Date of this Version
The purpose of this design project is to recover a co-solvent mixture that is used to remove oil and water from metal machine parts. The cleansing solvents used are n-propylbromide (NPB) and isopropyl alcohol (IPA), which respectively remove oleic acid and water. The solvent mixture starts at an IPA/NPB molar ratio of 56/44 and can be used for cleaning machinery until the water reaches 7.5% by mole. This spent cleaning mixture is then delivered for reclamation of IPA and NPB so that it can be used for cleaning again. A 6,240 gallon truckload is delivered every two days, and the mixture to be cleaned has a molar composition of 47.6% IPA, 37.4% NPB, 7.5% oleic acid, and 7.5% water. The goals of the project are to completely remove the oleic acid, reduce the water molar composition to below 2.5%, maximize co-solvent recovery, and maximize profitability.
A major challenge of the project is the non-ideal behavior of the components, which includes multiple azeotropes and distillation boundaries. Another important characteristic of this design project is the unusually small scale: one 6,240 gallon quantity of used co-solvent mixture must be processed every two days. Due to this scale, batch processes were investigated as well as continuous processes.
The continuous alternative utilizes three major separation units: a 15-tray distillation column, a decanter for the distillate, and an evaporator for the bottoms. 93.8% of the original IPA and 99.2% of the original NPB is recovered. There is no oleic acid and 0.5% by mole of water in the product. Pure NPB and IPA are added at the end of the separation to compensate for the lost co-solvents, and to restore the IPA/NPB ratio to 56/44. A $0.45/lb selling price of reclaimed co-solvent returns an IRR of 45.6% and an NPV of $2,657,300 in year 10 of production.
The batch alternative utilizes a batch distillation column with multiple receivers and recovers 95.4% of the original IPA and 99.7% of the original NPB. There is 0.8% water by mole and no oleic acid in the product. Pure NPB and IPA also must be added to restore the original ratio. Because the composition of water is higher in the batch product than in the continuous, the co-solvent mixture is sold at a lower $0.42/lb, resulting in an IRR of 37.2% and an NPV of $2,452,500 in year 10. However, the batch process has significant down time and can potentially handle up to four times the solvent demand (2 trucks of solvent/day), resulting in an IRR of 130% and an NPV of $22,494,500 at the same selling price. Because of the batch plant’s ability to handle demand growth, its flexibility in separating different co-solvent ratios, and its robust economic potential, we recommend the construction of the batch co-solvent reclamation plant.
Date Posted: 20 August 2012