The present invention relates to a method for removing an acidic gas component from a raw gas, and relates to an additive having antifoaming and corrosion inhibition effects to be added to a high concentration of an amine solution (preferably 2-(2-aminoethoxy)ethanol, referred to as “diglycolamine” hereinafter) for removing an acidic gas such as carbon dioxide and hydrogen sulfide.
A natural gas or an offgas produced from oil factory contains water, and an acidic gas such as carbon dioxide and hydrogen sulfide. Water and an acidic gas must be removed to prevent corrosion and obtain heat from a treated gas. An aqueous solution of an amine such as monoethanolamine and diethanolamine has been used for a long time to remove an acidic gas. An attempt to produce a smaller-size apparatus for removing an acidic gas has been made to reduce an operational cost and increase productivity by using a higher concentration of an amine solution. However, when the amine concentration is increased, problem that apparatus materials comprised of carbon steel and stainless steel become corroded occur, and an aqueous diglycolamine solution, considered to be relatively anticorrosive, has been used. However, when an aqueous solution containing 25% or higher of diglycolamine was used, it was observed that the apparatus materials become more corroded. Particularly, it was observed that a stainless steel of an amine regenerating tower, to remove an absorbed acidic gas from an amine solution by heating, was much corroded. High alloy “Carpenter 20Cb3” was used but corrosion could not be prevented.
On the other hand, foaming frequently occurred in an acidic gas absorbing tower in which a natural gas and diglycolamine are countercurrently contacted, and corrosion was increased by an uneven flow of an amine solution. A variety of antifoaming agents were used but they failed. There is a concern that an excessive addition of an antifoaming agent caused an increase in foaming.
Because a corrosion product stabilizes and increases foaming, and foaming increases corrosion, an additive having antifoaming and corrosion inhibition effects has been required.
An addition of chromium as a corrosion inhibition agent is not preferable because chromium is not environmentally acceptable as apparent from the pollution problems caused by hexavalent chromium in recent years. An apparatus for removing an acidic gas, capable of being used for a long period of time and in which a high concentration of a diglycolamine solution can be used, has been demanded. In addition, foaming frequently occurred at a step of removing an acidic gas conducted at a high temperature, and an antifoaming agent was added for increasing productivity. However, there was no effective antifoaming agent. There was a case in which an addition amount of an antifoaming agent was increased to be 5,000 ppm or higher, a silicone component was adhered to the apparatus, and adversely a problem of low-productivity was caused.
There are prior art publications of Kohyo (Jpn. Unexamined Patent Publication) No. 2002-519171 and Kokai (Jpn. Unexamined Patent Publication) No. 7-53206.
The object of the present invention is to provide an additive used for an alkanolamine (referred to an amine hereinafter) solution for removing an acidic gas, which has a corrosion inhibition effect on a material of an apparatus for removing an acidic gas, such as carbon steel and stainless steel, even when the concentration of the amine solution for removing an acidic gas, preferably 2-(2-aminoethoxy)ethanol (referred to as “diglycolamine” hereinafter) is 40% or higher, and which has an antifoaming effect during the acidic gas removing process, and to provide a method for removing an acidic gas component from a raw gas.
An acidic gas can be effectively removed while foaming is suppressed, and corrosion of an apparatus for removing the gas can be reduced by adding a mixture of an organopolysiloxane having a polyoxyalkylene group, and a fine silica powder to a high concentration of an aqueous amine solution for removing an acidic gas which contains 40% by mass or more of an amine.
Because the organopolysiloxane having a polyoxyalkylene group is dispersed well in water, there is less corrosion of apparatus materials after it has been used for a long period of time, and it can be satisfactorily used in apparatuses comprising carbon steel or stainless steel.
The fine silica powder is used to maintain an antifoaming effect. If the powder has a BET specific surface area of 50 m2/g or more, an antifoaming effect is maintained for a long period of time.
More corrosion inhibition and antifoaming effects can be obtained by adding a surfactant to increase a dispersing property of the mixture of the organopolysiloxane having a polyoxyalkylene group and the fine silica powder in an aqueous solution.
The embodiment of the present invention is explained with the drawings.
The present invention is further described as below. An apparatus can be effectively operated by using an aqueous solution, containing 40% by mass or more of an amine, preferably a 60 to 65% by mass aqueous amine solution, as an acidic gas absorbing solution. As a silicone-type additive to be added therein, a mixture of 50 to 99% by mass of an organopolysiloxane, having a polyoxyalkylene group, represented by formula (1) with 1 to 50% by mass of a fine silica powder, having a BET specific surface area of 50 m2/g or more, is useful. With the additive, antifoaming effect can be remarkably improved as compared to the case using a conventional emulsified mixture of dimethylpolysiloxane and a fine silica powder. A corrosion inhibition effect on the apparatus materials such as stainless steel and carbon steel was also recognized.
R12XSi—(R12SiO)m—(R12SiO)n—SiR12X (1)
(provided that R1 represents a monovalent hydrocarbon group having 1 to 6 carbon atoms; Y represents —R2O—(CpH2pO)q—R3; X represents an alkoxy group having 1 to 4 carbon atoms, an acyl group, a hydroxyl group, R1 or Y; R2 represents a divalent hydrocarbon group having 3 to 6 carbon atoms; R3 represents a hydrogen atom, a hydrocarbon group having 1 to 4 carbon atoms, or an acyl group; m is an integer of 10 to 200, n is 0 or an integer of 1 to 50, p is an integer of 2 to 4, q is an integer of 5 to 50, provided that when n is 0, X is Y)
The dispersion property of a conventional additive of dimethylpolysiloxane emulsion in a water system is poor at a high temperature, in a basic condition. A silicone oil was precipitated, due to a repeated addition of the additive after an antifoaming effect was reduced, and the silicone oil was adhered to the inside of the apparatus tubes, whereby productivity was adversely reduced. There are additives which increase corrosion of the apparatus materials such as carbon steel and stainless steel. However, it was found that the organopolysiloxane having a polyoxyalkylene group is excellent in dispersing in water, has no problems as described above, there is less corrosion observed in the apparatus materials after the organopolysiloxane having a polyoxyalkylene group had been used for a long period of time, and the organopolysiloxane having a polyoxyalkylene group can be sufficiently used for carbon steel and stainless steel.
A preferable organopolysiloxane having a polyoxyalkylene group, used in the present invention, is represented by formula (1), in which R1 represents a hydrocarbon group having 1 to 6 carbon atoms. Examples of the hydrocarbon group include an alkyl group such as methyl group, an ethyl group, a propyl group, a butyl group, and a hexyl group, and an aryl group such as a phenyl group. A preferable example is a methyl group. X represents an alkoxy group having 1 to 4 carbon atoms, a hydroxyl group, R1, or Y which is a group having a polyoxyalkylene group represented by formula (2).
—R2O—(CpH2pO)q—R3
In the formula, R2 represents a divalent hydrocarbon group having 3 to 6 carbon atoms, R3 represents a hydrogen atom, a hydrocarbon group having 1 to 4 carbon atoms, or an acyl group, m is an integer of 10 to 200, n is 0 or an integer of 1 to 50, p is an integer of 2 to 4, q is an integer of 5 to 50, provided that when n is 0, X is Y. The polyoxyalkylene group includes a polyoxyethylene, polyoxypropylene or polyoxybuthylene unit, which can be a copolymer comprising at least one of the units. Particularly, an organopolysiloxane having 8% by mass or more of an organic group having the polyoxyethylene group in which p is 2 is preferably used, when dispersion thereof in the acid gas absorbing solution is considered.
M represents an integer of 10 to 200. When m is less than 10, an antifoaming property is poor, and when m is more than 200, the viscosity of the solution becomes high and workability is reduced, both of which are not preferable. m preferably ranges from 15 to 150. n is preferably 0 or an integer of 1 to 50. When n is 0, a polyoxyalkylene-containing group represented by formula (2) is present at both ends thereof. When n is more than 50, siloxane segments in the entire structure are reduced whereby an antifoaming property becomes poor, which is not preferable. q is preferably an integer of 5 to 50. When q is less than 5, dispersing property in water becomes poor, and when q is more than 50, an antifoaming property is reduced, both of which are not preferable. Particularly, q is preferably 7 to 40 when antifoaming and dispersing properties are considered. Examples include the compounds represented as below, in which Me represents a methyl group, EO represents an oxyethylene group, and PO represents an oxypropylene group.
It is preferable that, as a material to exhibit an antifoaming property, a mixture of 50 to 99% by mass of the organopolysiloxane having a polyoxyalkylene group, represented by formula (1) and 1 to 50% by mass of the fine silica powder having a BET specific surface area of 50 m2/g or more is present in an amount of 0.1 to 5000 ppm in the additive for removing an acidic gas. The fine silica powder is used to maintain an antifoaming property. The type of the fine silica powder is not limited as long as it has a BET specific surface area of 50 m2/g or more. The fine silica powder to be used may be wet or dry silica. Examples of the fine silica powder include precipitated silica, silica xerogel, humed silica and silica comprising a surface modified by an organic silyl group, specifically aerogel (Nippon Aerogel K. K. Trade Name), Nipsil (Nippon Silica K. K. Trade Name), and Cabosil (Cabot Trade Name) The fine silica powder preferably has a BET specific surface area of 50 m2/g or more. If a BET specific surface area thereof is less than 50 m2/g, an antifoaming property is reduced. Particularly, the silica having a BET specific surface area of 100 m2/g or more is preferable in terms of an antifoaming property. The mixing ratio of the organopolysiloxane having a polyoxyalkylene group to the fine silica powder is preferably 50 to 99/50 to 1% by mass in terms of workability and a stable antifoaming property. If the fine silica powder accounts for less than 1% by mass, an antifoaming property cannot be maintained. If it accounts for more than 50% by mass, the mixture with the organopolysiloxane having a polyoxyalkylene group becomes highly viscous, and is not industrially applicable. Particularly, the preferable content of the fine silica powder is 2 to 40% by mass.
A surfactant can be added to be contained in an amine solution for removing an acidic gas to improve dispersion of the mixture of the organopolysiloxane having a polyoxyalkylene group and a fine silica powder in the aqueous solution. The surfactant to be used includes a nonionic, cationic, or anionic surfactant, preferably a nonionic surfactant in terms of dispersion. Examples thereof include polyoxyethylene alkylether, polyoxyethylene alkylphenylether, a sorbitan fatty acid ester, a glycerin fatty acid ester, a sucrose fatty acid ester, a polyoxyethylene higher fatty acid ester, a polyoxyethylene ricinus ester, an alkylbenzene sulfonic acid salt, and a higher alkylsulfonic acid salt. When these surfactants are used, a mixture having a specific blending amounts of 50 to 98% by mass of the organopolysiloxane having a polyoxyalkylene group, represented by formula (1), 1 to 50% by mass of the fine silica powder having a BET specific surface area of 50 m2/g or more, and 1 to 40% by mass of a surfactant. If an added amount of a surfactant is more than 40% by mass, an antifoaming property is reduced which is not preferable.
The present invention further provides a method for removing an acidic gas by providing an amine solution for removing an acidic gas, which is an aqueous solution containing 40% by mass or more of an amine and comprises a mixture of the organopolysiloxane having a polyoxyalkylene group and the fine silica powder in an amount of 0.1 to 5000 ppm, from the top part of an acid gas absorbing tower, introducing a natural gas containing an acidic gas from the bottom part of the absorbing tower, and subjecting them to countercurrent contact at 60 to 85° C. (see
However, it was found that corrosion can be remarkably reduced by using an additive for an amine solution for removing an acidic gas of the present invention. Corrosion environment was produced in a test tube contained in an autoclave in a laboratory to determine corrosion inhibition effect by dipping a carbon steel test piece therein. A corrosion rate was calculated based on a reduced mass of the carbon steel piece after the dipping test. The result is shown in
The mechanism of anticorrosion has not been elucidated. However, it is presumed that a mixture, of the present invention, of the organopolysiloxane having a polyoxyalkylene group and the fine silica powder, was thermally decomposed during the operation of the apparatus to form a water-repellent methyl siloxane protective film on the surface of carbon steel and stainless steel of apparatus materials, whereby an anticorrosion property was exhibited.
On the other hand, it was determined that foaming, which frequently occurred from the beginning of the operation in an acidic gas absorbing tower in which a natural gas and an aqueous amine solution is countercurrently contacted, causing various problems such as low workability, was reduced by using an additive for a solution for removing an acidic gas of the present invention. It is presumed that foaming was suppressed because the mixture, of the present invention, of the organopolysiloxane having a polyoxyalkylene group and the fine silica powder has a remarkable antifoaming property, and an amount of a silicone adhered to the apparatus was reduced due to a reduced amount of the additive added to the aqueous amine solution.
The present invention is further described with examples, but is not limited thereto.
Using an acidic gas absorbing apparatus shown in
Against foaming in the operation, 12.5 ml of the above-described silicone mixture 1 was diluted with water and added. An amount of an aqueous 65% diglycolamine solution is 28 kl. The change in number of times of the silicone 1 mixture addition is shown in
An aqueous diglycolamine solution which had absorbed an acidic gas was collected from the plant immediately, one month and two months after starting addition of the silicone mixture 1, and change in corrosion level of stainless steel was determined. The collected aqueous diglycolamine solution was put in an autoclave and a cell to which a test piece was attached was rotated at a high speed at a high temperature and in a stream of carbonic acid gas to calculate the corrosion rate based on a reduced mass of the test piece. The result is shown in
A thickness of a remained steel (SUS 304L steel) of the regenerating tower shell 24 months after starting the addition was determined. The result is shown in
Using an acidic gas absorbing apparatus shown in
The average number of addition times per day of the silicone mixture 2 (which equals to the times of foaming) was 20 and several. However, in the later period of the operation, it was more than 70 and it was determined that the mixture 2 was not effective to reduce foaming which was required to operate the plant safely.
The thickness of a remaining steel (SUS 304 L steel) of the regenerating tower shell 24 months after starting the addition was determined. The result is shown in
Using an acidic gas absorbing apparatus shown in
Against foaming during the operation, 62.5 ml of the above-described silicone emulsion 1 was diluted with water to be added. The amount of the aqueous 65% diglycolamine solution was 28 kl.
A thickness of a remaining steel (SUS 304L steel) of the regenerating tower shell 24 months after starting the addition was determined. The result is shown in
Number | Date | Country | Kind |
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2002-343611 | Nov 2002 | JP | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/JP03/15083 | 11/26/2003 | WO | 4/18/2005 |