Brine flow rate

Although the MCr/MD units introduce a thermal energy requirement, the water-recovery factor is increased up to 92.8% for the IS5 without a relevant increase of the cost if waste thermal energy is already available for the process at the same time brine flow rate is significantly reduced while the fresh water flow rate is increased (Table 12.2) [10]. 

Cooling water/chiUed brine flow rate ... 

F. Current Efficiency Equation for Membrane Cells. The chlorine and caustic efficiency expressions derived from the material balance for the various pertinent species across the electrolyzer are given by Eqs. (49) to (53). These equations need further simplification to eliminate the F/I term and the brine flow rate terms, especially the depleted brine flow rate q, because of the practical difficulties associated with its measurement. 

The ratio of the feed brine flow rate to the depleted brine flow rate can be determined from the various material balances described earlier. Thus, ANa can be expressed in terms of ACl, ACb and AOH, employing Eqs. (25), (40), (46) and (54) as ... 

Thus, Eqs. (81) and (88) represent the chlorine and caustic efficiency expressions derived rigorously from the appropriate material balances while accounting for the BCLs. Equations (81) and (88) can be simplified, by expressing the depleted brine flow rate in terms of caustic current efficiency as ... 

Equation (98) can be further simplified. The feed brine flow rate is almost equal to the depleted brine flow rate, since the water loss in the cell is about 0.5 kg min. Hence, p = q. Thus, can be rewritten as ... 

Keating [119] has estimated the brine flow over a 28-month time span to be about 200,000 kg m of membrane, at a brine flow rate of 200-250 kg day" and at a current density of 5 kA m. If the impurity concentration is 1 ppm, then the amount of impurities that are lodged in the membrane can be as much as 200 g. This is 50% of the total membrane weight of 400 gm . If the total weight of the impurities in the membrane is to be restricted to a number of grams, the impurity levels must be in the ppb range. 

TABLE 7.9. Reduction of Brine Flow Rate by Deposition of Magnesium and Calcium Impurities... 

The filter medium is usually graded by size. Three or four layers of different screen cuts, with the finest material on top, are common. Sand is often placed above one or two layers of gravel or pebbles. The top layer is the deepest as well as the finest. An upflow filter then has a solids capacity of20-30kg m , compared with the 5-15 kg obtained in downflow. The brine flow rate must be kept below about 0.3 m min . The fluidization limit, which will depend on the size and density of the filter medium and the properties of the brine, is an absolute hmit which must not be exceeded in an attempt to overcome upsets or to compensate for filter downtime. Some form of assurance that excessive flow will not occur should be part of the design. 

Cell room feed brine temperature Cell room feed brine flow rate Cell room feed caustic temperature Cell room feed caustic flow rate Electrolyzer feed brine temperatures Electrolyzer feed brine flow rates Cell room depleted brine temperature Electrolyzer depleted brine temperatures Electrolyzer feed caustic temperatures... 

The decrease in salt concentration Ac is determined by the current /, the brine flow rate M, and the electrochemical equivalent f. 

Factor 2. Decreasing brine flow rate to a cell increases the conversion of sodium chloride to sodium hydroxide and raises the hydroxide concentration in the catholyte because of reduced overflow from the cell. The decreased flow rate of brine through the diaphragm allows increased migration of hydroxide ions into the anolyte. These factors decrease cell efficiency. 

Factor 3. The condition of the diaphragm is extremely important. Nonuniformity in the diaphragm results in high flow rates of brine through thin or loosely compacted areas and low flow rates through thick or compacted areas. In the areas where there is a low brine flow rate, back migration of hydroxide is increased. 

Typical cell rooms are shown in Figures 42 (bipolar cells) and 43 (monopolar cells). A cell in normal operation requires little attention. The critical requirement is that the brine flow rate is sufficient to maintain an anolyte level above the cathode. 

A change in porosity may necessitate a change in brine flow rate. If the increase in porosity is severe, the ceil may be replaced or doped with an asbestos slurry or inorganic salts. 

A cell operated with the anolyte level at the maximum value and the lowest catholyte level is called a sleeper. To gain additionai diaphragm life after a cell has entered the sleeper position, the brine flow rate must be decreased below normal. This is not normally a recommended practice because current efficiencies of these cells are usually low. 

The chlorine header should be maintained at positive pressure to permit detection and correction of any chlorine piping leaks. The hydrc en header is also maintained at a positive pressure to avoid pulling air into the hydrogen, creating a potentially explosive mixture. The brine header pressure should be maintained to give the desired caustic concentration in the cell liquor. Normal practice is to adjust individual brine feed valves so that each cell receives the correct brine flow rate. 

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