Organic compounds -- part of the class 532-570 series – Organic compounds – Sulfur containing
Utility Patent
1999-04-20
2001-01-02
Vollano, Jean F (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Sulfur containing
C568S021000
Utility Patent
active
06169210
ABSTRACT:
TECHNICAL FIELD
This invention pertains to a novel and highly efficient method for the production of dialkyl disulfides.
BACKGROUND
Dialkyl disulfides are useful as intermediates in the production of various end products. For example, dimethylthiotoluenediamine (DMTDA), a compound used extensively in many injection molding processes, can be produced in a relatively cost efficient manner using dimethyl disulfide prepared pursuant to this invention.
The production of dialkyl disulfides by the metal catalyzed oxidation of mercaptans in the presence of elemental oxygen is not new to the chemical industry. However, the presently utilized processes are still deficient in important areas.
In particular, the large quantities of heat released in the reaction combined with the high efficiency of modem catalysts place an extreme burden on the cooling method employed. A present method of cooling the oxidation reaction involves continuously feeding alkyl mercaptan into the reaction zone in great excess relative to oxygen, with the expectation that unoxidized alkyl mercaptan will function to absorb the heat of reaction generated by the oxidized mercaptan. The above method suffers from a deficiency in that the limiting reagent is oxygen. In order to keep the reaction zone temperature within a specified range, it is necessary that only a fraction of the mercaptan introduced into the reaction zone be converted to dialkyl disulfide; the unoxidized alkyl mercaptan molecules function as a coolant and pass from the reaction zone. As a result, the mercaptan to dialkyl disulfide conversion efficiency after a single pass through the reaction zone is significantly less than one, and corrosive mercaptan must be continuously recycled and rehandled.
Another problem with the presently practiced methods of producing dialkyl disulfides from mercaptans is that a convenient carbon dioxide recycle process for producing mercaptans from hydrogen sulfide and methanol yields a product containing extremely high amounts of carbon dioxide. In order to obtain pure mercaptan to feed into the reaction zone, it is necessary to separate the carbon dioxide from the mercaptan.
Another problem with currently practiced methods for converting alkyl mercaptans to dialkyl disulfides is that the reactor is typically not a simple adiabatic reactor. Due to the fact that external coolant is generally necessary, the reactor is of a relatively expensive and complicated nonadiabatic design.
It would represent a significant advance in the state of the art if a method could be discovered which (1) improves the one-pass conversion efficiency of the oxidation reaction, thus eliminating the recycling and rehandling of the corrosive mercaptan, (2) allows direct utilization of the carbon dioxide recycle process product, thus eliminating the separation of mercaptan and carbon dioxide, and (3) enables the use of a simple and inexpensive adiabatic reactor design.
SUMMARY OF THE INVENTION
Pursuant to this invention, carbon dioxide is used as a coolant in the place of the mercaptan component used heretofore in the reaction of alkyl mercaptan and oxygen to give dialkyl disulfide. By so doing, the alkyl mercaptan conversion efficiency after one pass through the reaction zone can be raised to nearly one hundred percent. Moreover, carbon dioxide can be readily and easily separated from the dialkyl disulfide product, and this in turn greatly facilitates the removal of and recycling of the coolant. In addition, mercaptan which is contaminated with high amounts of carbon dioxide, a mixture which is available from other industrial processes, can be fed into the reaction zone without first separating out the carbon dioxide. Thus, this invention provides a convenient and highly efficient process for making dialkyl disulfides from alcohols, hydrogen sulfide, and oxygen. Additionally, since carbon dioxide functions as an internal coolant, a simple adiabatic reactor design can be used.
Accordingly, one embodiment of this invention is a process for producing dialkyl disulfides, which process comprises:
A) continuously introducing into a reaction zone components comprising (i) a mercaptan, (ii) elemental oxygen, and (iii) carbon dioxide to form a reaction mixture and contacting said mixture in said reaction zone with a solid phase catalyst such that a product mixture comprising dialkyl disulfide and carbon dioxide is formed; and
B) continuously withdrawing product mixture from said reaction zone.
As noted above, this invention enables the use of carbon dioxide contaminated feedstreams. In fact, the carbon dioxide contaminant can be present in either or both reactant feedstreams.
After functioning as a coolant in the reaction, it is preferable to separate the carbon dioxide from the dialkyl disulfide product, cool the carbon dioxide, and reintroduce it into the reactor with incoming reactants to give continued cooling benefits.
The repeated removal and recycling of carbon dioxide results in the accumulation of a substantial recycled stream during continuous operation. Likewise, a source of “fresh” carbon dioxide can be used instead of or in addition to a recycled stream of carbon dioxide. Thus, it can be desirable to begin the process with fresh carbon dioxide as the coolant, and decrease its usage as a recycle stream is generated. If a reactant source contains carbon dioxide, dependence upon fresh carbon dioxide can be reduced more quickly.
Thus in another embodiment, this invention provides for a process for producing dialkyl disulfides, which process comprises:
A) continuously introducing into a reaction zone components comprising (i) a mercaptan, (ii) elemental oxygen, and (iii) carbon dioxide to form a reaction mixture and contacting said mixture in said reaction zone with a solid phase catalyst such that a product mixture comprising dialkyl disulfide and carbon dioxide is formed;
B) continuously withdrawing product mixture from said reaction zone;
C) separating from said mixture all or a portion of the carbon dioxide present therein; and
D) recycling all or a portion of the carbon dioxide from C) to A).
An additional aspect of the process of this invention is that it enables the efficient linking of the dialkyl disulfide formation reaction with the mercaptan formation reaction. Such integration is especially convenient when the mercaptan source is contaminated with carbon dioxide.
In commonly owned co-pending Ser. No. 09/295,622, filed Apr. 20, 1999, an efficient process for producing alkyl mercaptans in which carbon dioxide is used as an internal coolant is disclosed, which process comprises: A) continuously introducing into a reaction zone components comprising (i) an alkanol, (ii) hydrogen sulfide, and (iii) carbon dioxide to form a vapor phase reaction mixture and contacting said mixture in said reaction zone with a solid phase catalyst such that a vapor phase product mixture comprising alkyl mercaptan and/or dialkyl monosulfide is formed; and B) continuously withdrawing from said reaction zone a mixture comprising alkyl mercaptans and carbon dioxide.
If the alkyl mercaptan is formed in a process which utilizes the addition of carbon dioxide as a coolant, it is preferable to recycle all or a portion of the carbon dioxide separated from the dialkyl disulfide product back into the mercaptan formation reaction. On the other hand, if it is the case that the mercaptan is formed in a process in which the carbon dioxide has other origins, e.g., a contaminated reactant stream, it can be preferable to recycle all or a portion of the carbon dioxide separated from the dialkyl disulfide product back into the dialkyl disulfide formation reaction. Both types of recycle can be used if it is deemed advantageous to do so.
If desired, the mercaptan/carbon dioxide mixture resulting from the mercaptan formation reaction can be cooled before introduction of said mixture into the dialkyl disulfide formation reaction zone in order to control the temperature in the zone. Additionally or alternatively, the temperature in the dialkyl disulfide reaction zone can be controlled by
Pippenger Philip M.
Vollano Jean F
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