Ind. Eng. Chem. Res. 1990,29, 2389-2401
and even in controlling a trickle bed reactor.
2389
Subscripts
G = gas L = liquid a = either gas or liquid (G or L)
Nomenclature A = cross-sectional area of constriction, m2 Bo = bond number, [d?(pL - p ~ ) g ] / udimensionless , Ca = capillary number, p V / u , dimensionless d , = nominal particle size, m d , = hydraulic diameter of constricted passage, 4 A / P , m F = superficial mass flow rate, kg/(s m2) g = acceleration of gravity, m/s2 h = length of test section, m 1, = total length of model packed bed, m n, = number of passages in the width of the model P = wetted perimeter of constriction, m Q = volumetric flow rate Re = Reynolds number, p V D / p , dimensionless v = superficial velocity, m/s up = pulse velocity, m/s & = rate of increase of pulse frequency with Reynolds number of phase CY, Au/ARe,, s-l u = frequency of pulses, s-l p = viscosity, kg/(m s) p = density, kg/m3 u = surface tension, kg/s2 ud = standard deviation T = pulse travel time, s { = d L / @ G , dimensionless
Literature Cited Blok, J. R.; Drinkenburg, A. A. H. Hydrodynamic Properties of Pulses in Two-Phase Downflow Operated Packed Columns. Chem. Eng. J . 1982,25, 89-99. Blok, J. R.; Varkevisser, J.; Drinkenburg, A. A. H. Transition to Pulsing Flow, Holdup and Pressure Drop in Packed Columns with Cocurrent Gas-Liquid Downflow. Chem. Eng. Sei. 1983, 38, 687-99. Christensen, G.; McGovern, S.J.; Sundaresan, S.Cocurrent Downflow of Air and Water in a Two-Dimensional Packed Column. AIChE J . 1986, 32, 1677-82. Chou. T. S.: Worlev. F. L.. Jr.: Luss. D. Transition to Pulsed Flow in MixedLPhase Cocurrent Downflow through a Fixed Bed. Ind. Eng. Chem. Process Des. Deu. 1977, 16, 424-27. Melli, T. R.; de Santos, J. M.; Kolb, W. B.; Scriven, L. E. Cocurrent Downflow in Networks of Passages. Microscale Roots of Macroscale Flow Regimes. Ind. Eng. Chem. Res. 1990, preceding paper in this issue. Rao, V. G.; Drinkenburg, A. A. H. Pressure Drop and Hydrodynamic Properties of Pulses in Two-Phase Gas-liquid Downflow through Packed Beds. Can. J. Chem. Eng. 1983,61, 158-67. Stanley, W. D.; Dougherty, G. R.; Dougherty, R. Digital Signal Processin,g;Restom Publishing Company, Inc.: Reston, VA, 1984.
Receiued for review November 2, 1989 Revised manuscript received May 10, 1990 Accepted May 23, 1990
Investigation of Acidulation and Coating of Saudi Phosphate Rocks. 1. Batch Acidulation Said S. Elnashaie,* Tariq F. Al-Fariss, Salah M. Abdel Razik, and Hazem A. Ibrahim Phosphoric Acid Group (PAG),Chemical Engineering Department, College of Engineering, King Saud University, P.O. Box 800, Riyadh 11421, Saudi Arabia
A mathematical model is used for the simulation of the acidulation of phosphate rock using both sulfuric acid and sulfuric acid/phosphoric acid mixtures. T h e attack on the rock by the acid and crystallization of the dihydrate are taking place in the same reactor vessel. T h e model has been tested by using a batch laboratory-scale reactor in a wide range of conditions. The matching between the model and the experimental results is used to obtain the effective diffusivity for the system. The effect of different parameters and the speed of acidulation on the coating of the rock particles is also investigated. 1. Introduction Sulfuric acid acidulation of phosphate rocks is already an old process for phosphoric acid production, and a broad experimental knowledge has been accumulated in the plants and in the laboratories. However, modeling the phosphoric acid reactor, which is the main stage of the process, remains a difficult and uncertain task, and design still relies to a great extent on empirical principles. Several reactions take place simultaneously in this reactor, and the effective rates of these reactions are very sensitive to the solution composition, agitation, temperature, and rock characteristics. As a result, very few papers in the literature use the physical modeling approach. Gioia et al. (1977) put forward a model that considers the two main reactions involved in the reactor: apatite acidulation through the action of H+ ions; calcium sulfate
* Author t o whom correspondence should be addressed.
crystallization in the hemihydrate state. However, no comparison between the experimental results and the computed data was presented; thus, Gioia et al.’s model remains purely theoretical. Shakourzadeh et al. (1980) put forward a model for the dihydrate system. The authors included a comparison between the experimental results and the computed data. Shakourzadeh et a1.k model was designed to study the influence of phosphate rock impurities using a continuous laboratory-scale reactor. The investigation covered only a very narrow range of sulfate ion concentration. Shakourzadeh et al.’s model did not consider the effect of the calcium sulfate layer, which forms around the rock surface, on the diffusion of ions to and from the rock surface and subsequently did not investigate thoroughly the coating phenomenon, which has a critical effect on the rate of the reaction. In Gioia et al.’s model, a diffusion coefficient was assumed to be on the order of magnitude of 7 X lo4 m2.h-’.
os8a-5aa5/90/2629-a~a9~02.50/0 0 1990 American Chemical Society
2390 Ind. Eng. Chem. Res., Vol. 29, No. 12, 1990
In Shakourzadeh et al.3 model where the diffusion of hydrogen ions from solution to the rock particle surface is considered to be the controlling step, the value of the diffusion coefficient of the H+ ions was arbitrarily taken as 0.18 X lo-: m2*h-*,which was claimed to be adjusted later in order to make the model fit the experimental results. The purpose of the present paper is to develop a model and use it to study the kinetics of the acidulation of phosphate rocks and the effect of the process parameters on both the diffusion process and the coating phenomenon, using a series of batch experiments. The results of the model are compared with the experimental results. The tests were performed using, for the first time, Saudi phosphate ore in a wide range of conditions to evaluate the adjustable parameters, under batch operating conditions. 2. Chemistry of the Process The production of phosphoric acid is accomplished in industrial practice by using different processes (Becker, 1983, pp 35-59), the most important and applicable processes being the traditional dihydrate process (DH), the hemihydrate process (HH), and the hemidihydrate process (HDH). These are the most common processes used in industrial practice. The differences between the DH and HH processes are the strength of the produced phosphoric acid (2842% w/w P205for DH and 4042% w/w P205 for HH), the reaction temperature (70-82 "C for DH and 90-125 "C for HH), and finally, the form of calcium sulfate produced. In the dihydrate process, gypsum (CaSO4. 2H20),with large crystals and good filtration characteristics, is produced, while in the HH process, hemihydrate calcium sulfate (CaS04-0.5H20),with small crystal size and bad filtration characteristics, is produced. In the third process (HDH), the reaction takes place in the hemihydrate mode (90-125 "C and 40-5470 w/w P205),followed by filtration to separate the hemihydrate cake (CaS04-0.!5H20),and finally, the recrystallization of the hemihydrate cake is carried out to produce gypsum. The common factor among these processes is the formation of calcium sulfate, which causes the coating phenomenon during the reaction between the phosphate rock and sulfuric acid. Many chemical reactions take place in the reactor depending upon the ore complexity, but only two main reactions are considered here. The first is the fluoroapatite ore attacked by sulfuric acid and/or phosphoric acid/ sulfuric acid mixture according to
-
12Ca5(P04)3F+ 60H2S04+ 120H20 + Si02 36H3P04+ 60CaSO4-2H20+ H2SiF6+ 6HF + 2H20 (1) Notice that reaction 1 also includes the reaction for one of the main impurities, silica. The second reaction is between calcite and sulfuric acid: CaC03 + H2S04+ H 2 0
-
CaS04-2H20+ C 0 2 (2)
The detailed mechanism can be described by the following steps (Becker, 1983, p 69): 1. Ionization of H2S04,which is a very fast process described by the equation H2S04
-
2H+ + S O : -
(3)
2. Diffusion of H+ through the liquid toward the exposed surface of the rock particle (and through the coating film when coating takes place).
Figure 1. Microscopic picture for the coating of the rock particle with gypsum (magnification, 17.05X).
3. H+ ion attack of the phosphate rock particles according to the equation nH+ + Ca5(P04)3F+ CaC03 3H3P04+ C 0 2 + H 2 0 + H F + (n - 12)H+ + 6Ca2+
-
(4)
The H+ ions participating in this reaction "belong" to the sulfuric acid as well as to the phosphoric acid in the slurry. In the commercial reactors (dihydrate process), the liquid phase of the reaction slurry is composed of 30% P205and 1.6-1.8% H2S04(weight basis); i.e., there are about 25 times more H3P04than H2S04molecules. 4. Diffusion of products from the reaction sites to the bulk of the liquid. 5. Reaction between Ca2+and S042-: Ca2+ + S042-+ XH20
-
CaSO4.XH2O
(5)
The form of calcium sulfate that crystallizes from the solution depends on the reaction temperature and the liquid-phase composition (H3P04and H$04) (Slack, 1967, pp 95-96). In fact, steps 3 and 5 represent some simplification from the actual situation that involves the formation of calcium dihydrogen phosphate (CDHP); however, the inclusion of this intermediate compound requires certain extra information that is being gathered a t the present time in this laboratory through the acidulation of the phosphate ore using phosphoric acid (clean phosphoric acid process) (van der Sluis et al., 198'7). As Ca2+diffuses from the rock particles into the liquid phase, it will be surrounded by a crowd of SOa2-ions in the liquid phase. The reaction between S042-and Ca2+ is the slowest of the three reactions. This fact causes SO4* and Ca2+ion accumulation in the liquid phase, causing a certain degree of supersaturation. The supersaturation in the bulk liquid phase or locally near the surface of the phosphate particle is the main cause of coating. 3. Coating Effect during Acidulation The reaction between phosphate rocks and acids is essentially a surface reaction (Janikowski e t al., 1964), in which the rate is largely controlled by the reaction temperature, the hydrogen ion concentration, the diffusion of reactants to the surface and products from the surface, the agitation power, and the surface area of the rock available for reaction. In the case where sulfate ions are present, i.e., as in the normal "wet" phosphoric acid process, the kinetics are further complicated by the fact that solid layers of calcium sulfate may form around the rock particle (coating phenomenon). This solid layer drastically reduce the rate of transfer of ions to and from the rock surface and thus tends to cause the reaction to be blinded. This
Ind. Eng. Chem. Res., Vol. 29, No. 12, 1990 2391 Table I. Operating Conditions of the Experiments Performed agitation speed, initial % initial % 104D,, temp, rph X exDt H,SO," H,PO," m "C 10-4 1 01.92 00.00 1.275 70 4.80 2 26.30 23.23 1.275 70 4.80 2.385 70 4.80 3 26.30 23.23 4 26.30 23.23 3.610 70 4.80 5 31.00 23.23 2.385 70 4.80 2.385 78 4.80 6 31.00 23.23 2.385 70 4.80 7 31.00 23.23 8 31.00 23.23 2.385 70 3.00 9 07.10 00.00 2.385 70 4.80 10 31.00 00.00 2.385 70 4.80
" w/w
in the liquid phase.
phenomenon is illustrated in Figure 1. Figure 1 is a microphotograph, showing a rock particle that had been exposed to an excessive concentration of sulfate; the outer layer coating the particle consists of calcium sulfate. This outer layer slows down the reaction, or, in extreme cases, stops it altogether. In the phosphoric acid industry, the reactor and the crystallizer represent the heart of the plant, where many phenomena (chemical reactions, diffusion, crystallization, etc.) take place. The controlling parameters in the phosphoric acid process are listed as follows: sulfate ion concentration, P205concentration, content of solids, and reaction temperature. Of these parameters, the sulfate concentration is the most important parameter for the overall process, because it governs the crystallization quality, crystal shape, and crystal size. Also, the sulfate ion concentration in the reaction slurry affects to a large extent the cocrystallized P2O5 losses (the highest P,05 losses among other P,05 loss routes) plus the unattacked P205losses due to the coating effect (Becker, 1983, p 144). The batch experiments are the most suitable way to follow up the effect of sulfate concentration on the process, especially the coating effect. Special emphasis is given to the investigation of the effect of sulfate concentration on the inhibition of the reaction (coating effect) and subsequently on the diffusion process and the effective diffusion coefficient used to describe it. When the gypsum crystals are in equilibrium with the solution, the ionic product or solubility product at 75 "C for crude phosphoric acid is given by Becker (1983, p 73) as
K, = (70CaO),,(% S04)eq= 0.83
(6)
Accordingly, the degree of supersaturation may be expressed as
G = (70 CaO)(% S04)/K,
(7)
and when the solution becomes supersaturated with Ca2+ and SO:- ions, the solubility product of the supersaturation state is given as (Becker 1983, p 76)
K,, = (% CaO)(% SO4) = 1.3
From the K, and K, values, the degree of supersaturation is 1.57; accordingly, if 1 < G < 1.57, the rock dissolves, and CaS04.2Hz0crystallizes on nuclei and crystals existing in the bulk of the liquid. And if G > 1.57, the rock dissolves and CaS04.2H,0 crystallizes directly on the external surface of the dissolving particles. Beyond the supersaturation limit (C > 1.57), nuclei start to be produced at a very high rate, and blocking of the reaction may occur. From the computed results obtained during experiment 1, we found that G is always less than 1.57; i.e. the rock dissolves, and CaS04.2H20 crystallizes on nuclei and crystals existing in the bulk of the liquid. Experiment 1 is the only experiment in which the rock particles are not exposed to coating by CaS04.2H20. In experiment 9, G was greater than 1.57; during the three periods of the reaction, under these conditions, CaS04-2H20crystallizes on the rock particles; i.e., blocking of the reaction does occur. Table I11 shows the values of G at different residence times obtained during experiment 9. This sequence of events is based on conditions in the bulk liquid phase; however, due to diffusional resistances between the solid surface and the bulk of the fluid, the local conditions in the neighborhood of the solid surface may differ in this respect from the bulk conditions, giving instantaneous nucleation and coating on the solid surface, while the bulk conditions do not predict the possibility of this type of coating. This problem will be addressed in full detail in a forthcoming paper dealing with the modeling of the crystallization step. In industrial practice, blocking of the reaction could occur due to differential reasons: 1. Bad design, where the rock and sulfuric acid are introduced into the reactor in such manner that two streams, one rich in Ca2+ and the other rich in contact each other. 2. Low agitation power, since agitation plays an important role in the dispersion of the reactants and prevents any localized concentration gradients. 3. Low slurry recycle rate. Hence, the concentration of S042-in the recycle acid will be high, and supersaturation may occur when it gets in contact with the rock. 4. Weak control of the reactants feed system. In practice, control of the solid fed is not an easy task, and quite often gives an error, which creates unsteady-state conditions for the concentration of SO:- inside the reactor. Some plants use a ratio controller between the sulfuric acid and the rock feed; in addition, analysis is carried out for concentration at short intervals, and adjustments according to these analyses are made. The bad control for
Table 11. Values of the Diffusion Coefficients Obtained at the Three Stages of the Reaction computed diffusion coeff obtained within the 3 time for each period in the reactor, min periods of the reaction, m 2 W expt Dei De, De, 1" 26 3c 1.14-05.0 05.0-120.0 1.0 x 104 0-1.14 1 1.0 x 104 1.0 x 106 2 3.5 x 10-9 2.0 x 10-11 5.0 x 0-0.30 0.30-05.0 05.0-060.0 0.30-24.0 24.0-120.0 3 1.0 x 10-8 2.0 x 10-12 1.0 x 10-11 0-0.30 0.30-24.0 24.0-060.0 4 3.5 x 10'8 1.0 x 10-11 1.0 x 10-11 0-0.30 5 1.0 x 10-8 1.0 x 1 0 4 3 1.5 X lo-" 0-0.30 0.30-24.0 24.0-120.0 6 5.0 X 3.0 X 3.0 x 0-0.09 0.09-10.0 10.0-120.0 7 1.0 x 10-8 2.0 x 10-12 1.5 X lo-" 0-0.36 0.36-24.0 24.0-120.0 9.0 x 10-9 2.0 x 10-12 1.0 x 10-11 0-0.36 0.36-24.0 24.0-120.0 36.0-120.0 8 9 3.5 x 10-7 3.0 X lo-" 1.5 X lo-" 0-0.42 0.42-36.0 10 8.3 x 10-7 3.5 x 10-11 1.5 X lo-" 0-0.06 0.06-04.8 04.8-060.0 a
0 - t l , the period of the initial stage.
(8)
t , - t,, the period of the intermediate stage.
t,
-
tend,the period of the final stage.
2392 Ind. Eng. Chem. Res., Vol. 29, No. 12, 1990 Table 111. Values of the Degree of Supersaturation, C (Dimensionless), Obtained during Experiment 9 by Using Eouation 7 residence time, min G residence time, min G 0.6 5.04 20.1 5.51 0.9 5.14 30.0 5.60 3.0 5.20 40.5 5.78 5.1 5.19 60.0 6.33 10.9 5.32 80.1 6.46
either the sulfuric acid stream or the phosphate rock stream could cause a very bad and difficult operation and sometimes causes a complete shutdown for phosphoric acid plants (reaction and filtration sections); hence, the plant stream factor falls down. To avoid these difficulties, the control of reactant feed rates should be carefully chosen. In industrial practice, a batch weigher screw feeder or a continuous endless belt weigher feeder is used to control the phosphate rock feed rate. The latter type is the most practical and suitable one, while control of the sulfuric acid feed rate is accomplished by using both variable orifice and electromagnetic flow meter controllers (MFC). The operator should not depend completely upon the controlling system, but he has to measure the stocks at certain time intervals (e.g., every week) and carry out calibration tests for the controllers; then corrections for the system are carried out. By these simple checks, the efficiency of the plant could be fixed a t the highest value. Janikowski et al. (1964) found that there is a linear relationship between crystalline P205losses and the reciprocal of the sulfate concentration in the reaction slurry. The work presented in this paper can be of great use for the design of new reactors and also for the improvement of the operation of existing ones. For a design of a new reactor, certain factors should be considered. We summarize these factors (according to their importance) as follows: 1. The reactants inlet points should not be designed to be at certain fixed locations, but a flexible arrangement for reactants feeding should be allowed for the plant operator to chose the most suitable feeding points according to his experience with the plant. 2 . The phosphate rock should be dissolved in the phosphoric acid of the recycle slurry liquid phase before contact with sulfuric acid inside the reactor. 3. The reactor’s agitators should be of the variable-speed type to overcome any changes that may occur in the nature of the phosphate rock, due to changes in the mining area or the source of the supply. For existing reactors, if the analytical results indicate high unreacted P205losses, we recommend an increase of the recycle ratio to decrease the concentration of S042-ions in the recycle slurry, hence avoiding a high degree of supersaturation, which causes rock coating. This can be done easily by changing the recycle pump with another one with a higher pumping capacity. Also another alternative solution can be carried out to avoid high unreacted P205 losses, by using simple cyclone on the top of the reactor to mix the phosphate rock with the recycle slurry before entering the reactor and contact with sulfuric acid. 4. The Change of Physical Properties and the Mass Balance Equations In order to simulate the course of the batch acidulation experiments, the change of physical properties with conversion should be included in the model. This requires material balance equations for all the components involved in terms of the conversion, which is computed as a function of time from the main differential equation of the model
Table IV. Chemical and Physical Characteristics of Saudi Phosphate Rock Used in This Work % w/w
constituents p205
CaO Fez03 A1203
MgO Na,O KzO CdO Ti02
(dry basis) 35.000 51.000 0.210 0.230 0.190 0.860
