Process for direct etherification of a dehydrogenation zone effluent stream

Abstract: Dehydrogenation of C₃-C₅ paraffins is integrated with conversion of the resulting olefins to ether to improve efficiency for ether production by directly charging all but the lightest components of the dehydrogenation zone effluent to the etherification zone. A particularly preferred arrangement of this invention uses an etherification zone for direct production of MTBE from the dehydrogenation zone effluent and two fractionation columns to yield an overhead stream of light ends (C₁ to C₃) and of oxygenates, a stream of recycle isoparaffins, recycle methanol, and a product stream of MTBE. The arrangement also simplifies the removal of undesired oxygen-containing compounds such as H₂O and DME from the recycle paraffin stream to the dehydrogenation step by allowing the use of only a depropanizer to remove both oxygenates and light ends without the additional cost of a separate oxygenate recovery unit.

Description: This invention relates broadly to processes for the production of ethers by the reaction of olefins contained in an effluent stream from a dehydrogenation step without separating light ends. The invention more directly relates to processes for the direct etherification of a dehydrogenation effluent stream recovery and the recycle of dehydrogenatable materials from the etherification zone effluent to the dehydrogenation zone. BACKGROUND OF THE INVENTION Processes for producing olefins by the dehydrogenation of saturated hydrocarbons are well known. A typical dehydrogenation process mixes the feed hydrocarbons with hydrogen and heats the resulting admixture by indirect heat exchange with the effluent from the dehydrogenation zone. Following heating, the feed mixture passes through a heater to further increase the temperature of the feed components before it enters the dehydrogenation zone where it is contacted with the dehydrogenation catalyst. The catalyst zone may be operated with a fixed bed, a fluidized bed, or a movable bed of catalyst particles. After heat exchange with the feed, the dehydrogenation zone effluent passes to product separation facilities. The product separation facilities will typically produce a gas stream, made up primarily of hydrogen, a first product stream that includes the desired olefin products, and a second potential product stream comprising light hydrocarbons. The light hydrocarbon stream typically has fewer carbon atoms per molecule than the desired olefin product. Light hydrocarbons are generally removed from the product stream in order to reduce flow volume, operating pressures, and undesirable side reactions in downstream process units that receive the olefin product. A portion of the hydrogen stream is typically recycled to the dehydrogenation zone to provide hydrogen for the combined feed stream. The product stream usually contains unconverted dehydrogenatable feed hydrocarbons in addition to the product olefin. These unconverted hydrocarbons may be withdrawn in the separation facilities for recycle to the dehydrogenation zone or passed together with the product olefins to an etherification zone for conversion of the product olefins to ethers. Etherification processes are currently in great demands for making high octane compounds which are used as blending components in lead-free gasoline. These etherification processes will usually produce ethers by combination of an isoolefin with a monohydroxy alcohol. The etherification process can also be used as a means to produce pure isoolefins by cracking of the product ether. For instance, pure isobutylene can be obtained for the manufacture of polyisobutylenes and tert-butyl-phenol by cracking methyl tertiary butyl ether (MTBE). The production of MTBE has emerged as a predominant etherification process which uses C₄ isoolefins as the feedstock. A detailed description of processes, including catalyst, processing conditions, and product recovery, for the production of MTBE from isobutylene and methanol are provided in U.S. Patents 2,720,547 and 4,219,678 and in an article at page 35 of the June 25, 1979 edition of Chemical and Engineering News. The preferred process is described in a paper presented at The American Institute of Chemical Engineers, 85th National Meeting on June 4-8, 1978, by F. Obenaus et al. Another etherification process of current interest is the production of tertiary amyl ether (TAME) by reacting C₅ isoolefins with methanol. Due to the limited availability of olefins for etherification, it has become common practice to produce them by the dehydrogenation of isoparaffins and to pass the dehydrogenation effluent to an etherification process. General representations of flow schemes where a dehydrogenation zone effluent passes to an etherification zone are shown in U.S. Patents 4,118,425 and 4,465,870. More complete representations of a flow arrangement where the dehydrogenation zone effluent passes to an etherification zone are given in U.S. Patent 4,329,516 and at page 91 of the October, 1980 edition of Hydrocarbon Processing. The latter two references depict the typical gas compression and separation steps that are used to remove hydrogen and light ends from the dehydrogenation zone effluent before it passes to the etherification zone. A typical effluent from an etherification zone includes an ether product, unreacted alcohol, and unreacted hydrocarbon. These effluent components enter separation facilities that yield the ether product, alcohol for recycling to the etherification zone, hydrocarbons for further processing into dehydrogenation. This recycle stream of C₄ or C₅ isoparaffins, prior to recycling to the dehydrogenation zone, is usually treated to recover methanol and remove other oxygenates which are harmful to the dehydrogenation catalyst. As evidenced by the foregoing references, the light materials that are present with the effluent from the dehydrogenation zone are viewed as undesirable and have been removed ahead of the etherification processes. These undesirable light materials, in the case of C₄ olefin conversion to produce butyl ethers, will normally include hydrogen, methane, and ethane. In the case of C₅ olefin conversion in the production of aryl ethers, the undesirable materials can include C₄ hydrocarbons. GB-A-2,147,290 discloses a process for producing isopropyl alcohol, secondary butyl alcohol, methyl tertiary butyl ether or a mixture of at least two thereof from liquid petroleum gas by dehydrogenating propane, n-butane, iso-butane or a mixture of at least two thereof in the gas to propylene, n-butylene and/or iso-butylene respectively and then etherifying with methonol and/or hydrating the dehydrogenated gas to yield isopropyl alcohol, secondary butyl alcohol, methyl tertiary butyl ether or a mixture of at least two thereof. The document does not teach or suggest any separation step between the dehydrogenation and the etherification but teaches that the entire liquid petroleum gas fraction is dehydrogenated and then this stream is passed to the etherification zone. Moreover, it does not teach or suggest any separation of light paraffins such as are contained in the etherification zone product prior to recycle but teaches simply that unconverted paraffins are recycled to the dehydrogenation zone. AU-A-563,178 discloses, inter alia a process for the manufacture of methyl tertiary butyl ether in which a mixture comprising isobutene and ethane is reacted with methanol. However, this document is concerned with the treatment of a transhydrogenation effluent, which contains essentially no hydrogen, rather than a dehydrogenation zone effluent. Moreover, it fails to teach the presence of light oxygenates as by-products in the etherification zone effluent and proposes no separation step for the removal of light oxygenates prior to recycle of isobutene to the dehydrogenation step. It is a broad object of this invention to improve the arrangement and operation of an etherification process that receives the dehydrogenating feed stream of dehydrogenated hydrocarbons. A more specific object of this invention is to reduce the capital and utility cost associated with the separation and recycle of components from the effluents of the combined processes for dehydrogenating hydrocarbons and the production of ethers. Another object of this invention is to simplify the separation facilities in a combined process for the dehydrogenation of dehydrogenatable hydrocarbons and the etherification of the hydrogenated hydrocarbons. BRIEF SUMMARY OF THE INVENTION It has now been discovered that capital and operating costs associated with the etherification of dehydrogenation zone effluents having undesirable light end materials can be improved by a process that allows some of these materials to be passed through the etherification zone. Thus, in a broad aspect, this invention charges the liquid effluent still containing some methane and C₂ hydrocarbon from a dehydrogenation separator, that recovers hydrogen from the dehydrogenation zone effluent, to an etherification zone. The effluent entering the etherification zone will contain saturated and unsaturated C₄ or C₅ hydrocarbons including isoolefins and C₃ hydrocarbons as well as C₂ hydrocarbons and some methane. In the etherification zone essentially all of the isoolefins reacts with C₁-C₅ monohydric alcohol to produce an ether product and produce an etherification zone effluent that contains an ether product and is deficient in the reacted olefins. The etherification effluent, also containing isoalkane, alcohols and C₃ and lighter hydrocarbons, is first separated to recover the ether product. That portion of the etherification zone effluent that contains hydrocarbons suitable for recycle to the dehydrogenation zone passes through an alcohol (methanol) recovery zone for the recovery of alcohol (methanol) and is further fractionated in another separation zone to remove C₃ and lighter hydrocarbons as well as oxygenates and to produce a stream of saturated C₄ or C₅ hydrocarbons for recycle to the dehydrogenation zone. The stream of C₃ and lighter hydrocarbons will also contain essentially all of the light oxygenates from the etherification effluent that are not removed by the methanol recovery zone. In a typical etherification process, the recycle hydrocarbon stream, if untreated, may contain 100 to 1000 wt. ppm of dimethylether, produced by the decomposition of methanol over the etherification catalyst, and this can detrimentally affect the operation of the dehydrogenation step. Thus, a process according to the invention for producing an ether from an effluent stream from a dehydrogenation zone, said effluent stream comprising one or more C₄ or C₅ isoolefins and isoalkanes, one or more C₃ hydrocarbons, one or more C₂ hydrocarbons, methane, and hydrogen, comprises the steps of: (a) separating essentially all the hydrogen and a substantial portion of the methane from said dehydrogenation zone effluent stream without separating all the C₂ hydrocarbons from said effluent stream; (b) passing the residual dehydrogenation zone effluent stream containing one or more C₄ or C₅ isoolefins and isoalkanes, one or more C₃ hydrocarbons, one or more C₂ hydrocarbons, and methane into an etherification zone for reaction with one or more C₁ - C₅ monohydric alcohols for essentially complete conversion of the isoolefin(s), thereby producing an etherification zone effluent stream comprising isoalkane(s), alcohol(s), ether(s), C₃ hydrocarbon(s), and C₂ hydrocarbon(s); (c) passing the etherification zone effluent stream to a first separation zone and recovering therefrom at least a first stream comprising an ether product and a second stream comprising isoalkane(s), alcohol(s), and C₃ and lighter hydrocarbons; (d) recovering alcohol(s) from the second stream in an alcohol recovery zone and returning at least a portion of the recovered alcohol(s) to the etherification zone; (e) passing the resulting alcohol-depleted second stream from the alcohol recovery zone to a second separation zone to separate C₃ and lighter hydrocarbons and oxygenates from said alcohol-depleted second stream, thereby obtaining a recycle stream consisting essentially of isoalkane(s); and (f) recycling the recycle stream to the dehydrogenation zone. In a more specific embodiment, this invention provides a process for producing MTBE. Practice of this process includes combining a recycle stream and a feed stream to provide a dehydrogenation zone input stream containing isobutane and hydrogen. Contacting the input stream with a dehydrogenation catalyst at dehydrogenation conditions in the dehydrogenation zone yields a mixed stream of isobutane, isobutene and hydrogen which also contains C₃ and lighter hydrocarbons. The dehydrogenation zone effluent enters a hydrogen recovery section. After substantial depletion of hydrogen, the dehydrogenation zone effluent still containing some methane and C₂ hydrocarbon directly enters an etherification zone. Admixture with methanol and contact with an etherification catalyst at etherification conditions in the etherification zone effects an essentially complete conversion of isobutene into MTBE and produces an etherification zone effluent containing MTBE, isobutane, methanol, C₃ and lighter hydrocarbons and oxygenates such as DME. A first separation zone receives the etherification zone effluent and separates it into an MTBE product stream and a separation stream containing methanol, isobutane, and C₃ and lighter hydrocarbons. A methanol recovery zone removes methanol from the separation stream and transfers the remainder of the stream to a second separation zone. The second separation zone separates the separation stream into an isobutane fraction that forms the recycle stream for the dehydrogenation zone and an off gas stream including C₃ and lighter hydrocarbons. Oxygenates that may otherwise be present in the recycle stream and may interfere with the operation of the dehydrogenation zone such as dimethyl ether are also removed with the C₃ and lighter hydrocarbons. Additional embodiments, aspects, and details of this invention are set forth in the following detailed description. BRIEF DESCRIPTION OF THE DRAWING The drawing schematically shows a highly integrated dehydrogenation and etherification process. This process includes a dehydrogenation reactor section 10, a hydrogen recovery section 12, an MTBE reactor section 14, and an MTBE product separator 16, a methanol recovery unit 18, and a depropanizer 20. DETAILED DESCRIPTION OF THE INVENTION The operation of this invention uses at least a portion of the effluent from a dehydrogenation zone or reaction section for the production of olefins from dehydrogenatable hydrocarbons. Dehydrogenatable hydrocarbons for this invention include isoalkanes having 4 or 5 carbon atoms. A suitable feed of dehydrogenatable hydrocarbons will often contain light hydrocarbons (i.e., those having less carbon atoms than the primary feed components) which, for the purpose of this invention, serve as contaminants. In most cases, olefins are excluded from the dehydrogenation zone recycle in order to avoid the formation of dienes which produce unwanted by-products in many of the olefin conversion processes. Along with the dehydrogenatable hydrocarbons, the feed to the dehydrogenation zone of the present invention comprises an H₂ rich stream, preferably containing at least 75 mole percent H₂. The presence of H₂ within the dehydrogenation zone serves several purposes. First, the H₂ acts to suppress the formation of hydrocarbonaceous deposits on the surface of the catalyst, more typically known as coke. Secondly, H₂ can act to suppress undesirable thermal cracking. Because H₂ is generated in the dehydrogenation reaction and comprises a portion of the effluent, the H₂ rich stream introduced into the reaction zone generally comprises recycle H₂ derived from separation of the dehydrogenation zone effluent. Alternately, the H₂ may be supplied from suitable sources other than the dehydrogenation zone effluent. The dehydrogenatable hydrocarbon stream and H₂ stream are introduced into a dehydrogenation reaction zone. The dehydrogenation reaction zone of this invention preferably comprises at least one radial flow reactor through which the catalytic composite gravitates downwardly to allow a substantially continuous replacement of the catalyst with fresh and/or regenerated catalyst. A detailed description of the moving bed reactors herein contemplated may be obtained by reference to U.S. Patent 3,978,150. The dehydrogenation reaction is a highly endothermic reaction which is typically effected at low (near atmospheric) pressure conditions. The precise dehydrogenation temperature and pressure employed in the dehydrogenation reaction zone will depend on a variety of factors such as the composition of the paraffinic hydrocarbon feedstock, the activity of the selected catalyst, and the hydrocarbon conversion rate. In general, dehydrogenation conditions include a pressure of from about 0 to about 35 bars and a temperature of from about 480°C (900°F) to about 760°C (1400°F). A suitable hydrocarbon feedstock is charged to the reaction zone and contacted with the catalyst contained therein at a liquid hourly space velocity of from about 1 to about 10 hr.⁻¹ Hydrogen, principally recycle hydrogen, is suitably admixed with the hydrocarbon feedstock in a mole ratio of from about 0.1 to about 10. Preferred dehydrogenation conditions, particularly with respect to C₄-C₅ paraffinic hydrocarbon feedstocks, include a pressure of from about 0 to about 5 bars and a temperature of from about 540°C (1000°F) to about 705°C (1300°F), a liquid hourly space velocity of from about 1 to about 5 hr.⁻¹, and a hydrogen/hydrocarbon mole ratio of from about 0.5:11 to about 2:1. The dehydrogenation zone of this invention may use any suitable dehydrogenation catalyst. Generally, the preferred catalyst comprises a platinum group component, an alkali metal component, and a porous inorganic carrier material. The catalyst may also contain promoter metals which advantageously improve the performance of the catalyst. It is preferable that the porous carrier material of the dehydrogenation catalyst be an absorptive high surface area support having a surface area of about 25 to about 500 m²/g. The porous carrier material should be relatively refractory to the conditions utilized in the reaction zone and may be chosen from those carrier materials which have traditionally been utilized in dual function hydrocarbon conversion catalysts. A porous carrier material may therefore, be chosen from an activated carbon, coke or charcoal, silica or silica gel, clays and silicates including those synthetically prepared and naturally occurring, which may or may not be acid-treated as, for example, attapulgus clay, diatomaceous earth, kieselguhr, bauxite; refractory inorganic oxides such as alumina, titanium dioxide, zirconium dioxides, magnesia, silica alumina, alumina boria, etc.; crystalline alumina silicates such as naturally occurring or synthetically prepared mordenite or a combination of one or more of these materials. The preferred porous carrier material is a refractory inorganic oxide, with the best results being obtained with an alumina carrier material. The aluminas, such as gamma alumina, give the best results in general. The preferred catalyst will have a gamma alumina carrier which is in the form of spherical particles having relatively small diameters on the order of about 1/16 inch (1.588 mm). The preferred dehydrogenation catalyst also contains a platinum group component. Of the platinum group metals, which include palladium, rhodium, ruthenium, osmium and iridium, the use of platinum is preferred. The platinum group component may exist within the final catalyst composite as a compound such as an oxide, sulfide, halide, oxysulfide, etc., or an elemental metal or in combination with one or more other ingredients of the catalyst. It is believed that the best results are obtained when substantially all the platinum group components exist in the elemental state. The platinum group component generally comprises from about 0.01 to about 2 wt.% of the final catalytic composite, calculated on an elemental basis. It is preferred that the platinum content of the catalyst be between about 0.1 and 1 wt.%. The preferred platinum group component is platinum, with palladium being the next preferred metal. The platinum group component may be incorporated into the catalyst composite in any suitable manner such as by coprecipitation or cogelation with the preferred carrier material, or by ion-exchange or impregnation of the carrier material. The preferred method of preparing the catalyst normally involves the utilization of a water-soluble, decomposable compound of a platinum group metal to impregnate the calcined carrier material. For example, the platinum group component may be added to the support by commingling the support with an aqueous solution of chloroplatinum or chloropalladic acid. An acid such as hydrogen chloride is generally added to the impregnation solution to aid in the distribution of the platinum group component throughout the carrier material. Additionally, the preferred catalyst contains an alkali metal component chosen from cesium, rubidium, potassium, sodium, and lithium. The preferred alkali metal is normally either potassium or lithium, depending on the feed hydrocarbon. The concentration of the alkali metal may range from about 0.1 to 5 wt.%, but is preferably between 1 and about 4 wt.% calculated on an elemental basis. This component may be added to the catalyst by the methods described above as a separate step or simultaneously with the solution of another component. With some alkali metals, it may be necessary to limit the halogen content to less than 0.5 wt.% and preferably less than 0.1 wt.%, while others may have higher halogen content. As noted previously, the dehydrogenation catalyst may also contain promoter metal. One such preferred promoter metal is tin. The tin component should constitute about 0.01 to about 1 wt.% tin. It is preferred that the atomic ratio of tin to platinum be between 1:1 and about 6:1. The tin component may be incorporated into the catalytic composite in any suitable manner known to effectively disperse this component in a very uniform manner throughout the carrier material. Thus, the component may be added to the carrier by coprecipitation. A preferred method of incorporating the tin component involves coprecipitation during the preparation of the preferred carrier material. This method typically involves the addition of a suitable soluble tin compound, such as stannous or stannic chloride to an alumina hydrosol, mixing these ingredients to obtain a uniform distribution throughout the sol and then combining the hydrosol with a suitable gelling agent and dropping the resultant admixture into an oil bath. The tin component may also be added through the utilization of a soluble decomposable compound of tin to impregnate the calcined porous carrier material. A more detailed description of the preparation of the carrier material and the addition of the platinum component and the tin component to the carrier material may be obtained by reference to U.S. Patent 3,745,112. Operation of the dehydrogenation zone will produce a mixture of hydrogen and hydrocarbons. Normally, a portion of the hydrocarbons will include an equilibrium mixture of the desired isoolefin and its isoalkane precursor. Additional hydrocarbons having fewer carbon atoms than the desired isoolefin also form part of the effluent, originate as impurities in the feed or are produced by side reactions in the dehydrogenation zone. These additional hydrocarbons will usually comprise methane, ethane, ethylene, propylene and propane. Where the dehydrogenation effluent goes to an etherification process for the reaction of C₅ isoolefins to produce ethers, such as tertiary amyl ether (TAME), C₄ hydrocarbons may be part of the additional hydrocarbons which enter the etherification zone. Effluent from the dehydrogenation reaction section passes to a hydrogen recovery section. This separation section removes hydrogen from the effluent and recovers it in high purity for recycle to the dehydrogenation reaction section. Separation steps for the removal of hydrogen will normally include cooling and compressing with subsequent cooling and flashing in a separation vessel. Such methods for the separation of hydrogen and light gases are well known by those skilled in the art. The advantages of this invention can be realized by operating the hydrogen recovery section to allow essentially all C₃ and higher hydrocarbons to pass through the olefin conversion zone. At minimum, these steps will remove primarily hydrogen and methane from the dehydrogenation zone effluent. These separation facilities are preferably designed to reduce the concentration of hydrogen and methane in the effluent with minimum loss of C₄+. Reduction of hydrogen and methane will, as explained later in more detail, allow the etherification zone to operate without an excessive increase in pressure over that required for operation of etherification process with a more complete removal of light end materials. In other embodiments these facilities can be designed to remove substantial quantities of C₁ and C₂ hydrocarbons in addition to hydrogen. To the extent that liquid phase conditions are desired in the etherification zone, removal of these light gases will permit reduction of the etherification zone operating pressure. The advantages associated with the removal of additional C₂ hydrocarbons must be balanced against the loss of additional product hydrocarbons such as C₄ and higher hydrocarbons. After removal of at least the hydrogen, substantial methane, and some ethane/ethylene the remaining light hydrocarbons and undehydrogenated hydrocarbons are passed with the olefins to an etherification zone. In the etherification zone, olefins are combined with one or more monohydric alcohols to obtain an ether compound having a higher boiling point than the olefin precursor. In order to obtain complete conversion, an excess of the alcohol is usually present in the etherification zone. It has now been found that the presence of hydrocarbons having fewer carbon atoms than the olefin reactants will not unduly interfere with the operation of the etherification zone. The major changes in the etherification zone resulting from the presence of the additional light materials such as methane, ethane, ethylene, etc. will be an increased pressure and additional throughput. It has also been discovered that these changes will be relatively small and will not interfere with the olefin reactions or increase the operational utilities, particularly, when substantial methane is removed with hydrogen. Another characteristic of most etherification processes that contributes to the advantages of this invention is that they can convert essentially all of the isoolefins having a particular range of carbon numbers to a higher boiling ether. A preferred etherification process is one for the production of MTBE. Converting essentially all of the isobutene to MTBE eliminates the need for separating that olefin from isobutane. As a result, downstream separation facilities are simplified and operated more economically since these facilities need to handle a reduced volume of closely boiling materials. Several suitable etherification processes have been described in the available literature, with these processes being presently used to produce MTBE. The preferred form of the etherification zone is similar to that described in U.S. Patent 4,219,678. In this instance, the isobutene or other isoolefin, methanol or other feed alcohol, and a recycle stream containing recovered excess alcohol are passed into the etherification zone and contacted with an acidic catalyst while maintained at etherification conditions. A wide range of materials are known to be effective as etherification catalysts for the preferred reactants including mineral acids such as sulfuric acid, boron trifluoride, phosphoric acid on kieselguhr, phosphorus-modified zeolites, heteropoly acids, and various sulfonated resins. The use of a sulfonated solid resin catalyst is preferred. These resin type catalysts include the reaction products of phenolformaldehyde resins and sulfuric acid and sulfonated polystyrene resins including those crosslinked with divinylbenzene. Further information on suitable etherification catalysts may be obtained by reference to U.S. Patents 2,480,940, 2,922,822, and 4,270,929 and the previously cited etherification references. A wide range of operating conditions are employed in processes for producing ethers from olefins and alcohols. Many of these include vapor, liquid or mixed phase operations. Processes operating with vapor or mixed phase conditions may be suitably employed in this invention. The preferred etherification process uses liquid phase conditions. The range of etherification conditions for processes operating in liquid phase still includes a broad range of suitable conditions including a superatmospheric pressure sufficient to maintain the reactants as a liquid phase, generally below about 50 bars, and a temperature between about 30°C (85°F) and about 100°C (210°F). Even in the presence of additional light materials, pressures in the range of 10 to 40 bars are sufficient. A preferred temperature range is from 50°C (120°F) to 100°C (210°F). The reaction rate is normally faster at higher temperatures but conversion is more complete at lower temperatures. High conversion in a moderate volume reaction zone can, therefore, be obtained if the initial section of the reaction Zone, e.g., the first two-thirds, is maintained above 70°C (160°F) and the remainder of the reaction zone is maintained below 50°C (120°F). This may be accomplished most easily with two reactors. The ratio of feed alcohol to isoolefin should normally be maintained in the broad range of 1:1 to 2:1. With the preferred reactants, good results are achieved if the ratio of methanol to isobutene is between 1.05:1 and 1.5:1. An excess of methanol, above that required to achieve satisfactory conversion at good selectivity, should be avoided as some decomposition of methanol to dimethylether may occur which may increase the load on separation facilities. The effluent from the etherification zone includes at least product ethers, C₃ hydrocarbons, dehydrogenatable hydrocarbons, and any excess alcohol. The effluent includes C₁-C₂ hydrocarbons, and may include small amounts of hydrogen that were dissolved with the feed components and small amounts of other oxygen-containing compounds (oxygenates) that were formed in the etherification zone such as dimethyl ether. The effluent from the etherification zone passes from the etherification zone to a separation section for the recovery of product. The first task in the separation section is to separate the ether product from the effluent of the etherification zone. The product ethers are typically withdrawn as a bottoms stream from a fractionation column. The initial separation between the ether products and the remainder of the etherification zone effluent can be performed in a single column. Depending upon the specification for the ether product, it may be suitable for use as withdrawn from the bottom of the separation column or may require additional separation to remove methanol which may be present in the form of an azeotrope mixture of the product ether. The column will also provide at least one additional separator stream made up of a lighter fraction that contains reactants for the dehydrogenation zone such as isoalkane and alcohol reactants for use in the etherification zone which make up in part the remainder of the etherification zone effluent. Alcohol present in the separator stream is unreacted excess alcohol in an amount equivalent to its azeotropic composition with the hydrocarbons. Any alcohol, in excess of the amount taken as an azeotrope with the separator stream, will leave the separator with the ether product and may be recovered by additional fractionation steps as previously described. The cut containing the reactants will also contain C₃ hydrocarbons and C₁-C₂ hydrocarbons and in most cases may contain some hydrogen. The separation section can be arranged to further separate hydrogen and C₁-C₂ hydrocarbons from the cut containing the reactants. This can be done, for example, in a reflux system on the top of the distillation column that condenses the heavier components of the reactant cut for liquid recycle to the column and venting of the lighter hydrogen and hydrocarbon gases. In the preferred embodiment of this invention, a reactant stream deficient only in the etherification product is recovered from the etherification separation section. The reactant cut from the etherification separation section enters a methanol recovery unit. The methanol recovery unit extracts methanol from the reactant cut. The methanol recovery unit can use any methanol recovery technique that effect a substantially complete recovery of methanol and reduces its concentration in the reactant cut to approximately less than 10 wt. ppm. The preferred alcohol recovery system will be a water washed system that absorbs alcohol from the remaining hydrocarbons in the reactant stream and includes a separation column for recovery of the methanol and recycle of the water. Another type of methanol recovery unit will use a solid adsorbent to preferentially adsorb the alcohol component from the reactant cut. At least a portion of the alcohol separated in the methanol recovery unit is recycled to the etherification zone to provide a portion of the methanol reactant. After separation of alcohol, the remainder of the reactant cut enters another separation section. This second section divides the reaction cut into a recycle stream made up of isoalkanes that will be recycled to the dehydrogenation zone and a lighter fraction having a lower boiling point then the recycled isoalkanes. Where the etherification zone produces MTBE, the second separation zone will function as a depropanizer and recover an isobutane bottoms stream for recycle to the dehydrogenation zone. A relatively lighter hydrocarbon stream made up of C₃ and lighter hydrocarbons is recovered overhead. In most cases, the second separation zone can be designed as a single column with the recycle stream recovered as a bottoms streams and the lighter hydrocarbons taken overhead. This separator can also be operated to remove the unwanted oxygen-containing compounds that may be formed as by-products in the etherification zone, or that were not removed in the ether and alcohol separation steps. These materials are referred to collectively herein as oxygenates. One such compound that can be removed overhead by the second separator is dimethyl ether which has a lower boiling than propane. Where a water-wash system is used for the methanol recovery unit, the second separation zone can also be operated to remove entrained as well as soluble water from the dehydrogenatable hydrocarbons. When separating the isoalkanes or dehydrogenatable hydrocarbons the separation facilities normally need not provide a good cut between the light ends and the dehydrogenatable hydrocarbons. Since the dehydrogenation zone can normally tolerate these light hydrocarbons, allowing some light hydrocarbons to pass with dehydrogenatable hydrocarbons eases the severity of the separation zone. This invention will be further described in the context of an example for the production of MTBE. The description of this invention in terms of this specific process example is not meant to limit this invention to the particular details disclosed herein. This example is based on engineering calculations and experience with the operation of similar process units. The Figure provides a schematic drawing for this type of operation. The drawing shows only the equipment that is useful in the description of the process. The utilization of other miscellaneous hardware such as heaters, coolers, valves, reboilers, pumps, instrumentation, and controls have been omitted as not essential to a clear understanding of the process, the use of such hardware being well within the purview of one skilled in the art. Referring then to the drawing, a hydrocarbon input stream comprising isobutane is charged to line 21 from a deisobutanizer column which is not shown. The input stream is combined with a hereinafter described recycle isobutane stream 22 to obtain a dehydrogenation zone feed stream 23 which passes through a dehydrogenation separation section 12. In separation section 12, the dehydrogenation zone feed stream is heat exchanged and transported to dehydrogenation reactor section 10 by way of line 24 at a temperature of about 40°C (100°F) and at a pressure of about 3 bars (44 psig). A hydrogen-rich recycle stream from line 26 provides hydrogen to dehydrogenation section 10. The recycle hydrogen rate is set to provide a hydrogen/hydrocarbon ratio within the range previously specified. Within dehydrogenation section 10, hydrogen is mixed with the feed stream and the combined stream is further heat exchanged with the effluent from the dehydrogenation reactor effluent. After heat exchange, the combined stream is further heated to the desired reaction temperature before entering the reactors in zone 10. Preferably, dehydrogenation reactor section 10 comprises multiple stacked or side by side reaction zones, and a combined stream of hydrogen and hydrocarbon feed is processed serially through said zones each of which contains a particulate catalyst disposed as an annular-form bed movable downwardly through said zones. The combined stream is then processed through said annular-form beds in a substantially radial flow and, since the dehydrogenation reaction is endothermic in nature, intermediate heating of the reactant stream between zones is the preferred practice. The moving catalyst bed permits a continuous addition of fresh and/or regenerated catalyst and the withdrawal of spent catalyst. The moving bed system herein contemplated is illustrated in U.S. Patent 3,647,680 in conjunction with a continuous catalyst regeneration system, and in U.S. Patent 3,978,150 with reference to the dehydrogenation of paraffinic hydrocarbons. Regardless of the actual reactor details, the hot effluent stream is heat exchanged with the combined feed as previously described and recovered from the dehydrogenation section 10 through line 28. The composition of the effluent taken by line 28 is given in the Table. The reactor section effluent stream, at a temperature of about 95°C (200°F) and a pressure slightly above atmospheric is passed to dehydrogenation separation section 12. In separation section 12, the dehydrogenation effluent stream is cooled and compressed, and again cooled to obtain a dried reactor effluent vapor phase stream for further cooling and condensing where it is exchanged against feed stream 23 and finally introduced into one or more separators. The separators yield a liquid hydrocarbon phase and a hydrogen-rich vapor phase which, after heat exchange, exits separation section 12 at a temperature of about 40°C (100°F) and a pressure of about 3.4 bars (50 psig). A portion of the hydrogen-rich vapor phase, substantially equivalent to the net hydrogen product, is taken from separation section 12 through line 30 and processed for further use. The remainder of the hydrogen-rich vapor stream continues through line 26 and enters dehydrogenation reactor section 10 as previously described. The liquid hydrocarbon phase is pumped from separation section 12 through line 32 at a pressure of about 10 to 20 bars and at a temperature of about 65°C (150°F). The contents of line 32 have the relative composition given in the Table. Compositions in mol % Line 28 Line 32 Line 38 Line 42 Line 52 Line 54 Line 22 H₂ 53.0 Trace Trace -- TR TR -- C₁ 8.6 2 2 -- 3.5 30.3 -- C₂ 0.6 1 1 -- 1.73 15.2 -- C₃ 2.0 4 4 -- 7.0 52.3 1.1 isobutane 18.5 48 47.5 <0.5 82.6 1.8 93.0 isobutene 16.5 43 1 <0.5 1.73 -- 2.0 Other C₄'s 0.8 2 2 <0.5 3.5 -- 3.9 C₅ and heavier hydrocarbons -- -- -- -- -- -- -- MEOH -- -- 1 TR TR -- -- MTBE -- -- 41.5 99.0 -- -- -- DME and Other Oxygenates -- -- -- .5 0.05 0.4 TR 100 100 100 100 100 100 100 The contents of line 32 enter MTBE reaction section 14 to which methanol is added via line 34 to provide a 1:1 to 1.1:1 ratio of methanol to isobutylene. The added methanol consists of fresh methanol and recycle methanol added via line 36. The combined reactants pass through a sulfonic resin catalyst at temperature of 65°C (150°F) and a pressure of 10 to 15 bars (145 to 220 psig). An etherification zone effluent is withdrawn by line 38 and has the composition given in the Table. Line 38 carries the etherification zone effluent to etherification separation section 16 at a temperature of 45-70°C and a pressure of 5 to 15 bars. The etherification separation section 16 includes an ordinary tray-type column 40 of conventional design that receives the contents of line 38 at a tray elevation located at or below the column midpoint and divides the etherification zone effluent into three fractions. An MTBE product fraction at a purity of 99 plus % leaves the bottom of the column through line 42 and its composition is given in the Table. The remainder of the etherification zone effluent, withdrawn overhead via line 44, is cooled in exchanger 46 to a temperature of 40°C and split between reflux which line 48 returns to the column and a net overhead withdrawn via line 50. Methanol recovery section 18 receives the net overhead from line 50. The methanol recovery unit consists of a water wash system that extracts essentially all of the methanol. The amount of methanol withdrawn depends on the azeotropic composition at the overhead operating conditions. Line 36 returns the extracted methanol to the etherification zone in a manner previously described. Passage through section 18 reduces the concentration of methanol in the net overhead stream to less than 10 wt. ppm. Line 52 carries the net overhead stream, which has the composition given in the Table, at a temperature of 40°C and a pressure of 15-25 bars to depropanizer 20. Depropanizer column 20 is a fractionation column of ordinary construction. Column 52 splits the contents of line 52 into a bottoms stream and a net overhead stream. Line 22 carries the bottoms stream back to the dehydrogenation zone as the previously described recycle. The net overhead is withdrawn by line 54 and has the composition given in the Table. The Example shows that a high yield of MTBE product at relatively high purity can be obtained by the method of this invention. The flow requires only two simple separation facilities to obtain the MTBE product, an isobutane recycle and the removal of light materials following the etherification of the hydrogen deficient dehydrogenation zone recycle. In addition, the use of the depropanizer as a means of removing the water and oxygen-containing compounds allows the process to operate with only a methanol recovery unit and does not require an additional recovering unit for other oxygen containing compounds.

Claims: A process for producing an ether from an effluent stream (28) from a dehydrogenation zone (10), said effluent stream comprising one or more C₄ or C₅ isoolefins and isoalkanes, one or more C₃ hydrocarbons, one or more C₂ hydrocarbons, methane and hydrogen, which process comprises the steps of: (a) separating essentially all the hydrogen and a substantial portion of the methane from said dehydrogenation zone effluent stream (28) without separating all the C₂ hydrocarbons from said effluent stream; (b) passing the residual dehydrogenation zone effluent stream (32) containing one or more C₄ or C₅ isoolefins and isoalkanes, one or more C₃ hydrocarbons, one or more C₂ hydrocarbons and methane into an etherification zone (14) for reaction with one or more C₁ - C₅ monohydric alcohols (34) for essentially complete conversion of the isoolefin(s), thereby producing an etherification zone effluent stream (38) comprising isoalkane(s), alcohol(s), ether(s), C₃ hydrocarbon(s), and C₂ hydrocarbon(s); (c) passing the etherification zone effluent stream to a first separation zone (16) and recovering therefrom at least a first stream (42) comprising an ether product and a second stream (44) comprising isoalkane(s), alcohol(s) and C₃ and lighter hydrocarbons; (d) recovering alcohol(s) from the second stream (44) in an alcohol recovery zone (18) and returning at least a portion of the recovered alcohol(s) (36, 34) to the etherification zone (14); (e) passing the resulting alcohol-depleted second stream (52) from the alcohol recovery zone (18) to a second separation zone (20) to separate C₃ and lighter hydrocarbons and oxygenates (54) from said alcohol-depleted second stream, thereby obtaining a recycle stream (22) consisting essentially of isoalkane(s); and (f) recycling the recycle stream (22) to the dehydrogenation zone (10). A process as claimed in claim 1, characterised in that methanol is used as monohydric alcohol. A process as claimed in claim 2, characterised in that the dehydrogenation zone effluent stream (32) contains primarily C₄ isoolefin(s) and methyl tertiary butyl ether is produced. A process as claimed in any of claims 1 to 3, characterised in that the residual dehydrogenation zone effluent stream (32) comprises not less than 1 mol % C₂ hydrocarbons. A process as claimed in any of claims 1 to 4, characterised in that the etherification zone (14) contains a sulfonated solid resin catalyst and operates at a temperature in the range of from 30-100°C (85-210°F) and a pressure of from 10-14 mPa s (10-40 bars).

Inventors: SCOTT NORMAN H, VORA BIPIN VIRPAL, SCOTT, NORMAN H., VORA, BIPIN VIRPAL
Applicants: UOP INC, UOP
Application year: 1989
Main classification: C07C43
IPC codes: C07C43, C07C41