Kinetic separation of vaporous alcohol-water mixtures : modeling, experiments, feasibility study

Abstract

Separation processes play an important role in the petrochemical and chemical industry and are to a large extent responsible for the high energy consumption in this sector. For this reason the chemical industry is continuously looking for more efficient separation processes. Besides energy consumption also the replacement or reduction of the use of hazardous chemicals is an important issue. The work performed in this thesis focuses on the development of the kinetic separation process FricDiff. The aim is to get more insight in this separation technology and to examine if FricDiff, both from an economical and environmental perspective, can be an attractive technology for the chemical industry. FricDiff can be classified as a kinetic separation process, because separation is achieved as a result of differences in transport velocities of the components of a gas or vapor mixture when diffusing through an auxiliary component. This auxiliary component is intentionally added to the system to achieve a separation and is referred to as the sweep gas or separating agent. The main focus of this thesis is to study the separation of alcohol-water vapor mixtures with FricDiff using nitrogen or carbon dioxide as the sweep gas. A typical FricDiff unit consists of two compartments separated by a porous, nonselective screen. The vaporous mixture is introduced to one compartment and the sweep gas to the other. While flowing through the unit, material is selectively exchanged between the two compartments. Two product streams leave the unit, one enriched in the slower diffusing mixture component(s) and one enriched in the faster diffusing component(s). In the unit multi-component mass transfer through the porous screen plays an important role and it is examined in more detail in Chapter 3. In this chapter six models for multi-component mass transport through pores and porous media are compared. Differences between the models are observed, but they are generally small and for engineering purposes most probably of minor importance. In Chapter 4 three models are developed to describe the separation process in a tubular FricDiff module. With these three models the influence of process conditions and barrier characteristics on the performance of the module are examined. It is shown that the mode of operation (co-current vs. countercurrent), the sweep gas to feed mixture ratio, absolute pressure level, pressure gradients over the porous barrier, type of sweep gas, barrier thickness and barrier pore size all influence the separation process and can be used for module and process optimization. Chapter 5 shows that for FricDiff modules equipped with thin barriers of high porosity concentration boundary layer in the compartments have a large impact on the separation process. These boundary layers give rise to additional resistances to mass transfer. In order to describe these resistances multi-component Sherwood correlations are derived that give an accurate description of the transport of mixture components through these boundary layers. In Chapter 6 experimental results are presente on the separation of helium-argon gas mixtures and the separation of isopropyl alcohol-water vapor mixtures with nitrogen as the separating agent. The main focus of this chapter is the validation of the numerical models developed in Chapter 4 with experimental data. With a value of the porosity-tortuosity (??/??2) parameter fitted and a value of the pore size determined from permeation experiments, generally a good agreement is obtained between experimental data and numerical results. The performance of a unit in which the FricDiff separation process is combined with condensation of vapors on the wall of the sweep gas compartment is studied numerically in Chapter 7. It is shown that the condensation of vapor components within the unit can have a positive effect on the separation when the faster diffusing component also condenses at a higher rate. Another goal of the thesis is to search for viable applications for FricDiff in industrial separation processes. For this reason an exergy analysis is performed in Chapter 8 to determine the thermodynamic efficiency of pervaporation, distillation, a single FricDiff unit and a cascade of FricDiff units. For the separation of an isopropyl alcohol-water mixture the lowest efficiencies are obtained with the cascade of FricDiff units. This is a result of the irreversibilities inherent to the separation process (intermingling of mixture components and sweep gas) and the irreversibilities associated with the heat transfer steps in each stage of the cascade. The entropy production in a single FricDiff unit is limited and therefore hybrid distillation-FricDiff configurations are studied in Chapter 9. This chapter studies several process configurations for isopropyl alcohol-water and ethanol-water dehydration processes. Application of FricDiff as a final purification step for ethanol-water separations can be beneficial from an energetic perspective, but the extra investments that have to be made, make the process economically unattractive.