B through line 126, per FIG. 2. Column A is purged by heating the bed with unit 138. The heated gaseous effluent continues to be withdrawn and passes through line 130, manifold unit 132 and lines 136/137 where it is returned back to the front end of the hydrocarbon treatment plant and recombined with hydrocarbon feed, per FIG. 3. Next, the molecular sieves in Column A are regenerated by flowing hot residue gas (135° F.; 380 psig; 16 MMSCFD) through the column for 3  to 4.5 hours and passing the to the flares, per FIG. 4. The molecular sieves are then cooled by flowing residue gas or sweetened propane through Column A and the effluent is discharged to the flare, per FIG. 5. Column A is then refilled with sweetened propane and the effluent is passed to the front end of the hydrocarbon recovery plant for further recovery of the propane vapors which form as the liquid propane cools the molecular sieves to a temperature below the boiling point of propane, at operating pressure, so that the column can be put back onto adsorption mode, per FIG. 6. 
     In this novel process, a substantial amount of sponged propane is removed and recovered. COS breakthrough during the propane purge step occurs simultaneously with a precipitous drop in one propane concentration in the effluent gas, signalling the removal of virtually all the propane from the bed of molecular sieves. 
     Column A performs as expected in sweetening the propane when the feed stream was diverted from Column B back to Column A. Column B is then regenerated in a similar manner and with similar results.