One of the unit operations in process engineering refers to the separation of a mixture containing at least two components into two distinct fractions.[90] In general, separation processes can be classified into two main categories, namely thermal and mechanical separation processes.
Thermal separation processes are based on the utilization of mass and/or heat transfer that occurs at interfaces between two different phases that are far from being at equilibrium state.[91] For example during distillation processes, a homogenous liquid phase tries to achieve equilibrium state with a gas phase by mass transfer from the liquid into the gas phase. By mass exchange between these two phases a separation effect can be achieved. Mostly homogenous mixtures containing different components are separated by thermal separation processes. Such processes encompass different techniques like liquid-liquid solvent extraction, distillation and rectification, adsorption and absorption. In chemical industry, distillation processes are one of the most commonly used separation techniques.[92,93] A possible application of adsorption and absorption processes is the purification of gases.
In contrast, mechanical separation processes are based on the application of mechanical forces to heterogeneous mixtures of different components. Examples of such processes are sedimentation, centrifugation or filtration. Due to the time required for sedimentation processes, centrifugation or filtration is often preferred in chemical industry.[92] Due to the fact that much of the work performed in the course of this thesis is closely related to filtration processes to remove solid particles from a fluid, the principles of filtration will be introduced in the following.
Separation by filtration
Filtration processes are used for the separation of heterogeneous mixtures of at least two different phases. Although many filtration operations exist that correspond to the separation of a liquid phase from a gas, in most cases filtration is referred to the separation of a solid phase from a fluid phase, while the fluid can either be a liquid or a gas.[94] In general, filtration processes are performed for different reasons in industrial applications. For example, a valuable fluid may be treated to remove a solid impurity or a desired solid may be collected from a fluid. Depending on the purpose of the process different kinds of filters or filtration systems have to be applied. A typical filtration operation normally exhibits different individual stages such as pre-treatment, solids-concentration, separation and post-treatment. The pre-treatment step corresponds to a modification of the suspension that has to be filtered such as by adjustment of the pH-value or by addition of flocculant. During concentration of the
18
suspension, the liquid is partially removed to reduce the volume that has to be filtered. Post-treatment processes mostly refer to improvements of quality of the separated products.[95] However, the most essential part of a filtration operation is given by the separation stage based on the use of a filter medium. Purchas and Sutherland define a filter medium as follows:[1]
“A filter medium is any material that, under the operating conditions of the filter, is permeable to one or more components of a mixture, solution or suspension, and is impermeable to the remaining
components.”
Filter media can be obtained from a variety of different materials such as polymer fibers, metals, glass, ceramic materials or carbon.[96–100] These materials are transformed into a permeable form to serve as filter media.[1] Each filter medium exhibits a set of characteristic properties that makes each medium suitable for a specific application. Among others, such properties include thermal and chemical stability, wettability, particle sizes that can be retained, filtration efficiency, dirt-holding capacity and resistance to flow of the fluid medium resulting in a pressure building up before and after the filter medium and finally cost.
In real life applications many compromises have to be made in the design of a filter medium for a specific application. A filter medium should meet the following requirements to be favorable for a specific application: 1. Capability of removing up to all undesired contaminants from the fluid regardless of the particle size. 2. Low resistance of the filter medium to an applied flow. 3. Large dirt holding capacity. 4. Very small in size. 5. Low costs for production of the filter medium.[3]
Mechanisms of filtration
In general, separation of solid particles from a fluid can be classified into four different mechanisms.
The first mechanism refers to the so-called surface straining. Thereby, the particles are greater than the pore size of the filter medium and once the particle, which is carried by the fluid flow, reaches the filter medium it is separated close to the surface and blocks a pore of the filter (see Figure 1.11 (A)).
Particles separated by cake filtration (see Figure 1.11 (B)) can be smaller than the size of the individual pores. However, bridging processes lead to an accumulation of particles (filter cake) close to the surface without fully blocking them. As the filter cake builds up, an additional filtration effect is generated by the deposited particles. A filtration system that is based on cake filtration usually involves an initial period of time after the filtration operation started during which the filter does not reach the desired filtration efficiency. In contrast to the first two mechanisms, depth filtration (see Figure 1.11 (C)) is based on particle deposition inside the filter medium after the individual particle entered
19
through a pore. Thereby, particle sizes can be smaller than the pores of the filter medium. In most cases, a filtration process mostly consists of a combination of these three mechanisms. In addition, another separation process exists that corresponds to a combination of surface straining and depth filtration, the so-called depth-straining. It is based on variations of the pore diameter as the particle passes through the filter medium. The diameter of the pore might become too small for such a particle resulting in the blocking of the pore and deposition inside the filter medium.[3,95]
Figure 1.11: Basic filtration mechanisms for the separation of solid particles from a fluid stream.
Surface straining (A) refers to the deposition of particles close to the surface of the filter if the particle size is larger than the pores of the filter medium. In cake filtration (B) the particles are smaller than the pores, but are gathered close to the surface by bridging of the pores. Accumulation results in filter cake formation. In case of depth filtration (C) the particles are smaller than the pore and attach to the wall of the pore inside the filter. (Figure is based on ref[95])
Depending on the desired application for a filtration system, filters are designed to feature specific filtration mechanisms. For example, the recovery of a valuable solid from the fluid is usually accomplished by filtration systems that are based on surface straining or cake filtration. Due to the fact that the particles are deposited near the surface of the filter, the solid particles can easily be removed from the filter. In contrast, clarification of a fluid can also be achieved by depth filtration and depth straining mechanisms. Apart from the valuable fraction of the filtered mixture to be obtained from the process, other factors such as the particle concentration in the feed have to be considered.[95]
Usually filters based on cake filtration mechanism are fed with high concentrations of solid material suspended in the fluid and the filtration system is cleaned after reaching a limiting resistance of the filter to the flow.
The complexity of important influences in the deposition of particles by depth filtration mechanism exceeds those for the other three processes. Most of the work performed is concerned with fibrous media. Thereby, depth filtration is one very important separation mechanism. In consequence, the concept of depth filtration will be introduced in more detail.
Flow direction
A C
Flow direction B
Flow direction