CA2039863C - Biological process for conversion of hydrogen sulphide - Google Patents
Biological process for conversion of hydrogen sulphideInfo
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- CA2039863C CA2039863C CA 2039863 CA2039863A CA2039863C CA 2039863 C CA2039863 C CA 2039863C CA 2039863 CA2039863 CA 2039863 CA 2039863 A CA2039863 A CA 2039863A CA 2039863 C CA2039863 C CA 2039863C
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Abstract
A microbiological method of desulphurizing gases is disclosed. Chemoautrotrophic bacteria of the Thiobacillus genus are used to remove sulphides from gases. More specifically, the bacteria Thiobacillus thiooxidans in a low pH, aerobic atmosphere oxidizes hydrogen sulphide to sulphur or sulphate compounds in a contacting column in which countercurrent flow of gas and aqueous medium occurs.
Description
203~~~3 APPLICATION FOR PATENT
INVENTOR: Hector M. Lizama and Bruce M. Sankey TITLE: BIOLOGICAL PROCESS FOR CONVERSION OF
HYDROGEN SULPHIDE
SPECIFICATION
BACKGROUND OF THE INVENTION
1. Field of the Invention This invention relates to the desulphurization of gases and, more specifically, relates to a micro-s biological method for desulphurizing a gas stream containing hydrogen sulphide.
INVENTOR: Hector M. Lizama and Bruce M. Sankey TITLE: BIOLOGICAL PROCESS FOR CONVERSION OF
HYDROGEN SULPHIDE
SPECIFICATION
BACKGROUND OF THE INVENTION
1. Field of the Invention This invention relates to the desulphurization of gases and, more specifically, relates to a micro-s biological method for desulphurizing a gas stream containing hydrogen sulphide.
2. Description of the Prior Art Natural gas removed from a well frequently contains undesirable levels of hydrogen sulphide.
Hydrogen sulphide is a toxic gas which must be removed from the natural gas prior to marketing or combustion.
After removal, the toxic hydrogen sulphide must be converted to a non-toxic product. Generally, this is accomplished by the Claus reaction conversion of hydrogen sulphide to elemental sulphur. However, treatment of hydrogen sulphide in a Claus Plant is only economical on a large scale. For treatment of natural gas streams with low concentrations of hydrogen sulphide, alternative technology is needed.
Natural microbes which oxidize hydrogen sulphide exist. Some bacteria of the group Thiobacillus, such as Thiobacillus thiooxidans and Thiobacillus ferrooxidans, oxidize sulphide through the reaction:
S2 + 202 ----~ S042 2o39gs3 These bacteria are most suitable for the oxidation of hydrogen sulphide in an industrial process.
Thiobacillus thiooxidans is especially suitable being biologically active at an extremely low pH (e. g. <1.0), resulting from the fact that sulphuric acid is the ultimate end-product of hydrogen sulphide oxidation.
Thiobacillus thiooxidans was discovered and named by Waksmann & Joffe, Science LIII: 216, 1921. Bacterium from the group Thiobacillus thiooxidans require reduced sulphur compounds such as elemental sulphur or sulphide as their sole source of energy.
Several microbial processes for the oxidation of hydrogen sulphide have been described in the patent literature. U.S. Patent No. 4,879,240 discloses a process for controlling hydrogen sulphide production using the bacterium Thiobacillus denitrificans under aerobic conditions to oxidize hydrogen sulphide to sulphate compounds. U.S. Patent No. 4,760,027 discloses the use of the bacterium Thiobacillus denitrificans to desulphurize a gas stream by converting hydrogen sulphide to sulphate compounds.
U.S. Patent No. 4,869,824 discloses an apparatus and process for the biological purification of outgoing air and waste water. U.S. Patent No. 4,723,968 discloses a method for the purification of waste air containing biologically decomposable impurities. U.S. Patent No.
4,179,374 discloses an apparatus for the treatment of waste water by denitrification using faculative organisms.
The aforementioned prior art methods are deficient in that these methods do nat operate at a low pH. This is a particularly desirable trait since sulphuric acid is the end-product of hydrogen sulphide oxidation.
Thus, the need exists for a method operable at a low pH
which will convert hydrogen sulphide to non-toxic, manageable products. Another benefit of a low PH
environment is the fact that most bacteria will not _ 3 _ 2Q39863 tolerate this condition, hence contamination from undesirable bacterial species is virtually eliminated.
SUMMARY OF THE INVENTION
According to an embodiment of the present invention, there is provided a microbiological process for desulphurizing a gas containing hydrogen sulphide comprising contacting said gas with a culture of the bacterium Thiobacillus thiooxidans. This reaction takes place in an acidic environment with the pH < 3 and oxidizes the sulphides to sulphur or sulphates.
Preferably, this reaction takes place in a countercurrent flow by injecting gas in the bottom of the chamber and injecting liquid bacterial medium in the top.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is an illustration of the laboratory scale flow scheme.
Fig. 2 is a flow diagram of the preferred embodiment of the invention.
Fig. 3 is a plot showing improvement in bacterial performance with successive exposures to hydrogen sulphide.
Fig. 4 is a graph illustrating the difference in rates of hydrogen sulphide removal and sulphate formation.
Fig. 5 is a graph showing the effect of gas flow rate on hydrogen sulphide removal.
Fig. 6 is a plot showing improvement in hydrogen sulphide removal with time.
DETAILED DESCRIPTION OF THE INVENTION
The present invention utilizes a chemoautotrophic bacterium to convert sulphides to sulphates. Although several chemolithotrophic bacteria are capable of oxidizing sulphides and using sulphur compounds as their source of energy, the bacterium Thiobacillus thiooxidans has been discovered to be uniquely suitable for achieving the objects of the present invention.
This chemolithotrophic bacterium is also acidophilic, surviving only at a pH < 3. In addition, Thiobacillus thiooxidans is capable of oxidizing hydrogen sulphide gas in an oxygen-containing environment and at room temperature. These unique conditions and the use of the bacterium Thiobacillus thiooxidans sets the present invention apart from the prior art.
The method of culturing used in the present invention is well known in the art. Any of the known methods of culturing bacteria may be used. An example of a suitable method would be to culture the bacteria in a countercurrent contacter containing inert support medium.
The culturing temperature is generally any temperature that enables the bacteria to be cultured.
Generally, the temperature is in the range of about 10°C to about 50°C; preferably in the range of about 20°C to about 40°C.
The process of the present invention may be conducted generally at any pressure that enables the bacteria to be cultured and allows the bacteria to desulphurize gas. Generally, the pressure is in the range of from about 0 psig to about 100 psig;
preferably in the range of from about 0 psig to about 10 psig.
The support medium used in the present invention is also well known in the art. Any inert material may be used, e.g. glass beads, silica beads, ceramic chips, etc., or any of the commercial media developed for the application of biofilm growth.
The amount of bacteria used in the present invention is from about 106 cells/ml to about 1010 cells/ml; preferably, about 10$ cells/ml to about lOlo cells/ml bacteria are used.
The pH of the aqueous solution is from about 0.8 to 3Ø Preferably, the pH is from about 2.0 to 2.5.
Fig. 1 is an illustration of the present invention. Hydrogen sulphide is removed from the feed gas stream 10 by any established techniques, such as extraction or absorption, providing a purified gas stream for use or sale. The concentrated hydrogen sulphide stream is oxygenated and then fed to a contacter 20 wherein the bacterium Thiobacillus thiooxidans oxidize the hydrogen sulphide to either sulphur or sulphate. Preferably, countercurrent contact is used arid is accomplished by injecting the gas stream into the contacter bottom 30 while the aqueous bacterial medium is injected at the contacter top 40, flows to the bottom and is recycled. Part of the gas stream may be recycled around the contacter for increased efficiency.
For commercial scale operation, 2 or more contacters 50 could be connected in series for the gas flow, as shown in Fig. 2. Sulphur could be recovered continuously from the aqueous stream recycled out of container 60 by a separation method, such as gravity settling, filtration or centrifugation.
The following examples are given for illustrative purposes only and are not meant to limit the scope of the present invention.
EXAMPLES
Example 1 The experiment consisted of passing hydrogen sulphide by way of a recirculating gas mixture through a column packed with glass beads. The bacterial strain SM-7 (Thiobacillus thiooxidans) was constantly recirculated in the presence of a medium containing O.lg KH2P04, 0.4g (NH4)2S04, 0.4g MgS04 ~ 7 H20 per liter, adjusted to pH 2.3 with H2S04.
Microbiologically-induced rates of hydrogen sulphide removal were compared with spontaneous chemical removal rates (control). Two columns with identical air/hydrogen sulphide mixtures were run and hydrogen sulphide levels were measured over time. Sulphide, sulphate and pH were determined at the start and end of the experiment. Bacterial viability was confirmed by the growth of a sulphur flask inoculated with effluent from the bacterial column. The results are summarized in Table 1.
Table 1 Conversion of hydrogen sulphide to sulphate by bacteria and control columns Bacteria Control H2S (mmoles) start 2.26 2.26 5042-(mmoles) start 1.84 0.84 end 3.33 0.78 pH start 1.67 3.00 end 1.44 7.70 The bacterial strain SM-7 oxidized hydrogen sulphide to sulphate whereas the control column did not. The decrease in pH seen in the bacterial column further indicates that oxidation of sulphide is taking place, producing sulphuric acid. This was not seen in the control column. In addition, sulfur deposition was observed in the bacterial column but not in the control.
Example 2 A constant stream of hydrogen sulphide in air was passed through a column packed with glass beads and onto a scrubber. The bacterial strain SM-7 (Thiobacillus thiooxidans) was constantly recirculated countercurrent to the gas flow. Microbiologically-induced rates of hydrogen sulphide removal were compared with spontaneous chemical removal rates (control). Two columns with identical air/hydrogen aossss3 _7_ sulphide mixtures were run. Inflow and outflow hydrogen sulphide levels were monitored at various times. The results are summarized in Tables 2 and 3.
Table 2 Summary of initial trials with gas flowthrouqh system using bacteria Duration Gas Flow Hydrogen sulphide (ppm) Conversion (hours) (mL/miny In Out lmmol,/hour~
16 15 3500 60 0.13 23 15 3500 55 0.13 Medium changed 88 20 3250 45 0.16 Sulphur deposit observed 94 50 3250 900 0.29 112 50 3250 150 0.38 Medium changed 117 50 3250 150 0.38 Volume of liquid was 70 mL, recirculated at a rate of 63 mL/min.
NM = not measured Table 3 Control experiments showing chemical hydrogen sulphide conversion rates using a gas flowthrough system Run Gas Flow Hydrogen sulphide (ppm) Conversion ~_m~~,~min)~ In out (mmol,/hour~, Volume of liquid was 70 mL, recirculated at a rate of 63 mL/min.
Good hydrogen sulphide conversion rates were observed (see Table 2) which were higher than those seen in the gas recycle experiments. After only a few days, sulfur could be seen accumulating at the bottom of the column bed.
_8_ Example 3 An experiment was run as per Example 1 but only with the column containing the bacterial strain SM-7.
The column was run until all the hydrogen sulphide was removed from the recirculating gas mixture, then hooked up to a second gas reservoir and run as before without changing the medium or reinoculating with bacteria. At the end of the experiment bacterial activity was confirmed by the growth of a sulfur flask inoculated with effluent from the bacterial column. Consumption of hydrogen sulphide took place in both runs. The second run gave a faster hydrogen sulphide removal rate (Fig. 3). As shown in Table 4, the bacterial oxidation rate almost doubled from about 0.06 mmoles/hour to 0.10 mmoles/hour. This experiment involved recycling of air/hydrogen sulphide gas mixtures using two 10 L gas reservoirs sequentially. This is a clear indication of adaptation by the bacterial strain SM-7 to hydrogen sulphide, enabling the organism to oxidize it faster.
Table 4 Effect of subsequent exposure of bacteria to hydrogen sulphide Run 1 Run 2 Hydrogen sulphide (mmoles) start 2.46 2.50 end 0.01 0.01 Time required for H2S
removal (hours) 42 27 Bacterial oxidation rate 0.06 0.10 (mmoles H2S/hour) A repetition of this experiment under identical conditions gave similar results. As before, the bacterial hydrogen sulphide oxidation rate doubled from 0.06 to 0.11 mmoles/hour (see Fig. 3). As in other experiments, elemental sulphur could be seen in the liquid medium.
_g_ Example 4 An experiment was run as per Example 1 but only with the column containing the bacterial strain SM-7.
The column was run well past the point when all the hydrogen sulphide was removed from the recirculating gas mixture. This allowed all of the elemental sulphur to be oxidized all the way to sulphate. Sulphate was determined at the start of the experiment, once all the hydrogen sulphide was gone, and at time intervals after that. The bacterial hydrogen sulphide removal rate was 0.07 mmoles/hour and is comparable to past observed rates. The formation of sulphate, however, was much slower at 0.02 moles/hour. Thus, the rate of hydrogen sulphide removal was 4 times that of sulphate formation (see Fig. 4). This immediately suggests the accumulation of an intermediate, namely elemental sulfur which was visible in the column liquid.
A repetition of this experiment using twice the usual volume of air/hydrogen sulphide gas mixture gave similar results. the rate of hydrogen sulphide oxidation was 0.20 mmoles/hour while the rate of sulphate formation was only 0.05 mmoles/hour.
Examgl,e 5 An experiment was run per example 2 but with both columns containing the bacterial strain SM-7. One column contained glass beads that were 3 mm in diameter while the other contained glass beads that were 5 mm in diameter. Both columns were run in parallel, using the same source of air/hydrogen sulphide gas mixture, the same volume of liquid, and the same liquid flow rates.
Various gas flow rates were used and the corresponding bacterial hydrogen sulphide removal rates were determined. All the measurements were carried out within 9 hours to minimize any effect of performance improvement over time as seen in past experiments. The results obtained are shown in Table 5 and Fig. 5.
Table 5 Influence of gas flow rate on hydrogen sulphide conversion rates Column 1 Column 2 Gas rate Conversion Gas rate Conversion (mL,/min) (mmoljhour) (mLlmin) (mmol,/hour) 12 0.08 18 0.16 18 23 0.19 23 0.10 34 0.28 38 0.28 38 0.14 38 0.12 100 0.26 Columns 1 and 2 contained 3 mm and 5 mm glass beads, respectively. Each time the flow rate was changed the system was allowed to equilibrate for at least 30 minutes prior to taking a gas sample. The entire experiment was carried out within 9 hours.
At similar gas flow rates the difference between conversion rates correlated with the differences in total reaction surface area for each column. With both columns having the same bed volume, the difference in glass bead size accounts for a difference in reaction surface area by a factor of about two. Column 1 which had almost twice the surface area as column 2 (3mm beds versus 5 mm beads) also had about double the conversion rate.
The rate of hydrogen sulphide removal increased with gas flow rate. This is to be expected, up to the point where mass transfer from the gas to liquid phases no longer represents the rate-limiting step.
Examgle 6 An experiment was run as per Example 5 but maintaining a constant gas flow rate through both columns for several days in order to investigate the variation in H2S conversion with time. The results are shown in Table 6 and Figure 6.
-11- ~~9863 Table 6 Performance of two columns with different size cLlass beads Column 1 Column 2 Duration Gas rate Conversion Gas rate Conversion ,Lhours) (mL/min~ (mmol,/hour) ~(mL~/minJi ~(mmol~hour) 1.5 12 0.09 34 0.25 16 12 0.09 10 21 34 0.26 23 12 0.10 24 34 0.25 45 34 0.29 46 12 0.11 15 63 34 0.30 64.5 12 0.11 Columns 1 and 2 contained 3 mm and 5 mm glass beads, respectively.
For each column, conversion rates were observed to increase with time. The bacterial activity (in this case hydrogen sulphide conversion rate) is related to their concentration and can be used to represent population size. Bacteria reproduce by binary fission, hence their populations increase exponentially. In such cases the best way to describe the growth rate of a bacterial population is its generation time: the time required for the population to double in size. In this case, the generation time would be the time required for the hydrogen sulphide conversion rate to double.
The rate increases with time were extremely slow for both columns (see Fig. 6). Doubling times were estimated at about 200 hours. These generation times are extremely low; when grown in sulfur flasks, Thiobacillus thiooxidans shows typical generation times of 6 to 10 hours. Sulfur, however, is a much more desirable substrate than hydrogen sulphide which must dissolve in water in order to be accessible to the bacteria. The main significance of this is that the majority of the active microorganisms in the columns came from the original inoculum and not from growth.
This is a great advantage from a process standpoint since the rate of biomass increase will be manageable.
Example 7 The experiment consisted of passing hydrogen sulphide by way of a constant stream through two columns arranged in series and onto a scrubber. Each column had the bacterial strain SM-7 (Thiobacillus thiooxidans) constantly recirculating in the present of medium as per Example 1. Hydrogen sulphide was supplied as a mixture with air. Column 1 containing 3 mm beads was first on line followed by column 2 packed with 5 mm beads. Inflow and outflow hydrogen sulphide levels were monitored upstream of column 1 (inflow) and downstream of column 2 (outflow). The results are summarized on Table 7.
Table 7 Total removal of hydrogen sulphide by columns in series.
2 0 Column 1 Column 2 Total H2S Gas Flow HZS Con- H S Con- Con-Duration In Rate out version ~ut version version Jhours) (vumZ (mL/minl fpm) (mmoJhr Lj,ppm) (mmol/hr) (mmol/hr) 0 3000 23 17 0.17 0 0.001 0.17 2 5 24.5 2100 58 330 0.25 0 0.05 0.30 95.5 2100 58 550 0.22 0 0.08 0.30 T a co umna were arranged n ser es.
Putting the two columns in series was successful 30 in removing all of the hydrogen sulphide from the gas stream. Most of the hydrogen sulphide was removed in column 1, but this is to be expected since the amount of hydrogen sulphide entering column 2 was extremely low to begin with. In a separate experiment the 35 positions of the columns were reversed to that the gas stream passed through column 2 followed by column 1.
The results were analogous with those shown on Table 7 in that most of the hydrogen sulphide was removed in the first column (column No. 2 in this case).
Hydrogen sulphide is a toxic gas which must be removed from the natural gas prior to marketing or combustion.
After removal, the toxic hydrogen sulphide must be converted to a non-toxic product. Generally, this is accomplished by the Claus reaction conversion of hydrogen sulphide to elemental sulphur. However, treatment of hydrogen sulphide in a Claus Plant is only economical on a large scale. For treatment of natural gas streams with low concentrations of hydrogen sulphide, alternative technology is needed.
Natural microbes which oxidize hydrogen sulphide exist. Some bacteria of the group Thiobacillus, such as Thiobacillus thiooxidans and Thiobacillus ferrooxidans, oxidize sulphide through the reaction:
S2 + 202 ----~ S042 2o39gs3 These bacteria are most suitable for the oxidation of hydrogen sulphide in an industrial process.
Thiobacillus thiooxidans is especially suitable being biologically active at an extremely low pH (e. g. <1.0), resulting from the fact that sulphuric acid is the ultimate end-product of hydrogen sulphide oxidation.
Thiobacillus thiooxidans was discovered and named by Waksmann & Joffe, Science LIII: 216, 1921. Bacterium from the group Thiobacillus thiooxidans require reduced sulphur compounds such as elemental sulphur or sulphide as their sole source of energy.
Several microbial processes for the oxidation of hydrogen sulphide have been described in the patent literature. U.S. Patent No. 4,879,240 discloses a process for controlling hydrogen sulphide production using the bacterium Thiobacillus denitrificans under aerobic conditions to oxidize hydrogen sulphide to sulphate compounds. U.S. Patent No. 4,760,027 discloses the use of the bacterium Thiobacillus denitrificans to desulphurize a gas stream by converting hydrogen sulphide to sulphate compounds.
U.S. Patent No. 4,869,824 discloses an apparatus and process for the biological purification of outgoing air and waste water. U.S. Patent No. 4,723,968 discloses a method for the purification of waste air containing biologically decomposable impurities. U.S. Patent No.
4,179,374 discloses an apparatus for the treatment of waste water by denitrification using faculative organisms.
The aforementioned prior art methods are deficient in that these methods do nat operate at a low pH. This is a particularly desirable trait since sulphuric acid is the end-product of hydrogen sulphide oxidation.
Thus, the need exists for a method operable at a low pH
which will convert hydrogen sulphide to non-toxic, manageable products. Another benefit of a low PH
environment is the fact that most bacteria will not _ 3 _ 2Q39863 tolerate this condition, hence contamination from undesirable bacterial species is virtually eliminated.
SUMMARY OF THE INVENTION
According to an embodiment of the present invention, there is provided a microbiological process for desulphurizing a gas containing hydrogen sulphide comprising contacting said gas with a culture of the bacterium Thiobacillus thiooxidans. This reaction takes place in an acidic environment with the pH < 3 and oxidizes the sulphides to sulphur or sulphates.
Preferably, this reaction takes place in a countercurrent flow by injecting gas in the bottom of the chamber and injecting liquid bacterial medium in the top.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is an illustration of the laboratory scale flow scheme.
Fig. 2 is a flow diagram of the preferred embodiment of the invention.
Fig. 3 is a plot showing improvement in bacterial performance with successive exposures to hydrogen sulphide.
Fig. 4 is a graph illustrating the difference in rates of hydrogen sulphide removal and sulphate formation.
Fig. 5 is a graph showing the effect of gas flow rate on hydrogen sulphide removal.
Fig. 6 is a plot showing improvement in hydrogen sulphide removal with time.
DETAILED DESCRIPTION OF THE INVENTION
The present invention utilizes a chemoautotrophic bacterium to convert sulphides to sulphates. Although several chemolithotrophic bacteria are capable of oxidizing sulphides and using sulphur compounds as their source of energy, the bacterium Thiobacillus thiooxidans has been discovered to be uniquely suitable for achieving the objects of the present invention.
This chemolithotrophic bacterium is also acidophilic, surviving only at a pH < 3. In addition, Thiobacillus thiooxidans is capable of oxidizing hydrogen sulphide gas in an oxygen-containing environment and at room temperature. These unique conditions and the use of the bacterium Thiobacillus thiooxidans sets the present invention apart from the prior art.
The method of culturing used in the present invention is well known in the art. Any of the known methods of culturing bacteria may be used. An example of a suitable method would be to culture the bacteria in a countercurrent contacter containing inert support medium.
The culturing temperature is generally any temperature that enables the bacteria to be cultured.
Generally, the temperature is in the range of about 10°C to about 50°C; preferably in the range of about 20°C to about 40°C.
The process of the present invention may be conducted generally at any pressure that enables the bacteria to be cultured and allows the bacteria to desulphurize gas. Generally, the pressure is in the range of from about 0 psig to about 100 psig;
preferably in the range of from about 0 psig to about 10 psig.
The support medium used in the present invention is also well known in the art. Any inert material may be used, e.g. glass beads, silica beads, ceramic chips, etc., or any of the commercial media developed for the application of biofilm growth.
The amount of bacteria used in the present invention is from about 106 cells/ml to about 1010 cells/ml; preferably, about 10$ cells/ml to about lOlo cells/ml bacteria are used.
The pH of the aqueous solution is from about 0.8 to 3Ø Preferably, the pH is from about 2.0 to 2.5.
Fig. 1 is an illustration of the present invention. Hydrogen sulphide is removed from the feed gas stream 10 by any established techniques, such as extraction or absorption, providing a purified gas stream for use or sale. The concentrated hydrogen sulphide stream is oxygenated and then fed to a contacter 20 wherein the bacterium Thiobacillus thiooxidans oxidize the hydrogen sulphide to either sulphur or sulphate. Preferably, countercurrent contact is used arid is accomplished by injecting the gas stream into the contacter bottom 30 while the aqueous bacterial medium is injected at the contacter top 40, flows to the bottom and is recycled. Part of the gas stream may be recycled around the contacter for increased efficiency.
For commercial scale operation, 2 or more contacters 50 could be connected in series for the gas flow, as shown in Fig. 2. Sulphur could be recovered continuously from the aqueous stream recycled out of container 60 by a separation method, such as gravity settling, filtration or centrifugation.
The following examples are given for illustrative purposes only and are not meant to limit the scope of the present invention.
EXAMPLES
Example 1 The experiment consisted of passing hydrogen sulphide by way of a recirculating gas mixture through a column packed with glass beads. The bacterial strain SM-7 (Thiobacillus thiooxidans) was constantly recirculated in the presence of a medium containing O.lg KH2P04, 0.4g (NH4)2S04, 0.4g MgS04 ~ 7 H20 per liter, adjusted to pH 2.3 with H2S04.
Microbiologically-induced rates of hydrogen sulphide removal were compared with spontaneous chemical removal rates (control). Two columns with identical air/hydrogen sulphide mixtures were run and hydrogen sulphide levels were measured over time. Sulphide, sulphate and pH were determined at the start and end of the experiment. Bacterial viability was confirmed by the growth of a sulphur flask inoculated with effluent from the bacterial column. The results are summarized in Table 1.
Table 1 Conversion of hydrogen sulphide to sulphate by bacteria and control columns Bacteria Control H2S (mmoles) start 2.26 2.26 5042-(mmoles) start 1.84 0.84 end 3.33 0.78 pH start 1.67 3.00 end 1.44 7.70 The bacterial strain SM-7 oxidized hydrogen sulphide to sulphate whereas the control column did not. The decrease in pH seen in the bacterial column further indicates that oxidation of sulphide is taking place, producing sulphuric acid. This was not seen in the control column. In addition, sulfur deposition was observed in the bacterial column but not in the control.
Example 2 A constant stream of hydrogen sulphide in air was passed through a column packed with glass beads and onto a scrubber. The bacterial strain SM-7 (Thiobacillus thiooxidans) was constantly recirculated countercurrent to the gas flow. Microbiologically-induced rates of hydrogen sulphide removal were compared with spontaneous chemical removal rates (control). Two columns with identical air/hydrogen aossss3 _7_ sulphide mixtures were run. Inflow and outflow hydrogen sulphide levels were monitored at various times. The results are summarized in Tables 2 and 3.
Table 2 Summary of initial trials with gas flowthrouqh system using bacteria Duration Gas Flow Hydrogen sulphide (ppm) Conversion (hours) (mL/miny In Out lmmol,/hour~
16 15 3500 60 0.13 23 15 3500 55 0.13 Medium changed 88 20 3250 45 0.16 Sulphur deposit observed 94 50 3250 900 0.29 112 50 3250 150 0.38 Medium changed 117 50 3250 150 0.38 Volume of liquid was 70 mL, recirculated at a rate of 63 mL/min.
NM = not measured Table 3 Control experiments showing chemical hydrogen sulphide conversion rates using a gas flowthrough system Run Gas Flow Hydrogen sulphide (ppm) Conversion ~_m~~,~min)~ In out (mmol,/hour~, Volume of liquid was 70 mL, recirculated at a rate of 63 mL/min.
Good hydrogen sulphide conversion rates were observed (see Table 2) which were higher than those seen in the gas recycle experiments. After only a few days, sulfur could be seen accumulating at the bottom of the column bed.
_8_ Example 3 An experiment was run as per Example 1 but only with the column containing the bacterial strain SM-7.
The column was run until all the hydrogen sulphide was removed from the recirculating gas mixture, then hooked up to a second gas reservoir and run as before without changing the medium or reinoculating with bacteria. At the end of the experiment bacterial activity was confirmed by the growth of a sulfur flask inoculated with effluent from the bacterial column. Consumption of hydrogen sulphide took place in both runs. The second run gave a faster hydrogen sulphide removal rate (Fig. 3). As shown in Table 4, the bacterial oxidation rate almost doubled from about 0.06 mmoles/hour to 0.10 mmoles/hour. This experiment involved recycling of air/hydrogen sulphide gas mixtures using two 10 L gas reservoirs sequentially. This is a clear indication of adaptation by the bacterial strain SM-7 to hydrogen sulphide, enabling the organism to oxidize it faster.
Table 4 Effect of subsequent exposure of bacteria to hydrogen sulphide Run 1 Run 2 Hydrogen sulphide (mmoles) start 2.46 2.50 end 0.01 0.01 Time required for H2S
removal (hours) 42 27 Bacterial oxidation rate 0.06 0.10 (mmoles H2S/hour) A repetition of this experiment under identical conditions gave similar results. As before, the bacterial hydrogen sulphide oxidation rate doubled from 0.06 to 0.11 mmoles/hour (see Fig. 3). As in other experiments, elemental sulphur could be seen in the liquid medium.
_g_ Example 4 An experiment was run as per Example 1 but only with the column containing the bacterial strain SM-7.
The column was run well past the point when all the hydrogen sulphide was removed from the recirculating gas mixture. This allowed all of the elemental sulphur to be oxidized all the way to sulphate. Sulphate was determined at the start of the experiment, once all the hydrogen sulphide was gone, and at time intervals after that. The bacterial hydrogen sulphide removal rate was 0.07 mmoles/hour and is comparable to past observed rates. The formation of sulphate, however, was much slower at 0.02 moles/hour. Thus, the rate of hydrogen sulphide removal was 4 times that of sulphate formation (see Fig. 4). This immediately suggests the accumulation of an intermediate, namely elemental sulfur which was visible in the column liquid.
A repetition of this experiment using twice the usual volume of air/hydrogen sulphide gas mixture gave similar results. the rate of hydrogen sulphide oxidation was 0.20 mmoles/hour while the rate of sulphate formation was only 0.05 mmoles/hour.
Examgl,e 5 An experiment was run per example 2 but with both columns containing the bacterial strain SM-7. One column contained glass beads that were 3 mm in diameter while the other contained glass beads that were 5 mm in diameter. Both columns were run in parallel, using the same source of air/hydrogen sulphide gas mixture, the same volume of liquid, and the same liquid flow rates.
Various gas flow rates were used and the corresponding bacterial hydrogen sulphide removal rates were determined. All the measurements were carried out within 9 hours to minimize any effect of performance improvement over time as seen in past experiments. The results obtained are shown in Table 5 and Fig. 5.
Table 5 Influence of gas flow rate on hydrogen sulphide conversion rates Column 1 Column 2 Gas rate Conversion Gas rate Conversion (mL,/min) (mmoljhour) (mLlmin) (mmol,/hour) 12 0.08 18 0.16 18 23 0.19 23 0.10 34 0.28 38 0.28 38 0.14 38 0.12 100 0.26 Columns 1 and 2 contained 3 mm and 5 mm glass beads, respectively. Each time the flow rate was changed the system was allowed to equilibrate for at least 30 minutes prior to taking a gas sample. The entire experiment was carried out within 9 hours.
At similar gas flow rates the difference between conversion rates correlated with the differences in total reaction surface area for each column. With both columns having the same bed volume, the difference in glass bead size accounts for a difference in reaction surface area by a factor of about two. Column 1 which had almost twice the surface area as column 2 (3mm beds versus 5 mm beads) also had about double the conversion rate.
The rate of hydrogen sulphide removal increased with gas flow rate. This is to be expected, up to the point where mass transfer from the gas to liquid phases no longer represents the rate-limiting step.
Examgle 6 An experiment was run as per Example 5 but maintaining a constant gas flow rate through both columns for several days in order to investigate the variation in H2S conversion with time. The results are shown in Table 6 and Figure 6.
-11- ~~9863 Table 6 Performance of two columns with different size cLlass beads Column 1 Column 2 Duration Gas rate Conversion Gas rate Conversion ,Lhours) (mL/min~ (mmol,/hour) ~(mL~/minJi ~(mmol~hour) 1.5 12 0.09 34 0.25 16 12 0.09 10 21 34 0.26 23 12 0.10 24 34 0.25 45 34 0.29 46 12 0.11 15 63 34 0.30 64.5 12 0.11 Columns 1 and 2 contained 3 mm and 5 mm glass beads, respectively.
For each column, conversion rates were observed to increase with time. The bacterial activity (in this case hydrogen sulphide conversion rate) is related to their concentration and can be used to represent population size. Bacteria reproduce by binary fission, hence their populations increase exponentially. In such cases the best way to describe the growth rate of a bacterial population is its generation time: the time required for the population to double in size. In this case, the generation time would be the time required for the hydrogen sulphide conversion rate to double.
The rate increases with time were extremely slow for both columns (see Fig. 6). Doubling times were estimated at about 200 hours. These generation times are extremely low; when grown in sulfur flasks, Thiobacillus thiooxidans shows typical generation times of 6 to 10 hours. Sulfur, however, is a much more desirable substrate than hydrogen sulphide which must dissolve in water in order to be accessible to the bacteria. The main significance of this is that the majority of the active microorganisms in the columns came from the original inoculum and not from growth.
This is a great advantage from a process standpoint since the rate of biomass increase will be manageable.
Example 7 The experiment consisted of passing hydrogen sulphide by way of a constant stream through two columns arranged in series and onto a scrubber. Each column had the bacterial strain SM-7 (Thiobacillus thiooxidans) constantly recirculating in the present of medium as per Example 1. Hydrogen sulphide was supplied as a mixture with air. Column 1 containing 3 mm beads was first on line followed by column 2 packed with 5 mm beads. Inflow and outflow hydrogen sulphide levels were monitored upstream of column 1 (inflow) and downstream of column 2 (outflow). The results are summarized on Table 7.
Table 7 Total removal of hydrogen sulphide by columns in series.
2 0 Column 1 Column 2 Total H2S Gas Flow HZS Con- H S Con- Con-Duration In Rate out version ~ut version version Jhours) (vumZ (mL/minl fpm) (mmoJhr Lj,ppm) (mmol/hr) (mmol/hr) 0 3000 23 17 0.17 0 0.001 0.17 2 5 24.5 2100 58 330 0.25 0 0.05 0.30 95.5 2100 58 550 0.22 0 0.08 0.30 T a co umna were arranged n ser es.
Putting the two columns in series was successful 30 in removing all of the hydrogen sulphide from the gas stream. Most of the hydrogen sulphide was removed in column 1, but this is to be expected since the amount of hydrogen sulphide entering column 2 was extremely low to begin with. In a separate experiment the 35 positions of the columns were reversed to that the gas stream passed through column 2 followed by column 1.
The results were analogous with those shown on Table 7 in that most of the hydrogen sulphide was removed in the first column (column No. 2 in this case).
Claims (16)
1. A microbiological process for desulphurizing a gas containing hydrogen sulphide comprising the steps of:
providing a column having a packing disposed therein;
passing a liquid medium containing a culture of Thiobacillus thiooxidans through the column, the liquid medium having a pH in the range of from 0.8 to 3.0;
passing the gas containing hydrogen sulphide through the column to contact the liquid culture for conversion of hydrogen sulphide to elemental sulphur; and separating at least a portion of the elemental sulphur from the liquid medium.
providing a column having a packing disposed therein;
passing a liquid medium containing a culture of Thiobacillus thiooxidans through the column, the liquid medium having a pH in the range of from 0.8 to 3.0;
passing the gas containing hydrogen sulphide through the column to contact the liquid culture for conversion of hydrogen sulphide to elemental sulphur; and separating at least a portion of the elemental sulphur from the liquid medium.
2. The process according to claim 1, wherein the temperature is in the range of from 10°C to 50°C.
3. The process according to claim 1, wherein the temperature is in the range of from 20°C to 40°C.
4. The process according to claim 1, 2 or 3, wherein the pressure is in the range of from 0 psig to 100 psig.
5. The process according to claim 1, wherein oxygen is introduced into the column along with the hydrogen sulphide-containing gas.
6. The process according to claim 1, wherein oxygen is introduced into the column by saturating the liquid medium.
7. The process according to claim 5 or 6, wherein oxygen is supplied in the form of air.
8. The process according to claim 5 or 6, wherein oxygen is supplied in the form of pure oxygen.
9. The process according to claim 1, wherein the Thiobacillus thiooxidans is immobilized as a biofilm on the packing.
10. The process according to claim 1, 2 or 3, wherein the pressure is in the range of from 0 psig to 10 psig.
11. The process according to any one of claims 1 to 10, wherein the gas flows upwardly through the column while the liquid medium flows downwardly.
12. The process according to any one of claims 1 to 11, wherein the liquid medium is recycled through the column.
13. The process according to any one of claims 1 to 12, wherein a portion of the gas is recycled to the column.
14. The process according to any one of claims 1 to 13, wherein the amount of Thiobacillus thiooxidans is in the range of from 10 6 cells per milliliter to 10 cells per milliliter.
15. The process according to any one of claims 1 to 14, wherein the pH is in the range of from 2.0 to 2.5.
16. The process according to any one of claims 1 to 13, wherein the amount of Thiobacillus thiooxidans is in the range of from 10 8 cells per milliliter to 10 10 cells per milliliter.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA 2039863 CA2039863C (en) | 1991-04-05 | 1991-04-05 | Biological process for conversion of hydrogen sulphide |
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| CA 2039863 CA2039863C (en) | 1991-04-05 | 1991-04-05 | Biological process for conversion of hydrogen sulphide |
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| CA2039863C true CA2039863C (en) | 1999-09-28 |
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