Process for purifying unprocessed C4 hydrocarbon mixtures

Abstract

The invention relates to a method for purifying an unprocessed C4 hydrocarbon mixture, the unprocessed C4 hydrocarbon mixture comprises at least 2% by weight of isobutene, at least 23% by weight of butene other than isobutene, less than 3% by weight of butadiene, at least 1.5 ppm by weight of a catalyst deactivator selected from the group of polar nitrogen-containing compounds and mixtures thereof, and the balance of unprocessed C4 hydrocarbon mixtures. Wherein the sum of all the components in the C4 hydrocarbon mixture is 100% by weight, comprising the steps of (d) contacting the unprocessed C4 hydrocarbon mixture in countercurrent with an aqueous stream in an extraction unit to obtain an intermediate C4 hydrocarbon mixture, (e) discharging at least a portion of the intermediate C4 hydrocarbon mixture from the extraction unit, and (f) dehydrating the discharged intermediate C4 hydrocarbon mixture to obtain a purified C4 hydrocarbon mixture having a catalyst deactivator content of up to 1 ppm by weight. The invention further relates to a process for obtaining isobutene from a C4 hydrocarbon mixture containing isobutene in a plant comprising an etherification unit, a first distillation unit, an ether cracking unit and a second distillation unit.

Description

[0001] The present invention relates to a process for purifying a crude C4 hydrocarbon mixture comprising at least 2% by weight of isobutene, at least 23% by weight of butenes other than isobutene, less than 3% by weight of butadiene, at least 1.5 ppm by weight of a catalyst deactivator selected from the group of polar nitrogen-containing compounds and mixtures thereof. The present invention further relates to a process for obtaining isobutene from a C4 hydrocarbon mixture containing isobutene in a plant comprising an etherification unit, a first distillation unit, an ether cracking unit and a second distillation unit.

[0002] The C4 fraction from a steam cracker or fluid catalytic cracker (FCC) unit is essentially composed of butadiene, isobutylene, 1-butene and 2-butene and the saturated hydrocarbons isobutane and normal butane. Conventional post-processing methods for such C4 fractions worldwide include the following steps: First, the main part of butadiene is removed. The hydrocarbon mixture is called raffinate 1, which includes saturated hydrocarbons and isobutylene, 1-butene and 2-butene residues. A possible way to remove isobutylene from this mixture is to react with primary alcohols to form alkyl tert-butyl ethers. This leaves saturated hydrocarbons and linear butenes. The C4 mixture obtained after removing butadiene and isobutylene is called raffinate 2.

[0003] Document EP 0003305 A2 discloses a method for removing isobutene from a C4 hydrocarbon mixture containing isobutene, the method comprising (a) reacting the mixture with a primary alcohol in the presence of an acidic ion exchange resin to form an alkyl tert-butyl ether; (b) distilling the reaction mixture to obtain an overhead product comprising unconverted hydrocarbons and a bottom product comprising the alkyl tert-butyl ether; (c) feeding the bottom product to an ether cracking unit to decompose the alkyl tert-butyl ether to obtain isobutene and a primary alcohol; (d) distilling the mixture of isobutene and primary alcohol produced in step (c) to obtain an overhead product comprising isobutene and a bottom product comprising the primary alcohol; and (e) recycling the bottom product of step (d) to step (a).

[0004] Document CN 1158228 C discloses a method for producing isoolefins and / or tertiary alkyl ethers by reacting a mixed hydrocarbon stream containing isoolefins with an alcohol to obtain a tertiary alkyl ether product. The tertiary alkyl ether product is separated in a distillation column, wherein a high-purity tertiary alkyl ether product is discharged from the stripping section of the distillation column in the form of a side draw, so as to reduce the equipment cost and energy consumption for preparing tertiary alkyl ethers and / or isoolefins.

[0005] Document CN 1239444 C discloses a method for producing isoolefins, which comprises (a) feeding a hydrocarbon mixture containing isoolefins and alcohols into an etherification reactor, (b) separating the obtained product mainly containing tertiary alkyl ethers in a first fractionator and a second fractionator, (c) heating the product by a heater, and then feeding it into an ether cracking reactor to obtain a product mainly containing unreacted tertiary alkyl ethers, isoolefins and alcohols, and (d) subjecting the product to high-boiling fraction removal through a third fractionator to obtain isoolefins.

[0006] Document US 5,446,231 A discloses a method for removing pollutants from a hydrocarbon stream, in particular from a C5 hydrocarbon stream. The C5 hydrocarbon stream is washed with a mixture comprising 50% methanol and 50% water in a countercurrent manner to extract nitrites from the C5 hydrocarbons into a water-methanol mixture. In addition, a method for recovering methanol from the extract stream by hydrogenating the nitrite to form amines is disclosed.

[0007] Document US2011 / 0282092 A1 discloses a method for reducing nitrogen-containing Lewis bases in molecular sieve oligomerization, wherein the nitrogen-containing Lewis base acts as a poison for the molecular sieve catalyst. Reducing its presence in the feed before it comes into contact with the molecular sieve extends the catalyst life.

[0008] Even if plants for reactive separation of isobutylene from C4 hydrocarbon mixtures are installed, there are still some challenges in operating these plants in terms of their operating window (i.e., the range of process conditions that should be met in order to ensure stable operation of the plant). One problem encountered during operation of such plants is the deactivation of the catalyst in the etherification reactor over time. Once the activity of the catalyst drops below a certain limit, the catalyst must be replaced, which requires reactor shutdown and additional costs.

[0009] The object of the present invention is to provide a process for the reactive separation of isobutene from C4 hydrocarbon mixtures via the formation and reverse cleavage of alkyl tert-butyl ethers which is more robust with respect to potential catalyst deactivation.

[0010] This object is achieved according to the invention by a process for purifying a crude C4 hydrocarbon mixture according to claim 1. This object is further achieved according to the invention by a process for obtaining isobutene from an isobutene-containing C4 hydrocarbon mixture according to claim 9. Advantageous variants of these processes are presented in claims 2 to 8 and 10.

[0011] A first subject of the present invention is a process for purifying a crude C4 hydrocarbon mixture comprising at least 2% by weight of isobutene, at least 23% by weight of butenes other than isobutene, less than 3% by weight of butadiene, at least 1.5 ppm by weight of a catalyst deactivator selected from the group of polar nitrogen-containing compounds and mixtures thereof, wherein the sum of all components in the C4 hydrocarbon mixture is 100% by weight,

[0012] The method comprises the following steps

[0013] (a) contacting a crude C4 hydrocarbon mixture in countercurrent with an aqueous stream in an extraction unit to obtain an intermediate C4 hydrocarbon mixture,

[0014] (b) discharging at least a portion of the intermediate C4 hydrocarbon mixture from the extraction unit, and

[0015] (c) dehydrating the discharged intermediate C4 hydrocarbon mixture to obtain a purified C4 hydrocarbon mixture having a catalyst deactivating agent content of at most 1 ppm by weight.

[0016] A second subject of the present invention is a process for obtaining isobutene from a C4 hydrocarbon mixture containing isobutene in a plant comprising an etherification unit, a first distillation unit, an ether cracking unit and a second distillation unit, the process comprising:

[0017] (i) contacting a C4 hydrocarbon mixture with a primary alcohol and reacting the mixture with the primary alcohol in the presence of an acidic catalyst in an etherification unit to form the corresponding alkyl tert-butyl ether;

[0018] (ii) distilling the reaction mixture from the etherification unit in a first distillation unit, removing the C4 hydrocarbon raffinate as an overhead product and the alkyl tert-butyl ether as a liquid or vaporous bottom product and, if the bottom product is removed in liquid form, evaporating it;

[0019] (iii) reacting the vapor bottoms in an ether cracking unit in the presence of an acidic catalyst to obtain isobutene and a primary alcohol as reaction products; and

[0020] (iv) The reaction mixture from the ether cleavage unit is distilled in a second distillation unit, isobutene is removed as overhead product and the primary alcohol is removed as bottom product and recycled to the etherification unit.

[0021] According to the present invention, the C4 hydrocarbon mixture supplied to the etherification unit is obtained by pretreating an unprocessed C4 hydrocarbon mixture containing a catalyst deactivating agent, the pretreatment comprising the steps of: (a) contacting the unprocessed C4 hydrocarbon mixture with an aqueous stream in countercurrent in an extraction unit to obtain an intermediate C4 hydrocarbon mixture, (b) discharging at least a portion of the intermediate C4 hydrocarbon mixture from the extraction unit, and (c) dehydrating the discharged intermediate C4 hydrocarbon mixture to obtain a C4 hydrocarbon mixture having a catalyst deactivating agent content of at most 1 ppm by weight, wherein the catalyst deactivating agent is selected from the group of amines, acetonitrile, ammonia, dimethylformamide and mixtures thereof.

[0022] It has been found that pre-treating an unprocessed C4 hydrocarbon mixture by washing out potential catalyst deactivating agents and reducing their content in the purified C4 hydrocarbon mixture to a value of at most 1 ppm significantly increases the catalyst life in the subsequent etherification reaction. Furthermore, providing a purification step prior to the etherification reaction allows the use of an unprocessed C4 hydrocarbon mixture having a much higher content of potential catalyst deactivating substances than would be possible without a purification step. This allows a more flexible operation of the overall process, since feeds having a higher and variable content of catalyst deactivating agents can be processed.

[0023] The unprocessed C4 hydrocarbon mixture suitable for the process of the present invention is obtained, for example, by thermal cracking or catalytic cracking of petroleum products, by pyrolysis of liquefied petroleum gas (LPG), naphtha, gas oil, etc., or by catalytic dehydrogenation of n-butane and / or n-butene. Typically, these C4 hydrocarbon mixtures contain C4 olefins and C4 paraffins in addition to isobutylene. They can also contain butadiene and acetylene, for example, 1-butyne and butene acetylene. The C4 hydrocarbon mixture containing butadiene can be used as it is or after removing butadiene from the C4 hydrocarbon mixture, for example, by extraction with a selective solvent. Typically, the C4 hydrocarbon mixture containing isobutylene contains 2% to 77% by weight, preferably 10% to 70% by weight, and in particular 20% to 60% by weight of isobutylene. Preferably, a C4 hydrocarbon mixture containing, in addition to isobutene, n-butane, isobutane, 1-butene, trans-2-butene and cis-2-butene (with or without 1,3-butadiene) is used. More preferably, for the process of the invention, a C4 hydrocarbon mixture without 1,3-butadiene is used, which is referred to as "raffinate 1".

[0024] Preferably, the unprocessed C4 hydrocarbon mixture comprises less than 1% by weight of components having less than four carbon atoms and less than 1% by weight of components having at least five carbon atoms.

[0025] Such unprocessed C4 hydrocarbon mixtures typically contain polar nitrogen compounds which may deactivate the acidic catalysts used for the etherification of isobutene with primary alcohols. Catalyst deactivators present in unprocessed C4 hydrocarbon mixtures are in particular amines, acetonitrile, ammonia, dimethylformamide and mixtures thereof.

[0026] Primary alcohols suitable for use in the process of the present invention are those which can react with isobutylene to form the corresponding alkyl tert-butyl ethers. Preferably, the primary alcohol is selected from the group consisting of methanol, ethanol, isopropanol and isobutanol. More preferably, the primary alcohol is isobutanol.

[0027] In a first embodiment of the second subject of the present invention, the primary alcohol is methanol and the alkyl tert-butyl ether is methyl tert-butyl ether (MTBE). A method for obtaining isobutene from a C4 hydrocarbon mixture containing isobutene in a plant comprising an etherification unit, a first distillation unit, an ether cracking unit and a second distillation unit comprises the following steps:

[0028] i) contacting a C4 hydrocarbon mixture with methanol and reacting the mixture with methanol in the presence of an acidic catalyst in an etherification unit to form methyl tert-butyl ether (MTBE);

[0029] ii) distilling the reaction mixture from the etherification unit in a first distillation unit, removing the C4 hydrocarbon raffinate as overhead product and removing MTBE as liquid or vaporous bottoms product and, if the bottoms product is removed in liquid form, evaporating it;

[0030] iii) reacting the vapor bottoms in an ether cracking unit in the presence of an acidic catalyst to obtain isobutene and methanol as reaction products; and

[0031] iv) distilling the reaction mixture from the ether cleavage unit in a second distillation unit, removing isobutene as overhead product and removing methanol as bottom product and recycling it to the etherification unit;

[0032] The C4 hydrocarbon mixture supplied to the etherification unit is obtained by pretreating an unprocessed C4 hydrocarbon mixture, the pretreating comprising the steps of: (a) contacting the unprocessed C4 hydrocarbon mixture with an aqueous stream in countercurrent in an extraction unit to obtain an intermediate C4 hydrocarbon mixture, (b) discharging at least a portion of the intermediate C4 hydrocarbon mixture from the extraction unit, and (c) dehydrating the discharged intermediate C4 hydrocarbon mixture to obtain a C4 hydrocarbon mixture having a catalyst deactivator content of at most 1 ppm by weight, wherein the catalyst deactivator is selected from the group consisting of amines, acetonitrile, ammonia, dimethylformamide and mixtures thereof.

[0033] In a second embodiment of the second subject of the present invention, the primary alcohol is ethanol and the alkyl tert-butyl ether is ethyl tert-butyl ether (ETBE). A process for obtaining isobutene from a C4 hydrocarbon mixture containing isobutene in an apparatus comprising an etherification unit, a first distillation unit, an ether cracking unit and a second distillation unit comprises the following steps:

[0034] i) contacting a C4 hydrocarbon mixture with ethanol and reacting the mixture with ethanol in the presence of an acidic catalyst in an etherification unit to form ethyl tert-butyl ether (ETBE);

[0035] ii) distilling the reaction mixture from the etherification unit in a first distillation unit, removing the C4 hydrocarbon raffinate as overhead product and removing ETBE as liquid or vaporous bottoms product and, if the bottoms product is removed in liquid form, evaporating it;

[0036] iii) reacting the vapour stream of step (c) in the presence of an acidic catalyst in an ether cracking unit to obtain isobutene and ethanol as reaction products; and

[0037] iv) distilling the reaction mixture from the ether cleavage unit in a second distillation unit, removing isobutene as overhead product and removing ethanol as bottom product and recycling it to the etherification unit;

[0038] The C4 hydrocarbon mixture supplied to the etherification unit is obtained by pretreating an unprocessed C4 hydrocarbon mixture, the pretreating comprising the steps of: (a) contacting the unprocessed C4 hydrocarbon mixture with an aqueous stream in countercurrent in an extraction unit to obtain an intermediate C4 hydrocarbon mixture, (b) discharging at least a portion of the intermediate C4 hydrocarbon mixture from the extraction unit, and (c) dehydrating the discharged intermediate C4 hydrocarbon mixture to obtain a C4 hydrocarbon mixture having a catalyst deactivator content of at most 1 ppm by weight, wherein the catalyst deactivator is selected from the group consisting of amines, acetonitrile, ammonia, dimethylformamide and mixtures thereof.

[0039] In a third embodiment of the second subject of the present invention, the primary alcohol is isopropanol and the alkyl tert-butyl ether is isopropyl tert-butyl ether (IPTBE). A process for obtaining isobutene from a C4 hydrocarbon mixture containing isobutene in a plant comprising an etherification unit, a first distillation unit, an ether cracking unit and a second distillation unit comprises the following steps:

[0040] i) contacting a C4 hydrocarbon mixture with isopropanol and reacting the mixture with the isopropanol in the presence of an acidic catalyst in an etherification unit to form isopropyl tert-butyl ether (IPTBE);

[0041] ii) distilling the reaction mixture from the etherification unit in a first distillation unit, removing a C4 hydrocarbon raffinate as an overhead product and removing IPTBE as a liquid or vaporous bottom product and, if the bottom product is removed in liquid form, evaporating it;

[0042] iii) reacting the vapour stream of step (c) in the presence of an acidic catalyst in an ether cracking unit to obtain isobutylene and isopropanol as reaction products; and

[0043] iv) distilling the reaction mixture from the ether cleavage unit in a second distillation unit, removing isobutene as overhead product and removing isopropanol as bottom product and recycling it to the etherification unit;

[0044] The C4 hydrocarbon mixture supplied to the etherification unit is obtained by pretreating an unprocessed C4 hydrocarbon mixture, the pretreating comprising the steps of: (a) contacting the unprocessed C4 hydrocarbon mixture with an aqueous stream in countercurrent in an extraction unit to obtain an intermediate C4 hydrocarbon mixture, (b) discharging at least a portion of the intermediate C4 hydrocarbon mixture from the extraction unit, and (c) dehydrating the discharged intermediate C4 hydrocarbon mixture to obtain a C4 hydrocarbon mixture having a catalyst deactivator content of at most 1 ppm by weight, wherein the catalyst deactivator is selected from the group consisting of amines, acetonitrile, ammonia, dimethylformamide and mixtures thereof.

[0045] In a fourth embodiment of the second subject of the invention, the primary alcohol is isobutanol and the alkyl tert-butyl ether is isobutyl tert-butyl ether (IBTBE). A process for obtaining isobutene from a C4 hydrocarbon mixture containing isobutene in a plant comprising an etherification unit, a first distillation unit, an ether cracking unit and a second distillation unit comprises the following steps:

[0046] i) contacting a C4 hydrocarbon mixture with isobutanol and reacting the mixture with the isobutanol in the presence of an acidic catalyst in an etherification unit to form isobutyl tert-butyl ether (IBTBE);

[0047] ii) distilling the reaction mixture from the etherification unit in a first distillation unit, removing a C4 hydrocarbon raffinate as an overhead product and removing IBTBE as a liquid or vaporous bottom product and, if the bottom product is removed in liquid form, evaporating it;

[0048] iii) reacting the vapour stream of step (c) in the presence of an acidic catalyst in an ether cracking unit to obtain isobutene and isobutanol as reaction products; and

[0049] iv) distilling the reaction mixture from the ether cleavage unit in a second distillation unit, removing isobutene as overhead product and removing isobutanol as bottom product and recycling it to the etherification unit;

[0050] The C4 hydrocarbon mixture supplied to the etherification unit is obtained by pretreating an unprocessed C4 hydrocarbon mixture, the pretreating comprising the steps of: (a) contacting the unprocessed C4 hydrocarbon mixture with an aqueous stream in countercurrent in an extraction unit to obtain an intermediate C4 hydrocarbon mixture, (b) discharging at least a portion of the intermediate C4 hydrocarbon mixture from the extraction unit, and (c) dehydrating the discharged intermediate C4 hydrocarbon mixture to obtain a C4 hydrocarbon mixture having a catalyst deactivator content of at most 1 ppm by weight, wherein the catalyst deactivator is selected from the group consisting of amines, acetonitrile, ammonia, dimethylformamide and mixtures thereof.

[0051] Purification steps

[0052] The unprocessed C4 hydrocarbon mixture is contacted with an aqueous stream in a countercurrent in an extraction unit to obtain an intermediate C4 hydrocarbon mixture. The extraction unit may include any device known in the art capable of extracting components from an organic phase into an aqueous phase by intimate contact of the C4 hydrocarbon mixture and the aqueous stream. For example, the extraction unit may include a tube equipped with a static mixer capable of achieving intimate mixing of the phases, and a subsequent phase separation device, such as a settler.

[0053] In a preferred embodiment, the extraction unit comprises an extraction column. The extraction column may be equipped with internal components, such as trays or packing. Preferably, the unprocessed C4 hydrocarbon mixture is fed to the lower part of the extraction column, more preferably to the lowest part of the internal components or the bottom of the column. Preferably, the aqueous stream is fed to the upper part of the extraction column, more preferably to the highest part of the internal components or the top of the column. The countercurrent of the upwardly flowing organic phase and the downwardly flowing aqueous phase enables the two phases to be closely mixed and the catalyst deactivating components to be transferred from the organic phase to the aqueous phase.

[0054] Preferably, the extraction column is operated at a pressure of 4 to 7 bara and a temperature of 30 to 60°C.

[0055] The aqueous phase is enriched in the lower part of the extraction column and is removed via the bottom outlet of the column. In a preferred embodiment, a portion of the aqueous bottom outlet stream is recycled to the extraction column. Preferably, 40 to 80% by weight of the bottom stream discharged from the bottom of the extraction column is recycled to the extraction column, more preferably to the upper part of the extraction column. It is further preferred that a portion of the recycle stream is fed to a stripping unit, wherein organic components are removed from the recycle stream. Preferably, fresh water is added directly to the recycle stream or to the extraction column in an amount corresponding to the amount of organic phase removed from the stripping unit.

[0056] The aqueous stream fed to the extraction unit preferably comprises 90 to 100% by weight water.More preferably, the aqueous stream is water.

[0057] Extracting the catalyst deactivating components from the unprocessed C4 hydrocarbon mixture into the aqueous phase produces an intermediate C4 hydrocarbon mixture containing water due to the extraction process. At least a portion of the intermediate C4 hydrocarbon mixture is discharged from the extraction unit. Preferably, the intermediate C4 hydrocarbon mixture is completely discharged from the extraction unit.

[0058] The discharged intermediate C4 hydrocarbon mixture is dehydrated to obtain a purified C4 hydrocarbon mixture. In a preferred embodiment, the intermediate C4 hydrocarbon mixture discharged from the extraction unit is supplied to a phase separation unit for dehydration. The phase separation unit preferably includes a filter, a coalescer and / or a phase separator. The filter, the coalescer and the phase separator can be provided according to the physical properties of the organic-aqueous mixture of the intermediate C4 hydrocarbon mixture. For example, if the water droplets in the organic-aqueous dispersion are quite large, the phase separator is sufficient to remove the aqueous phase from the organic phase. If the droplets are quite small, it is advantageous to provide a sequence of a filter-coalescer-phase separator so as to first increase their size before separating the droplets. It can also be advantageous to cool the intermediate C4 hydrocarbon mixture before entering the phase separation unit.

[0059] Etherification unit

[0060] The etherification is based on the selective reaction of primary alcohols with isobutene contained in an isobutene-containing C4 hydrocarbon mixture (such as raffinate 1). The product formed is the corresponding alkyl tert-butyl ether. Other C4 hydrocarbons do not participate in the etherification reaction. The etherification can be carried out, for example, in one or more stirred tanks or in one or more fixed bed reactors, the latter being preferred.

[0061] Diisobutylene is formed as the main by-product during the etherification reaction. Tert-butyl alcohol may be formed as a further by-product, especially if water and isobutylene are present over an acidic catalyst.

[0062] The etherification reaction is carried out in the presence of an acidic ion exchange resin (which acts as a heterogeneous etherification catalyst). The acidic ion exchange resin is a cation exchanger in acid form. In one embodiment, the acidic ion exchange resin comprises a sulfonic acid or phosphoric acid ion exchange resin. Preferably, the acidic ion exchange resin comprises a macroreticular ion exchange resin. Examples of suitable ion exchange resins are sulfonated phenolic resins, sulfonated resins derived from coumarone-indene condensation products, and particularly sulfonated polystyrene resins. In a preferred embodiment, the acidic ion exchange resin comprises a copolymer of styrene and divinylbenzene functionalized with sulfonic acid groups, such as a crosslinked styrene-divinylbenzene copolymer.

[0063] In embodiments, the acidic ion exchange resin may have a concentration of acidic ion exchange groups of at least about 1 milliequivalent of H+ / gram of dry resin. Typically, the amount of ion exchange resin is 0.01 to 1 liter total volume / liter reactor volume.

[0064] The etherification reaction is an equilibrium reaction. Therefore, a certain residence time is required to reach equilibrium. However, from a practical point of view, it is preferred to carry out the etherification continuously, in which case the quotient of the volume of the reaction zone (in volume units) and the flux (in volume units) / hour is generally 0.01 to 5 hours, preferably 0.02 to 1 hour, especially 0.03 to 1 hour.

[0065] Typically, the etherification reaction results in a conversion of not less than 90%, preferably not less than 95%, in particular not less than 96% of the isobutene contained in the C4 hydrocarbon mixture into alkyl tert-butyl ether.

[0066] The molar excess of primary alcohol relative to isobutylene is advantageous for achieving high conversion of isobutylene and suppressing the formation of isobutylene oligomers. As the molar ratio of primary alcohol to isobutylene increases, the conversion increases. Preferably, the molar ratio of primary alcohol to isobutylene contained in the C4 hydrocarbon mixture is 100:1 to 1:1, more preferably 20:1 to 1.2:1, especially 4:1 to 1.3:1.

[0067] The etherification can be carried out at atmospheric pressure. However, it is advantageous to carry out the etherification under excessive pressure, for example at 1.01 to 30 bar, in particular at 2 to 20 bar. Depending on the pressure and temperature, the isobutene-containing C4 hydrocarbon mixture can be used in liquid or gaseous form. Preferably, a liquid isobutene-containing C4 hydrocarbon mixture is used. The pressure will be maintained in the range of 12 to 20 bar to ensure that no gasification occurs in the etherification unit.

[0068] Preferably, the outlet temperature of the reaction mixture from the etherification unit is 25°C to 65°C, preferably 30°C to 60°C, especially 30°C to 50°C. Etherification is an exothermic reaction. Ether formation is favored at low temperatures. In order to achieve high reaction rates and high isobutene conversions and low by-product formation, the reactor system is preferably cascaded and a temperature of less than 70°C is applied. In an embodiment, a plurality of adiabatic fixed bed reactors are used in series, for example, three adiabatic fixed bed reactors. Typical reactor inlet temperatures are in the range of 30°C to 40°C. The conversion is highest in the first reactor, the second reactor converts the remaining isobutene, and the last reactor has a greater residence time to achieve equilibrium conditions for the etherification reaction.

[0069] As the catalyst ages, the major contribution to the overall conversion shifts from the first reactor to the second reactor. The reactor inlet temperatures are adjusted to achieve the desired conversion and depend on the activity of each catalyst. The inlet temperature of the third reactor will generally be the lowest and maintained as low as possible while still achieving equilibrium conditions at the outlet of the reactor.

[0070] Typically, the catalyst of the first reactor will be replaced more frequently than the catalysts of the second and third reactors because contaminants in the feedstock will deactivate the catalyst of the first reactor with a higher probability, and conversion is typically highest in the first reactor.

[0071] Providing parallel reactors in a reactor stage allows replacement of the catalyst without shutting down the entire etherification unit.Parallel connected reactors, such as two first reactors, two second reactors and / or two third reactors, may be provided in any reactor stage.

[0072] First distillation unit

[0073] The reaction mixture discharged from the etherification unit contains alkyl tert-butyl ether, diisobutylene, unconverted hydrocarbons and unreacted primary alcohols. The C4 hydrocarbons that do not participate in the etherification reaction are separated from the alkyl tert-butyl ether and the excess primary alcohol in the first distillation unit. The top product removed is a C4 hydrocarbon raffinate that is substantially free of isobutylene. Typically, the isobutylene content is 5% by weight or less, preferably 2.5% by weight or less, and especially 1.5% by weight or less. The isobutylene content in the top product is determined by the conversion rate in the etherification unit and the initial composition of the isobutylene-containing C4 hydrocarbon mixture (e.g., raffinate 1). The isobutylene content in the top product can be reduced by recycling a portion of the top product to the etherification unit.

[0074] Preferably, the combined amount of alkyl tert-butyl ether and / or diisobutyl ether in the top product is not more than 200 ppm by weight. The top product is also designated as "raffinate 2".

[0075] Preferably, the raffinate 2 product stream is discharged as a side draw at the top of the distillation column. Components having a lower boiling point than the boiling points of the raffinate 2 components are preferably discharged from the top condenser of the distillation column as waste gas. Those lighter components may include nitrogen, C3 hydrocarbons or potentially formed tertiary butanol.

[0076] The bottom product from the first distillation unit comprises mainly alkyl tert-butyl ether and diisobutylene and components having a higher boiling point than alkyl tert-butyl ether. The bottom product may or may not contain excess primary alcohols. Advantageously, a bottom product containing not more than 1,000 ppm by weight, preferably not more than 500 ppm by weight, in particular not more than 100 ppm by weight of C4 hydrocarbons is removed.

[0077] Conveniently, the first distillation unit is operated at a pressure of about 4 to 8 bar and the bottom temperature is from 165°C to 200°C, for example about 170°C.

[0078] In one embodiment of the process according to the invention, the alkyl tert-butyl ether-containing bottom product from the first distillation unit is discharged in the gas phase, for example in the form of a vapor side draw from a distillation column.

[0079] In another embodiment, the bottom product containing the alkyl tert-butyl ether from the first distillation unit is discharged in the liquid phase or in the form of a two-phase gas-liquid stream. In this case, the bottom product is vaporized. Possible vaporizers are all conventional types of vaporizers, such as falling film evaporators, spiral tubes, thin film evaporators, natural convection evaporators with external or internal circulation (e.g. Robert evaporators), or forced circulation evaporators. Preference is given to Robert evaporators or falling film evaporators.

[0080] In a preferred variant of this embodiment, the bottom product from the first distillation unit is vaporized in an evaporator, from which a purge stream containing high-boiling components having a higher normal boiling point than the boiling point of the alkyl tert-butyl ether is discharged.

[0081] It is further preferred to superheat the gas phase in order to prevent condensation on the ether cleavage catalyst due to endothermic reactions and pore condensation.

[0082] In a first preferred embodiment, the first distillation unit comprises a distillation column, the bottom product is discharged as a side stream from the distillation column at a stage below the feed stage, and a purge stream rich in high-boiling components is discharged from a sump of the distillation column. The side stream can be discharged in vapor or liquid form. Discharging the bottom product as a side stream has the following advantage: the high-boiling components in the bottom product that may cause degradation of the ether cracking process are greatly reduced.

[0083] For this embodiment, it is further preferred that the purge stream from the sump of the distillation column in the first distillation unit is supplied to the by-product separation unit. The purge stream containing high boiling point components can be separated into valuable products in the by-product separation unit. This improves the overall efficiency of the isobutylene separation process by reducing the loss of by-products.

[0084] In a second preferred embodiment, the bottom product from the first distillation unit is vaporized in an evaporator, and a purge stream containing high-boiling components having a normal boiling point higher than that of the alkyl tert-butyl ether is discharged from the evaporator. Removing high-boiling components in the evaporator before the ether cracking unit has the advantage of preventing potential accumulation of those high-boiling components in a closed process, which results in an increased capacity of the isobutylene separation process. In addition, potential catalyst deactivation in the ether cracking unit is prevented.

[0085] For this embodiment, it is further preferred that the evaporator is a natural circulation evaporator, in particular a Robert type evaporator, and that the purge stream is discharged from the liquid phase in the bottom of the evaporator. Thus, high boiling point components can be removed in an easy and efficient manner.

[0086] For this embodiment, it is further preferred that the purge stream from the evaporator is fed to the by-product separation unit. The purge stream containing high boiling point components can be separated into valuable products in the by-product separation unit. This improves the overall efficiency of the isobutylene separation process by reducing the loss of by-products.

[0087] Ether cracking unit

[0088] In the ether cleavage unit, the alkyl tert-butyl ether is decomposed into isobutylene and primary alcohols at elevated temperature in the presence of an acid catalyst. Preferably, the bottom product containing the alkyl tert-butyl ether from the first distillation unit is transferred to the ether cleavage unit without removing any excess primary alcohol that may be present. Alternatively, part or all of the primary alcohol may be removed.

[0089] The decomposition of the alkyl tert-butyl ether is carried out in the gas phase over an acid catalyst. It can be carried out batchwise, but is preferably carried out continuously.

[0090] The ether cleavage reaction is an equilibrium reaction and high temperature is favorable for cleavage. Typical conversion is greater than 90%. The ether cleavage reaction can be carried out in one or more reactors connected in series and / or in parallel. Available reactors include heated tubular reactors, such as steam heated tubular reactors, or two reactor systems consisting of a heated tubular reactor and a second heated tubular reactor or an adiabatic fixed bed reactor. As an alternative, the ether cleavage reaction can be carried out in a two-phase system using a two-phase reactor.

[0091] Examples of suitable acid catalysts are ion exchangers in acid form, for example sulfonated coals, sulfonated phenolic resins, sulfonated resins derived from coumarone-indene condensation products and, in particular, sulfonated polystyrene resins, for example sulfonated, crosslinked styrene-divinylbenzene copolymers.

[0092] Other catalysts that can be advantageously used are solid phosphoric acid catalysts comprising monophosphoric acid or preferably polyphosphoric acid on a solid support. Examples of suitable supports for phosphoric acid catalysts are aluminum oxide, silicon dioxide, activated carbon, diatomaceous earth (kieselguhr) or pumice (pumice). Silica gel is a preferred support.

[0093] Other suitable acid catalysts are metal sulfates, for example, sodium bisulfate, calcium bisulfate, aluminum sulfate, nickel sulfate, copper sulfate, cobalt sulfate, cadmium sulfate and strontium sulfate. These sulfates can be used without support, but are preferably used on a support. Examples of suitable supports are silica gel, activated carbon, aluminum oxide and pumice.

[0094] Further suitable catalysts for the decomposition are silica gel or alumina as such.

[0095] In a further embodiment of the method according to the invention, metal phosphates, in particular metal hydrogen phosphates, are used as acid decomposition catalysts. These phosphates may also contain an excess of phosphoric acid, for example up to 65%, preferably 1% to 50%, in particular 10% to 20%, than the amount corresponding to the stoichiometric composition of the acidic metal phosphate. Examples of such metal phosphates are magnesium phosphate, calcium phosphate, strontium phosphate, barium phosphate, manganese phosphate, nickel phosphate, copper phosphate, cobalt phosphate, cadmium phosphate, iron (II) phosphate, chromium phosphate and in particular aluminum phosphate. The metal phosphate catalyst can be used as is or on a support. Examples of suitable supports are aluminum oxide, silicon dioxide, activated carbon and zinc oxide.

[0096] The amount of acid catalyst is generally about 0.01 to 1 kg, preferably about 0.03 to 0.3 kg per kg of alkyl tert-butyl ether passing through the reactor per hour. Preferably, a fixed bed reactor is used for the decomposition of the alkyl tert-butyl ether.

[0097] The decomposition temperature of the tertiary ether varies with the nature of the acid catalyst and the contact time, but is generally 50° C. to 350° C., preferably 80° C. to 300° C., especially 100° C. to 250° C. If a metal phosphate or phosphoric acid catalyst is used as the decomposition catalyst, the decomposition is generally carried out at 80° C. to 350° C., preferably 90° C. to 260° C., especially 170° C. to 210° C.

[0098] The contact time of the vaporized alkyl tert-butyl ether is advantageously from 0.1 to 20 seconds, preferably from 1 to 10 seconds.

[0099] The decomposition of the alkyl tert-butyl ether can be carried out at atmospheric pressure, but is generally carried out at excess pressure, for example at up to 30 bar, preferably up to 20 bar. Advantageously, the decomposition of the alkyl tert-butyl ether is carried out at a pressure of 2 to 15 bar, preferably 3 to 12 bar, in particular 4 to 12 bar. However, the decomposition can also be carried out under reduced pressure.

[0100] In an embodiment, the ether cleavage unit comprises a first ether cleavage reactor and a second ether cleavage reactor connected in series. Due to the high initial activity of the first ether cleavage reactor, almost complete conversion is achieved in this reactor. The absence of cleavable ether in the output of the first reactor may cause undesirable side reactions in the second reactor, such as dehydration of primary alcohols to water and isobutylene. When the activity of the first ether cleavage reactor decreases over time and the output of the first ether cleavage reactor contains a predetermined concentration of alkyl tert-butyl ether, the second ether cleavage reactor is put into operation.

[0101] In a preferred embodiment, the first ether cleavage reactor and the second ether cleavage reactor are periodically alternated in sequence and / or the flow direction through the first ether cleavage reactor and / or the second ether cleavage reactor is periodically changed. The possibility to switch between the two reactors and to periodically change the flow direction through the reactors can lead to a more uniform deactivation over the length of the reactor and a better reaction control. Ultimately, the overall run time can be improved.

[0102] Second distillation unit

[0103] The reaction mixture obtained from the ether cleavage unit, which contains isobutene and primary alcohols as reaction products, is fed to a second distillation unit. In the second distillation unit, high-purity isobutene is separated from heavier components such as primary alcohols, unreacted alkyl tert-butyl ethers and further heavier compounds such as diisobutene by distillation.

[0104] In a first embodiment of the subject matter of the present invention, wherein the primary alcohol is methanol and the alkyl tert-butyl ether is methyl tert-butyl ether (MTBE), the second distillation unit preferably comprises a methanol extraction unit and an isobutylene purification column. The reaction mixture from the ether cleavage unit is supplied to the methanol extraction unit, where it is contacted with a solvent, preferably water as a solvent, in countercurrent. A stream rich in water and methanol is discharged from the bottom of the extraction unit for further processing of methanol and recycling of methanol to the etherification unit. An isobutylene-rich stream is discharged from the top of the extraction unit and supplied to an isobutylene purification column, where isobutylene is separated from higher boiling components and discharged in the form of an overhead product of the isobutylene purification column. Preferably, the isobutylene product stream is discharged as a side draw at the top of the isobutylene purification column, while the column is operated under total liquid reflux. Components with a boiling point lower than that of isobutylene are preferably discharged from the top condenser of the column as offgas. Those lighter components may contain nitrogen or C3 hydrocarbons.

[0105] In a second embodiment of the subject matter of the present invention, wherein the primary alcohol is ethanol and the alkyl tert-butyl ether is ethyl tert-butyl ether (ETBE), the second distillation unit preferably comprises an ethanol extraction unit and an isobutylene purification column. The reaction mixture from the ether cleavage unit is supplied to the ethanol extraction unit, where it is contacted with a solvent, preferably water as a solvent, in countercurrent. A stream rich in water and ethanol is discharged from the bottom of the extraction unit for further processing of the ethanol and recycling of the ethanol to the etherification unit. An isobutylene-rich stream is discharged from the top of the extraction unit and supplied to the isobutylene purification column, where isobutylene is separated from higher boiling components and discharged in the form of an overhead product of the isobutylene purification column. Preferably, the isobutylene product stream is discharged as a side draw at the top of the isobutylene purification column, while the column is operated under total liquid reflux. Components with a boiling point lower than that of isobutylene are preferably discharged from the top condenser of the column as offgas. Those lighter components may contain nitrogen or C3 hydrocarbons.

[0106] In a third embodiment of the subject matter of the present invention, in which the primary alcohol is isopropanol and the alkyl tert-butyl ether is isopropyl tert-butyl ether (IPTBE), the second distillation unit preferably comprises an isopropanol extraction unit and an isobutylene purification column. The reaction mixture from the ether cleavage unit is supplied to the isopropanol extraction unit, where it is contacted with a solvent, preferably water as a solvent, in countercurrent. A stream rich in water and isopropanol is discharged from the bottom of the extraction unit for further processing of ethanol and recycling of ethanol to the etherification unit. An isobutylene-rich stream is discharged from the top of the extraction unit and supplied to the isobutylene purification column, where isobutylene is separated from higher boiling components and discharged in the form of an overhead product of the isobutylene purification column. Preferably, the isobutylene product stream is discharged as a side draw at the top of the isobutylene purification column, while the column is operated under total liquid reflux. Components with a boiling point lower than that of isobutylene are preferably discharged from the top condenser of the column as offgas. Those lighter components may contain nitrogen or C3 hydrocarbons.

[0107] In a fourth embodiment of the subject matter of the invention, in which the primary alcohol is isobutanol and the alkyl tert-butyl ether is isobutyl tert-butyl ether (IBTBE), the second distillation unit preferably comprises a distillation column supplied with the reaction mixture from the ether cracking unit. Isobutene is discharged from the column as an overhead product, isobutanol and diisobutene as a bottom product and are recycled to the etherification unit. The bottom temperature is preferably from 150° C. to 200° C. and the pressure in the column is preferably from 4 to 8 bar. In a preferred variant, the isobutene product stream is discharged as a side draw at the top of the column, while the column is operated under total liquid reflux. Components having a lower boiling point than that of isobutene are preferably discharged from the top condenser of the column as offgas. Those lighter components may contain nitrogen or C3 hydrocarbons.

[0108] Advantageously, the top product contains not less than 99.3% by weight, preferably not less than 99.5% by weight, in particular not less than 99.7% by weight of isobutene. Preferably, isobutene containing not more than 500 ppm by weight, preferably not more than 100 ppm, in particular not more than 50 ppm by weight of primary alcohols is removed as the top product.

[0109] Preferably, the high purity isobutylene product is removed from a side draw from the second distillation unit near the top.Any water that may be produced as a by-product in the process, for example in the reflux drum of the top condenser of the distillation column, can be removed from the system.

[0110] The bottom product contains mainly primary alcohol, alkyl tert-butyl ether and diisobutylene. Advantageously, the bottom product contains 80 to 85% by weight of primary alcohol, 8 to 10% by weight of alkyl tert-butyl ether and 4 to 5% by weight of diisobutylene. The main part of the bottom product is recycled to the etherification unit. If necessary, the recycling stream can be supplemented with fresh primary alcohol.

[0111] By-product separation unit

[0112] In a preferred embodiment, the apparatus further comprises a by-product separation unit supplied by a portion of the purge stream of the first distillation unit and / or the bottom product of the second distillation unit. In case a portion of the bottom product of the second distillation unit is supplied to the by-product separation unit, the weight ratio of the bottom product directed to the by-product separation unit to the bottom product recycled to the etherification unit is preferably in the range of 1:20 to 1:5, more preferably about 1:10.

[0113] Providing a by-product separation unit supplied by a portion of the purge stream of the first distillation unit and / or the bottom product of the second distillation unit significantly increases the operating window of the apparatus for obtaining isobutene from a C4 hydrocarbon mixture containing isobutene. Removing a portion of the purge stream of the first distillation unit and / or the bottom product of the second distillation unit enables continuous removal of high boiling impurities from the system. Removal of high boiling components from the system can be accomplished in different ways. In a preferred embodiment, the by-product separation unit is supplied by the bottom purge stream of the first distillation unit. In another preferred embodiment, the by-product separation unit is supplied by a portion of the bottom product of the second distillation unit. In another preferred embodiment, the by-product separation unit is supplied by the bottom purge stream of the first distillation unit and a portion of the bottom product of the second distillation unit.

[0114] The process according to the invention is more robust to internal disturbances, like accumulation of impurities, as well as to external disturbances, like changes in feed composition. Furthermore, separation of the discharged part of the bottom product of the second distillation unit into three different fractions enables more specific recycling strategies, higher process integration and thus reduced operating costs. In contrast to known processes which discharge the valuable by-product diisobutene, diisobutene is easily recovered.

[0115] It is further preferred that the alcohol product stream enriched in primary alcohols is separated from the stream fed to the by-product separation unit.

[0116] More preferably, the primary alcohol is recycled to the etherification unit. The recycling of the primary alcohol improves the overall efficiency of the isobutene separation process by reducing the amount of fresh primary alcohol required for isobutene separation. In processes according to the prior art, the amount of primary alcohol contained in the purge stream taken from the second distillation unit is typically discharged.

[0117] In a further preferred embodiment, the mass fraction of primary alcohols in the alcohol product stream is at least 90 wt.-%, preferably at least 95 wt.-%. When the second by-product stream is recycled to the etherification unit, the high purity of the primary alcohols increases the overall efficiency of the isobutene separation process by reducing the amount of fresh primary alcohol required for isobutene separation.

[0118] In a preferred embodiment, the alkyl tert-butyl ether contained in the bottom product stream from the second distillation unit is separated in a by-product purification unit and recycled to the evaporator for evaporation in step (c), to the ether cracking unit, or to both the evaporator and the ether cracking unit. Separating and recycling the alkyl tert-butyl ether contained in the bottom product stream from the second distillation column improves the overall efficiency of the isobutylene separation process by reducing the loss of the valuable product isobutylene contained in the alkyl tert-butyl ether. In the process according to the prior art, the purge stream taken from the second distillation unit is typically discharged.

[0119] In a further preferred embodiment, the bottom product of the second distillation unit supplied to the by-product separation unit is divided into at least three by-product streams, wherein the first by-product stream is rich in diisobutylene, the second by-product stream is an alcohol product stream rich in primary alcohols, and the third by-product stream is rich in components having a normal boiling point above 110° C. Separating the discharged portion of the bottom product of the second distillation unit into three different fractions enables a more specific recycling strategy, higher process integration and thereby reduced operating costs. In contrast to known processes that discharge valuable by-product diisobutylene, diisobutylene is easily recovered.

[0120] Methanol / MTBE

[0121] In a first embodiment of the inventive subject matter, wherein the primary alcohol is methanol and the alkyl tert-butyl ether is methyl tert-butyl ether (MTBE), the by-product separation unit preferably comprises at least two distillation columns.

[0122] In one embodiment, the first distillation tower of the by-product separation unit is supplied by a portion of the purge stream of the first distillation unit and / or the bottom product of the second distillation unit, and the bottom product mainly contains methanol, MTBE and diisobutylene. A second by-product stream rich in methanol is discharged from the top of the first tower. The second by-product stream may further contain MTBE. The bottom product stream of the first tower is supplied to the second distillation tower of the by-product separation unit. The first by-product stream rich in diisobutylene is discharged from the top of the second tower, and the third by-product stream rich in components with a normal boiling point higher than 110°C is discharged from the bottom of the second tower. The third by-product stream may contain high boiling point components, like triisobutylene.

[0123] In another preferred embodiment, the by-product separation unit comprises at least three distillation towers. The first distillation tower of the by-product separation unit is supplied by a portion of the purge stream of the first distillation unit and / or the bottom product of the second distillation unit, which mainly contains methanol, MTBE and diisobutylene. The MTBE-rich stream is discharged from the top of the first tower. Components with a higher standard boiling point than MTBE are discharged from the bottom of the first tower and supplied to the second distillation tower of the by-product separation unit. A second by-product stream rich in methanol is discharged from the top of the second tower. The bottom product stream of the second tower is discharged and supplied to the third distillation tower of the by-product separation unit. The first by-product stream rich in diisobutylene is discharged from the top of the third tower, and a third by-product stream rich in components with a standard boiling point higher than 110°C is discharged from the bottom of the third tower. The third by-product stream may contain high boiling point components, such as triisobutylene.

[0124] Ethanol / ETBE

[0125] In a second embodiment of the inventive subject matter, wherein the primary alcohol is ethanol and the alkyl tert-butyl ether is ethyl tert-butyl ether (ETBE), the by-product separation unit preferably comprises at least two distillation columns.

[0126] In one embodiment, the first distillation tower of the by-product separation unit is supplied by a portion of the purge stream of the first distillation unit and / or the bottom product of the second distillation unit, and the bottom product mainly contains ethanol, ETBE and diisobutylene. The second by-product stream rich in ethanol is discharged from the top of the first tower. The second by-product stream may further contain ETBE. The bottom product stream of the first tower is supplied to the second distillation tower of the by-product separation unit. The first by-product stream rich in diisobutylene is discharged from the top of the second tower, and the third by-product stream rich in components with a normal boiling point higher than 110°C is discharged from the bottom of the second tower. The third by-product stream may contain high boiling point components, like triisobutylene.

[0127] In another preferred embodiment, the by-product separation unit comprises at least three distillation columns. The first distillation column of the by-product separation unit is supplied by a portion of the purge stream of the first distillation unit and / or the bottom product of the second distillation unit, and the bottom product mainly contains ethanol, ETBE and diisobutylene. The first distillation column is a pre-fractionation column, where a stream rich in ETBE and ethanol is discharged from the top of the column and a stream rich in diisobutylene and higher boiling point components is discharged from the bottom of the column. The top product stream is supplied to the second distillation column of the by-product separation unit, where a stream rich in ETBE is discharged from the top of the second column, and a second by-product stream rich in ethanol is discharged from the bottom of the second column. The bottom product stream of the first column is supplied to the third distillation column of the by-product separation unit, where a first by-product stream rich in diisobutylene is discharged from the top of the third column, and a third by-product stream rich in components with a normal boiling point higher than 110°C is discharged from the bottom of the third column. The third by-product stream may contain high boiling point components, such as triisobutylene.

[0128] Isopropyl alcohol / IPTBE

[0129] In a third embodiment of the inventive subject matter, wherein the primary alcohol is isopropanol and the alkyl tert-butyl ether is isopropyl tert-butyl ether (IPTBE), the by-product separation unit preferably comprises at least two distillation columns.

[0130] In one embodiment, the first distillation tower of the by-product separation unit is supplied by a portion of the purge stream of the first distillation unit and / or the bottom product of the second distillation unit, and the bottom product mainly contains isopropanol, IPTBE and diisobutylene. The second by-product stream rich in isopropanol is discharged from the top of the first tower. The bottom product stream of the first tower is supplied to the second distillation tower of the by-product separation unit. The first by-product stream rich in diisobutylene is discharged from the top of the second tower. The first by-product stream may further contain IPTBE. A third by-product stream rich in components with a standard boiling point higher than 110° C. is discharged from the bottom of the second tower. The third by-product stream may contain high boiling point components, such as triisobutylene.

[0131] In another preferred embodiment, the by-product separation unit comprises at least three distillation columns. The first distillation column of the by-product separation unit is supplied by a portion of the purge stream of the first distillation unit and / or the bottom product of the second distillation unit, the bottom product mainly containing isobutanol, IPTBE and diisobutylene. The first distillation column is a pre-fractionation column, where a stream rich in IPTBE and isopropanol is discharged from the top of the column and a stream rich in diisobutylene and higher boiling point components is discharged from the bottom of the column. The top product stream is supplied to the second distillation column of the by-product separation unit, where a second by-product stream rich in isopropanol is discharged from the top of the second column, and a stream rich in IPTBE is discharged from the bottom of the second column. The bottom product stream of the first column is supplied to the third distillation column of the by-product separation unit, where a first by-product stream rich in diisobutylene is discharged from the top of the third column, and a third by-product stream rich in components with a normal boiling point higher than 110°C is discharged from the bottom of the third column. The third by-product stream may contain high boiling point components, such as triisobutylene.

[0132] Isobutyl alcohol / IBTBE

[0133] In a fourth embodiment of the inventive subject matter, wherein the primary alcohol is isobutanol and the alkyl tert-butyl ether is isobutyl tert-butyl ether (IBTBE), the by-product separation unit preferably comprises at least two distillation columns.

[0134] In one embodiment, the first distillation column of the by-product separation unit is fed by a portion of the purge stream of the first distillation unit and / or the bottom product of the second distillation unit, which mainly contains isobutanol, IBTBE and diisobutylene. A first by-product stream rich in diisobutylene is discharged from the top of the first column. The bottom product stream of the first column is fed to the second distillation column of the by-product separation unit. A second by-product stream rich in isobutanol is discharged from the top of the second column, and a third by-product stream rich in components with a normal boiling point higher than 110° C. is discharged from the bottom of the second column. The third by-product stream mainly contains IBTBE and may contain additional high boiling point components, such as diisobutyl ether (DIBE) and / or triisobutylene.

[0135] In an alternative embodiment with two columns, the first distillation column of the by-product separation unit is fed by a portion of the purge stream of the first distillation unit and / or the bottom product of the second distillation unit, which mainly contains isobutanol, IBTBE and diisobutylene. A third by-product stream rich in components with a normal boiling point above 110° C. is discharged from the bottom of the first column. The third by-product stream mainly contains IBTBE and may contain additional high-boiling components like diisobutyl ether (DIBE) and / or triisobutylene. The top product stream of the first column is fed to the second distillation column of the by-product separation unit. A first by-product stream rich in diisobutylene is discharged from the top of the second column, while a second by-product stream rich in isobutanol is discharged from the bottom of the second column.

[0136] In another alternative embodiment with two columns, the first distillation column of the by-product separation unit is fed by the purge stream of the first distillation unit and / or a portion of the bottom product of the second distillation unit, which bottom product mainly contains isobutanol, IBTBE and diisobutylene. A third by-product stream rich in components with a normal boiling point above 110° C. is discharged from the bottom of the first column. The third by-product stream mainly contains IBTBE and may contain additional high-boiling components, like diisobutyl ether (DIBE) and / or triisobutylene. A second by-product stream rich in isobutanol is discharged from the first column as a side draw. The top product stream of the first column is fed to the second distillation column of the by-product separation unit. A first by-product stream rich in diisobutylene is discharged from the bottom of the second column. The top product of the second column is recycled back to the first column.

[0137] More preferably, the by-product separation unit comprises at least three distillation columns.

[0138] In one embodiment with three columns, the first distillation column of the by-product separation unit is fed by a portion of the purge stream of the first distillation unit and / or the bottom product of the second distillation unit, which mainly contains isobutanol, IBTBE and diisobutylene. A third by-product stream rich in components with a normal boiling point higher than 110° C. is discharged from the bottom of the first column. The third by-product stream mainly contains high-boiling components, such as triisobutylene. The top product stream of the first column is fed to the second distillation column. A second by-product stream rich in isobutanol is discharged from the bottom of the second column. The top product stream of the second column is fed to the third distillation column, where a first by-product stream rich in diisobutylene is discharged from the bottom of the column. The first by-product stream may also contain IBTBE and / or diisobutyl ether (DIBE).

[0139] In a preferred embodiment, the first by-product stream is further divided into at least two further by-product streams, wherein the fourth by-product stream is enriched in diisobutylene and the fifth by-product stream is enriched in alkyl tert-butyl ether. Providing further separation of the first by-product stream increases the flexibility of the isobutylene separation process, for example in terms of process integration by recycling the stream to a dedicated process unit. In addition, the overall efficiency of the process is improved.

[0140] For processes in which the first by-product stream is further divided into at least two further by-product streams, it is preferred that the mass fraction of diisobutene in the first by-product stream is at least 30 wt.-%, and the mass fraction of diisobutene in the fourth by-product stream is at least 90 wt.-%, more preferably at least 95 wt.-%, most preferably at least 98 wt.-%, in particular at least 99 wt.-%. Providing a by-product stream with high to very high purity diisobutene enables the use of this component in various applications and thereby improves the overall efficiency of the isobutene separation process.

[0141] For processes in which the first by-product stream is further divided into at least two further by-product streams, it is preferred to recycle the fifth by-product stream at least partially to the evaporator and / or the ether cracking unit for evaporation. By recycling the fifth product stream enriched in alkyl tert-butyl ether, the loss of the valuable product isobutene contained in the alkyl tert-butyl ether is reduced. This improves the overall efficiency of the isobutene separation process.

[0142] The invention is explained in more detail below with reference to the accompanying drawings. These drawings should be interpreted as presentations in principle. These drawings do not constitute any limitation of the invention, for example with respect to specific embodiments. In these figures:

[0143] Figure 1 A process flow diagram of an embodiment for purifying a crude C4 hydrocarbon mixture according to the present invention is shown.

[0144] Figure 2 A block diagram showing a first embodiment of the method according to the invention for obtaining isobutene from a C4 hydrocarbon mixture containing isobutene.

[0145] Figure 3 A block diagram showing a second embodiment of the method according to the invention for obtaining isobutene from a C4 hydrocarbon mixture containing isobutene.

[0146] Figure 4 A block diagram showing a third embodiment of the process according to the invention for obtaining isobutene from a C4 hydrocarbon mixture containing isobutene.

[0147] Figure 5 Experimental data according to examples are shown.

[0148] List of reference symbols used:

[0149] 10…Extraction unit

[0150] 11…Extraction tower

[0151] 12…Unprocessed C4 hydrocarbon streams

[0152] 13…Water flow

[0153] 14…Aqueous recirculation stream

[0154] 15…Aqueous bottom flow

[0155] 16…Intermediate C4 hydrocarbon stream

[0156] 20…Phase separation unit

[0157] 21…Filter

[0158] 22…Coalescer

[0159] 23…Phase separator

[0160] 24…purified C4 hydrocarbon stream

[0161] 30…etherification unit

[0162] 31…Primary alcohol feed stream

[0163] 32… Reaction mixture flow

[0164] 40…First distillation unit

[0165] 41…C4 hydrocarbon raffinate stream

[0166] 42…Alkyl tert-butyl ether bottoms

[0167] 43…High boiling point purge stream

[0168] 50…Ether cracking unit

[0169] 51… Reaction mixture flow

[0170] 60…Second distillation unit

[0171] 61…isobutylene product stream

[0172] 62…Primary alcohol recycle stream

[0173] 63…Byproduct separation unit feed stream

[0174] 70…Byproduct separation unit

[0175] 71…First by-product stream

[0176] 72…Second byproduct stream

[0177] 73…Third byproduct stream

[0178] 74…Fourth byproduct stream

[0179] 75…Fifth byproduct stream

[0180] Figure 1A process flow diagram of an extraction unit 10 as an exemplary embodiment for purifying an unprocessed C4 hydrocarbon mixture according to the present invention is shown. The extraction unit 10 includes a phase separation unit 20, which is represented by a dotted box surrounding the corresponding process unit. The extraction unit 10 further includes an extraction column 11, which is equipped with an inlet in the upper part of the column, an inlet in the lower part of the column, an inlet between the upper inlet and the lower inlet, a top outlet and a bottom outlet. An unprocessed C4 hydrocarbon stream 12 containing an unprocessed C4 hydrocarbon mixture is supplied to the extraction column 11 through the lower inlet. An aqueous stream 13 is supplied to the extraction column 11 through the upper inlet. The unprocessed C4 hydrocarbon mixture flowing upward is contacted with the aqueous stream in the extraction column 11 in countercurrent to produce an intermediate C4 hydrocarbon mixture, which is discharged from the top of the column as an intermediate C4 hydrocarbon stream 16.

[0181] The countercurrent of the upwardly flowing organic phase and the downwardly flowing aqueous phase enables intimate mixing of the two phases and transfer of catalyst deactivating components from the organic phase to the aqueous phase discharged through the bottom outlet. A portion of the aqueous stream discharged from the bottom of the extraction column 11 is recycled as aqueous recycle stream 14 and fed to the column through an intermediate inlet. The remainder is discharged from the extraction unit 10 as waste water in aqueous bottom stream 15.

[0182] Due to the close contact of the organic phase and the aqueous phase in the extraction column 11, the intermediate C4 hydrocarbon stream 16 contains water removed in the phase separation unit 20 including the filter 21, the coalescer 22 and the phase separator 23. The intermediate C4 hydrocarbon stream 16 is fed to the filter 21, where very small droplets that cannot be coalesced in the subsequent units are removed. The filtered droplets are removed from the filter 21 as a wastewater stream ( Figure 1 not shown).

[0183] The remainder of the intermediate C4 hydrocarbon stream is fed to a coalescer 22 where small droplets are coalesced into larger droplets. The resulting mixture is fed to a phase separator 23 where the organic phase is separated from the aqueous phase. The aqueous phase is collected in the phase separator 23 and removed continuously or discontinuously ( Figure 1 The aqueous stream is not shown). A purified C4 hydrocarbon stream 24 is discharged from the phase separator 23 and can be used in subsequent process steps.

[0184] Figure 2A block diagram of a first embodiment of a method for obtaining isobutene from a C4 hydrocarbon mixture containing isobutene according to the present invention is shown. In a first step, a raw C4 hydrocarbon mixture containing isobutene is pretreated in an extraction unit 10 to obtain a purified C4 hydrocarbon mixture. A raw C4 hydrocarbon stream 12 and an aqueous stream 13 are contacted in countercurrent in the extraction unit 10 to obtain an intermediate C4 hydrocarbon mixture. At least a portion of the intermediate C4 hydrocarbon mixture is discharged from the extraction unit and dehydrated to obtain a purified C4 hydrocarbon mixture stream 24. The remaining aqueous phase is discharged from the extraction unit as a wastewater bottom stream 15. The extraction unit 10 can be configured as described according to Figure 1 The illustrated process.

[0185] The purified C4 hydrocarbon feed stream 24 containing isobutylene and the primary alcohol feed stream 31 are fed to an etherification unit 30 comprising at least one reactor with an acidic catalyst, preferably an ion exchange resin. Advantageously, the etherification unit 30 comprises a fixed bed reactor, such as a flow tube or loop reactor or a combination of both types. The C4 hydrocarbon mixture is contacted with the primary alcohol and the mixture reacts in the presence of the acid catalyst to form the corresponding alkyl tert-butyl ether. Diisobutylene is obtained as a by-product.

[0186] The obtained reaction mixture 32 is supplied to the first distillation unit 40. The C4 hydrocarbon raffinate stream 41 is discharged as the top product of the first distillation unit 40. The bottom product stream 42 discharged from the first distillation unit 40 mainly contains alkyl tert-butyl ether and diisobutylene. Excess primary alcohols and heavy components having a normal boiling point typically higher than 110° C. may also be present in the bottom product stream 42. The bottom product stream 42 is discharged as a liquid or vapor stream. In the case of discharging it as a liquid stream, it is gasified in an evaporator. A purge stream 43 containing high-boiling components having a higher normal boiling point than that of the alkyl tert-butyl ether may be discharged from the first distillation unit 40.

[0187] The vaporous alkyl tert-butyl ether stream 42 is fed to an ether cracking unit 50 comprising at least one reactor with an acidic catalyst, preferably an ion exchange resin. Advantageously, at least one reactor in the ether cracking unit 50 is a fixed bed reactor. Isobutene and primary alcohols are obtained as reaction products.

[0188] The obtained reaction mixture 51 is supplied to the second distillation unit 60. A highly pure isobutylene product stream 61 is discharged as the top product of the second distillation unit 60. The bottom product discharged from the second distillation unit 60 mainly contains primary alcohols and diisobutylene. Heavier components having a normal boiling point typically above 110° C. may also be present in the bottom product. The bottom product of the second distillation unit 60 is recycled to the etherification unit 30 as a primary alcohol recycling stream 62. If necessary, the recycling stream may be supplemented with fresh primary alcohol.

[0189] Figure 3 A block diagram of a second embodiment of the process according to the invention for obtaining isobutene from a C4 hydrocarbon mixture containing isobutene is shown. In this embodiment, the main part of the bottom product of the second distillation unit 60 is recycled to the etherification unit 30 as a primary alcohol recycle stream 62, while the remaining smaller part of the bottom product of the second distillation unit 63 is supplied to a by-product separation unit 70. In this unit, the by-product separation unit feed stream 63 is divided into at least three by-product streams, preferably in three interconnected distillation columns. The first by-product stream 71 is rich in diisobutene and is removed from the device. The second by-product stream 72 is rich in primary alcohols and is recycled to the etherification unit 30. The third by-product stream 73 is rich in components with a normal boiling point above 110° C. and is also removed from the device.

[0190] Figure 4 A block diagram of a third embodiment of a method for obtaining isobutene from a C4 hydrocarbon mixture containing isobutene according to the present invention is shown. In this embodiment, the first by-product stream 71 is further divided into at least two further by-product streams, wherein the fourth by-product stream 74 is rich in diisobutene and the fifth by-product stream 75 is rich in alkyl tert-butyl ether. The fifth by-product stream 75 is at least partially recycled to the ether cracking unit 50. The first distillation unit 30 includes a distillation column, a bottom product 42 is discharged from the distillation column as a side stream at a stage below the feed stage, and a purge stream 43 rich in high-boiling components is discharged from the sump of the distillation column. The high-boiling purge stream 43 from the sump of the distillation column in the first distillation unit 30 is supplied to the by-product separation unit 70.

[0191] Examples

[0192] The unprocessed C4 hydrocarbon mixture is Figure 1 and Figure 2 Purified and further processed in the equipment. The unprocessed C4 hydrocarbon mixture is the raffinate 1 stream, whose composition changes over time and whose main components are as follows (values ​​in weight percentage):

[0193]

[0194] The unprocessed C4 hydrocarbon mixture contains up to 3 ppm and an average of more than 1.5 ppm of catalyst deactivators over time. The main component of the catalyst deactivator is amine.

[0195] The unprocessed C4 hydrocarbon mixture 12 is fed to the lower part of the extraction column 11 at a mass flow rate of from 17000 to 20000 kg / h. An aqueous stream 13 containing 99% to 100% of water by weight is fed to the upper part of the extraction column 11 at a mass flow rate of 2000 kg / h. The extraction column is operated at a pressure of 4 bar (absolute pressure) at the top of the column. At the bottom of the column, the pressure is 6.5 bar (absolute pressure) and the temperature is 38.2° C. An aqueous recycle stream 14 is recycled from the bottom of the column to the intermediate feed point at a mass flow rate of 5800 to 6000 kg / h.

[0196] The intermediate C4 hydrocarbon stream 16 is discharged from the top of the extraction column 11 and supplied to a phase separation unit 20 including a filter 21, a coalescer 22 and a phase separator 23. The dehydrated purified C4 hydrocarbon stream 24 is supplied to an etherification unit 30, where it is contacted with isobutyl alcohol to form isobutyl ether. The content of the catalyst deactivating agent is less than 1 ppm by weight, less than 0.5 ppm by weight on average over time. Among the catalyst deactivating components, amines constitute a major portion of less than 0.9 ppm by weight measured, less than 0.4 ppm by weight on average over time.

[0197] Figure 5 The conversion of isobutylene in the first reactor of the etherification unit 30 is shown over time. At the time point indicated by "t_start", the extraction unit 10 is put into operation. Before this time point, the unprocessed C4 hydrocarbon mixture is supplied to the etherification unit without purification. Figure 5 As can be seen in Figure 1, the removal of the catalyst deactivator by the purification method according to the invention enables a more robust and stable operation of the etherification unit. The life of the etherification catalyst increased from 3 months before the start-up of the extraction unit to more than 23 months thereafter.

Claims

1. A method for purifying an unprocessed C4 hydrocarbon mixture, the unprocessed C4 hydrocarbon mixture comprising at least 2% by weight of isobutylene, at least 23% by weight of butenes other than isobutene, Less than 3% by weight of butadiene, at least 1.5 ppm by weight of a catalyst deactivator selected from the group of polar nitrogen-containing compounds and mixtures thereof, wherein the sum of all components in the C4 hydrocarbon mixture is 100% by weight, The method comprises the following steps (a) contacting the crude C4 hydrocarbon mixture (12) in countercurrent with an aqueous stream (13) in an extraction unit (10) to obtain an intermediate C4 hydrocarbon mixture (16), (b) discharging at least a portion of the intermediate C4 hydrocarbon mixture (16) from the extraction unit, and (c) dehydrating the discharged intermediate C4 hydrocarbon mixture to obtain a purified C4 hydrocarbon mixture (24) having a catalyst deactivating agent content of at most 1 ppm by weight.

2. The method according to claim 1, wherein: The extraction unit (10) comprises an extraction column (11), the unprocessed C4 hydrocarbon mixture (12) is fed to the lower part of the extraction column (11), and the aqueous stream (13) is fed to the upper part of the extraction column (11).

3. The method according to claim 2, wherein: The extraction column (11) is operated at a pressure of 4 to 7 bara and a temperature of 30°C to 60°C.

4. The method according to claim 2 or 3, wherein: 40 to 80% by weight of the bottom stream discharged from the bottom of the extraction column is recycled to the extraction column (11), preferably to the upper part of the extraction column (11).

5. The method according to any one of claims 1 to 4, wherein: The aqueous stream (13) in step (a) contains from 90 to 100% by weight of water.

6. The method according to any one of claims 1 to 5, wherein: The intermediate C4 hydrocarbon mixture (16) discharged from the extraction unit (10) is fed to a phase separation unit (20) comprising a filter (21), a coalescer (22) and / or a phase separator (23).

7. The method according to any one of claims 1 to 6, wherein: The unprocessed C4 hydrocarbon mixture (12) contains less than 1% by weight of components having less than four carbon atoms and less than 1% by weight of components having at least five carbon atoms.

8. The method according to any one of claims 1 to 7, wherein: The catalyst deactivator is selected from the group consisting of amines, acetonitrile, ammonia, dimethylformamide and mixtures thereof.

9. A process for obtaining isobutene from a C4 hydrocarbon mixture (24) containing isobutene in a plant comprising an etherification unit (30), a first distillation unit (40), an ether cracking unit (50) and a second distillation unit (60), the process comprising: (i) contacting the C4 hydrocarbon mixture (24) with a primary alcohol (31), and reacting the mixture with the primary alcohol in the presence of an acidic catalyst in the etherification unit (30) to form the corresponding alkyl tert-butyl ether; (ii) distilling the reaction mixture (32) from the etherification unit (30) in the first distillation unit (40), removing the C4 hydrocarbon raffinate as an overhead product (41), removing the alkyl tert-butyl ether as a liquid or vaporous bottom product (42), and evaporating the bottom product (42) if it is removed in liquid form; (iii) reacting the vapor bottom product (42) in the presence of an acidic catalyst in the ether cleavage unit (50) to obtain isobutylene and the primary alcohol as reaction products; (iv) distilling the reaction mixture (51) from the ether cleavage unit (50) in the second distillation unit (60), removing isobutene as the top product (61), removing the primary alcohol as the bottom product (62) and recycling it to the etherification unit (30); The process is characterized in that the C4 hydrocarbon mixture (24) supplied to the etherification unit (30) is obtained by pretreating an unprocessed C4 hydrocarbon mixture (12) containing a catalyst deactivating agent, the pretreatment comprising the steps of: (a) contacting the unprocessed C4 hydrocarbon mixture (12) with an aqueous stream (13) in countercurrent in an extraction unit (10) to obtain an intermediate C4 hydrocarbon mixture, (b) discharging at least a portion of the intermediate C4 hydrocarbon mixture from the extraction unit (10), and (c) dehydrating the discharged intermediate C4 hydrocarbon mixture to obtain the C4 hydrocarbon mixture (24) having a catalyst deactivating agent content of at most 1 ppm by weight, wherein the catalyst deactivating agent is selected from the group consisting of amines, acetonitrile, ammonia, dimethylformamide and mixtures thereof.

10. The method according to claim 9, wherein: The unprocessed C4 hydrocarbon mixture comprises: at least 2% by weight of isobutylene, at least 23% by weight of butenes other than isobutene, Less than 3% by weight of butadiene, at least 1.5 ppm by weight of the catalyst deactivator, Therein the sum of all components in the crude C4 hydrocarbon mixture is 100% by weight.

11. The method according to claim 9 or 10, wherein: The primary alcohol is isobutanol and the alkyl tert-butyl ether is isobutyl tert-butyl ether (IBTBE).

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