Customer Requirements Propane to Acrylic Acid design plant proses

14 Separation After acrylic acid is produced, it must be separated from the reactants oxygen, nitrogen, and propane and the by-products of the reaction acetic acid, water, and propylene. While a simple flash drum will be able to separate the reactants from acrylic acid, the final product separation from acetic acid and water is difficult due to three separate azeotropes as seen in Figure 3.2: In order to determine the most economic and efficient method of separation, three different processes were evaluated and the results compared. The first proposed method of separation, displayed in Figure B.2 located in Appendix B, page 133, involved the use of a liquid-liquid extraction process to allow better separation of organic components from water. The product stream from the reactor was fed to a flash drum and a distillation column, which separated and recycled the gaseous components. The bottoms product was separated in an eight- tray column into a pure acrylic acid stream. The distillate contained water, acetic acid, and residual acrylic acid and was fed into a decanter. The decanter was operated with n-propyl Figure 3.2: Azeotrope Diagram for Acetic AcidWaterAcrylic Acid. The diagram was produced using Aspen, based on the NRTL-RK model. 15 acetate, an extraction solvent recommended by U.S. Patent Number 5,315,037. 11 The decanter succeeded in separating the mixture into a water stream containing 97 by mass water and the balance organic components and an organic stream. Through a series of two more distillation columns a majority of the solvent which can be recycled, and 96 of the total acrylic acid were recovered. Ultimately, this method of separation was not selected because the additional costs associated with the decanter and the solvent feed of n-propyl acetate did not result in better acrylic acid separation. The second method of separation proposed was an extractive distillation process performed in a distillation column, which eliminated the use of a decanter but still required an entrainer. 12 An Aspen diagram of the separation process can be seen in Figure B.3 located in Appendix B, page 134. The extractive agent is introduced near the top of the column and its presence alters the relative volatility of the compounds to allow a greater degree of separation. After a separation using different entrainers based on suggestions from U.S. Patent 5,154,800 12 was designed in Aspen, dimethylsulfoxide was selected as the optimal extractive agent. As with the decanter process, this method of separation was not selected because it did not result in higher acrylic acid separation despite the added cost of solvent and keeping the number of distillation towers four used the same. The final separation process evaluated was a network of four distillation towers which maneuvered around the azeotropes within the system. This processes proved to be better than the other two alternatives based on cost, optimal separation of acrylic acid and final stream purity. This separation scheme is discussed later on in detail. 11 Sakamoto, K., Tanaka, H., Ueoka, M., Akazawa, Y., Baba, M. 1994. U.S. Patent No. 5,315,037. 12 Berg, L. 1992. U.S. Patent No. 5,154,800.