Crystallization as a Separations Process - American Chemical Society

Chapter 9

Double Draw-Off Crystallizer Major Player in the Acid Rain Game? 1

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Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 8, 2015 | http://pubs.acs.org Publication Date: September 21, 1990 | doi: 10.1021/bk-1990-0438.ch009

Alan D. Randolph, S. Mukhopadhyay , B. C. Sutradhar, and Ross Kendall Department of Chemical Engineering, University of Arizona, Tucson, AZ 85721

A novel Double Draw-Off (DDO) crystallizer has been designed in order to improve the particle size distribution in the precipitation of CaSO • / H O from simulated Flue Gas Desulfurization (FGD) liquor. The effects of DDO ratio and residence time on the mean particle size were studied. Industrial conditions were maintained in all experiments as far as practical. Significant improvement in mean particle size was achieved. The performance of an actual industrial DDO crystallizer (DuPont) for gypsum crystallization was reported. 1

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Acid rain control is an important issue in the U.S. today. One of the major pollutants that has been linked to acid rain is sulfur dioxide. S 0 control measures (e.g. Clean Air Act Amendment of 1977) for single source power plants have been passed and are likely to be strengthened in the fiiture. Considerable technology has been developed for efficient and cost-effective S 0 removal. The rapid growth of coal-fired power plants has provided incentives for work in this area. Thus, a number of S 0 removal methods have been developed, including new space-saving dry processes as well as traditional wet scrubbing processes. However, the most widely-used method is still the wetscrubbing lime/limestone process. The lime/limestone wet scrubbing process involves the reaction of acidic S 0 in the flue gas with alkaline lime and/or limestone in the scrubbing liquor to form the solid products C a S 0 « l / 2 H 0 or CaS0 »2H 0 (gypsum). Sulfate can be the major product if forced oxidation is used. There is always some excess air present in the flue gas and, depending on its amount, different proportions of sulfite or sulfate are formed. Tlje solid precipitates of 2

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Current address: Chemistry Department, State University of New York at Buffalo, Buffalo, NY 14214 Current address: E. I. du Pont de Nemours and Company, Wilmington, DE 19714-6090

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0097-6156/90/0438-0115$06.00/0 © 1990 American Chemical Society

In Crystallization as a Separations Process; Myerson, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

CRYSTALLIZATION AS A SEPARATIONS PROCESS

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C a S 0 « l / 2 H 0 and/or CaS0 «2H 0 are filtered off as a waste product. The cost-effectiveness of this process is greatly influenced by the cost of the dewatering step which in turn is strongly influenced by the mean particle size and particle size distribution. The crystallization kinetics of calcium sulfite and calcium sulfate are such that the former is crystallized as a finer product and filters poorly. The sulfate form (gypsum) usually gives larger crystals that filter easier, but exhibits severe fouling (e.g. wall scaling) problems. Good dewatering properties not only ease solid liquid separation, but also decrease water loss, waste disposal load and reduce free moisture content of the filter cake. Research on particle size improvement of the precipitate from flue gas desulfiirization (FGD) liquor has been conducted at the University of Arizona for several years. Crystallization kinetics studies of both calcium sulfate and sulfite have been reported (I)(2). Etherton studied the growth and nucleation kinetics of gypsum crystallization from simulated stack gas liquor using a one-liter seeded mimnucleator with a Mixed Suspension Mixed Product Removal (MSMPR) configuration for the fines created by the retained parent seed. The effect of pH and chemical additives on crystallization kinetics of gypsum was measured. This early fundamental study has been the basis for later CSD studies. Thus, the CSD of gypsum from simulated F G D liquors was studied in a bench-scale Double-Draw-Off (DDO)-type crystallizer (2). They studied the effect of variables such as overflow/underflow ratio, agitator rpm, overall liquid phase retention time, pH, rate of make, additive (citric acid) and crystallizer configuration (DDO or MSMPR) on the product CSD. The results were simulated using Etherton's reported kinetics. Experimental and simulated results were in reasonable agreement. Chang and Brna also studied gypsum crystallization in a DDO configuration from a forced oxidation F G D liquor in pilot plant scale (4). Predicted increased particle size and improved dewatering properties were confirmed. Nucleation/growth rate kinetics and growth and nucleation modification by use of chemical additives on the crystallization of C a S 0 » l / 2 H 0 from simulated F G D liquors have been studied by Keough and Kelly, respectively (2)(5). An MSMPR crystallizer was used for all their studies. The effect of total dissolved solids composition and concentration on the crystallization kinetics of C a S 0 » l / 2 H 0 was studied by Alvarez-Dalama (6). The presence of high total dissolved solids from CI" solutions ( > 100K ppm) was found to increase nucleation whereas high total dissolved solids from SO4 " solutions were found to inhibit nucleation. Benson et al. (2) showed that high M g ion concentration also inhibits nucleation during the precipitation of calcium sulfite hemihydrate from simulated F G D liquors. However, the largest mass mean particle size reported thus far for the C a S 0 « l / 2 H 0 system is only about 32 microns. The objectives of the present work were to demonstrate a method of achieving improved calcium sulfite particle size (by use of the DDO configuration) while showing the industrial practicality of this crystallizer configuration. The particular industrial case


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RANDOLPH ETAL.

Double Draw-Off Crystallizer

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illustrated was the production of large gypsum crystals with minimal process fouling from an in-process stream of weak sulfuric acid. CSD Improvement; The DDO Crystallizer The CSD studies of C a S 0 « l / 2 H 0 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 (2)(4)(8)(2)(1Q). These studies demonstrated that the mean size from a continuous crystallizer can be significantly increased using the DDO configuration. Among the advantages of a DDO design are:


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Increased mean particle size. Less vessel fouling. High per-pass yields of solute.

The present study demonstrates significant particle size increases for the sulfite system using the DDO crystallizer. In addition, the industrial success of this configuration to improve gypsum size while reducing vessel fouling is reported. Experimental A novel laboratory DDO crystallizer has been designed in order to obtain an overflow containing a small amount of fines (Figure 1). (In fact, fines overflow is necessary for the DDO configuration to produce larger-size crystals). Also shown in Figure 1 is the MSMPR configuration and the characteristic form of the population density-size plot for each configuration. The configuration embodies a conical section attached to a cylindrical bottom. The bottom contains a draft tube and four baffles. Agitation is provided by an upflow marine propeller. The total volume of the crystallizer is 10 liters. The objective of using the conical upper part is to aid the settling of fines that must be removed in this configuration. The rpm of the propeller is adjusted to confine agitation mainly to the cylindrical section. The detailed experimental set-up is shown in Figure 2. Simulated flue gas (92% nitrogen and 8% S0 ) is passed through a sparger of sintered glass placed inside an absorption tank, where it comes into contact with the liquor from the crystallizer. Liquor continuously recirculates from the crystallizer through the adsorption tank. Lime is pumped in slurry form directly into the crystallizer. Precipitation of CaS0 » 1/2H 0 under normal oxidation conditions usually results in 10-15% of the sulfite hemihydrate being oxidized to sulfate hemihydrate, incorporated in the crystal product as a crystalline solid solution. In order to obtain a representative S 0 / S 0 crystal product, a sulfate concentration of 15,000 ppm was maintained in the feed liquor, thus simulating industrial conditions. 2

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M S M P R CRYSTALLIZER Qj,Feed log

Qp, Product


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DDO CRYSTALLIZER log Qj, Feed

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/Qo, overflow Q , underflow u

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Figure 1. Comparison of MSMPR and D D O Configurations. pH Meter/ Controller

^ o m Crystallizer Absorption Tank'

From ^ ,/Absorption * Tank Absorption Tank DDO Crystallizer *

Feed Liquor

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o D r a , n

Slaked Thiosorbic Lime

Figure 2. Schematic Diagram of Experimental Setup for F G D Process.


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RANDOLPH ETAL.

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Double Draw-Off Crystallizer

In process operations with high magnesium-containing lime (thiosorbic lime of Dravo Lime Company, U.S.A.), a high magnesium ion concentration develops. Therefore, in the present work a Mg** concentration of approximately 5,000 ppm was used in the feed liquor. The sulfate and magnesium ion concentration are maintained by using a solution of MgS0 »7H 0 and MgCl #6H 0 in the make-up feed tank. The present crystallizer set-up is equipped with a pH indicator-controller and a temperature indicator. The temperature in the crystallizer is controlled by heat supplied to the feed and liquor recirculated through the absorption tank. In our laboratory runs, recirculation of the liquor from the absorption tank to the crystallizer using a 3,600 rpm centrifugal pump adversely affected particle size. This problem was overcome by use of a peristaltic pump with liquor withdrawal from and return to the crystallizer as shown in Figure 3. Liquor is withdrawn through four fines-traps placed at the surface of the liquor and returned through four manifold ports. This arrangement plus use of the peristaltic pump minimized the amount of crystals contacted by high shear and/or high velocity mechanical parts, thus reducing secondary nucleation. The experimental DDO crystallizer was shown in Figures (2-3). A run was made for each set of conditions, e.g. DDO ratio, residence time and recycle ratio. The run was continued until a steady state CSD was obtained. (Size analysis was made using a PDI E L Z O N E 80 X Y Particle Counter). Steady state was ascertained by analyzing particle size in sample intervals of one hour. 4


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Results and Discussion The main operating parameters which were varied were residence time (based on feed rate) and DDO ratio. Table I indicates the range of operating conditions used for all the experiments performed. Table II summarizes the particle size obtained using these conditions. Table I. Operating Conditions Residence Time 32-80 min DDO ratio 9-21.4 Recycle Rate (through absorption tank) 2.9 £/min Temperature (crystallizer) 55 ± 1°C Stirrer R P M 210 ± 5 R P M Crystallizer p H 7.0 ± 0.2 Fines Cut Size 20-28/xm Rate of Make (ROM) 0.121 g CaS0 *Vai20 per liter per min. 3


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Table II. Mass Mean Particle Size Obtained for Different Residence Time and DDO Ratio (Mixed Product)


Residence Time, min 80 70 60 60 60 60 45 45 30

Mass Mean Size, Micron

DDO Ratio

53.2 59.85 81.6 74.0 72.6 75.5 71.2 59.8 55.0

9 9 21.4 16 12.8 11.3 11 9 9

Both residence time and DDO ratio have significant effects on the particle size. Figure 4 shows the effect of DDO ratio on the mass mean size of the mixed product from the crystallizer for an overall residence time of 60 min. Similar behavior was observed for other residence times (not shown on plot). It was found that the particle size increased with an increase in DDO ratio, as expected. However, high DDO ratios in the laboratory size crystallizer lead to solids accumulation in the cylindrical section of the crystallizer and in plugging of the inlet and outlet ports. Figures 5-8 show strange behavior of the last (largest) five population density points. These points indicate that the population density not only doesn't decrease with size, but actually increases at a high DDO ratio. This behavior, highlighted by the dashed lines in the population density plot, might be qualitatively explained with conventional population balance mechanics (e.g. size-dependent growth rate) wherein the growth rate for crystals above about 60/xm is severely retarded. A necessary condition for positive-sloped semi-log population density plots to occur would be:

Crystallization as a Separations Process - American Chemical Society

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