Hybrid separation processes

Advances in fundamental knowledge of adsorption equihbrium and mass transfer will enable further optimization of the performance of existing adsorbent types. Continuing discoveries of new molecular sieve materials will also provide adsorbents with new combinations of useflil properties. New adsorbents and adsorption processes will be developed to provide needed improvements in pollution control, energy conservation, and the separation of high value chemicals. New process cycles and new hybrid processes linking adsorption with other unit operations will continue to be developed. 

At present, there is one main commercial application of pervaporation, the production of high purity alcohol by a hybrid process which also incorporates distillation. Such separations use cellulose-acetate-based composite-membranes, with an active layer of polyvinyl alcohol, for example. Membrane fluxes are in the range 0.45-2.2 kg/m2 h. Pervaporation... 

In simple experiments, particulate silica-supported CSPs having various cin-chonan carbamate selectors immobilized to the surface were employed in an enantioselective liquid-solid batch extraction process for the enantioselective enrichment of the weak binding enantiomer of amino acid derivatives in the liquid phase (methanol-0.1M ammonium acetate buffer pH 6) and the stronger binding enantiomer in the solid phase [64]. For example, when a CSP with the 6>-9-(tcrt-butylcarbamoyl)-6 -neopentoxy-cinchonidine selector was employed at an about 10-fold molar excess as related to the DNB-Leu selectand which was dissolved as a racemate in the liquid phase specified earlier, an enantiomeric excess of 89% could be measured in the supernatant after a single extraction step (i.e., a single equilibration step). This corresponds to an enantioselectivity factor of 17.7 (a-value in HPLC amounted to 31.7). Such a batch extraction method could serve as enrichment technique in hybrid processes such as in combination with, for example, crystallization. In the presented study, it was however used for screening of the enantiomer separation power of a series of CSPs. 

Naturally, there exist a variety of membrane separation processes depending on the particular separation task [1]. The successful introduction of a membrane process into the production line therefore relies on understanding the basic separation principles as well as on the knowledge of the application limits. As is the case with any other unit operation, the optimum configuration needs to be found in view of the overall production process, and combination with other separation techniques (hybrid processes) often proves advantageous for large-scale applications. 

In conclusion, a greater knowledge of the effect of the key controlling parameters of this powerful separation technique, as well as improvement in membrane life time of the currently available commercial electromembranes and reduction in their costs, would ensure further growth beyond desalination and salt production and foster ED applications in the food sector, as well as in the chemical, pharmaceutical, and municipal effluent treatment areas. This will of course need extensive R D studies and will highly likely result in hybrid processes combining ED to other separation techniques, such as NF, IE, and so on, so as to shorten present downstream and refining procedures. 

Dewatering of glycol is a difficult separation by distillation alone, so a hybrid process of the type shown in Figure 9.13(a) has been proposed. The product of the distillation step is approximately 90% glycol/10 /i- water. This mixture is then sent to a pervaporation unit to remove most of the water as a discharge-able product. The glycol concentrate produced by the pervaporation unit contains 1-2 wt% water and can be sent to an optional adsorption dryer if further dehydration is required. 

Pervaporation as a standalone technique is still to be developed industrially, but as part of a hybrid process, combined with for example, distillation (Figure 3.3), it is very promising for difficult separations and may yield considerable energy savings. 

Development of hybrid production-separation processes may be the way to successful solutions especially in the case of higher value added compounds. 

Blahusiak, M., Schlosser, S. and Martak, J. (2009) Simulation of hybrid fermentation-separation process for production of butyric acid. Submitted for publication in Chemical Papers. 

Homogeneous hybridization assays are analyses for the detection of a given DNA/RNA sequence by hybridization without separation processes (B/F separation). These assays form a basis of real-time PCR as described below. Furthermore, sufficiently sensitive homogeneous hybridization assays could be done without PCR and enable, for example, direct detections of certain mRNA forms expressed in cells. This type of experiment has been actually performed with some organic fluorophores (Peng et al., 2005 Santangelo et al., 2006). 

The possibility of combining two different separation units into one, hybrid, process has not been considered in this chapter. Hybrid processes are quite novel and have only very recently been considered by industry and have, therefore, so far not made it into the standard textbooks. A hybrid process has the combined benefits of both of the component units and the benefits should theoretically outweigh the disadvantages. An example is a hybrid of a distillation column and a pervaporation unit for azeotropic separation, where the distillation unit alone is limited by the azeotropic point. Again, a lot of research is currently devoted to this type of operation and it is generally believed that it will become more widely used in the future. 

It can be concluded that there is a potential for membrane separation of almost any gas from a mixture of gases if physical and chemical properties are carefully considered as well as material properties and durability, possible transport mechanisms, and optimum process conditions evaluated. Creative reflection and advanced research will be able to develop this environmental friendly separation technique for applications within many areas in the future, and hopefully be able to displace old, energyconsuming (and not so clean) technology or combine with them in hybrid process solutions. The costs of the final solution will always be a major issue for commercialization. 

Very low capital and operating cost The separation could be made more economical by using a hybrid membrane process, i.e., a combination of distillation and pervaporation processes. Thus, a part of the total separation employs distillation where it is economical. PV replaces the subsequent separation where distillation becomes expensive. The overall operating cost of such a hybrid process is much lower than that of distillation alone. 

Reactive distillation, as the name implies, refers to a distillation process that incorporates a reaction and a separation step within a distillation column. The technique offers a key opportunity for improving the structure of a process. - It is a so-called hybrid process, i.e. it merges two different unit operations in a single apparatus, namely reaction and distillation. But the combination of distillation and reactions is possible only if the conditions of both unit operations can be combined. This means that the reactions have to show reasonable data for conversions at pressure and temperature levels that are compatible with distillation conditions. Because of the limited hold-up in distillation column, those reactions having a conversion half-time of 10-30 min are preferred. So, the judicious use of the chemical equilibrium constant is the basis for the design of reactive distillation processes. 

Hybrid Extraction Processes Hybrid processes employ an extraction operation in close association with another unit operation. In these processes, the individual unit operations may not be able to achieve all the separation goals, or the use of one or the other operation alone may not be as economical as the hybrid process. Common examples include the following. 

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