Organic compounds -- part of the class 532-570 series – Organic compounds – Sulfur containing
Reexamination Certificate
1999-04-20
2001-03-06
Vollano, Jean F. (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Sulfur containing
C568S061000, C568S038000
Reexamination Certificate
active
06198003
ABSTRACT:
TECHNICAL FIELD
This invention pertains to a novel, highly efficient method for the production of alkyl mercaptan and dialkyl monosulfides.
BACKGROUND
Alkyl mercaptan and dialkyl monosulfides are useful as intermediates in the production of various end products. For example, methionine, an important component in poultry feed, can be prepared in a relatively cost-efficient manner using methyl mercaptan prepared pursuant to this invention.
The reaction of alkanols and hydrogen sulfide in the presence of catalytic materials to produce alkyl mercaptan and dialkyl monosulfides has been known for almost ninety years. Modern commercial methods for the production of alkyl mercaptan and dialkyl monosulfides from alkanols and hydrogen sulfide generally employ alumina-based catalysts. As the reaction rate in the presence of these modem catalysts is quite high and the reactions themselves are extremely exothermic, heat is generated at a high rate. In addition, the reaction rate rises with temperature, thus in the absence of efficient cooling, a small temperature rise may be quickly amplified. It is therefore desirable to employ a cooling method which has not only a high heat removal capacity, but also the ability to quickly stabilize the reaction temperature in the event of even a small temperature rise.
Some present commercial methods of producing methyl mercaptan and dimethyl sulfide utilize an evaporative cooling method by supplying methanol as a liquid/vapor mixture. However, in practice, fine control of reaction temperature is limited by the difficulty in providing precise mixtures and altering said mixtures quickly and accurately. Furthermore, such methods strongly recommend the partitioning of the reaction zone into smaller regions in order to obtain sufficient cooling efficiency.
Other cooling methods utilized in modern commercial processes involve a non-adiabatic transfer of heat across an interface to an external coolant. The hot reaction products transfer heat to the cooling medium only indirectly through an intermediate separator. Reactors so cooled are susceptible to spatially uneven cooling, leading to “hot spots” which can dramatically reduce the life of the catalyst. In addition, the inability of coolant and reactant molecules to mix, as well as the indirectness of reactant-coolant heat transfer force a thermodynamic limitation on the rate and energy-efficiency of cooling attainable with such methods. As a result, the ability to respond quickly and efficiently to small temperature changes is also limited. Such non-adiabatic reactor designs are also significantly more expensive than simpler adiabatic designs.
Furthermore, sources of hydrogen sulfide tapped for industrial use can contain very high amounts of contaminants. In particular, carbon dioxide can be present in amounts of 50% by mole or higher. Carbon dioxide and hydrogen sulfide react efficiently in the presence of aluminum oxide catalysts to form another contaminant, carbonyl sulfide (J. Lavalley et al.
J. C. S. Chem. Comm.
1979, pgs. 146-148)—at high carbon dioxide to hydrogen sulfide ratios, the formation of carbonyl sulfide is deemed very likely (M. Suckow et al.
Separation Technology
1994, pgs. 143-151). In order to produce a product of acceptable purity, it would seem advisable to remove the carbon dioxide contaminant before the reaction process is carried out. However, not only does separation of carbon dioxide from reactants of similar weights and vapor pressures require time and energy, but once removed, the resultant carbon dioxide is a resource which may be wasted unless the costly steps of storing it and/or transporting it to another utility are undertaken.
It would thus be a significant advance in the state of the art if a process for producing alkyl mercaptan and/or dialkyl monosulfides could be found which has among its benefits increased cooling efficiency, improved capacity to respond quickly and efficiently to temperature changes, simple reactor design and beneficial utilization of the carbon dioxide contaminant, while maintaining a high rate and quality of product output.
SUMMARY OF THE INVENTION
A process has been devised which can achieve the above-mentioned benefits while providing for high-yield, carbonyl sulfide-free production of alkyl mercaptan and/or dialkyl monosulfides. It has been found, pursuant to this invention, that when carbon dioxide gas is used as an internal coolant in the aluminum oxide catalyzed reaction of alkanols and hydrogen sulfide to produce alkyl mercaptan and dialkyl monosulfides, a surprisingly low, often undetectable amount of carbonyl sulfide is formed.
Additionally, the direct contact and complete mixing of the hot reaction product with the coolant increases the rate and energy efficiency of cooling. If the carbon dioxide-hydrogen sulfide concentration ratio supplied to the reaction zone is raised/lowered in response to temperature increase/decrease, a highly responsive mode of reaction zone temperature control can be realized.
Also, the internal usage of carbon dioxide allows the reaction to be carried out in a simple adiabatic reactor. Expensive designs which require partitioning of reaction zone and external coolants are thus eliminated.
Furthermore, the separation of carbon dioxide from the vapor phase product mixture is generally easier than separation from the vapor phase reaction mixture due to increased weight and decreased vapor pressure of products relative to reactants.
Accordingly, in one of its embodiments, this invention provides a process for producing alkyl mercaptan and/or dialkyl monosulfide, 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 mercaptan and/or dialkyl monosulfides.
The carbon dioxide may already be present in the feed stream of one or both of the reactants. After functioning as a coolant in the reaction, the carbon dioxide can be separated from the heavier alkyl mercaptan and dialkyl monosulfide reaction products, cooled, and reintroduced into the reactor with incoming reactants to give continued cooling benefits. The repeated removal and recycling of carbon dioxide can be used to increase the amount of carbon dioxide relative to hydrogen sulfide, resulting in an increased cooling capacity.
If a reactant stream contains carbon dioxide, it may be desirable to achieve elevated carbon dioxide levels by separating it from the vapor phase product mixture, cooling it, and reusing it without employing any additional sources of carbon dioxide. However, a source of fresh carbon dioxide can be used instead of or in addition to the recycled carbon dioxide. By “fresh”, it is meant that the carbon dioxide comes from a source independent of the present process. Thus, it may be desirable to begin the process with fresh carbon dioxide as the coolant, and decrease its amount as a recycled stream is generated. If a reactant source contains carbon dioxide, dependence upon fresh carbon dioxide can be reduced more quickly.
Highly sensitive temperature control can be realized by the adjustment of reactor input of recycled and/or fresh carbon dioxide. Typically, temperature control at a constant reactor pressure is effected by altering the mole ratios of carbon dioxide to reactants introduced into the reaction zone.
Thus, in another embodiment, this invention provides a process for producing alkyl mercaptan and/or dialkyl monosulfide, 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 with a solid phase catalyst such that a vapor phase product mixture comprising alkyl me
Boone James E.
Booth McGee Sharon D.
Lin Kaung-Far
Matthews Michael D.
Prindle, Jr. John C.
Albemarle Corporation
Pippenger Philip M.
Vollano Jean F.
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