I n d . Eng. C h e m . Res. 1987, 26, 2063-2069
2063
A Rotary Kiln Process for Making Calcium Carbide Jacob J. Mu* U.S. Borax Research, Anaheim, California 92801
Robert A. Hard Cabot Corporation, Boston, Massachusetts 02110
A preliminary economic analysis has indicated that the making of acetylene, via a kiln calcium carbide process, can be cost competitive with ethylene if used to produce vinyl chloride monomer. The work presented here includes (1) the kinetics of carbide formation from reduction of lime, (2) the characterization of this carbide product, and (3) a process demonstration in a prototype bench-scale rotary kiln. From the study of the reduction chemistry, it was found that reduction rates of lime to carbide are fast, needing less than 30 min to achieve conversions of 5 0 4 0 % (1740-1870 "C). An increase of carbon monoxide pressure in the surrounding atmosphere tends to inhibit the reaction. The calcium carbide thus Droduced is Dorous and stable in air UP to 650 "C. It reacts with water to generate impurity-free acetylene readily. The results from both the continuous and the batch tests are found to be in good agreement.
For many years, acetylene was the preferred starting point for making vinyl chloride monomer as well as other unsaturated Cz products such as vinyl acetate. However, the associated high energy and capital costs for making acetylene eventually caused the industry to switch to petroleum-derived ethylene as the starting material for these products. Occidental Research Corporation has investigated the concept of reducing lime to calcium carbide in a fuel-fired rotary kiln and then treating the subsequent product with water to make acetylene (Figure 1). This method of making acetylene will cost considerably less than the conventional electric furnace method because the kiln process does not require large amounts of electric power. The low-cost acetylene thus produced may provide an alternative route to products deriving from steamcracked petroleum naphtha (Shih et al., 1984). Basically, the formation of calcium carbide in the kiln process is carried out at somewhat lower temperatures than electric furnace processes (2000-2200 "C). The reaction scheme is given by the overall reaction CaOw + 3%
==5
CaC,,,, + CO,,)
(1)
The carbide is reacted with water to generate acetylene, with calcium hydroxide obtained as a byproduct.
-
CaCz(s)+ 2H&)
C2Hz(g) + Ca(OH)z(s)
(2)
Calcium hydroxide can be recycled to form pellets with coke to continue the calcium carbide production. The rates of conversion and equilibrium conversions of lime to calcium carbide depend strongly on the reduction temperature and the surrounding CO partial pressure (Brookes, 1972; Ershov, 1970). In addition to desirable reaction 1, the decomposition reaction
-
CaCZ(s)
Ca(g)+ 2C,S)
(3)
is also involved (Tagawa and Sugawara, 1962; Mukaibo and Yamanaka, 1953). The reaction rates and the extent of reactions 1and 3 were individually measured at various temperatures and CO partial pressures by Mukaibo and Yamanaka (1953) and Tagawa and Sugawara (1962). They obtained a linear relationship between the amount of carbide decomposed and the time of reaction between 1600 and 1800 "C, at CO partial pressures of 50-200 mmHg. Brookes et al. (1975) also reported the kinetic results on the formation of solid-state calcium carbide between 1650 and 1750 "C at a 0888-588518712626-2063$01.50/0
reduced CO partial pressure of 50 mmHg. By thermogravimetric measurement, wet chemical analysis, and microscopic observation of CaCz product, they were able to describe the reaction process in detail. The shrinking core reaction scheme coupled with the temperature gradient between the furnace and the sample was proposed for the reaction mechanism. The present work investigates the reaction kinetics in a broader temperature range (1580-1950 "C)and under various CO partial pressures (0-1 atm). A variety of carbonaceous sources ranging from low-grade coals to petroleum coke were also tested. Some of the chemical and physical properties of the calcium carbide product were also determined. A prototype of a continuous rotary kiln was also assembled to evaluate this conceptual process.
Experimental Section Apparatus. The major apparatus used for the kinetic studies includes an induction furnace, a high-temperature thermogravimetric analyzer (TGA), and a bench-scale rotary reactor. The details of the individual units are briefly described as follows. (1) Induction Furnace. The general experimental arrangement of the induction furnace is illustrated in Figure 2. A graphite crucible (1-in. i.d.) used as a magnetic receptor is located coaxially in a magnesia crucible (4-in. i.d.) which encloses lamp-black as the thermal insulator. The whole sample crucible assembly is supported by an alumina cylinder. A quartz reaction tube (6-in. i.d.) is used to isolate the reaction system from the surrounding atmosphere. The reaction temperature is controlled by means of a Lepel25-kW high-frequency generator operating at 450 kHz. Reaction temperature is accurately measured by a W-5% Re vs. W-26% Re thermocouple which is enclosed in a molybdenum sheath. An optical pyrometer (Precision Pyrometers, Model 83) is also used to measure the reaction temperature and to observe the shape of the pellets in the reduction process. The gas composition and flow rates are controlled by a Matheson mass flow meter and controller. Cooling water is circulated in the brass flanges to prevent the metal parts from overheating. The heating rates and cooling rates by induction heating are as high as 150 "C/min to minimize the nonisothermal period during the experiments. (2) Thermogravimetric Analyzer. The TGA module assembled in the laboratory consists of six main units-the balance assembly (Cahn lOOO), furnace assembly, tem1987 American Chemical Society
2064 Ind. Eng. Chem. Res., Vol. 26, No. 10, 1987 Pet Coke
Ca(OHI2
1 ) Product Hopper 2) ThermocoupleIViewing Port 3 ) Product Gale 4) Product Hopper 5) Feed-gas Part
CaC03 Make-up
To Scrub
a
Stack
6) Alumina Reactor l u b e 7) Flange 8) Furnace Chamber Assembly 9) Drive Assembly, Motor,
1 1 ) Exhaust Port 1 2 ) Vibrating Feeder w l Hopper 1 3 ) Cyclone
Bell, Putly 1 0 ) Rotary Seal and Bellows
f
GC
Water Steam
Purge Gas Water
Figure 3. Schematic diagram of a continuous bench-scale rotary reactor.
To Kiln
Table I. Composition of Various Carbon Sources moisture, volatiles, fixed bituminous coals % ash, % % c, % Wyoming 8.6 5.9 40.0 46.5 4.3 8.3 33.4 54.0 Kentucky No. 9 23.3 67.1 Lower Eagle 0.9 8.1 Spurlock 2.3 3.7 36.4 57.3 0.9 3.9 17.6 77.0 Virginia Poco High Splint 3.8 3.2 34.9 59.9 0 0.3 6.4 89.5 green coke 3.8 9.5 4.6 81.9 anthracite
Water
To Recycle + Bleed
Ca(OH)2
4
Figure 1. Conceptual flow sheet of the kiln carbide process for calcium carbide and acetylene. Optical Pyrometer
\
Prism, W - R e Thermocouple Pressure G a u g e
Induction C o i l
[Cooling
Chamber
Figure 2. Schematic diagram of an induction furnace.
perature controller, gas mixing system, sample boat assembly, and recording system. The graphite furnace gives the TGA a range of 2500 "C in a reducing and/or inert atmosphere and 1850 "C in oxidizing conditions. (3) Bench-Scale Rotary Reactor. The bench-scale continuous flow rotary reactor (Figure 3) is controlled by induction heating with a maximum operating temperature of about 1850 "C. The induction coil is charged by a Lepel radio-frequency generator. Usually, a 10-cm hot zone of uniform temperature can be obtained. By controlling the solid feed rate, rotation speed, and inclination angle, the solid residence time in the hot zone can be adjusted as required. Temperature measurements are made with a Pt/Pt-1370 Rh thermocouple and an optical pyrometer.
s, % 0.2 0.3 0.1 0 3.8 0.1
Table 11. Various Parameters of Reduction Kinetic Study reaction temp 1580-1950 " C 0-100% co gas composition 0-100'70 excess C ratio 0-60 min reaction time 0-0.24 cm/min/g purge gas velocity 0.95-5.1 cm diameter pellet size Ca(OH),: -200 mesh particle size C: -325 mesh listed in Table I C source
Supplies. The chemical analyses of various carbonaceous materials, such as bituminous coals, anthracite, and green coke, are shown in Table I. The calcium hydroxide (99% pure) has a particle size of less than 200 mesh. Procedures. ( 1 ) Induction Furnace Tests. Three pellets (=lo g) made in a hydraulic mold at 5000 psi were loaded into the induction furnace at room temperature. The furnace was evacuated and then pressurized with the gas mixture of the desirable ratio (CO/Ar) until 1atm was reached. The gas exit valve was then opened and a steady-state flow condition was maintained. The same procedure was repeated to remove any possible oxygen contamination. The power supply of the furnace was regulated to give the desired temperature profile of 500 "C for 10 min, 1000 " C for 10 min, and then the selected reduction temperatures (1600-2000 "C) for certain periods at a heating rate of approximately 100-150 "C/min. A temperature of 500 "C was chosen to complete the calcium hydroxide dehydration and 1000 "C for removing all the volatiles in the carbon source. Before the experiment was terminated, CO gas was shut off to minimize any possible back reaction as shown in eq 1. The products were weighed and the acetylene was generated from them in a gas evolution device. The extent of conversion from lime to carbide could then be calculated. The slurry solution was dried and burned at 1000 "C in a Leco muffle furnace. The remaining solid residue was practically all lime. Therefore, the calcium balance could be computed. The various parameters studied during this investigation are shown in Table 11. The purpose of these runs was to evaluate the relative effect
Ind. Eng. Chem. Res., Vol. 26, No. 10, 1987 2065
90
1
x
l800Y
& 174O.C
ao
A 100% CO
1700
I
1
I
1800
1900
2000
Reaction Temperalure.
20
OC
Figure 5. Effect of CO dilution in the surrounding atmosphere on the reduction of lime to carbide. Reaction time = 30 min.
10
0
0
10
20
30
Reduction Time, min
Figure 4. Reduction of lime to carbide between 1580 and 1870 O C in 1 atm of CO.
on the carbide formation of various parameters, such as solids composition, temperature, gas composition, and carbon source. (2) Thermogravimetric Balance Tests. A few TGA runs of the same pellets were also conducted to permit a direct on-line analysis and to facilitate the kinetic investigation. I t was found that this newly assembled TGA system could provide more accurate kinetic data and reveal some reaction characteristics which could not be detected in a batch experiment. (3) Induction Rotary Reactor Tests. The heating rate of the alumina tube could not exceed 300 "C/h, and the power input to the high-frequency generator was controlled so that a slow heat-up could be obtained. Usually 6-7 h was required to reach 1800 OC. Feed gas flow rate and composition were continuously monitored with a mass flow meter and controller. The off-gas was continuously monitored by using on-line GC. Typically, GC data were used to study the effect of various reaction parameters. (4) Purity of C2H2from CaC2. A GC analysis of gas synthesized from calcium carbide was performed. A Hewlett-Packard 5830A GC equipped with dual TCDs and FIDs was used to perform this analysis. (5) Other Tests. Ignition tests of the carbide product were performed in a Du Pont high-temperature differential thermal analyzer (DTA) and a Du Pont thermogravimetric analyzer (TGA). The carbide product was also subjected to X-ray diffraction and SEM analyses. The pellet strength as a function of composition and temperature was determined by a Chatillon press.
Results and Discussion Kinetics of Reduction in the Induction Furnace. Reduction of lime to calcium carbide is fast. It generally takes about 30 min for the reaction to attain a steady state. Figure 4 shows that the reduction yield is a strong function of temperature (1580-1870 "C) in 1atm of CO. Dilution of CO in the surrounding atmosphere should favor the reduction according to thermodynamic analysis. This phenomenon is confirmed by the kinetic results shown in Figure 5. Higher reaction temperature and lower CO pressure generally reduce the thermal stability of calcium carbide. Calcium loss due to the decomposition of calcium carbide is shown in Figure 6. The extent of decomposition appears to increase with time at a temperature greater than 1800 OC (Figure 7). Control of residence time in a reactor to enhance reduction yield, while minimizing CaCz decomposition, is thus important.
oB --o -a-
c 0 1870'
C
A 18W C A 1740. C 0 1670' C
0
o
I
I 20
I
40
E3
Percenl of
I
eo
I 100
CO h Arm
Figure 6. Effect of temperature on calcium oxide recovery (calcium loss due to the decomposition of calcium carbide).
t
I;: 5
c
0 18708
40
c
A 1800'C A 1740O C 0 1670° C
0
15
30 Reacfion M e . mm.
45
60
Figure 7. Effect of reduction time on calcium oxide recovery (calcium loss due to the decomposition of calcium carbide).
No melting was observed in all the reacted residues, and the shape and size of the pellets remained rather constant after the reaction. This behavior would make the rotary kiln process much easier to control. Various carbonaceous materials including subbituminous and bituminous coals, anthracite, and petroleum coke were tested for lime reduction to form calcium carbide. The results are shown in Figure 8. Higher conversions achieved by using low-grade coals may be due to the higher reactivity of carbon and larger surface area exposed by the release of volatiles. No melting was observed despite the high ash content present in the low-grade coals. Other findings are summarized as follows: (i) excess carbon did not appear to enhance the reduction yield; (ii)
2066 Ind. Eng. Chem. Res., Vol. 26, No. 10, 1987 0.2 atm CO
+ 81.5%
E
Conve,rsion
B'min
40
I 2o
t
0'
-
50
Kentucky + 9 Antracite
0
0
33.4 4.6 36.4
Spurlock
I
45
Lower Eagle Green Pet Coke
I
I
I
I
I
*r
23.3
Moisture Removal
6.4
Dehydration of Ca(OHIz
CaCZ Formation
"
0
400
BOO 1200 Temperature. *C
1600
t.4 ,3336
Figure 10. Results of reduction of lime to carbide using green petroleum coke from TGA.
-
-cm
>,
-
60
/
i 40-
I
E
cn
t
01
rp
n' 0 I
I
I
I
I
1
rMOIstUre t // Removal
E
,
I
30 mln
E
l
/
Dehydration of Ca(OH), +Loss of Volatiies
0 0
c
U
0
400
800
1200
Temperature
1600 l C l O O O - - - - Y
,t
Figure 9. Results of reduction of lime to carbide using bituminous coals from TGA.
the effect of gas purge velocity (0-2.4 cm/min/g) on the yield and rates was negligible; and (iii) the effect of pellet size on the yield and rates was insignificant. Measurement of Reduction Rates by TGA. A hightemperature thermogravimetric analyzer (TGA) was assembled for measuring the reduction rates of lime. Several runs on samples containing various carbon sources, such as bituminous coals and green petroleum coke, were obtained, and the results were proved to be consistent with the reduction data from the induction experiments reported in the previous section. A typical dynamic TGA run at successive heating rates of 15 OC/min (400-1300 "C)
and then 45 "C/min (1300-1800 "C) in a CO atmosphere is shown in Figure 9. The temperature was maintained at 1800 OC for 30 min, and then the CO partial pressure was reduced to 0.1 atm for 30 min. The test sample was composed of 41.3% calcium hydroxide and 58.7% Wyoming coal. The instantaneous sample weight and rate of weight change with time were recorded as functions of temperature. The secondary reactions such as moisture removal, dehydration of Ca(OH)2, and volatilization of organic matters, etc., were observed before the main reduction reaction. It may be possible that some of the other minor reactions occur because of the trace amount of impurities present. The reduction reaction proceeded rather rapidly over the first 40-45% reduction (10 min), then gradually slowed down (50% conversion) after 20 min, and finally reached a steady state (56% in 30 min). These phenomena appeared repeatedly in every sample run in an induction furnace as shown before. Much higher conversion (84% in 10 min) was obtained very rapidly when the CO partial pressure was reduced to 0.1 atm. Because of the lower CO partial pressure, a slow CaC2 decomposition was triggered. It became indistinguishable from the further reduction reaction after 15-min reaction time. It was also observed in the reduction experiments in the induction furnace. The weight gain during the cooling down period in the presence of remaining CO was probably due to the back reaction of CaC2with CO. In the graphite furnace of the TGA system, the cooling rate was limited to below 50 OC/min. Higher back reaction was possible by comparison with that in the induction furnace (cooling rate of 150 OC/min). The back reaction was terminated in the temperature range 1300-1350 "C. Another TGA result using a sample of green coke as the carbon source ((C/CaO)/M = 4.0) is shown in Figure 10. The maximum conversion at a temperature of 1835 "C and 0.2 atm of CO was approximately 81.5% in a reaction time of 6 min. The reaction started at a temperature as low as 1350 "C, and the reaction rates became appreciable at a temperature of 1600 "C. The strong dependence of the
Ind. Eng. Chem. Res., Vol. 26, No. 10, 1987 2067 Table 111. Mix Order for ComDactinP:Tests
wt % Ca(0HL wt '70 CaCO, wt % pet coke (86% C)
100-4 57 0 43
75-4 40.7 18.3 41.0
50-4 25.9 35.0 39.1
100-3.5 60.2 0 39.8
75-3.5 42.9 19.3 37.8
conversion and the reaction rates on the temperature and CO partial pressure was again indicated. Purity of C2H2from CaC2 The analysis of acetylene gas generated from the reaction between calcium carbide and water by GC showed only two peaks: one at 1.30 min (air)and another at 5.36 min (C2H&.Air was used to dilute the gas generated by CaC2and H20in the generator apparatus. None of the eight calibrated gases were detected, and there were no other peaks. This suggests that, aside from air, the acetylene produced was greater than 99.5 vol
mix no. 50-3.5 100-3 27.3 63.9 36.9 0 35.8 36.1
75-3 45.4 20.4 34.2
50-3 28.7 38.8 32.5
100-2.25 70.2 0 29.8
75-2.25 49.2 22.4 28.0
50-2.25 31.3 42.2 26.4
Raw Material Table Feeder
Fines h F e e d
Force F e e d e r
%.
Product Characteristics. The calcium carbide product produced by solid-state reduction looks considerably different from commercial fused calcium carbide. The material is black, has a low bulk density, and is very porous according to SEM pictures. It contains about 50-60% calcium carbide, 10-15% free carbon, and the balance lime. Commercial calcium carbide is a much denser material, lighter in color, being approximately gray, with no free carbon, about 80% calcium carbide, and 20% lime, with small amounts of impurities such as alumina and silica. Because of the light, porous nature of the product, it was suspected that the product might be pyrophoric. This would create problems when the material exits from the reactor into the air. To check this point, several experiments were carried out in a Du Pont differential thermal analyzer (DTA) in which samples of the product were heated while measuring the thermal effects caused by reaction with air. DTA tests on commercial calcium carbide indicated that ignition occurred at about 600-635 "C on finely divided material. With the solid-state reduced material, it was found that the ignition occurred at approximately 650 "C. The unreacted carbon was the first to oxidize at 500-550 "C, followed by the calcium carbide at 650 "C. TGA results were consistent with the DTA analysis. Both the laboratory prepared calcium carbide and the commercial calcium carbide were submitted for X-ray diffraction analysis. These data indicated the existence of calcium carbide, calcium oxide, and graphite as expected. A trace amount of lime hydrate was also identified due to the hydroscopic nature of the calcium carbide in moist air. Bench-Scale Rotary Reactor Studies Feed Preparation. In order to evaluate the reduction of lime to calcium carbide under rotary kiln conditions, a bench-scale rotary reactor was assembled. The mixture of calcium hydroxide, coke, and limestone had to be pelletized before entering the reactor. A compacting machine was found to be appropriate to do this job. Such a machine is shown in Figure 11. It consists of a pair of pressurized rolls which are force-fed with the loose mixture of ingredients. The material passes through the rolls and is pressed into flakes which are then broken in a flake breaker. After screening, the material goes to a storage bin and the fines are recycled to the process. No binders need to be added to the system, as the calcium hydroxide itself acts as a binder when put under high pressure. A series of tests were carried out at the Allis Chalmers Test Laboratories in Oak Creek, WI, to determine the
Flake Breaker
Granulators Barrels
to Screen Fines
Product
Figure 11. Schematic of a pilot-scale compacting process. Table IV. Pellet Strength (Button Tests) break strength, psi, for pellets calcined (at mix no. green dry (at 100 "C) 1000 "C) 100-4 564 512 795 75-4 534 495 730 392 562 458 50-4 520 437 1075 100-3.5 528 746 75-3.5 437 441 487 554 50-3.5 100-3 594 418 877 75-3 489 787 489 386 612 367 50-3 100-2.25 447 948 566 390 801 602 75-2.25 677 308 50-2.25 328
physical characteristics of the agglomerates made by the compacting system which might be suitable for a kiln process. Table I11 shows the various mixtures that were used, in which the main compositional variables tested were the ratio of fixed carbon to calcium hydroxide and the ratio of limestone to calcium hydroxide. The initial tests, which were called "button" tests, were made on a small laboratory machine in order to bracket the conditions necessary and to get some indication of the relative strengths. The test results are displayed in Table IV and essentially indicate that all of the tested mixtures had sufficient strength to be of interest. The strength of these pellets, whether green, dry, or calcined, was generally well above the necessary 100 psi and therefore should withstand the abrasion in the kiln reasonably well. The
2068 Ind. Eng. Chem. Res., Vol. 26, No. 10, 1987 Table V. Pilot-Scale ComDacting Tests run 1 (75-4) 2 (75-4) 3 (75-3.5) 4 (75-3.5) 5 (50-3.5) 6 (50-3.5) 7 (75-4) 8 (75-4)
recycle 0 30% of fines (-4 mesh) from run 1 0
30% of fines (-4 mesh) from run 3 0 -6 mesh coke 0 granulates (-6 mesh) from runs 1 + 2
moisture, % 4.4 4.2 5.2 4.8 5.0 5.0 8.0 5.0
Table VI. Results on Tumbling Tests oroduct size anal. screen size micrometers % passing Cold Tumble Test" 3 J 8 in. 9525 18.93 3 mesh 6680 8.61 4 mesh 4700 5.25 6 mesh 3350 4.19 8 mesh 2362 3.48 10 mesh 1700 3.07 14 mesh 1168 2.78 20 mesh 833 2.54 28 mesh 589 2.25 70 passing product screen size micrometers feed Hot Tumble Testb 3 / 4 in. 19050 72.6 ' I z in. 12 700 28.9 36.1 3 / s in. 9 525 5.3 19.6 in. 6 350 1.9 14.0 4 mesh 4 700 11.6 6 mesh 3 327 9.9 10 mesh 1651 9.2 28 mesh 589 "Feed size = 100% +4 mesh, dried a t 100 "C; 50 revolutions in the tumble tester. *Feed size = 100% +4 mesh, dried a t 100 "C; 10 rpm, 60 min a t 1000 "C.
main conclusion to be drawn from these button tests is that the agglomerates become weaker as the amount of calcium hydroxide is reduced. This is to be expected as calcium hydroxide is believed to be the principal binding agent in the system. Following these initial tests, three different mixtures were selected for subsequent testing on the pilot-scale compactor as shown in Figure 11. The results from the compacting tests are shown in Table V. The initial runs produced thin, weak flakes which were screened from undersize material on a 4-mesh screen. The recoveries were only 48430% because these mixes were not deaerated before compacting. However, by recycling the fines, which had been somewhat densified in the first pass, the recoveries could be increased and a somewhat stronger flake produced. Runs 1, 3, and 5 showed essentially the same results on the three different mixes, producing these thin, weak flakes with rather low recoveries. The best run was run 8, in which a granulated material from runs 1 and 2 was rerun through the rolls. This gave a 90% recovery with very strong, thick flakes. In run 7, water was added to assist in deaerating the mix, but this still produced a relatively weak flake. The run 8 product was subjected to cold and hot tumbling tests, and the results of their physical durability can be found in Table VI. The conclusion reached from these runs was that any of the mixtures could produce good flakes if they had been deaerated or predensified first. Some recycle is always expected, but if it is too large, then the investment must
discharge, lb/h 784 1085 997 1294 1518 1333 1423 1521
flake thickness, in. 0.06 0.13 0.11 0.11 0.13 0.17 0.065 0.215
Table VII. Results solid solid fill, rate, run 70 g/min 0.8 1 30 0.5 2' 15 0.8 3 10 4 15 0.6 5 20 0.9 1.0 6 30
flake strength weak weak weak weak weak medium weak strong
recovery, % 53 (+4 mesh) 60 (+4 mesh) 60 (+4 mesh) 48 (+4 mesh) 80 (+4 mesh) 68 (+4 mesh) 54 (+4 mesh) 90 (+4 mesh)
from Rotary Reactor Testso reaction conversion, temp, CO, COz, time, "C % % min % 1720 35 0 15 70 0 12 80 1750 55 0 5 40 1680 25 10 65 30 1750 20 9 55 0 1700 30 0 12 64 1730 40
"Material: 40.770 Ca(OH),, 18.370 CaC03, and 41.0% green coke. Size of flakes: -4 to +12 mesh. Kiln rotation speed: 0.5-1.5 rpm. Solid fill: 10-30%. Solid throughout: 0.5-1.0 g/min. Kiln gas flow rate: 150-400 mL/min, 20-55% CO, balance N,.
be greater in compacting machines. Therefore, it is desirable to hold the recycle down to less than 25%, if possible. Deaerating devices are often employed but were not used in this run. Induction Rotary Kiln. To test the above compacted material and to better evaluate the reduction of lime to calcium carbide under rotary kiln conditions in the laboratory, a small rotating furnace heated by means of an induction coil was assembled. This allowed the reactor to achieve temperatures up to 1850 "C while passing a controlled atmosphere simulating a combustion atmosphere over the pellets. Thus, one can observe the effects of the carbon burnout reaction which is an undesirable side reaction represented by the equation C + COz = 2CO. The results of these continuous tests are summarized in Table VII, where the temperature ranged from about 1680 to 1750 OC. As can be seen, reductions of 40-80% have been obtained. These tests were made using material which had been compacted a t Allis Chalmers and granulated into particles of approximately 4-12 mesh in diameter. In run 4, C 0 2 was introduced into the kiln atmosphere, which caused some carbon burnout. As was hoped, there was no indication of melting, even through some free lime was exposed due to the carbon burnout. This run is compared with run 2, in which there was no C 0 2 present and 80% conversion was achieved. A t this time, the only conclusion which can be drawn from these runs is that the compacted feed material prepared at Allis Chalmers seems to react quite well and that reductions comparable to previous batch tests can be obtained without melting in the unit. Future tests will give us more indication as to the effects of carbon burnout on the reduction yield. Conclusion
Laboratory studies have established a good foundation for the basic chemistry of the reduction of lime to calcium carbide. The optimal conditions for producing solid calcium carbide in a rotary kiln have been approximately defined. Some of the chemical and physical properties of this calcium carbide have been determined. The next stage will comprise gaining a better knowledge of the high-tem-
Ind. Eng. Chem. 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)
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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
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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
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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)
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found for this work. The reaction sequence leads to the overall reaction shown by reaction 6. Reaction 5 is
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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
