Feeds, salt concentration

The ratio of the mean solution concentration to the feed salt concentration is plotted in Figure 2 as a function of volume flux. Two extreme cases will be of interest. First, when qd/5 - , the right-hand side of Eq, (32) approaches unity. This means that for all values of r, the mean solution concentration becomes the feed salt concentration as the volume flux becomes infinite. The second case is when qd/Dg is very small. In this case Eq. (32) becomes indeterminant. Hence, L Hospltal s Theorem must be applied to find the limiting value. It gives two different... 

For a perfectly selective membrane the permeate salt concentration, cJe = 0 and R = 100%, and for a completely unselective membrane the permeate salt concentration is the same as the feed salt concentration, ck = cJo and R = 0%. The rejection coefficient increases with applied pressure as shown in Figure 2.9, because the water flux increases with pressure, but the salt flux does not. 

Figure 5.2 Effect of pressure, feed salt concentration and feed temperature on the properties of good quality seawater desalination membranes (SW-30) [14]...

The As rejection decreased by increasing the feed salt concentration and the valence of the added anion. This phenomenon can be attributed to the competition between arsenate and other anions (such as phosphate, silicaUte, carbonate commonly present in water) for binding sites on the pol)mer. A similar behavior has been also observed in the As removal by using ion-exchange resins containing ammonium groups (Berdal et al, 2000). 

Cj = feed salt concentration %SP = percent salt passage CFG = concentration of the feed-concentrate ... 

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... 

Consider Figure 6.3.28(a) and an initial volume of salt water of salt concentration Cm- Let the volume of salt water remaining after the process is over be Vfg let the corresponding salt concentration be Cug. Let the permeate salt concentration required be Cip. The salt rejection required of the membrane, Ri,reqd, may be defined with respect to the final feed salt concentration Coe (an alternative definition may be the average feed concentration. 

To meet the requirement of a particular salt concentration in the permeate, the quantity iJ reqd was defined by (6.3.173) based on the final salt concentration, Cue- Define the ili,reqd instead based on an average feed salt concentration,... 

Calculate the final feed salt concentration. Cue, in ppm, corresponding to the recoveries specified in (3) above. 

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. 

Electrodialysis. In electro dialysis (ED), the saline solution is placed between two membranes, one permeable to cations only and the other to anions only. A direct electrical current is passed across this system by means of two electrodes, causiag the cations ia the saline solution to move toward the cathode, and the anions to the anode. As shown ia Figure 15, the anions can only leave one compartment ia their travel to the anode, because a membrane separating them from the anode is permeable to them. Cations are both excluded from one compartment and concentrated ia the compartment toward the cathode. This reduces the salt concentration ia some compartments, and iacreases it ia others. Tens to hundreds of such compartments are stacked together ia practical ED plants, lea ding to the creation of alternating compartments of fresh and salt-concentrated water. ED is a continuous-flow process, where saline feed is continuously fed iato all compartments and the product water and concentrated brine flow out of alternate compartments. 

Process Flow The schematic in Fig. 22-56 may imply that the feed rates to the concentrate and diluate compartments are equal. If they are, and the diluate is essentially desalted, the concentrate would leave the process with twice the salt concentration of the feed. A higher ratio is usually desired, so the flow rates of feed for concentrate and feed for diluate can be independently controlled. Since sharply differing flow rates lead to pressure imbalances within the stack, the usual procedure is to recirculate the brine stream using a feed-and-bleed technique This is usually true for ED reversal plants. Some nonreversal plants use slow flow on the brine side avoiding the recirculating pumps.. Diluate production rates are often 10X brine-production rates. 

Membrane Characterization Membranes are always rated for flux and rejection. NaCl is always used as one measure of rejection, and for a veiy good RO membrane, it will be 99.7 percent or more. Nanofiltration membranes are also tested on a larger solute, commonly MgS04. Test results are veiy much a function of how the test is run, and membrane suppliers are usually specific on the test conditions. Salt concentration will be specified as some average of feed and exit concentration, but both are bulk values. Salt concentration at the membrane governs performance. Flux, pressure, membrane geome-tiy, and cross-flow velocity all influence polarization and the other variables shown in Fig. 22-63. 

Osmotic Pinch Ejfect Feed is pumped into the membrane train, and as it flows through the membrane array, sensible pressure is lost due to fric tion effects. Simultaneously, as water permeates, leaving salts behind, osmotic pressure increases. There is no known practical alternative to having the lowest pressure and the highest salt concentration occur simultaneously at the exit of the train, the point where AP — AH is minimized. This point is known as the osmotic pinch, and it is the point backward from which hydrauhe design takes place. A corollary factor is that the permeate produced at the pinch is of the lowest quality anywhere in the array. Commonly, this permeate is below the required quahty, so the usual prac tice is to design around average-permeate quality, not incremental quahty. A I MPa overpressure at the pinch is preferred, but the minimum brine pressure tolerable is 1.1 times H. 

Hydrophobic interaction chromatograph (HIC), while very attractive in principle, has proved difficult to scale up for processing. A recent series of articles explores some of the unique problems associated with process-scale HIC. Load sample preparation20 must be carefully examined to prevent protein aggregate formation in the presence of the relatively high salt concentrations used in this technique. Successful scale-up also requires the setting of wide specifications to accomodate routine variations in the feed.21 The effect of the salt concentration on capacity may be somewhat more... 

Mass spectrometers that use electrospray ionization (ESI) do not function well if the eluent contains low volatility salts. This is a major concern when an ion-exchange column is used as a first-dimension column and the salt concentration is used to modulate the retention in this column. In this case, another valve can be connected between the second-dimension column and the detector so that any salt from the second-dimension elution process that is either unretained or weakly retained can be diverted prior to feeding zones to the mass spectrometer. 

A method for simultaneous desulfurization and desalting of fossil fuels by mixing the aqueous biocatalytic solution with the feed under conditions for both processes to occur. Both inorganic salts, the originally present in the fuel and the produced from the conversion of the organic sulfur compounds are solubilized by the added water, resulting in an aqueous phase with a salt concentration greater than 0.5 wt%. The biocatalyst consists of Rhodococcus sp. ATCC 53968, its mutants or cell-free fractions. 

Here, single prime refers to the feed side (high pressure) and double prime refers to the product side (low pressure) and the lower case c s refer to the bulk concentrations. Even if the membrane is homogeneous, it is possible to have a variable diffu-sivlty, which may be a function of salt concentration. 

Pharmacia s Q- and S-Sepharose anion- and cation-exchange resins For the anion-exchange process it was found that two step changes, simultaneous in pH and salt concentration were necessary to carry out the anion-exchange separation. A 0.01 M sodium acetate buffer, pH 5.8, was used forthe starting state orfeed loading buffer. After the whey feed was loaded onto the column, one column volume of this... 

End of Pipe" Treatment - It is possible to use reverse osmosis and ultrafiltration to concentrate or "dewater" mixed effluent streams in order to reduce the hydraulic loading to down stream treatment processes. Typically, at least 90% of the feed volume can be purified and often returned to the process, with the salts concentrated in the remaining 10%. 

A 500-gallon water tank has a continuous feed and discharge rate of 10 gpm and 15 lb of NaCl are added to the tank in a batch method. The tank has uniform mixing to maintain a uniform salt concentration at all times. Estimate the amount of salt in the tank after 2 hours. 

A thermodynamic analysis of the energy requirements of desalting processes is presented, to clarify the conditions under which such calculations are valid. The effects of departure from isothermal operation, finite product recovery, differential as opposed to single-stage operation, and salt concentration in the feed are examined. A comparison shows that there is essentially no difference between the energy requirements for a distillation and a freezing process. The minimum heat consumption and maximum number of efFects for a multiple-effect evaporation plant are calculated. The above analysis leads to the conclusion that efficiencies in the range 10 to 20% will be very difficult to achieve. 

The salt flux through the membrane is given by the product of the permeate volume flux. /,. and the permeate salt concentration c,p. For dilute liquids the permeate volume flux is within 1 or 2% of the volume flux on the feed side of the membrane because the densities of the two solutions are almost equal. This means that, at steady state, the net salt flux at any point within the boundary layer must also be equal to the permeate salt flux Jvcip. In the boundary layer this net salt flux is also equal to the convective salt flux towards the membrane Jvc, minus the diffusive salt flux away from the membrane expressed by Fick s law (Didcildx). So, from simple mass balance, transport of salt at any point within the boundary layer can be described by the equation... 

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