Continuous crystallizers

Since zeolites are metastable crystallization products tliey are subject to Ostwald s mle which states tliat metastable phases are initially foniied and gradually transfonii into tlie tlieniiodynaniically most stable product. The least stable zeolitic phase (tliat witli tlie lowest framework density) is tlierefore foniied first and consumed with furtlier syntliesis time at tlie expense of a more stable phase due to a continuous crystallization/redissolution equilibrium. 

Spain, Tmbia nea Oviedo 1972 spray chamber as primary roaster, plate reactor as secondary stage continuous electrolysis of filtered electrolyte, continuous crystallization 2,000 112... 

Continuous vacuum pans have been successhiUy developed for raw sugar crystallization, and are widely appHed in South Africa, AustraHa, South America, and the United States. Continuous crystallizers, developed for beet sugar manufacture, are being adapted for use in cane sugar factories. 

Several features of secondary nucleation make it more important than primary nucleation in industrial crystallizers. First, continuous crystallizers and seeded batch crystallizers have crystals in the magma that can participate in secondary nucleation mechanisms. Second, the requirements for the mechanisms of secondary nucleation to be operative are fulfilled easily in most industrial crystallizers. Finally, low supersaturation can support secondary nucleation but not primary nucleation, and most crystallizers are operated in a low supersaturation regime that improves yield and enhances product purity and crystal morphology. 

Although evidence exists for both mechanisms of growth rate dispersion, separate mathematical models were developed for incorporating the two mechanisms into descriptions of crystal populations random growth rate fluctuations (36) and growth rate distributions (33,40). Both mechanisms can be included in a population balance to show the relative effects of the two mechanisms on crystal size distributions from batch and continuous crystallizers (41). 

The dominant crystal size, is most often used as a representation of the product size, because it represents the size about which most of the mass in the distribution is clustered. If the mass density function defined in equation 33 is plotted for a set of hypothetical data as shown in Figure 10, it would typically be observed to have a maximum at the dominant crystal size. In other words, the dominant crystal size is that characteristic crystal dimension at which drajdL = 0. Also shown in Figure 10 is the theoretical result obtained when the mass density is determined for a perfectiy mixed, continuous crystallizer within which invariant crystal growth occurs. That is, mass density is found for such systems to foUow a relationship of the form m = aL exp —bL where a and b are system-dependent parameters. 

Mass Balance Constraints. Erom the schematic diagram of a continuous crystallizer shown ia Eigure 11, the foUowiag mass balance on solute can be constmcted ... 

Preferential Removal of Crystals. Crystal size distributions produced ia a perfectiy mixed continuous crystallizer are highly constraiaed the form of the CSD ia such systems is determined entirely by the residence time distribution of a perfectly mixed crystallizer. Greater flexibiUty can be obtained through iatroduction of selective removal devices that alter the residence time distribution of materials flowing from the crystallizer. The... 

The reaction mixture is filtered. The soHds containing K MnO are leached, filtered, and the filtrate composition adjusted for electrolysis. The soHds are gangue. The Cams Chemical Co. electrolyzes a solution containing 120—150 g/L KOH and 50—60 g/L K MnO. The cells are bipolar (68). The anode side is monel and the cathode mild steel. The cathode consists of small protmsions from the bipolar unit. The base of the cathode is coated with a corrosion-resistant plastic such that the ratio of active cathode area to anode area is about 1 to 140. Cells operate at 1.2—1.4 kA. Anode and cathode current densities are about 85—100 A/m and 13—15 kA/m, respectively. The small cathode areas and large anode areas are used to minimize the reduction of permanganate at the cathode (69). Potassium permanganate is continuously crystallized from cell Hquors. The caustic mother Hquors are evaporated and returned to the cell feed preparation system. 

Production of potassium permanganate in the CIS is beheved to be from potassium manganate. Electrolysis of potassium manganate in a continuous-flow electrolytic cell with turbulent electrolyte flow and continuous crystallization has been reported (72). 

For large-scale continuous crystal production, several alternate types of crystallizer vessel are available (Figure 3.3). 

Given expressions for the crystallization kinetics and solubility of the system, the population balance (equation 2.4) can, in principle, be solved to predict the performance of both batch and of continuous crystallizers, at either steady- or unsteady-state... 

In addition to induction time measurements, several other methods have been proposed for determination of bulk crystallization kinetics since they are often considered appropriate for design purposes, either growth and nucleation separately or simultaneously, from both batch and continuous crystallization. Additionally, Mullin (2001) also describes methods for single crystal growth rate determination. 

Tailoring of the particle size of the crystals from industrial crystallizers is of significant importance for both product quality and downstream processing performance. The scientific design and operation of industrial crystallizers depends on a combination of thermodynamics - which determines whether crystals will form, particle formation kinetics - which determines how fast particle size distributions develop, and residence time distribution, which determines the capacity of the equipment used. Each of these aspects has been presented in Chapters 2, 3, 5 and 6. This chapter will show how they can be combined for application to the design and performance prediction of both batch and continuous crystallization. 

It has been shown that an increase in crystallizer residence time, or decrease in feed concentration, reduces the working level of supersaturation. This decrease in supersaturation results in a decrease in both nucleation and crystal growth. This in turn leads to a decrease in crystal surface area. By mass balance, this then causes an increase in the working solute concentration and hence an increase in the working level of supersaturation and so on. There is thus a complex feedback loop within a continuous crystallizer, illustrated in Figure 7.11. 

Continuous crystallizers find widespread application for the production of bulk chemicals such as fertilizers, fibre intermediates, salts and sugars. Production rates are typically 100 kTe/yr, or more with vessel capacities typically exceeding 20 m. Typical continuous crystallizers are depicted in Figure 9.18 and are described in more detail in Chapter 3. 

Figure 9.18 Continuous crystallizers, a) draft-tube and baffle (DTB), (b) single effect forced-circulation evaporative, (c) Oslo or Krystal type after Rohani, 2001)...

Abegg, C.F., Stevens, J.D. and Larson, M.A., 1968. Crystal size distribution in continuous crystallizer when growth rate is size-dependent. American Institmte oj Chemical Engineers Journal, 41, 188. 

Hostomsky, J., 1987. Particle size distribution of agglomerated crystal product from a continuous crystallizer. Collection of Czechoslovakian Chemical Communications, 52, 1186-1197. 

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

Lei, S., Shinnar, R. and Katz, S., 1971a. The stability and dynamic behaviour of a continuous crystallizer with a fines trap. American Institute of Chemical Engineers Journal, 17, 1459-1470. 

Sheikh, A.Y. and Jones, A.G., 1998. Optimal synthesis of stage wise continuous crystallization networks. American Institution of Chemical Engineers Journal, 44, 1637-1645. 

K. C. Lim, M. A. Hashim, B. Sen Gupta. Monte Carlo simulation of transient crystal size distribution in a continuous crystallizer using the ASL model. Cryst Res Technol 33 249, 1998. 

So far the structure of pure metals has been discussed with reference to bulk characteristics and continuous crystals. However, corrosion is essentially a surface phenomenon and it is necessary to consider how the structure and defects already described interact with free surfaces. At this stage it is convenient to consider only a film-free metal surface, although of course in most corrosion phenomena the presence of surface films is of the utmost importance. Furthermore, it is at free surfaces that the hard sphere model of metals... 

It should therefore not be surprising that for relatively small-scale operations involving solids handling within the fine and intermediate chemicals industry, batch operation is preferred. Similarly, continuous processes that involve precipitation or crystallization, a common unit operation in fine chemicals, are rare. Small-scale examples are known, for instance, a continuous crystallization process was used by Bristol-Myres Squibb in order to improve dissolution rates and bioavailability of the product [12]. The above does indicate that not all process or parts thereof are suited for conversion from B2C, given the current technology. 

The anodic dissolution of metals on surfaces without defects occurs in the half-crystal positions. Similarly to nucleation, the dissolution of metals involves the formation of empty nuclei (atomic vacancies). Screw dislocations have the same significance. Dissolution often leads to the formation of continuous crystal faces with lower Miller indices on the metal. This process, termed facetting, forms the basis of metallographic etching. 

The choice of crystallizer for a given separation will depend on the method used to bring about supersaturation. Batch and continuous crystallizers can be used. Continuous crystallizers are generally preferred, but special circumstances often dictate the use of batch operation, as will be discussed further in Chapter 14. The methods used to bring about supersaturation can be classified as ... 

A well-mixed continuous crystallizer is to be used to separate potassium sulfate from an aqueous solution by cooling crystallization. The solubility of potassium sulfate can be represented by the expression ... 

From Table 14.2, it can be concluded that both batch and continuous crystallization have their relative advantages and... 

Randolph, A.D., Larson, M.A. (1988) Theory qfparticulateprocesses, analysis and techniques of continuous crystallization, Academic Press, 2nd edn. 

Chevron (2) A process for separating/i-xylene from its isomers by continuous crystallization, using liquid carbon dioxide in direct contact with the xylene as the refrigerant. Developed by the Chevron Research Company in 1966. 

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