Noncatalytic Heterogeneous Kinetics in the Engel-Precht Potassium Carbonate Process Gene L. Smithson Saskatchewan Research Council, Saskatoon, Canada S7N OX 7
Narendra N. Bakhshi' Department of Chemistry and Chemical Engineering, University of Saskatchewan, Saskatoon, Canada S7N OW0
The Engel-Precht process of K2CO3 manufacture was abandoned in Germany for economic and technical reasons. This study attempts to identify and solve the process control problems and provide the kinetic data necessary for converting the original batch process into a continuous flow process. The two reactions investigated here are the formation of Engel salt (KHC03.MgC03.4H20) and the decomposition of this insoluble salt into a solution of KHC03 plus solid MgCO3-3H20.The kinetics and mechanism of the Engel salt formation reaction (KCI, CO2, and MgC03.3H20 reacted in an aqueous slurry) have been established along with the optimum conditions for the reaction. Similarly, the kinetics, mechanism, and optimum conditions for the decomposition of Engel salt have also been determined. This paper completes a study of all reactions in the Engel-Precht process and together with the results obtained in two previous publications (the hydration of MgO and the carbonation of MgO) presents all the data required for the design of an efficient, continuous flow process.
Introduction The Engel-Precht process of manufacturing K2C03consists of the reaction of Con, KC1, and MgC03-3H20 in an aqueous slurry to form the insoluble double salt (KHC03.MgC03. 4&0, Engel salt) which can be separated and decomposed in water to produce a solution of KHC03 or K2C03and solid magnesium carbonate. As long as the magnesium carbonate is recovered as MgC03.3H20 it can be recycled in the process. This method of K2C03manufacture was utilized in Germany from the turn of the century until 1938 when it was superceded by the Formate process (Wiedbrauck,1942).The main reasons for its demise were problems in process control and the high operating costs of the batch process. The purpose of this study was to identify and solve these processing problems and to provide sufficient kinetic data for the design of a continuous flow process. The chemical reactions encountered in the Engel-Precht process are as follows: MgO
+ H20
Mg(OH)2
followed by
+ COS + 2H20 MgC03.3H20 MgO + COn + 3H20 s MgCOy3H20 3(MgC03*3H20)+ CO2 + 2KC1 2(KHC03*MgC03*4H20)+ MgC12 2(MgC03*3H20)+ 2KC1+ MgO + 2C02 + 3H20 s 2(KHC03*MgC03*4H20)+ MgCl2 Mg(OH)2
F!
F!
KHCOyMgC03.4H20 s KHC03 MgC03.3H20
+
+ H2O
(1) (2)
(3) (4) (5)
The results of our studies of the first two reactions have been published previously (Smithson and Bakhshi, 1969, 1973). This paper deals with reactions 3,4, and 5 . These reactions have been investigated by Bayliss and Koch (1952) and Bahavnagary (1962) with respect to phase equilibria, the equilibrium yield of Engel salt, and methods of decomposing Engel salt. There are no data available on the kinetics of the 450
Ind. Eng. Chem., Process Des. Dev., Vol. 15, No. 3, 1976
formation of Engel salt. The present study was intended to supply the required information on the kinetics and mechanisms of these reactions and to establish the optimum conditions for each reaction.
Experimental Section A. Formation of Engel Salt. The MgO used to prepare the MgC03-3Hz0 required in reactions 3 and 4 was sample no. 3 of the MgO described before (Smithson and Bakhshi, 1969). The normal method of preparing MgC03.3H20 was to add the required amounts of MgO and water (usually 10 g of MgO, 250 ml of H20) to the reactor and bubble in COz gas through a sintered glass dispersion tube while stirring a t 1600 rpm with a four-blade propeller stirrer. This reactor has previously been described (Smithson and Bakhshi, 1973). The reaction was continued for 1h at which time the appropriate amount of KC1 was added and the CO2 flow and stirring were continued resulting in the formation of Engel salt. Temperature control in all Engel salt formation and decomposition reactions was maintained by immersing the reactor to within 1-2 cm from its top in a constant temperature bath. The CO:! flow rate was adjusted so that there was a slight froth produced on the surface of the slurry. Preliminary runs using this method resulted in severe thickening and frothing when KCl was added. This was found to be caused by the fine particle size of the freshly formed MgC03.3H20. By allowing the MgC03 precipitate to digest in the reactor (maintaining reduced CO2 flow and stirring) for 6-12 h, large crystals developed which did not cause thickening problems when KC1 was introduced. All subsequent Engel salt production runs were performed with these digested MgC03.3H20 crystals. The first series of Engel salt production runs was designed to determine the effect of temperature and initial KC1 concentration on the equilibrium conversion of KCl. In each case the reaction was allowed to proceed to near equilibrium (124 h). The extent of reaction was determined by measuring the amount of unreacted K+ remaining in solution. The reaction slurry was centrifuged and the solids washed with cold water. The filtrate and washings were combined, diluted to a definite volume, and analyzed for potassium by flame emic-:on spectrometry. From the amount of KCl added a t the start of the
reaction and the amount remaining a t the end, it is possible to calculate the conversion to Engel salt. Once these preliminary runs had established the optimum range of experimental conditions the method of sampling was changed so that reliable rate curves could be obtained. Sample aliquots were withdrawn from the reacting slurry a t definite time intervals for analysis. A clear sample of the reacting solution was obtained by immersing the sintered glass tip of a gas dispersion tube in the slurry and drawing into it approximately 1 ml of liquid. A medium porosity gas dispersion tube filters out all crystalline solids. Because of its sensitivity and hence the requirement of very small sample volumes, the flame photometric determination of potassium was used to follow the course of the reaction. I t was also necessary to accurately estimate the total volume of the solution in the slurry so that the potassium concentration could be used to calculate the amount of unreacted potassium. This meant taking into account all additions Gr removals of water and dissolved ionic species. For reaction 3 there is no uptake or release of water in the reaction. The major changes to be considered are the solution volume changes in going from KCl to MgC12, the volume due to the dissolved magnesium carbonate, and the volume of solution removed with each sample aliquot. From these corrections an equation was derived (Smithson, 1973) which has been used to calculate the fractional conversion of KC1 to Engel salt from the measured concentration of Kf in solution and the original volume as corrected for previously removed aliquots. (xE)n
=
X,,Wo
- VACn
[ Wo - cn[ vo - u(n - 1) - J ]
titer X 0.025 N 256.3 X volume filtrate X 1000 weight E. salt X volume aliquot X
100% = % decomposition
(8)
The acid titration for carbonate-bicarbonate content is preferred to the flame photometric determination of potassium as a means of calculating percent decomposition because of the higher speed and precision of the titration. In several experimental runs, Engel salt was decomposed at elevated temperatures under a positive pressure of COz in a Parr low-pressure, catalytic reactor. Constant temperature operation was maintained by circulating water from a constant temperature bath through a copper coil surrounding the glass reactor bottle. The reaction was carried out for a definite time interval after which the COS pressure was released and the slurry centrifuged and analyzed as before. The kinetics of the decomposition reaction were followed by sampling the solution at time intervals and analyzing for COB- and HC03- or K+. The reactor and sampling procedure were the same as those used for continuously monitoring the Engel salt formation reaction. The COz flow rate was maintained at a level just sufficient to produce mild bubbling at the surface of the slurry (approximately 200 ml/min) and the stirring rate was 1600 rpm. Temperature control was maintained by placing the reactor in a constant temperature bath. To calculate the percent decomposition of Engel salt it is necessary to know the concentration of KHC03 in solution and the total volume of solution. The total volume can be calculated from the initial volume of water by correcting for all additions and removals. These volume corrections are for dissolved KHC03 and MgCOr3HzO and the volume removed with each sample aliquot. In addition a correction is required for the amount of KHCOs removed with the sample aliquots. These corrections have been incorporated into an equation which gives the fractional decomposition of Engel salt (Smithson, 1973).
The term Cn [ V O- u ( n - 1) - J ] is the correction for volume changes due to dissolved magnesium bicarbonate and for the water removed in each sample aliquot. The term
where
is the correction for the amount of dissolved KC1 removed in previous sample aliquots. B. Decomposition of Engel Salt. Decomposition reactions were performed in the same reactor used for the preparation of Engel salt. Measured amounts of Engel salt, water, and any decomposition additive required were placed in the reactor and stirred at the maximum stirrer speed (1600 rpm). The decomposition of Engel salt is normally carried out as in reaction 5, but when MgO is used as a decomposition agent the reaction proceeds as follows.
The term V, is the correction for the volume increase due to dissolved MgCO3-3H20. Strictly speaking this correction increases gradually for the first 10-20% reaction and then decreases through the remainder of the reaction as the increasing concentration of KHC03 suppresses the amount of dissolved MgC03-3H20. Rather than introducing this complex correction, a constant value of V, is used throughout the reaction. The error introduced by this simplification is less than 1%over the range 0 I X D I1.00.
The decomposition was continued for a definite time period and then the slurry was centrifuged and the solids were washed with cold water. After measuring the volume of filtrate plus washings, an aliquot was taken for analysis. The carbonate and bicarbonate content were determined by titrating with 0.025 N HzS04 to the phenolphthalein and methyl purple end points. The total titer (methyl purple end point) was calculated as HC03- which was used to calculate the extent of decomposition as follows.
Results and Discussion A. Preparation of Engel Salt. Equilibrium Conversion. From the phase data compiled and expanded upon by Bayliss and Koch (1952) it is possible to calculate the equilibrium conversion of KC1 to Engel salt. These calculated conversions are plotted in Figure 1 for the KC1 concentration range 0-90 mo1/1000 mol of HzO at both 18 and 25 "C. To confirm these values and to check on the extent of the equilibrium conversion achieved in 24 h and over, three runs were performed a t KC1 concentrations ranging from 29 to 72.5 mo1/1000 mol of HzO. The temperature was 25 "C. These values are plotted as the broken line in Figure 1. The close agreement between the two 25 "C curves indicates that near equilibrium has been Ind. Eng. Chem., Process Des. Dev., Vol. 1 5 , No. 3, 1976
451
$60 L
2 40 8
20
'0
10
20
K)
40
M
Inifla1 KCI COncenfrafiOn
60
70
80
- moler/1000 moles H,O
TEMPERATURE- 'C
Figure 1. Conversion of KC1 to Engel salt vs. initial KC1 concentration at 18 and 25 "C, 1atm of COP:0,calculated from Bayliss and Koch's phase data; +, experimental data this study (24-25 "C).
achieved in the experimental reaction period. From these calculated and experimental curves it is seen that the highest equilibrium yield is obtained a t an initial KC1 concentration of 50 mo1/1000 mol of H20 (at both 18 and 25 "C). Another series of runs was performed a t 12,18, and 25 "C to determine the effect of temperature on the equilibrium conversion of KC1. These results, obtained a t the optimum KC1 concentration of 50 mo1/1000 mol of H20, are shown in Figure 2. The equilibrium conversion of KC1 is seen to decrease with increasing temperature. This is a result of the thermal instability of Engel salt a t the higher temperatures. Consequently, the reaction should be conducted a t the lowest temperature which still provides an acceptable rate of reaction. In addition to reaction temperature and the initial concentration of KCl, a third variable, the pressure of COz on the reacting system, has a pronounced affect on equilibrium conversion. As a suitable pressurized reactor was not available for this study, it was necessary to calculate the expected equilibrium yields using the phase data and method of Bayliss and Koch (1952). They used the equilibrium relationship for eq 3
and the equilibrium equation for the dissolution of MgC033Hz0 by COz
+
(MgCO3.3HZO COz F! Mg2+
+ 2HC03- + 2H20)
namely
(12) and the equation relating ionic equilibrium between the four species [Cl-]
+ [HC03-] = [K+] + 2[Mg2+]
(13)
These three equations are solved simultaneously by trial and error and the value of [K+] thus obtained was then used to calculate the conversion of KC1 to Engel salt. Details of the calculation method are given by Smithson (1973). These calculations have been performed for COz pressures of 2,3,5,10, and 20 atm a t a temperature of 18 "C and are plotted in Figure 3. The equilibrium conversion increases with increasing COZ pressure but levels off rapidly a t pressures above 10 atm. Stirred tank reactors with mechanical pressure seals on the 452
Ind. Eng. Chem., Process Des. Dev., Vol. 15,No. 3, 1976
Figure 2. Conversion of KC1 to Engel salt vs. temperature (1atm of COz; initial KC1, 50 mo1/1000 mol of H20).
0
5
10
15
20
25
COS Pressure -atm
Figure 3. Influence of CO2 pressure on the maximum conversion of KC1 to Engel salt (18 "C).
stirrer shafts are normally designed for pressures under 20 atm. For this reason and the small increase in conversion above 10 atm it is probable that the optimum pressure range would be 10-20 atm of COz. An added benefit of operating a t positive COz pressure should be an increase in reaction rate due to the increased concentration of Mg(HC03)2 in solution. From eq 12 it has been calculated that a tenfold increase in COz pressure would approximately double the Mg(HC03)2 concentration. The increase in reaction rate produced by this doubled concentration would depend on the nature of the rate-controlling process (mass transfer or chemical' reaction). From the foregoing calculations and experimental results it may be concluded that the optimum range of conditions for the production of Engel salt via eq 3 would be: a temperature of 18 "C or lower, an initial concentration of KCl of 50 mol/ 1000 mol of HzO or higher, and a pressure of 10-20 atm of COz. Temperatures below 18 "C will produce higher conversions of KCl but lower rates of reaction. Conversely, concentrations of KCl greater than 50 mo1/1000 mol of H20 will result in lower conversions of KCl but higher rates of reaction. An increase in COZ pressure should result in an increase in both the rate of reaction and the conversion of KCl. Rate of Engel Salt Formation. The effect of temperature, initial KC1 concentration, and the method of adding MgC03. 3H20 on the rate of Engel salt formation were investigated in a series of six experimental runs (Figure 4). In all cases, the
"-
TIME
- hours
TIME
- hours
Y
Figure 4. Conversion of reactable KC1 to Engel salt vs. time: 0, MgC0~3H20added gradually throughout reaction; +, all MgCO? 3H20 added at start of reaction (1atm of COz in all runs; initial KC1, 50 mol/lOOO mol of H20 unless otherwise noted).
percent conversion is expressed as a percentage of the equilibrium conversion. A common characteristic of these rate curves is the existence of an induction period prior to the formation of Engel salt. This induction period varies from approximately 1.5 h a t 12 "C to 4 h a t 25 "C. The rate of reaction increases with increasing KC1 concentration and with decreasing temperature. The rate of reaction is also increased by adding the MgCOY3H20 gradually throughout the reaction rather than all a t the beginning. The cause of the induction period prior to the formation of Engel salt is the slow rate a t which nuclei of this solid are generated. Formation of crystalline Engel salt requires the simultaneous incorporation of K+, Mg2+,HC03-, C03- and 4H20 into the crystal lattice. The probability of spontaneous nucleation in the solution will be very low. I t is much more likely that nucleation begins on the surface of MgC03.3Hz0 by interaction with K+ and HCO3-. Once formed these nuclei can continue growing by uptake of all four ionic species and water molecules. It has previously been noted that Engel salt formation occurs only when MgC03.3HzO is used as the magnesium carbonate reactant (Precht, 1900). This would be explained if the MgCO:l.3H20 crystal lattice is topotaxically compatable with the Erigel salt lattice. The induction period is reduced as the concentration of KC1 is increased because of the increase in the production and growth of nuclei with increased K+ concentration. The induction period also decreases as the temperature is lowered. This is most likely due to the increase in dissolved HCOB- with decreasing temperature and the effect of this increased concentration on the rate of nuclei formation and growth. From the two lower rate curves a t 25 "C it is seen that the rate of reaction increases approximately 70% for a 2.3-fold increase in KC1 concentration. I t is most probable that the reaction is first order with respect to [K+] but this is not observed here because of the inhibition of diffusion caused by the high solids content of the slurry. The inhibition of mass transfer produced by a high level of MgC03-3H20 is readily observed in the upper two 25 "C rate curves and the two 18 "C rate curves. In each case the lower curve is the result of adding all the MgC03.3H20 a t the start of the reaction, whereas the upper curve is observed when the MgC03-3HzO is added gradually throughout the reaction, At 25 "C an increase in
Figure 5. Comparison of several Engel salt formation rate curves with eq 15; 18" rate curve: 0 , MgCOr3HzO added gradually; 25" rate curves: +, MgC0~3H20all at start, 21.75 mol of KClilOOO mol of H20; 0,MgCOr3HzO all at start, 50 mol of KC1/1000 mol of HzO.
reaction rate of 80% is observed while the increase is 60% a t 18 "C for the gradual addition of MgC03.3H20. The increase in reaction rate with decreasing temperature is the result of two factors. The first is that as the temperature is lowered the concentration of dissolved HC03- increases with a corresponding increase in the rate of Engel salt formation. The second is the result of the thermal instability of Engel salt. A t lower temperatures (12-18 "C) the rate of decomposition of Engel salt is much less than its rate of formation. As the temperature is increased the rate of the chemically controlled decomposition reaction increases faster than the rate of the formation reaction resulting in a net decrease in the rate of Engel salt production. At some temperature slightly above 27 "C the rates of the formation and decomposition reactions become equal resulting in zero net production of Engel salt. Rate Equation for Engel Salt Formation. The shape of the rate curves obtained in the conversion of KCI to Engel salt is typical of reactions which are rate controlled by nucleation and crystal growth. In such a reaction the rate of formation of the precipitated solid will depend on its own surface area and the concentration of the reacting species. For the fractional conversion XBthe remaining reactant will be proportional to 1 - XBand the surface area of solid will be proportional to x B 2 / 3 . From this, the rate of reaction can be expressed as rate = k (1 - x B ) x B 2 ' 3
(14)
Integration of this expression results in the following integrated rate equation.
+
1
tan-' - = k t
4
(15)
Plots of X B vs. t/tl,z for the experimental rates curves were compared to the corresponding plot for eq 15 in Figure 5. Although the experimental rate curves tend to bracket eq 15, there is no general agreement with this relationship. A second relationship was derived in which the effects of Ind. Eng. Chem., Process Des. Dev., Vol. 15,No. 3, 1976
453
nuclei production and nuclei growth were introduced. Instead of the simple proportionality between rate and surface area (rate a x B 2 i 3 ) the rate was assumed to be proportional to the total number of nuclei, A , plus a simple function of the surface area, B X B . The term X B is used instead of XBzi3 because it has been shown that where a solid has a log-normal size distribution the surface area is more closely proportional to XB than X J ~ "(Smithson, ~ 1973).Substituting the expression A BXB for XBzi3 eq 14 results in
+
+
rate = k (1- XB)(A BXB)
10
08
06
x, 04
(16) 02
Integration of this expression results in the following integrated rate equation. In
A
+ BXB - In A = ht
0
30
20
1-xB
Plots of X B vs. t/tliz for eq 17 with various ratios of BIA are shown in Figure 6. For small ratios of BIA, that is when a large number of nuclei are present initially or the rate of nuclei production is very rapid, eq 17 reduces to
- In (1 - XB) = ht
Ind. Eng. Chem., Process Des. Dev., Vol. 15, No. 3, 1976
Figure 6. Variation of XB vs. t / t l / z for the equation In [ ( A+ BXB)/(1 - XB)] - In A = kt with various ratios of B / A
(18)
In this case the rate of reaction is proportional to the concentration of reactant B in the solution. As the ratio BIA increases the rate curves become more S-shaped approaching a step function a t infinitely large values of BIA. This final condition represents the case of a super-cooled melt which undergoes sudden, complete crystallization when nucleation is induced. The most appropriate value of BIA for a particular set of rate data can be determined by comparing the plot of X B vs. tltllz with the curves in Figure 6 and selecting the ratio corresponding to the closest match. Once BIA has been determined it is possible to calculate the left-hand side of eq 17 for each value of X B and plot this value vs. time. A straight line plot indicates good agreement between the experimental data and the derived rate equation. The linear plots observed in Figure 7 indicate reasonable agreement between the 18 and 25 "Crate data and eq 17. The 12 "C rate curve does not correspond to a single value of B / A . The initial portion of this rate curve is best fitted by BIA = 10 000 (nucleation control) whereas the mid and latter portions correspond to BIA < 0.001 (crystal growth controlling). This may be the result of both nucleation and crystal growth being limited by solution mass transfer after a certain number of nuclei have been generated. A t Bo, the rate curve corresponding to the addition of all MgC03-3H20 a t the start of the reaction has a smaller value of B / A than when the M g C 0 ~ 3 H 2 0is added gradually, indicating a greater dependence on crystal growth or mass transfer. Likewise, a t 25 "C the rate curves for the low KC1 concentration and for the complete, initial addition of MgC03.3HzO have lower values of B / A than the gradual addition rate curve confirming the reduction in mass transfer under these conditions. It is evident from these results that as the temperature is lowered or the concentration of KC1 is increased or the addition of MgC03.3Hz0 is regulated to increase the rate of reaction, nucleation of Engel salt becomes rate limiting. Under these conditions it should be possible to further increase the rate of reaction by adding sufficient seed crystals of Engel salt a t the start of the reaction. Seeded Engel Salt Formation Reaction. A set of experimental runs were performed a t 12,18, and 25 "C to observe the effect of adding seed crystals of Engel salt a t the start of the reaction. In each run, 10 g of finely ground Engel salt (approximately 100 mesh) was added to 250 ml of 2.6 M KC1 and the appropriate amount of MgC03-3H20 (depending on temperature) was added gradually throughout the reaction. 454
10
0
2
4
6
8 IO TIME - hours
12
14
16
-
TIME hours
+
Figure 7. Plots of In [ ( A BXB)/(1 - XB)] - In A vs. time for the experimental Engel salt formation rate data. Selected best f i t of B / A indicated on each curve.
The observed rate data are plotted in Figure 8. From these rate curves it is seen that the induction period has been completely eliminated at all three temperatures. That is, nucleation is no longer a rate-determining factor. Consequently, eq 18 should be applicable under these circumstances. Plots of -In (1 Xp,) vs. time for the observed rate curves are shown in Figure 9. The excellent linear agreement indicates that eq 18 is obeyed. Equation 18 is a special case of the general eq 17 in which surface area is so large that mass transfer of reactant B to this surface becomes rate limiting. The small increase in the rate constant in going from 12 to 18 "C (Figure 9) corresponds to an activation energy of approximately 2 kcal/g-mol. This confirms that mass transfer is the rate-controlling process for the formation of Engel salt under these conditions. At higher temperatures (i.e., 25 "C) the thermal decomposition of Engel salt which is a chemically controlled reaction causes a reduction in the net formation of Engel salt. As previously mentioned, a temperature is eventually reached (27
TIME - hours
Figure 8. Conversion of reactable KCI to Engel salt vs. time (seeded
reaction). Temperature
2
- "C
Figure 10. Rate of formation of Engel salt vs. temperature (initial KCl concentration, 50.2 mo1/1000 mol of H20). 2
-
I
X"
c I
I
I
(
1
2
3
4 5 TIME -hours
6
7
8
Figure 9. Integrated rate equation, -In (1 - X B ) vs. time for the seeded, Engel salt formation reaction.
"C) a t which the decomposition reaction exceeds the formation reaction. At or above this temperature Engel salt cannot be produced by the foregoing reaction scheme. A comparison of the reaction rates for the seeded and unseeded reactions can be made on the basis of the maximum slope of each rate curve after correcting for the total amount of KCl converted. The maximum slopes for the curves in Figure 4 and 8 have been computed in terms of moles/(literhour) and are plotted in Figure 10. The rate of the seeded reaction increases rapidly from 25 to 18 "C and then decreases gradually a t temperatures below 17 "C. The gradual increase in reaction rate in going from 12 to 18 "C corresponds to the mass transfer control region. Above 18 "C, the chemical rate-controlled decomposition reaction begins to limit the rate of reaction. The unseeded reaction rate increases gradually with decreasing temperature but must level off below 12 "C as the rate of the unseeded reaction cannot exceed that of the seeded reaction. In addition, it must be remembered that the induction periods have not been taken into account when determining these maximum rates for the unseeded reactions.
Therefore, this simple comparison is greatly biased in favor of the unseeded reactions. In summary, the production of Engel salt can be optimized with respect to the maximum conversion of KC1 and the maximum rate of reaction by the following means. A significant increase in the rate of reaction is accomplished by adding MgC03.3Hz0 gradually throughout the reaction rather than all a t the beginning. Two other means of increasing the rate of reaction are to increase the concentration of KC1 and operate the reaction a t a positive pressure of COS.Increasing the concentration of KCl shifts the reaction from the maximum conversion value but the decrease in conversion is small in comparison to the increase in rate obtainable. An increase in KCl concentration from 50 to 80 mo1/1000 mol of HzO would decrease the conversion of KC1 by 4%but should increase the rate of reaction by up to 50%. Operating a t a positive pressure of CO2 will increase the concentration of HC03- and thereby increase the rate of reaction. At 10 atm of CO2 the concentration of HC03- is more than double that a t 1 atm of COz. Increasing the pressure of COn has the added benefit of increasing the conversion of KC1 as well as the rate of reaction. In going from 1 atm of CO2 to 20 atm of COz the conversion of KCl a t 18 "C is increased from 69% to 89%. Also, the conversion of KC1 can be increased by lowering the temperature from 18 to 12 "C. At 18 "C, 1atm of COz the conversion is 69% whereas a t 1 2 "C, 1 atm of COz it increases to 78%. Using the foregoing optimized conditions it should be possible to increase the rate of reaction 2.5-fold, or from 1 g-mol/l.-h to 2.5 g-moi/l.-h. In addition, the conversion of KCl could probably be increased from 69% to approximately 90%. B. Decomposition of Engel Salt. Decomposition Method. The two most commonly referred to methods of decomposing Engel salt are by heating or adding MgO to a slurry of the crystals in water (Bhavnagary, 1962; Bayliss and Koch, 1952; French Patent, 1931; Heimann, 1955; Precht, 1900; Precht, 1901; Weidbrauck, 1973). All attempts a t using either of these methods or variations thereof resulted in failure. In all cases the M g C 0 ~ 3 H 2 0was contaminated with basic magnesium carbonate and in many cases only basic magnesium carbonate was produced. Other methods of decomposing the double salt were attempted, such as the addition of KOH or NH40H to an aqueous Engel salt slurry. Neither of these Ind. Eng. Chem., Process Des. Dev., Vol. 15, No. 3, 1976
455
methods was successful. Another attempted method was the application of vacuum to the slurry to cause decomposition by removal of COz. 2(KHC03.MgC03-4HzO)
+ 2(MgC03*3H~O)
+ KzC03 + coz + 3H20
(19)
This method resulted in the immediate formation of basic magnesium carbonate which caused severe thickening of the slurry and a maximum decomposition of 42%. From these preliminary experiments it became evident that the recovery of M g C 0 ~ 3 H 2 0was favored by low temperatures and low pH'i. On careful examination of Bayliss and Koch's phase diagram for this system (Figure 11)the reason for the requirement of a low pH became apparent. The region in which MgC03.3HzO occurs as a stable solid phase lies close to the HC03- axis of the phase diagram. A high ratio of HC03- to COS- necessitates a low pH. If this ratio is decreased by increasing the pH, decomposition will take place in the region where basic magnesium carbonate is the stable solid phase. The easiest method of maintaining a low pH and high HC03- concentration is to saturate the solution with COS. A few trial runs were performed in which a positive pressure of COz (1-2 atm) was applied to an aqueous slurry of Engel salt heated in a Parr low-pressure hydrogenation apparatus. For all temperatures up to 65 "C the only solid decomposition product was MgC03-3H20. Above this temperature the M g C 0 ~ 3 H 2 0was contaminated with basic magnesium carbonate. In all cases a solution of KHC03 was obtained as the other decomposition product. Because of the time required to assemble and disassemble the Parr reactor, it was not possible to obtain accurate measurements of the rate of decomposition. Consequently, rate measurements were performed in an open stirred tank reactor as described in the Experimental Section. The ratio of Engel salt to water was selected such that there was just sufficient water t o allow all Engel salt to decompose. The concentration of this equilibrium solution changes with temperature and was determined by slurrying an excess of Engel salt in water a t the desired temperature and analyzing the composition of the final solution. The concentration of KHCO3 in these solutions at 31, 41, 51,61, and 71 "C is shown in Figure 12. The rate curves obtained for the decomposition of Engel salt under these conditions are shown in Figure 13. Normally, it will be desirable to decompose the Engel salt a t the highest temperature practical as this gives the maximum rate of decomposition and the highest concentration of KHC03 in solution. The upper limit will be dictated by the maximum temperature a t which MgC03.3HzO can be produced free of basic magnesium carbonate. In the trial runs with the Parr reactor, pure MgCOy3H20 was produced a t 65 "C (2-h reaction period) but a trace of basic magnesium carbonate was formed at 70 "C (2-h reaction period). In the open stirred tank runs no basic magnesium carbonate was evident up to 71 "C. The difference between these two cases is that the reaction time was only 20 min in the open stirred tank reactor. Therefore, it may be concluded that a decomposition temperature of 70 "C may be used as long as the reaction time is not much over 20 min. Decomposition of Engel salt a t 70 "C gives a maximum KHC03 concentration of 14% when saturated with COz and 15% when no COz is applied (Figure 12). This difference is caused by the higher level of dissolved Mg(HC03)2 present in a solution saturated with COS. The dissolved Mg(HC03)z suppresses the solubility of KHC03. It is possible to circumvent this problem by saturating the Engel salt slurry with COz a t the start of the reaction and then cease further addition of COz. Once the decomposition is started in the region where MgC03.3HzO is the stable phase, the KHCO.1released in so456
Ind. Eng. Chem.,Process Des. Dev., Vol. 15, No. 3, 1976
I
1
I
/
L
i - 4
/
$';
I
Figure 11. Janecke phase diagram for the system K+-Mg2+HC03--C0s2- at 25 "C from Bayliss and Koch.
0
' 3
0
4
I
0
v
I
)
6
I
0
7
TmprOtW
I0
-Y
8
I
0
I
9
100
0
Figure 12. Temperature dependence of the maximum solution concentration of KHC03 attainable from the decomposition of Engel salt.
I
I
I
f
-
16-
60q Enqcl 5011
I
10
I
I
20
30
I
TIME- min
Figure 13. Decomposition of 60 g of Engel salt in that volume of water which produces the maximum attainable KHC03 concentration a t 31,41,51,61, and 71 OC.
lrition will maintain the concentration of HC03- within the desired region. R a t e Equation for Engel Salt Decomposition. The rate of decomposition of Engel salt could be rate controlled by either a chemical reaction at the surface of the solid or by mass transfer into solution. In the first case the rate of reaction would be mainly dependent on the surface area of the solid. For a solid composed of a distribution of particle sizes the integrated rate equation would be (Smithson and Bakhshi, 1969) -In (1 - XB) = k t
IO
9
8 7
-EL
I-x.
6 5
(20)
4
In order to make plots involving XBit is necessary to convert the amount reacted in Figure 13 to a fraction of the maximum equilibrium concentration. Plots of -In (1 - XB) vs. time, using these fractional equilibrium conversions, are not linear. Therefore, surface chemical reaction control may be rejected. Mass transfer rate control for the dissolution or decomposition of a granular solid is dependent on the surface area of the solid and the concentration differential of the dissolving species between the solid surface and the bulk solution concentration. Surface area is proportional to (1 - Xg)2'3 but when a distribution of particle sizes exists, the change in surface area is more closely proportional to (1 - X B ) . The difference in concentration of the diffusing species a t the surface of the solid and in the bulk of the solution will be proportional to (1 - X B ) . From these two proportionalities the rate may be expressed as
3
rate = k ( 1 - x B ) ( 1
-XB)
2 I
I
'0
3
2
~
4 5 TIME- min
7
6
9
8
F i g u r e 14. Integrated rate equation plots [X,/'(l - XB)] = k t for the Engel salt decomposition rate data.
(21)
Integratinn of eq 21 results in the following integrated rate equation. (22)
Plots of XB/(1 - X,) vs. time for the fractional equilibrium conversions calculated from Figure 13 are shown in Figure 14. The straight line agreement of the experimental data in these plots indicate that mass transfer is the probable rate-controlling process. It is possible to confirm mass transfer control if the activation energy of the reaction is below 6 kcal/g-mol. An Arrhenius plot of the rate constants determined in the Figure 14 plot is presented in Figure 15. A straight line relationship exists from 41 to 71 "C and results in an activation energy of 2.3 kcal/g-mol. This definitely confirms mass transfer as the rate-controlling process in the decomposition of Engel salt at temperatures above 40 "C. Below 40 "C the activation energy appears to increase dramatically. It is in this region that the decomposition reaction becomes chemical rate controlled. This has been pointed out previously in the formation of Engel salt that as the temperature is increased above 18 "C the decomposition of Engel salt (a chemically controlled reaction) increases rapidly, reducing the net rate of its formation. At some temperature above 27 "C the rate of the decomposition reaction exceeds that of the formation reaction. As the temperature is increased further, the rate of the chemically controlled decomposition reaction increases until it is limited by mass transfer (above 40 "C). The relationship between the mass transfer formation region, the chemical reaction controlled decomposition region, and the mass transfer controlled decomposition region are illustrated in Figure 16. In summary, the optimum conditions for the decomposition of Engel salt are 100 g of Engel salt/243 ml of water heated to 70 "C. The water must be saturated with COn a t the start of the reaction to ensure the formation of MgC03-3H20 throughout the decomposition. No further C02 need be added during the course of the decomposition as the dissolution of
-20
I
I
I
I
I
28
29
30
31
32
33
pT x 10' F i g u r e 15. Arrhenius plot of rate constants, k , from the Engel salt decomposition reaction (Figure 14).
I ' I
'I
W
W.
I
70.
I
I
f
I
I
BO'
50.
40'
W
I
I
1
I
kT
10.
20'
I
I
QC
I
I
10'
F i g u r e 16. Descriptive representation of the various reaction rate control regimes in the formation and decomposition of Engel salt. Ind. Eng. Chern., Process
Des. Dev., Vol.
1 5 , No. 3, 1976
457
KHC03 will maintain the required HC03- concentration. The time required for complete decomposition is from 20 to 30 min and results in a solution of 15 wt % KHCOs.
Conclusions 1. The rate of formation and the yield of Engel salt produced from a slurry of MgC03.3HzO in a C02 saturated solution of KC1 are favored by: a low solids (MgC03.3H20) content, a high level of KC1, a positive pressure of COz, the presence of seed crystals of Engel salt, and a low temperature (12-18 "C). Under optimized conditions it should be possible to,achieve a rate of production of 2.5 g-mol/l.-h and a 90% conversion of KCl to Engel salt. The seeded Engel salt formation reaction is mass transfer controlled a t temperatures up to 18 "C. Above 18 "C, the rate of reaction becomes limited by the chemically controlled thermal decomposition of Engel salt. The rate of thermal decomposition increases with temperature causing the net rate of Engel salt formation to decrease to zero a t approximately 27 "C. 2. The thermal decomposition of an aqueous slurry of Engel salt is rate controlled by a chemical reaction up to 40 "C and by mass transfer above this temperature. Mass transfer rate control above 40 "C is confirmed by an activation energy of 2.3 kcal/g-mol. Optimum decomposition conditions are 100 g of Engel salt/243 ml of water a t a temperature of 70 O C resulting in a 15% (w/v) solution of KHC03. By initially saturating the decomposition slurry with COz the solution composition is maintained in the phase region where M g C 0 ~ 3 H 2 0 is the only stable solid. Once the decomposition is initiated in this phase region the addition of COz can be discontinued as the production of dissolved KHC03 maintains the solution composition within the desired region. Acknowledgment This project was carried out in the laboratories of the Saskatchewan Research Council. The authors thank Brad Gunn and J. H. Hudson for their interest and time spent in discussing the interpretation of data in this study. Nomenclature A = a factor proportional to the total number of Engel salt nuclei present a t a given time B = a factor proportional to the surface area of Engel salt crystals a t a given time (CK), = the concentration of dissolved KHC03 a t the time of the removal of the n t h sample aliquot, g/ml
458
Ind. Eng. Chem., Process Des. Dev., Vol. 15, No. 3, 1976
C, = the concentration of KC1 in solution a t the time of the removal of the nth sample aliquot, g/ml J = the correction for the decrease in volume near the end of the reaction when all solid MgC03.3HzO has been consumed and the concentration of Mg(HC03)z in solution begins to decrease; J = L ( X E - X E ~ ml; ) , J = 0 when X E -XE1
