Julian M . M avity, Earl E. Z c t ter h o lm , and George L . Hervert U n iv e r s a l O il P r o d u c t s C o m p a n y , R iv e r s i d e , III.
Laboratory data are presented on the catalytic dehydro
genation of ethylbenzcne over a chromia-alnminn catalyst in the presence of benzene. Reduction of the partial pres
sure of ethylbenzcne by the diluent has the same favorable clTccls as subatmospheric pressures; in addition, the benzene serves as a heat carrier for the cndothcrmic dchy- drogcnation reaction. Benzene is relatively stable under the conditions used and has no deleterious effect on the catalyst. Weight ratios of benzene to ethylbenzene vary
ing from about 10 to 0 were investigated. Dilution de
creased conversion at high ethylbenzene space velocities, increased it at low space velocities. At a given conversion, dilution improved styrene ultimate yield and rcdixced car
bon formation. Reduction of pressure below atmos
pheric, in addition to the use of benzene diluent, further improved the selectivity of the dehydrogenation reaction.
Under preferred conditions (benzene to ethylbenzene weight ratio of 5, atmospheric pressure, 3.8 ethylbenzene space velocity, and 581° C. at the catalyst bed inlet) the average conversion in a series of simulated recycle runs was 34.8%, styrene ultimate yield, 89.7 weight % .
A
PREVIOUS paper (4) presented laboratory scale data on the catalytic dehydrogcnation of ethylbenzene to styrene and demonstrated the advantages of carrying out the reaction at pressures below atmospheric. Another method which has been used to produce similar effects but which avoids the necessity of sub- atmospheric pressures involves carrying out the dehydrogenation in the presence of some added material; the partial pressure of the ethylbenzene is thereby reduced. This method has an added advantage in that the diluent can be used as a heat carrier for the endothermic dehydrogenation reaction.
The use of various inert gases, such as carbon dioxide (2, 6,10), aitrogen (2, 6, 10), and methane (2), has been cited in a number of patents, both domestic and foreign. The Dow process (1, 6) now in extensive commercial use employs steam. The use of benzene for this purpose has been suggested by Smith (6).
Benzene was well adapted for present work, since it has no deleterious effect on the chromia-alumina type of catalyst em
ployed, is relatively stable under the conditions covered, and is readily separated from styrene and ethylbenzene by virtue of its lower boiling point. This work comprised a laboratory scale study of some of the factors involved in the use of benzene dilu
ent, with particular regard to the effect of the process variables oh yields.
Dow ethylbenzene was fractionated in a fifty-tray, bubblc-cap glass column, a small forerun and residue being discarded. The benzene was Merck’s reagent grade.
Fresh U.O.P. dehydrogenation catalyst (Vs X 1/t inch cylin
drical pellets) was used in each of the simulated recycle runs.
In all other runs the catalyst had been preaged by 21 days of service in pilot plant dehydrogenation of butane. Separate samples were used for each run.
Two block furnace assemblies were described in a previous paper (4). The simulated recycle runs were made with the unit containing the stainless steel reaction tube of 1.25-inch inside diameter. The 16.4-mm. i.d. quartz tube unit was used for all other tests.
I / L H S V CON E T H Y L B E N Z E N E O N L Y )
Figure 1. Effect of Ethylbenzene Concentration and Space Velocity on Conversion at 600° C., Atmospheric
Pressure, and 30-Minute Process Period
■>7. C O N V E R S IO N
Figure 2. Effect of Ethylbenzene Concentration on Ultimate Yield of Styrene at 600° C., Atmospheric
Pressure, and 30-Minute Process Period 829
Ta b l e I . Ef f e c t o f Pr o c e s s Va r i a b l e s a t 6 0 0 ° C . Bl o c k from degradation of the ethylbenzene was calculated (4) from the amounts of gaseous hydrocarbons pro
duced; and the remaining main portion was con
sidered as the quantity of benzene diluent originally processed, any conversion of the benzene itself being disregarded. It should be apparent that if some decomposition of the benzene has occurred during processing, or if hydrocarbon gases have been pro
duced by routes other than simple splitting of ethyl
benzene to produce one molecule of benzene or toluene and one molecule of gaseous hydrocarbon, the errors in the reported ultimate yields because of these factors are on the conservative side.
S T A B I L I T Y O F B E N Z E N E Atmospheric Pressure, and 30-
Minute Process Period
The gases were analyzed by conventional absorption and com
bustion methods (7).
High boiling residue in the recovered liquid from each test was determined by distilling a weighed sample at room temperature by gradually reducing the pressure to a value of about 2 mm. and obtaining the weight of residual material on an analytical bal
ance.
The styrene content of the vacuum distillate was determined by bromine number (8). In most cases samples larger than the customary 1 ml. were used in order to obtain increased accuracy.
Carbon on the used catalyst from each run was obtained from a combustion analysis (5) of a representative ground sample.
For the calculation of ultimate yields of styrene obtainable by recycling, the amount of unconverted ethylbenzene was con
sidered as the difference between the quantity of recovered liquid and the combined quantities of benzene, toluene, styrene, and
Mattox and Grosse (3) reported a 25 to 30% con
version of benzene to biphenyl, gas, and carbon at 600° C. over a chromia-alumina catalyst, using an extremely low space velocity of 0.06. This reaction is of minor concern, however, in the space velocity range involved in the present work. In a run at a space velocity of 1.0, which is at the lower end of the range covered, pure benzene was proc
essed over the catalyst at 600° C. and at atmospheric pressure.
The products consisted of 0.15 weight % of high boiling residue, 0.85 weight % of catalyst carbon, and 3.2 mole % of gas, based
velocities, dilution with benzene decreased conversion (Figure 1;.
This effect may be anticipated since the diluent decreases the time of contact of the ethylbenzene with the catalyst. The ob
served reversal of this trend as space velocity is decreased is at
tributed to the fact that the conversions involved are largely a re
flection of the extent of the reversible dehydrogenation reaction, which is capable of proceeding further at lower ethylbenzene par
tial pressures (i.e., at the higher dilutions) as thermodynamic equilibrium is approached.
Improved selectivity by dilution with benzene is demonstrated in Figure 2, in which styrene ultimate yield is shown as a function of conversion. Whereas a yield of about 80% is indicated with
7 o C O N V E R S IO N
Figure 4. Effect of Pressure on Ultimate Yield of Styrene, with 16.5 Weight % Ethylbenzene Feed at 600° C. and 30-
Minute Process Period
August, 1946 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 831
7 » C O N V E R S IO N
Figure 5. Effect of Pressure on Ratio of Carbon to Styrene, with 16.5 Weight % Ethylbenzenc Feed at 600° C. and 30-Minute Process Period
the undiluted feed at 40% conversion, dilution with ten parts of benzene brings the yield above 90% at the same conversion level.
A parallel effect appears in Figure 3 in which the ratio of catalyst carbon to styrene produced at a given conversion is shown to de
crease with increasing proportions of diluent.
P r e s s u r e . Selectivity is further improved by reduction of
pressure. Figure 4 compares ultimate yields of styrene obtain
able over a wide range of conversion at atmospheric pressure and at 250 mm. of mercury absolute from ethylbenzene diluted with five parts of benzene. At the lower pressure, ultimate yields of
better than 90% are indicated at conversions as high as 70%. A marked lowering of carbon formation is also effected at the lower pressure (Figure 5).
Conversion, on the other hand, at higher space velocities is adversely affected by decreasing pressure (Figure 6) owing, presumably, to decreased contact preferred operating conditions and to determine by actual fractionation the extent of toluene formation, a simulated recycle operation was carried out in five passes. Approximately 1700 ml. of hydrocarbon (ethyl
benzene plus benzene) were processed in each pass.
The liquid product from each run was fractionated at atmospheric pressure in a fifty-tray, bubble-cap, vacuum-jacketed, glass column, the benzene being taken overhead along with any toluene formed and a small portion of the unconverted ethylbenzene. The distillate was blended with fresh ethylbenzene and a small amount of benzene required to compensate for losses of the latter, and was processed in the
succeeding run. Three methods of determining yields were employed.
Method A was used in the process variable investigation and was described under the heading ‘ ‘Analyses and Calculations” . This was used in the last pass only.
Method B involved removal of the diluent in the fifty-tray bubble-cap distillation and determination of styrene in the resi
due from tho bromine number of the vacuum distillate from a small weighed sample. Also included in the styrene yield was the amount of polymer formed during the atmospheric pressure distillation, as calculated from the difference in the concentration of high boiling residue found in the atmospheric pressure distilla
tion residue and in the original recovered liquid. Benzene and toluene yields were calculated from the amount and composition of the gas produced, and recycle ethylbenzene was found by dif
ference.
Method C utilized the atmospheric pressure distillation of method B. The major portion of the styrene in the residue was determined by polymerization (5 hours at 200° C. in a sealed tube), recycle ethylbenzene by fractionation (corrected for any unpolymerizcd styrene as determined by bromine number), and toluene by fractionation. Actually, toluene was allowed to build up in the recycle material, and the amount found after the fifth pass was distributed equally over tho five passes.
Ta b l e II. Sim u l a t e d Re c y c l e Op e r a t io n
Figure 6. Effect of Pressure and Space Velocity on Conversion»
with 16.5 Weight % Ethylbenzene Feed at 600° C. and 30-Minute Process Period
A composite yield summary for the series, based chiefly on the distillation method, is shown in Table II, where a value of 89.7 weight % is given as the ultimate yield of styrene. Devia
tion from this value did not exceed 3.0 for any one pass by any of the methods. In the fifth pass, where ultimate yield was de
termined by all three methods, the values were 91.4, 91.5, and 91.7 weight %, respectively.
A C K N O W L E D G M E N T
The authors express their appreciation to Louis S. Kassel at whose suggestion the work with benzene diluent was initiated.
L I T E R A T U R E C I T E D
Separation of fine, suspended particles from hot, concen
trated caustic solution presents a difficult problem. Be
cause of the large volumes of cooking liquor required in a pulp mill, a method similar to the continuous filtration of water is desirable. Investigation revealed that anthracite
coal as a filter medium successfully removes finely divided particles and is sufficiently inert to hot, concentrated sodium hydroxide. Filters in operation are removing finely divided carbon (largely under 15 microns) from 11%
caustic solution at the rate of 100 gallons per minute.
W T E C E S S IT Y for clarifying hot, concentrated sodium hydrox
ide solution has led to the develop
ment of a continuous filter system
bonate is leached from the black ash, the larger carbon particles settle out and are repioved, but the fine parti
cles remain suspended in the sodium carbonate solution which goes to the causticizers. Subsequent to the causticizing operation, the liquor passes through a Dorr clarifier where the calcium carbonate pro
duced by causticizing is removed. clarifier. So much carbon is pres
ent that the liquor is black and Figure 1. Effect of Filtration on Caustic liquor varies greatly. More than