Reverse osmosis flow rate

Reverse osmosis/electrodeionization (RO/EDI) plants are available in modular form to suit any desired input-output water quality and flow rate. A RO/EDI system should be capable of producing high-purity water of perhaps 5 to 20 xS/cm conductivity (0.2-0.05 MO/cm resistance). By providing a second EDI stack in series, it is possible to achieve even higher quality of up to 0.055 xS/cm conductivity (18.2 Mfl/cm resistance). 

Procedure Flavonoids are then further purified with 2 ml of methanolic HC1 (2 N), followed by centrifugation (2 min, 15 600 g), hydrolyzation of 150 il of suspension in an autoclave (15 min, 120 C). A reverse osmosis-Millipore UF Plus water purification system is used in high performance liquid chromatography (HPLC) with an autosampler. After injections of 5 pg of samples, the mobile phases flow at a rate of 1 ml/minute with isocratic elution in a column at 30 C. 

The thermodynamic approach does not make explicit the effects of concentration at the membrane. A good deal of the analysis of concentration polarisation given for ultrafiltration also applies to reverse osmosis. The control of the boundary layer is just as important. The main effects of concentration polarisation in this case are, however, a reduced value of solvent permeation rate as a result of an increased osmotic pressure at the membrane surface given in equation 8.37, and a decrease in solute rejection given in equation 8.38. In many applications it is usual to pretreat feeds in order to remove colloidal material before reverse osmosis. The components which must then be retained by reverse osmosis have higher diffusion coefficients than those encountered in ultrafiltration. Hence, the polarisation modulus given in equation 8.14 is lower, and the concentration of solutes at the membrane seldom results in the formation of a gel. For the case of turbulent flow the Dittus-Boelter correlation may be used, as was the case for ultrafiltration giving a polarisation modulus of ... 

The reverse osmosis membranes were tested in the standard experimental set-up (10). The experiments were carried out at three different pressures 17.4, 40.8 and 102 bars the corresponding sodium chloride concentrations were 3500 ppm, 5000 ppm and 29000 ppm. Before the reverse osmosis runs, membranes were thermally shrunk for 10 minutes in water and subsequently pressurized at 15-20% higher pressures than those used during the reverse osmosis experiments. A feed flow rate of 400 ml/mln was used giving a mass transfer coefficient k = 40 x 10 cm/s on the high pressure side of the membrane. 

Srinivasan and Tien (18) have made an analytical study on the mass-transfer characteristics of reverse osmosis in curved tubular membranes. The increase in mass-transfer due to secondary flow resulted in a substantial reduction in the wall concentration (the polarization modulus) for Np =100 and a/R=0.01 (see Figure 39). Further, the production capacity (permeation rate) was markedly increased (see Figure 40). 

In ultrafiltration and reverse osmosis, in which solutions are concentrated by allowing the solvent to permeate a semi-permeable membrane, the permeate flux (i.e. the flow of permeate or solvent per unit time, per unit membrane area) declines continuously during operation, although not at a constant rate. Probably the most important contribution to flux decline is the formation of a concentration polarisation layer. As solvent passes through the membrane, the solute molecules which are unable to pass through become concentrated next to the membrane surface. Consequently, the efficiency of separafion decreases as fhis layer of concentrated solution accumulates. The layer is established within the first few seconds of operation and is an inevitable consequence of the separation of solvent and solute. 

The detailed process design is familiar to students of chemical engineering, and includes specifying the source of the raw material water the equipment to be used, such as filtration, reverse osmosis, charcoal absorption, ozone treatment, ion exchanger, and pumps the processing conditions, such as flow rates and temperatures and the plant flow sheet. The detailed product design plan for this simplest of products includes the composition of this bottled water, with special attention to the concentrations of compounds such as sodium and carbon dioxide, suspended matter, and microbes, with special emphasis on the appearance and smell. 

Therefore, an effective water system is required. Nowadays, several techniques can be used to obtain water of high pharmaceutical quality. These include ionexchange treatment, reverse osmosis, distillation, electrodialysis, and ultrafiltration. However, there is no single optimum system for producing high-purity water, and selection of the final system is dependent on factors such as the quality of raw water, intent of its use, flow rate, and costs. In the pharmaceutical industry, the different water classes normally encountered are well water, potable water, purified water, and specially purified grades of water, such as water for injection (e.g., MilliQ water). 

It is the rate of separation rather than the efficiency of salt retention that is the primary practical issue in the development of reverse osmosis desalination. In addition to a variety of other factors, the rate of reverse osmotic flow depends on the excess pressure across the membrane. Therefore the problem of rapid flow is tied into the technology of developing membranes capable of withstanding high pressures. The osmotic pressure of sea water at 25 °C is about 25 atm. This means that no reverse osmosis will occur until the applied pressure exceeds this value. This corresponds to a water column about 840-ft high at this temperature. 

Reverse-Osmosis Experiments. All reverse-osmosis experiments were performed with continuous-flow cells. Each membrane was subjected to an initial pure water pressure of 2068 kPag (300 psig) for 2 h pure water was used as feed to minimize the compaction effect. The specifications of all the membranes in terms of the solute transport parameter [(Dam/ 6)Naci]> the pure water permeability constant (A), the separation, and the product rate (PR) are given in Table I. These were determined by Kimura-Sourirajan analysis (7) of experimental reverse-osmosis data with sodium chloride solution at a feed concentration of 0.06 m unless otherwise stated. All other reverse-osmosis experiments were carried out at laboratory temperature (23-25 °C), an operating pressure of 1724 kPag (250 psig), a feed concentration of 100 ppm, and a feed flow rate >400 cmVmin. The fraction solute separation (/) is defined as follows ... 

A conventional wastewater treatment system with an average flow rate of 160,000 gpd produces effluent suitable for NPDES discharge. Metal hydroxide sludges are dewatered in a 15 cu. ft filter press producing more than one half ton of filter cake per day. The filter cake is further dewatered in a 7 cu. ft, batch-type sludge dryer. Based upon recommendations by their consultant, the firm also uses the sludge dryer to dehydrate nickel strip solutions. Two reverse osmosis systems are used for partial nickel recovery. Trivalent chromium is recovered by drag-out control and evaporation. 

Direct flow filtration has certain Umitations. The flux (filtration flow rate per unit membrane area) decreases over time as the process continues because the filtering media is loaded with more contaminant particles, as illustrated in Figure 14.1. Moreover, when the concentration of the contaminant in the feed stream is high, the filtering media must be replaced very frequently, which can be economically impractical. Also when the contaminant matter to be separated is small in size, requiring ultrafiltration or reverse osmosis membranes with much smaller pores, then direct filtration is less feasible as the flux declines very rapidly over time, again requiring frequent filter replacement. 

FIGURE 41.10 Flow rate and conductivities obtained with the Toray reverse osmosis membrane. 

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