Design of a monomer recovery plant for the Polyols Polyols plant

Identifier:  TFG:3011

Handle: https://hdl.handle.net/20.500.11797/TFG3011

Authors:  Solanas Martín, Marc

Abstract:
The company REPSOL has a plant for the production of Polymer Polyols that generates a residual stream formed by water and remnants of unreacted monomer (acrylonitrile and styrene). Currently, this effluent is not used and is burned entirely due to the toxicity of the monomer, which means a great environmental impact as well as a loss of benefits. The developed project consists of the use of part of the monomer contained in the residual stream by means of a Flash distillation under vacuum pressure of 0,02 kg/cm2 (abs). Despite the fact that a small amount of the desired acrylonitrile is lost mixed with water and part of the styrene at the bottom of the flash column, this reduces the amount of water in the stream, thus obtaining another with a higher concentration of monomer absorbed per column head thanks to the vacuum system. Reducing the amount of water makes it easier for acrylonitrile to separate in a decanter at atmospheric pressure. In this case, the difference in the densities of the liquids causes the acrylonitrile to be in the highest position and to overflow. Both the aqueous stream obtained at the bottom of the column and the decantation process are sent to the existing cremation process, which is outside the limits of the project, with a notable reduction in its monomer content. This entire process is capable of recovering 85% of the acrylonitrile initially fed. The source of income of the project is in turn the key to its success, and resides in the same use of acrylonitrile. The fact of being able to recover the acrylonitrile to be sent to its origin where it will be used to produce Polymer Polyols produces a benefit of 2,36·105 €/year and an environmental saving of 7,2·103 €/year. It economically reduces product losses from not recovering a monomer that could be used to produce more product. Environmentally, the CO2 emission is reduced by 85,45% compared to what it supposed to burn all the residual current. 3,49·105 € is needed for the implantation of the designed plant, which will be recovered in a 1.3-year period. The economic study reveals a NPV of 1,15·106 and an IRR of 78%, thus confirming the viability of the project.