Fundamentals of Membranes and Membrane Processes

Introduction

Separation, purification, and concentration are some of the major unit operations that are highly important in many industrial applications. One of the major challenges in the separation of molecular mixtures is to obtain products with high recovery and high quality. Over the last couple of centuries, the development in these operations has been snowballing with multi-scale techniques being established such as the distillation, centrifugation, adsorption, extraction, crystallization, etc. which is continuously progressed to overcome this challenge. The more recent development in these processes has been in the membrane which acts as a semipermeable barrier that allows the separation of components.

Since membranes have variation in their properties, structures, and usages, a comprehensive definition of the membrane which relates all of its aspects is challenging. In the general context of this book, the membrane is a synthetic film which acts as a selective barrier between two phases and controls the transport of various chemical components through it in a specific manner. A membrane may be porous or nonporous, symmetric or asymmetric, thin or thick, homogeneous or heterogeneous, solid or liquid, or neutral or carries positive or negative charges. The mass transport across a membrane may be due to convective flow or diffusion of individual molecules. Membrane processes comprised of different techniques which are mainly classified by the membrane pore sizes and the separation driving force. The membrane thickness can range from less than 100 nm to more than a centimetre, and the membrane pore diameter can be less than 0.1 nm to more than several micrometres. The membrane processes can be driven by the force gradients in terms of applied pressure, osmotic pressure, concentration, electrical, or thermal, or a combination of the driving forces. Although membrane can be described in a wide range of ways, a membrane can fit into more than one of the above-mentioned classes. For example, a membrane can be porous, asymmetric, and carries electrical charges.

The selection of suitable membrane or membrane process depends on some factors such as the characteristics of the mixture feed, degree of separation intended, and the volume of feed needs to be handled. Each membrane is unique, as there are no two membranes that are completely identical; each one will have differences to an extent in terms of the membrane structure and basic characteristics such as the porosity, pore sizes and pore size distributions, surface roughness, tortuosity, and thickness, even if they are made from the same type of material and fabrication method. These characteristics are important to classify the different types of membranes which are suitable for different applications. Furthermore, the characteristics of a membrane influence the parameters obtained such as the membrane flux, the membrane permeability, the achievable retention and recovery rate, and consequently the overall performance.

Figure 1.

The basic of membrane and membrane process