(375b) Reaction Engineering of Urea Alcoholysis

Organic carbamates and carbonates are a class of compounds having variety of applications. Dialkyl carbonates (DAC) form an important branch of organic carbonates and are used as precursor to synthesize higher organic carbonates. Synthesis of DACâ??s by the reaction of urea and alcohol is an attractive â??greenâ?? route¹. In this work we have investigated the reaction engineering aspects of urea alcoholysis to Methyl carbamate.

Urea alcoholysis is a reversible reaction generating alkyl carbamate and ammonia as products and it can be performed non-catalytically or in presence of catalyst. Since ammonia is highly soluble in methanol, it needs to be stripped out from the reaction zone in order to achieve substantial conversion. The first step in Reaction engineering analysis is to finalize the reactor configuration and the decision making factor to decide the reactor configuration here is to decide the mode of stripping. If the reaction is severely equilibrium limited, then it is necessary strip ammonia continuously to drive the reaction forward whereas if the value of equilibrium constant is closer to or more than 1 then equilibrium conversion is nearly complete without the need of reaction and separation taking place simultaneously. Thus the reaction can also be performed with reaction and separation taking place in sequential manner where number of reactor can be arranged in series with separation taking place between each of them removing ammonia generated in previous reactor.

Simple mathematical models were developed and simulations were performed to check the feasibility of reaction and separation happening in simultaneous and sequential manner for different values of equilibrium constant. Our analysis shows that for this reaction sequential reactor-separation is sufficient to get substantial conversion of urea. In sequential reaction-separation the reactor is still operating in the equilibrium dominated region and we would essentially get equilibrium conversions in subsequent reactors. However among the reactor types the equilibrium conversion is achieved faster in PFR than in CSTR. Also given the advantages of using a tubular reactor with low cost and easy operation the tubular reactor â??separator in sequential manner was chosen.

An experimental setup comprising tubular reactor and separator was built. Reaction of urea and methanol yielding Methyl carbamate (MC) and ammonia was selected as a model reaction. Since the reaction is slow the residence time requirements of the reaction demand operation in laminar region. To achieve near plug flow conditions a coiled type tubular reactor was built. Experiments were performed with the tubular reactor configuration for different flow rates and temperatures. A kinetic analysis was performed where data obtained from experiments was fitted against the reactor model and kinetic parameters are reported. Although the urea alcoholysis to alkyl carbamate reaction is reversible, it is not severely equilibrium limited and therefore it is possible to get good conversion in sequential reactor-stripper configurations with appropriate alcohol to urea ratio. Given sufficient residence time, equilibrium conversion can be easily achieved in subsequent reactors. The analysis presented in this work can be readily applied to a general class of reversible reactions with volatile products and the methodology used to develop and design reactors for them.

References

1. Ranade, V.V.; Kelkar, A.A.; Rane, V.H.; Kinage, A.K.; Shingote, S.K.; Roy L.S.; Synthesis of methyl carbamate and dimethyl carbonate (dmc) in presence of stripping with inert gas or superheated vapours and a reactor for the same, US 20150315134A1.