Thermally driven membrane processes

Most membrane transport processes are isothermal processes with either concentration, pressure or electrical potential difference as the driving force. 

When a membrane separates two phases held at different temperatures, heat will flow from the high-temperature side to the low-temperature side. This transpon of heat can be expressed by a simple phenomenological equation, i.e. Fourier s law (see chapter 1.5), where the heat flow is related to the corresponding driving force, the temperature difference. The process of heat conduction across a homogeneous membrane is shown schematically in figure VI - 47. The heat flux is given by 

Integratioa of eq. VI -107 across the membrane at steady-state flow and constant X gives 

In addition to the heat flow a mass flow also occurs, a process called thenno-osmosis or thermo-diffusion. No phase transitions occur in these processes. 

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If a temperature difference is maintained across the two sides of the membrane, a water vapour difference appears. For this, MD is a thermally driven membrane process, which is similar to OMD, (discussed in Section 2.7), using the same basic mechanism of separation the driving force in both cases is the water vapour pressure difference. 

Another thermally driven membrane process is membrane distillation. Here, a porous membrane separates two liquids which do not wet it. If the liquids differ in temperature, the resulting vapour pressure differencecauses vapour molecules to permeate from the high-temperature (high vapour pressure) side to the low-temperature (low vapour pressure) side. The basic concept of membrane distillation will be described below. 

While membrane processes have infiltrated many process schemes, prohahly the most extensive replacement has been in the area of water treatment. For example, reverse osmosis has largely replaced distillation/evaporation for the production of drinking water from seawater [41,42]. Clearly, with the right mix of material properties, efficient process designs, and economic drivers, energy-efficient membrane processes can replace conventional thermally driven separation processes. Later in this chapter, desalination will be a featured example. 

The range of application of the three pressure-driven membrane water separation processes—reverse osmosis, ultrafiltration and microfiltration—is illustrated in Figure 1.2. Ultrafiltration (Chapter 6) and microfiltration (Chapter 7) are basically similar in that the mode of separation is molecular sieving through increasingly fine pores. Microfiltration membranes filter colloidal particles and bacteria from 0.1 to 10 pm in diameter. Ultrafiltration membranes can be used to filter dissolved macromolecules, such as proteins, from solutions. The mechanism of separation by reverse osmosis membranes is quite different. In reverse osmosis membranes (Chapter 5), the membrane pores are so small, from 3 to 5 A in diameter, that they are within the range of thermal motion of the polymer... 

Membranes can contribute significantly to new concepts in more energy-efficient and low C02 emission power generation, and the following section explores some of these cases as alternatives to conventional amine-absorption-based thermally driven processes. 

Vaccum membrane distillation, such as any membrane distillation process, is a thermally driven process in which a feed solution is bought into contact with one side of a microporous membrane and a vacuum is created on the opposite side to create a driving force for transmembrane flux (Figure 19.8). The microporous membrane only acts as a support for a vapor-liquid interface, and does not affect the selectivity associated with the vapor-liquid equilibrium [76]. 

A range of membrane processes are used to separate fine particles and colloids, macromolecules such as proteins, low-molecular-weight organics, and dissolved salts. These processes include the pressure-driven liquid-phase processes, microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO), and the thermal processes, pervaporation (PV) and membrane distillation (MD), all of which operate with solvent (usually water) transmission. Processes that are solute transport are electrodialysis (ED) and dialysis (D), as well as applications of PV where the trace species is transmitted. In all of these applications, the conditions in the liquid boundary layer have a strong influence on membrane performance. For example, for the pressure-driven processes, the separation of solutes takes place at the membrane surface where the solvent passes through the membrane and the retained solutes cause the local concentration to increase. Membrane performance is usually compromised by concentration polarization and fouling. This section discusses the process limitations caused by the concentration polarization and the strategies available to limit their impact. 

Membrane processes have been widely adopted over the last 30 years in spite of inherent hmitations such as fouHng, thermal and chemical resistance and maximum achievable purity. One reason for their wide success is the emergence of integrated/hybrid membrane processes for several appHcations, some of which are discussed in Chapter 3. The principal characteristics of membrane processes are Hsted in Table 1.3. AH processes except dialysis, ED and EDI are pressure driven, and aU except PV and MD do not involve a phase change. 

MD is one of the emerging nonisothermal membrane separation processes, known for abont 50 years but still reqniring development for its industrial implementation. MD refers to the thermally driven transport of vapor through porous hydrophobic membranes, the driving force being the vapor pressure difference between the two sides of the membrane pores. Simnltaneous heat and mass transfer occurs in this process, and different MD contignrations include direct contact MD, sweeping gas MD, vacuum MD, and air gap MD. 

The first separation example is seawater desalination. Traditionally, desahnation was done by distillation or simple evaporation/condensation [55]. Today, thermally driven desalination has been largely replaced by the membrane process reverse osmosis. In reverse osmosis an applied pressure exceeding the osmotic pressure of the salt solution causes water to permeate through a dense membrane. Hydrated salt ions are relatively large compared to water and have a lower permeability through the membrane resulting in relatively salt-free water being collected as the reverse osmosis permeate. 

Micro filtration (MF) and ultrafiltration (MF) are typical low-driven pressure membrane processes widely applied in various chemical and biochemical processes thanks to their advantages over traditional filtration methods. They are generally a thermal and simple in concept and operation and do not involve phase changes or chemical additives. Additionally, they are modular, easy to scale-up and characterized by low energy consumptions (Mulder, 1998). 

First we will illustrate the minimum energy required to separate a small amount of mixture for the following processes evaporation of water from a saline solution recovery of water by reverse osmosis separation of an ideal binary gas mixture by membrane permeation. Then we will consider the definition of net work consumption for thermally driven processes. Next we will consider a variety of separation processes vis-k-vis their minimum energy requirement for separation. 

In the late 1980s, it was observed that the same membranes that had been used in thermal membrane distillation could be used in a concentration-driven process called OMD [36]. Both these processes show great similarity in employing hydrophobic membranes, where a liquid-vapor interface is formed on both sides of the membrane pores. The source for the driving force (vapor pressure difference) is different in both cases. It is a temperature difference in the case of thermal membrane distillation, while it is a concentration difference in the case of OMD [37]. 

The control over supersaturation is one of the essential aspects of crystallisation. Because of the limited thermal stability of many biopharmaceutical products, evaporation of the solvent is often a less desired method since the heat transfer to the system is associated with temperature gradients. Therefore, alternative methods to remove the solvent have been proposed. One of these techniques is osmotic dewatering in which solvent removal is a pressure driven transport of solvent through solvent-selective membranes. The membrane part of the process is analogous to ultrafiltration for macromolecules or to reverse osmosis for small solutes. 

The majority of the commercial membranes for pressure-driven processes are made from hydrophobic polymers with high thermal, chemical, and mechanical stabilities. Because of the hydrophobicity of these materials, they are prone to adsorption of the fouling substances. It has been well documented that membranes with hydrophilic surfaces are less susceptible to fouling (Fane and Fell 1987 Hilal et al. 2005). 

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