Membrane process product quality

While the plant is in operation, but not producing water that is of the required quality, two additional discharge streams can be produced (Mauguin and Corsin 2005). This usually occurs during the startup of the plant. The first stream is the pretreated water which has not reached an acceptable quality to pass through the membrane unit. Its composition is largely similar to that of the feed, and if membrane processes are not used as pretreatment, its salinity should be the same as feed. The second stream is the permeate which has not yet reached the desired quality of the final water product. This stream will have a lower salinity than the feed, and should not contain great quantities of harmful chemicals. Both these streams should be safe for disposal in the concentrate stream. 

High hydrostatic pressure (HHP) processes have been used mainly for sauces or seafood and proven effective at reducing microbial populations without adverse effects on product quality (Considine et al., 2008 Brinez et al., 2006). HHP treatment causes bacterial inactivation by damaging the cell membrane, which affects membrane permeability and intracellular enzyme inactivation and possibly ruptures the plant cell wall (Kniel et al.,... 

However, the possibility to integrate various membrane operations in the same process or in combination with conventional separation units, allows, in many cases better performance in terms of product quality, plant compactness, environmental impact, and energy use to be obtained. 

The field of chemical process miniaturization is growing at a rapid pace with promising improvements in process control, product quality, and safety, (1,2). Microreactors typically have fluidic conduits or channels on the order of tens to hundreds of micrometers. With large surface area-to-volume ratios, rapid heat and mass transfer can be accomplished with accompanying improvements in yield and selectivity in reactive systems. Microscale devices are also being examined as a platform for traditional unit operations such as membrane reactors in which a rapid removal of reaction-inhibiting products can significantly boost product yields (3-6). 

Undoubtedly, this new kind of integrated approach is well representative of what should be membrane engineering, with final objectives clearly defined, the right hypothesis and choice of simple equations for modeling, a realistic representation of real complex solutions and the set-up of efficient simulation tools involving successive intra- and extrapolation steps. It appears to be easily extended to other membrane operations, in other fields of applications. It should provide stakeholders with information needed to make their decision costs, safety, product quality, environment impact, and so on of new process. Coupled with the need to check the robustness of the new plant and the quality of final output, it should constitute the right way to develop the use of membranes as essential instruments for process intensification with industrial units at work. 

Highly integrated membrane processes, combining various membrane operations suitable for separation and conversion units, are an attractive opportunity because of the synergic effects that can be attained. Practically, there are a lot of opportunities for membrane separation processes in all areas of industry [8]. The most interesting developments for industrial membrane technologies are related to the possibility of integrating various membrane operations in the same industrial cycle, with overall important benefits in terms of product quality and plant compactness. 

Application of membrane processes during production of purified food proteins is a mild treatment which ensures that the functional properties of the native proteins are retained. (1 ) These properties are mostly found to be superior to those of denatured proteins. However, not all possible needs of the modern food industry are fulfilled by using native proteins instead of denatured ones. Therefore, enzymatic modification of proteins has been demonstrated as a possible means of meeting the needs of the food industry for high-quality protein ingredients ( ), (13), (14). 

Discussion. In the above-mentioned examples membrane processes are found useful for both concentration and desalination. One reason for recommending hyperfiltration instead of evaporation in this area is the economical factors. Multi-step-evaporators are still more economic than reverse osmosis in very big plants, but the production of protein hydrolysates will in all probability be distributed between a number of middle-sized plants requiring new investments. At a time with increasing costs of energy, hyperfiltration is recommendable in such plants. Also, the freedom of choosing membranes which may improve the quality of the proteins, for example by removing of off-flavours and salt, speaks for hyperfiltration. 

Reverse osmosis is a cross-flow membrane separation process which separates a feed stream into a product stream and a reject stream. The recovery of a reverse osmosis plant is defined as a percentage of feedwater that is recovered as product water. As all of the feedwater must be pretreated and pressurized, it is economically prudent to maximize the recovery in order to minimize power consumption and the size of the pretreatment equipment. Since most of the salts remain in the reject stream, the concentration of salts increases in that stream with increased recovery. For instance, at 50% recovery, the salt concentration in the reject is about double that of the feed and at 90% recovery, the salt concentration in the reject is nearly 10 times that of the feed. In cases of sparingly soluble salts, such as calcium sulfate, the solubility limits may be exceeded at a high recovery. This could result in precipitation of the salt on the membrane surface resulting in decreased flux and/or increased salt passage. In addition, an increase in recovery will increase the average salt concentration in the feed/reject stream and this produces a product water with increased salt content. Consequently, the recovery of a reverse osmosis plant is established after careful consideration of the desired product quality, the solubility limits of the feed constituents, feedwater availability and reject disposal requirements. 

It is understood that the economical success of any membrane process depends primarily on the quality of the membrane, specifically on flux, selectivity and service lifetime. Consideration of only the transport mechanisms in membranes, however, will in general, lead to an overestimation of the specific permeation rates in membrane processes. Formation of a concentration boundary layer in front of the membrane surface or within the porous support structure reduces the permeation rate and, in most cases, the product quality as well. For reverse osmosis. Figure 6.1 shows how a concentration boundary layer (concentration polarization) forms as a result of membrane selectivity. At steady state conditions, the retained components must be transported back into the bulk of the liquid. As laminar flow is present near the membrane surface, this backflow is of diffusive nature, i.e., is based on a concentration gradient. At steady state conditions, the concentration profile is calculated from a mass balance as... 

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