Membrane process power consumption

A few trial calculations show that a process using a feed-gas compressor, even if coupled with an energy-recovery turbine on the residue side, cannot produce low-cost oxygen because of the quantity of electricity consumed. All the feed air must be compressed but only a small portion permeates the membrane. The power consumption of a vacuum pump is less because the only gas evacuated by the vacuum pump is the oxygen-enriched product that permeates the membrane. 

Electrodialysis. Electro dialytic membrane process technology is used extensively in Japan to produce granulated—evaporated salt. Filtered seawater is concentrated by membrane electro dialysis and evaporated in multiple-effect evaporators. Seawater can be concentrated to a product brine concentration of 200 g/L at a power consumption of 150 kWh/1 of NaCl (8). Improvements in membrane technology have reduced the power consumption and energy costs so that a high value-added product such as table salt can be produced economically by electro dialysis. However, industrial-grade salt produced in this manner caimot compete economically with the large quantities of low cost solar salt imported into Japan from Austraha and Mexico. 

As indicated in Fig. 17.2, the membrane process has long been characterised by substantial reductions in electric power consumption, through constant advances in membrane, electrolyser and electrode technologies. In the early years of its commercial establishment, some 25 years ago, it yielded a caustic soda concentration of 20% or lower, with less than 90% current efficiency. Today, the caustic soda concentration is 33%, the current efficiency is 97%, and the ohmic drop of the membrane has been lowered by approximately 1.0 V. During the same period, advances in electrolyser design have improved the uniformity of intracell electrolyte concentra-... 

To date, the Improved B-1 with an F-8934 membrane has been operated as a full-sized pilot cell. Some essential data at 8 kA m-2 are in the process of being collected. Improved B-1 combined with F-8934 shows significantly lower power consumption, as is shown in Fig. 19.12. F-8934 shows a value of less than 2300 (d.c.) kWh tonne-1... 

Innovative process designs are being developed to reduce the size of the membrane unit and the energy needed to separate, condense and inject the carbon dioxide. It seems possible to reduce the energy consumption of the membrane process to about 20-25% of the power plant output. If this work is successful and these membrane plants are built, this application will dwarf all other gas-separation membrane processes. 

Many conventional wastewater treatment processes that have long been in use are now considered impractical because they require a large amount of space, a large number of unit operations, and are affected by problems associated with odor and other emissions. Recent years have seen an increasing trend toward process intensification, which has led to the development of advanced membrane processes that are simple to construct and operate, have well-defined flow patterns, better dispersion effects, relatively low power consumption, lower emissions, and high mass-transfer performance, which are compact and recyclable. 

Energy Consumption. Electric power consumption of electrolysis is the major part of the energy consumption in a chlor-alkali process. The power consumption of the membrane process has recently been greatly reduced by various improvements. The latest performance of Asahi Chemical s membrane process realized at a commercial plant and also in an industrial scale cell is shown in relation to current density in Figure 13 (82). 

The most important step in the membrane separation process is the selection of the membrane material, its properties, and its selectivity to the different components of the gas stream. The membrane area and operational power consumption are the major factors to be considered in the design of a membrane gas-separation system once the suitable membrane is selected. Capital investment is affected by the membrane area, while the operating cost could be altered by the power requirements. However, the individual (specific to the separation problem) design parameters (e.g., pressure ratio between feedstock and permeate stream, reflux fraction for a recycle permeator, and relative areas for a cascade system) must be considered during the design process. 

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. 

The current efficiency of acid/base generation and the purity of the acid and base made with bipolar membranes drops off as concentrations increase, because Donnan exclusion diminishes with increasing solution concentrations. Further, the production rate is limited by the rate of diffusion of water into the bipolar membrane. Nevertheless, there are substantial advantages to the process. Since there are no gases evolved at the bipolar membranes, the energy associated with gas evolution is saved, and the power consumption is about half that of electrolytic cells. Compared to the electrodes used in conventional electrolytic cells, the bipolar membranes are inexpensive. Where dilute (e.g., 1 N) acids or bases are needed, bipolar membranes offer the prospect of low cost and minimum unwanted by-products. 

Accurate cost figures for processes early in development are impossible to project. However, it is possible to roughly estimate the power and capital requirements to assess viability. The power consumption is overwhelmingly due to tile cell current, which is near stoidtiometric. Cell voltage, as shown earlier, can be estimated with reasonable accuracy. Capital costs can be estimated by analogy with MCFC stacks, whose design these membrane cells will mimic. 

The membrane s benefit in the food industry was first realized when reverse osmosis membranes were developed for the purification of water, a process known as desalination. After this application, membranes were introduced in various conventional processes, such as concentration by UF instead of evaporation. Membranes allow the development of processes and entirely new products. Among the main benefits of using membranes in the food industry are the separation of molecules and microorganisms, the absence of thermal damage to the products and microorganisms, and low power consumption [5]. 

The viability of a membrane process for potable water production depends on the energy consumption. The power input reflects the pressure energy required to pump water molecules through a size/charge selective membrane and is expressed as SEC in kWh/m of product water. The foUowing relationships are used to calculate energy consumption ... 

Merkel et al. (2012) assessed such configuration considering an H2 membrane operating at 150 °C in which the retentate stream is treated in a low-temperature process in which high-purity CO2 is recovered by phase separation. Although the complete plant was not simulated, they estimated a reduction of both parasitic power consumptions and capital cost with respect to the benchmark process with CO2 separation by physical absorption. The optimal configuration proposed in Merkel et al. (2012) would also include a C02-selective polymeric membrane to recover the CO2 released with the vapors of the low-temperamre knockout drum. 

Chlor-Alkali Industry. ) The replacement of the mercury process in the chlor-alkali industry by a nonmercury process is considered to be urgent, for the elimination of mercury pollution and for energy conservation. The membrane process has been proven as an effective alternative to the mercury process because of its important advantages, such as 1) Freedom from mercury pollution. 2) Lower electric power consumption. 3) Small steam consumption. 

Calculate the membrane area and. power consumption necessary to produce 10 m /h of 95% Nj in a single-stage process using asymmetric poly(phenyIene oxide) membranes. TTie characteristics of the membrane and the process data are given in the table... 

Note that the energy consumption is related to the third power of the velocity, P = f v, (in fact to V- 7 5 j where f is a friction factor). The membrane area and power consumption are listed in table below for both the single-stage and the rwo-stage process. 

This example shows that both the membrane area required and the power consumption are lower for the two-stage process. On the other hand, the capital cost will be higher for the two-st e process. Funheimore. by increasing the cross-flow velocity from 1 to 3 m/s. the membrane area is reduced by more than a ctor of twa whereas the energy consumpdon increases by one order of magnimde. These data can be used to calculate the actual process costs, where power consumpdon and membraiK area are important parameters. 

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