Gas Phase Chlorination of Coal - Advances in Chemistry (ACS

43 Gas Phase Chlorination of Coal

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Construction and Evaluation of Apparatus and Reaction S. C. SPALDING, JR. and J. O. BURCKLE Chemstrand Co., Decatur,

Ala.

W. L. TEISER Esso Research

&

Engineering

Co., Florham Park, N. J.

To evaluate the chemical behavior of coal in a free radical environment when the coal solids themselves are in a condition of maximum free radical content, a fluidized bed gas phase reactor was set up; it is suitable for handling and heating the reactants to approximately 600°C. Evaluation runs indicate that the coal will undergo "chlorinolysis," and among the products will be carbon tetrachloride, phosgene, and possibly a chlorinated sulfur derivative together with a subliming solid as yet uncharacterized.

y h e extensive literature on the thermal degradation of variously ranked coals indicated that interesting products might be obtained if this reaction (depolymerization) were conducted in a chlorine environment. Because of its high reactivity, chlorine poses problems of containment, especially if it becomes wet or contaminated with hydrogen chloride or oxygen. Elevated temperatures exacerbate the containment problem as do heating and cooling of these gaseous mixtures. Experimental Self-bonded silicon carbide was used in that part of the apparatus where high temperatures were necessary together with high heat transfer rates. Borosilicate glass for tubing and condenser surfaces together with polypropylene plastic pipe were also used. In this way temperatures up to 6 0 0 ° C . could be tolerated by the reactor section, and heat transfer occurred easily by conduction through the reactor wall using Scotchlite beads as a fluidized bed material. 677 Given; Coal Science Advances in Chemistry; American Chemical Society: Washington, DC, 1966.

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COAL SCIENCE

The design temperature of 6 0 0 ° C . was chosen b y considering coal as a free radical source. Electron spin resonance studies of coals (2) indicate that a rather sharp maximum in nonspin-paired electron content occurs i n coals when processed i n this temperature zone—i.e., 5 0 0 ° - 5 5 0 ° C . Chlorine moreover can be converted to atomic chlorine by several energy sources—corona discharge, silent electric discharge, ca. 4700 A . light, etc. The coal was processed under these conditions using the experimental setup shown i n Figures 1 a n d 2. In Figure 1 the process flow diagram is presented with indicator and/or controller points for both temperature a n d flow. The pressure indicator controller ( P I C ) was a cylinder regulator. A double diaphragm regulator was used on the nitrogen supply. T h e l i q u i d chlorine maintained a constant cylinder pressure, a n d a double diaphragm valve was not necessary for constant flow to the system. Ammonia was also l i q u i d i n the

GAS

RECYCLE U N E

VENT

LIQUID SUMP TANK

CHLORINE

SUPPLY

Figure I .

Process flow diagram

cylinder, and no difficulty was encountered i n controlling these flows b y simple, single diaphragm cylinder regulator valves. T h e pressure recorder ( P R ) was used to indicate the pressure differential across the fluidized bed a n d was a very effective monitor of the degree of fluidization. T h e flow recorder controller ( F R C ) represents closed loop flow control and recording of the gases used to fluidize the bed. T h e differential pressure across an orifice meter actuated pneumatic flow control-record system with reset and proportional band operation, which i n turn actuated a pneumatic motor valve for delivering chlorine, nitrogen, or air. to the system. A i r and nitrogen were delivered

Given; Coal Science Advances in Chemistry; American Chemical Society: Washington, DC, 1966.

43.

SPALDING ET AL.

Gas Phase Chlerinatien

PRESSURE

679

TAP

REFRAX

CYCLONE

ASSEMBLY

DISENGAGEMENT SILICON

CARBIDE

CYLINDER

FLUID KT

BED

SECTION

SILICON

CARBIDE

GAS

CYLINDER

DISTRIBUTION SUPPORT

MANIFOLD

PLATE

PLATE

G A S THERMOCOUPLE

SCCTION

REFRAX

INLET

ASSEMBLY

TUBE WATER COAL

J A C K E T

S A M P L E

PORT

INLET

Figure 2.

Reactor diagram

through the same piping, and only one F R C system was required. Chlorine was delivered separately by its own piping and F R C system. T h e temperature control and/or indication was carried out more or less separately. Because the heating was electric, fairly close top temperature limits h a d to be observed. Monitoring and closed loop control were carried out at the critical surfaces-heating element faces or hot walls. N i c k e l may not be used i n excess of 9 5 0 ° F . i n chlorine duty; heater sheath temperatures carry 1 2 0 0 ° F . and 1 5 0 0 ° F . duty limits, and heating tapes may not exceed 8 0 0 ° F . Therefore, to achieve 5 0 0 - 5 2 5 ° C . for the gas temperature, the reactor was held at 6 5 0 ° C . using a temperature indicator-controller ( T I C ) system which was closed loop control i n three stages. Initially the thermocouple signal unbalance was magnetically amplified for output to a small magnetic amplifier w h i c h generated a control signal for a saturable core reactor i n series with the heating load. This control was characterized by limited reset and proportional band operation. This mode of temperature control was used for the two gas preheater furnaces. T h e reactor space temperature was obtained by a thermocouple inserted into a bulb which was about 8 inches above the reactor gas


COAL SCIENCE

680 Table I. Bed Temp., °C. Initial Final

Run No. 6 8 9 11 13 14 15

394 250 27 25 475 525 520

2

403 250 54 78 390 470 500

5

Operational Data

Corona Excitation

Coal in from

No Yes Yes Yes Yes Yes Yes

Bottom Bottom Bottom Bottom Bottom Top Top

4

S

Charged Volume in ml. Coal to pass screen No. Coal Beads 250 250 250 250 250 200 150

β

1500 1500 1500 1500 1500 1500 1500

200 200 200 200 200 200 50

7

8

Figure 3.

Infrared

Wavelength

distribution plate. This bulb was a two-piece assembly of silicon carbide inserted into the reaction space with a nickel coupling welded into the column support. There was no taper on these threads; hence, when the ceramic was screwed down tight, no compression was achieved, and gas leakage occurred with severe consequences to the thermocouple. T h e monitoring instrument was a 2200-ohm, 0 - 5 0 microammeter movement, standard 4-inch square face meter, which was calibrated using a standard thermocouple potentiometer. A ten point commutator was used for switching from point to point. Figure 2 is a schematic diagram of the two-section reactor body and accessories. T h e fluid b e d section was made of self-bonded silicon carbide, 16 inches high by 6 % inches o.d. with a recessed flange. T h e recess accom modated a 120-mesh porosity silicon carbide gas distribution plate. T h e nickel manifold assembly was topped by a heavy support flange. This manifold sup-


43.

SPALDING ET AL.

Gas Phase Chlorination

681

plied several services: pressure tap, gas inlet, cooled coal inlet, bed solids sample outlet, and thermocouple tube. T h e upper section, head, cyclone, etc. are well illustrated in Figure 1. T h e corona discharge electrode was inserted through the ceramic pipe with the label "ground spherical joint/' A 1-inch glass tubing line with an outlet for the corona electrode coming straight out also connected this ceramic pipe to the product condenser as shown i n Figure 1. Chlorine excitation was obtained by a corona discharge on platinum wire points at 40 kv., 2 ma., and 4 - 5 M c . from the tesla coil output of a vacuum system leak tester. According to Steacie (3) more than 4 0 % atomic chlorine

microns spectrum for residual gases, Run

#15

can be formed using such techniques; however, without using such techniques, thermal dissociation is inconsequential at these temperatures. M e t h o d . T h e reactor was brought on stream i n the following manner: air was used to fluidize the bed while the heaters brought the bulb temperature in the bed space to between 5 0 0 ° a n d 5 3 5 ° C . This corresponds to thermal equilibrium with a heater sheath to reactor wall temperature of 6 5 0 ° C , the control point. Prepurified ( 9 9 . 8 % ) grade nitrogen was then used to displace the air from the system. T h e nitrogen, i n turn, was displaced from the assembly by chlorine. T h e reactor bulb temperature site was monitored by spot temperatures taken by periodically inserting the thermocouple into the bulb. The reason for not leaving the thermocouple in the bulb throughout the reaction has been explained above, and mention here need only be made that the chlorine leakage consequent to the assembly method destroyed thermocouples if they were left i n contact for any period of time. T h e complete displacement of nitrogen was indicated by condensation of chlorine i n


682

COAL SCIENCE cm.

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683


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the dry ice-methanol chilled condenser which led to the first tar chiller. W h e n this time point was achieved, coal was slowly introduced by whichever feed device was used, chlorine of course was being passed through the apparatus continuously and was not recycled. Results In a given run for more than 3 hours as much as 2.5 liters of liquid chlorine accumulated i n the last tar chiller. This large reservoir of chlorine introduced operational problems i n the system and forced early termination of the runs. The actual experiments reported here are exploratory and certainly qualitative in character. Table I shows some of the usual parameters for measuring and logging reactions. T h e unreported runs represented either failure of some part of the apparatus because of the extreme corrosiveness of the system studied or failure of the system to achieve design temperatures or runs using oxygen mixtures. The significant runs, from the standpoint of characterized products, are N o . 14 and N o . 15. A n infrared absorption spectrum (Figure 3) was obtained using the residual gases after chlorine was removed by distilling l i q u i d chlorine. F o r comparison Figure 4 shows the spectra for thionyl chloride, Figure 5 for carbon tetrachloride at several concentrations, and Figure 6 for phosgene. G o o d evidence exists i n Figure 3 for small amounts of phosgene, somewhat more carbon tetrachloride (ca. 0.25 atm.), together with a third compound


43.

SPALDING ΒΤ AL


685

microns spectra for

phosgene

closely related to thionyl chloride, as the triplet at 1360 c m . ' shows. Yet it otherwise is different. Also, neither carbon-hydrogen stretch frequencies nor carbonyl-oxygen frequencies were evident. T h e other product was a subliming white solid of cedary fragrance which was also difficult to dissolve. T h e remaining coal solids were readily burned out by air at a bed temperature of 5 0 0 ° C . T h e beads were left with a very pink residue which d i d not erode dur ing the tumbling action of the bed but h a d to be dissolved i n concentrated nitric acid then washed free of acidity by repeated washings with distilled water. T h e beads were washed free of acidity so that on reuse there w o u l d be no doubt concerning the presence of proton catalysis of the reaction rather than free radical reaction. T h e presence of phosgene i n coal chlorination prod ucts when using ultraviolet light and lower temperatures has been observed by Pasha ( 2 ) . Under these conditions phosgene was about three or more times more concentrated. However, i n the products of these experiments no evidence was seen i n the absorption spectra for either carbon tetrachloride or the sulfur compound. 1

Summary Production as measured by lower molecular weight products is presently about 5% of the coal charged. So far, the evidence has favored only perchlorinated products. Provisions have been made to recycle the chlorine i n order that longer runs would permit a closer approach to exhaustive chlorina tion of coal.


686

COAL SCIENCE

Acknowledgment T h e authors are deeply indebted to the extensive support provided b y the National Science Foundation which made grants to obtain the apparatus and to support the research of M r . Burckle. Literature

Cited

(1) Lowry, H . H., E d . , "Chemistry of Coal Utilization," Suppl. E d . , p. 78, John Wiley and Sons, New York, 1963. (2) Pasha, M . , Master's Thesis, University of Louisville, 1964. (3) Steacie, E . W . R., "Atomic and Free Radical Reactions," p. 37, Reinhold Publishing Co., New York, 1946. RECEIVED October 5, 1964. Contribution from the Chemical Engineering Department, Speed Scientific School, University of Louisville, Louisville, Ky.

Discussion Bhupendra K . M a z u m d a r : I do not see how a significant amount of useful products could be obtainable by degradation i n the presence of chlorine. W e have found that treatment by chlorine before or during pyrolysis inhibits tar formation completely. Consequently, 9 3 - 9 4 % of the carbon i n the coal is fixed in the char. Thus, only a relatively small proportion of carbon could be devolatilized, leading to the formation of small amounts of "interesting products." S. C . Spalding: Concerning the necessarily simple product spectrum from the chlorination, I would comment that the physical evidence on yields from the run at 5 0 0 ° C . would seem to preclude the possible formation of a structurally simple product. It seemed that about one-third of the coal appeared as an o i l . Some white solid substance sublimed out of the reaction zone and precipitated i n the cooler zones.


Gas Phase Chlorination of Coal - Advances in Chemistry (ACS

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