Composition, feed solutions

Effects of Impurities nd Solvent. The presence of impurities usually decreases the growth rates of crystalline materials, and problems associated with the production of crystals smaller than desired are commonly attributed to contamination of feed solutions. Strict protocols should be followed in operating units upstream from a crystallizer to minimize the possibiUty of such occurrences. Equally important is monitoring the composition of recycle streams so as to detect possible accumulation of impurities. Furthermore, crystalliza tion kinetics used in scaleup should be obtained from experiments on solutions as similar as possible to those expected in the full-scale process. 

Feed Composition. Feed composition has a substantial effect on the economics of a distillation. Distillations tend to become uneconomical as the feed becomes dilute. There are two types of dilute feed cases, one in which the valuable recovered component is a low boiler and the second when it is a high boiler. When the recovered component is the low boiler, the absolute distillate rate is low but the reflux ratio and the number of plates is high. An example is the recovery of methanol from a dilute solution in water. When the valuable recovered component is a high boiler, the distillate rate, the reflux relative to the high boiler, and the number of plates all are high. An example for this case is the recovery of acetic acid from a dilute solution in water. For the general case of dilute feeds, alternative recovery methods are usually more economical than distillation. 

Elution Chromatography The components of the mobile phase supphed to the cohimn ter feed introduction have less affinity for the stationary phase than any of the feed solutes. Under trace conditions, the feed solutes travel through the cohimn as bands or zones at different velocities that depend only on the composition of the mobile phase and the operating temperature and that exit from the cohimn at different times. 

The copper(II) transport rate increases, as a rule, as Cu + initial concentration in the feed solution increases. The increase of the caiiier s concentration from 10 to 30 vol.% results in a decrease of both metal fluxes and in an increase of Cu transport selectivity. The increase of TOA concentration in the liquid membrane up to 0.1 M leads to a reduction of the copper(II) flux, and the platinum(IV) flux increases at > 0.2 M. Composition of the strip solution (HCl, H,SO, HNO, HCIO, H,0)does not exert significant influence on the transport of extracted components through the liquid membranes at electrodialysis. 

A common starting point is that the process engineer is given a brief from which to determine a crystallization plant design viz. some specification of the product and process (e.g. mean particle size, production rate) and characteristics of the feed solution (e.g. composition, temperature etc.). Figure 9.1. 

Metal hydroxides (e.g., Fe, Mn, Al) can also be a problem (Rauten-bach and Albrecht, Membrane Processes, Wiley, New York, 1989). A chemical analysis of the feed solution composition along with consideration of solubility products allows one to determine the significance of precipitation. Solubility products can be affected by temperature, pH, and ionic strength. Seasonal temperature variations must be considered. Concentrations of silica need to be < 120 mg/L in the feed. 

Ion exchange (IX) is a very useful technique for the concentration, the purification and the separation of chemically similar metallic elements present in an aqueous solution. In its most popular form of application, the metal-bearing aqueous solution is passed through a bed of solid organic resin in a particulate form wherein the sorption of the metal ions on the resin takes place by ion-exchange reactions. The pregnant resin is washed free of the entrapped feed solution and then brought into contact with an eluant of suitable composition and volume so that the resin releases the metal ions back to the eluant. The ratio of the volume of the feed and that of the eluant determines the extent of concentration that can be achieved. Purification and separation are achievable if the resin is selective or specific with respect to the metal ions of interest in comparison to impurity ions. 

Dense membranes are used for pervaporation, as for reverse osmosis, and the process can be described by a solution-diffusion model. That is, in an ideal case there is equilibrium at the membrane interfaces and diffusional transport of components through the bulk of the membrane. The activity of a component on the feed side of the membrane is proportional to the composition of that component in the feed solution. 

The initial requirement in the development of a solvent extraction process for the recovery or separation of metals from an aqueous solution is knowledge of the solution composition, pH, temperature, and flow rate. Both pH and temperature can be adjusted, within certain economic limits, before feeding to the solvent extraction circuit, but only in a few cases can the leaching or dissolution conditions be dictated by the extraction process. Consequently, no serious development work on the extraction process can be carried out before the leaching conditions or the type of feed solution are established. 

The nature of the feed composition can be a major determining factor as to whether crud will form in the subsequent extractive operations [33,34]. The presence and concentration of certain cations, such as Fe, Si, Ca, Mg, or Al, with sufiicient shear in the mixing process can produce stable cruds [32,34]. Solids must be absent from most solvent extraction circuits, and clarification is usually aimed at achieving about 10 ppm of solids. One of the major causes of crud is the lack of good clarification of the feed solution, with the result that solids get through to the solvent extraction circuit. The presence of... 

The development conditions that are feasible for at-source processes are somewhat different from those of the centrally located facility regarding the freedom in feed materials and end products. This is especially characteristic for at-source processes, where the feed solution is dependent on optimal working conditions in the main process and therefore the composition is relatively constant and well defined. In addition, the product solution must have an accurate specification in order not to disturb the main production. So in the development work, a good knowledge of the main operation is necessary. 

Albany International Research Co. has developed an advanced hollow fiber composite reverse osmosis membrane and module under the name of Quantro II . This composite membrane is comprised of a porous hollow fiber substrate on which has been deposited a rejection barrier capable of fluxes of commercial importance at high rejection of dissolved salts at elevated temperatures. Resistance to active chlorine has been demonstrated. Proprietary processes have been developed for spinning of the fiber, establishment of the rejection barrier and processing of the fiber to prepare modules of commercial size. Prototype modules are currently in field trials against brackish and seawater feed solutions. Applications under consideration for this membrane include brackish and seawater desalination as well as selected industrial concentration processes. 

A typical test loop includes a pump capable of pressure development to 1000 psig and sufficient valving and piping to permit multistation installation of fiber samples. Feed solutions are delivered from reservoirs which are thermostated and isolated in order to maintain constancy of temperature and composition. The facility is so established that both permeate and concentrate are returned continuously to the reservoir. 

Figure 6.3 shows the time course of ethanol production as a function of feed composition (Table 6.3). The best feed solution was feed D which permitted the bed to run for eight hours without quenching and resulted in a higher ethanol yield (0.46) than feed A (0.35) which contained only glucose. Potassium and magnesium ions are known to stimulate fermentation (Jones and Greenfield, 1984 Maynard, 1993). Feed C caused the bed to quench after only two hours because, under anaerobic conditions, the yeast extract supplied was unable to be assimilated sufficiently quickly and the remainder was therefore... 

Table 6.3 The composition of glucose-based feed solutions used for anaerobic ethanol production (Hayes, 1998).

Fig. 23. Swelling ratios of NIPA gel and volume fluxes across the composite membrane at various temperatures. Pressure difference between permeate and feed solutions in the permeation experiments is 10 [kg cm-2]...

A simplified flow scheme for a brackish water reverse osmosis plant is shown in Figure 5.24. In this example, it is assumed that the brackish water is heavily contaminated with suspended solids, so flocculation followed by a sand filter and a cartridge filter is used to remove particulates. The pH of the feed solution might be adjusted, followed by chlorination to sterilize the water to prevent bacterial growth on the membranes and addition of an anti-sealant to inhibit precipitation of multivalent salts on the membrane. Finally, if chlorine-sensitive interfacial composite membranes are used, sodium sulfite is added to remove excess chlorine before the water contacts the membrane. Generally, more pretreatment is required in plants using hollow fiber modules than in plants using spiral-wound modules. This is one reason why hollow fiber modules have been displaced by spiral-wound systems for most brackish water installations. 

The total costs of the electrodialytic water dissociation with bipolar membranes are the sum of fixed charges associated with the amortization of the plant investment costs and of the operating costs which include energy and maintenance costs and all pre- and post-treatment procedures. The total costs are a function of the membrane properties, of the feed-solution composition, the required acid and base concentrations, and several process and equipment design parameters such as stack construction and operating current density. 

The simulator used was a DISMOL, described previously by Batistella and Maciel (2). All explanations of the equations used, the solution methods, and the routine of solution are described in Batistella and Maciel (5). DISMOL is a simulator that permits changes in feed composition, feed temperaturethe evaporation rate, as well as feed flow rate. The effective rate of surface evaporation is obtained from the kinetic theory of gases. The liquid film thickness is obtained by mass balance and geometry of the evaporator. The temperature in the liquid obeys the Fourier-Kirchhoff equation. The solution of the velocity profile requires knowledge of the viscosity and the liquid film thickness over the evaporator. The solution for the temperature and the concentration profiles requires knowledge of the velocity profiles, which determine the convective heat and mass fluxes. 

Since a SAM is a densely packed system where cooperative phenomena may occur, the overall strength of the next-neighbor interactions is difficult to foresee. For this reason, it is not possible to ensure that a feeding solution made of two, or more, thiols in any well-defined molar ratio will produce mixed monolayers with the same composition. The experimental parameters and conditions may be changed to a certain extent to tune the composition of the resulting monolayer. For instance, the solvent may be chosen to favor a polar thiol over an apolar one.40 43 44... 

Specification of the separation. A separation is specified by defining column feed flow rate and composition, overhead solute concentration (alternatively, solute recovery), and the concentration of solute (if any) in the lean solvent. If the purpose of absorption is to generate a specific solution, as in acid manufacture, the solution concentration completes the separation specification. For all other purposes, one specifying variable (e.g., rich solvent concentration or solvent flow rate) remains to be specified and is usually set by optimization as outlined below. 

The distribution coefficient (Kd) is dependent upon temperature, metal composition, salt composition, and solute concentration. The salt-to-metal ratio (s/m) is dependent upon the weights of the salt and metal feed. The fraction of equilibrium (F) is dependent upon the time and degree of mixing, and the side reaction term (3) is dependent upon the amount and kind of salt and metal insoluble impurities present in the system. 

A typical composition of feed solution and the fractional distribution of the feed solution components into the various exit streams are shown in Table I. The feed solution is usually the product of a Cleanex batch solvent extraction ( T0), a process... 

TABLE I. TYPICAL COMPOSITION OF FEED SOLUTIONS AND DISTRIBUTION OF COMPONENTS TO EXIT SOLUTIONS... 

Feed Adjustment. The tantalum-lined evaporator used to collect the actinide product solutions from a series of double oxalate precipitation runs also serves to adjust the composite product to a feed solution for a series of oxide production runs. Excess acid is removed by boiling the solution slowly to near dryness. The temperature is held at greater than 119°C for 5 h or more during boiling to ensure the destruction of all oxalates. After approximate dryness has been reached, the evaporator is cooled, 0.01 M HNO3 is added to dilute the actinides to less than 10 g/L (usually to 10-15 L, total volume) and a sample is taken to determine the acidity (typically 0.05-0.10 M) and to verify that the actinides are still in solution. The adjustment is completed by the addition of acid and evaporation to give an actinide concentration of about 10 g/L and an acid concentration of 0.20-0.35 M at the final volume. 

---