Crystallizers fines removal

For the filtration of very small quantities of crystals, the simple apparatus shown in Fig. 46 is often used. It consists of a fine glass rod (sometimes termed a filtration nail ) which is flattened at one end, the flattened surface being preferably roughened. It fits as shown into a small funnel which replaces F (Fig. 45). A circular piece of filter-paper is cut e-g.y with a clean sharp cork-borer) so as to fit completely and snugly over the flat end. After draining, the nail is raised and the filter-paper and crystals are removed with forceps and dried. 

Char-liquor advance is simply the removal of mother Hquor from the crystallizer without simultaneous removal of crystals. The primary objective of fines removal is preferential withdrawal from the crystallizer of crystals whose size is below some specified value. Such crystals may be redissolved and the resulting solution returned to the crystallizer. Classified-product removal is carried out to remove preferentially those crystals whose size is larger than some specified value. 

As an idealization of the classified-fines removal operation, assume that two streams are withdrawn from the crystallizer, one corresponding to the product stream and the other a fines removal stream. Such an arrangement is shown schematically in Figure 14. The flow rate of the clear solution in the product stream is designated and the flow rate of the clear solution in the fines removal stream is set as (R — 1) - Furthermore, assume that the device used to separate fines from larger crystals functions so that only crystals below an arbitrary size are in the fines removal stream and that all crystals below size have an equal probabiHty of being removed in the fines removal stream. Under these conditions, the crystal size distribution is characterized by two mean residence times, one for the fines and the other for crystals larger than These quantities are related by the equations... 

For systems following invariant growth the crystal population density in each size range decays exponentially with the inverse of the product of growth rate and residence time. For a continuous distribution, the population densities of the classified fines and the product crystals must be the same at size Accordingly, the population density for a crystallizer operating with classified-fines removal is given by... 

Figure 15 shows how the population density function changes with the addition of classified-fines removal. It is apparent from the figure that fines removal increases the dominant crystal size, but it also increases the spread of the distribution. 

Fig. 15. Population density function for product from crystallizer with classified-fines removal. Cut size Lp = 150 /tm R = 3.7.

Classified removal of course material also can be used, as shown in Figure 16. In a crystallizer equipped with idealized classified-product removal, crystals above some size ate removed at a rate Z times the removal rate expected for a perfecdy mixed crystallizer, and crystals smaller than are not removed at all. Larger crystals can be removed selectively through the use of an elutriation leg, hydrocyclones, or screens. Using the analysis of classified-fines removal systems as a guide, it can be shown that the crystal population density within the crystallizer magma is given by the equations... 

Although many commercial crystallizers operate with some form of selective crystal removal, such devices can be difficult to operate because of fouling of heat exchanger surfaces or blinding of screens. In addition, several investigations identify interactions between classified fines and course product removal as causes of cycling of a crystal size distribution (7). Often such behavior can be rninirnized or even eliminated by increasing the fines removal rate (63,64). 

Crystallizers with Fines Removal In Example 3, the product was from a forced-circulation crystallizer of the MSMPR type. In many cases, the product produced by such machines is too small for commercial use therefore, a separation baffle is added within the crystallizer to permit the removal of unwanted fine crystalline material from the magma, thereby controlling the population density in the machine so as to produce a coarser ciystal product. When this is done, the product sample plots on a graph of In n versus L as shown in hne P, Fig. 18-62. The line of steepest ope, line F, represents the particle-size distribution of the fine material, and samples which show this distribution can be taken from the liquid leaving the fines-separation baffle. The product crystals have a slope of lower value, and typically there should be little or no material present smaller than Lj, the size which the baffle is designed to separate. The effective nucleation rate for the product material is the intersection of the extension of line P to zero size. 

FIG. 18-62 Plot of Log N against L for a crystallizer with fines removal. 

An Oslo surface-cooled crystallizer is illustrated in Fig. 18-71. Supersaturation is developed in the circulated liquor by chilling in the cooler H. This supersaturated liquor is contacted with the suspension of ciystals in the suspension chamber at E. At the top of the suspension chamber a stream of mother hquor D can be removed to be used for fines removal and destruction. This feature can be added on either type of equipment. Fine ciystals withdrawn from the top of the suspension are destroyed, thereby reducing the overall number of ciys-tals in the system and increasing the particle size of the remaining product ciystals. 

One method for improving the size distribution still further is to employ fines destruction (Jones etal., 1984 Jones and Chianese, 1988). In this technique, a classified stream of suspension, i.e. containing only fine not coarse crystals, is removed from the crystallizer in an elutriation leg (Figure 7.6(a)). 

A fines removal system is installed on the crystallizer designed in the first example. Assuming that the cut size for the fines removal system is 50 im and the ratio of mean residence times for product and fines, rp/rp( = 7), is 10, calculate the mean product residence time now required to produce the same dominant size of 600 pm at the same production rate and suspension density. 

Fine-mesh screen printing, 9 221 Fine ore drums, 15 453 Fine particles, suspensions of, 22 54 Fine particulate matter (PM2.s), 1 799 Fine-pore wick structure, 13 232 Fine precipitated alumina hydroxides, 2 430 properties of commercial, 2 429t Fine quicklime, 15 27 Fines removal, in crystallization, 8 124 Fine structural properties, of polyester fibers, 20 5... 

Fines removal, the selective removal of small crystals from a well-mixed crystalliser, is generally used to remove excessive fines, thus increasing the average size. An increase in the fines removal rate immediately reduces the number of small crystals contained in the reactor. 

As a result of the selective removal of the largest crystals, the specific surface area tends to increase, which imposes a decrease in the crystal growth rate and eventually causes a decrease in the average crystal size. Therefore a product clcussification step should preferably be combined with fines removal. 

In this paper, three methods to transform the population balance into a set of ordinary differential equations will be discussed. Two of these methods were reported earlier in the crystallizer literature. However, these methods have limitations in their applicabilty to crystallizers with fines removal, product classification and size-dependent crystal growth, limitations in the choice of the elements of the process output vector y, t) that is used by the controller or result in high orders of the state space model which causes severe problems in the control system design. Therefore another approach is suggested. This approach is demonstrated and compared with the other methods in an example. 

The method of lines can handle size-dependent growth rates, fines removal and product classification and is not restricted in the choice of the elements of the output vector y (t). The population densities at the grid points are system states, thus moments, L, CV, population densities at the grid points and the number or mass of crystals in a size range can be elements of y (t). 

Example. Consider an evaporative, isothermal. Class II crystallizer with fines removal. It is assumed that the growth rate is size-independent and that there is no growth rate dispersion. Because we assume fines removal, the method of moments can not be applied. 

Industrial crystallizers are often equiped with an annular zone for fines removal. This fines removal system gives rise to a mass accumulation in the large annular zone (4), which affects the process dynamics at least under unsteady conditions. It is shown, that implementation of this fines destruction system into the simulation program has some implications for the algorithm used. The Influence of the mass accumulation in the annular zone on the process dynamics is also discussed. 

In this section the model for a continuous evaporative crystallizer is discussed. The crystallizer is of the draft tube baffled (DTB) type and is equiped with a fines removal system consisting of a large annular zone on the outside of the crystallizer (see Figure 1). In order to vary the dissolved fines flow without changing the cut-size of the fines removal system, the flow in the annular zone is kept constant and the flow in the dissolving system is varied by changing the recycle flow rate. The model assumptions are ... 

As discussed in the previous section and summarized in Table II, a drawback of the MCFT method is that the mass accumulation in the fines removal system cannot be simulated, therefore we examined whether this mass accumulation has a notlcable effect on the process dynamics. In the simulation the fines removal is simulated with a cut sizes of 150 ]m. The fines flow rate and the recycle flow rate Q were 1.25 and. 75 liter per second. The results are sho%m in Flgi e 7 It is clear that the mass accumulation has Indeed an effect on the process dynamics. Even on the mean size of the crystals a clear shift in the response is seen. It appears that the effect is strongly dependent on the value of the recycle flow rate (not shown). The conclusion from these results is that the effects of mass accumulation in the fines system are present, and can only be neglected at low cutsizes and low fines recycle rates. 

A warm solution of 5.57 g. (30 mmoles) of p-nitrobenzoyl chloride in 50 ml. of dry pyridine is cooled to 0° under stirring, and to the stirred suspension is added, in small portions over a period of 15 min., 0.08 g. (6 mmoles) of finely divided 2-deoxy-D-n o-hexose. Stirring is maintained at 0a for 1.5 hr., and the mixture is set aside in a refrigerator for 4 days. The excess p-nitrobenzoyl chloride is neutralized by the careful addition of 60 ml. of saturated, aqueous sodium hydrogen carbonate, and the mixture is added to 11. of ice-water. The precipitate is removed by filtration, washed with water, and dried in the open at room temperature and then in a desiccator over phosphorus pentaoxide. Most of the water of crystallization is removed by heating the material at 110°/0.1 mm. for 16 hr. The dry product is crystallized from nitromethane (decolorized) giving 3.46 g. (76%) of 2-deoxy-l, 3,4,6-tetra-0-p-nitrobenzoyl-D-n o-hexose, m.p. 201-202°, of satisfactory quality for the following conversion. Its specific rotation is +106° in chloroform. 

SYNTHESIS To a solution of 19.7 g 2,5-dimethoxy-4-ethylbenzaldehyde (see the recipe for 2C-E for its preparation) in 72 g glacial acetic acid there was added 6.5 g anhydrous ammonium acetate and 10.2gnitroethane. After heating for 1.75hon the steam bath, the reaction mixture was cooled in a wet ice bath, diluted with 10 mL H20, and seeded with a small crystal of product. The yellow crystals were removed by filtration (7.6 g wet with acetic acid) and another 2.25 g was obtained from the mother liquors with additional H20. The combined fractions were recrystallized from 25 mL boiling MeOH, to give 6.5 g fine yellow crystals of l-(2,5-dimethoxy-4-ethyl)-2-nitropropene, with a mp of67.5-68.5 °C. Anal. (C1(Hi N04) C,H,N. 

Crystallizers are made more flexible by the introduction of selective removal devices that alter the residence time distributions of materials flowing from the crystallizer. Three removal functions—clear-liquor advance, classified-fines removal, and classified-product removal— and their idealized removal devices will be used here to illustrate how design and operating variables can be manipulated to alter crystal size distributions. Idealized representations of the three classification devices are illustrated in Fig. 17. 

Clear-liquor advance from what is called a double draw-off crystallizer is simply the removal of mother liquor without simultaneous removal of crystals. The primary action in classified-fines removal is preferential withdrawal from the crystallizer of crystals of a size below some specified value this may be coupled with the dissolution of the crystals removed as fines and the return of the resulting solution to the crystallizer. Classified-product removal is carried out to remove preferentially those crystals of a size larger than some specified value. In the following discussion, the effects of each of these selective removal functions on crystal size distributions will be described in terms of the population density function n. Only the ideal solid-liquid classification devices will be examined. It is convenient in the analyses to define flow rates in terms of clear liquor. Necessarily, then, the population density function is defined on a clear-liquor basis. 

A simple method for implementation of classified-fines removal is to remove slurry from a settling zone in the crystallizer. The settling zone can be created by constructing a baffle that separates the zone from the well-mixed portion of the vessel—recall, for example, the draft-tube-baffle crystallizer described in Section V—or, in small-... 

FIGURE 18 Population density plot for product from crystallizer with idealized classified-fines removal. 

---