Separation of binary homogeneous azeotropic mixtures using pervaporation.

Abstract

The separation of mixtures containing homogeneous azeotropes is often complex and requires the use of enhanced distillation techniques. This leads to a significant increase in capital and operating costs. The use of membrane separation techniques to separate azeotropic mixtures is favoured over extractive distillation, azeotropic distillation and absorption as this is an effective low energy and low-cost alternative. Pervaporation is a membrane-based separation technique often used in industry to dehydrate alcohol-water azeotropes, to remove water from organic solvents or to remove organics from water. The process requires a liquid feed at a pressure high enough to maintain its phase while being depleted of components contained within the feed to form a liquid retentate. A membrane is typically selective for one component with finite permeability for the remaining components in the feed. A vapour phase must be maintained on the permeate side of the membrane by applying a vacuum downstream thereby creating a pressure gradient. A pervaporation unit generally consists of a series of membrane cells grouped together in modules, and interstage heat is applied to the feed of subsequent modules. This investigation focused on the dehydration of alcohols (ethanol, propan-1-ol and propan-2-ol) using a poly(vinyl alcohol) based membrane. An experimental study on ethanol-water under various operating conditions was performed. The effect of permeate pressure (2‒5 kPa), feed temperature (338.15‒348.15 K) and feed water concentration (1‒5 wt.%) are reported in terms of flux and permeate quality. Results confirmed that pervaporation is a suitable method to break an azeotrope. Due to technical issues encountered with the equipment, the experimental determination of pervaporation performance was not pursued further. This prompted an extensive simulation study whereby semi-empirical models were developed for the alcohol-water systems using Aspen Custom Modeler® before exporting to Aspen Plus® for simulation and optimization. Dehydration of an industrial grade propan-2-ol aqueous solution (85 wt.% propan-2-ol) using pervaporation was then rigorously simulated as the final objective, as this is not explored in detail in the literature. Various interstage heat temperatures (363.15, 368.15, 373.15 K) and module arrangements (3, 5 and 8 cells per module) were considered to produce the required retentate stream of less than 2 wt.% water. A total of nine design cases were developed to meet the industry purification requirements (>98 wt.% propan-2-ol in retentate). An economic evaluation (inclusive of operating, investment, and maintenance cost) of the separation was performed. It was confirmed that a membrane setup of 3 modules with 3 cells per module including interstage heating to 373.15 K presented the lowest. total cost of 174.27 $/t. This arrangement provided the most feasible configuration for propan-2-ol dehydration using a PVA-based membrane and when compared to azeotropic distillation from literature, it was found that a saving of 34% could be achieved using pervaporation, assuming a pre-concentrator cost of 1/3 of the total process costs from the literature studies. The comparative economic analysis performed across various processes was based on the total cost per ton of propan-2-ol product, which served as a standardized cost. Two procedural assumptions were applied; an operational time of 300 days per year and 24 hours a day for an industrial plant, and a production rate of 257.69 kg.h-1 propan-2-ol, as per the optimal design case.