WO2020173828A1 - Method for combined removal of siloxanes, sulfur and vocs - Google Patents

Method for combined removal of siloxanes, sulfur and vocs Download PDF

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Publication number
WO2020173828A1
WO2020173828A1 PCT/EP2020/054638 EP2020054638W WO2020173828A1 WO 2020173828 A1 WO2020173828 A1 WO 2020173828A1 EP 2020054638 W EP2020054638 W EP 2020054638W WO 2020173828 A1 WO2020173828 A1 WO 2020173828A1
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Prior art keywords
siloxanes
gas
alcohol
sulfur
catalyst
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PCT/EP2020/054638
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French (fr)
Inventor
Niklas Bengt Jakobsson
Kresten Egeblad
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Haldor Topsøe A/S
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Publication of WO2020173828A1 publication Critical patent/WO2020173828A1/en

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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/14—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols by absorption
    • C—CHEMISTRY; METALLURGY
    • C10—PETROLEUM, GAS OR COKE INDUSTRIES; TECHNICAL GASES CONTAINING CARBON MONOXIDE; FUELS; LUBRICANTS; PEAT
    • C10L—FUELS NOT OTHERWISE PROVIDED FOR; NATURAL GAS; SYNTHETIC NATURAL GAS OBTAINED BY PROCESSES NOT COVERED BY SUBCLASSES C10G, C10K; LIQUEFIED PETROLEUM GAS; ADDING MATERIALS TO FUELS OR FIRES TO REDUCE SMOKE OR UNDESIRABLE DEPOSITS OR TO FACILITATE SOOT REMOVAL; FIRELIGHTERS
    • C10L3/00—Gaseous fuels; Natural gas; Synthetic natural gas obtained by processes not covered by subclass C10G, C10K; Liquefied petroleum gas
    • C10L3/06—Natural gas; Synthetic natural gas obtained by processes not covered by C10G, C10K3/02 or C10K3/04
    • C10L3/10—Working-up natural gas or synthetic natural gas
    • C10L3/101—Removal of contaminants
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2257/00—Components to be removed
    • B01D2257/30—Sulfur compounds
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2257/00—Components to be removed
    • B01D2257/55—Compounds of silicon, phosphorus, germanium or arsenic
    • B01D2257/556—Organic compounds
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2257/00—Components to be removed
    • B01D2257/70—Organic compounds not provided for in groups B01D2257/00 - B01D2257/602
    • B01D2257/708—Volatile organic compounds V.O.C.'s
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2258/00—Sources of waste gases
    • B01D2258/05—Biogas
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/34—Chemical or biological purification of waste gases
    • B01D53/74—General processes for purification of waste gases; Apparatus or devices specially adapted therefor
    • B01D53/75—Multi-step processes
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/34—Chemical or biological purification of waste gases
    • B01D53/74—General processes for purification of waste gases; Apparatus or devices specially adapted therefor
    • B01D53/86—Catalytic processes
    • B01D53/8603—Removing sulfur compounds
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/34—Chemical or biological purification of waste gases
    • B01D53/74—General processes for purification of waste gases; Apparatus or devices specially adapted therefor
    • B01D53/86—Catalytic processes
    • B01D53/8678—Removing components of undefined structure
    • B01D53/8687—Organic components
    • C—CHEMISTRY; METALLURGY
    • C10—PETROLEUM, GAS OR COKE INDUSTRIES; TECHNICAL GASES CONTAINING CARBON MONOXIDE; FUELS; LUBRICANTS; PEAT
    • C10L—FUELS NOT OTHERWISE PROVIDED FOR; NATURAL GAS; SYNTHETIC NATURAL GAS OBTAINED BY PROCESSES NOT COVERED BY SUBCLASSES C10G, C10K; LIQUEFIED PETROLEUM GAS; ADDING MATERIALS TO FUELS OR FIRES TO REDUCE SMOKE OR UNDESIRABLE DEPOSITS OR TO FACILITATE SOOT REMOVAL; FIRELIGHTERS
    • C10L2290/00—Fuel preparation or upgrading, processes or apparatus therefore, comprising specific process steps or apparatus units
    • C10L2290/54—Specific separation steps for separating fractions, components or impurities during preparation or upgrading of a fuel
    • C10L2290/541—Absorption of impurities during preparation or upgrading of a fuel
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02A—TECHNOLOGIES FOR ADAPTATION TO CLIMATE CHANGE
    • Y02A50/00—TECHNOLOGIES FOR ADAPTATION TO CLIMATE CHANGE in human health protection, e.g. against extreme weather
    • Y02A50/20—Air quality improvement or preservation, e.g. vehicle emission control or emission reduction by using catalytic converters

Definitions

  • the present invention deals with the removal of siloxanes from biogas, especially landfill gas.
  • Biogas is typically a waste product from sources including anaerobic digesters, municipal waste treatment plants and landfill sites, and it often contains siloxanes which have a poisoning effect on catalysts and cause degradation of engine lubrication oil.
  • Siloxanes further cause silica build-up on spark plugs for engines and deposits on gas turbine blade tips.
  • Siloxanes are organosilicon compounds comprising silicon, carbon, hydrogen and oxygen which have Si-O-Si bonds.
  • Si loxanes can be linear as well as cyclic. They may be pre- sent in digester or landfill gas because they are used in various beauty products, such as cosmetics and shampoos that are washed down drains or otherwise disposed of, so that they end up in municipal wastewater and landfills.
  • Si loxanes are not broken down during anaerobic digestion, and as a result, waste gas captured from treatment plants and landfills is often heavily contaminated with these com pounds. It is known that siloxanes can be removed using non-regenerative packed bed adsorption with activated car bon or porous silica as sorbent.
  • Regenerative sorbents can also be used as well as units based on gas cooling to very low temperatures to precipitate the siloxanes from the gas. Further, liquid extraction technologies are used. In addi tion, these technologies can be used in combination.
  • a silanol is a functional group in silicon chemistry with the connectivity Si-O-H. It is related to the hydroxy func tional group C-O-H found in all alcohols. So a major issue in the utilization of raw gas from land fills and anaerobic digesters is to provide a gas stream with a low sulfur content, i.e. less than a few hundred ppm, and with a very low content of siloxanes, typically linear or cyclic dimethyl Si-O-Si compounds, and silanols.
  • any catalysts installed to control ex haust gas emissions are sensitive to S1O2 and particularly unconverted siloxanes entrained in the gas stream, in fact even more so than the engine itself.
  • SCR selective catalytic reduction
  • the siloxane tolerance can be as low as 250 ppb .
  • landfill gases that vent gases directly to the atmosphere can have emission control needs which go beyond siloxane re moval.
  • landfill gases that are going to be vented to the atmosphere need to be scrubbed before release to remove not only siloxanes, but also reduced sulfur compounds (e.g. 3 ⁇ 4S) and volatile organic compounds (VOCs) as well as other contaminants that may be carried in the gas.
  • reduced sulfur compounds e.g. 3 ⁇ 4S
  • VOCs volatile organic compounds
  • WO 2012/006729 A1 describes an integrated biogas cleaning system designed to remove water, siloxanes, sulfur, oxygen, chlorides and VOCs from biogas sources such as landfills and digesters for heat and power generation systems.
  • the biogas system cools a biogas stream to partially remove contaminants, blends in a small concentration of hydrogen gas and combusts the re maining oxygen to heat the biogas and leave sufficient hy- drogen suitable for a downstream sequence of further con taminant conversion and removal in stages using a hydro desulfurization bed and adsorbent media beds. Siloxanes, chlorine, oxygen and sulfur are allegedly removed down to ppb levels.
  • US 2006/0225571 A1 discloses a siloxane removal process in which biogas released from landfills and sewage treatment plants is freed of siloxane contaminants by passing the bi ogas through a bed containing activated alumina, which ab sorbs the siloxanes.
  • the absorption capacity of the acti vated alumina can be recovered by passing a regeneration gas through the bed of activated alumina.
  • a method for removing siloxanes from a gas stream includes flowing the gas stream carrying siloxanes through an adsorbent media to remove at least part of the siloxanes from the gas stream, said adsorbent media comprising lignite-based activated carbon .
  • WO 2017/140662 A1 describes a process for combined removal of siloxanes and sulfur-containing compounds from biogas streams from landfills and anaerobic digesters, com prising the steps of heating the biogas and feeding it to an absorber containing a siloxane absorption bed, feeding the siloxane-free gas effluent from the absorber to a con- verter containing a sulfur monolith catalyst, where organic and inorganic sulfur compounds are oxidized to SO2, cooling the hot exit gas from the converter and removing SO2 from the cooled gas.
  • a purified gas stream free of sulfur compounds and siloxanes is obtained.
  • the subject matter of the present invention differs from that of WO 2017/140662 A1 in that a step of siloxane re moval is performed prior to the siloxane absorption step. So the present invention relates to a method for the com bined removal of gas-phase siloxanes, sulfur and optionally also VOCs from various gases, such as industrial gas, land fill gas or digester gas.
  • the method makes use of Applicant's SiloxFree® sorbent, which is a highly activated and acidic alumina sorbent that operates at elevated temperatures and converts the siloxane components to silica.
  • the silica is trapped in the pore system of the SiloxFree sorbent.
  • the SiloxFree system is thus a sacrificial media. For high siloxane contents (of around 50 to 150 mg Si/m 3 ) , the consumption of sorbent be comes prohibitive from a process economy point of view.
  • the preferred embodiment of the inventive method is to use a regenerative siloxane removal system upstream from the SiloxFree unit and use the SiloxFree unit as a polisher.
  • the regenerative systems suffer from very complex operation cycles, high operation costs associated with flaring the siloxane-containing regeneration gas and also a high capex.
  • the challenge is to provide a system that can effect bulk removal of siloxanes down to a level of 5- 15 mg Si/m 3 upstream from the SiloxFree system.
  • the idea underlying the procedure of the present invention is to use an alcohol scrubber where the raw gas is being contacted with an alcohol liquid to effect absorp tion of the siloxanes.
  • the absorbed siloxanes are subsequently burnt and subjected to a further scrubbing, while the process gas is polished in a SiloxFree polisher.
  • the invention relates to a process for the combined removal of siloxanes, sulfur and VOCs from a raw gas, said process comprising the steps of
  • the catalyst in the catalyst unit is preferably a sulfur monolith catalyst (SMC) type catalyst.
  • the polisher is a SiloxFree® unit.
  • the alcohol composition in the scrubber liquid is prefera- bly methanol, ethanol, propanol, butanol or mixtures thereof, more preferably methanol and ethanol or mixtures thereof, and most preferably pure methanol.
  • the invention relates to a process for the combined re moval of siloxanes, sulfur and VOCs from a raw gas, which can be a biogas, especially a landfill gas.
  • a scrubber is used, in which the gas is contacted with a liquid, said liquid containing methanol, ethanol, propanol, butanol or mixtures thereof.
  • the vapor pressure of the alcohol or of the mixture of alcohols, which is used results in a sufficiently low vapor pressure of alco hol in the scrubber liquid when considering that the gas will have a significant water content.
  • the scrubber will operate at atmospheric pressure, and a certain part of the siloxanes will dissolve in the alcohol at atmospheric pres sure . From the scrubber, the gas is led to the SiloxFree unit, where the remaining part of the siloxanes is removed, and an SMC catalyst is placed directly downstream from the Si loxFree sorbent bed to oxidize any residual alcohol that the gas may have picked up in the scrubber.
  • the mixture of alcohol and water should be chosen such that the uptake of alcohol in the gaseous phase is minimized, i.e. similar to azeotrope distillation for vapor/liquid systems.
  • a blow-down from the scrubber is necessary to maintain a suitable siloxane level in the scrubber liquid.
  • the liquid is combusted in a burner, and the small volume of flue gas is mixed into the main gas stream to be further processed.
  • a low temperature bag filter is inserted between the mixing point and the SiloxFree unit to remove silica from liquid combustion flue gas. This process provides a robust bulk removal of siloxanes which can preferably be combined with the SiloxFree technology and sorbent.
  • the method of the invention for combined removal of silox- anes, sulfur and optionally VOCs is illustrated in further detail in the below example with reference to the enclosed figure .
  • Raw biogas for cleaning is fed to an alcohol scrubber A via line a. Any necessary make-up alcohol is fed to the scrub ber A via line b. In the alcohol scrubber A, absorption of siloxanes takes place. To maintain a suitable siloxane level in the scrubber liquid, a siloxane-rich alcohol blow down from the scrubber is fed to a burner B along line c, and air is supplied to the burner via line d. Hot flue gas containing S1O2 passes through a hot gas filter H and leaves the filter at around 600°C. This hot gas f is fed to a further scrubber S via a feed/effluent heat exchanger Hex. From the scrubber S, an effluent h is discharged.
  • Process gas from the scrubber A lean in siloxanes but con taining alcohol and water, is passed to a heater C via line e, and from heater C the gas proceeds to the feed/effluent heat exchanger Hex. After passing the heat exchanger, the gas is fed to a SiloxFree polisher P. The polished gas g is then fed to a catalyst unit D containing an SMC type cata lyst (SMC stands for sulfur monolith catalyst) , which oxi dizes 3 ⁇ 4S selectively to SO2 and converts residual methanol along with sulfur and VOCs without converting methane.
  • SMC sulfur monolith catalyst

Abstract

A process for combined removal of siloxanes, sulfur and VOCs from a raw gas comprises feeding the raw gas to an alcohol scrubber to absorb the siloxanes, performing an alcohol blow-down to control the build-up of siloxanes and other components, which are soluble in the alcohol, and separating the process gas from the siloxane-rich scrubber liquid, heating the process gas, now lean in siloxanes, removing traces of siloxanes in a polisher, and feeding the treated process gas to a catalyst unit containing a catalyst converting residual alcohol along with sulfur and VOCs without converting methane.

Description

Method for combined removal of siloxanes, sulfur and VOCs
The present invention deals with the removal of siloxanes from biogas, especially landfill gas. Biogas is typically a waste product from sources including anaerobic digesters, municipal waste treatment plants and landfill sites, and it often contains siloxanes which have a poisoning effect on catalysts and cause degradation of engine lubrication oil. Siloxanes further cause silica build-up on spark plugs for engines and deposits on gas turbine blade tips.
Siloxanes are organosilicon compounds comprising silicon, carbon, hydrogen and oxygen which have Si-O-Si bonds. Si loxanes can be linear as well as cyclic. They may be pre- sent in digester or landfill gas because they are used in various beauty products, such as cosmetics and shampoos that are washed down drains or otherwise disposed of, so that they end up in municipal wastewater and landfills. Si loxanes are not broken down during anaerobic digestion, and as a result, waste gas captured from treatment plants and landfills is often heavily contaminated with these com pounds. It is known that siloxanes can be removed using non-regenerative packed bed adsorption with activated car bon or porous silica as sorbent. Regenerative sorbents can also be used as well as units based on gas cooling to very low temperatures to precipitate the siloxanes from the gas. Further, liquid extraction technologies are used. In addi tion, these technologies can be used in combination. A silanol is a functional group in silicon chemistry with the connectivity Si-O-H. It is related to the hydroxy func tional group C-O-H found in all alcohols. So a major issue in the utilization of raw gas from land fills and anaerobic digesters is to provide a gas stream with a low sulfur content, i.e. less than a few hundred ppm, and with a very low content of siloxanes, typically linear or cyclic dimethyl Si-O-Si compounds, and silanols. Pipeline specifications for natural gas are even stricter. In this case, ¾S must be removed to a residual concentra tion below 5 ppm, and CO2 and N2 need to be removed as well. Combustion of sulfur containing compounds leads to formation of sulfur trioxide which will react with moisture in the gas to form sulfuric acid, which can condense in cold spots and lead to corrosion. However, particularly si loxanes give rise to problems because they are converted to S1O2 during combustion, leading to build-up of abrasive solid deposits inside the engine and causing damage, re- duced service time and increased maintenance requirements for many components such as compressors, fans, blowers, burner nozzles, heat recovery surfaces in boilers and for gas engine components such as spark plugs, valves, pistons etc. In addition to causing damage and reduced service time to the engine, also any catalysts installed to control ex haust gas emissions are sensitive to S1O2 and particularly unconverted siloxanes entrained in the gas stream, in fact even more so than the engine itself. For an SCR (selective catalytic reduction) catalyst, for example, the siloxane tolerance can be as low as 250 ppb . For the reasons outlined above it is very desirable to re move siloxanes, silanols and sulfur-containing compounds from gas streams. The presence of siloxanes in biogas causes an increased maintenance cost, frequent catalyst replacements and a re duced power production.
Landfills that vent gases directly to the atmosphere can have emission control needs which go beyond siloxane re moval. Thus, landfill gases that are going to be vented to the atmosphere need to be scrubbed before release to remove not only siloxanes, but also reduced sulfur compounds (e.g. ¾S) and volatile organic compounds (VOCs) as well as other contaminants that may be carried in the gas.
Regarding prior art in the field, WO 2012/006729 A1 de scribes an integrated biogas cleaning system designed to remove water, siloxanes, sulfur, oxygen, chlorides and VOCs from biogas sources such as landfills and digesters for heat and power generation systems. The biogas system cools a biogas stream to partially remove contaminants, blends in a small concentration of hydrogen gas and combusts the re maining oxygen to heat the biogas and leave sufficient hy- drogen suitable for a downstream sequence of further con taminant conversion and removal in stages using a hydro desulfurization bed and adsorbent media beds. Siloxanes, chlorine, oxygen and sulfur are allegedly removed down to ppb levels.
US 2006/0225571 A1 discloses a siloxane removal process in which biogas released from landfills and sewage treatment plants is freed of siloxane contaminants by passing the bi ogas through a bed containing activated alumina, which ab sorbs the siloxanes. The absorption capacity of the acti vated alumina can be recovered by passing a regeneration gas through the bed of activated alumina.
In WO 2014/099422 A1 , a method for removing siloxanes from a gas stream is described. The method includes flowing the gas stream carrying siloxanes through an adsorbent media to remove at least part of the siloxanes from the gas stream, said adsorbent media comprising lignite-based activated carbon .
Finally, WO 2017/140662 A1 describes a process for combined removal of siloxanes and sulfur-containing compounds from biogas streams from landfills and anaerobic digesters, com prising the steps of heating the biogas and feeding it to an absorber containing a siloxane absorption bed, feeding the siloxane-free gas effluent from the absorber to a con- verter containing a sulfur monolith catalyst, where organic and inorganic sulfur compounds are oxidized to SO2, cooling the hot exit gas from the converter and removing SO2 from the cooled gas. This way, a purified gas stream free of sulfur compounds and siloxanes is obtained.
The subject matter of the present invention differs from that of WO 2017/140662 A1 in that a step of siloxane re moval is performed prior to the siloxane absorption step. So the present invention relates to a method for the com bined removal of gas-phase siloxanes, sulfur and optionally also VOCs from various gases, such as industrial gas, land fill gas or digester gas.
The method makes use of Applicant's SiloxFree® sorbent, which is a highly activated and acidic alumina sorbent that operates at elevated temperatures and converts the siloxane components to silica. The silica is trapped in the pore system of the SiloxFree sorbent. The SiloxFree system is thus a sacrificial media. For high siloxane contents (of around 50 to 150 mg Si/m3) , the consumption of sorbent be comes prohibitive from a process economy point of view. In the market, there are several regenerative systems availa ble to remove siloxanes, which can provide bulk removal of siloxanes but cannot remove siloxanes to a sufficiently low level to eliminate issues with catalyst poisoning, mainte nance cost and reduced power production.
The preferred embodiment of the inventive method is to use a regenerative siloxane removal system upstream from the SiloxFree unit and use the SiloxFree unit as a polisher.
However, the regenerative systems suffer from very complex operation cycles, high operation costs associated with flaring the siloxane-containing regeneration gas and also a high capex. Thus, the challenge is to provide a system that can effect bulk removal of siloxanes down to a level of 5- 15 mg Si/m3 upstream from the SiloxFree system.
Therefore, the idea underlying the procedure of the present invention is to use an alcohol scrubber where the raw gas is being contacted with an alcohol liquid to effect absorp tion of the siloxanes. The absorbed siloxanes are subsequently burnt and subjected to a further scrubbing, while the process gas is polished in a SiloxFree polisher. More specifically, the invention relates to a process for the combined removal of siloxanes, sulfur and VOCs from a raw gas, said process comprising the steps of
- feeding the raw gas to an alcohol scrubber together with make-up alcohol to absorb the siloxanes,
- performing an alcohol blow-down to control the build-up of siloxanes and other components, which are soluble in the alcohol, and separating the process gas from the siloxane- rich scrubber liquid,
- heating the process gas, which is now lean in siloxanes, and removing traces of siloxanes in a polisher, and - feeding the treated process gas to a catalyst unit con taining an SMC type catalyst, which converts residual alco hol along with sulfur and VOCs without converting methane.
The catalyst in the catalyst unit is preferably a sulfur monolith catalyst (SMC) type catalyst.
Preferably the polisher is a SiloxFree® unit.
The alcohol composition in the scrubber liquid is prefera- bly methanol, ethanol, propanol, butanol or mixtures thereof, more preferably methanol and ethanol or mixtures thereof, and most preferably pure methanol. So the invention relates to a process for the combined re moval of siloxanes, sulfur and VOCs from a raw gas, which can be a biogas, especially a landfill gas. A scrubber is used, in which the gas is contacted with a liquid, said liquid containing methanol, ethanol, propanol, butanol or mixtures thereof. It is preferred that the vapor pressure of the alcohol or of the mixture of alcohols, which is used, results in a sufficiently low vapor pressure of alco hol in the scrubber liquid when considering that the gas will have a significant water content. The scrubber will operate at atmospheric pressure, and a certain part of the siloxanes will dissolve in the alcohol at atmospheric pres sure . From the scrubber, the gas is led to the SiloxFree unit, where the remaining part of the siloxanes is removed, and an SMC catalyst is placed directly downstream from the Si loxFree sorbent bed to oxidize any residual alcohol that the gas may have picked up in the scrubber. The mixture of alcohol and water should be chosen such that the uptake of alcohol in the gaseous phase is minimized, i.e. similar to azeotrope distillation for vapor/liquid systems.
A blow-down from the scrubber is necessary to maintain a suitable siloxane level in the scrubber liquid. The liquid is combusted in a burner, and the small volume of flue gas is mixed into the main gas stream to be further processed.
A low temperature bag filter is inserted between the mixing point and the SiloxFree unit to remove silica from liquid combustion flue gas. This process provides a robust bulk removal of siloxanes which can preferably be combined with the SiloxFree technology and sorbent. The method of the invention for combined removal of silox- anes, sulfur and optionally VOCs is illustrated in further detail in the below example with reference to the enclosed figure .
Example
Raw biogas for cleaning is fed to an alcohol scrubber A via line a. Any necessary make-up alcohol is fed to the scrub ber A via line b. In the alcohol scrubber A, absorption of siloxanes takes place. To maintain a suitable siloxane level in the scrubber liquid, a siloxane-rich alcohol blow down from the scrubber is fed to a burner B along line c, and air is supplied to the burner via line d. Hot flue gas containing S1O2 passes through a hot gas filter H and leaves the filter at around 600°C. This hot gas f is fed to a further scrubber S via a feed/effluent heat exchanger Hex. From the scrubber S, an effluent h is discharged.
Process gas from the scrubber A, lean in siloxanes but con taining alcohol and water, is passed to a heater C via line e, and from heater C the gas proceeds to the feed/effluent heat exchanger Hex. After passing the heat exchanger, the gas is fed to a SiloxFree polisher P. The polished gas g is then fed to a catalyst unit D containing an SMC type cata lyst (SMC stands for sulfur monolith catalyst) , which oxi dizes ¾S selectively to SO2 and converts residual methanol along with sulfur and VOCs without converting methane.

Claims

Claims :
1. A process for the combined removal of siloxanes, sulfur and VOCs from a raw gas, said process comprising the steps of
- feeding the raw gas to an alcohol scrubber together with make-up alcohol to absorb the siloxanes, - performing an alcohol blow-down to control the build-up of siloxanes and other components, which are soluble in the alcohol, and separating the process gas from the siloxane- rich scrubber liquid, - heating the process gas, which is now lean in siloxanes, and removing traces of siloxanes in a polisher, and
- feeding the treated process gas to a catalyst unit con taining a catalyst, which converts residual alcohol along with sulfur and VOCs without converting methane.
2. Process according to claim 1, wherein the catalyst in the catalyst unit is a sulfur monolith catalyst (SMC) type catalyst.
3. Process according to claim 1, where the polisher is a SiloxFree® unit.
4. Process according to claim 1, where the preferred alcohol composition in the scrubber liquid is methanol, ethanol, propanol, butanol or mixtures thereof.
5. Process according to claim 4, where the preferred alcohol composition in the scrubber liquid is methanol and ethanol or mixtures thereof.
6. Process according to claim 5, where the preferred alcohol composition in the scrubber liquid is methanol.
PCT/EP2020/054638 2019-02-28 2020-02-21 Method for combined removal of siloxanes, sulfur and vocs WO2020173828A1 (en)

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Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN114076321A (en) * 2021-11-15 2022-02-22 镇海石化工程股份有限公司 Method for treating petrochemical VOCs waste gas by using sulfur recovery device incinerator
CN114076321B (en) * 2021-11-15 2024-04-30 镇海石化工程股份有限公司 Method for treating petrochemical VOCs waste gas by utilizing sulfur recovery device incinerator

Citations (6)

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