Mineral oils: processes and products – Refining – Sulfur removal
Reexamination Certificate
2002-01-09
2004-05-25
Langel, Wayne A. (Department: 1754)
Mineral oils: processes and products
Refining
Sulfur removal
C423S228000, C423S229000, C423S242600, C423S242700
Reexamination Certificate
active
06740230
ABSTRACT:
The present invention relates to a process for removing mercaptans from fluid streams comprising same, especially from hydrocarbon gas streams, for example, natural gas, synthesis gas from heavy oil or heavy residues or refinery gas, or else from liquid hydrocarbons, for example LPG (liquefied petroleum gas).
Numerous processes in the chemical industry give rise to fluid streams comprising acid gases, for example CO
2
, H
2
S, SO
2
, CS
2
, HCN, COS or mercaptans as impurities.
The LPG or gas streams in question here can be for example hydrocarbon gases from a natural gas source, synthesis gases from chemical processes or, say, reaction gases involved in the partial oxidation of organic materials, for example coal or petroleum. The removal of sulfur compounds from these fluid streams is of particular importance for various reasons. For instance, the level of sulfur compounds in natural gas has to be reduced by suitable processing measures immediately at a natural gas well, since the natural gas will normally also contain a certain fraction of entrained water as well as the above-recited sulfur compounds. In aqueous solution, however, these sulfur compounds form acids and have a corrosive effect. To transport natural gas in a pipeline, therefore, predetermined limits must be complied with for the sulfur-containing impurities. In addition, numerous sulfur compounds are malodorous and—with hydrogen sulfide (H
2
S) a prime example—extremely toxic even at low concentrations.
Similarly, the CO
2
content of hydrocarbon gases, such as natural gas, customarily has to be significantly reduced, since high concentrations of CO
2
reduce the calorific value of the gas and may likewise cause corrosion to pipework and fittings.
There are therefore numerous processes already in existence for removing acid gas constituents from fluid streams such as hydrocarbon gases or LPG. In the most widely used processes, the fluid mixture containing acid gases is contacted with an organic solvent or an aqueous: solution of an organic solvent as part of a gas scrub process.
There is extensive patent literature on gas scrub processes and the scrubbing solutions used in these processes. In principle, two different kinds of gas scrub solvents can be distinguished:
On the one hand there are physical solvents, which rely on a physical absorption process, i.e., the acid gases dissolve in the physical solvent. Typical physical solvents are cyclotetramethylene sulfone (sulfolane) and its derivatives, aliphatic acid amides, NMP (N-methylpyrrolidone), N-alkylated pyrrolidones and corresponding piperidones, methanol and mixtures of dialkylethers of polyethylene glycols (Selexol®, Union Carbide, Danbury, Conn., USA).
On the other hand, there are chemical solvents which work on the basis of chemical reactions which convert the acid gases into compounds which are simpler to remove. For instance, the most widely used chemical solvents in industry, aqueous solutions of alkanolamines, form salts when acid gases are passed through, and these salts can either be decomposed by heating and/or stripped off by means of steam. The alkanolamine solution is regenerated in the course of the heating or stripping, so that it can be re-used. Preferred alkanolamines used for removing acid gas impurities from hydrocarbon gas streams include monoethanolamine (MEA), diethanolamine (DEA), triethanolamine (TEA), diisopropylamine (DIPA), diglycolamine (DGA) and methyldiethanolamine (MDEA).
Primary and secondary alkanolamines are particularly suitable for gas scrubs where the purified gas has to have a very low CO
2
content (e.g., 10 ppm
v
of CO
2
). To remove H
2
S from gas mixtures having a high original CO
2
content, however, it has been found to be disadvantageous that the effectiveness of the solution for removing H
2
S is much reduced by an accelerated absorption of CO
2
. In addition, the regeneration of solutions of primary and secondary alkanolamines consumes large volumes of steam.
The European patent application EP-A-0 322 924 discloses, for example, that tertiary alkanolamines, especially MDEA, are particularly suitable for a selective removal of H
2
S from gas mixtures containing H
2
S and CO
2
.
The German patent application DE-A-1 542 415 proposes increasing the effectiveness not only of physical solvents but also of chemical solvents by addition of monoalkylalkanolamines or of morpholine and its derivatives. The German patent application DE-A-1 904 428 describes the addition of monomethylethanolamine (MMEA) as an accelerant to improve the absorption properties of an MDEA solution.
U.S. Pat. No. 4,336,233 describes one of the currently most effective scrubbing solutions for removing CO
2
and H
2
S from a gas stream. It is an aqueous solution of about 1.5 to 4.5 mol/l of methyldiethanolamine (MDEA) and 0.05 to 0.8 mol/l of piperazine as absorption accelerant (&agr;MDEA®, BASF AG, Ludwigshafen). The removal of CO
2
and H
2
S using MDEA is further described in greater detail in the following patents of present assignee: U.S. Pat. Nos. 4,551,158; 4,553,984; 4,537,753; 4,999,031, CA 1 291 321 and CA 1 295 810. The removal of mercaptans from gas streams containing same is not mentioned in these property rights.
Mercaptans are substituted forms of H
2
S in which a hydrocarbyl R takes the place of one of the hydrogen atoms. Their general formula is therefore RSH. The properties of mercaptans depend substantially on the length of the hydrocarbon chain. Mercaptans in aqueous solution likewise act as acids, but are significantly weaker than H
2
S, for example. With increasing length of the hydrocarbon chain, therefore, mercaptans behave like hydrocarbons, which makes their removal from hydrocarbon gas streams particularly difficult. For instance, there is a report in the literature that MEA and DEA solutions will remove about 45 to 50% of methyl mercaptan, but only 20 to 25% of ethyl mercaptan and about 0 to 10% of propyl mercaptan (A. Kohl, R. Nielsen: “Gas Purification”, 5
th
Edition, 1997, p.155). “Gas Conditioning and Processing”, Vol. 4: “Gas Treating and Liquid Sweeting”, 4
th
Ed., J. M. Campbell & Company, 1998, states on page 51 that aqueous amine solutions have little if any utility with regard to the removal of mercaptans from gas streams. Mercaptans occur in some natural gas sources, especially on the North American continent, and are typically present in most liquid or liquefied refined hydrocarbon products (LPG). However, because of their corrosive and malodorous properties, mercaptans must likewise be substantially removed from hydrocarbon gases or liquids. Treated and purified hydrocarbons for polymerization reactions, for example, should customarily contain not more than 1-20 ppm of mercaptans.
The literature contains a wide variety of proposals for removing mercaptans from fluid streams containing same:
U.S. Pat. No. 4,808,765 describes a three-stage process for removing acid gases from a gaseous hydrocarbon stream. The first step is an absorption process in which an aqueous solvent which contains MDEA as a selective absorbent for H
2
S and DIPA as a selective absorbent for COS, to remove substantially all the H
2
S and a portion of the COS. The second step, which utilizes an aqueous alkaline solution of a primary alkanolamine as scrubbing solution, removes a substantial portion of the remaining COS. The third step, finally, removes mercaptans with the aid of an aqueous caustic solution (NaOH). This process is very complicated in terms of apparatus, since the individual scrubbing solutions have to be regenerated separately. Moreover, the hydrocarbon gas stream has to be subsequently additionally scrubbed with water to remove remnants of the caustic solution.
U.S. Pat. No. 4,462,968 states that, although traditional alkanolamine solutions are capable of removing H
2
S down to concentrations of less than 4 ppm, these processes are not suitable for removing mercaptans. U.S. Pat. No. 4,462,968 therefore proposes a scrubbing solution for the removal of mercaptans which consists of hydrogen peroxide or a combination of
Grossmann Christoph
Holst Thomas S.
Hugo Randolf
Wagner Rupert
BASF - Aktiengesellschaft
Langel Wayne A.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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