Rejects composition

The PRO and BRO unit recovery was 75% and 50%, respectively, and the overall recovery was 87.5% with the BRO feed (PRO reject) composition as TDS = 1300 ppm, hardness = 825 ppm, 504 = 360 ppm, and sihca=16ppm. The BRO unit was a one-stage array with three pressure vessels (3 0) in parallel and three elements per pressure vessel (9 elements total). The RO elements were Hydranautics ESPAl (40 cm dia.). 

The water recovery was increased to 95% using a triple membrane process a NF unit was added to purify the BRO reject stream, as shown in Figure 3.41 [69]. The PRO, BRO and NF unit recovery was 75%, 60% and 50%, respectively, resulting in the overall water recovery of 95%. The BRO feed (PRO reject) composition was TDS = 6650 ppm. 

SEM images of surface of (a) virgin and (b)Ti02-dip-coatedTFC-SR (thin film composite selective rejection) composite RO membrane (Fluid Systems Company). Membrane material PVA/polyaryl sulfone/ polyester. (Reprinted from Madaeni and Ghaemi, 2007 with permission from Elsevier.)... 

This equation relates yp, the permeate composition, to x , the reject composition, and the ideal separation factor a is defined as... 

The pressure selected for use are p , = 190 cm Hg and p = 19 cm Hg. Again, assuming the complete-mixing model, calculate the permeate composition, the reject composition, and the area. 

This equation relates the permeate composition y to the reject composition x at a point along the path. It is similar to Eq. (13.4-5) for complete mixing. Hwang and Kammermeyer (HI) give a computer program for the solution of the above system of differential equations by numerical methods. 

To continue, consider the expression for the dimensionless C-value or permeate-reject composition distribution function as derived in Chapter 3,... 

The overriding consideration is that of convenience in other words, what simplifies or eases the modus operandi for the calculations. And, as is further explained and demonstrated for analytical calculations, it is by far preferable to start calculations at the feedstream location, assuming that the feedstream composition, and preferably the reject composition, are identical. In this way, the overall material balance is automatically satisfied, even for an integral number of stages or cells in each section, which, not so incidentally, is a fundamental feature for the analytical calculation. 

Interestingly, as shown in Example 3.1 at least, the calculated permeate flux V" does not vary appreciably, even as the parameter V/F is changed. Nor do the resulting permeate and reject compositions change appreciably. That is, at least in this particular example, there are only minimal changes in composition. 

More rigorously speaking, if the feedstream composition is to be made identical to the reject composition for stage + 1 = w -l- 1, then there is a relationship between L /V and L/V that involves the composition of product streams B and D. That is to say, if L/V and are determined by a specification and calculation on the rectifying section for integral number of stages, then this dictates a constraint between L/V and Xg. At the same time, the overall material balance is automatically satisfied. 

Both the lower limits (y,), and (x,), correspond to the permeate and reject compositions from a bubble-point type calculation of the feedstream at... 

Note The feed and reject compositions can be assumed to be equal. The separation factor can be defined as the ratio of the composition of / to / in the permeate, divided by the ratio of i to in the reject. In terms of mole or mass fractions for a binary system, where is that for the feed-reject and y, is that for the permeate, as per Chapter 2, it follows that... 

Let us now consider a few examples for the use of this simple representation. A grand composite curve is shown in Fig. 14.2. The distillation column reboiler and condenser duties are shown separately and are matched against it. Neither of the distillation columns in Fig. 14.2 fits. The column in Fig. 14.2a is clearly across the pinch. The distillation column in Fig. 14.26 does not fit, despite the fact that both reboiler and condenser temperatures are above the pinch. Strictly speaking, it is not appropriately placed, and yet some energy can be saved. By contrast, the distillation shown in Fig. 14.3a fits. The reboiler duty can be supplied by the hot utility. The condenser duty must be integrated with the rest of the process. Another example is shown in Fig. 14.36. This distillation also fits. The reboiler duty must be supplied by integration with the process. Part of the condenser duty must be integrated, but the remainder of the condenser duty can be rejected to the cold utility. 

Fig. 7. (a) Impurity elements are rejected into the Hquid between the dendritic solidification fronts, (b) Corresponding impurity concentration profiles. Cq, weld metal composition k, impurity partitioning coefficient in the Hquid maximum impurity soHd solubiHty eutectic composition at grain... 

Interfdci l Composite Membra.nes, A method of making asymmetric membranes involving interfacial polymerization was developed in the 1960s. This technique was used to produce reverse osmosis membranes with dramatically improved salt rejections and water fluxes compared to those prepared by the Loeb-Sourirajan process (28). In the interfacial polymerization method, an aqueous solution of a reactive prepolymer, such as polyamine, is first deposited in the pores of a microporous support membrane, typically a polysulfone ultrafUtration membrane. The amine-loaded support is then immersed in a water-immiscible solvent solution containing a reactant, for example, a diacid chloride in hexane. The amine and acid chloride then react at the interface of the two solutions to form a densely cross-linked, extremely thin membrane layer. This preparation method is shown schematically in Figure 15. The first membrane made was based on polyethylenimine cross-linked with toluene-2,4-diisocyanate (28). The process was later refined at FilmTec Corporation (29,30) and at UOP (31) in the United States, and at Nitto (32) in Japan. 

The first reverse osmosis modules made from cellulose diacetate had a salt rejection of approximately 97—98%. This was enough to produce potable water (ie, water containing less than 500 ppm salt) from brackish water sources, but was not enough to desalinate seawater efficiently. In the 1970s, interfacial composite membranes with salt rejections greater than 99.5% were developed, making seawater desalination possible (29,30) a number of large plants are in operation worldwide. 

Table 1. Rejections by Aromatic Polyamide Composite Membrane ...

The suitabiHty and economics of a distillation separation depend on such factors as favorable vapor—Hquid equiHbria, feed composition, number of components to be separated, product purity requirements, the absolute pressure of the distillation, heat sensitivity, corrosivity, and continuous vs batch requirements. Distillation is somewhat energy-inefficient because in the usual case heat added at the base of the column is largely rejected overhead to an ambient sink. However, the source of energy for distillations is often low pressure steam which characteristically is in long supply and thus relatively inexpensive. Also, schemes have been devised for lowering the energy requirements of distillation and are described in many pubHcations (87). 

When acetylene is recovered, absorption—desorption towers are used. In the first tower, acetylene is absorbed in acetone, dimethylformarnide, or methylpyroUidinone (66,67). In the second tower, absorbed ethylene and ethane are rejected. In the third tower, acetylene is desorbed. Since acetylene decomposition can result at certain conditions of temperature, pressure, and composition, for safety reasons, the design of this unit is critical. The handling of pure acetylene streams requires specific design considerations such as the use of flame arrestors. 

The next stage in the zone-refining process is to move the furnace slowly and steadily to the right. The left-hand end of the bar will then cool and refreeze but with the equilibrium composition /cCq (Fig. 4.4c). As the furnace continues to move to the right the freezing solid, because it contains much less impurity than the liquid, rejects the surplus impurity into the liquid zone. This has the effect of inereasing the impurity concentration in the zone, which in turn then increases the impurity concentration in the next layer of freshly frozen solid, and so on (Fig. 4.4d). Eventually the concentrations ramp themselves up to the situation shown in Fig. 4.4(e). Flere, the solid ahead of the zone has exactly the same composition as the newly frozen solid behind the zone. This means that we have a steady state where as much impurity is removed from the... 

When a metal is cast, heat is conducted out of it through the walls of the mould. The mould walls are the coldest part of the system, so solidification starts there. In the Al-Si casting alloy, for example, primary (Al) crystals form on the mould wall and grow inwards. Their composition differs from that of the liquid it is purer, and contains less silicon. This means that silicon is rejected at the surface of the growing crystals, and the liquid grows richer in silicon that is why the liquid composition moves along the liquidus line. 

The limitations of SIMS - some inherent in secondary ion formation, some because of the physics of ion beams, and some because of the nature of sputtering - have been mentioned in Sect. 3.1. Sputtering produces predominantly neutral atoms for most of the elements in the periodic table the typical secondary ion yield is between 10 and 10 . This leads to a serious sensitivity limitation when extremely small volumes must be probed, or when high lateral and depth resolution analyses are needed. Another problem arises because the secondary ion yield can vary by many orders of magnitude as a function of surface contamination and matrix composition this hampers quantification. Quantification can also be hampered by interferences from molecules, molecular fragments, and isotopes of other elements with the same mass as the analyte. Very high mass-resolution can reject such interferences but only at the expense of detection sensitivity. 

In a permeation experiment, an HERO module with a membrane area of 200 m is used to remove a nickel salt from an electroplating wastewater. TTie feed to the module has a flowrate of 5 x IQ— m /s, a nickel-salt composition of 4,(X)0 ppm and an osmotic pressure of 2.5 atm. The average pressure difference across the membrane is 28 atm. The permeate is collected at atmospheric pressure. The results of the experiment indicate that the water recovery is 80% while the solute rejection is 95%. Evaluate the transport parameters Ay and (D2u/KS). 

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