Method for producing liquid hydrocarbons from synthesis gas

Abstract

1. A method for producing liquid hydrocarbons from a synthesis gas, the method comprising: providing a first synthesis gas containing hydrogen cyanide; converting at least a portion of the hydrogen cyanide in the first synthesis gas to ammonia to provide a second synthesis gas enriched in ammonia and depleted in hydrogen cyanide; passing the second synthesis gas through a scrubber and contacting the second synthesis gas with scrubber liquor, whereby at least a portion of the ammonia contained in the second synthesis gas is retained in the scrubber liquor to form a third synthesis gas depleted in ammonia and hydrogen cyanide; and passing the third synthesis gas through a Fischer-Tropsch reaction chamber to produce the liquid hydrocarbon product, wherein passing the third synthesis gas through the Fischer-Tropsch reaction chamber to produce the liquid hydrocarbon product comprises contacting the third synthesis gas with a catalyst comprising a metal selected from cobalt, iron and ruthenium, and the scrubber liquor comprises by-product water separated from products recovered from the Fischer-Tropsch reaction chamber.

Description

[Technical field]

[0001] The present invention relates to a method for producing liquid hydrocarbons from synthesis gas. [Background technology]

[0002] The Fischer-Tropsch process is a set of chemical reactions that convert a mixture of carbon monoxide and hydrogen into liquid hydrocarbons. These reactions take place in the presence of a metal catalyst, typically at temperatures between 150 and 300 °C and pressures ranging from one to several tens of atmospheres. The Fischer-Tropsch process ideally involves the reaction of the following reaction product: n H 2n+2 ), a series of chemical reactions that produce various hydrocarbons with different alkane groups. The more useful reactions produce alkanes as follows: (2n+1)H2+nCO→C n H 2n+2 +nH2O where n is typically 1-100 or more. Formation of methane (n=1) is undesirable. Most of the alkanes produced tend to be straight chain and suitable for being upgraded to produce middle distillate fuels such as diesel and jet fuel. In addition to alkane formation, competing reactions result in small amounts of alkenes, as well as alcohols and other oxygenated hydrocarbons. By-product water is a by-product that is separated from the product of the Fischer-Tropsch reaction. The Fischer-Tropsch reaction is highly exothermic, with a standard enthalpy of reaction (ΔH) of -165 kJ / mol CO total.

[0003] The synthesis gas (syngas) feed to a Fischer-Tropsch unit can be derived from many feedstocks, for example, natural gas by steam reforming and / or autothermal reforming, municipal solid waste and biomass by high-temperature gasification, or carbon dioxide and hydrogen by reverse water gas shift. The synthesis gas produced by these processes typically contains ppm levels of hydrogen cyanide and ammonia that deactivate the Fischer-Tropsch catalyst, therefore ideally hydrogen cyanide and ammonia are removed to single digit ppb levels. To remove these species from the synthesis gas, hydrogen cyanide is typically converted to ammonia via hydrolysis, and the ammonia is then removed using wet scrubbers. Achieving ppb levels of ammonia by conventional scrubbing is technically challenging.

[0004] US Patent No. 6,107,353 describes a process for removing hydrogen cyanide and ammonia from synthesis gas prior to a hydrocarbon synthesis reaction. The process involves passing the synthesis gas through a catalyst to hydrolyze the hydrogen cyanide to ammonia. The hydrolysis gas removed from the hydrolysis zone is then contacted with water in a scrubbing zone to dissolve the ammonia. However, to achieve ppb levels of hydrogen cyanide and ammonia, the process requires the use of a hydrogen cyanide absorption zone prior to entering the synthesis gas into the Fischer-Tropsch reactor. The use of such a hydrogen cyanide absorption zone increases the complexity of the process.

[0005] The present invention aims to address at least some of the problems associated with the prior art, or at least to provide a commercially acceptable alternative solution. Summary of the Invention

[0006] One aspect of the present disclosure is a method for producing liquid hydrocarbons from synthesis gas, comprising: Providing a first synthesis gas containing hydrogen cyanide; converting at least a portion of the hydrogen cyanide in the first synthesis gas to ammonia to provide a second synthesis gas enriched in ammonia and depleted in hydrogen cyanide; passing the second synthesis gas through a scrubber and contacting the second synthesis gas with scrubber liquor, whereby at least a portion of the ammonia contained in the second synthesis gas is retained in the scrubber liquor to form a third synthesis gas depleted in ammonia and hydrogen cyanide; and passing the third synthesis gas through a Fischer-Tropsch reaction chamber to produce a liquid hydrocarbon product, wherein passing the third synthesis gas through the Fischer-Tropsch reaction chamber to produce the liquid hydrocarbon product comprises contacting the third synthesis gas with a catalyst comprising a metal selected from cobalt, iron and ruthenium, and wherein the scrubber liquor comprises by-product water separated from the product recovered from the Fischer-Tropsch reaction chamber. [Brief description of the drawings]

[0007] [Figure 1] FIG. 2 is a flow diagram of an embodiment of a method according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] In a first aspect, the present disclosure provides a method for producing liquid hydrocarbons from a synthesis gas, comprising: Providing a first synthesis gas containing hydrogen cyanide; converting at least a portion of the hydrogen cyanide in the first synthesis gas to ammonia to provide a second synthesis gas enriched in ammonia and depleted in hydrogen cyanide; passing the second synthesis gas through a scrubber and contacting the second synthesis gas with scrubber liquor, whereby at least a portion of the ammonia contained in the second synthesis gas is retained in the scrubber liquor to form a third synthesis gas depleted in ammonia and hydrogen cyanide; and passing the third synthesis gas through a Fischer-Tropsch reaction chamber to produce a liquid hydrocarbon product, wherein passing the third synthesis gas through the Fischer-Tropsch reaction chamber to produce the liquid hydrocarbon product comprises contacting the third synthesis gas with a catalyst comprising a metal selected from cobalt, iron and ruthenium, and wherein the scrubber liquor comprises by-product water separated from the product recovered from the Fischer-Tropsch reaction chamber.

[0009] Each aspect or embodiment defined in this specification may be combined with any other aspect(s) or embodiment(s) unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature(s) indicated as being preferred or advantageous.

[0010] Advantageously, in contrast to conventional processes, the use of by-product water as scrubber liquor, which is carried out as a final treatment of the syngas upstream of the Fischer-Tropsch reaction chamber, reduces the risk of catalyst poisoning present in conventional scrubbing processes. The ammonia removal capacity of the scrubber liquor is also improved by the presence of carbon dioxide dissolved in the by-product water. If the scrubber water is not enriched with carbon dioxide, more scrubbing stages or more scrubber liquor may be required to reduce the ammonia level. It is therefore possible to reduce the ammonia and hydrogen cyanide content of the syngas to single digit ppb levels. As a result, the poisoning of the Fischer-Tropsch catalyst is reduced, which means that the process is more efficient as the Fischer-Tropsch catalyst does not have to be regenerated or replaced less regularly. The process uses the third syngas as feed to the chamber of the Fischer-Tropsch reaction without further treatment. The invention thus avoids the use of downstream hydrogen cyanide and ammonia absorption beds, thereby resulting in a more simplified and cost-effective process compared to other processes.

[0011] As used herein, the term "liquid hydrocarbon" may include species formed from carbon and hydrogen that are liquid at room temperature and pressure. Hydrocarbons typically include alkanes and typically contain from 5 to 100 or more carbon atoms per molecule.

[0012] The term "syngas" or "synthesis gas" as used herein may encompass a gas mixture containing hydrogen and carbon monoxide. In the method of the present invention, the first synthesis gas contains carbon monoxide (i.e., CO), hydrogen (i.e., molecular hydrogen H2), hydrogen cyanide (i.e., HCN), and typically less than 1% by volume of carbon dioxide (CO2). The synthesis gas may contain other gases, such as water, methane, ammonia, and sulfur-containing gases, such as hydrogen sulfide (i.e., H2S), as well as solid species, such as dust and coke. The synthesis gas is typically produced from the gasification of carbonaceous materials. In some configurations, providing the first synthesis gas containing hydrogen cyanide may include gasification of biomass, municipal waste, or coal, steam reforming of a hydrocarbon feedstock, or recovering the first synthesis gas from a reverse water gas shift unit. In the present invention, the first synthesis gas is preferably non-fossil fuel-based, i.e., produced by gasification of biomass and / or municipal waste. Such feeds are more sustainable than fossil fuels, but create additional problems with catalyst poisoning. Alternatively, the first synthesis gas may be recovered from a reverse water gas shift unit used to convert carbon dioxide, such as carbon dioxide separated from combustion gases, air, or other chemical processes, to carbon monoxide along with hydrogen. The composition of the synthesis gas varies depending on its production method and the starting materials used.

[0013] The method includes passing the second syngas through a scrubber and contacting the second syngas with a scrubber liquor. Scrubbers and scrubber liquors are known in the art. The efficiency of ammonia removal can be improved by increasing the residence time in the scrubber or by increasing the surface area of ​​the scrubber liquor, for example, by using trays, structured packing, or random packing.

[0014] The third synthesis gas is passed through a Fischer-Tropsch reaction chamber containing a Fischer-Tropsch catalyst. Fischer-Tropsch reaction chambers are known in the art.

[0015] The scrubber liquor includes by-product water, which may be separated from the products recovered from the Fischer-Tropsch reaction chamber using conventional separation equipment.

[0016] The by-product water preferably contains at least 0.01 mol / L of carbon dioxide, more preferably at least 0.02 mol / L of carbon dioxide. Such a concentration of carbon dioxide may result in a particularly high ammonia removal capacity of the scrubber liquor.

[0017] In a preferred embodiment, the by-product water is preferably saturated with carbon dioxide under the temperature and pressure conditions of the scrubber.

[0018] The liquid hydrocarbon products preferably comprise alkanes, more preferably alkanes having from 5 to 100 or more carbon atoms. Such hydrocarbon products may be particularly desirable. Furthermore, the cobalt catalysts typically used to produce such product mixtures may be particularly susceptible to poisoning by hydrogen cyanide.

[0019] The by-product water is preferably recovered from the Fischer-Tropsch reaction chamber. As mentioned above, in the Fischer-Tropsch reaction, hydrogen and carbon monoxide are converted to hydrocarbons and water. By using the by-product water stream, rather than having to use another water source such as boiler feed water or desalinated water, the process is simplified and the operating costs are reduced. Furthermore, because the by-product water is recovered from the Fischer-Tropsch reaction chamber, the water does not contain poisons. Thus, the risk of introducing poisons that could deactivate the catalyst is reduced over comparable processes using other scrubber liquor sources. In addition, carbon dioxide contained in the third synthesis gas, as well as carbon dioxide produced by side reactions in the Fischer-Tropsch reaction chamber, typically becomes dissolved in the by-product water, which can increase the ammonia scrubbing capacity of the scrubber liquor.

[0020] The method may further include recovering gas from the Fischer-Tropsch reaction chamber, which may be recycled to the Fischer-Tropsch reaction chamber. Such gas may include, for example, carbon monoxide, carbon dioxide, and / or hydrogen. The recycled gas including carbon monoxide and / or hydrogen may increase the efficiency of the process and improve the yield. Since carbon dioxide is inert, the recycled gas including carbon dioxide may increase the concentration of carbon dioxide in the gas passing through the Fischer-Tropsch reaction chamber, i.e., to more than 0.5% by volume, typically to about 5% by volume. This may cause the by-product water produced in the Fischer-Tropsch reaction chamber to be saturated with carbon dioxide. As a result, the efficiency of ammonia removal may be increased when recovering the carbon dioxide-enriched water from the Fischer-Tropsch reaction chamber. The use of water recovered from the hydrocarbon synthesis reaction chamber as a scrubber liquor is described, for example, in U.S. Pat. No. 6,107,353 (see discussion above). However, in US Patent No. 6,107,353, the recovered water is first stripped with natural gas to remove organic acids and oxygenates so that they do not contaminate the final hydrogen cyanide / ammonia absorption zone. Such stripping also removes carbon dioxide. In contrast, the present process does not require stripping of the water prior to use as a scrubber liquor. Furthermore, the present process can reduce hydrogen cyanide and ammonia in the synthesis gas to ppb levels without the use of a downstream hydrogen cyanide / ammonia absorption zone between the scrubbing stage and the Fischer-Tropsch reaction chamber.

[0021] Providing a first synthesis gas containing hydrogen cyanide preferably comprises gasification of biomass and / or municipal waste. Biomass and municipal waste are low cost and becoming more widely available, and synthesis gas produced from these species may be particularly suitable for the production of liquid hydrocarbons. Gasification is a technique known in the art. During gasification, biomass and / or municipal waste are blown with oxygen and steam (water vapor) while being heated (and possibly also pressurized). It is essential that the oxidant provided is insufficient for complete oxidation (combustion) of the fuel. During the mentioned reaction, oxygen and water molecules oxidize the biomass and / or municipal waste, producing a gaseous mixture of carbon dioxide, carbon monoxide, water vapor, and molecular hydrogen. Advantageously, heat can be recovered from the gasification for use in other steps of the method. Alternatively, providing a first synthesis gas containing hydrogen cyanide is preferably performed by steam reforming of a hydrocarbon feedstock or by recovering synthesis gas from a reverse water gas shift unit. Synthesis gas produced from these species may be particularly suitable for the production of liquid hydrocarbons.

[0022] Preferably, the first synthesis gas is subjected to a carbon dioxide removal step. This may be accomplished using conventional carbon dioxide removal equipment such as an acid gas removal unit (AGRU). The removal of carbon dioxide is preferably carried out by absorption using a suitable liquid to remove the carbon dioxide from the first synthesis gas. This can make the process more efficient as the reduced volume of inert gas reduces the energy required to perform any heating or cooling steps.

[0023] The first synthesis gas may further comprise ammonia. The method of the present invention is particularly suitable for use with synthesis gas containing ammonia in view of the high ammonia removal capacity of the scrubber liquor.

[0024] The first synthesis gas may further comprise a sulfur-containing gas. In this case, the method preferably further comprises removing at least a portion of the sulfur-containing gas from the synthesis gas prior to the step of converting at least a portion of the hydrogen cyanide to ammonia. Removing at least a portion of the sulfur-containing gas from the synthesis gas preferably comprises contacting the synthesis gas with a solvent at a pressure of at least 1 MPa to dissolve at least a portion of the sulfur-containing gas. Removing at least a portion of the sulfur-containing gas from the synthesis gas preferably comprises contacting the synthesis gas with a suitable sulfur absorbent. This contacting can be effectively achieved simultaneously with carbon dioxide removal using an acid gas removal unit (AGRU).

[0025] The conversion of at least a portion of the hydrogen cyanide to ammonia to provide a second synthesis gas preferably comprises catalytic hydrolysis of hydrogen cyanide. Catalytic hydrolysis is a simple method of converting hydrogen cyanide to ammonia, since it can be carried out at a relatively low temperature by adding water. Thus, the simplicity and efficiency of the process can be improved. Water for hydrolysis can be sprayed into the synthesis gas stream before being fed to the hydrolysis bed, or added to the synthesis gas as steam. The equilibrium reaction is as follows:

[0026] [ka]

[0027] The hydrolysis is preferably carried out at a temperature above 100° C., more preferably between 150° C. and 300° C. Lower temperatures may result in undesirably low levels of hydrolysis. Higher temperatures may increase the energy costs of the process without significant improvement in hydrogen cyanide conversion.

[0028] The hydrolysis is preferably carried out using an alumina catalyst, more preferably an activated alumina catalyst, which may provide particularly high conversions and / or allow operation at advantageously low temperatures.

[0029] Following hydrolysis, the second synthesis gas is preferably cooled, more preferably to a temperature below 40° C., even more preferably to ambient temperature, before contacting the scrubber.

[0030] The second synthesis gas preferably contains less than 10 ppbv hydrogen cyanide. Such low levels of hydrogen cyanide may result in particularly low levels of poisoning of the Fischer-Tropsch catalyst.

[0031] The third synthesis gas preferably contains less than 10 ppbv ammonia. Such low levels of hydrogen cyanide may result in particularly low levels of poisoning of the Fischer-Tropsch catalyst.

[0032] Despite upstream processing of the first syngas, the second syngas may contain trace amounts of sulfur compounds which are desirably removed. Preferably, the method further comprises passing the second syngas through a sulfur guard bed to remove sulfur compounds from the second syngas prior to passing the second syngas through the scrubber. The sulfur guard bed preferably comprises a zinc oxide catalyst. Removal of residual sulfur-containing species from the second syngas can reduce poisoning of the Fischer-Tropsch catalyst by sulfur.

[0033] The temperature of the Fischer-Tropsch reaction chamber is preferably between 150°C and 300°C. Lower temperatures may result in undesirably low levels of liquid hydrocarbons being produced. Higher temperatures may increase the energy costs of the process without significantly increasing the levels of liquid hydrocarbons produced.

[0034] Passing the third synthesis gas through a Fischer-Tropsch reaction chamber to produce the liquid hydrocarbon product comprises contacting the third synthesis gas with a catalyst comprising a metal selected from cobalt, iron and ruthenium, preferably cobalt. Such catalysts are particularly effective in catalyzing the Fischer-Tropsch reaction and / or allow the reaction to proceed at advantageously low temperatures and / or in high yields.

[0035] The hydrocarbon products from the Fischer-Tropsch reaction chamber may be separated from by-product water and unreacted gases and then converted to liquid hydrocarbon fuels, for example by hydrocracking.

[0036] The invention will now be described with reference to the following non-limiting examples. EXAMPLES

[0037] FIG. 1 shows a flow diagram of one embodiment of the method according to the invention. In the exemplary method, a first syngas is produced by gasification of biomass (not shown). The first syngas then undergoes acid gas removal (not shown). After acid gas removal, the first syngas 10 is heated to about 125° C. in a Syngas Polishing Interchanger 12 using hot gas from a downstream sulfur guard bed 26. Following this, the syngas passes through another Syngas Polishing Interchanger 14 where it is heated to about 190° C. with gas from a Hydrogen Cyanide Hydrolysis Bed 20. The syngas is further heated to 250° C. by a Hydrogen Cyanide Hydrolysis Bed Preheater 18 using saturated high pressure steam. There is a boiler feedwater supply 18 upstream of the Hydrogen Cyanide Hydrolysis Bed 20 to provide sufficient water for the hydrolysis of COS and HCN. After cooling the second syngas 22 to 150°C through the syngas polishing interchanger 14, the cooled second syngas 24 enters the final purification sulfur guard bed 26 containing zinc oxide catalyst. The desulfurized second syngas 28 is then cooled in the syngas polishing interchanger 12, which then cools the syngas 30 to 40°C in the syngas washing drum cooler 32. Ammonia present in the gas or formed by hydrogen cyanide hydrolysis is washed from the syngas 34 in the syngas washing drum 36 using FT by-product water 38 from the downstream FT unit (not shown). The syngas washing water 46 from the syngas washing drum bottom is sent for treatment. The purified third syngas is recovered from the syngas washing drum 36 via line 44 and sent directly to the FT unit (not shown). The syngas scrubbing drum 36 contains either trays, structured or random packing 40 to provide mass transfer surface area, and a demister 42 to prevent liquid carryover. The water scrubbing rate and number of mass transfer stages are set to achieve the required ammonia removal level, i.e., less than 10 ppbv.

[0038] The foregoing detailed description has been provided for purposes of explanation and illustration and is not intended to limit the scope of the appended claims. Many variations of the presently preferred embodiments described herein will be apparent to those of ordinary skill in the art and remain within the scope of the appended claims and their equivalents.

[0039] The process shown in Figure 1 was modelled using standard process software. The stream conditions and compositions were as follows:

[0040] [Table 1]

Claims

1. 1. A method for producing liquid hydrocarbons from synthesis gas, comprising the steps of: Providing a first synthesis gas containing hydrogen cyanide; converting at least a portion of the hydrogen cyanide in the first synthesis gas to ammonia to provide a second synthesis gas enriched in ammonia and depleted in hydrogen cyanide; passing the second synthesis gas through a scrubber and contacting the second synthesis gas with scrubber liquor, whereby at least a portion of the ammonia contained in the second synthesis gas is retained in the scrubber liquor to form a third synthesis gas depleted in ammonia and hydrogen cyanide; and passing the third synthesis gas through a Fischer-Tropsch reaction chamber to produce a liquid hydrocarbon product, wherein passing the third synthesis gas through a Fischer-Tropsch reaction chamber to produce a liquid hydrocarbon product comprises contacting the third synthesis gas with a catalyst comprising a metal selected from cobalt, iron and ruthenium, and wherein the scrubber liquor comprises by-product water separated from products recovered from the Fischer-Tropsch reaction chamber.

2. 2. The method of claim 1, wherein the by-product water contains at least 0.01 mol / L of carbon dioxide, preferably at least 0.02 mol / L of carbon dioxide.

3. 3. The method of claim 1 or claim 2, wherein the by-product water is saturated with carbon dioxide under temperature and pressure conditions of the scrubber.

4. The process of any one of claims 1 to 3, wherein the liquid hydrocarbon product comprises an alkane.

5. 5. The method of any one of claims 1 to 4, wherein providing a first synthesis gas containing hydrogen cyanide comprises gasification of biomass and / or municipal waste, or by steam reforming of a hydrocarbon feedstock, or the first synthesis gas is recovered from a reverse water gas shift unit.

6. The method according to any one of claims 1 to 5, wherein the first synthesis gas has been subjected to a step of carbon dioxide removal.

7. The method of any one of claims 1 to 6, wherein the first synthesis gas further comprises ammonia.

8. 8. The method of any one of claims 1 to 7, wherein converting at least a portion of the hydrogen cyanide to ammonia to provide the second synthesis gas comprises catalytic hydrolysis of the hydrogen cyanide.

9. The process according to claim 8, wherein the hydrolysis is carried out at a temperature above 100°C, preferably between 150°C and 300°C.

10. 10. The process according to claim 8 or claim 9, wherein the hydrolysis is carried out using an alumina catalyst, preferably an activated alumina catalyst.

11. 11. The method of any one of claims 1 to 10, wherein the second synthesis gas contains less than 10 ppbv hydrogen cyanide.

12. 12. The method of any one of claims 1 to 11, wherein the third synthesis gas comprises less than 10 ppbv ammonia.

13. 13. The method of any one of claims 1 to 12, wherein the method further comprises passing the second syngas through a sulfur guard bed to remove sulfur compounds from the second syngas prior to passing the second syngas to the scrubber.

14. The method of any one of claims 1 to 13, wherein the temperature of the Fischer-Tropsch reaction chamber is from 150°C to 300°C.

15. 15. The method of any one of claims 1 to 14, wherein passing the third synthesis gas through a Fischer-Tropsch reaction chamber to produce liquid hydrocarbon products comprises contacting the third synthesis gas with a catalyst comprising cobalt.

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