Fines, removal

Crystallizers of the draft tube baffle type that employ fines removal and destruction devices can also be analyzed by the techniques described previously, as shown in Perry (1973) and by Larson (1978). 

The weight size fraction from a crystallizer of the DTB type for particles up to size X is 

The calculated size distribution for various Lp sizes at constant 

With control techniques such as fines destruction, the particle size distribution within a crystallizer body may be varied through relatively wide ranges by changing the velocity behind the baffle and hence, the diameter of the particle withdrawn. The quantity of liquor removed with the particle separated and its residence time within the crystallizer body is another important variable. Experience has shown that such systems, when pushed beyond their capacity, can produce cyclic crystal size behavior due to homogeneous nucleation that occurs as the supersaturation rises beyond the metastable limit. 

Numerous attempts have been made by operators and designers to stabilize such systems the normal technique for doing this is by an appropriate selection of the maximum particle size separated by the baffle Lp) and the flow through the fines destruction system. A pioneering paper by Randolph et al. (1973) developed the mathematical basis for instability in systems of this type, and predicted instability in such systems as a function of the fines dissolving parameter and the nucleation sensitivity parameter. Experimental verification of these concepts was demonstrated by Randolph and Beckman (1977) for a KCl crystallizer. 

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Fine powders Fines removal Fingernail polish Fining... 

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As an idealization of the classified-fines removal operation, assume that two streams are withdrawn from the crystallizer, one corresponding to the product stream and the other a fines removal stream.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... 

Fig. 14. Simplified schematic diagram of classified-fines removal.

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). 

In batch classification, the removal of fines (particles less than any arbitrary size) can be correlated by treating as a second-order reaction K = (F/Q)[l/x(x — F)], where K = rate constant, F = fines removed in time 0, and x = original concentration of fines. 

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. 

For a given set of assumptions it is possible to calculate the characteristic curves for the product from the ciystaUizer when it is operated at various levels of fines removal as characterized by Lj. This has been done for an ammonium sulfate crystalhzer in Fig. 18-63. Also shown in that figure is the actual size distribution obtained. In calculating theoretic size distributions in accordance with the Eq. (18-41), it is... 

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

The use of the internal baffle permits operation of the ciystaUizer at a slurry consistency other than that naturally obtained by the cooling of the feed from the initial temperature to the final mother-liquor temperature. The baffle also permits fines removal and destruction. 

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. 

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. 

With fines removal the system growth rate is given by... 

Figure 3-4. Flow diagram of an Exxon flexicoking unit (1) reactor, (2) scrubber, (3) heater, (4) gasifier, (5) coke fines removal, (6) HgS removal.

In this application of the log-normal plot, note that the "n chanical" separation of "fines" has created anew particle distribution with 62 = 2 p. Even the value of 02 differs from that of the major particle distribution. In the "fines" fraction, it appears that the largest pcirticle does not exceed about 5 p. Needless to say, lamps prepared from this phosphor were inferior in brightness. Armed with this information, one could then recommend that the method of "fines" removal be changed. 

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... 

Based on predicted weathering and erosion rates of the region, we estimate the profile to be several million years old. Because the soil has developed in situ, the topmost grains have reacted with water for the greatest extent of time. With depth, the total "lifetime" of the particles as soil decreases. This implies that the topmost quartz surfaces should be "reactively mature" (all fines removed, deep grown-together etch pits) and the bottom-most quartz surfaces should be "reactively young" (plentiful fines, fresh surfaces). ... 

Of the various mechanical properties of a formed catalyst containing zeolite, attrition resistance is probably the most critical. This is particularly the case for FCC catalysts because of the impact on the addihon rate of fresh catalyst, particulate emissions of fines and overall catalyst flow in the reactor and regenerator. Most attrition methods are a relative determination by means of air jet attrition with samples in the 10 to 180 xm size range. For example the ASTM D5757 method attrites a humidified sample of powder with three high velocity jets of humidified air. The fines are continuously removed from the attrition zone by elucidation into a fines collection assembly. The relative attrition index is calculated from the elutriated fines removed at a specific time interval. 

Thin Layer Chromatography Procedures. Silica gel G (Suppelco, Redi-coat, 5 X 20 cm X 0.25 mm) plates were activated by heating in an oven for 1 hour at 110°C. Toxic products were spotted onto duplicate plates at concentrations corresponding to their previously determined LD99 levels. The plates were developed to 14 cm with chloroform, methanol and (6N) ammonium hydroxide (90 9.5 0.5) or chloroform, methanol, and water (60 35 8). Plates were developed and visualized by spraying with 50% aqueous sulfuric acid and charring. Undeveloped plates were scraped, the silica gel fines removed and the extracts concentrated to a residue under a nitrogen gas stream. 

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. 

This paper presents the grade-efficiency curves of a 75 n flat bottom cyclone (RWB 1613) provided by the Amberger Kaolin Werke (AKW). It is tested for the ammonium sulfate-water system for both fines removal and product classification. Its results will be compared with the results for fines removal obtained when using an... 

In fines removal, both the cut size and the grade efficiency are difficult to assess because of the limited accuracy of the sieve analysis technique and the problems Involved in the determination of the solids concentration in the overflow. For a. 65 m cyclone, whilst using a 20 mm vortexfinder diameter, an apex diameter of 16 mm and a feed flow of 1.6 1/s, solids recovery is over 99 % This recovery corresponds to a cut size between 50 - 100 pm. Typical distributions of size by weight, for the feed flow as well as the overflow are shown in Figure 5 Results are summarised in Table 2. 

Table 2 Experimental results and conditions Fines removal...

Figure 55 Typical size distributions by volume, for the feed flow and the overflow, if the hydrocyclone is operated for fines removal. The ordinate value is defined by volume percentage divided by interval width.

However, the differences between an experiment whereby 1.4 1/s of the total fines flow of 1.7 1/s is dissolved and an experiment at the same residence time of 1.25 hrs without fines dissolution, are negligible. This is probably due to the small cut size which is attained with the present configuration. Increasing the cut size by using an insert in the annular zone, may be one method of increasing the effect of fines removal. 

Fines removal. The annular zone enables a sharp separation to be made because of the low solids concentration which prevails in the upper part of this zone. Furthermore, it is a simple separation technique and, as such, is very successful. As a result of the low cut size, its effect on the average size produced in the crystalliser is negligible. At a higher solids concentration a reasonable separation is also attained using the hydrocyclone. 

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). 

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