Patent Application
20220306948
The present invention relates to a process for the treatment of a gaseous material mixture which has been obtained by catalytic conversion of synthesis gas, this material mixture containing at least alkenes, possibly alcohols and possibly alkanes, and also possibly nitrogen as inert gas and unconverted components of synthesis gas, comprising hydrogen, carbon monoxide and/or carbon dioxide, in which, after the catalytic conversion of the synthesis gas, at least one stage is provided in which a separation of the product mixture obtained in this reaction into a gas phase and a liquid phase is carried out.
Recovery of Synthesis Gas from Smelter Gases
WO 2015/086154 A1 describes a process for the operation of an integrated plant for steel production, in which such an integrated plant comprises, inter alia, a blast furnace for pig iron production and a converter steel works for crude steel production. In the blast furnace process, CO, CO2, hydrogen and steam are produced as products of the reduction reactions, the blast furnace waste gas drawn off from the blast furnace process exhibiting, in addition to the abovementioned constituents, still a high content of nitrogen. A blast furnace waste gas comprises, for example, from 35% to 60% by volume of nitrogen, from 20% to 30% by volume of carbon monoxide, from 20% to 30% by volume of carbon dioxide and from 2% to 15% by volume of hydrogen.
In a converter steel works, which is arranged downstream of the blast furnace process, pig iron is converted into crude steel. By blowing oxygen onto liquid pig iron, troublesome impurities, such as carbon, silicon, sulfur and phosphorus, are removed. A converter gas which exhibits a high content of carbon monoxide and also comprises nitrogen, hydrogen and carbon dioxide is drawn off from the steel converter. A typical converter gas composition exhibits from 50% to 70% by volume of carbon monoxide, from 10% to 20% by volume of nitrogen, approximately 15% by volume of carbon dioxide and approximately 2% by volume of hydrogen.
A coking plant can furthermore be operated in such an integrated plant, in which coal is converted into coke by a coking process. In the coking of coal to give coke, a coke oven gas is obtained, which gas comprises a high hydrogen content and considerable amounts of methane. Typically, coke oven gas comprises from 55% to 70% by volume of hydrogen, from 20% to 30% by volume of methane, from 5% to 10% by volume of nitrogen and from 5% to 10% by volume of carbon monoxide.
WO 2015/086154 A1 describes the possibility of using blast furnace waste gas, converter gas and coke oven gas, which are also described in their totality as smelter gas, for the production of chemical compounds, it being possible for the crude gases, individually or in combination as a mixed gas, to be recovered and then to be supplied as synthesis gas to a chemical plant. In concrete terms, methanol or generally and nonspecifically “other hydrocarbon compounds” are mentioned as possible chemical compounds which can be produced from such a synthesis gas obtained from smelter gases. Furthermore, a biochemical use of the synthesis gas for the production of ethanol or butanol via a fermentation is mentioned. However, the production of higher alcohols from the synthesis gas is not treated in more detail in this publication.
In the treatment of these crude gases, a gas purification is generally carried out for the separation of troublesome ingredients, in particular tar, sulfur, sulfur compounds, aromatic hydrocarbons and high boiling point aliphatic hydrocarbons. In addition, a gas conditioning is for the most part carried out, in which the proportion of the components carbon monoxide, carbon dioxide and hydrogen within the crude gas is changed. The plan of the use described above of smelter gases for the production of chemical products is also denoted as Carbon2Chem® process.
State of the Art Concerning the Production of Alcohols and Alkenes by Catalytic Conversion of Synthesis Gas
EP 0 021 241 B1 discloses a process for the production of mixtures of acetic acid, acetaldehyde, ethanol and alkenes with two to four carbon atoms by conversion of synthesis gas comprising carbon monoxide and hydrogen in the gas phase over supported catalysts, the catalysts comprising rhodium and from 0.1% to 5.0% by weight of sodium or potassium. The oxygen-containing compounds and the alkenes are formed in a molar ratio of 1:1 to 2.5:1. The selectivity for the alcohols of the catalysts used is comparatively poor. Conversion at a pressure of 20 bar, a temperature of 275° C. and a ratio of carbon monoxide to hydrogen in the synthesis gas of 1:1 produces more than 20% of acetic acid, approximately 12-20% of acetaldehyde, about 5% to 10% of ethene, a comparatively high proportion of propene of sometimes more than 20%, varyingly large proportions of methane and only a few percent of ethanol, about 2.5% to 7%. This known process seeks to produce mixtures of oxygen-containing C2 compounds and a high proportion of low molecular weight alkenes and to reduce the proportion of methane. Processes for the separation of the families from the mixture obtained are not described
U.S. Pat. No. 6,982,355 B2 describes an integrated Fischer-Tropsch synthesis for the production of linear and branched alcohols and alkenes, in which initially a light fraction and a heavy fraction are separated from one another, the light fraction is brought into contact with a dehydration catalyst in order to obtain a light fraction comprising alkenes and alkanes which is then further divided into fractions comprising C5-C9 and C10-C13 alkenes and alkanes, which are then reacted with synthesis gas partly to give the aldehydes with the corresponding chain lengths. The aldehydes present in the alkane fraction are subsequently reacted with hydrogen to produce the corresponding alcohols which remain in the alkane fraction. In a first distillation these alcohols are separated from the alkanes and, in a further distillation, the individual alcohols are obtained from the C5-C9 fraction and the C10-C13 fraction. The alkanes of the corresponding fractions can be dehydrogenated to give the alkenes. Use is made, as catalysts in the Fischer-Tropsch synthesis, of cobalt, iron, ruthenium or other group VIIIb transition metals, optionally on an oxide support, such as silicon dioxide, aluminum oxide or titanium oxide.
CN108067235A describes catalysts for the production of alkenes from synthesis gas which comprise cobalt and cobalt carbide as active component, lithium as additive and one or more further metals selected from manganese, zinc, chromium and gallium. Besides the alkenes, the reaction also produces higher alcohols. When using these catalysts, the selectivity for an alkene mixture is said to be up to 40% and that for a mixture of alcohols at 30%. Straight-chain alkenes with 2 to 30 carbon atoms and primary alcohols with corresponding chain lengths are obtained. The product mixture comprises predominantly alkanes and alkenes and, depending on the catalyst, about 20% to 25% of alcohols, both methanol, alcohols with 2 to 5 carbon atoms as well as higher alcohols with 6 or more carbon atoms being produced, the latter group of alcohols forming the predominant proportion and usually being formed to more than 50%. The publication does not elaborate on the separation of the various products present in the mixture.
CN108014816A describes catalysts for the reaction of carbon monoxide with hydrogen for the production of mixed primary alcohols and alkenes. Use is made of catalysts based on cobalt, in particular dicobalt carbide, and manganese, on an activated carbon support which can comprise additions of cerium, copper, zinc or lanthanum. Primary alcohols and alkenes with 2 to 30 carbon atoms are formed. The catalysts used here are said to have a high selectivity for alkenes, it being mentioned that alkenes produced can be further converted to alcohols by hydroformylation. Depending on the type of catalyst used, the catalytic conversion of the synthesis gas produces about 23% to 28% by weight of alkanes, about 36% to 41% by weight of alkenes and about 20% to 21% by weight of higher alcohols, at the same time about 8% by weight of methane and about 2% to 5% by weight of carbon dioxide and about 1% to 2% by weight of methanol being produced.
U.S. Pat. No. 8,129,436 B2 describes a process for the production of an alcohol mixture from synthesis gas, a mixture of alcohols and oxygen-containing compounds being obtained. It is proposed to strip the product mixture with a methanol-containing stream in order to remove a proportion of the carbon dioxide as well as inert gases present in the product stream. It is also possible to carry out a downstream dehydration in order to convert a portion of the ethanol and also possibly propanol produced into the corresponding alkenes. Potassium-modified molybdenum sulfide catalysts are used in the conversion of the synthesis gas. This known process gives very complex product mixtures which do not comprise alkenes but do comprise relatively small amounts of alkanes, besides C2-C5 alcohols, sometimes relatively high proportions of methanol and many other oxygen-containing compounds, such as aldehydes, carboxylic acids, ketones, esters or ethers, and also mercaptans and alkyl sulfides.
US 2010/0005709 A1 describes alternative fuel compositions comprising ethanol, isopropanol and butanols, in which synthesis gas is initially converted by a Fischer-Tropsch synthesis into a C2-C4 alkene stream and subsequently these alkenes are hydrated. The alcohols obtained can be blended with gasoline in order to obtain fuel compositions. The conversion of synthesis gas described in this document gives only about 39% of hydrocarbons with 2 to 4 carbon atoms while about 40% of higher hydrocarbons, cycloalkanes and aromatic C5 to C20 compounds, such as are typically present in gasoline or diesel, are formed. This hydration of the C2-C4 alkenes can only give a maximum of 39% of alcohols, which, besides ethanol, are secondary alcohols and also tertiary butanol. Methanol, 1-propanol and 1-butanol are not formed. This known process employs, in a variant of the Fischer-Tropsch synthesis, iron-manganese catalysts with contents of zinc oxide and potassium oxide. This reaction produces almost exclusively C2-C4 alkenes and 9.6% of methane and 15.7% of C2-C4 alkanes while primarily not forming any alcohols, so that the alcohols are only acquired in a further stage through hydration of the alkenes. Since the product mixture is subsequently mixed with gasoline to give a fuel, it is not absolutely necessary to separate the alkanes or the compounds with 5 carbon atoms or longer carbon chains.
U.S. Pat. No. 5,237,104 A describes a process for the hydroformylation of a hydrocarbon-containing feed stream using a cobalt-containing catalyst. The aim of the hydroformylation is the production of higher alcohols and aldehydes by chain lengthening, in which here the hydrocarbon-containing feed stream is reacted with a synthesis gas. This known process is concerned, in the course of the treatment of the product mixture, with separating the cobalt compounds from the mixture. For this, the volatile cobalt compounds are brought into contact with an olefinic absorbent which exhibits a relatively high molecular weight and comprises, for example, a chain with 10 to 14 carbon atoms.
U.S. Pat. No. 4,510,267 A describes a process for the production of alkenes from synthesis gas in which ruthenium catalysts on a cerium oxide support are used. It is described that the catalyst is selective for the production of alkenes with a low proportion of methane. In order to keep the yield of methane low, a low molar ratio of hydrogen to carbon monoxide in the synthesis gas is recommended. Besides methane, C2-C6 alkenes and, in relatively low amounts, C2-C6 alkanes are produced. Separating processes for the separation of the product mixture are not described in this document. Alcohols (methanol and ethanol) are only formed in very low amounts.
US 2014/0142206 A1 describes a process for the production of a catalyst comprising cobalt and molybdenum on a carbon support. The catalyst is used for the production of alcohols from synthesis gas, in which a high yield of C2 and C3 alcohols is described. The conversion is carried out under the conditions of a Fischer-Tropsch synthesis. Separating processes for the recovery of individual product groups or individual compounds from the product mixture obtained after the conversion are not described in this document.
In the abovementioned Carbon2Chem® process, smelter gases are used and treated such that a synthesis gas is produced which is suitable in principle for the production of chemical compounds. This synthesis gas can be converted in a catalytic synthesis to give higher alcohols. Because of the different composition of the synthesis gas and the differences in the conditions of the subsequent conversion of the synthesis gas, the product mixture obtained in the process described in the present patent application differs from that of a conventional Fischer-Tropsch synthesis, which starts out from natural gas.
After the catalytic synthesis of higher alcohols by conversion of a synthesis gas recovered from smelter gases, a gaseous mixture is obtained which comprises hydrogen, carbon monoxide, nitrogen, carbon dioxide, methane and water as well as various alkanes, in particular ethane, propane, butane, pentane, the corresponding alkenes, in particular ethylene, propylene, 1-butylene, 1-pentylene, and alcohols, in particular methanol, ethanol, 1-propanol, 1-butanol and also possible isomers of these alcohols, in particular 2-propanol, isobutanol, tert-butanol, 2-butanol, the latter not being explicitly looked at in more detail here since their behavior corresponds essentially to that of 1-propanol or 1-butanol. Only a few percent of the gaseous product stream are useful materials, in particular the alcohols and the alkenes. For example, up to 90% or more are, on the other hand, light gases which noticeably impede the condensation of the product mixture.
The object of the present invention consists in making available an improved process for the treatment of a gaseous material mixture which has been obtained by catalytic conversion of synthesis gas, in which the light gases are largely separated and then the product mixture is so treated further that the useful materials present in this, in particular the alcohols and alkenes, can be recovered as completely as possible.
The abovementioned object is achieved by a process for the treatment of a gaseous material mixture of the type mentioned at the outset with the characteristics of claim 1.
The treatment of the gaseous material mixture comprises, according to the invention, at least the following stages:
If, in the context of the present invention, a synthesis gas recovered from smelter gases might be used, it is possible, for example, to proceed as follows. Blast furnace waste gas and converter gas are optionally purified, for example in order to remove solids and/or catalyst poisons present in these gases. If coke oven gas is also used, then the purification thereof is for example advantageously by a pressure swing adsorption (PSA), so that predominantly hydrogen is recovered from the coke oven gas. The proportion of hydrogen of the synthesis gas can optionally be increased, by adding hydrogen produced by electrolysis using electricity from renewable energy sources. The carbon dioxide content of the synthesis gas can optionally be reduced by a reverse water gas shift reaction, in which the carbon dioxide is converted into carbon monoxide. The treated synthesis gas obtained in this way from smelter gases comprises, as main components, carbon monoxide, hydrogen and in addition nitrogen.
A catalytic conversion is subsequently carried out with this synthesis gas to give higher alcohols, which is also denoted in the technical literature as “mixed alcohol synthesis” (see by way of example thereof WO 2008/048364 A2). This catalytic synthesis of the higher alcohols from synthesis gas can according to the invention be carried out, for example, at reaction temperatures of 200° C. to 360° C., preferably at temperatures of 220° C. to 340° C., more preferably at 240° C. to 320° C., in particular at 260° C. to 300° C., for example at about 280° C. In addition, this reaction can be carried out, for example, at a reaction pressure of 10 bar to 110 bar, in particular at 30 bar to 90 bar, preferably at 50 bar to 70 bar, for example at about 60 bar.
In this synthesis, a product stream is obtained which is at a temperature for example in the abovementioned range and a high pressure of the abovementioned order of magnitude, so that this product stream is preferably initially, for example by reduction in pressure in a turbine, converted to a manageable form. Electrical energy is recovered in this reduction in pressure to a pressure of for example 5 bar to 15 bar, in particular to about 10 bar, which energy can be used to meet most of the electricity requirement of the process.
The composition of the product mixture obtained in this catalytic synthesis of higher alcohols can vary strongly. The product mixture comprises in particular alcohols, alkenes, alkanes, carbon monoxide and hydrogen from unconverted synthesis gas, carbon dioxide, nitrogen and methane. The composition of the product phase depends on the composition of the synthesis gas used, in particular the proportion of inert gas thereof (proportion of nitrogen), and the degree of conversion in the catalytic reaction, through which results the remaining proportion of carbon monoxide and hydrogen and also possibly carbon dioxide and the ratio of the products to the residual gas. Furthermore, the selectivity of the catalyst used determines the distribution of the desired products, that is of the alcohols, alkenes, of the proportion of alkanes, and also the proportion of gaseous byproducts, which are produced in the reaction, in particular methane and carbon dioxide.
The synthesis gas not converted in the reaction is preferably led back to the stage before the conversion, so that amounts of hydrogen and carbon monoxide present in this gas can be used in a renewed conversion. If the synthesis gas used comprises, for example, a proportion of 30% by volume of nitrogen, which is inert with regard to the conversion sought, less unconverted synthesis gas has to be recycled than if the synthesis gas used comprises, for example, a proportion of 50% by volume of nitrogen. If the proportion of nitrogen in the synthesis gas is lower and amounts to, for example, only 10% by volume, it is possible, for example, to separate nitrogen before the actual separation process using an additional membrane.
The amount of absorption medium which is used in the first separation stage depends on the degree of conversion and accordingly on the product composition resulting therefrom. If a relatively large amount of absorption medium is used, more alkenes and alkanes are separated from the product mixture, but it is also to be considered though that simultaneously more undesirable gases, in particular nitrogen, carbon dioxide, carbon monoxide and hydrogen, are coabsorbed.
Consequently, in the process according to the invention, through the first separation stage, which can also be denoted as gas-liquid separation, initially a separation of the greater part of the gases, in particular nitrogen, carbon monoxide, carbon dioxide, hydrogen and methane, is achieved, since these gases are barely absorbed in the absorption medium, the high boiling point hydrocarbon or hydrocarbon mixture. The gases separated can be lead back to the preceding process stage of the catalytic conversion of synthesis gas. Two liquid phases are formed, namely an aqueous phase and an organic phase comprising the absorption medium. Organic compounds which are regarded as useful materials in the process according to the invention, in particular alcohols and alkenes and also alkanes, go predominantly into the organic phase, a lesser portion also into the aqueous phase. Short-chain alcohols, in particular methanol and ethanol, go predominantly into the aqueous phase. In a second separation stage, the aqueous phase is separated from the organic phase of the absorption medium, preferably by decanting. Subsequently, alcohols, alkenes and possibly alkanes are desorbed from the absorption medium.
According to a preferred development of the process according to the invention, the mixture obtained after the desorption, comprising alkenes, possibly alkanes and possibly alcohols, is subsequently separated through a first distillation into a fraction comprising predominantly the hydrocarbons and a fraction comprising predominantly the alcohols.
This distillation is preferably carried out at an elevated pressure, which preferably is in a pressure range from about 10 bar to about 40 bar.
According to preferred further development of the process according to the invention, the hydrocarbons separated in the first distillation are subsequently separated, in an extractive distillation with water, from residues of alcohol and water present in the hydrocarbons. This extractive distillation is also denoted in the present application, for the purpose of differentiation, as second distillation. This second distillation is used for the further purification of the hydrocarbons separated previously. In this extractive distillation in a column, the hydrocarbons are obtained at the top of the column, while the water is also partly obtained in the bottom of the column, alcohols still generally being dissolved in this water. The product mixture from the bottom of the column is preferably led back in order for the alcohols present in the water to be recovered.
According to a preferred development of the process according to the invention, alcohols, which are present in the aqueous phase after the separation of the aqueous phase from the organic phase, preferably after the decanting, are separated from the water by distillation, the alcohols being obtained as an azeotrope with water at the top. This distillation, which is used to recover additional alcohols from the aqueous phase, is also denoted, to better distinguish it from the other separation stages in the present patent application, as third distillation.
Preferably, the alcohols separated from the water in this third distillation are conveyed to the mixture obtained after the desorption and, with this mixture, separated from the hydrocarbons through the first distillation. In this way, it is possible, in the first distillation which is used for the separation of the alcohols from the hydrocarbons, to recover a higher proportion of alcohols since even those alcohols are recovered which, in the decanting, went into the aqueous phase, without a separate distillation apparatus being necessary for this.
According to a preferred development of the invention, the alcohol fraction obtained in the first distillation, in which the alcohols are separated from the hydrocarbons, subsequently preferably has water removed from it by means of a molecular sieve. The alcohol fraction can, after the first distillation, for example exhibit a water content with an order of magnitude of about 10% or less. In order to remove water from the alcohol fraction, mention is made later still of alternative possibilities to the removal of water by means of a molecular sieve mentioned here. After the removal of water, an alcohol mixture is obtained which comprises in particular methanol, ethanol, propanols and butanols.
According to a preferred development of the process according to the invention, this alcohol mixture obtained after the first distillation can be subsequently separated, for example, through one or more additional distillation stages into alcohol fractions with each time a different number of carbon atoms, in particular into a C1 fraction, a C2 fraction, a C3 fraction and a C4 fraction.
A possible variant of the process according to the invention provides for alkenes present in the hydrocarbon mixture obtained after the first distillation, optionally after a further treatment, to be converted to alcohols by hydration. By this measure, the yield of alcohols obtained in the process can on the whole be increased. In addition, there is the advantage that alcohols can be separated better from alkanes, while alkanes and alkenes, because of their chemical similarity, are difficult to separate.
The hydration of alkenes to give the corresponding alcohols is a known reaction for the preparation of alcohols and is used industrially, for example, for the production of isopropanol from propene. With the exception of ethene, the hydration of linear alkenes results predominantly in the formation of secondary alcohols. Isobutene is hydrated to give tertiary butanol, a tertiary alcohol.
There exist essentially two well-known industrial processes for the hydration of alkenes to give the corresponding alcohols; on the one hand direct hydration and alternatively indirect hydration.
In direct hydration, the alkene is reacted with water over an acidic catalyst to give the respective alcohol. The hydration of alkenes to give alcohols is an equilibrium reaction. High pressures and low temperatures shift the equilibrium of the exothermic reaction to the product side in favor of the alcohols. The indirect hydration of alkenes is carried out in a two-stage reaction. The alkene is initially reacted with sulfuric acid to give mono- and dialkyl sulfates and subsequently hydrolyzed to give the alcohol.
Industrially, ethanol is produced predominantly by fermentation of carbohydrates, for example of sugars from corn, sugar beet, grain or wheat. Synthetic ethanol can be produced from ethene by direct hydration. The direct hydration of ethene is carried out in the gas phase over “solid” phosphoric acid (SPA catalysts), for example at 250-300° C. and 50-80 bar. The hydration of ethene is an equilibrium reaction, in which high pressures and low temperatures favor the exothermic formation of ethanol. The indirect hydration of ethene is no longer carried out industrially.
Various well-known processes are available for the direct hydration of propene: low-temperature/high-pressure processes (130-180° C., 80-100 bar) with for example sulfonated polystyrene ion-exchange catalysts, high-temperature/high-pressure processes (270-300° C., 200 bar) with for example reduced tungsten oxide catalysts and processes in the gas phase (250° C., 250 bar, WO3—SiO2 catalyst, ICI process/170-260° C., 25-65 bar, phosphoric acid catalyst on SiO2, Hüls process). The direct hydration of propene with steam under high pressure is carried out in Canada, Mexico and Western Europe. In the indirect hydration of propene, it is also possible to use, besides propene, the C3 stream from refinery offgas with a propene concentration of 40-60%.
2-Butanol (secondary butyl alcohol) can be produced from butene or MTBE raffinate by direct hydration or indirect hydration. 2-Butanol is used for the production of methyl ethyl ketone (MEK).
According to a first variant of the invention, the use of a catalyst with a high selectivity for alcohols and alkenes is preferred since a recovery of these product families as useful materials is in the foreground, while the formation of alkanes and also oxygenated compounds, such as aldehydes, esters, ethers, carboxylic acids and the like, which are obtained on using Fischer-Tropsch catalysts often in higher proportions, is not desired in the process according to the invention. Furthermore, the subsequent separation processes in the treatment of the product mixture after the conversion of the synthesis gas become more expensive as the product mixture becomes more complex. In the conversion of synthesis gas using Fischer-Tropsch catalysts, this is less problematic since the product mixtures are often not separated into individual families but the mixture is used as such as additive in fuels.
The synthesis of higher alcohols generally provides a mixture of primary alcohols. Through the inclusion of hydration in the process, it is possible to selectively form secondary alcohols and accordingly to widen the product spectrum. A more uniform product is thus produced from the complex product mixture, which leads to advantages in the purification process and in marketing logistics.
For the process according to the invention, not only smelter gases but also any other suitable synthesis gas sources are suitable in principle. By using smelter gases, the process according to the invention has the merit that a high proportion of nitrogen frequently present in the smelter gases can be satisfactorily separated by the separation stage of the absorption in a high boiling point hydrocarbon or hydrocarbon mixture. Nitrogen as inert gas in the present process absolutely has to be separated since a high proportion of nitrogen would make more difficult the subsequent treatment of the product mixture.
In the context of the present invention, an overall process has been developed which makes it possible to produce higher alcohols (with two or more carbon atoms) with good yield starting from synthesis gas. In the present application, processes are described which, starting from the product mixture obtained in the conversion of synthesis gas, comprising carbon monoxide and/or carbon dioxide and hydrogen, offer advantages economically, technologically and/or ecologically compared with known processes, in particular as regards simply a separation with subsequent individual marketing of the products/substance groups. In this connection, particular attention was paid to the optimization of the product separation in harmony with the synthesis stages. This relates inter alia to the respective physical process conditions (pressure, temperature) and also to the establishment of preferred or technically tolerable reactant ratios for the synthesis stages while considering in particular economic circumstances.
Due to the large plant capacities which are necessary for example for the utilization of significant amounts of smelter gas but also with other synthesis gas sources, use is preferably made of processes which lead to products with sufficiently large (potential) markets. It is accordingly particularly advantageous here to consider commodity chemicals which can be used, for example, in the plastics or fuels sectors.
According to a possible preferred variant of the process according to the invention, from the first mixture of alkanes, alkenes and alcohols obtained after the catalytic conversion of synthesis gas and preferably after separation of the unconverted synthesis gas, initially the alkanes and alkenes are separated from the alcohols and only then are the alkenes hydrated in this second mixture.
The alcohols can be separated from the alkenes and alkanes at little cost. In comparison, the alkenes can be separated from the alkanes only at considerable cost. The consecutive hydration of the alkenes to give alcohols accordingly facilitates the operation of separating alkenes and alkanes.
The separation of the alkene/alkane mixture into the individual Cx cuts or alkenes can optionally also be advantageous since this makes possible the separate hydration of the individual alkenes. Alkenes, the respective hydration products of which are particularly suitable for the fuel market, or alkenes which can be hydrated under mild reaction conditions or inexpensively, can be selectively converted to the respective alcohols. Alkenes for which there is an appropriate alkene market can be separated from the respective C cut and marketed. Furthermore, the reaction conditions for the hydration of the individual Cx cuts or alkenes can be chosen independently of one another. For example, a hydration of the C2 cut or of ethene could be dispensed with and the ethene could instead be used for other applications in the chemical industry. Moreover, in this way a relatively pure alkane stream can be obtained which can be used for the production of synthesis gas or the generation of energy. With this way of proceeding though, a separate plant for the hydration of the alkenes is required for each C cut or a batchwise hydration of the different fractions must be carried out.
According to an alternative preferred variant of the process according to the invention, the second mixture comprising the alkanes and alkenes comprises a mixture of C2-C4 alkenes or a mixture of C2-C5 alkenes which is subsequently hydrated in the mix to give the corresponding alcohols. Consequently, in this variant, the hydration of an alkane/alkene mixture is carried out without providing beforehand a separation of this mixture into different fractions with different numbers of carbon atoms.
With regard to the reaction conditions for the hydration of such an alkene/alkane mixture, it must be taken into consideration that the conventional industrial processes are optimized for the conversion of the individual alkenes and differ from one another in the choice of the catalyst and of the reaction conditions. For this stage of the hydration of the alkene/alkane mixture, therefore preferably process conditions are to be used in this variant which make possible the conversion of all alkenes or promote the conversion of the favored alkenes to the respective alcohols. As regards the abovementioned process variant, the hydration of the alkene mixture offers the advantage that merely one plant is required for the hydration or a batchwise hydration of the various fractions can be dispensed with.
After the hydration, the alkanes are separated from the alcohols formed. The alkane stream remaining after the separation of the alcohols can then be used, for example, for the production of synthesis gas or energy.
According to a preferred further development of the process according to the invention, the conditions for the hydration of the alkane/alkene mixture with regard to the choice of the catalyst and of the reaction conditions, in particular of the temperature and pressure, at which the hydration reaction is carried out, are chosen such that the hydration of propene and/or 1-butene is favored over that of ethene. It was established that, with the catalysts which were used in the context of the present invention in the production of higher alcohols by catalytic conversion of synthesis gas, predominantly propene is formed as alkene. As a rule, the CO selectivity of the conversion of synthesis gas to the alkenes decreases in the order 1-propene>1-butene>ethene. The industrial processes for the hydration of 1-propene and 1-butene proceed under milder reaction conditions than those of ethene so that in this process variant it might optionally be advantageous to concentrate on the hydration of propene and to neglect the hydration of ethene or optionally to dispense with the hydration of ethene.
According to a preferred further development of the process according to the invention, the direct hydration is carried out at elevated temperatures and at elevated pressure. Wide temperature ranges and wide pressure ranges are in principle possible here, depending on which other conditions are selected. The hydration is as a rule carried out in the presence of an acid acting as catalyst.
For example the hydration of the alkenes can be carried out at temperatures above 80° C., in particular above 100° C., for example at temperatures in the range from 100° C. to 180° C., preferably at 120° C. to 150° C., and/or at a pressure of 5 bar to 150 bar, in particular at a pressure of 10 bar to 100 bar, preferably at a pressure of 50 bar to 100 bar, for example at a pressure of 70 bar to 80 bar. The hydration of propene and of 1-butene proceed under similar reaction conditions each time, for example at the abovementioned temperatures and pressures. In the industrial direct hydration of propene, conversions of for example up to about 75% per pass are achieved. For the direct hydration of the alkene/alkane mixture, the invention therefore proposes to be guided by the reaction conditions of the hydration for propene and 1-butene. It is also conceivable in principle to carry out the hydration of the alkene/alkane mixture in several consecutive reactors with different catalysts and/or different reaction conditions and separation at intervals of the alcohols formed.
A third possible preferred variant of the process according to the invention provides for carrying out the hydration of the alkenes with the mixture of alkanes, alkenes and alcohols without the alcohols being separated from this mixture beforehand. The hydration of the alkenes in the mixture of alcohols, alkenes and alkanes obtained in the conversion of the synthesis gas, without preceding separation of the alcohols present in this mixture, can, for example, offer the advantage that the reaction mixture is already at a comparatively high pressure of for example about 60 bar and therefore merely has to be preheated to the reaction temperature.
At a reaction temperature of for example about 150° C., the hydration of the alkenes to the alcohols is thermodynamically preferred. Experiments in the context of the synthesis of the higher alcohols with specific catalysts and subsequent hydration and also calculations for an equilibrium reactor clearly show that, on carrying out the hydration at elevated temperature (for example of up to 150° C.) and an elevated pressure of for example from 2 bar to 100 bar, a conversion of the alkenes and of the primary C3+ alcohols to the secondary alcohols takes place. In particular, propene and 1-butanol are converted predominantly to isopropanol and 2-butanol. Ethene is hydrated to give ethanol.
According to a preferred further development of the process according to the invention, after the hydration, the alkanes are separated from the product mixture obtained and the remaining mixture of alcohols is optionally purified and/or separated into individual fractions of alcohols or individual alcohols. In turn, the advantage exists here that in principle only alcohols and alkanes are present after the hydration, even in the variant described above in which the alcohols already obtained in the conversion of the synthesis gas in the first stage are not separated before the second stage of the hydration. Accordingly, in turn, only two substance classes are present in the mixture, which can easily be separated from one another, while the separation of alkenes and alkanes would be substantially more difficult.
According to the process according to the invention, the stage is provided, preferably before the hydration of the alkenes to give the corresponding alcohols and after the catalytic conversion of the synthesis gas, in which the product mixture obtained in this reaction is separated into a gas phase and a liquid phase, the liquid phase being used for the subsequent hydration of the alkenes to give the alcohols. The gas phase separated at this point can for example comprise unconverted CO and H2 and also CO2, CH4 and N2. The gas phase obtained in this separation operation, which as a rule comprises the unconverted gases mentioned, can, according to a preferred variant of the process according to the invention, be at least partially led back to the stage of the catalytic conversion of the synthesis gas in order in this way to increase the yield of the overall process through the renewed conversion of the recycled reactant gases to higher alcohols.
Alternatively to this, it is also possible in principle to carry out the hydration of the alkenes before a separation of the product mixture obtained after the conversion of the synthesis gas into a gas phase and a liquid phase. In this case, the hydration is carried out for example directly in a reactor arranged downstream of the synthesis of higher alcohols and without preceding separation of the product mixture. Propene and butene can for example be hydrated at about 150° C. while higher temperatures of for example about 230° C. to 260° C. are advantageous for the hydration of ethene. The hydration can be carried out at a lower temperature than the preceding conversion of the synthesis gas, it being possible for temperatures of for example 120° C. to 150° C. to be selected for the hydration. It can therefore be advantageous to cool the product mixture for the hydration to temperatures of this order of magnitude.
Experiments, calculations and the simulation of the synthesis of higher alcohols with subsequent hydration show that, under the reaction conditions of the synthesis of higher alcohols, the dehydration of the alcohols to give the alkenes is thermodynamically preferred. When the reaction conditions of the synthesis of higher alcohols are for example about 280° C. and about 60 bar, a virtually complete conversion of the alcohols into the corresponding alkenes is possible.
At a reaction temperature of the order of magnitude of for example about 50° C., the hydration of the alkenes to give the alcohols is, in comparison, thermodynamically preferred. Experiments in the context of the synthesis of the higher alcohols with specific catalysts and calculations or simulations of the subsequent hydration for an equilibrium reactor clearly show that, on carrying out the hydration at for example about 50° C. and a pressure of about 60 bar, a conversion of the alkenes and of the primary alcohols to the secondary alcohols takes place. In particular, propene and 1-butanol are converted predominantly to isopropanol and 2-butanol. Ethene is hydrated to give ethanol.
However, it should be taken into consideration, in this purely thermodynamic analysis, that the industrial processes for hydration as a rule are carried out at reaction temperatures of 130-260° C. It can therefore be assumed that the reaction at 50° C. proceeds at a markedly reduced reaction rate. This process variant is therefore less suitable for the hydration of the alkenes (or can be carried out only under certain circumstances).
Instead, preference is consequently to be given to one of the abovementioned variants in which initially, after the synthesis of higher alcohols, the separation into a gas phase and a liquid phase is carried out, the product mixture being cooled after the synthesis of higher alcohols from the synthesis gas.
In the process according to the invention, there are in particular the three preferred process variants subsequently mentioned:
In variant 1, the process preferably comprises the stages of:
In variant 2, the process preferably comprises the stages of:
In variant 3, the process preferably comprises the stages of:
Alternatively to this, a fourth process variant is possible in which the hydration of the alkenes is already carried out after the conversion of the synthesis gas and before the separation of the product mixture obtained into a gas phase and a liquid phase.
In variant 4, the process preferably comprises the stages of:
In all four of the abovementioned process variants, an at least partial recycling of the gas phase to the synthesis of the higher alcohols after the gas-liquid separation is advantageous.
Besides the abovementioned process variants, it is also possible to carry out the hydration of the alkenes by a combination of two or more of the abovementioned process variants. For example, the composition of the product mixture of higher alcohols (with at least two carbon atoms) and alkenes initially obtained by catalytic conversion of synthesis gas can be shifted by process variant 4 and, after separation of the product mixture then obtained into a gas phase and a liquid phase, the alkenes present in the liquid phase are hydrated for example by means of one of the process variants 1, 2 or 3 to give the corresponding alcohols. The combination of the two process variants can for example favor the isomerization of the primary alcohols to give secondary alcohols. The isomerization of the primary alcohols to give the secondary alcohols proceeds via the dehydration of the primary alcohols to give the corresponding alkenes as intermediate products. The dehydration preferably proceeds at higher temperatures than the hydration.
The provision of the synthesis gas for the catalytic conversion to alcohols according to the invention can comprise, besides the preparation of the synthesis gas, also the purification and the conditioning of the synthesis gas. Both fossil fuels, such as natural gas, coal, but also CO- and CO2-rich gases, for example from steel or cement works, and hydrogen, can be used as feed.
It is also possible to obtain the synthesis gas used from biomass. The hydrogen is preferably produced in a sustainable manner by means of renewable energy sources and/or low CO2 emissions, for example by water electrolysis or methane pyrolysis. The electricity for operating the hydrogen production is preferably generated using renewable energy sources.
The liquid phase comprises predominantly the alcohols, alkenes and possibly alkanes formed. Reducing the pressure to less than 50 bar, in particular to less than 30 bar, preferably to less than 20 bar, more preferably to less than 10 bar, for example from 3 to 1 bar, preferably to about 1 bar, makes it possible for example to evaporate the alkenes and alkanes and to separate them from the product mixture. However, other methods known to a person skilled in the art for the separation of alkenes and alkanes from alcohols are likewise suitable here. It is optionally advantageous, for economic and/or ecological optimization of the process, to convert the alkanes into synthesis gas, for example via a partial oxidation, steam reforming or autothermal reforming, and to lead back into the process. The alkanes can optionally also be dehydrogenated to give the corresponding alkenes and subsequently likewise hydrated in order to increase the yield of alcohols. The alcohols remain in the liquid phase and, after separation of the water formed as connected product, are optionally marketed as product mixture, for example as fuel additive, or separated in a distillation into the individual alcohols.
Furthermore, there exists the option of hydrating the alkenes after the separation of the alkanes from the respective Cx cuts. This gives the advantage of a comparatively pure reactant concentration and the possibility of carrying out the hydration under conditions well-known industrially for the respective alkene. Due to the expenditure on equipment and energy of the separation of alkanes and alkenes, this option though can only be carried out under certain circumstances.
The various options for integration of the consecutive conversion of the alkenes to alcohols into the process concept for the synthesis of higher alcohols differ each time in the composition of the reaction mixture and the prevailing process conditions, such as temperature and pressure, and also in the manner and time of the separation of the alcohols, alkenes and alkanes from the synthesis gas. The possibility exists, through the integration of the hydration of the alkenes into the process concept for the synthesis of higher alcohols, of using the already existing temperature and pressure levels of the catalytic synthesis of higher alcohols for the hydration.
Preferably, primary alcohols are formed in the catalytic synthesis of higher alcohols from synthesis gas. The formation of secondary alcohols is hardly observed. In comparison, the hydration of linear alkenes preferentially results in the formation of secondary alcohols, such as isopropanol and 2-butanol (with the exception of ethanol). The synthesis of higher alcohols and the consecutive hydration of the alkenes therefore differ in their product spectrum.
If the isomerization of primary alcohols to give secondary alcohols is desired, a suitable process concept is to be selected for it which guarantees the isomerization of primary alcohols to give secondary alcohols.
Due to the possible isomerization of the primary alcohols to give secondary alcohols, a separation of the alcohols from the hydrocarbon mixture (alkenes, alkanes) presents itself, i.e. the abovementioned process variants 1 and 2 preferably present themselves for the hydration. The alcohols can be separated from the alkenes and alkanes at little cost. In comparison, the alkenes can be separated from the alkanes only at considerable cost. The consecutive hydration of the alkenes to give alcohols accordingly facilitates the operation of separating alkenes and alkanes.
According to a preferred further development of the process according to the invention, the hydrocarbon mixture obtained after the first distillation is separated into fractions with each time the same number of carbon atoms, in particular into a C3 fraction, a C4 fraction and a C5 fraction.
According to a possible variant of the process according to the invention, the removal of water from the alcohol fraction can be carried out by, from the alcohol fraction obtained in the first distillation, separating the lower alcohols methanol and ethanol in a column with a little water, and the remaining alcohol mixture is treated with a higher hydrocarbon and is separated in a decanter into an organic phase and an aqueous phase. In this process variant, it can be advantageous to use a comparatively large amount of the higher hydrocarbon as absorption medium; accordingly, most of the alcohols go into the organic phase. The aqueous phase can be sent back to the abovementioned decanter for treatment.
Preferably, the alcohols are stripped out from the organic phase in an additional column and residual water present in the alcohols is subsequently removed using a molecular sieve.
According to an alternative variant of the process according to the invention, from the alcohol fraction obtained in the first distillation, the lower alcohols methanol and ethanol are separated from the higher alcohols in an extractive distillation with a hydrophilic substance, in particular with ethylene glycol, the higher alcohols are subsequently separated from the hydrophilic substance in an additional distillation column and water present in the higher alcohols is then optionally removed as azeotrope.
According to an alternative variant of the process, from the alcohol fraction obtained in the first distillation, the water can also be selectively removed for example by pervaporation through a membrane and be withdrawn as permeate in vapor form.
Finally, according to a further alternative variant of the process, from the alcohol fraction obtained in the first distillation, the water can also be removed for example by azeotropic distillation with a selective additive, in particular with a higher hydrocarbon, preferably with butane or pentane.
Mention has already been made above that a principal concern of the present invention consists in recovering the useful materials present in the product mixture obtained after the catalytic conversion of the synthesis gas, in particular the alcohols and alkenes being supposed to be recovered here. Accordingly it is preferably sought according to the invention for at least the alcohols methanol, ethanol, propanol and butanol, and also the C4 olefins and C5 olefins and possibly C3 olefins and C2 olefins, to be recovered from the material mixture obtained after the catalytic conversion of synthesis gas.
Various substances are suitable for the absorption medium which is used in the separation stage for the separation of the inert gases from the product stream. Preferably, the high boiling point hydrocarbon used as absorption medium or the hydrocarbon mixture comprises a diesel oil or an alkane, in particular a dodecane, with a viscosity of less than 10 mPas at ambient temperature and/or a boiling point of more than 200° C.
Preferably, the subsequent desorption of the alkenes, possibly alcohols and possibly alkanes from the absorption medium is carried out in a distillation column, preferably at a pressure of 1 bar to 5 bar.
According to an advantageous further development, after the desorption, the absorption medium is led back after heat exchange into the separation stage of the absorption.
Preferably, the gas phase of the gases not absorbed in the absorption medium, which gas phase is separated from the product mixture after the catalytic conversion of the synthesis gas, comprises at least the gases nitrogen, hydrogen, carbon monoxide, carbon dioxide and methane.
Mention has already been made above that alkenes present in the hydrocarbon mixture obtained after the first distillation according to an optional variant of the process according to the invention, optionally after a further treatment, can be converted by hydration to alcohols, in order in this way to increase the proportion of alcohols in the product mixture and to simplify the separation of the product families. For example, such a hydration of the reaction mixture consisting of alcohols, alkenes and alkanes can be carried out at a temperature of about 150° C. As can be demonstrated from simulations and calculations of the thermodynamic equilibrium, the mixture of alkenes and primary alcohols is, under these reaction conditions, virtually completely converted to secondary alcohols. The isomerization of the primary alcohols to the secondary alcohols presumably takes place via the formation of the alkenes as intermediates. The hydration of the product mixture of the synthesis of higher alcohols from alcohols and alkenes accordingly offers the possibility of shifting the product spectrum in the direction of the secondary alcohols. The industrial hydration of propene and 1-butene proceeds at reaction temperatures of for example 120 to 150° C.
The consecutive hydration of the alkenes formed as byproducts in the catalytic synthesis of higher alcohols makes it possible, with suitable reaction management, to increase the alcohol yield. Furthermore, this equilibrium reaction offers the possibility, in principle, of converting the complex reaction mixture of primary alcohols and alkenes to secondary alcohols (with the exception of ethanol) by means of dehydration and hydration stages. The reduction in the products results in a small number of stages of purification of the individual products and facilitates the marketing of the products of the synthesis of higher alcohols.
The present invention is described in more detail below on the basis of exemplary embodiments with reference to the enclosed drawings. In this connection:
Subsequently, a description is given, with reference to the simplified reaction scheme according to
Removal of Inert Gas and Gas-Liquid Separation
After the catalytic conversion of a synthesis gas stream under the conditions of the process according to the invention, a product stream is present at a temperature of 280° C. and a pressure of 60 bar. The latter is initially reduced in a turbine (not represented in
The subsequent gas-liquid separation is used in particular for the separation of the inert gases (nitrogen) and unconverted components of the synthesis gas (hydrogen, carbon monoxide and possibly carbon dioxide, and also the byproducts carbon dioxide and methane) and is carried out by introducing the crude product gas stream 10 into an absorption apparatus 11, in which the absorption of the product stream in a diesel oil (reference component dodecane) or alternatively in an alkane or a hydrocarbon mixture with a comparatively low viscosity of for example less than 10 mPas at ambient temperature and with preferably a comparatively high boiling point of in particular more than 200° C. is carried out. The water is in this connection not absorbed but for the most part condensed as second liquid phase. The separated abovementioned gaseous components can be led back via the line 12 to the catalytic conversion of the synthesis gas (not represented here) and be converted there once again.
The two liquid phases (organic phase and aqueous phase) obtained in this absorption operation, which can be carried out at a pressure of for example about 10-20 bar, can subsequently be separated in a decanter 13, little of the hydrocarbons but a portion of the alcohols going into the aqueous phase. The separation of the two liquid phases in the decanter 13 can for example likewise be carried out still at the abovementioned pressure of about 10-20 bar. The organic phase separated in this connection comprises the hydrocarbons, at least a portion of the alcohols, the absorption medium, possibly a portion of the water and also possibly amounts still remaining of inert and unconverted gases, in particular nitrogen and carbon monoxide, and is led to a desorption apparatus 15, via a line 14, in which the alcohols and hydrocarbons are desorbed from the absorption medium for example at low pressure, for example at a pressure of about 1 bar, and a bottom temperature of 216° C. A column, for example, can be used as desorption apparatus 15. With relatively low amounts of inert gases in the product stream of the catalytic conversion of synthesis gas, a condensation of the low boiling point components may alternatively also be suitable. The absorption medium (dodecane, diesel oil) can be led back to the absorption apparatus 11 via the line 16.
Separation of Alcohols/Hydrocarbons
The organic phase desorbed from the absorption medium in the desorption apparatus 15, which comprises the alcohols, hydrocarbons, small amounts of water and possibly amounts of unconverted and inert gases, is subsequently conveyed via a line 18 to a first distillation column 17. The separation of alcohols and hydrocarbons is carried out by distillation in this first column, preferably at a high pressure of for example 10 bar to 40 bar, for example at about 36 bar, and a bottom temperature of for example 221° C., so that the C3 constituents remain still condensable even in the presence of inert gas residues possibly present. This separation is preferably operated in such a way that the hydrocarbons are virtually completely removed from the alcohol fraction at the bottom, while relatively small contents of alcohol (in particular methanol) can be tolerated in the hydrocarbons. This process can optionally be assisted by a solubility-driven membrane. The alcohols obtained in this first distillation, which still comprise a proportion of water, can be withdrawn from the first distillation column 17 via the line 19 from the bottom and dried, as is more fully explained subsequently.
Preparation of the Hydrocarbons
The hydrocarbons obtained in the first distillation column 17 at the top can be conveyed, via a line 20, to a second distillation column 21, in which they are recovered at elevated pressure of preferably 5 bar to 20 bar, for example at a pressure of about 10 bar, and a bottom temperature of 102° C. at the top of the distillation column 21, and can be withdrawn, via the product line 22, for further purification and optionally separation into individual carbon fractions (not represented in
The remaining water and also the alcohols dissolved therein are obtained in the bottom of this second distillation column 21. This stream is separated and can be led back, via the line 23, for the recovery of the alcohols in a third distillation column 24. The condenser of the column can, for example, be a partial condenser. The outputs of the column are a gas phase of hydrocarbons and inerts, a liquid phase of hydrocarbons and also an aqueous phase which can be returned to the column as reflux.
The aqueous phase separated from the organic phase in the decanter 13 is conveyed, via the line 28, to the third distillation column 24, which aqueous phase can comprise amounts of the alcohols since these are at least partially soluble in water, in particular the lower alcohols, such as methanol and ethanol. The distillation in this third distillation column 24 can, for example, be carried out at a pressure of about 2 bar and at a temperature in the bottom of, for example, about 120° C. The alcohols present in this aqueous fraction are recovered at the top of the third column and conveyed, via the line 29, to the first distillation column 17 and combined there with the mixture of alcohols and hydrocarbons from the organic phase, so that these alcohols recovered from the aqueous phase can be separated in the first distillation column 17 with the rest of the alcohols and subsequently, for example, dried via the molecular sieve 25. The water separated in this third distillation column can, for example, be withdrawn from the plant via the line 30 as wastewater.
In this way, the useful materials, alkenes and alcohols, can be recovered each time as separate product groups by means of the separation scheme represented in exemplary and simplified fashion in
Removal of Water from the Alcohol Fraction
The alcohol fraction from the first distillation column 17 can have a water content of for example about 10%. This water can, for example, be removed using a molecular sieve 25. In this process variant, the alcohols are conveyed, via the line 19, to the molecular sieve 25, by means of which water is removed from them, it being possible for the water to be withdrawn from the plant via the line 26. The alcohols dried in this way can be withdrawn via the product line 27 and optionally further separated, for example into individual carbon fractions (isomeric alcohols with each time the same number of carbon atoms) or into individual specific alcohols.
Suitable as alternative method for removing the water from the alcohol fraction is extractive distillation, for example with ethylene glycol, which however requires a further separation stage since the water is pulled from the ethylene glycol into the bottom while the alcohols methanol and ethanol proceed via the top virtually free from water. About half of the propanol and all of the butanol remain in the bottom and these C3-C4 alcohols must likewise be removed via the top from the ethylene glycol in a subsequent column.
Pervaporation is possible as third alternative. In this connection, water passes selectively through a membrane and is withdrawn as permeate in vapor form. The energy consumption is even lower than in a molecular sieve.
A further alternative method would be an azeotropic distillation, for example with butane or pentane as selective additive.
Examples for the Composition of the Product Mixture Obtained After the Catalytic Conversion of the Synthesis Gas
In the context of the present invention, it has been investigated how the variation in different parameters has an effect on the composition of the gaseous material mixture obtained after the catalytic conversion of the synthesis gas. The results are reproduced in the following examples.
The following example 1 gives an exemplary product composition which was obtained in the catalytic conversion of synthesis gas according to the process according to the invention. The catalyst used exhibited a high C2-C4 selectivity, alcohols, alkenes and alkanes being formed. A catalyst which comprises grains of nongraphitic carbon with cobalt nanoparticles dispersed therein was used. The CO selectivity with regard to the conversion to alcohols is about 28% and the CO selectivity with regard to the conversion to alkenes is about 32%. The precise CO selectivities of the catalytic conversion of the synthesis gas are apparent from the following table 1. The selectivities given in table 1 were normalized to the products detected in the catalytic tests (C1-C5 alcohols, C1-C5 alkenes and C1-C5 alkanes, CO2). The analysis of the CO conversion allows it to be concluded therefrom that, besides named products detected, long-chain C6+ alcohols, C6+ alkenes and C6+ alkanes, and also possibly aldehydes, are also formed.
A pulverulent catalyst was used in this example. The catalyst can alternatively also be pressed into tablets, for example.
Table 1 above shows that, in the catalytic conversion of the synthesis gas according to the invention, a comparatively high proportion of alcohols compared with the alkenes can be obtained if a suitable catalyst is used. The proportion of alkanes in the product mixture is lower in comparison thereto. The alkenes can likewise be converted to alcohols in the following hydration stage so that, inclusive of the following hydration stage, the synthesis gas can be converted overall into alcohols with a CO selectivity of virtually 60%, primary alcohols (methanol, ethanol, 1-propanol and 1-butanol) being obtained from the alcohol synthesis and ethanol and secondary alcohols (2-propanol, 2-butanol and possibly 2-pentanol) being obtained from the hydration stage and the methanol content being comparatively low. Such an alcohol mixture is suitable, for example, as fuel additive for blending with gasoline. The separation into the individual alcohols is alternatively possible.
For comparison, a CO0.126Mo0.255C catalyst was used, as described in the US document US 2014/0142206 A1 in Example 2. The H2 to CO ratio in the synthesis gas was 1:1. After the conversion of the synthesis gas, a composition according to the following table 1a was obtained.
The above table 1a shows that, on using this catalyst, predominantly alcohols are formed, the proportion of methanol being comparatively high while only a little 1-butanol is formed. C2-C6 hydrocarbons are only formed in small amounts.
For further comparison, a catalyst with high selectivity for olefins was used, as described in Example 2 of U.S. Pat. No. 4,510,267 A. After the conversion of the synthesis gas, a composition according to the following table 1b was obtained. The composition of the synthesis gas was in this case H2:CO=1:1. The selectivities (% by weight) given in U.S. Pat. No. 4,510,267 A and in table 1b were recalculated in CO selectivities for comparison with the results in Table 1 and for the simulation of the separation process. The selectivity for CO2 was determined from the difference from 100% and the sum of all products. For the simulation of the separation process, the C11+ alkanes and the C11+ alkenes were assumed to be undecane or undecene.
The above table 1b shows that here predominately alkenes are formed but also a comparatively high proportion of methane. However, only a small amount of alcohols, namely ethanol, is formed.
For further comparison, the conversion with a Fischer-Tropsch reactor and a catalyst was simulated, as described in the literature by Syed Naqvi, SRI Consulting, Menlo Park, Calif. 94025, in PEP Review, 2007-2, December 2007, on page 5 in the right-hand column of table 1. The composition of the product mixture is reproduced in the following table 1c.
13-C18 (Oxygenates)
The above table 1c shows that here a multitude of compounds are produced, namely higher alkanes with up to 20 carbon atoms, higher alkenes, aromatic hydrocarbons, and alcohols with up to 19 carbon atoms. The proportion of methane was 8%. A total of 30% of compounds with C2-C4, 36% of compounds with C5-C11, 12% of compounds with C12-C19, 5% of compounds with more than 19 carbon atoms and 5% of oxygenates were formed. The selectivity of the formation of CO2 is not represented.
In this example, the distribution of the compounds of the product mixture which arose after the synthesis of higher alcohols in the three phases which are formed after the separation stage with the absorption medium (in this example dodecane) is clarified, a mixture being separated which arose after the conversion with a catalyst according to example 1, table 1. The catalyst comprised grains of nongraphitic carbon with cobalt nanoparticles dispersed therein. In the example according to table 2a, the assumed CO conversion was 50% while that in the example according to table 2b was 75%. The different CO conversions in the individual examples are achieved through the catalytic conversion of the synthesis gas in one or more reactors placed in series with addition of hydrogen at intervals for adjustment of the H2:CO ratio of H2:CO=1:1.
In the following example, the proportion of inert gas in the feed gas stream of the synthesis gas, which was catalytically converted to higher alcohols, i.e. the nitrogen content, varied with 10%, 20% and 30% (examples 3a, 3b and 3c). In this connection, the result was that the proportion of the lower alkenes and alkanes absorbed in the stage of the absorption of the product mixture in the high boiling point hydrocarbon of the organic liquid phase in each case decreases with increasing proportion of inert gas. This is valid for ethane, propane, ethene and propene, while the higher alkanes and alkenes from C4 in each case pass 100% into the organic liquid phase.
In the following exemplary embodiment, the amount of substance of the absorption medium used (in this instance dodecane) was varied each time. A product gas mixture which was obtained in the catalytic conversion of a synthesis gas mixture with the composition given in example 1 according to table 1 was subjected to the separation stage. In this connection, in four different simulations, 25%, 50%, 100% or 150% of the absorption medium was used. The results are reproduced in the following tables of examples 4a to 4d.
The amount of the absorption medium used (in the examples, dodecane was used) was varied in the above examples 4a to 4d with 886 kmol/h, 1767 kmol/h, 3528 kmol/h or 5289 kmol/h, it being possible to show that the amount of the alkenes and alkenes absorbed in the absorption medium increases approximately linearly with increasing molar flow, as expected the higher alkenes and alkanes (for example propene, propane) being more strongly absorbed than the lower alkenes and alkanes (ethene, ethane).
The alcohols methanol, ethanol, propanol and butanol, in the absorption operation, pass partly into the aqueous phase and partly into the dodecane phase, methanol and ethanol, as expected, passing predominantly into the aqueous phase while butanol, even at a low molar flow, already passes predominantly into the organic phase. The gases H2, CO, CO2, CH4 and N2 remain in the gas phase in this separation stage. At a low molar flow of the absorption medium, though, the lower alkenes and alkanes and sometimes also amounts of the higher alkenes (propene, 1-butene) and alkanes (propane, butane) pass into the gas phase. This, however, changes with increasing molar flow of the absorption medium. Thus, already at a molar flow of 3528 kmol/h, about half of the propene and 100% of the 1-butene passes into the organic liquid phase. At a still higher molar flow of 5289 kmol/h, the proportion of the propene absorbed in the organic phase increases even further. At a higher molar flow of the absorption medium, though, higher amounts of methanol and ethanol and also small amounts of CO2 and CH4 can also pass into the organic phase.
In the following exemplary embodiment, the catalyst was varied. A product gas mixture which was obtained in the catalytic conversion of a synthesis gas mixture with the composition given in example 1 according to table 1b was subjected to the separation stage. For the simulation, the selectivity for CO2 was determined from the difference from 100% by weight and the sum of all products. For the simulation of the separation process, the C11+ alkanes and the C11+ alkenes were assumed to be undecane or undecene.
The results of this exemplary embodiment show that even an olefin-rich product gas mixture, which comprises a low proportion of alcohols, can be subjected to the separation stage. With the exception of the short-chain alkanes and alkenes (ethane, ethene, propane, propene), the alkanes and alkenes are virtually completely absorbed in the liquid phase and, after the phase separation, are virtually completely present in the organic liquid phase.
10 feed inlet for the crude product gas stream
11 absorption apparatus
12 line for the discharge of the separated gaseous constituents
13 decanter
14 line for organic phase to the desorption apparatus
15 desorption apparatus
16 line for recycling the absorption medium
17 first distillation column
18 line for the feeding of the organic phase to the distillation column
19 line to the drying for alcohols separated in the distillation
20 line for hydrocarbons to the second distillation
21 second distillation column
22 line for the discharge of the hydrocarbons
23 return line for the recycling of the alcohols
24 third distillation column
25 molecular sieve
26 line for discharged water
27 product line for discharged alcohols
28 line for aqueous phase
29 line for alcohols
30 line for wastewater
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2019 213 493.9 | Sep 2019 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2020/062858 | 5/8/2020 | WO |
