February, 1941
INDUSTRIAL AND ENGINEERING CHEMISTRY
pounds per square inch and a mole ratio of 19-22:l (Figure 7) revealed that a t 330" C. (626" F,) polychloropentanes and hexachlorobutadiene constituted a substantial portion of the product. As the temperature was elevated to 375" C. (707' F.) the polychloropentane content of the product became essentially nil; however, hexachlorobutadiene still constituted 24 per cent of the product. At 400" C. (752O F.) a maximum carbon tetrachloride and hexachloroethane content of 89 per cent was obtained, and a t this temperature the hexachlorobutadiene content was 10 per cent. It might be anticipated that further increases in temperature would be attended by a continued decrease in the hexachlorobutadiene content of the product. Actually it was found that the products from runs made a t 450" and 500" C. (842" and 932" F.) contained larger amounts of hexachlorobutadiene than did those runs made a t 400" C. This apparent anomaly is probably due to the decreased chlorine concentration in the liquid phase a t the higher temperatures.
185
hexachlorobutadiene. I n addition to these conversions to the desired products, it was shown that hexachlorobutadiene can be converted to hexachloroethane by further chlorinolysis. The over-all yields of carbon tetrachloride and hexachloroethane would approach the theoretical value in a process involving further conversion of hexachlorobutadiene.
Acltnowledgment The authors are indebted to the Sharples Solvents Corporation for defraying the expense of this investigation. J. F. Olin (Sharples Solvents Corporation) and G. A, Hawkins (Purdue University) made suggestions concerning this investigation which were of considerable value.
Evaluation of the Chlorinolysis Reaction
Literature Cited
The observation was repeatedly made that the proportion of carbonaceous material in the product and the carbonaceous deposit on the reaction tube walls were always very low when the optimum operating conditions were employed. These observations are in accord with the fact that it has been possible to isolate 95 per cent of the carbon contained in the starting material as carbon tetrachloride, hexachloroethane, and
(1) Brooks, B. T., IND. EN*. CHEM.,17, 752 (1925). (2) Grebe, J., Reilly, J., and Wiley, R., U. S. Patent 2,034,292 (March 17, 1936). (3) Hartmann, E., Be?., 24, 1011 (1891). (4) E ~ K.,~ Ibid., ~ , (1877). BASEDupon a thesis submitted b y Earl Pierson t o the faaulty of P u r d u e University in partial fulfillment of the requirements for the degree of doctor of philosophy, 1941.
HIGH-PRESSURE CHLORINATION OF PARAFFINS E. T. MCBEE, H. B. HASS, AND J. A. PIANFETTI Purdue University and Purdue Research Foundation, Lafayette, Ind.
T
H E discovery (8) that liquid-phase chlorination of paraffin hydrocarbons yields a higher percentage of primary substitution products than are obtainable in the vapor phase at the same temperature led to an investigation of highpressure chlorination a t elevated temperatures (1). During this investigation i t was found that in the chlorination of propane at 300" C. there is a n increase in the yield of the primary substitution product with a n increase in pressure. This discovery was advanced tentatively as chlorination rule 11: "In vapor-phase chlorination of saturated hydrocarbons, increased pressure causes increased relative rates of primary substitution.'' Since this work is not only of considerable theoretical interest but also of potential practical value in obtaining higher yields of the generally more desirable primary substitution products, the investigation-has been extended with several hydrocarbons to include pressures as high as 4000 pounds per square inch. Figure 1 shows a drawing of the apparatus used. A charge was prepared by first introducin the material to be chlorinated into the mixing and supply tan%, 9. To obtain a known and desired amount of chlorine, a tank of liquid chlorine was attached
Liquid-phase chlorination of paraffin hydrocarbons yields a higher percentage of primary substitution products than are obtainable in vapor phase at the same temperature. Thus, in the thermal chlorination of paraffin hydrocarbons, high pressures may be desirable in order to maintain liquid phase. In vapor-phase chlorination of saturated hydrocarbons, increased pressure causes increased relative rates of primary substitution. Since there seems to be a direct correlation between the change in the relative chlorination rate and in the molar volume of the hydrocarbon with increasing pressures, it appears that pressure alters the relative chlorination rate by effecting a greater absolute concentration of the hydrocarbon in the vapor phase.
at 2 and the vapor was allowed to flow into a cooled receiver 1, where it condensed. The receiver was weighed before and aker filling by detaching it from the apparatus at union 3. When the desired amount had been collected, the valves in the line were opened t o allow the chlorine to vaporize and pass into tank 9 where solution with the material t o be chlorinated occurred. A bath of warm water was applied t o receiver 1 to facilitate com-
INDUSTRIAL AND ENGINEERING CHEMISTRY
186
plete and rapid transfer of the chlorine. After the reactants had been added to tank 9, the stirrer was operated for a short time to ensure homogeneity. TWOtanks of nitrogen were usually connected to the system; one tank was nearly full and the other partially discharged. The valve on the partially discharged cylinder was opened, and uitrogen was allowed to flow into t,he apparatus until the pressure in the two was the same. Still higher pressures were obtained by admitting nitrogen from the other cylinder. When a pressure above that found in commercial nitrogen cylinders was desired, nitrogen from a cylinder mas picked up by a booster pump, 35, and compressed until the predetermined working premire was attained. The pressure in the apparatus was measured by gages 32 and 17. Valve 30 was closed when sufficient nitrogen litid been added to the syskm.
Relative Chlorinolion Role
EFFECTOF PRESSURE ON RELATIVE CHLORATEOF HYDROGEN ATOIIBIN PROPANE AT CON-
FIGURE^. RINATION
ST.4NT TEJIPERATURE
The reactor vias maintained in an electrically heated oil bath. 22, a,nd the temperature was controlled by a recording potentiometer. After the temperature and pressure had been properly regulated, valve 6 xvas opened and the reactants were pumped into the reactor by a proportioning pump, 12. This ump sorvrd ta.0 purposes: It pumped the reactants against tge predetermined nitrogen pressure, and the rate at which the reactants ir-ere pumped into the system could be controlled by adjusting the length of the piston stroke. Considerable difficulty was experienced a t the outset in pumping mixtures containing liquid chlorine against high pressures. These difficulties were circumvented by maintaining a column of sulfuric acid in a vertical pipe (not shown in Figure 1) which extended from pump 12 to the pair of check valves, 11. By means of this arrangement, the reactants passed through the c,heck valves without coming in contact with the pump and thus avoided the troublesome deleterious effects of the hydrocarbon and chlorine on the packing material. The reaction products were cooled by condenser 20 (beyond the re-
Vol. 33. No. 2
actor) and collected in receiver 27. The material was removed from the apparatus through valve 29. Valve 24 was provided so that the product from the reactor could be tested for free chlorine with potassium iodide solution. The rate of pumping and therefore tho optimum exposure time could be determined by observing the amount of free chlorine present. The exposure time should be just, long enough to permit substantially all of the chlorine to react. All of the metal parts which were in contact with the reactants or the products were made either of R'lonel or nickel. Iron is not satisfactory becaure ferric chloride, which is readily produced, is a catalyst for splitting off hydrogen chloride from the chlorohydrocarbons formed in tlie reaction.
REPRESENTATIVE data obtained by the chlorination of propane, n-pentane, and 7dieptane are given in Table I and summarized in Figures 2 to 5 . These data indicate that the absolute concentrations of the reactants in the chlorination of paraffin hydrocarbons affect the ratio of the isomeric m o n o c h 1o r i d e s , a n d hence the relative chlorination rates are calculated from the percentage of the isomeric monochlorides and the number of hydrogen atoms of e a c h t y p e . T h e method of computation is best illustrated by a specific example: The percentages of l-chloropropane and 2-chloropropane from a given chlorination were 48 and 52, respectively. A primary hydrogen atom is arbitrarily assigned a relative chlorination rate of 1.00. The relative chlorination rate of a secondary hydrogen atom is X (to be calculated). Since there are six primary and two secondary hydrogen atoms in propane, the value of X is found to be 3.25--Viz.,
1000 Ib./sq,in
3c
26
26 e
6
e
e E
24
t" 221
201
181 f
5 ative
3.0 Chlorinotion
5 Rote
FIGURE 3. EFFECT OF PRESTEMPERATURE ON RELATIVE CHLORINATION RATE OF HYDROGEN Arroiis IN PROPANE SURE AND
187
INDUSTRIAL AND ENGINEERING CHEMISTRY
February, 1941
I
Chlorination rule 11 is further substantiated by data shown graphically in Figure 2 . Propane was chlorinated at pressures ranging from atmospheric to 4000 pounds per square inch and a t approximately 225' C. (437" F.). Increased pressure causes increased relative rates of primary substitution. The relative chlorination rate decreases from 3.6 at atmospheric pressure to 2.65 at 4000 pounds pressure. It is significant t h a t a substantial part of this change (3.6 to 3.0) occurs with a n increase in pressure from atmospheric to 500 pounds, and that each additional increment of pressure becomes increasingly less effective in causing a decrease in the relative chlorination rate. A plot showing the pressure-volume relation of one mole of propane a t 225" C. gives a curve much like that of Figure 2. Since there seems t o be a direct correlation between the change in the relative chlorination rate and in the molar volume of propane with increasing pressures, it appears that pressure alters the relative chlorination rate by effecting a greater absolute concentration of propane in the vapor phase. The theoretical interpretation of this phenomenon is still not entirely clear. The hypothesis of two or more competing mechanisms, a t least one of which is affected by the absolute concentration of the reactants, seems plausible. The chain mechanism ( I ) ,
-
+ + +
c1, --e+ e1 e1 R HCl RC1 C1
+ C1R + Clz
Rh
probably accounts for most of the chlorination, particularly a t low temperatures and pressures. At high temperatures
and high pressures some substitution involving a bimolecular metathesis may occur. High pressures should favor this reaction, and if it should happen to be relatively nonselective, this circumstance would give rise to the results observed in Figure 2. Propane was chlorinated over a temperature range of 185" to 300' C. (365" to 572' F.) and a t 1000, 2000, and 4000 pounds per square inch pressure (Figure 3). As the temperature is increased, there is a definite but slight decrease in the relative chlorination rate of secondary hydrogen atoms a t each of the three pressures. A considerably greater change was expected from the results of the chlorinatioii of propane in vapor phase a t atmospheric pressure (9). The three curves of Figure 3 are nearly parallel as should be the case since there is a constant difference in pressure over the same temperature range. From the practical point of view for obtaining high yields of primary substitution products, an increase in pressure above 1000 pounds per square inch seems undesirable because more expensive equipment would be required, and the ratio of the isomeric monochlorides is not altered appreciably. Also, little advantage results from chlorinating at high temperatures when high pressures are employed. The optimum temperature for practical operation seems to be that a t which a suitable rate of reaction is obtained. As is true for other types of chlorination, the rate of substitution a t high pressures is greatly accelerated by an increase in temperature.
2.8
I
I
,
Liqpid phose
1
A N D PRESSURE UPON THE TABLE I. EFFECTOF TEMPERATURE RELATIVE CHLORINATION RATESOF HYDROGEN ATOMS IS PARAFFIN HYDROCARBONS
Expt. No. 5-76 5-74 5-42 5-96
5-69
5-32 5-30 5-111 5-112 5-110 Hatch
5-43 5-49 5-35
5-116 5-48 5-53 5-118
5-8 J-9
5-23 5-21
5-40 5-36 J-99
5-117 5-67 5-71 J-70
240 230 185 200 210 225 225 240 250 260 300 200 200 225 260 185 185 225
5-68 5-89
1.2-2,6 0.4-0,s 5 .O-0. 0 1.6
10.5 9.2 15.7 13.4 12.0 15.5 13,4 12.2 11.2 15.8
0,5-0,8 0.4-1.0 0.4-1.0 1.0 0.3 0.3
.....
1R 0 1l.d
0.3-0.6 0.8
0.25 0.8 0.4 0.4
13.1 13.9 13.7 14.5 15.1 14.3
210 260
n-Pentane Chlorinated 1000 2.9 1000 3.0 1000 3.0 1000 2.95 1000 2.85 1000 2.9 1000 2.7 2000 2.6
5.0 5.0 2.0 1.3 0.4-1,O 0.25-0.4 0.4 0.2
20.2 18.2 16.3 16.0 15,5 14.7 14.2 15.0
175 200 225
%-Heptane Chlorinated 1000 2.3 1000 2.4 2000 2.3
0.4
0 6 0.4
12.5 13.2
150 150 150 180 180 195
CCla Used 5-64
Propane Chlorinated 250 3.0 500 2.9 1000 2.95 1000 2.9 1000 3.0 1000 2.9 1000 3.05 1000 2.85 1000 2.6 1000 2.85 1000 2.6 2000 2.8 2000 2.7 2000 2.7 2000 2.5 4000 2.7 4000 2.8 4000 2.65
88
190 226 190
0.3-0.5
17.5
Solvent for Reactants (Propane a n d Chlorine) 1000 1000 3000
2.7 2.5 2.6
1.2 0.6 0.0
14.5
15.2
12.5
I
2.3 .35
I
.w
I .a
.50
.55
.M)
Log Relative Chlorination Rate
FIGURE4. COMPARISOX OF LIQUID-PHASE AND HIGH-PRESSURE VAPOR-PHASE CHLORINATION OF PROPANE
T H E logarithm of the absolute temperature plotted against the logarithm of the relative chlorination rate (Figure 4) gives a straight line for the liquid-phase chlorination of propane. Above the critical temperature the position of the line depends upon the pressure. However, the lines in this region above the critical temperature approach the extrapolated liquid-phase line as the pressure is increased. The relative chlorination rates obtained by chlorinating propane using carbon tetrachloride as a solvent, fall upon this extra-
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 33, No. 2
FIGERE 5. COMPARISON However, this difference, if it is real, is only very slight. It is conceivable that factors such as pyrolysis or phase cause the effect observed in Figure 5 . Hence, the following generalization is tentatively advanced : I n high pressurehigh temperature chlorination of normal paraffin hydrocar280 bons, the relative rates of primary substitution increase with IKCH PRESSURE the length of the carbon chain. For example, in the chlorination of n-pentane and n-heptane a t 1000 pounds per square 260 inch pressure and 180" C., the relative chlorination rates were found to be 2.9 and 2.4, respectively. Since this m a temperature is below the critical one for both n-pentane and C j 240 n-heptane, the difference in relative chlorination rate appolated portion of the parently must be attributed t o an inherent difference in the 56 liquid-phase line; they two hydrocarbons. indicate that in these 220 Such differences in relative reaction rate can be accounted E e x p e r i m e n t s (which f for by differences in activation energy of the order of a few were above the critical hundred small calories. Such small differences are possible, temperature for both 200 -* but the present state of kinetics is not sufficiently refined to reactants but below the predict them. As the chain becomes longer, a steric repulcritical temperature of sion or screening of the secondary hydrogen atoms by other the solvent) chlorina180 parts of the chain may occur, which would inhence the end tion may have occurred methyl groups only slightly and would render the secondary a t the effective concenhydrogen atoms less available for collision with chlorine tration of the liquid I60 atoms; this would account for the observed differences in phase. the relative chlorination rates. Such an effect would probThese data lend supably be operative t o a greater extent in chains longer than five port to the hypothesis 140 carbon atoms, for in such cases the chains can double back W that differences in the in the form of a coil or some similar configuration. relative c h l o r i n a t i o n rates in liquid-phase Literature Cited and vapor-phase chlo- (1) Hass, MoBee, and Hatch, IND. Eaa. CHEM.,29,1335 (1937). rination a t a given temperature are due essentially t o concen(2) Hass, McBee, and Weber, Ibid., 28, 333 (1936). tration. PRESENTED as part of t h e paper on Recent Progress in Chlorination, 1937By a careful analysis of the reaction products formed when 1940 (see page 137 of this issue). This paper contains material abstracted chlorination is effected a t 1000 pounds per square inch and from a thesis submitted by J. A. Pianfetti t o the faoulty of Purdue Univera t elevated temperatures, i t was found that the relative sity in partial fulfillment of the requirements for the degree of doctor of chlorination rates vary with the length of the carbon chain. philosophy, 1941. 300-
.Propane n-Pentan on-Hepten
:
0
Y
OF RELATIVE CHLORINATION RATEOF HYDROGEN ATOMSIN PROPANE, 72-PENTANE, AND nH E P T A N EA T 1 0 0 0 POENDSPER SQUARE
!
I,
DESTRUCTIVE DISTILLATION OF MAPLE WOOD DONALD F. OTHMER AND W. FRED SCHURIG' Polytechnic Institute, Brooklyn, N. Y.
ARDWOOD, of which maple is a typical example, is an
H
agglomerate of highly complex carbohydrates and other molecular structures; and the action of heat upon it in the absence of oxygen results in a series of chemical reactions. This heat treatment and progressive thermal decomposition cause additional heat to be liberated upon the successive destruction of the various molecular structures. The exothermic reactions do not proceed for any great length of time, and the evolution of volatile products soon subsides unless the heating is continued. The temperature a t which each individual exothermic reaction starts is hard to define; but each particular reaction product is mainly formed during some definite temperature range. The aim of the commercial process is to obtain the greatest value of salable products a t minimum cost. To some extent these yields can be controlled by variation of the temperature of distillation and of the moisture content or seasoning time I
Present address, College of the City of New York, New York City.
'of the wood. While information on all of these variables is desired, the time of seasoning is particularly important. Usual practice requires the harvesting of wood a t least a year before its use. This entails high inventory costs, losses due to rotting, fire and theft in the woods, and sometimes additional handling costs. Predryers may eliminate these disadvantages t o some extent, but many plants are not so equipped. The temperature of destructive distillation influences materially the products formed and their quantity. Several stages in the distillation of the wood may be observed. First there is the evaporation of the uncombined moisture from the wood during which operation small amounts of volatile constituents steam distill. The first partial decomposition occurs between 200" and 250" C. (392' and 482" F ) . Next comes the exothermic stage or that part of the distillation in which the lignocelluloses decompose; finally, if the heating is continued, the high-boiling tarry materials are driven off from the charcoal to complete the decomposition, and a "dry" charcoal remains.
