Feed water composition

Characteristics of this membrane include 30 to 40% sodium chloride rejection, 85 to 90% magnesium sulfate rejection, 98% sucrose rejection, and 99% raffinose rejection. Water flux is 15 to 25 gfd at 35 to 50 psi, depending on feed-water composition. The effect of increasing salinity on salt rejection and water flux is similar to the behavior observed for NF-40 membrane as was illustrated in Figures 5.8 and 5.9. 

Once the feed water source has been determined, analysis of the feed water composition is necessary before a treatment system can be designed. Feed water constituents that must be analysed prior to designing a RO/NF membrane system as per ASTM Designation D4195-88 Standard Guide for Water Analysis for Reverse-Osmosis Applications are discussed in Chapter 6. Typical water treatment methods are summarised in Table 2.2. 

Reddy et al. (2008) cross-linked sodium alginate (SA)- and chitosan (CS)-blended membranes, pretreated with calcium chloride, with maleic anhydride for the separation of a 1,4-dioxane water mixture at 30°C by PV. The membrane performance exhibited a reduction in selectivity and an improvement in flux due to increased swelling with increasing feed water compositions. It was claimed by Reddy et al. that the membrane has a good potential for breaking the aqueous azeotrope 1,4-dioxane. 

Feed Water Composition (mol%) Aqueous Reflux (mol/min) Organic Reflux (mol/min) Reboiler Duty (KW) Entrainer Makeup (mol/min) Aqueous Draw (mol/min) Bottom Product (mol/min)... 

After deciding the inventory control strategy, there are three variables left that can be used in some composition control strategy. The three candidate variables for Inventory Strategy 1 are OR flow, aqueous reflux flow, and the reboiler duty. The three candidate variables for Inventory Strategy 2 are entrainer makeup, aqueous reflux flow, and the reboiler duty. The control objective is to hold the bottom and the top aqueous product specifications at base-case condition under +10% feed flow and +10% feed water composition changes. 

Figure 9.11 CSlQAwith +10% changes in the feed water composition.

The final objective of the alternative control structures is to maintain the bottom and top products specifications in spite of the load disturbances. Figures 9.15 and 9.16 compare the dynamic responses of these four control structures with +10% and -10% feed water composition changes, respectively. It is obvious from these two figures that CS2QA is the best control structure to reject feed water composition disturbances. Both bottom and top product compositions are quickly returned back to specifications much faster than when using the other three alternative control structures. 

Control stmcture CSIAO dynamically departs the furthest for both bottom and top product compositions from their specifications for the +10% feed water composition change. Control structure CSIQA dynamically departs the furthest for the -10% feed water composition change. In terms of the final steady-state value, control structure CS IQO gives the largest departure in bottom product purity (see Fig. 9.16). The CS IQO control structure keeps the aqueous reflux flowrate fixed during the dynamic runs. The ability to adjust the aqueous... 

Figure 9.15 Comparison under +10% feed water composition change.

From the dynamic responses of CS2QA (Fig. 9.14), another important observation can be made. With disturbances like +10% changes in the feed water composition, the aqueous reflux flowrate is adjusted upward or downward in order to maintain about the same overall water composition into the column. However, the reboiler duty actually returns back almost... 

Figures 9.18 and 9.19 show the closed-loop dynamic responses for CS3 and CS4 for +10% feed water composition changes. Notice that the dynamic responses are all quite satisfactory with all variables settled out at new steady-state values even faster than CS2QA (comparing to Fig. 9.14). The dynamic responses of the important bottom and top product compositions when using CS3 or CS4 are compared with results using CS2QA in Figures 9.20 and 9.21 for 4-10% and -10% changes in the feed water composition. Notice hrst that the scales of Figures 9.20 and 9.21 are much smaller than those in previous...

An optimum overall control strategy is also proposed for this column system to hold both bottom and top product specifications in spite of +10% feed rate and +10% feed water composition load disturbances. Several alternative control structures are compared using dynamic simulation. The proposed overall control strategy is very simple, requiring only one tray temperature control loop in the column. This simple overall control strategy can easily be implemented in industry. 

The proposed system consisted of a reverse osmosis step followed by a membrane distillation one. Seawater was treated with chemical agents and then fed to the RO unit after this first step a fraction of the RO permeate was sent to the MD module. As feed water composition the standard seawater composition at which As(III) and As(V) were added, was considered. 

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