Ind. Eng. C h e m . Res. 1987,26, 2069-2075 perature dynamic flow of combustion gases in a rotary kiln geometry and how it affects carbon burnout. This lead to the better control of the flow conditions in a commercial kiln for producing calcium carbide. Actual operation of this process in a fuel-fired rotary kiln pose refractory problems because Of the high temperatures required. Also, it is believed that even if this process is feasible, it will not be suitable for making shippable carbide such as is made by the electric furnace. Rather, it would only be suitable for low-quality carbide useful for on-site generation of acetylene.
2069
Literature Cited Brookes, C. Ph.D. Thesis, University of Waterloo, Waterloo, Ontario, Canada, 1972. Brookes, C.; Gall, C. E.; Hudgins, R. R. Can. J . Chem. Eng. 1975, Ershov, 53(5),v, 527. A. Ref. Zh. Khim. 1970, 4Ls8. Mukaibo, T.; Yamanaka, y. Kogyo Kagaku Zasshi 1953, 56, 73, Shih, c. c. J.; Mu, J.; D.; Sack, s. AIChE National Meeting a t Los Angeles, 1984. Tagawa, H.; Sugawara, H. Bull. Chem. SOC.Jpn. 1962, 35, 1276.
Received for review August 25, 1986 Revised manuscript received June 15, 1987 Accepted July 6, 1987
Registry No. CaC2, 75-20-7; C, 7440-440; Ca(OH)2, 1305-62-0.
Thermochemical Process of Producing Chlorine and Potassium Hydroxide from Potassium Chloride Norio Takeuchi Department of Energy Chemistry, National Chemical Laboratory for Industry, Tsukuba Research Center, Yatabe, Zbaraki 305, Japan
A new thermochemical process of producing chlorine and potassium hydroxide from potassium chloride has been devised. This process is composed of the five thermochemical reactions KC1 4/3HN03 KNOB 1/3NOCl + '/3Clz 2/3H20 (at 140 "C), l/,NOCl+ '/3HN03 NOz '/&lZ '/3HzO (at 80-100 "C), KNOB+ '/2FezO3 KFeOz + NO2 + 1/402 (at 850-900 "C), KFeOz + /2H2O KOH + '/2Fe203 (at 70 "C), 2N02 + '/z02HzO 2HN03 (at 25 "C), which sequence leads to the overall reaction KC1 ' / d o 2 1/zH20 KOH '/2C1z. Each reaction was experimentally investigated and verified about characteristics, operating conditions, and so forth. The material and heat flow sheet of the process was determined, and the heat requirement was estimated. If substantial thermal regeneration is included in the process, the thermochemical process of this work might become economically and energetically competitive with current electrolytic processes of producing chlorine and potassium hydroxide.
-
-
+
-
+
+
No process other than electrolytic has produced both chlorine and potassium hydroxide from potassium chloride. This work aims to devise a new thermochemical process to be less energy consuming than current electrolytic processes. There is a thermochemical process of producing chlorine and potassium nitrate from potassium chloride (Versar, Inc., 1979). Two facilities in the United States put this process into operation. Of those two, the Vicksburg Chemical plant is by far the larger. The second facility, operated by Mallinckrodt Chemical Company, produces only a small amount of reagent grade material. Therefore, the process is generally called the Vicksburg process. In the process, 64 wt % nitric acid is first made to react with potassium chloride according to reaction 1. This initial reaction is generally KC1 + 4/3HN03 KN03 + l/,NOCl + 1/3C12+ 2/3H20(1)
-
conducted below 10 "C to avoid corrosion problems involved in handling hot nitric acid-chloride based mixtures. The liberated gases, NOCl and C12,from the reaction are passed through a gas column reactor where they are brought into contact with nitric acid vapors at elevated temperatures to oxidize NOCl according to reaction 2. l/,NOCl + 2/3HN03 NOz + 1/6Clz+ 1/3H20 (2)
-
The Vicksburg process is evaluated to be technically viable for chlorine production. However, limited markets for the
potassium nitrate coproduct of the process make it unlikely 0888-5885/87/ 2626-2069$0l.50/0
+
-
-+ -+
+
+
that additional plants of this type will be constructed in the foreseeable future (Versar, Inc., 1979). If the potassium nitrate coproduct of the Vicksburg process is converted to potassium hydroxide and nitric acid for reaction 1, a new process may be devised for producing chlorine and potassium hydroxide from potassium chloride. Proposed Process. A promising process of producing C12and KOH was developed in this work. This process has been constructed by combining those reactions shown by reactions 1-5. Reactions 3 and 4 are available reactions
-
KC1 + 4/3HN03 KNOB+ l/,NOCl
+ 1/3C12+ 2/3Hz0 (1) l/,NOCl + 2/3HN03 NOz + 1/6Clz+ 1/3H20 (2) KNOB+ '/2FezO3 KFe02 + NO2 + (3) KFe02 + 1/2H20 KOH + '/zFezO3 (4) 2N02 + 1/202 + H 2 0 2HN03 (5)
-
-
-
found for this work. The reaction sequence leads to the overall reaction shown by reaction 6. Reaction 5 is
-
KC1 + 1/402 + 1/zH20
KOH
+ l/&12
(6)
well-known and presently used in the production of nitric acid. NOz is more efficiently absorbed by water at reduced temperature (Burdick and Freed, 1921; Spealman, 1965). In the current nitric acid producing process, absorption of NOz takes place at or above atmospheric pressure. The 1987 American Chemical Society
2070 Ind. Eng. Chem. Res., Vol. 26, No. 10, 1987
Ij R-q ~
CONDENSER
140
6 021
'C
v
130 "C
Figure 1. Apparatus for reaction 1.
pressurized absorption process of NOz is superior, both in absorption efficiency (98-99 % ) and in the concentration of the final nitric acid, to the atmospheric pressure process (Strelzoff, 1956). As already stated, reaction 1is conducted below 10 "C to avoid the corrosion problems which are encountered in the Vicksburg process. In the proposed process of this work, however, reaction 1 is preferably conducted at about 140 "C. Several kinds of engineering plastics for heat and chemical resistances are receiving practical application and are assumed to be used for reaction 1 in this proposed process. Reaction Characteristics and Experimental Verification Reaction 1. Reaction yields were determined as a function of time at various temperatures and aqueous concentrations of HNO, for the purpose of determining operating conditions. With the apparatus shown in Figure 1, warm 58-67 wt % HN03 aqueous solutions were added to solid KC1 in a glass reactor at a prescribed temperature. The generated C12and NOCl gases were absorbed into 500 mL of 0.26 mol/L NaOH aqueous solution. The amount of residual NaOH after absorption of the generated gases was determined by neutralization. Amounts of generated C1, and NOCl were calculated from the amount of the residual NaOH in the solution. The results are shown in Figure 2. Reaction yield becomes constant after reaching the maximum at about 20 min at every experimental temperature and concentration of HN03 Below 120 OC, reaction yield does not reach 100% even in 67 wt % HNO,. On the other hand, at 130 "C, reaction yield reaches almost 100% in 67 wt % HNO,. At 140 "C, the yield reaches almost 100% even in 64 wt % HNO,. Therefore, the advisable operating conditions for reaction 1 are 130-140 "C temperature and 64-67 wt % HN03 aqueous solution. Reaction 2. Generated gases from reaction 1 were passed through 58-67 wt % HNO, solutions in a glass tube as shown in Figure 3. The inlet of the glass tube in Figure 3 was connected by a glass connector with the outlet of a condenser attached to the reactor for reaction 1. The outlet of the glass tube was connected with an absorption bottle containing NaOH solution. NOCl gas reacted with HNO, in the glass tube. In order to generate NOC1, reaction 1was conducted at 140 "C for about 35 min in 64 wt % HNO,. O2gas was streamed at 200 mL/min to carry the generated gases from reaction 1 into HNO, in the glass tube. There was no specific reason why O2gas was chosen as a sweep gas in this experiment. The gases like air or nitrogen could be used instead of O2gas. Conversion yield from NOCl to C12is calculated according to eq 7. Since conversion yield of NOCl (mol %) = 100[(3[C1-1/2([OH-I,- [OH-IJ) - 21 (7)
0 6 lXHNO3
100
* ' 0°C
0
T i l30il 0 '0 20 40 TIME
(
MIN
>
Figure 2. Reaction yields of reaction 1. OUTLET
CONDENSER
GLASS TUBE
Figure 3. Reaction tube of glass for reaction 2.
each component gas of reaction 2 reacts with NaOH according to reactions 8-10, the amounts of both chloride ion and hydroxide ion are determined. NOCl + 2NaOH NaNOz + NaCl + HzO (8) Clz + 2NaOH NOz + NaOH
-
-
+ NaClO + HzO NaNO, + 1/zH20+ NaCl
(9) (10)
Figure 4 shows the effect of O2flow rate on the conversion yield. The conversion yield became constant after reaching the maximum at the Oz flow rate of 150 mL/min. Thus, O2 flow was set at a rate of 200 mL/min for determining the effects of both temperature and concentration of HNO, on the conversion yield. Figure 5 shows the effect of HN03 concentration on the conversion vield. HNO, solutions were keDt at 100 "C
Ind. Eng. Chem. Res., Vol. 26, No. 10, 1987 2071
w 0
601
' ' '
50l 0
50
0 2
'
I
'
'
I
150
100
I
200
250
FLOW R A T E ( m i / M l N )
Figure 4. Effect of O2 stream on the conversion yields from NOCl to C12 a t 100 "C in reaction 2.
Figure 7. Thermal analysis for reaction 3: DTA c w e s for (1)MgO, (2) KNOB+ MgO, (3) KN03/Fe203= 2.5, (4)KN03/Fe203= 2.0, (5) KN03/Fe203 = 1.5, and (6) KN03/Fe203 = 1.0 molar ratio at a heating rate of 5 OC/min in a stream of N2 gas. REAC-ICN TEMP ("C)
KNOa/FezCa
900 L
W
L
I
I
50 m t
900
0
-
0
57
59
61
63
65
9co
HN03 ( WTX )
Figure 5. Concentration effect of HNOBaqueous solution on the conversion yields from NOCl to Clz at 100 OC at a 02 flowrate of 200 mL/min in reaction 2. 100
,
900
I
9c0
A 850
'
A
A
2 0
A
8C0
0 20
40
60 TEMP. (
80
100
Figure 8. X-ray analysis of the solid products in reaction 3: (A) KFe02, (B)KFel1017, (C) K2Fe04,and (D)Fe203.
"C )
Figure 6. Temperature effect on the conversion yields from NOCl to C12 in 61 and 65 w t % HN03 aqueous solutions at a O2 flow rate of 200 mL/min in reaction 2.
during the experiment. NOCl is converted to C12at almost 100% in 66 wt % HNO,, while only 41% of it is converted in 57 w t % HNO,. Figure 6 shows the temperature effect on the conversion yield. The conversion yield became constant after reaching the maximum at 80 "C in every HNO, solution. From these results, the advisable operating conditions for reaction 2 are found to be 80 "C temperature and more than 66 wt % HN03. However, the conversion yield must depend on the contacting period of NOCl with HN03. If the reactor for reaction 2 is designed to enable NOCl to contact HNO, for a long period, the conversion yield may be 100% even under the experimental conditions of this work so that the conversion yield did not reach 100%. Reaction 3 (Thermal Analysis). Mixtures of KNOB + xFe2O3 (x = 0-1.0) were analyzed by differential thermal analysis (DTA) from 25 to 1000 "C in a stream of N2 gas at a heating rate of 300 "C/h. All mixtures used for the
thermal analysis were 100 mg of powder. The powdered Fe203used was identified with hematite by its X-ray diffraction pattern, and its particle size was 75-105 pm in diameter for 69 w t 70and less than 46 pm in diameter for the rest. Figure 7 shows the results. The endothermic peaks at 337 "C on the curves of 2-6 in Figure 7 correspond to the fusion of KN03, and an endothermic peak at about 380 "C on curve 1corresponds to dehydration from the MgO sample. Curves 2-6 show a gradual endothermic peak above 450 "C; this shifts to higher temperatures with increasing content of KNO, in the mixtures. The peaks for the mixtures at 650-800 "C indicate the end of reaction more clearly than the peak for pure KNO,. Both pure KNO, and the mixtures, however, have similar patterns of DTA curves. Therefore, it is estimated from the thermal analysis that reaction 3 is essentially a decomposition of KNO, in conjunction with Fe203. Reaction 3 (X-ray Analysis). In order to grasp the outline of reaction 3, an X-ray analysis was conducted on the solid phase. Mixed samples of KNOBand Fe203with various molar ratios (KN03/Fe203= 0.25-2.0) were heated
2072 Ind. Eng. Chem. Res., Vol. 26, No. 10, 1987
I
b--
ELECTRIC FURNACE 01
-
60-
i-
z 0 40L
J
> z
nn
L v -
3 0
M I X T U R E O F K X O 3 &Fe,O, Y
Figure 9. Apparatus for reaction 3.
h32
0
I'FeOz
I
-
C
o
C.5
K'vC3/Fez03
10
15
20
MOLAR 'IATI;
Figure 11. Effect of KN03/Fe203(molar ratio) on the conversion yields from KN03 to NO2 and KFe02 a t 850 "C for 30 min a t a 0, flow rate of 200 mL/min in reaction 3.
is constant at O2 flow rates of more than 100 mL/min, whereas that on the basis of solid product is almost constant a t every O2 flow rate. The yield on the basis of generated NO2 is higher than that on the basis of solid product at every O2 flow rate. These results may show that solid byproducts formed with KFe02 are hardly hydrolyzed. 0 ---I ____Figure 11 shows the conversion yields as a function of 2cc 3c3 U S C 5c'! molar ratio KN0,/Fe20,. The conversion yields in figure 3; L - O h RAT: TT Vh, 11 were determined on the bases of gaseous and solid products of reaction 3 conducted at 850 "C for 30 min at Figure 10. Effect of O 2 flow rate on the conversion yields from a O2 flow rate of 200 mL/min. As is obvious from Figure KNOBto NO2 and KFeO, a t 850 "C for 30 min a t KN03/Fe203= 1.0 (molar ratio) in reaction 3. 11, both conversion yields on the bases of generated NO2 and solid product are almost identical at a molar ratio KN03/Fe203= 2.0. It is already known from the results in a platinum boat in atmosphere of air a t 800, 850, and of X-ray analysis that only KFe02is formed at 850 "C from 900 "C. Each of the solid residues was examined by X-ray a mixture of KNO, and Fe203with molar ratio KNOB/ analysis. Fe203= 2.0 and that products different from KFeO, are Figure 8 shows the results. The solid residues obtained formed with KFeOz a t 850 "C at other molar ratios of from a mixture with molar ratio KN03/Fe203= 2.0 at 850 KN03/Fe203except 2.0. Therefore, the results in Figure and 900 "C show only the X-ray pattern of KFe02. The 11show that KFe02 is hydrolyzed almost completely while solid residues obtained from the mixtures with molar ratios the other solid products are hardly hydrolyzed. The best KN03/Fe203= 0.25, 0.5, 1.0, and 1.5 a t 900 "C and from KN0,/Fe20, ratio for reaction 3 is concluded to be 2.0 mol. a mixture with molar ratio KN03/Fe203= 2.0 at 800 " C On the other hand, none of the conversion yields in show the X-ray patterns of both KFe02 and KFel1017or Figures 10 and 11reach 100%. In order to find the reason both KFel1017and K,Fe04 and unchanged Fez03. KFe02 for this, potassium in the solid product from a mixture with may be regarded as the general component of the solid residues formed at 850 and 900 "C from a mixture with molar ratio KN0,/Fe203 = 2.0 was determined by flame molar ratio KN03/Fe203= 2.0. emission photometry. As a result, it was found that 83-84% of the potassium contained in the initial KNOB Reaction 3 (Conversion Yield). Conversion yields was converted to KFe02and that potassium of the residual from KNOBto KFe02 were determined on the bases of both amounts of generated gas and solid product. Acpercentage vaporized as KNOBwithout reacting with Fe203 cording to the equilibrium constant of reaction NO + 1/202 at the experimental temperatures and during elevation of temperature. If a conversion yield at a molar ratio e NOz, the species of nitrogen oxides generated in reaction KN03/Fe203= 2.0 in Figure 11 is corrected by the per3 is NO a t above 600 "C but is NO2 a t room temperature centage of KNO, vaporized without reacting with Fez03, (Takeuchi, 1985). NO2 is absorbed into NaOH aqueous solution more easily than NO. The effect of O2 flow rate it reaches almost 100%. on the conversion yield was first investigated. About 2 g The temperature effect on conversion velocity was of a mixture (KN03/Fe203.=1.0 molar ratio) in a platinum measured for a mixture with a molar ratio KN03/Fe203 boat was heated for 30 min at 850 "C in an atmosphere = 2.0 in a O2 flow rate of 200 mL/min at 800,850, and 900 of flowing 02.The flow rate of O2 gas was varied in the "C. Figure 12 shows the results. In Figure 12 the solid lines represent the curves drawn by smoothing experirange of 10-500 mL/min. The NO2 generated was absorbed a t room temperature into 450 mL of 0.26 mol/L mental plots and the broken lines represent the corrected NaOH aqueous solution with the apparatus shown in curves of experimental plots by the percentage of vaporized Figure 9. The amount of residual NaOH in aqueous KN03 in the initial amount of KNO,. Reaction is comsolution after absorption of NO2 was determined by new pleted in about 5 min a t 900 "C, in about 20 min a t 850 tralization. The solid product in the platinum boat was "C, and in about 45 min at 800 "C. Therefore, the prehydrolyzed, and the KOH formed was determined. Both ferred temperature for reaction 3 is more than 850 "C. Reaction 3 (Equilibrium Constant). As already of the amounts of absorbed NOz and KOH formed were compared with the initial amount of KNOBfor determistated, reaction NO + 1/202 G NO2 is completely displaced nation of the conversion yields. Figure 10 shows the reto the left side a t more than 600 "C but is completely sults. The conversion yield on the basis of generated NO2 displaced to the right side at room temperature. If reaction 9
n-
V Y
Ind. Eng. Chem. Res., Vol. 26, No. 10, 1987 2073 100
80
v
0 J
>
60
0
40
0 6 0 C' 0 50 C'
z ,
fx
7 0 C"
A 40
7 ,
C'
W
$ 20
A 3 0 C"
0 0
0
o
5 io
15
40
20 2 5 30 35 a 0 45 50 TIME
TIME
( MIN )
Figure 12. Temperature effect on the conversion velocity of reaction 3 a t KN03/Fe203 = 2.0 (molar ratio) a t a O2 flow rate of 200 mL/min.
50
60
( MIN )
Figure 13. Hydrolysis yields of KFeOz formed at 850 "C at KNO3/Fe2O3= 2.0 (molar ratio) in reaction 3. 50
Table I. Molal Standard Gibbs Free-Energy Changes ( P O ) of KFeOz and the Equilibrium Constants of Reactions 11 and 12 kcal/mol -196.528 -201.342 -206.157 -210.972 -215.786
ap
I 40 -
K,
P' K F ~ O ~ ( ~ ) ,
temp, K 800 900 1000 1100 1200
-
reaction 11 5.171 X 10" 3.218 X 1.453 X lo-' 7.790 X lo-' 5.024
reaction 12 1.881 X lo-' 5.528 X lo-' 2.191 1.578 1.848
1
'
0
fr 2 0 1 0
:
KNOB+ '/,Fez03
* KFeO, + NO + 3/402(11)
That is the reason why the heterogeneous temperature distribution in the system of reaction 11 has the effect which is equivalent to artificially removing the generated gases from the condensed phase. From the aforementioned point of view, reaction 3 is considered to be a kind of irreversible process comprising reaction 11 and reaction NO + '/z02 NOz. Therefore, the equilibrium constant of reaction 3 cannot be determined. On the other hand, the equilibrium constant of reaction 11 can be calculated in a homogeneous temperature system from the standard Gibbs free-energy change. Since the standard Gibbs free-energy change of KFeO, is not known,it was determined in this work by measuring the equilibrium constant of reaction 12. The equilibrium
*
+
1/2K&03 + '/zFezO3 s KFeOz l/zCOz (12) constant of reaction 1 2 is given by K,,(,?) = pcol 112. Therefore, eq 13 determines the standard Gibbs free-energy change of KFe0,. The equilibrium pressure of re-
-
p°KFeOz(s)1/2(p°K2co,(s,i)+ poFe2O3(s) - P ' C O ~ ( ~-) RT In pco,) (13) action 12, Pco2,was measured at 500-950 "C in this work. The thermodynamic values of the chemical species except KFeO, in reactions 11 and 1 2 are known (Barin and Knacke, 1973). Table I presents the standard Gibbs free-energy changes of KFeO, and the equilibrium constants of reactions 11 and 12. Reaction 4. As already clarified by X-ray analysis, the general component of the solid product in reaction 3 is
I
0 y
1
,
0
3 is conducted at more than 600 "C, the components in the gaseous phase of the reaction are NO and 0, as shown by reaction 11. However, if the temperatures are kept at above 600 "C in the condensed phase and at room temperature in the gaseous phase, the reaction spontaneously proceeds to the right irrespective of the equilibrium constant of reaction 11in a homogeneous temperature system.
I
l
6I 1 0l r 0
10
I
I
20
1
30
TIME
40
I
I
50
60
( Mllu )
Figure 14. Achievable concentration of KOH aqueous solution formed at 70 O C a t HZ0/KFeO2= 7.44 (molar ratio) in reaction 4.
KFeO, at 850-900 "C at a molar ratio KN03/Fe203= 2.0. Therefore, this study is focused on the hydrolysis of KFeO, formed at 850 "C at a molar ratio KN03/Fe203= 2.0. A 1.3-g samples of KFeO, was added to 100 g of water, and the suspension was reduced to half the volume by boiling. After cooling, OH- and C03,- in the suspension were determined with an automatic titrator. As a result of this analysis, it was concluded that 98.7-99.5 mol % of the KFeO, sample is converted to KOH by hydrolysis, if 84 mol 7'0 of the initial KNO, is assumed to have been converted to KFeO, in the preparation of the KFe0, sample. In order to detect undecomposed KFeO,, an X-ray analysis was conducted on the dry samples of solid residue after hydrolysis of the KFe0,. However, no X-ray diffraction patterns assignable to KFeOz were found; all the samples showed only the diffraction pattern of hematite. On the other hand, determination of potassium in the solid residues by flame emission photometry revealed that the solid residues contained 0.5-1.3 mol % of the potassium in the initial KNO,. Figure 13 shows the hydrolysis yield of KFeO, as a function of time. A sample containing 0.2 g of KFeO, and 20 g of water in a Teflon bottle was shaken and warmed in an oil bath at constant temperature. The features of the hydrolysis yield with respect to reaction period are similar above 60 "C in that it takes as short as 30 min to reach the maximum hydrolysis yield. On the other hand, the hydrolysis of KFeO, proceeds rather slowly below 50 "C. In any case, the maximum hydrolysis yield is more than 99% in view of the already stated fact that 16-17% of potassium in the initial KNOBwill not form KFeO, in the step of preparation of the KFeO, samples due to the vaporization of KNO,. If viewed as a matter of industrial manufacturing method, it will be important to know the highest achievable concentration of the KOH solution, which is obtained by
2074 Ind. Eng. Chem. Res., Vol. 26, No. 10, 1987 Table 11. Enthalpy, Entropy, and Gibbs Free-Energy ASo298K,and AGO) for Reactions 1; 2,b Changes (AH0298~, 4,' 5,d and 11' reaction 1 2 4 5
\
v
Y
3 5 -
'i,,
11
total
A-\S0298K7
AHo?QaK, kcal +4.080 +10.330 -16.723 -38.554 +74.873 +34.006
kcal/K +31.780 +35.710 -46.278 -92.491 +76.130 +4.851
AGO, kcal
-8.691 (at 413 K) -2.476 (at 373 K) -0.107 (at 340 K) -10.966 (at 298 K) -3.849 (at 1200 K)
0
2 14/ 3 ) H N 0 3 / K C \
3
' MOLAR
QP-IO
I
Figure 15. Effect of HN03/KC1 in reaction 1 on KC1 content in the assumed 45 wt 9i KOH aqueous solution produced by the process of this work.
the hydrolysis of KFe02. Figure 14 shows the concentrations of KOH solutions obtained after different hydrolysis periods. In this experiment, the calculated amount of water was added to each weighed amount of KFe02 so that a 30 wt % KOH solution may be formed when all of the KFeOz used was completely hydrolyzed. The KOH solution formed was separated from the solid residue by paper filtration. The determination of the hydrolysis rate curve of KFeOz between zero time and 1 min after starting the reaction is impossible because to start and to stop the reaction take some time. As obvious from Figure 14, the concentration of KOH reaches about 28 wt % within 1 min, a value which does not increase any more after then. In order to obtain a 33 wt % KOH solution, the calculated amount of water was added to the weighed amount of KFeOz sample. The reaction time and temperature were 30 min and 70 "C in this experiment, respectively. The reaction product mixture, in this case, looked like a wet precipitate from which any drop of KOH solution could not be squeezed out. A centrifugation at 1170g could not separate the liquid component from the reaction product mixture. These results, thus, indicate that the practically achievable maximum concentration of the KOH solution is about 28 wt YO. Unreacted KC1 in the KN03 in reaction 1is contained in the KOH aqueous solution formed by reaction 4 without change through reactions 3 and 4. Therefore, purity of the KOH solution can be estimated from the amount of unreacted KC1. After the end of reaction 1, unreacted KCI was determined in the KN03-HN03 mixed aqueous solutions or in the residual KN03wet crystals after filtration through filter paper. In estimating the purity, all the unreacted KC1 in KN03 was assumed to remain in the 28 wt % KOH solutions formed by reaction 4. Figure 15 shows the KC1 content in the assumed 45 wt % KOH solutions as a function of the molar ratio of KC1 and HNOB in reaction 1. In Figure 15, the 45 wt % KOH solutions were assumed to be produced from 28 wt % KOH solutions by evaporation of water. As is obvious from Figure 15, the KCl content is minimized within molar ratios 4/3HN03/KC1= 2-3. The least KC1 content is estimated to be less than 0.05 wt % in the assumed 45 wt % KOH solution in this process.
Discussion Thermodynamic Calculation. The enthalpy change, entropy change, and Gibbs free-energy change for each of the reactions involved in the present process were calculated based on thermodynamic data (Barin and Knacke, 1973). The results are shown in Table I1 and Figure 16.
C
200 400 600 8C0 IC00 1200
('C Figure 16. Gibbs free-energy change (AGO) vs. temperature plot for reactions 1, 2, 4, 5 , and 11. TEMP.
A
_
-A-
- _8 5
-C2
~
Figure 17. Material and heat flow sheet of the thermochemical process of this work the heat unit is in kilocalories, and the amounts of the chemicals are represented in moles
The thermodynamic values of reaction 11 in place of reaction 3 are shown in Table I1 and Figure 16, because reaction 3 is an irreversible process as already stated, and the thermodynamic values of reaction 11 were based on the data of KFe02 in Table I and the specific heats of KFeO, expressed by eq 14. These specific heats were measured by a calorimeter in this work. C, = 138.956 - 102.623 X 10-3T- 25.682 X 105'T2+ 96.012 X 10-6T2 298 K IT I 9 0 0 K (14)
Material and Heat Flow Sheet. A material and heat flow sheet for this process was estimated and was based on the previously mentioned thermodynamic data and experimental results. Figure 17 shows the estimated material and heat flow sheet. Q, at E3 contains the reaction
Ind. Eng. Chem. Res., Vol. 26, No. 10, 1987 2075 Although the practical heat requirements of the current processes depend on the types and operating conditions of the electrolytic cells, they are within the range of 4.54 X 106-6.05 X lo6kcal for the production of 1 ton of Cl, and 1.577 tons of KOH. On the other hand, if the released heat in the process is partially used for the process, the heat requirement of thisproposed process for the same products is much less than those of the current electrolytic processes. If substantial thermal regeneration is conducted in the thermochemical process of this work, the process might become economically and energetically competitive with current electrolytic processes of producing Clz and KOH. 1.0
1.0
0.8
0.6 0.4
0.2
0
f ( 3 ) IN Eq. 15
Figure 18. Heat requirements (Qrsq.) in production of 1 ton of Clz and 1.577 tons of KOH by this thermochemical process and the current electrolytic processes: (1) f(1) = 0.3, f ( 2 ) = 0.2, f(4) = 0.1; (2) f(1) = 0.4, f(z) = 0.3, f(4) = 0.2; (3)f ( i )= 0.5, f(z) = 0.4, f(4) = 0.3; (4) f(1) = 0.6, f ( 2 ) = 0.5, f(4) = 0.4; ( 5 ) f ( 1 ) = 0.7, f(z) = 0.6, f(4) = 0.5; (6) f ( 1 ) = 0.8, f ( 2 )= 0.7, f(4) = 0.6, in eq 15.
heat of NO + '/202 s NOz. Heat Requirement. The heat requirement for the process was calculated by using eq 15 formulated on the basis of the flow sheet in Figure 17. The following assumptions were made in the formulation of eq 15.
Each Qp is used in this process at the specific ratio for thermal regeneration. AHo at R1 is used in this process at the equivalent ratio for thermal regeneration to Qp(l)at El. at R4 is also used in this process at the equivalent ratio for thermal regeneration to Qp(4)at E4, because and are exothermic. The heat requirements for this process were estimated by substituting the assumed f(n's in eq 15. Figure 18 shows the estimated heat requirements for this process, and the practical and theoretical heat requirements for the current diaphragm and membrane-type electrolytic processes for production of 1 ton of C12with 1.577 tons of KOH. The heat requirements for this process do not include the energy for separation and transport of the process streams. In calculation of the practical and theoretical heat requirements for the current electrolytic process, this work assumed that the fossil fuel equivalent of electrical energy was 3 times the amount of electrical energy delivered to the current electrolytic processes. In calculating the theoretical heat requirement, the energy efficiency of the electrical process was assumed to be 100%. Since hydrogen is produced as a byproduct by the current electrolytic processes, the heat of the combustion of hydrogen is subtracted from the total practical and theoretical heat requirements to derive the heat requirements presented here.
Nomenclature C = molal heat capacity, J/(K.mol) [el-] = amount of C1- in NaOH aqueous solution after absorption of the gases in reaction 2, mol E = heat exchanger f(')= ratio for thermal regeneration at the ith reaction g = acceleration of gravity, 9.806 16 m/sz AGO, = Gibbs free-energy change of reaction at t("C), kcal AHo, = enthalpy change of reaction at t("C), kcal [OH-],, [OH-], = amount of OH- in NaOH aqueous solution before and after absorption of the gases in reaction 2, mol Kp(i)= equilibrium constant of the ith reaction p = pressure, atm QP = AHo, heat output (exothermic) for cooling the reaction products, kcal Q, = AH", - AHo2soc,heat input (endothermic) for heating the reactants, kcal Q,, = heat requirement for the processes of producing 1 ton of C12 and 1.577 tons of KOH, kcal R = gas constant R1-5 = reactor S = separator T = temperature, K Greek Symbol po =
molal standard Gibbs free-energy change, kcal/mol
Subscripts aq. = aqueous solution g = gaseous state 1 = liquid state s = solid state Registry No. KC1, 7447-40-7; KNOB,7697-37-2; NOCI, 2696-92-6; FezO3, 1309-37-1; KOH, 1310-58-3; CIZ, 7782-50-5.
Literature Cited Barin, I.; Knacke, 0 Thermochemical Properties of Inorganic Substances; Springer-Verlag,Berlin, 1973. Burdick, C. L.; Freed, E. S. J. Am. Chem. SOC.1921,43, 518. Spealman, M. L. Chem. Eng. 1965, 72, 198. Strelzoff, S. Chem. Eng. 1956, 63, 170. Takeuchi, N. Ind. Eng. Chem. Process Des. Dev. 1985, 24, 224. Versar, Inc. A Survey of Potential Chlorine Production Processes; National Technical Information Service: Springfield, VA, 1979.
Received for review October 21, 1986 Revised manuscript received June 23, 1987 Accepted July 23, 1987
