INDUSTRIAL AND ENGINEERING CHEMISTRY
September 1951
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C'. i n rases 1 a n d 2j particle radius radius variable inside pitrticle t = time T' = volume, of solut.ion iri rcnctor I T 7 = ni:iss of caatalyst iii+iilv iwirtoi, ( t i e f i r i c ~ c l ciiKc~i.rritlv in ( ' R S ~ S1 anti 2 , x = tlist :inee measured along re:tcbtor a .= f'r:ictional void volunie jn p:t(ckrti tube reactor y = i't~:ictionalvoid volume 111 I):ti.tirle p = p r t i c l e density
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L I T E R 4 T U R E CITED
(1) 13aun1an, TV. C . , a n d Is:ichorn, ,J., J . A m . ('hrnz. SOC.,69, 29;{ (1947). ( 2 ) D u n o y e r , J. N., Conzpt. r e n d . , 228, 1721) (19N). (3) Haskell. V. C . , a n d IIaiiimett, L. P., J . A m . C h e m . Soc., 71, 1284 (19411). ( 4 ) Levesque, C . I.., a n d Craig, A. &I., ISD. Es-o. ( ' H E M , , 40, !1fj 11948). a n d Amuiidson, N. R . , Ihid., 42, 1481 (1950). . Y., J . I'hys. Chcm. ( U . S . S . R . ) , 19, :37ii (lC1451, (7) ZM.,20, 1421 1946). ( 8 ) lbid., 21, 1019 (1947). (9) I'shezhetskii, S. Y . , a n d Rubinstein, It., Acta I ' i i ~ s i ~ ~ ;cuit .t C.R.S.S.,21, 1075 (1940). (10) Sanborri, C. E., P h . D . thesis, 1-nivcrsity of Minnesota, 1949. (11) Sussnian, S., 1x0.Ex;. CHEM.,38,1228 (1946). (12) Thiele, >:. JV., Ibid.,31,916 (1939). (13) T h o m a s , G . G., a n d Davies, C. TV., .t-nturc, 159, 372 (1947). 114) IVapiier, C., Z . p h y s i k Chem., 193, 1 (1943). (15) TVicke, IC., A7~gew.Chem., B19, 57 (1947). (16) Wicke, E., and Voigt, U., Anoew. Chem.. B19, 9 4 (1937). (17) Wilke, C . R., Chcm. Eng. Progress. 45, 218 (1949). (18) Zeldowitsch, J. B., Acta P h y s i c o c h i m . C.X.S.S., 10, 583 (l!L%l].
(modified Susselt number) K K C G I Y E DFebruary 2 4 , 1951. Based on part of the t h w i n < i i t i i r i i Norman L. Smith in partial fulfillinent of the reqnireiirents f o r t l i , of doctor of philosophy a t the University of Minnesota.
Improved Yields of p-Dichlorobemene ,SUBSTITUTIVE CHLORINATION OF BENZENE HERBERT F. WIEGANDT AND PETER R . LANTOSI C O R N E L L U N I V E R S I T Y , I T H A C A , N. Y.
In t h e substitutive chlorination of benzene 0- a n d pdichlorobenzenes form in varying ratios. Optimum conditions and proper catalysts are desired for preferential formation of t h e m o r e valuable para isomer. Increasing temperatures not onl>- piye a m o r e diffuse distribution among mono-, di-, and tri~hlorohenzenes b u t are ~rnfavorablet o t h e formation of para in the dichloro fraction. lligher para-ortho ratios experienced as chlorination progresses result i n part from a n inhibiting effect d n e t o the presence of o r t h o itself and from the relatively slow rate a t which para chlorinates. I , o w catal y s t concentrations sizch 21s 0.05 mole (j" of iron or a n t i mony pentachloride, t h e presence of polar compounds, a n d certain catalyst combinations are effective. These techniques niay be readily applied t o existing batch or continuous methods. Continued sttidy shoirlri suggest f u r t h e r inmprovenients. 1 PreRent address, Hnyoii Technical I)i~isions,15. I. dii I'ont d e C Co., U'ilrninpton. 1h.1.
Sieiiiiiiirh
I
INDUSTRIAL AND ENGINEERING CHEMISTRY
2168
Vol. 43, No. 9
Making some reasonable simplifying assumptions, MacMullin
TABLE I. 1949 PRODUCTION
AND SALES OF
CHLORINATED
BENZENES (18)
Chlorobenzene o-Dichlorobenzene 13,080,000 Production, lb. 251,503,000 10,959,000 Sales,lb. 32,445,000 $0.07 Unit value $0.06
p-Dichlorobenzene 33,6OO.000 33,546,000 $0.10
IMPROVED YIELDS OF p-DICHLOROBENZENE
The subject of the relative quantities of 0- and p-dichlorobenzenes formed has long been of interest to the industry. Thomas (17) reports t h a t when anhydrous aluminum chloride is the catalyst, the para isomer predominates, while ferric chloride favors formation of ortho. This is not always the case as will be seen later. Holleman (10) records para-ortho ratios of 2.26 and 1.4 with aluminum chloride and ferric chloride a s catalysts, respectively. Stoesser and Smith (16)considered the use of antimony trichloride and various combinations of it with sulfur, iron, and lead. Wide variations in the relative proportions of para and other polysubstituted products were observed, not only with benzene but also with toluene and biphenyl. Further considerations which take into account catalyst concentration, temperature, extent of chlorination, etc., are included in the present work. Brunjes (5) designed a special reactor to obtain maximum yields of either mono or dichlorobenzenes. An optimum temperature of 35" C. has been yeported for the production of chlorobenzene (5, 6 ) , while 75" C. is reported best for making dichlorobenzenes (5). Commercial chlorination of benzene to chlorobenzene is described by Lee (I$), to dichlorobenzene, by White ($0). A continuous chlorinator has been described (16)t h a t operates between the boiling points of benzene and chlorobenzene. Justoni (11) reviews theories and modern practice and includes a list of 102 references. It is generally agreed t h a t in liquid phase chlorination only traces, or very small quantities, of the meta isomer are formed (8-10).
(IS)derived expressions for the relative rates of product formation in batch chlorination, and for one- and two-stage continuous chlorinations. His analysis shows t h a t batch chlorination gives the most selective product distribution. I n other words, a t a n y given level of chlorination the maximum amounts of mono-, di-, or trichlorobenzene will exist by this method. He shows further t h a t a reaction system of a n infinite number of stages, with the liquid progressing from one vessel to the next and chlorine gas bubbling into each, gives a product distribution identical with t h a t obtained from batch chlorination. On the other hand, a single continuous reactor into which fresh benzene is flowing and chlorinated product is being removed will give the least selective product distribution. One assumption made by MacMullin is that the rate of chlorine introduction does not affect the product distribution. This was verified experimentally in these laboratories by a series of chlorinations with a n over-all sevenfold variation of chlorine flow in which no changes in product or isomer distributions were observed. Since at very high chlorine rates a.considerable amount of chlorine went unreacted, it can also be concluded t h a t the concentration of hydrogen chloride and chlorine in the vapor stream has no effect on the product or isomer distribution. The analysis of MacMullin can then be further extended to include truly concurrent and countercurrent operations. For commercial operation concurrent chlorination can most easily be visualized as taking place in a tube and countercurrent chlorination in a tower (probably packed), with upward passage of the gas and downward flow of liquid. Ideally, either of these methods would give results identical to a batch chlorinator. Longitudinal mixing would contribute some of the characteristics of a single kontinuous reactor. Since there is a minimum of such mixing in a long-tube reactor, it likely is the best type of continuous chlorinator.
T
J-1
Figure 1. Batch Chlorinator A. B. C.
D. E.
F. G.
H. J. K. L. M.
Chlorine cylinder Chlorine valve Drying tube Capillary flowmeter Differential manometer Static manometer Vent stopcock Chlorine inlet bulb Half-moon stirrer Mercury seal Stirrer motor Sampling tube
^U
N. Thermometer P. Sampling flask
Q. Reflux condenaer R. Chilled flask
Vacuum manometer Aspirator pump U . Immersion heater V. Thermostat W . Relay X. Stirrer Y. Crock Z. Flask S. T.
U
Figure 2.
A. B. C.
D. E. F.
G. H.
J.
K. L.
M. N. 0.
Continuous Chlorinator
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.
Mercury safety blow-out Notch-ground stopcock Capillary flowmeter Differentialmanometer Withdrawing siphon Product receiver Gas line Crock Thermostat Immersion heater Relay Thermometer CentrifuRal pump Vent to hood
IN Du
September 1951
sTR IA L A N D EN G IN EER IN
EXPERIMENTAL
Chlorinations were carried out in both batch and continuous laboratory equipment. The batch chlorinator consisted of a 3liter, 3-necked, round-bottomed flask immersed in a constant temperature bath. I t was equipped with a mercury-sealed agitator, chlorine inlet, reflux condenser, thermometer, and sampling tube. The continuous chlorinator made use of two waterjacketed glass condensers which were assembled vertically in series. The reaction took place in the lower one, while the upper one served simply as a condenser. Figure 1 and Figure 2 are diagrams of the reactor assemblies. The chemicals used were dried nitration-grade benzene; chlorine, p-dichlorobenzene (99.5%), and o-dichlorobenzene (80 to 85%) from Hooker Electrochemical Co.; chlorobenzene (99%), from Eastman Kodak Co.; catalysts (reagent or C.P. grade) from Baker and Adamson; and some additional o-dichlorobenzene from Paragon Laboratories (c.P. label). BATCHCHLORINATION. The reactor was charged with about 2000 grams of hydrocarbon and a carefully weighed quantity of catalyst was added. After 15 minutes of stirring to allow solution or suspension of the catalyst, a controlled steady flow of chlorine was introduced. Samples of about 100 grams were withdrawn every 3 to 6 hours depending on the temperature and chlorine flow rates. Hydrocarbons not returned by the overhead condenser were removed in an ice-cooled trap, measured, and returned to the reactor. CONTINUOUS CHLORINATION. A hydrocarbon-catalyst mixture was prepared and maintained in uniform suspension in the mixing flask. The desired flow rates of liquid to the top of the reaction tube, and chlorine gas to the bottom inlet were established, and a liquid level of 75 em. (100-ml. reactor volume) was maintained by adjustment of the take-off siphon. ANALYTICAL METHODS.Analytical methods were standardized for purposes of accuracy and, particularly, reproducibility. The following analytical scheme was adopted after standardization with known samples: The sample was washed with dilute (1C%) sodium hydroxide and with water in a separatorv funnel. The ureciuitate. which retained small quantities of hydrocarbons, was add-ed t o 'the organic layer and the mixture steam distilled under vacuum (160 mm. absolute pressure). The distillate was collected in an icechilled receiver, the organic condensate separated and dried. I n the event that a high concentration of p-dichlorobenzene was present, the total condensate was warmed t o dissolve all organic crystals before making the separation. The organic material was distilled in an efficient column (10 plates) with adjustable reflux control. A reflux ratio of 20 t o 1 was used in passing between plateaus. Fraction 1, 80' to 105' C. was benzene. Fraction 2, 105' t o 150' C., was chlorobenzene. I n order to avoid going to vacuum operation or resorting to high boilers, the column was drained when distillation became difficult and the residue subjected t o simple distillation. Fraction 3, for example, was obtained by simple distillation to 179" C., and constituted dichlorobenzenes. A portion of fraction 3, 168" t o 179" C., was taken for finding the percentage para isomer by freezing point determination and locating the composition on a eutectic curve such as Holleman's (10). If trichlorobenzenes were present, fraction 3 was completed by continuing the distillation from 179" to 190" C. The residue was considered trichlorobenzene.
G
c H E M I s T R'Y
2169
chlorination of benzene shows no evidence of a n induction period as experienced in alkylation or isomerization (7). Sulfur behaved in an irregular manner in that a long induction period, probably during which sulfur chlorides form, is required before the reaction gets under way. The concentrations of Table I1 were used a t only the one chlorine flow rate and do not represent optimums o r minimums. Figure 3 shows the immediate start of the chlorination reaction when benzene-catalyst mixtures are shaken in stoppered flasks in which the vapor space is originally filled with chlorine. c u 006
c
om g 004
Oo3 002 001
000
5 10 15 T i m . Minutes
0
Figure 3.
20
25
i t
5b
Initiation of Chlorination Reaction
Catalyst concentration in mole per cent
The only variable found to have an effect on the distribution curve for the progressive formation of mono-, di-, and trichlorobenzenes is temperature. Figure 4 averages 66 chlorination runs to show that increasing the temperature gives a broader product distribution at any chlorination level. Any successful catalyst, catalyst concentration, or chlorine rate can be used to produce any one of the curves a t the indicated temperature. These temperature effects agree with Bourion (1, 2) who concluded that for a 10' C. rise in temperature the rate coefficient for the chlorination of chlorobenzene is 8.5% higher than that for the chlorination of benzene.
RESULTS
Table I1 compares the effectiveness of a number of catalysts for a feed rate of 50 grams of chlorine per hour into the batch charge of 2000 grams of benzene. The ineffective catalysts are those which permitted appreciable amounts of unreacted chlorine to escape in t h e vapor and resulted in considerably reduced chlorination rates.
TABLE
11. CATALYST P E R F O R h l A N C E
Effective Catalysts, Mole % AlCla 0.058 FeCh', 0 , 011 11, 0.553 GbCls, 0.048 Fe owder 0 . 0 4 8 Sn&;r, 0 . 2 $ 6
Ineffective Catalysts, Mole yo Carbon 0 . 3 0 0 PClS 0 ' 5 0 0 cuciz,o 434 S 0.300 ZZCl,, 0.552
Although chlorination may be inhibited by the presence of small quantities of water or alkali, under normal conditions the
0
0.5
1.0 1.5 Chlorine Atoms 1 Mole .Benzene
2.0
Figure 4. Successive Formation of Chlorinated Benzenes at 30°, 50°, and 70" C,
On the other hand, the relative amounts of ortho and para isomers in the dichlorobenzene fraction are affected by a number of process variables. I t was observed early in this investigation that, regardless of catalyst, for a given chlorination a t constant temperature the concentration of para in the dichloro fraction increases as chlorination progresses. Figure 5 is a typical example. Above chlorination levels of two chlorine atoms per benzene molecule the percentage of para increases even more rapidly. This is because the ortho isomer chlorinates more rapidly than the para and, therefore, disappears much faster. This effect is noted in the literature (21) and is the subject of a patent by Britton ( 4 ) in which para is removed from the dichloro fraction by crystallization and the liquid now rich in ortho is chlorinated. The ortho is preferentially converted to trichlorobenzene and the dichloro
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Vol. 43, No. 9
iravtion may again be cooled to yield a n additional fiaction ( i t p t r a crystals. Figure 5 shows this effect. Bv chlorinating to
