Fractional water recovery

The treatment so far has heen based on a particular feed concentration, Q , in the reverse osmosis cell (Figure 6.3.28 (a)). As time progresses, water from the feed solution will be removed as permeate therefore the feed concentration of species i, e.g. NaCl, will increase. If we require the process to yield a particular concentration of salt in the permeated water, then the salt rejection required of the membrane, / i,reqd> will have to increase. Further, since the osmotic pressure of the feed solution increases with time, either the solvent flux will go down with time or the driving pressure difference, AP, has to go up. To these factors, one has to add the complication of concentration polarization. To illustrate the effect of increasing feed salt concentration with time, we will ignore first the effect of any concentration polarization and then focus on the consequence of different values of fractional water recovery, re. For the reverse osmosis cell shown in Figure 6.3.28(a), it is defined as... 

C,7, -I- Cjfe)/2) this, however, will vary with the fractional water recovery, re, defined by (6.3.172a) ... 

These results show how, as the fractional water recovery changes, the required salt rejection also changes considerably. 

Calculate the values of Pj.reqd for a seawater feed containing 35 000 ppm salt for the following values of re 0, 0.25, 0.3 and 0.5. The required C p corresponds to 500 ppm salt Comment on the requirement for pressure as the fractional water recovery is increased. 

Figure 7.2.3. (a) Details of a spiral-wound module. (After Ohya and Taniguchi (1975).) (b) Channels and flow configurations in an unwrapped spiral-wound module, (c) How mesh wires in the brine side spacer screen disturb the brine flow and create new boundary layers (schematically), (d) Fractional water recovery re as a function of normalized channel length L for various parametric values ofy, indicating the feed osmotic pressure level with respect to APf. 

If we define the fractional water recovery as re (definition (6.3.172b)), then, from the solute balance relation (7.2.33), valid for a membrane having a very high solute rejection. 

We wiU briefly illustrate graphically the productivity trend in a spiral-wound module. Figure 7.2.3(d) shows a plot of the behavioral trend exhibited by equation (7.2.43). The objective is to predict the fractional water recovery re as a function of the nondimensional membrane channel length for various parametric values of (hC,sj, y /APy) = 7 for a given value of the ratio ku/Aj APy F ). Figure 7.2.3(d) does not provide exact values corresponding to equation (7.2.43), but it provides the trends as illustrated by Ohya and Taniguchi (1975). 

The only unknown here is the fractional water recovery re, which has to be determined by trial and error. Now... 

In the above examples, the fractional water recovery is around 0.15. If one increases APy the fractional water recovery will increase, and so will the energy cost via the cost of pumping. On the other hand, usually a higher APyis used with a higher feed salt concentration therefore water recovery may not increase. To increase the water recovery, a number of spiral-wound modules are connected in series (as shown in Figure 7.2.4(a)) inside the pressure vessel. There is a brine seal between the module and the pressure vessel so that the brine is forced to go through the channels of the module. The permeate tubes are connected in series. Concentrated brine from one module enters the next module as feed and so on. The fractional water recovery is ultimately limited by the difference between the concentrated feed pressure and the osmotic pressure of the concentrate and the level of acceptable flux. 

Consider the spiral-wound module for reverse osmosis desalination described in Example 7.2.3. Determine the fractional water recovery if the feed brine has 10 000 ppm salt. All other conditions are as in the Example 7.2.3. What will be the fractional recovery if the feed brine has 20 000 ppm salt ... 

Consider reverse osmosis desalination in a spiral-wound module for a feed having negligible osmotic pressure. If the fractional water recovery, re, is such that the osmotic pressure of the concentration from the reverse osmosis process still has a negligible osmotic pressure vis-a-vis the liquid pressure, derive the result (7.2.44), i.e. 

The hydrocarbon mixture at the furnace outlet is quenched rapidly in the transfer line exchangers (2) (TLE or SLE), generating high-pressure steam. In liquid crackers, cracked gas flows to a primary fractionator (3) after direct quench with oil, where fuel oil is separated from gasoline and lighter components, and then to a quench water tower (4) for water recovery (to be used as dilution steam) and heavy gasoline production (end-point control). 

The pressure history of the secondary alkaline flood reflects the formation of a secondary oil bank behind the immiscible phase oil bank. This secondary oil bank results in an overall recovery which is above that obtained by secondary waterflooding or by secondary caustic flooding with an univalent ion of high electrolyte concentration. The concentration history of the fractional water production during the secondary calcium hydroxide flood represents the total consumption of the hydroxyl ion. This consumption curve is made up of consumption due to adsorption of the silica surfaces and consumption due to the in situ chemical reaction which forms the more oil-soluble, surface-active salt, calcium oleate. 

Pour the resulting dark reddish-brown liquid into 500 ml. of water to which 17 ml. of saturated sodium bisulphite solution has been added (the latter to remove the excess of bromine). Steam distil the resulting mixture (Fig. II, 41,1) , collect the first portion of the distillate, which contains a little unchanged nitrobenzene, separately. Collect about 4 litres of distillate. Filter the yellow crystalline solid at the pump, and press well to remove the adhering liquid. The resulting crude m-bromonitrobenzene, m.p. 51-52°, weighs 110 g. If required pure, distil under reduced pressure (Fig. II, 19, 1) and collect the fraction of b.p. 117-118°/9 mm. it then melts at 56° and the recovery is about 85 per cent. 

There are two methods available for aroma recovery. In one method, a portion of the water is stripped from the juice prior to concentration and fractionally distilled to recover a concentrated aqueous essence solution. Apple juice requires 10% water removal, peach 40%, and Concord grape 25—30% to remove volatile flavor as an essence. Fractional distillation affords an aqueous essence flavor solution of 100—200-fold strength, which means the essence is 100 to 200 times more concentrated in flavor than the starting juice. A second method of essence recovery is to condensate the volatiles from the last effect of the evaporator they are enriched in volatile flavor components (18). 

---