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Potassium ethyl xanthate |
Some chemicals are regarded as too difficult to work with on scale and processes that require them are nearly always automatically rejected for scale-up. Carbon disulfide is in that category. This low boiling liquid has an auto-ignition temperature of 90 C and so broad a flammability envelope (1.3-50 Vol. % in air), that a fire is likely to result if a bit of CS2 escapes from the process equipment and finds a source of ignition, which can apparently be as innocuous as a steam leak. This point was made in the article, Holistic Route Selection, by Ronald B. Leng, Mark V.M. Emonds, Christopher T. Hamilton, and James W. Ringer, Org. Process Res. & Dev. 2012, 16, 415-424. Nevertheless, these authors eventually stuck with advancing a process involving carbon disulfide reagent but only after adopting mitigation systems. To quote from the paper, “Ultimately a well-engineered CS2 management system using returnable CS2 containers, all-welded transfer lines, precise liquid metering, nitrogen blanketing, oxygen sensors, fugitive vapour sensors, and an automatic water deluge system were lines of defence that were incorporated into the process design…”
One lesson of this paper is that essentially any chemistry can be handle safely, if one is prepared to pay enough for the engineered system(s) needed to safeguard it. The problem is that unless the volume of the product is very high (their product was an agrochemical) and the profit margin good and stable, the up-front capital costs are likely to sink the project because of an inadequate discounted cash flow. Multipurpose plants simply cannot afford the required safety systems. It is not that the systems don’t exist.
This same paper points out that a large part of the risk using CS2 occurs as much from when the material is being transported, stored, moved, measured as from the residue that remains in the waste streams. The Dow Corporation developed a method of generating enough CS2 to serve as a reagent ‘in situ’ which eliminated the risks from transporting, storing, moving, measuring and transferring reagent into the reactor. They did this by acidification of the solid salt, potassium ethyl xanthate which created carbon disulfide in situ. Using this sourcing, they obtained the same yield in the requisite transformation; however, they found that in order to completely consume their expensive intermediate substrate a slight excess of carbon disulfide was needed and this left some of it in the post-reaction mixture which gave issues in the subsequent centrifugation and gas venting system. This particular problem would not eliminate this approach for other process steps involving carbon disulfide reagent where an excess is unnecessary or when scrubbing of an excess can be simple.
One lesson of this paper is that essentially any chemistry can be handle safely, if one is prepared to pay enough for the engineered system(s) needed to safeguard it. The problem is that unless the volume of the product is very high (their product was an agrochemical) and the profit margin good and stable, the up-front capital costs are likely to sink the project because of an inadequate discounted cash flow. Multipurpose plants simply cannot afford the required safety systems. It is not that the systems don’t exist.
This same paper points out that a large part of the risk using CS2 occurs as much from when the material is being transported, stored, moved, measured as from the residue that remains in the waste streams. The Dow Corporation developed a method of generating enough CS2 to serve as a reagent ‘in situ’ which eliminated the risks from transporting, storing, moving, measuring and transferring reagent into the reactor. They did this by acidification of the solid salt, potassium ethyl xanthate which created carbon disulfide in situ. Using this sourcing, they obtained the same yield in the requisite transformation; however, they found that in order to completely consume their expensive intermediate substrate a slight excess of carbon disulfide was needed and this left some of it in the post-reaction mixture which gave issues in the subsequent centrifugation and gas venting system. This particular problem would not eliminate this approach for other process steps involving carbon disulfide reagent where an excess is unnecessary or when scrubbing of an excess can be simple.
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