Crystallization material balances

Fig. 4. Drainage of salt crystals in a cylindrical screen pusher-discharge centrifuge (8), where the cake thickness is 3.3 cm, the centrifugal field = 320 U, and the crystals 14 wt % <250 p.m. ( ) Represents moisture in the discharge cake, and (° ) moisture in the cake by material balance with drainage flows line A...

WHITE, E. T. and RANDOLPH, A. D. AIChE Jl 33 (1984) 686-689. Graphical solution of the material balance constraint for MSMPR crystallizers. 

Figure 6 shows the size distributions for the samples taken from one of the runs, presented as the cumulative number oversize per ml of slurry. From the lateral shift of the size distributions, the growth rate can be determined. Figure 7 shows values of growth rate, G, supersaturation, s, and crystal content determined during the run. As a material balance check, the crystal contents were evaluated from direct measurements, from solution analyses and from the moments of the size distribution. The agreement was satisfactory. No evidence of size dependent growth or size dispersion was observed. 

A plant is to make 10,000 lb/hr of urea crystals from a solution that contains 75% dissolved salt. The material balance and operating conditions are shown on the sketch. Key crystallization data are given by Bennett (1981, p. 452) as... 

Figure 16.7. Material balancing of continuous stirred tank crystallizers (CSTC). (a) The single stage CSTC. (b) Multistage battery with overall residence time t = (lIQ) Si Ki-...

It is customary to carry out two, successive, batch crystallizations. The general pattern of the phase changes is indicated by a carbohydrate material-balance as shown in Scheme 1. 

SCHEME I.—Material Balance in the Crystallization of a -D-Glucose Monohydrate. (The numbers are mass units.)... 

For a steady-state crystallizer receiving solids-free feed and containing a well-mixed suspension of crystals experiencing neghgible breakage, a material-balance statement degenerates to a particle balance (the Randolph-Larson general-population balance) in turn, it simplifies to... 

There are three unknowns on the chart m, x, m2). We will assume that the solution leaving the crystallizer is saturated at 40 C Accordingly, the value of jc may be determined from the known solubility of KNO3 at that temperature, and the remaining two variables may be determined from material balances. From Figure 6.5-1, the solubility at 40 C is roughly 63 kg KNO3/IOO kg H O. The calculations follow. 

We are going to discuss a strategy of analysis of material balance problems that will enable you to understand, first, how similar these problems are, and second, how to solve them in the most expeditious manner. For some types of problems the method of approach is relatively simple and for others it is more complicated, but the important point is to regard problems in distillation, crystallization, evaporation, combustion, mixing, gas absorption, or drying not as being different from each other but as being related from the viewpoint of how to proceed to solve them. 

Figure 7-24 presents a unique resolution configuration. This configuration provides intrinsically balanced crystal growth rates, avoids optical contamination, and resolves the diasteromers in a simple, direct manner. The essential feature is to seed the crystallizer with the faster-growing diastereomer and leave the slower-growing diastereomer in the dissolver. The crystal growth rates are balanced automatically by the overall material balance. Optical contamination is not an issue either. 

The second step is to select an operating mode and to calculate the heat and material balances. For this example, an evaporative crystallizer will be used. Complete evaporation and crystallization of the feed will be assumed. 

Raw materials. A material balance is always needed to determine the amount of raw materials and their costs. The quality grade and the form of the raw material (powder, flake, crystal, or liquid) affect the cost. Quantity discounts on large contract amounts... 

The overall material balance for a binary system, wherein a pure solid component is crystallized by cooling or evaporation techniques, is derived and rearmnged in the following section to provide maximum solute yield and total suspended solids information. The approach described below is also useful for deriving material balance expressions for binary systems (hat forat hydrates and adducts. 

The fianl material balance computations for the case study are besed on a crystallizer temperature of 50°C nad are shown below ... 

An evaporative crystallizer operating under vacuum with suitable external heat Input will meet the material balance performance objectives. 

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