Ind. Eng. Chem. Res. 1992,31,2437-2446 Salmi, T.; Martikainen, P.; Paatero, E.; Hummeletedt, L.;D a m h , H.; Lindroos, T. Kinetic Model for the Synthesis of Monochloroacetic Acid. Chem. Eng. Sci. 1988,43, 1143. Salmi, T.; Paatero, E; Fageretolt, K. Kinetic Model for the Synthesis of a-Chlorocarboxylic Acids. Chem. Eng. Sci. 1992, in press. Sioli, G., Spaziante, P. M.; GiuffrB, L.Make MCA in Two Stages. Hydrocarbon Process. 1979,2, 111
2437
Vajda, S.; Valkb, P. Reproche-Regression Program for Chemical Engineers, Manual; European Committee for Computers in Chemical Engineering Education: Budapest, 1985,36 pp.
Receiued for reuiew April 28,1992 Revised manuscript received July 15, 1992 Accepted August 7, 1992
Reaction Kinetics of Ca-Based Sorbents with HCl Brian K. Gullett' Air and Energy Engineering Research Laboratory, US.Enuironmental Protection Agency, Research Triangle Park, North Carolina 27711
Wojciech Jozewicz Acurex Enuironmental Corporation, P.O. Box 13109, Research Triangle Park, North Carolina 27709
Leonard A. Stefanski Department of Statistics, North Carolina State Uniuersity, Raleigh, North Carolina 27695
The kinetics of the reaction between CaO and HC1 were investigated under conditions that minimize bulk mass transfer and pore diffusion limitations. Reactivity data from 0.2-to 1-sexposure to 5000 ppm HC1 in a fixed bed reactor were analyzed by a shrinking core model of diffusion and chemical reaction control, either singly or in combination. Between temperatures of 150 and 350 OC, the reaction is controlled by gaseous diffusion through the developing product layer. The apparent activation energy is about 28.1 kJ/mol(6.7 kcal/mol), and the reaction is first order with respect to HC1 concentration. Reactivity is a minor function of the measured particle size and surface area, likely due to the agglomerative nature of the individual grains that comprise the particle structure and complicate the interpretation of these measured values. Extrapolation of these results to the high-temperature, furnace sorbent injection process provides preliminary agreement with pilot-scale tests.
Introduction The reaction of Ca-based sorbents with hydrogen chloride (HC1) is of interest for control of acid gas emissions from combustion procesees, most notably municipal waste combustion (MWC) and hazardous waste incineration ("I It ) is also .likely that high-temperature removal of HC1 will limit the downstream formation of chlorinated dioxins and furans (Gullett, 1991). Current methods of HCl removal often use dry Ca-based sorbent injection into the post-flame region. This process closely parallels the work that has been done to develop sulfur dioxide (SO,) control technology by furnace injection of Ca-based sorbents. However, unlike the Ca/S02 reaction, fundamental information concerning the kinetics, the controlling mechanism, and the reaction products of the Ca/HC1 reaction i~limitsd. In the case of the Ca/SOz reaction, this information has proved crucial toward optimizing sorbent reactivity with SO, through optimizing design/operation parameters such as injection temperature, injector location, and sorbent loading and sorbent parameters such as particle size, porosity, and surface area. This work examines the kinetica of the reaction between Ca-based sorbents and HC1 with the intent of providing information that will be useful toward optimizing the removal of HC1 from flue gas. Injection of calcium hydroxide [Ca(OH),] into hot (>400 "C) flue gas results in the loss of H 2 0 through Ca(OH), F? CaO + H 2 0 (1) Similarly, injection of calcium carbonate (CaC03) (>650 "C) results in calcination: CaC03 s CaO + COz (2) The resulting calcium oxide (CaO) is more porous and of
higher surface area than the original Ca(OH), or CaC03, unless prolonged heating results in sintering. CaO will react with HC1 to presumably form calcium chloride (CaC1,) as such CaO + 2HC1 e CaCl, + HzO (3) Free energy calculations suggest that (3) is favorable over the full range of temperatures in a combustor. &uilibrium concentrations of about 500 ppm HC1(10% HzO) likely impose an upper limit on the reaction at around 800 "C. Since 782 OC is the melting point of CaCl,, this also may constrain the upper practical temperature range of the reaction. The reaction can be considered irreversible since C0.1 ppm HC1 is in equilibrium with the assumed CaCl, product at the peak teat temperature, 350 OC (from 1 to 10% HZO). Various types of Ca-based systems for HC1 removal are discussed in the literature (Buekens et al., 1984;MayerSchwinning and Laibold, 1989;Ellison, 1989;Schmal et al., 1989), and a few experimental results are reported, covering a wide range of operating conditions and systems (Verbeek et al., 1987;Gullett et al., 1989;Karlsson et al., 1981; Walters and Daoudi, 1987; Weinell et al., 1992). Numerous waste combustion facilities use some type of sorbent injection for acid gas control (White and Vancil, 1989),yet very little fundamental kinetic information is available to assist design efforts and enhance large-system performance. A few reported results elucidate some of the important parameters regarding the Ca/HC1 reaction. Karlsson et al. (1981)determined a fmt-order rate dependency upon HC1 concentration when running breakthrough analyses with HCl and Ca(OH), from 150 to 400 "C. Similarly,
Q8SS-~sS5/92/2631-2437$03.00/0 0 1992 American Chemical Society
2438 Ind. Eng. Chem. Res., Vol. 31, No. 11,1992 PROCESS GAS STREAM
SAMPLE BED HOLDER
I REACTOR BASE
Figure 1. The short time differential reactor (STDR).
Walters and Daoudi (1987) found the reaction of CaO and HCl to be first order with respect to gas-phase HC1 concentration (0.5-5 mol %). They determined that the intrinsic chemical reaction rate constant was governed by an Arrhenius expression with an activation energy of 20.9 kJ/mol(5 kcal/mol) between the temperatures of 500 and 650 'C. Weinell et al. (1992) studied the direct reaction of Ca(OH), with HC1 between 60 and lo00 "C, finding maximum sorption in the 500-600 "C range. The sorptive capacity was found to be independent of particle size over the range 2.12-20.5 pm, and only minor effects of initial surface area (range 8-20 m2/g) were observed. Reaction kinetica were controlled by gas diffusion through the solid product, governed by a diffusivity with an activation energy of 10-15 kJ/mol (2.4-3.6 kcal/mol). In this work, research was conducted to determine the controlling mechanism and kinetics of HCI removal with Ca-based sorbent injection for application in combustion systems. The effeds of reaction temperature, particle size, HC1 concentration, and surface area on reaction rate were investigated. Determination of the rate controlling mechanistic phenomena through fundamental kinetic experimentation will significantly aid the effectiveness and development of this technology.
Experimental Section Reactivity testa were conducted in the short time differential reactor (STDR), an isothermal, fixed bed reactor shown in Figure 1, described more fully elsewhere (Gullett et al., 1990). To prepare the sorbent for testing, a 4-mg sample was evenly dispemd on a quartz wool bed, placed on the sample bed holder, and pneumatically inserted by an air cylinder into the exiting process gas stream of an electric furnace. The sample temperature was monitored by a type R Pt/Rh thermocouple just above the sorbent bed. Test temperatures (150-350 "C), significantly lower than expeded for furnace injection (>6W "C), were chosen to slow the reaction down, enabling study of the controlling mechanism at the initial stages of the reaction, and to minimize potential transport limitations to the sorbent particles' reactive surfaces. The air cylinder control mechanism allows for a preset residence time of the sample in the heated process gas stream; this is measured more accurately by a photocell emitter/detector pair located in the reactor base (not shown in Figure l),and can be controlled for values exceeding 200 ms (+5 ms). Prior to and after reaction, the sorbent was held under N, sweep gas at 23 L/min STP. Reactive gas flow conditions, sample size, and HCl concentration were suffkient to minimike potential effects of external film diffusion resistance upon reactivity results. Gas flow rates were set by balancing the attainment of differentiality with respect to HCl versus potential loss of sample (by gas flow entrainment of the sorbent) through the quartz wool bed and STDR operational problems (e.&
backpreasure, temperature maintenance). Baseline teating of sorbent from 22 to 50 L/min STP at 350 "C determined conversion data which were statistically analyzed for significance of flow rate. Higher flow rata (>50L/min STP) resulted in loss of sorbent due to ita being swept through the quartz wool bed and unacceptable gas leakage due to STDR pressure buildup. While combustion field conditions will typically result in 10% H20 and C02in the flue gasea, these experimental testa were done without these gases to avoid their known sintering effects (Borgwardt, 1989) from obscuring the intrinsic reaction kinetics. Further testa should be done in which the effect of various H,O and CO, contents upon the reaction rate are assessed. Sorbent under baseline testing conditions was exposed to 40 L/min STP process gas consisting of HCI at .5ooo ppm, 5% Oh and the balance N2. The sample was reacted for times varying from 0.2 to 1 s and temperatures from 150 to 350 OC. Conversion versus time testa were run for each set of parameters, consisting of at least five to eight residence times with three to five repeats. Date reproducibility was ensured by repeat baseline condition testing throughout the program. The sorbent used in this research was derived from Ca(OH), supplied by Lmwood Lime and Cement Company (Davenport, IA),containing 90.1% (by thermogravimetric analysis, TGA) Ca(OH), This material has a m a s median diameter (D,) of 1.96 pm, a porosity of 0.131, and a speclfc surface area of 14.5 m2/g. Preparation of the sorbent to a low surface area for kinetic determinations minimized intraparticle diffusion resistances by making a large fraction of the reactive surface area reside on the external surface of the particle. This was effected by exposing the Ca(OH), to heated N, flow (23 L/min STP)at 870 'C for 30 min and resulted in test samples of -5 m2/g and a of 3.7-4.6 pm. The resulting CaO samparticle size (Dm) ples were used for reactivity testing to minimize the possible complication of intraparticle pore diffuion resistance and significant conversion-induced variation in reactive surface area. Particle size 'cuta" were produced from Linwood CaC03 with a Donaldson Accucut air classifier and an ATM sonic sifter to test the effect of reactivity on sorbent size. Each size cut of CaCO, particles was sintered at various temp e r a m and times to produce CaO of relatively equal and low surface areas (4.59-5.38 m2/g) to minimize internal reaction differences. The effect of surface area was determined by testing with sorbenta of varying initial surface area. These CaO sorbenta were produced by pretreating Ca(OH), for varying temperatures and residence times in a muffle furnace. Samples to be analyzed for conversion were removed from the STDR and transferred to a beaker containing 20 mL. of deionized water and a small amount of 2 N sulfuric acid. The sample was stirred for 15 min and was then filtered into a 100-mL volumetric flask to remove the quartz residue. From this original 100-mL dilution, aliquota were removed for determination of molar ration of CI-/Ca'+ (and from this, sorbent conversion) by ion chromatography (IC) and atomic absorption (AA) spectroscopy, respectively. Particle size distributions were determined by a Micromeritics Sedigraph using a gravity sedimentation method. Porosity and surface area were determined by a Mimmeritics ASAP 2600 using N, adsorption/desorption with a BrunauerEmmett-Teller (BET) method. Scanning electron microscopy and X-ray dispersive spectroscopy (SEM/XDS) analyses were conducted on an Amray scanning electron microscope. XRD analyses were con-
Ind. Eng. Chem. Res., Vol. 31, No. 11, 1992 2439 ducted on a Siemens diffractometer with a copper K a target source running at 50 kV and 40 mA. The entrance aperture was l.Oo and the detector slit was 0.05O. Spectra were identified by computer comparison with the Joint Committee for Powder Diffraction Spectra (JCPDS) spectral files.
Analysis When the reactivity of the sorbent particle is unaffected by gaseous diffusion through the internal pore structure or external reactant diffusion through the film layer around the particle, then the rate of reaction can be expected to follow a conversion time response indicative of the controlling mechanism at the grain surface. Such is the case for sorbent of small particle size, presintered to about 5 m2/g (to minimize pore diffusion and surface area variation), maintained in a flow environment sufficient to minimize reactant differentiality and film transfer limitations. The possibility of reaction control by film mass transfer is minimized by the small particle size. Increases in the gas flow rate will not have an appreciable effect upon the Sherwood number (it will remain =2), since the Reynolds number is so low at these small particle sizes. This is seen by determination of the mass-transfer correlation for a sphere (Ranz and Marshall, 1952):
”’- +
Sh=--
2 0.6Re1/2Sc1/3 (4) DAB Likewise, pore diffusion limitations are minimized by the low surface area (low porosity), low reaction temperatures, high gas concentrations, and small particle size of the sorbent. The criterion for elimination of pore diffusion control is (Levenspiel, 1972)
case, the expression is modified (Szekely et al., 1976) by the inclusion of 2 3(2 - [Z + (1 - Z)(1 - X)]2/31 - 3(1 - X ) 2 / 3= p ( X ) = 2-1 4 (10) TP
Reactions, however, are also likely to encounter a series combination of limiting mechanisms at these extents of reaction. In this case, both (6) and (8) [or (lo)] are used to model the mixed-control reaction: t/Tg
= g(x)+ a 2 p ( X )
(11)
where
or the ratio of diffusion to chemical reaction resistance. More elaborate models can be constructed to allow for changes in the controlling mechanism with time or conversion. Often, however, a single mechanism can sufficiently account for the full range of experimental data. These models, (6) and (8) [or (lo)], are used either singly (Borgwardt and Bruce, 1986; Gopalakrishnan and Seehra, 1990) or in combination (11) (Torres-Ordoiiezet al., 1989) to model the X-t data. Since X varies monotonically with t, (11) can be solved for X as a monotone function of t: x = f(t,Pl,B2) (13) where B1 = 7, and B2 = T,$. Although f cannot be obtained in closed form, it can be computed numerically. With this representation the nonlinear least-squares criterion for estimating B1 and B2 in the combined model becomes N
C [Xi - f(ti,P1,P2)12
(5)
This reaction is likely to be controlled by either the intrinsic rate of chemical reaction or mass transfer through the product layer. If the reaction is controlled by the chemical rate of conversion at the grain surface interface, the data can be modeled through use of the shrinking core model expression (Szekely et al., 1976): 1 - (1 - X)1/3 = g ( x ) = t / T , (6) where T,, the time required for complete conversion ( X = l),is (7)
assuming spherical particles of unchanging size; a single, isothermal, and irreversible reaction; and a first-order response to the reactant gas concentration. The validity of these assumptions will be revisited later. If the reaction is controlled by reactant diffusion through the product layer, it will follow the expression 1 - 3(1 - X ) 2 / 3+ 2(1 - X ) = p ( X ) = t / ~ (8) ~ where
As the reaction proceeds, formation of a product layer is likely, since the ratio of the molar volume of the assumed CaC12product to the CaO reactant (2)is 3.05. For this
i=l
(14)
The estimated coefficients Bi are then related to the model parameters by (15) p1 = T~ = l / k g and
b2 = T , U ~ = l / k p
(16)
Our least-squares criterion (14) differs from the criterion that has been used by others (Torres-Ordoiiezet al., 1989; Szekely et al., 1976). These authors reportedly estimate Bi by minimizing: N
i=l
Our decision to use (14) was guided by statistical principles related to the fact that the experimental error in the determination of X is greater than that in t. These coefficients can be used to fit the best model to the controlling mechanism. When, as in this case, the specific surface area, S,, is greater than that for a solid particle of radius R, the particle is considered as an assemblage of grains in which grain radius, R, becomes the physical dimension in (7), (9), and (12). R, is determined by Rg = ~/S,PC~O (18) The solution to thisproblem yields values of T and c? from which k, and De can be determined from (7), (9), and (12). Often, however, the data are statistically insufficient to
2440 Tnd. Eng. Chem. Res., Vol. 31, No. 11, 1992 25
I
I
I
I
20
15
-
15OoC 2oooc 2500c 275OC
0
--..-....- '
20
I
DATA MODEL
0.0445
8 . 0 . 8
/
-
I
I
I
15
E
0.0242
-
I
X
i
Qa
10
9 2
8
5
0.0026
0' 0
I
0.2
I
I
I
I
I
0.4
0.6
0.8
1
1.2
TIME, t Is)
TIME, t (s)
Figure 2. Reactivity of CaO (5 m2/g, =4 pm) as a function of temperature (6OOO ppm HCl, 40 L/min STP). Curves from (10)with k, values in 8-l.
... . .
Figure 4. Reactivity of CaO (5 m2/g, a 4 pm) as a function of HC1 concentration (275 "C, 40 L/min STP). Curves from (10) with k, values in 8-l.
40
30
35
25
DATA MODEL
zx
-
30
E i
2
$
20
X
25
$
20
15
E
15
10
0 0
10
5 5
'0
0.2
0.4
0.6
0.8
1
1.2
TIME, t (s)
Figure 3. Reactivity of CaO (5 m2/g, -4 pm) as a function of process gas flow rate (5000 ppm HCl, 350 "C). Curves from (10) with k, values in s-l. Curve of flow rate = m from (10) and (20).
suitably distinguish between controlling mechanisms (Szekely et al., 1976;Wen, 1968) because of the close similarity of the two expressions (6)and (8) [or (lo)]. In this case, alternative methods of elucidating the controlling mechanism should be employed to support these findings, such as tests of concentration, particle size, surface area, and flow rate effects. The value of (a2) represents the ratio of chemical reaction resistance to product layer diffusion resistance and is a form of the shrinking core reaction modulus. When C? 10, it is safe to assume that the reaction is under product layer diffusion control (external mass-transfer control may also play a secondary role unless adequate experimental steps are taken to minimize this contribution). Intermediate values of u2 (=1) suggest that the reaction is controlled by both chemical and product layer diffusion (Szekely et al., 1976).
Results Experiments were conducted to elucidate the rates and controllingmechanism of the CaO and HC1 reaction. The results of varying temperature, flow rate, HC1 concentration, surface area, and particle size are shown in Figures 2-6.
2o
Ds0 DATA MODEL
3pm 0 5pm A
-
28pm
15 4 i p m
A
-----. ---,._.._.
I
0
I
I
0.0195/0.0195-
Ind. Eng. Chem. Rea., Vol. 31, No. 11,1992 2441 Table I. Compariron of Rate Models &lo, and 11 Derived from Temperature-Variant Data of Figure 2 tamp, OC model SSE 81 se 82 se 150 200 250 275 300 350
11 6 10 11 6 10 11 6 10 11 6 10 11 6 10 11 6 10
0.0019 (280.9) 175.2 2750.7 1507.1 52.2 2.7 0.0052 390.2 32.0 0.0028 0.0248 (45.2) 43.4 263.1 182.2 22.0 1.4 0.0357 0.0270 81.2 8.4 0.0196 (37.2) 19.6 188.7 74.0 18.8 1.0 0.0364 0.0219 59.6 4.5 0.0318 (14.3) 4.8 96.6 16.0 0.0644 17.5 0.5 51.3 2.0 0.0346 0.0175 (19.3) 7.0 98.2 20.8 15.3 0.6 0.0487 41.3 1.8 0.0214 21.4 3.3 0.5 1.3 0.0706 11.0 0.3 0.1039 22.4 0.9 0.0706
creasing reaction rates at higher temperatures. Flow Rate. Testa were conducted to examine the potential limitations of extraparticle, HC1 maas transfer. Due to the greater temperature sensitivity of the chemical versus diffusion step, testa at higher temperaturea are more likely to be m888 transfer limited. For this reason, testing for potential mass-transfer limitations was conducted at the higheat test temperature, 350 "C. The effect of gas flow rate upon the rate of reaction was examined in Figure 3 at 350 "C. Variation of the process gas below (22 and 30 L/min STP) and above (50 L/min STP) the baseline testing flow rate (40L/min STP)indicated increases in the reaction rate with higher flow rates. Oxygen. Testa were conducted (not shown) to examine the effect of O2on (3). An analysis of variance test between reactivity resulta (350 "C, &5 s,5OOO ppm HC1) with and without the presence of 5% O2 showed that O2 has no apparent effect on the reaction rate under these test conditions. HCl Concentration. The dependency of the reaction rate on the HC1 concentration (5 m2/g, 275 "C) is shown in Figure 4. HC1 concentration was varied from lo00 to 7500 ppm. The reaction temperature of 275 "C was chosen to minimize potential mass-transfer limitations at higher temperatures and the resultant bias toward a reaction order, n,of unity. Testa between 0.2 and 1s showed increasing sorbent reactivity with higher HC1 concentration. Surface Area. The effect of initial surface area (from 5 to 40 m2/g) is shown in Figure 5. Increases in surface area resulted in increased sorbent conversion to 25% at 1s reaction time and 275 "C. Particle Size. Test results varying particle size (3-59 pm) during reactivity testing (275 "C, 5OOO ppm HC1) are shown in Figure 6. These testa show that smaller particlea react more quickly, although results for the 3- and 5-pm particlea are undiatinguishable under these test conditions.
Discussion Model Fit. The p ( X ) and g ( X ) rate expressions were applied both singly [(6) and (lo)] by minimizing (14) with respect to one of the Bi by setting the other to zero and in combination [(ll)] by minimizing (14) with respect to both Bi simultaneously, to determine the best fitting expression for the X-t data presented in Figures 2-6. The minimum value of (14), denoted SSE (sum of squared errore), measurea the quality of fit of the model to the data The best statistical fit to the experimental data of Figure 2 is produced by the mixed control model, (ll),in all cases as shown in Table I. This is a consequence of the fact that
12.01
36p%
3woc
ZSOQC
ZWQC
'
I
~-oo~/min'
150DC I
(2.51
1
c '2 a Y
E a = 28.1 kJ/mol
13.51
C
I62b014
0.0016
0.0018
0.002
0.0022
0.0024
0.0026
TEMPERATURE'~(K'')
Figure 7. Arrhenius plot of k, (1/&) values from Figures 2 and 3 (5OOO ppm HCl, 5 m2/g).
(14) is minimized over one additional free parameter in the case of fitting (11). More important, however, is the fact that the SSES for the product layer diffusion model [(lo)] are always less than those for the chemical control model [(S)], and are quite similar to those from the fit of the combined model [(ll)] at all temperatures. The indicated conclusions are the following: (i) of the two competing single models [(6) and (lo)], the data are better modeled by the product layer diffusion control model and (ii) there is little room for improvement in the product layer diffusion control model by inclusion of the chemical control mechanism. Thus the reaction over the temperature range of 150-350 "C is most meaningfully modeled by the product layer diffusion control mechanism alone [(lo)]. Similar conclusions are drawn for the X-t data from the flow rate, concentration, surface area, and particle size testing (Figures 3,4,5, and 6, respectively). Previous researchers (Torres-Ordoiiez et al., 1989; Szekely et al., 1976) have used the ratio of estimated coefficients = 2 from the fit of the combined model [(ll)] to infer the dominant form of the reaction model. However, this manner of inference is problematic, especially if the reaction is dominated by product layer diffusion. For then we expect & to be zero or at least a small positive number, and even small deviations in can induce large deviations in p2/&. Indeed for many of our fitted models, the estimate of & is negative or close enough to zero so that the lower bound of a confidence interval of the estimate (& f 2 se where se denotes the standard error of the estimate), lies below zero. In these cases, the ratio #?2/@1 is almost meaningless, yet it is evident that product layer diffusion is controlling the reaction. The finding that the reaction is beat modeled by a single controlling mechanism Lp(X)] provides reason for less concern about our initial assumption of a first-order reaction. This model is less sensitive to error in n in this case (Szekely et al., 1976). Temperature. The b2 coefficients shown in Table I were used to determine the values of De through (16) and (18). The temperature sensitivity of De was assumed to form an Arrhenius-type relationship:
and is plotted on Figure 7. From this figure, E, is determined to be 28.1 kJ/mol (6.7 kcal/mol), indicating a process that is relatively insensitive to temperature. A value of this magnitude is indicative of a procegs controlled
2442 Ind. Eng. Chem. Res., Vol. 31, No. 11, 1992 ,21,
1OPOpprn
17150 2 5 p O
50,OO 75,OO
,
, ,55
719
20 T / g
10 y / g
40 T / g
HCI CONCENTRATION, In[HCII (ppml
Figure 8. Estimation of apparent reaction order with k, (1/b2) values from Figure 4 (5 m2/g CaO, 275 "C).
by diffusion of gas and is consistent with results found by Walters and Daoudi (1987) of 20.9 kJ/mol (5 kcal/mol). Reactions controlled by true, ionic diffusion through the product layer or the rate of chemical reaction would be much more temperature sensitive resulting in higher values of E, (Jost, 1960). Flow Rate. The flow rate effects observed in Figure 3 were analyzed for their effect upon determining an E, independent of HC1 transport or depletion limitations. At 350 OC the reaction rate is more likely to be limited by these means than at the lower reaction temperatures. If the activation energy is artificially low as a result of insufficient supply of HC1, then experiments at increasing flow rate should significantly increase the E, value. The rate constanta determined from model analysis of Figure 3 are plotted in Figure 7. The rate constant at infinite gas flow rate is estimated by linear extrapolation of In( vs 1
i)
e
to l / Q = 0. When this value (shown on Figure 8) is accounted for, the E, increases to only 36.1 kJ/mol (8.6 kcal/mol), indicatingthat our resulta at flow rates less than infinity (40 L/min STP) are adequate to measure the temperature sensitivity of the reaction independent of film diffusion limitations. This is supported by comparison of reaction rates predicted by film-transfer limitation (4) and the maximum observed rate (from Figure 4). The observed rate is over 3 orders of magnitude lower than the rate predicted by film-transfer control. The relatively low E, is suggestive of a transport control process, and the model implicates a product layer diffusion mechanism. These interpretations would be consistent with a reaction mechanism limited by the rate of gaseous species diffusion through cracks in the product layer and the intergranular voids. HCl Concentration Effect. The value of n, or the order of the reaction with respect to HC1, is determined from the resulta of Figure 4. Values of k, are determined by fitting the product layer diffusion model to the reactivity reaulta at varying HC1 concentration, but constant T (275 OC) and S, (5 m2/g). These values are plotted against the concentration of HC1 assuming that the concentration of the reactant at the surface is proportional to a power of the bulk HC1 partial pressure: cs = IZOCHCln (21) Figure 8 shows the slope of this line (n)to be 0.97, with a correlation coefficient, r2, of 0.99, indicating that the reaction is first order with respect to bulk HC1 concentration. A value of unity is expected for a rate controlled
1.5
2
2.5
3
3.5
4
SURFACE AREA, Idsg](rn'ig) Figure 9. Estimation of surface area effect with k, (1/b2) values from Figure 5 (275 "C, 5000 ppm HCl).
by diffusion of HC1 gas and was the value assumed in the initial model. Thus it is apparent, from these results and the low apparent activation energy, that HC1 or C1- is the diffusing species that is limiting the rate of reaction. Tests with Enhanced Gas Diffusivity. Reactivity tests were conducted with helium (He) gas replacing the N2 carrier. The effect of He is to increase the bulk gas diffusivity of HC1 by a factor of 2.5 over that of the standard tests with a Nzatmosphere. Test results (not shown) indicate that the reaction rate increases in a statistically significant manner. This clearly suggests that the reaction as tested is limited by diffusion of a gaseous reactant. Initial Surface Area and Particle Size Effects. The effect of initial surface area upon the reaction rate is determined from the results of Figure 5. Values of k, are determined by fitting the product layer diffusion model to the reactivity results at varying surface area, but constant temperature (275 "C) and particle size (-3-4 wm, by Sedigraph). The results, shown in Figure 9, indicate a 0.47 order dependency (r2= 0.91). This order value is inconsistent with a value of 2 expected for reactions controlled by product layer diffusion [see (16) and (1811. It is also apparently inconsistentwith results of Weinell et al. (1992), who found very little surface area effect. This suggests that the higher surface areas [smaller grain sizes, see (l8)] are not as effective as anticipated by the model, suggesting that the HCl reactant is unable to freely diffuse through the particles' intergranular structure and react with the increased surface area. Also, the sorbents at s, > 5 m2/g are likely to be more sensitive to flow rate than the previously mentioned analysis would indicate. Decreased R, (increased S,) is more likely to be limited by intergranular transport, resulting in diffusion limitations that obscure the true surface area effect. Calculations based upon ( 5 ) with the maximum observed reaction rate (from Figure 3),a particle size of 0.5 pm based upon subsequent SEM evidence, and an effective diffusivity that accounts for bulk, Knudsen, and tortuosity/ porosity effects, resulta in a value that fits within the criterion (1)from (5), indicating pore diffusion limitations.
Ind. Eng. Chem. Res., Vol. 31, No. 11, 1992 2443 13.51,
:
3:"
28
41
59
I
111111111
l6lo
05
1
15
2
25
3
35
4
n
PARTICLE SIZE, lnlQ,121 Ipml
Figure 10. Estimation of particle size effect with k, (1/8.j values from Figure 6 ( 5 mz/g, 275 "C,5ooo ppm HCI).
Figure 12. Photomicrograph of the baseline test sorbent. 5 m2/g CaO, after reaction test conditions of 500 "C and 5 8 in the absence of HCI.
r--
.
-
,
r
..'1.
.
).
Figure 11. Photomicrograph of the baseline test sorbent. 5 mz/g
cso.
Figure 13. Photomicrograph of the baseline test sorbent, 5 mz/g CaO, after reaction test conditions of 500 "C, 5 s, 5000 ppm HCI.
This is consistent with the experimental reaults with lager particles and surface areas. Analysis of the p2 values from Figure 6 indicate a power dependenq of reciprocal reaction rate upon particle radius, R (or D,/2), of 0.48, as shown by the slope in Figure 10. This subunity value is inconsistent with values of 1-2 anticipated for expressions of the controlliig mechanism of the shrinking core model (Levenspiel, 1972). However, closer examination of the sedimentation results from which the D, values were obtained indicates considerable departure from a unimodal particle size. The larger D , particle sizes (-30,40, and 60 pm) show at least 20% of the sorbent mass within particles smaller than 10 pm. This difficulty in producing (or measuring) monosized sorbent cuts makes deterministic conclusions regarding the size effect tenuous, at best. In addition, since specific surfae area is the determinant [(ls)] for grain size, all of the particle size classes have approximately the same grain size of -0.2 pm (they are all -5 m2/g). Thus, each size cut should have the same reactivity, wuming sufficientlyfast diffusion to the grains. The fact that larger particle size cuts (larger grain agglomerates) had lower reactivity likely reflects diffusion limitations resulting in an intraparticle HC1 gradient. Subsequent evidence in the following section is used to discern the validity of our model wumptions and to more adequately discern the physical picture of the sorbents. SEM/XDS. SEM analyses of the baseline sorbent (5 m2/g CaO) in Figure 11 show that the sorbent appears as
agglomerated submicrometer particles of random, yet generally spherical, shape typical of most CaO. This ib consistent with grain size calculations of -0.2 pm. The discrete grain sizes are much smaller than expected from gravity sedimentation results of D, = 3.7-4.6 pm. Analyses (Figure 12) of the same sorbent exposed to high-temperature, nonreactive conditions (500 "C, 5 8, no HC1) show very little change in the sorbent morphology. This suggests that any apparent morphological change is due solely to reaction of CaO and HCl, and not due to thermal sintering. Analysis of a reacted particle [500OC, 5 s, 5OOO ppm HC1 (X> 25%)]in Figure 13 shows two distinct morphologies. The bottom of the particle appears crystalline, while the top retains the essential features of the unreacted sorbent. Examination with higher magnification shows that the more crystalline area at the bottom (Figure 14) has completely lost its granular, distinct particles. Although the change is less dramatic for the top portion (Figure 15). it is clear that the effect of reaction has been to increase the grain size and develop necldng between neighboring grains. These two latter fields were examined by XDS for their Cl/Ca atomic ratio, reflecting the amount of reaction. The more crystalline structure on the bottom (Figure 14) has a Cl/Ca ratio of 0.77 (-38% conversion to CaC1,) while the more granular field in the top (Figure 15) has a Cl/Ca ratio of 0.24 (=l2% conversion). The higher degree of reaction is clearly associated with the more crystalline structure. The discrete, granular sorbent particles coalesx
2444 Ind.
Eng. Chem. Rea., Vol. 31,No. 11.1992
r
-
e .
z
=ware 14. Cloee-up photomicrograph of the bottom of the samp
in Figure 13.
to discern kinetic parameters. XRD Results. An analysis of reacted sorbent was conducted to determine the compounds formed. Experimental conditions (500OC, 30 s,5OOO ppm HCI) were set to provide a Sample with a high percentage of reaction in a suffkiently short time as to prevent heat damage to the Macor parts of the reactor. A very complex XRD spectra resulted in products by approximate weight percentage: unreact&dCaO, 28; calcium chloride dihydrate, CaC1f2H20 (or sinjarite, JCPDS No. 1-989),57%; calcium chloride hydroxide, CaClOH (JCPDS No. 36-983), 10%; and a minor amount of calcium hypochlorite, Ca(CIO), (JCPDS No. 4&745), 5%. While quantitation of composition with XRD data is tenuous, these results suggest an overall conversion of X = 56%, given the assumption that the reaction product is composed of Ca and C1 in a 1/2 ratio. The XRD product distribution is supported by AA and IC analysis of the sample which showed a value of X = 58%. A value of X > 50% indicates that the reaction products must contain a compound with more than one CI atom bound to Ca, a fmding consistent with the major presence of CaCI&2H20.The hydrated form of CaC12may be due to reaction with ambient air humidity of this highly deliquescent ,sample during interlaboratory transfer. However, reaction with chemically formed H 2 0 is also a possibility. The presence of multiple prcducta identified by XRD, the widely varying Cl/Ca ratios found on single particles by EDX, and the differences in morphological appearance discovered by SEM exmaination suggest that (3)representa only an overall formula; reaction probably proceeds with a variety of intermediates and end products, Bome of which are identified above. Some proposed reactions include CaO + HCI CaClOH CaClOH + HCI CaCI2-Hz0 CaC12*H20 CaCI, + H20 CaC12.Hz0 + O2 Ca(C1O)z + HzO (22) Reaction Model Results. All of the temperature, HCI concentration, and grain size (surface area) results are normalized with
--
Figure 15. Close-up photomicrograph of the top of the sample in Figure 13.
and form more highly ordered, crystalline structures as a result of reaction with HCI. These results show that, as reaction progreeses (Cl/Ca increases), considerable morphological changes occur. Reaction to the more voluminous product (Z = 3.05)results in necking and coalescence of sorbent grains, drast i d y increasing the granular size and decreasing the initial value of surface area The physical observations emphasize the approximate nature of the model and the theoretidy predictable relationships derived from measurements of initial, unreacted surface areas and particle sizes (the latter for which SEM observations refute the values determined by gravity sedimentation). The widely disparate Cl/Ca ratios observed in the SEM figures suggest considerable inhomogeneity of reaction within single particles. This may be due to localized impurities in the sorbent catalyzing reaction with HC1 or preferential diffusion due to crystallographic orientation (Jost, 1960). Despite experimental differences and likely variations in analytical procedures, the variant CI/Ca raticm found here seem consistent with results of Weinell et al. (1992),who found Cl/Ca varying by greater than a factor of 2 a c r m a reacted particle diameter. The reacted solid analyses by IC and AA are thus only composite values. These findings are inconsistent with our assumption of uniform reaction through the sample given sufficiently small particles and sample size and differential HCI conditions. It also emphasizes the approximate nature of the theoretical model since composite conversions were used
--
by plotting the lefthand side against 1/T. The resulting rate exmession is
with units as the Nomenclature section. Quation 24 wm applied to Linwood Ca(OH), reactivity data collected from a pilot-de, natural gas furnace doped with HC1 (Figure 16). Sorbent was injected at 978 'C. The temperature gradient of the furnace (-3% 'C/S) waa iteratively modeled in 19.timestepsover the 0.93-sresidence time, with updatea of k, and CHa at each step. The model predictions assume that the Ca(OH), converts upon injection to CaO, r e a c t i i at a constant S The results sKowing the effect of Ca/C1 stoichiometric ratio upon HCI capture indicate that the pilot-sde data are aptly predicted by the model with a sorbent of S, between 15 and 20 m2/g. These values are consistent with surface areas of about 11-21 m2/g determined by Borbent sampling from another p i l o t - d e furnace operating with similar gases and injection (970 "C) conditions (Newton et al., 1988).
Ind. Eng. Chem. Res., Vol. 31,No. 11, 1992 2445
I
, 0 0
- -, 1
1 5 m2/g ------.
, 1
,
20 m2/g
2
3
4
Ca/CI RATIO
Figure 16. Comparison of kinetic model results [(lo) and (2411 at 5, 15, and 20 m2/g with pilotscale HC1-capture results [600ppm
HC1,978"C injection temperature, -326 OC/s quench rate, 0.93-8 reaction time, Linwood Ca(OH)zl. Clearly, numerous assumptions are made with this comparison. Specifically, the use of a controlling mechanism model determined at 150-350 OC, extrapolation of the kinetic constants up to the injection temperature of 978 OC, the aeeumption of a product formed with a Ca/C1 ratio of 1/2,and the approximation by a constant surface area all warrant further study. Nonetheless, the model comparison does provide a preliminary verification of the kinetics determined herein with high-temperature furnace results. Conclusions A study of the short-time (0.2-1 8) reaction kinetics of CaO with HC1 from 150 to 350 OC under conditions minimizing bulk transport and pore diffusion limitations suggesta that the reaction is controlled by diffusion of gaseous HC1 through the product layer. This conclusion is reached, in part, from analysis of the X-t data with a mixed chemical reaction and product layer diffusion control model. An activation energy of 28.1 kJ/mol (6.7 kcal/mol) and a fmt-order HC1 concentration dependency govern this reaction. The reaction is unaffected by the presence of 5 % 02,yet is significantly increased when He replaces N2 as the process gas carrier, thereby increasing the diffusivity of HC1. While both specific surface area and particle size have only a 0.5-order effect upon reactivity, the msaningfulnese of the former may be limited by experimental constraints and the latter is subject to question due to considerable doubt of the "effective" particle size. Indeed, SEM evidence indicates that all of the varied particle size c u b are simply agglomerates of submicrometer grains. SEM/XDS shows that the particles' grains react nonuniformly, resulting in considerable variation in Ca/Cl ratios across the particle surface. This is supportive of XRD evidence which shows multiple reaction products with different Ca/Cl ratios. Extrapolation of the kinetic model for a comparison with high-temperature furnace results of HCl removal by Ca(OHI2injection provides a p r e l i m i i verification of the parameters determined herein. Further work is suggestad on reactivity at higher temperatures, identification of the reaction products, and the effect of surface area variation during reaction. Acknowledgment The authors wish to thank Wojciech Kozlowski (Acurex Environmental Corp.) for reactor operation/analyses.
Special thanks to Frank E. Briden, Robert H. Borgwardt, and George R. Gillis (all of the U.S.EPA,AEERL)for XRD analyses, advice/commenta, and equipment construction/maintenance support, respectively. Nomenclature b = stoichiometric ratio of solid to gaseous reactanta, l/z (unitless) CHCl= bulk HCl concentration (ppm) C, = concentration of reactant C1 species at the surface (ppm) D m = molecular gas diffusivity (cm2/s) De = effective product layer diffusivity (cm2/s) E, = apparent activation energy (kcal/mol) g ( X ) = conversion function under chemical control (unitleas) k, = chemical reaction rate constant at surface (8-9 ko constant in (21) k , = product layer diffusion rate constant at surface ( 8 9 k, = intrinsic reaction rate constant, n = 1 (cm/s) m = reaction order with respect to grain size (unitless) Mcao = molecular weight of reactant, CaO (g/mol) n = reaction order with reapect to HCl concentration (unitleas) N = number of data points p ( X ) = conversion function under product layer diffusion control (unitless) Q = gas flow rate through reador (L/min STP) Yi = universal gas constant (1.987cal/(mol.K)) P = correlation coefficient (unitless) (-rA"')obs = observed rate of reaction (mol/cm3.s) R, = grain radius (cm) R = radius of particle (pm) Re = Reynolds number (unitless) Sc = Schmidt number (unitless) se = standard error (%) S = specific surface area (m2/g) Sk = S h e r w d number (unitless) SSE = sum of the squares of the errors (%'02) t = time (8) T = temperature ("C or K) X = conversion (%) 2 = molar volume ratio of product to readant, 3.05 (unitless) G;eek Letters a = Arrhenius preexponential constant, (19) pi = coefficients in the reaction model, defied by (15)(i = 1) and (16)(i = 2) peso = CaO reactant density (3.32 g/cm3) u2 = ratio of diffusional to chemical reaction resistance (unitless) T, = characteristic time equal to the time for complete conversion in the absence of diffusional resistance (8) T,, = characteristic time equal to the time for complete conversion in the absence of chemical reaction resistance (8) Registry No. CaO, 1305-78-8; HC1, 7647-01-0. Literature Cited Borgwardt, R. H. Calcium Oxide Sintering in Atmospheres Containing Water and Carbon Dioxide. Znd. Eng. Chem. Res. 1989, 28,493. Borgwdt, R. H.;Bruce, K.R. Effect of Specific Surface Area on the Reactivity of CaO with SO2. AIChE J. 1986,32 (2), 239. Buekens, A.; de Nijs, W.; Jacob, L.; Maniatis, K.; Schoeters, J. Measurement of the Performance of Dry Removal Systems for Gaseous Pollutanta on a Municipal Refuse Incinerator. In Proceedings: Eur. Abwasser-Abfallsymp., 6th, Gee. Foerd. Abwassertech., Vrije UNv. Brussel, B-1050: Brussels, Belgium, 19&1; pp 659-668. Ellison, W. Utilization of Flue-Gas Cleaning Technologies for Waste Incinerators in Europe. In Proceedings: 1989 Znternutional Conference on Municipal Waste Combustion, Volume 3; EPA600/R-92-052~;pp 7C-43 to 7C-61. GopaMu&mn, R; Seehra, M. S. Kinetics of the High-Temperature Reaction of SOz with CaO Particles Using Gas-Phase Fourier Transform Infrared Spectroscopy. Energy Fuels 1990, 4, 226.
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Ind. Eng. Chem. Res. 1992,31, 2446-2450
Gullett, B. K. Reduction of Chlorinated Organics in the Incineration of Wastes. U.S. Patent 5,021,229, June 4, 1991. Gullett, B. K.; Bruce, K. R.; Beach, L. 0. Formation Mechanisms of Chlorinated Organics and Impacts of Sorbent Injection. In Proceedings: 1989International Conference on Municipal Waste Combustion, Volume 3; EPA-600/R-92-052~;pp 8C-1 to 8C-25. Gullett, B. K.; Bruce, K. R.; Machilek, R. M. Apparatus for Short Time Measurements in a Fixed-Bed Gas/Solid Reactor. Rev. Sci. Instrum. 1990,61,904. Jost, W. Diffusionin Solids, Liquids, and Gases; Academic Press: New York, 1960; pp 285-323, 366-367. Karlsson, H. G.; Klingspor, J.; Bjerle, I. Adsorption of Hydrochloric Acid on Solid Slaked Lime for Flue Gas Clean Up. J. Air Pollut. Control Assoc. 1981, 31, 1177. Levenspiel, 0. Chemical Reaction Engineering, 2nd ed.; Wiley: New York, 1972; pp 357-408. Mayer-Schwinning,G.; Laibold, E. Basic Processes for Cleaning Flue Gases from Waste Incineration Plants. In Proceedings: 1989 International Conference on Municipal Waste Combustion, Volume 3; EPA-600/R-92-052~;pp 7C-1 to 7C-18. Newton, G. H.; Moyeda, D. K.; Kindt, G.; McCarthy, J. M.; Chen, S. L.; Cole, J. A.; Kramlich, J. C. ‘Fundamental Studies of Dry Injection of Calcium-Based Sorbents for SO2 Control in Utility Boilers”: EPA-600/2-88-069 (NTIS PB89-134142); December 1988; pp 1-6. Ranz, W. E.; Marshall, W. R. Evaporation from Drops. Chem. Eng. Pro#. 1952, 48 (31, 141-146. Schmd, D.; Verbeek, A,; van der Harst, C. Dry Techniques for Abatement of Acid Emissions in Flue Gases. Proceedings of the 8th World Clean Air Congress 1989, The Hague, The Netherlands,
Sept 11-15,1989. Published in Man and His Ecosystem; Brasser, L. J., Mulder, W. C., Eds.; Elsevier: Amsterdam, 1989; Vol. 4, pp 213-218. Szekely, J.; Evans, J. W.; Sohn,H. Y. Gas-Solid Reactions; Academic Press: New York, 1976; pp 65-107. Torres-Ordoiiez, R. J.; Longwell, J. P.; Sarofim, A. F. The Intrinsic Kinetics of CaS(s) Oxidation. Energy Fuels 1989, 3 (4), 506. Verbeek, A.; Schmal, D.; van der Harst, C. Abatement of HC1 and HF Emissions From Waste Incinerators by Injection of Hydrated Lime. Proceedings of the Second European Conference on Environmental Technology, Amsterdam, June 22-26,1987; Nijhoff: Amsterdam, 1987; pp 157-164. Walters, J. K.; Daoudi, M. The Removal of Hydrogen Chloride From Hot Gases Using Calcined Limestone. In Management of Hazardous and Toxic Wastes in the Process Industries; Kolaczkowski, S . T., Crittenden, B. D., Eds.; Elsevier: London, 1987; pp 574-583. Weinell, C. E.; Jensen, P. I.; Dam-Johansen, K.; Livbjerg, H. Hydrogen Chloride Reaction with Lime and Limestone: Kinetics and Sorption Capacity. Ind. Eng. Chem. Res. 1992,31, 164. Wen, C. Y. Noncatalytic Heterogeneous Solid Fluid Reaction Models. Ind. Eng. Chem. 1968, 60 (9), 34. White, D. M.; Vancil, M. A. Review of Dry Injection Technology for Reducing Emissions from Municipal Waste Combustors. In Proceedings: 1989International Conference on Municipal Waste Combustion, Volume 4 ; EPA-600/R-92-052& pp 1OC-31 to 1OC45.
Received for review April 27, 1992 Accepted August 10, 1992
Hydroformylation of Propylene Using Unmodified Cobalt Carbonyl Catalyst: Selectivity Studies Raghuraj V. Gholap, Oemer M. Kut, and John R. Bourne* Technisch Chemisches Laboratorium, ETH-Z, CH-8092 Zurich, Switzerland
Isomer distribution and kinetics of the formation of n-butyraldehyde and isobutyraldehyde have been determined for the hydroformylation of propylene using unmodified cobalt carbonyl catalyst. Effects of the propylene and catalyst concentrations and the partial pressures of carbon monoxide and hydrogen on the n/iso ratio as well as on the individual reaction rates have been measured in a temperature range of 383-423 K and a pressure range of 35-100 bar. An empirical rate model describing the intrinsic kinetics for each butyraldehyde has been proposed and ita kinetic parameters evaluated. Such information is useful for the preferential production of n-butyraldehyde.
Introduction It is known that hydroformylation of propylene, in the presence of cobalt carbonyl catalyst, gives a mixture of two isomeric aldehydes namely, isobutyraldehyde and nbutyraldehyde, although, under certain conditions, small amounts of side products are formed. The problem of directing this reaction to give preferential formation of n-butyraldehyde, o w i y to ita greater practical importance, has mainly been mentioned in the patent literature. Little has been reported on how to influence the product distribution of isomeric butyraldehydes (Falbe, 1973; Pino et al., 1977; Pruett, 1979; Cornils, 1980; Weissermel and Arpe, 1988; Piacenti et al., 1991). From the literature data, variations in the ratio of nbutyraldehyde to isobutyraldehydes from 1.6 to 4.4 were observed under widely different conditions of temperature, catalyst concentration, and partial pressures of carbon monoxide and hydrogen. The most influential parameter was found to be the partial pressure of carbon monoxide. The partial pressure of hydrogen has a small but repro-
* Author to whom correspondence should be addressed.
ducible effect on the product distribution. Temperature has a strong influence on the n/iso ratio, and conflicting results have been reported for the effect of catalyst concentration. Despite these invest,igations of the product distributiori in the hydroformylation of propylene, the factors influencingthe n-butyraldehydelisobutyraldehyde ratio are still rather obscure (Piacenti et al., 1991). Also, detailed kinetics of these individual reactions is lacking. Therefore, as a first step toward understanding the more complex problem of isomer distribution, the overall kinetics of the hydroformylation of propylene using unmodified cobalt carbonyl catalyst was investigated and modeled (Gholap et al., 1992). In the present work, the objectives were to investigate the effect of proceas variables on the isomer distribution during the hydroformylation of propylene using Co2(CO)8as a catalyst precursor and to determine the individual kinetics of the formation of n-butyraldehyde and isobutyraldehyde. For this purpose, experiments were carried out under different operating variables with unmodified cobalt carbonyl catalyst. The variables included temperature, propylene and catalyst concentrations, and partial pressures of carbon monoxide and hydrogen. On the basis of these
0888-5885/92/ 2631-2446$03.O0/0 0 1992 American Chemical Society
