WO2001025174A1

WO2001025174A1 - Process for producing ethylene

Info

Publication number: WO2001025174A1
Application number: PCT/US2000/021993
Authority: WO
Inventors: John J. Senetar; Lawrence W. Miller; Linda Shi Cheng; Mark M. Davis
Original assignee: Uop Llc
Priority date: 1999-10-04
Filing date: 2000-10-03
Publication date: 2001-04-12
Also published as: AU7825600A; US6303841B1; EG22225A; US6444869B2; US20020007101A1; NO20004985D0; MY128661A; NO20004985L

Abstract

A process is provided for the concentration and recovery of ethylene and heavier components from an oxygenate conversion process. A separation process such as a pressure swing adsorption (PSA) process is used to remove hydrogen and methane from a demethanizer overhead stream comprising hydrogen, methane, and C2 hydrocarbons and subsequently return the recovered C2 hydrocarbons to be admixed with the effluent from the oxygenate conversion process. This integration of a separation zone with a fractionation scheme in an ethylene recovery scheme using an initial deethanizer zone reduces capital and operating cost saving by the elimination of cryogenic ethylene-based refrigeration from the overall recovery scheme.

Description

"PROCESS FOR PRODUCING ETHYLENE"

FIELD OF THE INVENTION

The present invention relates to a process for the recovery of high purity ethylene from the reactor effluent of an oxygenate conversion process.

BACKGROUND OF THE INVENTION

Molecular sieves such as the microporous crystalline zeolite and non-zeolitic catalysts, particularly silicoaluminophosphates (SAPO), are known to promote the conversion of oxygenates to hydrocarbon mixtures. Numerous patents describe this process for various types of these catalysts: US-A-3,928,483, US-A-4,025,575, US-A- 4,052,479; US-A-4,496,786; US-A-4,547,616; US-A-4,677,243; US-A-4,843,183; US- A-4,499,314; US-A-4,447,669; US-A-5,095,163; US-A-5,191,141; US-A-5, 126,308; US-A-4,973,792; and US-A-4,861,938. US-A-4,861,938 and US-A-4,677,242 particularly emphasize the use of a diluent combined with the feed to the reaction zone to maintain sufficient catalyst selectivity toward the production of light olefin products, particularly ethylene.

The product produced by the oxygenate conversion process is a light gas stream containing lighter components (e.g. hydrogen, nitrogen, etc.) methane, ethane and a substantial quantity of hydrocarbons of higher molecular weight, for example, propane, butane, pentane and often their unsaturated analogs. Separation of these components to recover ethylene requires a complex energy intensive scheme, thus creating a need for more efficient separation processes which yields higher recovery levels of ethylene. In typical ethylene plant recovery sections, where the ethylene production is based on the pyrolysis of naphtha or gas oil, the use of cryogenic processes utilizing the principle of gas expansion through a mechanical device to produce power while simultaneously extracting heat from the system have been employed. The use of such equipment varies depending upon the pressure of the product gas stream, the composition of the gas, and the desired end results. In the typical cryogenic expansion-type recovery processes used in the prior art, a gas stream is withdrawn from the pyrolysis furnace, compressed, and cooled. The cooling is accomplished by heat exchange with other streams of the process and/or external sources of cooling are employed such as refrigeration systems. As the product gas is cooled, liquids are condensed, collected, and separated so as to thereby obtain desired hydrocarbons. The high-pressure liquid stream so recovered is typically transferred to a demethanizer column after the pressure is adjusted to the operating pressure of the demethanizer. In such a fractionating column, the high-pressure liquid stream is fractionated to separate the residual methane and lighter components from the desired products of ethylene and heavier hydrocarbon components. In the ideal operation of such separation processes, the vapors, or light cut, leaving the process contain substantially all of the methane and lighter components found in the feed gas and substantially no ethylene and heavier hydrocarbon components remain. The bottom fraction, or heavy cut, leaving the demethanizer typically contains substantially all of the ethylene and heavier hydrocarbon components with very little methane or lighter components which are discharged in the fluid gas outlet from the demethanizer. A typical combined gas expansion and fractionation process for the separation of hydrocarbon gas stream comprising components ranging from nitrogen through C₃- plus hydrocarbons into a methane and lighter stream and an ethylene and heavier stream is exemplified by US-A-4,895,584. A typical ethylene separation section of an ethylene plant containing both cryogenic and fractionation steps to recover an ethylene product with a purity exceeding 99.5% ethylene is described in an article by V. Kaiser and M. Picciotti entitled, "Better Ethylene Separation Unit," appeared in Hydrocarbon Processing, November 1988, pages 57-61 and is herein incorporated by reference.

Pressure swing adsorption (PSA) provides an efficient and economical means for separating a multi-component gas stream containing at least two gases having different adsorption characteristics. The more strongly adsorbable gas can be an impurity which is removed from the less strongly adsorbable gas which is taken off as product; or, the more strongly adsorbable gas can be the desired product, which is separated from the less strongly adsorbable gas. For example, it may be desired to remove carbon monoxide and light hydrocarbons from a hydrogen-containing feedstream to produce a purified (99+%) hydrogen stream for a hydrocracking or other catalytic process where these impurities could adversely affect the catalyst or the reaction. On the other hand, it may be desired to recover more strongly adsorbable gases, such as ethylene from a feedstream to produce an ethylene-rich product.

In PSA, a multi-component gas is typically fed to at least one of a plurality of adsorption zones at an elevated pressure effective to adsorb at least one component, while at least one other component passes through. At a defined time, the feedstream to the adsorber is terminated and the adsorption zone is depressurized by one or more cocurrent depressurization steps wherein pressure is reduced to a defined level which permits the separated, less strongly adsorbed component or components remaining in the adsorption zone to be drawn off without significant concentration of the more strongly adsorbed components. Then, the adsorption zone is depressurized by a countercurrent depressurization step wherein the pressure on the adsorption zone is further reduced by withdrawing desorbed gas countercurrently to the direction of the feedstream. Finally, the adsorption zone is purged and repressurized. The combined gas stream produced during the countercurrent depressurization step and the purge step is typically referred to as the tail gas stream. The final stage of repressurization is typically performed by introducing a slipstream of product gas comprising the lightest gas component produced during the adsorption step. This final stage of repressurization is often referred to as product repressurization. In multi-zone systems, there are typically additional steps, and those noted above may be done in stages. US- A-3, 176,444, US-A-3,986,849, and US-A-3,430,418 and US-A-3,703,068, among others, describe multi-zone, adiabatic PSA systems employing both cocurrent and countercurrent depressurization, and the disclosures of these patents are incorporated by reference in their entireties.

Various classes of adsorbents are known to be suitable for use in PSA systems, the selection of which is dependent upon the feedstream components and other factors generally known to those skilled in the art. In general, suitable adsorbents include molecular sieves, silica gel, activated carbon, and activated alumina. When PSA processes are used to purify hydrogen-containing streams, the hydrogen is essentially not adsorbed on the adsorbent. However, when purifying methane-containing streams, methane is often adsorbed on the adsorbent along with the impurity.

Numerous patents disclose the use of PSA in combination with fractionation to separate hydrogen and methane from heavier hydrocarbons. US-A-5,245,099, which is hereby incorporated by reference, discloses a process for the concentration and recovery of ethylene and heavier components from a hydrocarbon feedstream. A PSA process is used to remove from hydrocarbon feedstream light cut comprising hydrogen, carbon monoxide, and methane and subsequently concentrate a heavy cut comprising the ethylene and heavy components in the PSA tail gas. In one aspect of the invention, an FCC off gas is separated into a light cut and a heavy cut and the heavy cut is routed to an ethylene plant.

US-A-5,365,011, US-A-5,470,925, and US-A-5,744,687 disclose a process for the integration of a PSA zone containing an adsorbent selective for the adsorption of ethylene and propylene from a catalytic cracking process at an adsorption temperature above 50°C to about 250°C. The adsorbent is selected from the group consisting of zeolite 4 A, zeolite 5 A, zeolite 13X, and mixtures thereof.

The existing cryogenic and fractionation system in a typical ethylene recovery scheme can be employed in an oxygenate conversion process to recover ethylene, but the penalties of this operation are significant. The low level of light components such as hydrogen and methane relative to ethylene plant product compositions still are high enough to significantly raise the compression and refrigeration requirements in the recovery section of the oxygenate conversion plant for the incremental amount of ethylene recovered. Thus, recovering ethylene from oxygenate conversion effluent is an expensive and complex process involving extensive compression and fractionation to separate the light gases such as hydrogen and methane from the ethylene. Processes are sought which enable the concentration and recovery of the ethylene and heavier components from oxygenate conversion effluent without expensive compression and cryogenic separation steps to remove the lighter components.

SUMMARY OF THE INVENTION

It was discovered that the unique character of the oxygenate conversion process effluent stream allowed the use of a separation process to further concentrate and recover ethylene. For example, when a separation zone which employed a deethanization zone as an initial separation followed by a demethanization zone to recover a C₂ hydrocarbon stream was combined with a separation process to further process the demethanizer overhead stream, significant savings in the demethanizing step could be obtained by a warmer demethanizer operation, and the desorbed ethylene was returned to be combined with the oxygenate conversion effluent stream. In the conventional ethylene separation train, the demethanizer is required to operate at demethanizing conditions, including a demethanizer temperature which is sufficiently cold enough to provide a reasonable split between methane and ethylene. Typically, a demethanizer temperature less than about - 95°C (-140°F) is required to recover ethylene in the presence of a large amount of methane and hydrogen. It was discovered that by employing an initial deethanizer separation in the ethylene recovery zone with a separation process such as a PSA zone wherein the PSA zone contained an adsorbent which was selective for the adsorption of ethylene relative to methane and hydrogen, and the desorbed stream was recombined with the oxygenate conversion process effluent stream, the demethanizer temperature could be increased to about - 0°C with significant overall process benefits. This savings appears to be greatest in the separation of components such as ethylene from oxygenate conversion process effluent streams wherein the critical molar ratio of materials more volatile than ethylene, such as hydrogen and methane, to the total moles of ethylene and ethane is less than about 0.5.

In one embodiment, the present invention is a process for the production of ethylene from an oxygenate conversion effluent stream. The oxygenate conversion effluent stream comprises hydrogen, methane, ethylene, ethane, propylene, propane, and C₄-plus olefins. The oxygenate conversion effluent stream is passed to a deethanizer zone to provide a light hydrocarbon feedstream comprising hydrogen, methane, ethylene and ethane, and a deethanized stream comprising propylene, propane and C -plus olefins. The light hydrocarbon stream is passed to a demethanizer zone operating at a demethanizing temperature greater than about -45°C to provide a bottom stream comprising ethylene and ethane and an overhead stream comprising hydrogen, methane, and ethylene. The overhead stream is separated by adsorption, absorption and/or the use of a membrane to produce an adsorber first separation stream comprising hydrogen and methane and a second separation stream comprising ethylene. The bottom stream is passed to a C₂ splitter zone to produce an ethylene product stream and an ethane stream. At least a portion of the first separation stream is combined with the oxygenate conversion effluent stream prior to passing the oxygenate conversion effluent stream to the deethanizer zone BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic process flow diagram illustrating the process of the prior art for recovering light olefins from an oxygenate conversion process.

FIG. 2 is a schematic process flow diagram illustrating the process of the present invention.

FIG. 3 is a chart of adsorption isotherms for components of deethanizer overhead on silica gel.

FIG. 4 is a chart of adsorbent delta loadings comparing silica gel and zeolite 4A over a range of adsorption temperatures.

FIG. 5 is a chart of the relative ethylene recovery required in a PSA and in the demethanizer for a range of deethanizer overhead composition.

FIG. 6 is a chart of the ratio of ethylene recycled to the ethylene recovered over a range of deethanizer overhead composition.

DETAILED DESCRIPTION OF THE INVENTION

This invention relates to recovery schemes used in conjunction with a process for the catalytic conversion of a feedstock comprising one or more aliphatic hetero compounds comprising alcohols, halides, mercaptans, sulfides, amines, ethers, and carbonyl compounds or mixtures thereof to a hydrocarbon product containing light olefinic products, i.e., C₂, C₃ and/or C olefins. The feedstock is contacted with a catalyst, such as a silicoaluminophosphate molecular sieve, at effective process conditions to produce light olefins.

The oxygenate conversion step of the present invention is preferably conducted in the vapor phase such that the oxygenate feedstock is contacted in a vapor phase in a reaction zone with a molecular sieve catalyst at effective process conditions to produce hydrocarbons, i.e., an effective temperature, pressure, weight hourly space velocity (WHSV) and, optionally, an effective amount of diluent, correlated to produce hydrocarbons. The process is effected for a period of time sufficient to produce the desired light olefin products. Reaction conditions for the conversion of aliphatic hetero compounds can be determined by those skilled in the art and preferably, in accordance with the present invention, comprise a temperature of from about 200° to 600°C, more preferably from about 300° to 500°C, and a pressure of from about 7 kPa to about 1.4 Mpa (1 to 200 psia), more preferably from about 140 kPa to about 700 kPa (20 to 100 psia). Typical processes for producing light olefins are described US-A-4,499,327 and 4,873,390 cited above and hereby incorporated by reference.

The selection of a particular catalyst for use in the oxygenate conversion step depends upon the particular oxygenate conversion desired but in a preferred aspect of the present invention where the oxygenate feedstock is converted into light olefins, it is preferred that the catalysts have relatively small pores. Certain of the catalysts useful in the present invention have pores with an average effective diameter of less than 5 A. The average effective diameter of the pores of preferred catalysts is determined by measurements described in D. W. Breck, ZEOLITE MOLECULAR SIEVES by John Wiley & Sons, New York (1974), hereby incorporated by reference in its entirety. The term "effective diameter" is used to denote that occasionally the pores are irregularly shaped, e.g., elliptical, and thus the pore dimensions are characterized by the molecules that can be adsorbed rather than the actual dimensions. Preferably, the small pore catalysts have a substantially uniform pore structure, e.g., substantially uniformly sized and shaped pore. Suitable catalyst may be chosen from among layered clays, zeolitic molecular sieves, and non-zeolitic molecular sieves.

Non-zeolitic molecular sieves include molecular sieves which have the proper effective pore size and are embraced by an empirical chemical composition, on an anhydrous basis, expressed by the empirical formula:

(EL_xAl_yP_z)O₂ where EL is an element selected from the group consisting of silicon, magnesium, zinc, iron, cobalt, nickel, manganese, chromium and mixtures thereof, x is the mole fraction of EL and is at least 0.005, y is the mole fraction of Al and is at least 0.01, z is the mole fraction of P and is at least 0.01 and x + y + z = 1. When EL is a mixture of elements, x represents the total amount of the metal mixture present. Preferred elements (EL) are silicon, magnesium, and cobalt with silicon being especially preferred. The preparation of various ELAPOs is well-known in the art and may be found in US-A-5,191,141; US-A- 4,554,143; US-A-4,440,871; US-A-4,853,197; US-A-4,793,984, US-A-4,752,651 and US-A-4,310,440. A preferred embodiment of the invention is one in which the element (EL) content varies from about 0.005 to about 0.05 mole fraction. If EL is more than one element, then the total concentration of all the elements is between about 0.005 and 0.05 mole fraction. An especially preferred embodiment is one in which EL is silicon (usually referred to as SAPO). The SAPOs which can be used in the instant invention are any of those described in US-A-4,440,871; US-A-5, 126,308, and US-A-5,191,141. Of the specific crystallographic structures described in the '871 patent, the SAPO-34, i.e., structure type 34, is preferred. The SAPO-34 structure is characterized in that it adsorbs xenon but does not adsorb isobutane, indicating that it has a pore opening of about 4.2 A. Another SAPO, SAPO-17, as exemplified in Examples 25 and 26 of the '871 patent, is also preferred. The SAPO-17 structure is characterized in that it adsorbs oxygen, hexane, and water but does not adsorb isobutane, indicating that it has a pore opening of greater than about 4.3 A and less than about 5.0 A.

The molecular sieve catalyst for the oxygenate conversion zone preferably is incorporated into solid particles in which the catalyst is present in an amount effective to promote the desired hydrocarbon conversion. In one aspect, the solid particles comprise a catalytically effective amount of the catalyst and at least one matrix material, preferably selected from the group consisting of binder materials, filler materials, and mixtures thereof to provide a desired property or properties, e.g., desired catalyst dilution, mechanical strength, and the like to the solid particles. Such matrix materials are often, to some extent, porous in nature and may or may not be effective to promote the desired hydrocarbon conversion. Filler and binder materials include, for example, synthetic and naturally occuning substances such as metal oxides, clays, silicas, aluminas, silica- aluminas, silica-magnesias, silica-zirconias, silica-thorias, silica-berylias, silica-titanias, silica-alumina-thorias, silica- alumina- zirconi as, aluminophosphates, mixtures of these and the like. The preparation of solid particles comprising catalyst and matrix materials is conventional and well known in the art and, therefore, need not be discussed in detail herein.

The oxygenate conversion reaction is preferably carried out in a bubbling bed or a fluidized bed reaction zone wherein feed and catalyst are contacted at effective oxygenate conversion conditions. During the oxygenate conversion reaction, a carbonaceous material — i.e., coke — is deposited on the catalyst. The carbonaceous deposit material has the effect of reducing the number of active sites on the catalyst which thereby affects the extent of the conversion. During the conversion process, a portion of the coked catalyst is withdrawn from the reaction zone and regenerated to remove at least a portion of the carbonaceous material and returned to the oxygenate conversion reaction zone. Depending upon the particular catalyst and conversion, it can be desirable to substantially remove the carbonaceous material e.g., to less than 1 wt-%, or only partially regenerate the catalyst, e.g., to from about 2 to 30 wt-% carbon. Preferably, the regenerated catalyst will contain about 0 to 20 wt-% and more preferably from about 0 to 10 wt-% carbon. Additionally, during regeneration, there can be oxidation of sulfur and in some instances nitrogen compounds along with the removal of metal materials from the catalyst. Moreover, regeneration conditions can be varied depending upon catalyst used and the type of contaminant material present upon the catalyst prior to its regeneration.

The oxygenate conversion effluent comprises hydrogen, methane, carbon monoxide, carbon dioxide, oxygenates such as dimethyl ether (DME), ethylene, ethane, propylene, propane, and C -plus hydrocarbons such as butenes and butane. Preferably, the molar ratio of hydrogen and methane to the moles of ethylene and ethane in the oxygenate conversion effluent is less than about 0.5. More preferably, the molar ratio of hydrogen and methane to the moles of ethylene and ethane in the oxygenate conversion effluent is between about 0.01 to about 0.5. It is believed that this particular range of the ratio of moles of hydrogen and methane to the moles of ethylene and ethane in the oxygenate conversion effluent stream combined with the relatively high ratio of ethylene to ethane in the C hydrocarbons permits the incorporation of the separation zone for the separation of the ethylene from the demethanizer overhead stream. The separation zone can comprise a temperature swing adsorption zone, a pressure swing adsorption zone, a membrane separation zone, a vacuum swing adsorption zone, a liquid absorption system, and combinations thereof.

In the present invention, the oxygenate conversion effluent stream is withdrawn from the oxygenate conversion reactor and passed to a multi-stage effluent compressor to raise the pressure of the oxygenate conversion effluent stream to provide a compressed effluent stream. The compressed effluent stream passes through a series of zones as hereinafter described to recover or remove unreacted oxygenate such as DME to provide carbon dioxide, water to a content of less than about 5 ppm-wt., effluent stream before entering a series of fractionation zones to separate the individual olefins into high purity products. The initial fractionation zone comprises a deethanizer which separates the oxygenate conversion effluent into an overhead stream comprising hydrogen, carbon monoxide, methane, acetylene, ethylene, and ethane, and a deethanizer bottoms stream comprising propylene, propane, and heavier hydrocarbons. The deethanizer overhead stream is compressed to selectively hydrogenate or saturate acetylene in the presence of hydrogen and produce a treated deethanizer overhead stream which is essentially free of acetylene. Acetylene is preferably removed to a level of less than 1 ppm to minimize the possibility of an explosion hazard by the buildup of acetylenes in the ethylene column and to produce an ethylene product that is essentially free of acetylene. The treated deethanizer overhead stream, is passed to a demethanizer zone. The demethanizer zone operates at effective demethanizing conditions to provide a demethanizer bottoms stream comprising ethylene having at least a 50:1 ratio of ethylene to ethane. Preferably, the demethanizer zone comprises a primary ethylene recovery of greater than about 85 mol-% ethylene per mole of ethylene in the light hydrocarbon stream, or the feed to the demethanizer zone. The overhead stream from the demethanizer zone is passed to a separation zone.

The demethanizer overhead stream of the present invention preferably comprises a molar ratio of hydrogen and methane to C₂ hydrocarbons less than about 0.2. More preferably, the molar ratio of hydrogen and methane to C₂ hydrocarbons ranges between about 0.07 and about 0.17 which correspond to ethylene recycle ratios of between about 0.1 and 0.2 of the total ethylene produced. According to the present invention, it was unexpectedly discovered that for an overhead stream having a molar ratio within the above range, the demethanizer can be operated at a demethanizing temperature of greater than about - 5°C - see Claim 1, rather than the traditional cryogenic demethanizer temperature of about -95°C (-140°F). Thus, the overall ethylene separation objectives can be met without using an ethylene refrigerant, without a significant loss of ethylene product, without significant increases in ethylene recycle, and without considerable increases in operating cost and capital cost. When the above molar ratio range of exceeds about 0.5, the recovery requirements of the PSA zone or other type of separation zone increases to about 99.9+ percent, and the amount of ethylene returned to the reactor effluent compressor, or recycled within the entire ethylene recovery zone increases significantly. This is in contrast to a typical naphtha cracking stream wherein the molar ratio of hydrogen and methane to C₂ hydrocarbons ranges between about 1.2 to 1.4 and which would require an overall increase in ethylene recycle ratio to about 1.5 to about 1.7 (150-170 percent of the total ethylene recycle), which would significantly increase the capital and equipment requirements over a scheme according to the present invention. These overall benefits permit the demethanizer zone to be cooled with a propylene refrigerant (<-49°C) rather than an ethylene refrigerant-based cooling system (<-140°C) for recovering ethylene from the effluent from an oxygenate conversion zone.

The separation zone which can comprise a solvent absorption zone, a temperature swing adsorption zone, a pressure swing adsorption zone, a vacuum swing adsorption zone, or a membrane separation zone and combinations thereof which selectively separates ethylene and heavier components from a separation zone effluent stream comprising hydrogen, methane, and carbon monoxide which is withdrawn as a first fuel gas stream. The ethylene-rich stream is recycled to the product compressor to be combined with the oxygenate conversion effluent stream. When the separation zone comprises a PSA zone, the adsorbent comprises a selective adsorbent such as silica gel, activated carbon, alumina, zeolite, and mixtures thereof for the adsorption of ethylene and ethane. Such zeolites may include zeolite type X and zeolite type Y, ion exchanged forms thereof, and mixtures thereof. The zinc exchanged form of zeolite of zeolite X and the calcium exchanged form of zeolite Y may be included in the selective adsorbent. It is particularly preferred that the selective adsorbent for the PSA process comprise silica gel and that the adsorption temperature ranges between about - 18°C and about 100°C. More preferably, the adsorption temperature ranges between about -18°C and about 50°C. And most preferably, the adsorption temperature comprises less than about 49°C. It is believed that at adsorption temperatures above about 100°C, and especially at adsorption temperatures above about 50°C, silica gel becomes less selective for the adsorption of hydrogen and methane relative to C hydrocarbons. This adsorption of ethylene over silica gel, surprisingly, behaves in a manner which is opposite to the observed adsorption characteristics of ethylene over a zeolite adsorbent such as zeolite 4A. A comparison of the "delta loading" of ethylene on the adsorbent between the conditions of adsorption and desorption in a PSA cycle illustrated the different behavior of the adsorbents. As used herein, the term "delta loading" is the difference between the amount of adsorbed material on the adsorbent between the adsorption and desorption conditions, as measured in moles per unit weight of adsorbent. Generally, the delta loading is expressed in pound moles per 100 pounds of adsorbent, or gram moles per 100 grams of adsorbent. Silica gel was found to display an increasing delta loading as the adsorption temperature is reduced below about 100°C. On desorption, a desorbed stream comprising ethane and ethylene is withdrawn from the PSA zone. Preferably, the adsorption zone provides an adsorptive recovery of ethylene greater than about 95 mol-% ethylene.

The PSA process is an essentially adiabatic process for separating a multi- component fluid containing at least one selectively adsorbable component. The PSA process of the invention relates to conventional PSA processing in which each bed of an adsorption zone undergoes, on a cyclic basis, high-pressure adsorption, optional cocurrent depressurization to intermediate pressure level(s) with release of void space gas from the product end of the bed, countercurrent depressurization to lower desorption pressure with the release of desorbed gas from the feed end of the bed, with or without purge of the bed, and repressurization to higher adsorption pressure. The process of the present invention adds to this basic cycle sequence the use of a cocurrent depressurization, or vent step, in the adsorption zone in which the less readily adsorbable component is further and preferably essentially completely removed therefrom. The adsorption zone is then countercurrently depressurized to a desorption pressure that is at or above atmospheric pressure with the more adsorbable component(s) being discharged from the feed end thereof as a product of desired purity. Those skilled in the art will appreciate that the high-pressure adsorption step of the PSA process comprises introducing the PSA feedstream, or the demethanizer overhead stream, to the feed end of the adsorbent bed at a high adsorption pressure. The less readily adsorbable components (hydrogen and methane) pass through the bed and are discharged from the product end thereof. An adsorption front or fronts are established in the bed with said fronts likewise moving through the bed from the feed end toward the product end thereof. When the feedstream contains a less readily adsorbable component and a more readily adsorbable component, a leading adsorption front of the more readily adsorbable component will be established and move through the bed in the direction of the product or discharge end thereof. The PSA feedstream or demethanizer overhead stream of this invention comprises from about 10 to about 60 mol-% hydrogen, from about 5 to 50 mol-% methane, and from about 10 to 60 mol-% ethylene and heavier components. In the adsorption zone, the more readily adsorbable components are adsorbed at an adsorption pressure and adsorption temperature and the less readily adsorbable components are passed through the adsorption zone. The adsorption zone pressure ranges from about 350 kPa to about 3.5 MPa (about 50 to about 500 psia). The adsorption zone temperature is any temperature effective to adsorb the more readily adsorbable components in the feedstream, and preferably from about -18°C to less than about 50°C (about 0° to about 120°F). It is to be understood that the adsorption zones of the present invention contain adsorber beds containing adsorbent suitable for adsorbing the particular components to be adsorbed therein. As the capacity of the adsorber bed for the more readily adsorbable component is reached, that is, preferably before a substantial portion of the leading adsorption front has passed through the first adsorber bed, the PSA feedstream is directed to another bed in the adsorption zone. It is to be also understood that the term "countercurrent" denotes that the direction of gas flow through the adsorption zone, i.e., adsorber bed, is countercurrent with respect to the direction of PSA feedstream flow. Similarly, the term "cocurrent" denotes flow in the same direction as the PSA feedstream flow. The purge gas is at least partially comprised of an effluent stream, e.g., the adsorption effluent stream or the cocurrent displacement effluent stream, from the adsorption zone, as hereinafter described, which comprises the less readily adsorbable component. When hydrogen is the less readily adsorbable component, the purge gas is preferably rich in hydrogen at a higher concentration than available in the PSA feedstream. The term "enriched" is intended to be with reference to the PSA feedstream composition unless otherwise noted. Following the adsorption step, the bed undergoes a vent step wherein the bed is allowed to depressurize cocurrently to a vent pressure in a direction cocurrent to the feeding step. By the use of this cocurrent depressurization step, or vent step, gas the effluent stream is substantially reduced and preferably essentially free of the less readily adsorbable component, thus comprising more readily adsorbable components. The less readily adsorbable component that remains in the void spaces of the adsorbent bed ahead of the leading adsorption front can be essentially completely displaced from the bed and can enrich the more readily adsorbed component behind the adsorption front. This enables the more readily adsorbable component to be thereafter discharged from the feed end of the bed as a product of desired purity by countercurrently depressurizing the bed. The cocurrent depressurization step can be performed in conjunction with one or more cocurrent depressurization, or equalization, steps wherein the equalization gas is not vented, but passed to another bed. When a cocurrent depressurization step is used, it can be performed either before, simultaneously with, or subsequent to the vent step. The effluent stream from the cocurrent depressurization step, which is comprised primarily of less readily adsorbable components, can be used to partially repressurize or purge another adsorber bed. The combination of the desorption effluent resulting from the countercurrent depressurization step and the purge step are combined to have ethylene and heavier product purity of from about 20 to about 95 mol-%.

After the termination of the vent step and any desired cocurrent depressurization step(s), the adsorber bed is desorbed by reducing the pressure in a direction countercurrent to the PSA feed direction to a desorption pressure that is preferably from about atmospheric pressure to about 3.5 kg/cm absolute (about 50 psia). A portion of the desorption effluent stream recovered from the adsorption zone could be utilized as feed for the cocurrent displacement step following recompression.

A PSA cycle for a six-bed adsorption zone employing the vent, or cocurrent depressurization step following the adsorption step is shown in Table 1 below:

TABLE 1

Although any suitable PSA cycle known to those skilled in the art and incorporating the steps of adsorption, equalization, provide purge, purge, and repressurization may be employed in the PSA operation, it is preferred to operate the PSA zone with a cocurrent desorption step, or vent step, immediately following the adsorption step to provide a second fuel gas stream which is employed to regenerate the dryer zone. Preferably, the vent step is terminated at a vent pressure which is at least about 700 kPa (about 100 psia).

When the separation zone comprises a membrane separation zone, the membrane employed is selective for the permeation of ethylene and heavier components and the non-permeate stream comprises hydrogen and methane. US-A- 5,879,431, which is herein incorporated by reference, discloses the application of a rubbery and super glassy membrane at separation conditions which include a separation temperature between about 0°C and about -100°C for similar separations.

DETAILED DESCRIPTION OF THE DRAWINGS

The process of the present invention is hereinafter described with reference to FIGS. 1 and 2 which schematically illustrate various aspects of the process.

FIG. 1 illustrates the typical light olefins recovery scheme of the prior art. Referring to FIG. 1, an oxygenate conversion effluent stream in line 1 is passed to a compression zone 18 to raise the pressure of the oxygenate conversion effluent stream in line 1 to a deethanizer pressure of between about 1050 kPa (150 psia) and about 2860 kPa (400 psia) to produce a compressed effluent stream in line 5. More preferably, the compression zone raises the pressure of the oxygenate conversion effluent stream to produce a compressed effluent stream at a deethanizer pressure between about 1750 kPa (250 psia) and about 2450 kPa (350 psia). Generally, the compression zone 18 comprises a multi-stage compressor that, at a low interstage pressure, comprises a first interstage flash drum to remove a condensate stream in line 3 comprising primarily water and unconverted methanol that is returned to the oxygenate conversion reaction zone (not shown), and a second interstage flash drum to remove a C -plus stream in line 4 that is passed to a depropanizer (not shown) for further recovery of C₃ and C₄ hydrocarbons. The compressed effluent stream in line 5 is passed to an oxygenate removal zone 20 for the rejection of trace oxygenates including DME and methanol in line 6 that is returned to the oxygenate conversion reaction zone (not shown) and to produce an oxygenate removal zone effluent stream via line 7. The oxygenate removal zone effluent comprises less oxygenate than the oxygenate conversion effluent stream in line 5. Preferably, the oxygenate removal zone effluent comprises less than about 1000 ppm-wt oxygenates.

The oxygenate removal zone effluent in line 7 is passed to a carbon dioxide removal zone

22 wherein carbon dioxide is absorbed by contacting a caustic solution or by contacting an amine solution in combination with a caustic solution (not shown) in a conventional manner to remove the carbon dioxide in line 8 dissolved in the spent absorbent to provide a carbon dioxide depleted stream in line 9 that is depleted in carbon dioxide relative to the oxygenate conversion zone effluent and is saturated with water. The carbon dioxide depleted stream in line 9 is passed to a dryer zone containing a solid desiccant to remove water (shown as line 10) from the carbon dioxide depleted stream in line 9 to produce a dry deethanizer feedstream in line 11. The deethanizer feedstream in line 11 is passed to a deethanizer zone 26 to provide a light hydrocarbon stream in line 13 comprising hydrogen, methane, carbon monoxide, ethylene, and ethane, and a deethanized stream in line 12 comprising propylene, propane, and C -plus olefins. The deethanized stream in line 12 is passed to further separation and recovery of propylene in a conventional manner (not shown) employing, for example, a depropanizer to separate the C₃ hydrocarbon from the heavier C -plus olefins, and a C₃ splitter to provide a propylene product stream and a propane stream. The light hydrocarbon stream in line 13 is passed to a compression and selective saturation zone 28 to compress the light hydrocarbon stream to provide a compressed light hydrocarbon stream at a saturation pressure of about 2860 kPa (400 psia) and about 4200 kPa (600 psia). The compressed light hydrocarbon stream is contacted with a selective saturation catalyst in the presence of hydrogen to saturate any trace amount of acetylene which might be in the light hydrocarbon stream prior to passing the selectively saturated light hydrocarbon stream to a demethanizer zone 30 via line 14. Such selective saturation processes are well known to those skilled in the art.

Because the selectively saturated light hydrocarbon stream in line 14 comprises hydrogen and methane, the demethanizer zone must be cooled with an ethylene chiller and be operated at a demethanizing temperature of between about -100°C (-150TF) and about -

90°C (-130°F) in order to produce a C₂ hydrocarbon stream in line 16 comprising ethylene and ethane which can be further separated to produce a high purity ethylene product stream in a C₂ splitter column (not shown). An overhead stream comprising hydrogen and methane in line 15 is removed from the demethanizer zone 30 and withdrawn from the process as a fuel stream. The present invention recognizes the unique character of the oxygenate conversion effluent stream, in particular that the light hydrocarbon stream comprises a molar ratio of hydrogen and methane to C₂ hydrocarbons (ethylene and ethane) ranging from about 0.1 to about 0.5. According to the present invention, the demethanizer zone is operated at a significantly higher temperature (>-45°C) to produce an overhead stream which now, in addition to hydrogen and methane, comprises some ethylene and ethane. This ethylene-depleted overhead stream is passed to a separation zone, or adsorption zone to recover the ethylene and return at least a portion of the recovered ethylene as a desorbed stream. The desorbed stream comprising the recovered ethylene is returned to be combined with the oxygenate conversion effluent stream. By this combined separation/fractionation scheme, the ethylene product stream is recovered at an overall recovery greater than about 99.5 mol-% relative to the ethylene in the oxygenate conversion zone effluent.

Referring to FIG. 2, the oxygenate conversion effluent stream in line 100 is passed through zones 118, 120, 122, 124, 126 and 128 in substantially the same manner as described in Figure 1. The selectively saturated light hydrocarbon stream, now comprising less than about 5 ppm-wt acetylene, and preferably comprising less than about 1 ppm-wt acetylene is taken from zone 128 and passed to a demethanizer zone 130 operated at a demethanizing temperature of about greater than -45°C. Preferably, the demethanizer zone 130 is operated at a demethanizer temperature greater than or equal to about -40°C. A C₂ hydrocarbon stream comprising ethylene and ethane is withdrawn in line 116 from the bottom of the demethanizer zone 130 which is passed to a C₂ splitter zone (not shown) for conventional separation as required to produce a high purity (>99.5 mol-%) ethylene product stream. An overhead stream in line 115 is withdrawn from the demethanizer zone 130 and passed to a separation zone 132, such as an adsorption zone 132 that during an adsorption step provides a fuel gas stream 121 comprising hydrogen and methane, and, during a desorption step, provides a desorption stream comprising ethylene in line 119. At least a portion of the desorption stream is recycled via line 119 to the compression zone 118 wherein the desorption stream is combined with the oxygenate conversion effluent stream in line 100, introducing the desorption stream at the appropriate stage of compression. The adsorption zone 132 comprises a PSA zone or a temperature swing adsorption zone or a combination thereof wherein the purge gas may be heated to improve the recovery of adsorbed olefins from the selective adsorbent. Preferably, the adsoφtion zone 132 comprises a PSA zone which includes a cocurrent depressurization step, or vent step, to produce a vent stream immediately following the adsoφtion step and wherein the adsoφtion effluent and at least a portion of the vent stream withdrawn during the vent step are combined to produce the fuel gas stream 121. Preferably, the vent gas pressure comprises a vent gas pressure greater than about 700 kPa (100 psia) so that a portion of the vent gas stream can be employed to regenerate the dryer zone (not shown).

The following examples are only used to illustrate the present invention and are not meant to be limiting.

EXAMPLES EXAMPLE I

In a conventional process arrangement as depicted in FIG. 1 (Prior Art), the demethanizer is operated with a condenser temperature of about -95°C (-140°F) and a pressure of about 3200 kPa (465 psia) in order to recover at least 99.6 mol-% of the ethylene in the demethanizer bottoms product. A cooling system for operating the demethanizer in the above manner is required to provide cooling for the demethanizer condenser and to provide for chilling the demethanizer feed to a temperature of about - 68°C (-90°F). Pure ethylene refrigeration which can provide a cooling temperature of about -lore (-150°F) and is generally used in this service. For an ethylene production rate of about 500,000 metric tonnes per year, the ethylene refrigeration system requires a centrifugal compressor with a capacity of about 4350 Nm³/hr (2700 actual cubic feet per minute- inlet) at a capital cost of about 3.9 million dollars. For an ethylene production rate of about 120,000 metric tonnes per year, the ethylene refrigeration system requires two reciprocating compressors, each with a capacity of about 1030 Nm³/hr (640 actual cubic feet per minute-inlet) at a capital cost of about 1.9 million dollars. Thus, the ethylene refrigeration system represents a significant capital cost, a large portion of which is directly related to simply compressing the ethylene. EXAMPLE II

An engineering simulation of the demethanizer operating as shown in FIG. 2 according to the present invention indicates that the demethanizer can be operated at an overhead temperature of about - 0°C (-40°F) to provide about an 85 mol-% ethylene recovery and that this operation can be combined with a PSA zone containing silica gel as the selective adsorbent for the recovery of ethylene and ethane. The PSA zone recovers about 97.8 mol-% ethylene and about 20.5 mol-% methane. The ethylene-rich PSA tail gas is returned to the first stage of the oxygenate conversion compressor zone at a desoφtion pressure of about 117 kPa (17 psia). The overall recovery of the ethylene from the complex is greater than about 99.6 percent. The demethanizer bottoms comprises ethylene and ethane which is passed to an appropriate C₂ splitter for the production of high purity or polymer grade ethylene.

EXAMPLE III

A comparison is shown in Table 1 of the relative equipment size or capacity for the prior art scheme of FIG. 1 and the scheme of the present invention as depicted in FIG. 2. The relative size is expressed in term as a percent larger or smaller than the base case. Although the PSA inclusive scheme of the present invention required small increases in the compression capacity and process compressors, the ethylene compressor is completely eliminated. The capacity of the compressors is expressed in energy requirements and the size differences are expressed in terms of relative column inside diameter. The PSA case represents an overall capital cost savings of about 3.2 million dollars for a 120,000 metric tonnes per year ethylene production and about a 73,000 dollars per year utility savings.

EXAMPLE IV

Adsoφtion isotherms for the individual components typically found in the overhead stream withdrawn from a demethanizer operating according to the present invention are shown in FIG. 3 for adsoφtion on silica gel at an adsoφtion temperature of about 15°C (60°F). Isotherm data is shown for the following components: hydrogen(a), carbon monoxιde(b), methane(c), ethane(d), ethylene(e), and nιtrogen(f) In FIG. 3, the ethylene and ethane isotherms indicate a high selectivity over hydrogen, methane, and carbon monoxide over the silica gel adsorbent. This adsoφtion isotherm data were developed using standard high-piessure Sartoπus microbalance techniques well known to those skilled in the ait.

EXAMPLE V

Based on adsoφtion isotherm data developed from standard high-pressure Sartonus microbalance techniques for ethylene adsoφtion over silica gel and a zeolite 4A adsorbent, delta loadings expressed in grams/100 grams of adsorbent for the silica gel (a) and the 4A zeolite (b) at temperatures ranging from about 0°C to about 100°C were calculated and are presented in FIG 4 Of note is the greater delta loading or adsoφtion capacity for silica gel relative to the delta loading of the zeolite 4A at adsoφtion temperatures less than about 100°C Suφπsmgly, the delta loading of ethylene on the silica gel increases at adsoφtion temperatures less than about 100°C, while the delta loading values for the zeolite 4A are initially lower than that of the silica gel at 100°C and gradually decrease for temperatures below 100°C.

EXAMPLE VI

The operation of the demethanizer of the present invention operating at a demethanizer temperature of about -40°C as shown in FIG. 2 can be carried out over a range of recoveries according to the ratio of hydrogen and methane to C₂ hydrocarbons in the overhead stream. FIG. 5 illustrates the recovery of ethylene decreases as the molar ratio of hydrogen and methane to C₂ hydrocarbons increases. When the ratio falls within a range of about 0.07 to about 0.17 as produced in the MTO schemes, the recovery of the demethanizer is greater than about 85 percent and the PSA recovery is less than about 99 percent. If the overhead stream comprises a composition corresponding to a conventional naphtha cracker or pyrolysis ethylene plant, the molar ratio comprises a value above 1.2 which reduces the demethanizer recovery to less than about 40 percent and requires the PSA to recover essentially all of the ethylene. FIG. 6 illustrates the consequence of these operations on the overall complex. For a molar ratio as produced in the effluent of oxygenate conversion zones which ranges less than about 0.5, the amount of ethylene recycled in the overall complex is less than about 60 percent of the total ethylene product. As the molar ratio increases to about 1.2, the amount of ethylene recycled in the complex approaches 150 percent of the ethylene product produced, resulting in significantly greater capital and operating cost requirements.

Claims

CLAIMS:

1. A process for the production of ethylene from an oxygenate conversion effluent stream comprising hydrogen, methane, ethylene, ethane, propylene, propane, and C -plus olefins said process comprising: a) passing the oxygenate conversion effluent stream to a deethanizer zone to provide a light hydrocarbon feedstream comprising hydrogen, methane, ethylene and ethane, and a deethanized stream comprising propylene, propane, and C₄φlus olefins; b) passing the light hydrocarbon stream to a demethanizer zone operating at a demethanizing temperature greater than about -45°C to provide a bottom stream comprising ethylene and ethane and an overhead stream comprising hydrogen, methane, and ethylene; c) passing the overhead stream at effective separation conditions to a separation zone to produce a fuel gas stream comprising hydrogen, methane, and carbon monoxide, and an ethylene-rich stream comprising ethylene and heavier components; d) passing the bottom stream to a C splitter zone to produce an ethylene product stream and an ethane stream; and, e) combining at least a portion of the ethylene-rich stream with the oxygenate conversion effluent stream.

2. The process of claim 1 wherein the separation zone is selected from the group consisting of a pressure swing adsoφtion zone, a temperature swing adsoφtion zone, a solvent absoφtion zone, a membrane separation zone, and combinations thereof.

3. The process of Claim 1 wherein the overhead stream passes at effective adsoφtion conditions to an adsoφtion zone containing at least two adsoφtion beds, each of said adsoφtion beds containing a selective adsorbent to adsorb the ethylene on adsoφtion to produce an adsorber effluent stream comprising hydrogen and methane, and on desoφtion to produce a desorbed stream comprising ethylene; combining at least a portion of the desoφtion stream with the oxygenate conversion effluent stream prior to passing the oxygenate conversion effluent stream to the deethanizer zone.

4. The process of claim 1 or 3 wherein the overhead stream comprises a molar ratio of hydrogen and methane to the moles of ethylene and heavier components of less than about 0.5.

5. The process of claim 3 wherein the effective adsoφtion conditions include an adsoφtion temperature comprising less than about 49°C (120°F) and the selective adsorbent is selected from the group consisting of silica gel, activated carbon, alumina, zeolite, zinc X zeolite, calcium Y zeolite, and mixtures thereof.

6. The process of claim 1 or 3 wherein the light hydrocarbon stream comprises a molar ratio of hydrogen and methane to C₂ hydrocarbons of less than about 0.5 moles of hydrogen and methane to moles of ethylene and ethane.

7. The process of claim 1 or 3 wherein the ethylene product stream is recovered at a recovery of greater than about 99.5 mol-% relative to the ethylene in the oxygenate conversion effluent stream and the ethylene stream comprises 99.5% mol ethylene or greater.

8. The process of claims 1 or 3 wherein the demethanizer zone comprises a primary ethylene recovery of greater than about 85 mol-% ethylene per mole of ethylene in the light hydrocarbon stream and the adsoφtion zone provides an adsoφtive recovery of ethylene comprising greater than about 95 mol-% ethylene.

9. The process of claims 1 or 3 further comprising recovering a propylene product stream from the deethanized stream and the demethanizer zone is refrigerated to the demethanizing temperature with a refrigerant comprising propylene.

10. The process of claims 1 or 3 wherein the oxygenate conversion effluent stream comprises a reaction product of an oxygenate feedstock having been reacted at effective conversion conditions in the presence of a diluent in an oxygenate conversion reaction zone containing a small pore, non-zeolitic catalyst to convert essentially all of the oxygenate feedstock to produce the oxygenate conversion effluent stream.

11. The process of claims 1 or 3 wherein the oxygenate conversion effluent stream further comprises carbon dioxide and said process further comprises passing the oxygenate conversion effluent stream to a carbon dioxide removal zone to remove at least a portion of the carbon dioxide and subsequently passing a carbon dioxide depleted stream to a dryer zone containing a solid desiccant to remove water prior to passing the oxygenate conversion effluent stream to the deethanizer zone, regenerating the dryer zone with at least a portion of the adsorber effluent stream and at least a portion of a vent stream comprising hydrogen and methane, said vent stream withdrawn at a vent pressure from the adsoφtion zone in a cocurrent depressurization step immediately following the adsoφtion step and terminated prior to the breakthrough of ethylene from the adsoφtion zone.