JPH0320366B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for separating isobutylene from feedstock oil. Since butene is useful in gasoline blending and reactions to produce, for example, butyl rubber and lubricating oil additives, it is highly desirable to obtain products rich in isobutylene. It is well known that crystalline aluminosilicates are useful in hydrocarbon separations. In particular, certain X- or Y-type zeolites have been disclosed for the separation of olefins from paraffinic hydrocarbons and for the separation of butene-1 from isobutylene (U.S. Pat.
(See No. 4119678). However, zeolites tend to undergo dimerization or even polymerization of olefins, making them unsuitable for use as adsorbents for the separations of the present invention. The inventors have discovered that silicalite virtually completely eliminates the above-mentioned undesirable side effects of dipolymerization and polymerization, particularly when pentene-1 is used to displace normal _C4 hydrocarbons from silicalite. We have discovered that it is possible to separate positive _C4 hydrocarbons from isobutylene. In one embodiment, the present invention comprises contacting a feedstock containing isobutylene and at least one positive _C4 hydrocarbon with a zeolite-free molecular sieve comprised of silicalite at holding conditions to produce the positive _C4 .
Selective retention of hydrocarbons is provided, the isobutylene is removed from contact with the molecular sieve, and the positive _C4 carbonization is achieved by displacement from the molecular sieve with a displacement fluid consisting of pentene-1 at displacement conditions. It is a method for separating isobutylene from feedstock oil that consists of recovering hydrogen. In another embodiment, the invention provides for: (a) maintaining fluid flow in a unidirectional direction through a column of molecular sieves comprising at least three distinct and sequentially connected zones; (c) maintaining a retention zone bounded by the roughinate outlet as the boundary between the feedstock inlet and the downstream; (c) maintaining a refining zone bounded by the feedstock inlet as the boundary between the extract outlet and the downstream; d) maintaining a displacement zone bounded by the extract outlet as a boundary between a displacement fluid inlet and downstream; (e) displacing the feedstock;
(f) passing a raffinate stream containing isobutylene into the retention zone at retention conditions to retain the normal _C4 hydrocarbons; (g) removing _{an extract stream consisting of the normal C 4 hydrocarbons} and a displacement fluid from the displacement zone;
(h) periodically advancing the feedstock inlet, the roughinate outlet, the displacement fluid inlet, and the extract outlet through the column of the molecular sieve in a downstream direction with respect to fluid flow; The process comprises the steps of separating isobutylene from a feedstock containing isobutylene and at least one positive _C4 hydrocarbon using a zeolite-free molecular sieve made of silicalite. Other embodiments of the invention, including details about feedstock mixtures, molecular sieves, replacement materials and operating conditions, are described below. First, a description of terminology used herein will be provided to clarify the operation, purpose, and advantages of the present invention. A "raw oil mixture" is a mixture containing one or more extract components and one or more raffinate components to be separated by the method of the present invention. "Feed Stream" refers to the flow of the feed mixture through the molecular sieves used in this process. "Extract components" are the types of compounds that are more selectively retained by molecular sieves, and "raffinate components" are the types of compounds that are less selectively retained. In this process, the normal _C4 hydrocarbons are the extraction component and isobutylene is the ruffinate component. The term "displacement substance or fluid" generally refers to a substance capable of displacing extracted components. The terms "displacement stream" or "displacement fluid inlet stream" refer to the flow through which the displacement material flows to the molecular sieve. The term "ruffinate stream" or "ruffinate exit stream" refers to the stream through which the roughinate component is removed from the molecular sieve. The composition of the ruffinate stream can vary from substantially 100% substituted material to substantially 100% roughinate content. "Extract stream" or "Extract inlet stream"
The term refers to the flow through which the extracted material displaced by the displacing material is removed from the molecular sieve. The composition of the extract stream can similarly vary from substantially 100% replacement material to substantially 100% roughinate content. At least a portion of the extraction stream, preferably at least a portion of the raffinate stream, from the separation process is passed to a separation means, typically a rectification column, where at least a portion of the substituted material is separated to yield an extraction product and a raffinate product. . The terms "extract product" and "raffinate product" refer to products produced by a process that contain extract and raffinate components at higher concentrations than in the extract and raffinate streams, respectively. Although it is possible to produce high purity isobutylene with high recovery by the process of the present invention, the extract components are not completely adsorbed by the adsorbent and the raffinate components are also not completely adsorbed by the adsorbent. It is recognized that there is no non-adsorption. Thus, varying amounts of raffinate components can be found in the extract stream, as can extractive components in the raffinate stream. Extract and raffinate streams are further distinguished from each other and from the feedstock by the ratio of the concentrations of extract and raffinate components in a particular stream. More specifically, the positive effect on the concentration of isobutylene that is selectively retained less
The ratio of concentrations of _C4 hydrocarbons is lowest in the roughinate stream, followed by the feedstock mixture, and highest in the extract stream. Similarly, the ratio of the concentration of selectively less retained isobutylene to the concentration of selectively more retained positive _C4 hydrocarbons is highest in the raffinate stream, followed by the feedstock mixture, and lowest in the extract stream. be. The term "selective pore volume" of a molecular sieve is defined as the volume of the molecular sieve that selectively retains extractable components from the feed oil mixture. The term "non-selective empty volume" of a molecular sieve is the volume of the molecular sieve that does not selectively retain extract components from the feed oil mixture. This volume includes the recesses of the molecular sieves and the voids between the molecular sieves that can hold the roughinate component. The selective pore volume and the non-selective void volume are expressed together in terms of volume and provide the appropriate flow rate of fluid required to pass through the operating zone for efficient operation to occur for a given amount of molecular sieve. It is important to define When a molecular sieve is "passed" into the working zone used in this process embodiment, its non-selected empty volume, along with its selected pore volume, carries fluid into the zone. The non-selected empty volume is used to define the amount of fluid that is to enter the same area in a countercurrent direction to the molecular sieve and displace the fluid present in the non-selected empty volume. There is entrainment of liquid into a zone by the molecular sieve if the flow rate of the fluid entering the zone is less than the fraction of non-selected empty volume of the molecular sieve that enters the zone. This entrainment is fluid present in the non-selective empty volume of the molecular sieve, and therefore is often the feedstock component that is selectively retained less. The feedstocks that can be utilized in the process of this invention can be derived from any known refinery process. In particular, the feedstock oil is butene-1,
Isobutylene, trans-butene-2 and cis-
Includes normal _C4 hydrocarbons such as butene-2. The term "butene-2" includes both the cis- and trans-isomers of this hydrocarbon. The presence in the feedstock of other substances such as large amounts of paraffinic or naphthenic materials and in some cases low concentrations of aromatic hydrocarbons other than benzene and other contaminants such as bound sulfur-nitrogen compounds. Can be done. However, it is preferred that the amount of component that contributes to deactivation of the molecular sieve by blocking selective pore passages to the feedstock components is substantially reduced. A particular feedstock that can be utilized in the process of this invention is a feedstock containing approximately 35% VOL of butene-1, 32.5% isobutylene, and 32.5% isobutane. Other raw oil compositions are approximately 21VOL
% butene-1, 21VOL% isobutylene,
Includes feedstocks containing 16 VOL% trans-butene-2, 16 VOL% cis-butene-2, with the remaining components consisting of butane, such as isobutane or orthobutane. The feedstock may contain other paraffinic materials having high molecular weights such as heptane, hexanes, octanes, nonanes or higher molecular weight paraffins. It is preferred to utilize a feedstock containing about 15 VOL% or more of total olefins. Non-positive hydrocarbons other than isobutylene, such as isobutane, are removed by the silicalite adsorbent and together with isobutylene form a portion of the raffinate stream. This does not pose a problem as to whether pure isobutylene is desirable, since such hydrocarbons are easily separated from isobutylene by conventional distillation means. A difficult separation is the separation of isobutylene from positive hydrocarbons, especially positive olefins, and this separation is achieved by the present invention. The desorbent materials or displacement fluids used in various conventional separation processes vary depending on factors such as the type of operation used. In swing bed systems, where selectively retained feedstock components are removed from the molecular sieve by a barge flow displacement fluid, the choice is not critical and may be either gaseous hydrocarbons such as methane, ethane, etc. or nitrogen or hydrogen. Displacement materials consisting of other gases, such as, are used at elevated temperatures and/or reduced pressures to effectively purge retained feedstock components from the molecular sieve. However, in molecular sieve separation processes which are generally operated continuously at substantially constant pressure and temperature to ensure a liquid phase, the replacement material must be properly chosen to meet a number of criteria. First, the displacing material is passed through the molecular sieve at an appropriately large flow rate without strongly retaining itself in the next retention cycle to prevent extract components from unduly displacing the displacing material. Extract components should be replaced from Second, the replacement material must be compatible with the particular molecular sieve and the particular feedstock mixture. In particular, the critical selectivity of the molecular sieve over extract components with respect to raffinate components must not be reduced or destroyed. The replacement substance is also
The material must be easily separable from the feedstock mixture entering the process. Both the raffinate stream and the extract stream are removed from the molecular sieve mixed with the substituted material, and without a method to separate at least a portion of the substituted material, the purity of the extracted and raffinate products is not very high. , and the replacement material cannot be reused in the process. Therefore, the replacement material used in this process has an average boiling point substantially different from the feedstock mixture and at least a portion of the replacement material is extracted from the feedstock components in the extract and raffinate streams by simple fractional distillation. can be separated, thereby providing for reuse of the replacement material in the process. As used herein, the term "substantially different" means that the average boiling points of the replacement material and the feedstock mixture differ by at least about 5°C. The boiling range of the substituent may be higher or lower than that of the feedstock mixture. Finally, the replacement material must be readily available and cost-effective.
The inventors have found that in the preferred isothermal and isobaric liquid phase operation of the process of the present invention, substitution materials consisting of pentene-1 meet these requirements and are particularly effective. It is usually advantageous to mix the desorbent with a diluent, such as isooctane or cyclohexane, which is not selectively retained by the molecular sieve. Dynamic test equipment is used to test various molecular sieves with specific feed oil mixtures and replacement materials to determine molecular sieve properties such as retention capacity and degree of exchange. The apparatus consists of a molecular sieve chamber of approximately 70 c.c. volume with inlet and outlet sections at each end of the chamber. This chamber is contained within the temperature control means, and in addition, a pressure control device is used to operate the chamber at a constant predetermined pressure. Quantitative and qualitative analysis equipment, such as a refractometer, a polarimeter, and a chromatograph, is provided at the outlet of the chamber to quantitatively detect and qualitatively detect one or more components in the effluent stream exiting the molecular sieve chamber. used to measure Selectivity and other data for various molecular sieve systems are measured using this equipment and a pulse test performed using the following general method. The molecular sieve is filled with a particular substituent until equilibrium by passing the substituent into the molecular sieve chamber. At appropriate times, a pulse of feedstock containing a known concentration of a particular extract component or roughinate component diluted in a displacement fluid is injected for a period of several minutes. The extract component or the raffinate component (or both) is eluted as in a liquid-solid chromatographic operation. The effluent can be analyzed on-stream, or effluent samples are collected periodically and later analyzed individually by an analyzer and tracing of the envelope or corresponding component peak. From the information from this test, the performance of the molecular sieve can be evaluated in terms of empty volume, retention volume for extract or raffinate components, and degree of displacement of extract stream components by displacement fluid. The retention volume of the extract or roughinate component is characterized by the distance between the centers of the peak envelope of the extract or roughinate component and the peak envelope of the tracer component. It is expressed as the volume in cubic centimeters of displacement fluid pumped during the time interval represented by the distance between the peak envelopes. The rate of exchange of extract components with the displacement fluid can generally be characterized by the width of the peak envelope at half strength. The narrower the width of this peak, the faster the substitution rate. The displacement rate is characterized by the distance between the center of the tracer peak envelope and the vanishing point of the extract component just displaced. This distance is the volume of displacement fluid pumped during this time interval. The molecular sieve used in the process of this invention consists of silicalite. As mentioned above, silicalite is a molecular sieve of hydrophobic crystalline silica. Due to its aluminum-free structure, silicalite does not exhibit ion exchange behavior and is hydrophobic and organophilic. Thus,
Silicalite consists of molecular sieves,
It is not a hydrated aluminum or calcium silicate made of zeolite. Silicalite is
The reason why the pores are so sized and shaped that they can act as molecular sieves, i.e., admit molecules of positive _C4 hydrocarbons but not molecules of isobutylene into their channels or internal structures. and is particularly suitable for the separation process of the present invention. Silicalite is “Silicalite, A
New Hydrophobic Crystalline, Silica
“Molecular Sieve”; NATURE , Vol.271, 9
Discussed in detail in February 1978. The molecular sieve is used in the form of a dense compact fixed bed that is alternately contacted with the feed oil mixture and the substituent. In the simplest embodiment of the invention, molecular sieves are used in the form of a single static bed in the case of a semi-continuous process. In other embodiments, a set of two or more static beds is provided in which the feedstock mixture is passed through a bed of one or more molecular sieves and the replacement material is passed through another one or more of the sets. used in a fixed bed in contact with a suitable valve to pass through the bed. The flow of the feedstock mixture and displacement material may be either upward or downward through the displacement fluid. Any equipment used for static bed fluid-solid contact may be used. The particles of silicalite molecular sieve are preferably about 16
With particle sizes ranging from ~60 mesh (standard US mesh). Silicalite itself is a fine powder. Therefore, they must be combined to obtain the above particle sizes. Fluid-permeable organic polymers, particularly polystyrene divinylbenzene, are at least effective, although inorganic oxides such as alumina are also acceptable binders. This organic polymer also has the advantage of high resistance to dissolution of molecular sieves. Countercurrent moving bed or simulated moving bed countercurrent flow systems have significantly greater separation efficiency than fixed molecular sieve bed systems and are therefore preferred. In a moving bed or simulated moving bed process, the retention and displacement operations occur continuously, allowing for both continuous production of extract and raffinate streams and continuous use of feedstock and displacement fluid streams. One preferred embodiment of this process utilizes what is known as a simulated moving bed countercurrent flow system. The operating principles and results of such a flow system are described in US Pat. No. 2,985,589. In such systems, it is the forward movement of multiple liquid access points below the molecular sieve chamber that simulates the upward movement of the molecular sieve contained in the molecular sieve chamber. This will be explained below with reference to FIG. Four access lines are always active: feed stream 11, displacement fluid stream 12, roughinate stream 14, and extract stream 13.
The movement of liquid occupying the empty volume of the packed bed of molecular sieves coincides with this simulated upward movement of solid molecular sieves. The flow of liquid below the molecular sieve chamber is pumped to maintain countercurrent contact. As the liquid access point moves from the top to the bottom of the chamber throughout the cycle, the chamber's circulation pump moves through different zones requiring different flow rates. Scheduled flow controllers may be provided to set and adjust these flow rates. The liquid access pointer effectively divides the molecular sieve chamber into individual zones, each having a different function. In this embodiment of the process, it is necessary that three separate operating areas exist. In some cases, an optional fourth zone is used. Retention zone 1 is defined as a molecular sieve placed between feedstock inlet stream 11 and roughinate outlet stream 14. In this zone, the feedstock contacts the molecular sieve, the extract components are retained and the raffinate stream is removed.
Since the general flow through the holding zone 1 is from the feedstock stream 11 entering this zone to the roughinate stream 14 leaving this zone, the flow in this zone is in the direction of flow from the feedstock inlet to the roughinate outlet. It is recognized that there is. Immediately upstream in terms of fluid flow in the holding zone 1 is the purification zone 2 . Purification zone 2 is defined as a molecular sieve between extract outlet stream 13 and feedstock inlet stream 11. The basic operations carried out in purification zone 2 are the displacement of the non-selective empty volume of the molecular sieve by a circulating stream of raffinate material carried into purification zone 2 by the movement of the molecular sieve; The replacement of ruffinate material retained within the selective pore volume of the molecular sieve or on the surface of the particles of the molecular sieve. Purification is achieved by passing a portion of the material of extract stream 13 leaving displacement zone 3 into purification zone 2 at its upstream boundary to effect displacement of the raffinate material. The flow of material in the purification zone 2 is in the direction of flow from the extract outlet stream 13 to the feedstock inlet stream 11. Immediately upstream of the purification zone 2 with respect to the fluid flowing through the purification zone 2 is the displacement zone 3 . The displacement zone 3 is defined as a molecular sieve between the displacement fluid inlet stream 12 and the extract outlet stream 13. The action of displacement zone 3 is to cause the displacement material entering this zone to displace the extract components retained by the molecular sieve during contact with the feedstock in retention zone 1 in the previous cycle of operation. That's true. The fluid flow in displacement zone 3 is essentially in the same direction as in retention zone 1 and purification zone 2. In some cases, an optional buffer zone 4 is utilized.
This buffer zone 4 is defined as a molecular sieve between the roughinate outlet stream 14 and the displacement fluid inlet stream 12 and is located immediately upstream of the displacement zone 3 in terms of fluid flow. A portion of the raffinate stream removed from the retention zone 1 is passed through the buffer zone 4 so that the displacement material present in that zone can be displaced from that zone into the displacement zone 3. , used to store the replacement fluid used in the replacement step. Buffer zone 4 contains sufficient molecular sieves so that the raffinate material present in the raffinate stream leaving retention zone 1 and entering buffer zone 4 enters displacement zone 3 and is removed from displacement zone 3. is prevented from contaminating the extracted extract stream. When the presence of raffinate material in the raffinate stream passing from retention zone 1 to displacement zone 3 is detected, the direct flow from retention zone 1 to displacement zone 3 is changed to avoid contamination of the extract outlet stream 13. In order to be able to stop the holding area 1 to the buffer area 4
The flow of roughinate through the area must be carefully monitored. Propulsion of input and output flow cycles through a fixed bed of molecular sieves can be accomplished by utilizing a manifold system. In this system, valves in the manifold are operated in a continuous manner to effect input and output flow movement, thereby providing countercurrent flow of fluid with respect to the solid molecular sieve. Another mode of operation that can effect countercurrent flow of solid molecular sieves with respect to fluids involves the use of rotating disk valves in which input and output flows are connected to valves and lines that The flow paths of feedstock input, extract output, displacement fluid input, and roughinate output are directed in the same direction as they pass through the bed of molecular sieves. Manifold arrangements and disc valves are well known. In particular, the rotary disk valve that can be used for this operation is
No. 3040777 and No. 3422848.
These patents disclose rotary connecting valves in which the proper propulsion of various input and output flows from a fixed source can be accomplished without difficulty. Often one operating zone will contain significantly more molecular sieves than another operating zone. For example, in some operations, the buffer zone may contain a small amount of molecular sieve compared to that required in the retention and purification zone. If a displacement fluid is used that can easily displace the extracted material from the molecular sieve, it may be necessary to replace the molecular sieve in the buffer, retention, and/or purification zones in the displacement zone compared to the molecular sieve required. It can be appreciated that relatively small amounts of molecular sieve are required. Since the molecular sieve is not required to be placed in a single column, the use of multiple chambers or a series of columns is also within the scope of this invention. It is not necessary to use all input and output streams at the same time. In fact, in many cases some of the flows are stopped and others input or output material. The apparatus available for carrying out the process of this invention includes a series of connections in which input or output taps to which various input and output streams can be connected can be placed and moved alternately and periodically to effect continuous operation. It may also include a series of beds connected by conduits. In some cases, the connecting conduit is connected to a transfer tap that does not act as a conduit through which materials enter or exit the process during normal operation. It is contemplated that at least a portion of the ruffinate outlet stream may be passed through a separation means where at least a portion of the displacement fluid may be separated to produce a ruffinate product containing a reduced concentration of displacement material. Although perhaps not necessary for operation of this process, preferably at least a portion of the extract outlet stream is passed through separation means to obtain an extract product containing a reduced concentration of substituted material. This separation means is typically a fractionation column, the design and operation of which is well known. Reference is made to U.S. Patent No. 2985589 and Society of Chemical, Tokyo, April 2, 1969.
D. at the 34th Annual Meeting of Engineers.
“Continuous Adsorptive” by B. Broughton
Processing……A New Separation
Both liquid and vapor phase operations can be used in many adsorption or molecular sieve separation processes; Liquid phase operation is preferred because of the lower temperature requirements and because of the higher yields of extracted products obtained with liquid phase operation than those obtained with vapor phase operation.The holding conditions are about 20-200 °C, preferably about Includes a temperature range of 20 to 100° C. and sufficient pressure to maintain a liquid phase. Displacement conditions include the same ranges used for retention conditions. The process of this invention may be utilized. Unit sizes can vary from pilot plant size (US Pat. No. 3,706,812) to commercial scale, and can range in flow rates from a few cc per hour to thousands of gallons per hour. Next, the present invention will be explained with reference to examples.However, the present invention is not limited to these examples.Example In this example, isobutylene, n-butane, cis-
The ability of inorganic oxide-bonded silicalite to separate isobutylene from mixtures of trans-butene-2 and butene-1 was tested. I conducted two tests. In the first test, the molecular sieve was an alumina-bonded silicalite, the displacement fluid was isooctane, followed by a mixture of hexene-1 and isooctane in a 60:40 ratio, and the column temperature was
At 50°C, the feedstock flow pulse was 10 ml of a mixture of C4 isomers in isooctane. In the second test,
The molecular sieve used clay-bonded silicalite, the displacement fluid was a 50:50 mixture of pentene-1 and cyclohexane, the column temperature was 60 °C, and the feed oil flow pulse was 10 ml of a mixture of C4 isomers in the displacement fluid. Ta. The reason that the displacement in the first test was initiated with isooctane is that this separation was a molecular sieve action, i.e. all the C4 components of the feedstock except isobutylene were retained in the pores of the silicalite, but the isobutylene was to show that it was retained in the empty space and was easily flushed from there.
An aspect of this test was the comparative ability to displace the retained C4 moiety of hexene-1 or pentene-1 from silicalite in Tests 1 and 2. Differences in column temperature in the two tests are not expected to have a significant effect on the results. The results obtained from tests 1 and 2 are shown in the following table and in Figures 1 and 2. Figures 1 and 2 show the elution curves obtained by pulse tests 1 and 2, respectively.

[Table] The isobutylene peak in FIG. 1 is separated from other peaks and shown discontinuously because isobutylene is eluted with isooctane before use of the isooctane preflush and replacement fluid. The data in the table and the curves in the figure demonstrate the superior rate at which pentene-1 displaces the retained component (as evidenced by the lower retention volume and half-width) and the superior rate experienced with the use of hexene-1. Comparison shows the complete and "clean" replacement of such components obtained with pentene-1.

[Brief explanation of the drawing]

Figures 1 and 2 are the elution curves obtained in Test 1 and Test 2, respectively. FIG. 3 is an explanatory diagram of a specific example of the method of the present invention. In the figure, 1... retention area, 2... purification area, 3
... displacement zone, 4 ... buffer zone, 11 ... feedstock flow, 12 ... displacement fluid flow, 13 ... extract flow, 14 ... raffinate flow, 15 ... product flow, 21... Fixed adsorbent bed, 22... Fractionation column, 23... Rotary valve, 24... Circulation pump.

Claims (1)

[Claims] 1. A feedstock containing isobutylene and at least one positive _C4 hydrocarbon is heated at a temperature in the range of about 20 to 200°C and at a pressure sufficient to maintain a liquid phase to a zeolite-free compound consisting of silicalite. contact with a molecular sieve to selectively retain the positive C4 hydrocarbons, remove the isobutylene from contact with the molecular sieve, and maintain a temperature and liquid phase in the range of about 20-200°C. A method for separating isobutylene from a feedstock comprising recovering said C4 hydrocarbons by displacement from said molecular sieve with a displacement fluid consisting of pentene-1 at a pressure sufficient to cause 2. A method for separating isobutylene from a feedstock according to paragraph 1, wherein the displacement fluid comprises pentene-1 in a solution containing a diluent that is not selectively retained by the molecular sieve. 3 The displacement fluid contains approximately 10-90 LV% pentene.
2. The method for separating isobutylene from the feedstock oil according to item 2, comprising: 4. The method for separating isobutylene from a feedstock oil according to item 3, wherein the diluent is comprised of isooctane or cyclohexane. 5. The method for separating isobutylene from a feedstock oil according to item 1, wherein the normal C4 hydrocarbon comprises orifin. 6. The method for separating isobutylene from a feedstock oil according to item 1, wherein the molecular sieve is composed of silicalite combined with an inorganic oxide. 7. The method for separating isobutylene from a feedstock oil according to item 1, wherein the molecular sieve comprises silicalite combined with a fluid-permeable organic polymer. 8. The method for separating isobutylene from a feedstock oil according to item 1, wherein the fluid-permeable organic polymer comprises polystyrene divinylbenzene. 9 (a) maintain fluid flow in a unidirectional direction through a column of molecular sieves comprising at least three distinct and continuously connected zones; and (b) a roughinate as a feedstock inlet and downstream boundary. (c) maintaining a refining zone bounded by the feedstock inlet as an extract outlet and a downstream boundary; (d) a displacement fluid inlet and a downstream boundary; (e) maintaining the feedstock in the holding zone at a temperature in the range of about 20 to 200°C and at a pressure sufficient to maintain a liquid phase; to retain the normal C4 hydrocarbons and remove the raffinate stream containing the isobutylene and (f) replace the substituent consisting of pentene-1 with about 20 to
(g) displacing the normal C4 hydrocarbons from the molecular sieve by passing through the displacement zone at a temperature in the range of 200°C and at a pressure sufficient to maintain a liquid phase; removing an extract stream comprising a fluid from the displacement zone; (h) connecting the feedstock inlet, the ruffinate outlet, the displacement fluid inlet, and the extract outlet in a downstream direction with respect to the fluid flow to a column of the molecular sieve; A method for separating isobutylene from a feedstock according to claim 1, comprising the step of moving the molecular sieve periodically through the molecular sieve. 10. A method for separating isobutylene from a feedstock according to paragraph 9, wherein the raffinate stream is passed to a separation zone where the displacement fluid is removed from the stream to produce a substantially pure isobutylene product. . 11 a buffer zone is maintained immediately upstream of the substitution zone;
A method for separating isobutylene from a feedstock according to paragraph 10, wherein the buffer zone is defined as a molecular sieve placed between a displacement fluid inlet at its downstream boundary and a ruffinate outlet at its upstream boundary. . 12. A method for separating isobutylene from a feedstock according to paragraph 9, wherein the displacement fluid comprises pentene-1 in a solution containing a diluent that is not selectively retained by the molecular sieve. 13. A method for separating isobutylene from a feedstock according to paragraph 12, wherein the displacement fluid comprises about 10-90 LV% pentene-1. 14. The method for separating isobutylene from a feedstock oil according to item 13, wherein the diluent comprises isooctane or cyclohexane. 15. The method for separating isobutylene from a feedstock oil according to item 9, wherein the molecular sieve is composed of silicalite combined with an inorganic oxide. 16. A method for separating isobutylene from a feedstock oil according to item 9, wherein the molecular sieve comprises silicalite combined with a fluid-permeable organic polymer. 17. The method for separating isobutylene from a feedstock oil according to item 16, wherein the fluid-permeable organic polymer comprises polystyrene divinylbenzene.