MSMPR continuous crystallizer

Figure 6.19 Experimental set-up continuous MSMPR reaction-crystallizer (Zauner and Jones, 2000a)...

Jones, A.G. and Mydlarz, J., 1990a. Continuous crystallization of potash alum. MSMPR kinetics. Canadian Journal of Chemical Engineering, 68, 250-259. 

The CSD studies of CaS03 l/2H20 mentioned earlier all used the MSMPR configuration. In low solids systems, e.g. sulfite from FGD, the DDO crystallizer configuration is useful to increase particle size (3)(4)(8)(9)(10). These studies demonstrated that the mean size from a continuous crystallizer can be significantly increased using the DDO configuration. 

Chemically pure reagents were used. Cadmium was added as its sulfate salt in concentrations of about 50 ppm. Lanthanides were added as nitrates. For the experiments with other metal ions so-called "black acid from a Nissan-H process was used. In this acid a large number of metal ions were present. To achieve calcium sulfate precipitation two solutions, one consisting of calcium phosphate in phosphoric acid and the other of a phosphoric acid/sulfuric acid mixture, were fed simultaneously in the 1 liter MSMPR crystallizer. The power input by the turbine stirrer was 1 kW/m. The solid content was about 10%. Each experiment was conducted for at least 8 residence times to obtain a steady state. During the experiments lic iid and solid samples were taken for analysis by ICP (Inductively Coupled Plasma spectrometry, based on atomic emission) and/or INAA (Instrumental Neutron Activation Analysis). The solid samples were washed with saturated gypsum solution (3x) and with acetone (3x), and subsequently dried at 30 C. The details of the continuous crystallization experiments are given in ref. [5]. 

Randolph and Larson showed that for a continuous crystallizer with a (perfectly) mixed suspension, (perfectly) mixed product removal (MSMPR) crystallizer. 

Estimate the working volume of a continuous MSMPR cooling crystallizer, operating at 15 °C, to recover hydrated iron (II) sulphate (FeS04 7H2O) from an aqueous solution, saturated at 40 fed at the rate of 10 m h k The required product should contain at least 90 per cent by mass of crystals larger than 200 pm. 

The study started with a batch crystallization experiment using seeded method. The purpose of this batch experiment was to deteimine the parameters needed for the subsequent experiment, i.e. the seeded continuous crystallization experiment using an MSMPR (mixed-suspension-mixed-product-removal) crystallizer. These parameters were levels of supersaturation, residence time, stirring rate, and concentration of additives, respectively. 

Figure 9.7 Schematic of a MSMPR, a continuous crystallizer. The suspension is fed with fresh feed the removal of product is isokinetic, the particle size distribution is that of the suspension. The supersaturation is typically generated by evaporation.

The concept of a continuous crystallizer is abstracted by the MSMPR, mixed suspension, and mixed product removal reactor (Figure 9.7). 

While the population balance [9] forms the basis for the design and control of crystal sizes and crystal size distributions in continuous crystallizers, there is hardly any direct application in practice. The main reason for this is the poor transferability of the measured (apparatus-specific) proportionality constant (from = k MG ) in the following equation, presented here for the simple MSMPR (10], to the large-scale plant that is to be planned ... 

The CSD of the product from a continuous crystallizer is determined by a direct relationship between nucleation and crystal growth rates and magma residence time distributions. Since a priori prediction of crystallization kinetics is not yet possible, however, experimentally determined and statistically correlated nucleation and growth rates are needed for the design and analysis of industrial crystallizers. Analysis of the CSD from continuous MSMPR (mixed-suspension, mixed-product-removal) crystallizers has proved to be a popular way of inferring such crystallization kinetics. This approach has been widely... 

Similarly, several authors have presented MSMPR methods for kinetics determination from continuous crystallizer operation (Chapter 3), which have become widely adopted. In an early study, Bransom etal. (1949) anticipated Randolph and Larson (1962) and derived a crystal population balance to analyse the CSD from the steady state continuous MSMPR crystallizer for growth and nucleation kinetics. Han (1968) proposed a method of kinetics determination from the moments of the CSD from a cascade of continuous crystallizers and assessed the effect of sample position. Timm and Larson (1968) suggested the use of the extra information present in transient response data to determine kinetics, followed by Sowul and Epstein (1981), Daudey and de Jong (1984) and Jager etal. (1991). Tavare (1986) applied the j-plane analysis to the precipitation of calcium oxalate, again assuming nucleation and growth only. 

It was shown in Chapter 3 that the ideal continuous MSMPR crystallizer could be analysed using the population balance approach coupled with mass balances and crystallization kinetics to yield equations describing crystallizer performance in terms of the crystal size distribution, solids hold up etc. These concepts will now developed further to yield methods for continuous crystallizer design. Firstly, however, it is useful to consider how crystallization kinetics and crystallizer performance interact. 

The CSD from the continuous MSMPR may thus be predicted by a combination of crystallization kinetics and crystallizer residence time (see Figure 3.5). This fact has been widely used in reverse as a means to determine crystallization kinetics - by analysis of the CSD from a well-mixed vessel of known mean residence time. Whether used for performance prediction or kinetics determination, these three quantities, (CSD, kinetics and residence time), are linked by the population balance. 

A pilot-scale continuous MSMPR crystallizer of 10 litre capacity is used to crystallize potash alum from aqueous solution, supersaturation. This is being achieved using a 15-min residence time, a 100-ml slurry sample was taken and the crystals contained in this sample subjected to a size analysis. The results of this analysis are given below... 

Evidence for secondary nucleation has came from the early continuous MSMPR studies. MSMPR crystallization kinetics are usually correlated with supersaturation using empirical expressions of the form... 

Sengupta, B. and Dutta, T.K., 1990. Effect of dispersions on CSD in continuous MSMPR crystallizers. Chemical Engineering and Technology, 13(6), 426-431. 

Wojcik, J. and Jones, A.G., 1997. Experimental investigation into dynamics and stability of continuous MSMPR agglomerative precipitation of CaC03 crystals. Transactions of the Intitution of Chemical Engineers, 75, 113-118. 

Growth and nucleation interact in a crystalliser in which both contribute to the final crystal size distribution (CSD) of the product. The importance of the population balance(37) is widely acknowledged. This is most easily appreciated by reference to the simple, idealised case of a mixed-suspension, mixed-product removal (MSMPR) crystalliser operated continuously in the steady state, where no crystals are present in the feed stream, all crystals are of the same shape, no crystals break down by attrition, and crystal growth rate is independent of crystal size. The crystal size distribution for steady state operation in terms of crystal size d and population density // (number of crystals per unit size per unit volume of the system), derived directly from the population balance over the system(37) is ... 

As far as the gypsum crystals are concerned, the analysis is identical to that for a seeded MSMPR (34). The information required is the growth rate and the mean residence time. For the hemihydrate, the analysis is that for a continuous seeded MSMPR dissolver (35), which parallels that for the crystallizer. The information needed is the dissolution rate and the mean residence time. 

Experimental apparatus and procedure. Figure 1 is a schematic diagram of the experimental apparatus. The crystallizer was a 1 liter stirred tank reactor made of acrylic resin and is considered to be a continuous MSMPR reactor. The reactor was 0.1m in diameter and the liquid height 0.14m. The impeller used was of the 6-blade turbine type and operated at 450 rpm to... 

Reactive crystallization experiments of magnesium hydroxide were conducted to clarify the characteristics of reactive crystallization kinetics by a continuous MSMPR crystallizer. 

However, the mixing of the dispersed and continuous phases is considered here, and it is possible to apply the same way of thinking for plural dispersed phases. Additionally, the newly defined mixedness can be applied to judge whether the assumption of MSMPR (mixed suspension mixed product removal) in the crystallization operation is established. 

MSMPR crystallizer A vessel operating in a continuous manner in which crystallization occurs and whose contents are perfectly mixed. As a result of perfect mixing, all variables descriptive of the mother liquor and crystals are constant throughout the vessel and are identical to corresponding variables in the product stream leaving the vessel. 

Consider a liquid feed stream containing reactant species A entering an MSMPR crystallizer at a volumetric flow rate F. A solid reactant B is also charged continuously at a molar flow rate W. The solid reactant B dissolves, and reacts with A in the liquid phase in the presence of an inert solvent S. The reaction rate can be generally given as a function of the concentrations of the reactant species, (c). An amount of product P in excess of the solubility limit precipitates out of the solution as a solid product. 

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