Chemistry: molecular biology and microbiology – Process of utilizing an enzyme or micro-organism to destroy... – Treating gas – emulsion – or foam
Reexamination Certificate
1999-05-07
2001-04-24
Beisner, William H. (Department: 1744)
Chemistry: molecular biology and microbiology
Process of utilizing an enzyme or micro-organism to destroy...
Treating gas, emulsion, or foam
C435S264000, C210S610000, C210S614000
Reexamination Certificate
active
06221652
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for the removal of reduced sulphur compounds from a gas stream by scrubbing the gas with an aqueous washing liquid and treating the spent washing liquid with sulphide-oxidising bacteria in the presence of an electron acceptor in a reactor and recycling the treated liquid as a washing liquid.
BACKGROUND OF THE INVENTION
Such a process in known e.g. from WO 92/10270, WO 94/29227 and WO 96/30110. In these prior art processes oxygen is used as an electron acceptor. The oxygen is fed at a limited rate so as to direct the oxidation of sulphide to elemental sulphur rather than to sulphate. The use of oxygen as an electron acceptor, however, requires the presence of an aeration system including a compressor, pipings, spargers, and in most cases a separate reactor. The investment costs for such a system are relatively high, especially when small volumes of water or gas are to be treated or when e.g. high-pressure natural gas is to be desulphurised.
SUMMARY OF THE INVENTION
According to the invention, the spent washing liquid containing the reduced sulphur compounds is treated with sulphide-oxidising bacteria in an integrated scrubber/bioreactor, using nitrate as an electron acceptor.
In the present context, the term “reduced sulphur compound” is understood to comprise any gaseous or volatile sulphur compound wherein sulphur has the oxidation state-2. Such compounds include hydrogen sulphide, lower alkyl mercaptans such as methane-thiol, lower alkyl sulphides and disulphides such as dimethyl sulphide, carbonyl sulphide (COS) and carbon disulphide (CS
2
). Especially relevant are H
2
S, COS and CS
2
.
The biological oxidation reactions of hydrogen sulphide by colourless sulphur bacteria, such as the genus Thiobacillus, especially the species
T. denitrificans
, using nitrate as an electron acceptor are the following:
H
2
S+OH
−
→HS
−
+H
2
O (1)
H
2
S+HCO
3
−
→HS
−
+H
2
O+CO
2
(1a)
5HS
−
+2NO
3
−
+H
2
O→5S°+N
2
+7OH
−
(2)
5HS
−
+8NO
3
−
+H
2
O→
5
SO
4
2 −
+4N
2
+4N
2
+3OH
−
+H
2
O (3)
5H
2
S+2NO
3
−
→5S°+N
2
+2OH
−
4H
2
O (1+2)
5H
2
S+8NO
3
−
+2OH
−
→5SO
4
2 −
+4N
2
+6H
2
O (1+3)
Reaction (1) denotes a preliminary reaction, e.g. occurring in a gas scrubber, wherein gaseous hydrogen sulphide is dissolved as hydrosulphide anions. Reaction (2) describes the anoxic oxidation of sulphide to elemental sulphur, whereas reaction (3) represents the complete oxidation of sulphide to sulphuric acid. Reaction (1+2) is the total net reaction of hydrogen sulphide to elemental sulphur.
Carbonyl sulphide and carbon disulphide may hydrolyse to hydrogen sulphide according to the following reactions or their anionic equivalents:
COS+H
2
O→H
2
S+CO
2
(4)
CS
2
+2H
2
O→2H
2
S+CO
2
(5)
Alternatively or additionally, COS and CS
2
may be oxidised directly, according to the following reactions;
5COS+2NO
3
−
+H
2
O→5CO
2
+5S
0
+N
2
+2OH
−
(2′)
5COS+8NO
3
−
+H
2
O→5CO
2
+5SO
4
2−
+4N
2
+2OH
−
(3′)
5CS
2
+4NO
3
−
+2H
2
O→5CO
2
+10S
0
+2N
2
+4OH
−
(2″)
5CS
2
+16NO
3
−
+2H
2
O→5CO
2
+10SO
4
2−
+8N
2
+4H
+
(3″)
The reactions involving oxidation of thiosulphate with nitrate as electron acceptor are the following:
5S
2
O
3
2−
+8NO
3
−
+2OH
−
→10SO
4
2−
+4N
2
+H
2
O (6)
5S
2
O
3
2−
+2NO
3
−
+H
2
O→5S°+5SO
4
2−
+N
2
+2OH
−
(7)
Nitrate can be added as a solid salt, but preferably it is added as a concentrated solution of e.g. potassium nitrate, or a mixture of a nitrate salt and nitric acid. As the conversion of H
2
S and other reduced sulphur compounds to elemental sulphur produces alkali (equation 1+2/2/′/2″), and the conversion of H
2
S and other reduced sulphur compounds to sulphate consumes the same amount of alkali (equation 1+3/3′/3″), acid (preferably nitric acid replacing part of the nitrate) should be added in the preferred case where sulphide is predominantly converted to sulphur. Preferably, nitrate (and nitric acid) is added in a substantially stoichiometric amount for oxidation of reduced sulphur compound predominantly to sulphur, i.e. about 0.4 mole of nitrate per mole of H
2
S or COS, optionally allowing for minor oxidation to sulphate, i.e. 0.4-0.9, especially 0.4-0.6 mole of nitrate per mole of H
2
S or COS, and the double amount for CS
2
. An overdosis of nitrate should be avoided, because it destabilises the process due to an accumulation of nitrite (NO
2
−
). The nitrite concentration should remain below 1 mM, preferably below 0.5 mM.
The nitrate addition can be controlled using the redox potential of the aqueous solution, as described in WO 98/04503. Thus the redox potential of the medium of the oxidation is adjusted at a value below −150 mV (against an Ag/AgCl reference electrode), especially below −250 mV. The preferred redox potential range is form −300 to −390 mV, more preferably from −320 to −360 mV (against an Ag/AgCl reference electrode). The range of −300/−390 mV against Ag/AgCl corresponds to a range of −97/−187 mV against a H
2
reference electrode at 30° C. The redox setpoint values apply for a temperature of 30°C. and a pH of 8.
The temperature of the biological oxidation is between 10 and 85° C., the optimum being between 20 and 50° C., especially at about 30° C. The optimum pH is in the range of 7-9. If the solution does not contain nutrients, as is the case with gas scrubbing, these have to be supplied as well, This can be done at the same time as the nitrate supply. The electric conductivity of the washing liquid is preferably kept between 30 and 100 mS/cm.
The bacteria to be used in the present process can be taken from conventional sulphide-oxidising cultures. The bacteria are preferably neutrophilic bacteria and will typically include Thiobacillus species, especially T. denitrificans.
The process of the invention can be used for treating gases also containing carbon dioxide. The carbon dioxide contributes to the H
2
S loading capacity of the scrubbing liquid, especially at high pressures. As an example, the H
2
S loading capacity of a scrubbing liquid for scrubbing a pressurised gas (95 bar) having a CO
2
content of 1.1 vol. % is 200 to 300 g/m
3
. Also the carbon dioxide can be used as a carbon source for the sulphide-oxidising bacteria.
In the process of the invention for the removal of hydrogen sulphide and other reduced sulphur compounds from gas streams, the solution is recycled after oxidation of the reduced sulphur compounds to elemental sulphur, using the same reactor for scrubbing and for anoxic biological treatment. No liquid recirculation between different pressures is necessary. Further advantages are that the equipment can be relatively simple and inexpensive, and that the recycle ratio and thus the liquid residence time can be high so that any loss of biomass is compensated by bacterial growth. If required, the bacteria can be immobilised on a carrier, which carrier can be combined with a packing material in the scrubber. For a simple operation such immobilisation can be omitted.
REFERENCES:
patent: 1701825 (1929-02-01), Seil
patent: 4760027 (1988-07-01), Sublette
patent: 5196176 (1993-03-01), Buisman
patent: 5236677 (1993-08-01), Torres-Cardona et al.
patent: 5354545 (1994-10-01), Buisman
patent: 5366633 (1994-11-01), Buisman
patent: 5789236 (1998-08-01
Buisman Cees Jan Nico
Janssen Albert Jozef Hendrik
Beisner William H.
Paques Bio Systems B.V.
Young & Thompson
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