Low-Temperature Ketone Dewaxing of Mineral Oils by Direct Cooling C. A. Passut, P. Barton,* E. E. Klaus, and E. J. Tewksbury Fenske Laboratory, Pennsylvania State University, University Park, Pennsylvania 16602
Processes for ketone dewaxing of petroleum-based oils to pour point temperatures of -40 to -80 O F have been investigated, including crystallization by direct addition of chilled solvent to the oils. Methyl ethyl ketone-methyl isobutyl ketone blends were used at solvent-to-oil ratios of 5:l and 8:l. Yields of dewaxed oil ranged from 82 to 60%. Product yields and filtration rates were determined as functions of cooling rate and crystallization method. The effects of nature of the oil, mechanical shearing of wax crystals, and filter pressure drop on filtration were examined. The degree of filter media blinding was determined in a continuous rotary drum filtration pilot plant.
Synthetic fluids are being promoted to satisfy the needs for lubricants and hydraulic fluids with satisfactory fluidity a t temperatures less than -40 O F for use in internal combustion engines in the colder latitudes and in aerospace applications (Miller et al., 1974; Lestz and Bowen, 1975). These products retail a t several times the price of highly refined petroleum based oils. With proper refining, mineral oils can often be a viable alternative to synthetic fluids when the application demands fluidity and stability from temperatures of -70' or -40 OF to 700 OF (Klaus et al., 1962). Preparation of mineral oils with satisfactory viscosity characteristics over such wide temperature ranges can only be accomplished by dewaxing to temperatures lower than the conventional 0 OF. We have studied processes for low temperature ketone dewaxing of mineral oils (Barton et al., 1969; Passut, 1970). This included extension of a dilution crystallization technique (Livingston et al., 1959) to dewaxing to very low pour points. Crystallization is directly induced by adding previously chilled solvent to the mineral oil, eliminating the need for the conventional-type scraped-surface dewaxing crystallizers. Ketone dewaxings with this basic cooling technique to conventional dewaxing temperatures are being made on a commercial scale with the Dilchill process (Gudelis et al., 1973). The results of our work on ketone dewaxing of petroleumbased oils to pour point temperatures of -40 to -80 OF are reported here, including data on solvent selection to match the oil feed, product properties and yields, cooling rates, and filtration rates. Experimental Procedures The batch dewaxings were accomplished by diluting the oil with solvent, cooling the mixture and a t the same time crystallizing the wax, filtering the slurry, stripping the solvent from the separated products, and measuring product yields and properties. Indirect cooling was accomplished by placing the charge in an uncovered beaker in a cold box and then gradually lowering the temperature level in the cold box to control the cooling rate. Mild agitation was provided by a propeller stirrer. For cooling by direct solvent addition, prechilled solvent was added from a can fitted with a ys-in. plug valve to a charge comprising a 1:2 or 1:l solvent-to-oil blend precooled to a temperature above its cloud point. The cooling rate was varied by regulating the rate of chilled solvent addition. The slurry, contained in a dewar flask, was mildly agitated with a propeller stirrer. Cooling by direct injection of refrigerant was done by expanding the refrigerant from a pressurized cylinder through a needle valve and 0.055-in. i.d. tubing and bubbling the 120
Ind. Eng. Chem., Process Des. Dev., Vol. 16, No. 1, 1977
chilled refrigerant gas and liquid into ketone-oil blend in a dewar flask. Cooling by direct evaporation of fluid refrigerants from ketone4il mixtures was done in a thin-walled, insulated, 1.9-1. can placed inside a pressure vessel. Crystallization was done by evaporating off the pressurized refrigerant at a controlled rate through a valve. The pressure vessel was then dismantled to remove the dewaxing slurry. The filtrations were carried out in a cold box a t the dewaxing temperature with either sintered glass funnels or 0.1-ft2Eimco cloth test leaves. For top loadings, the cloth test leaves were fitted with rubber boots and the slurry dumped onto the filter. For dip tests with the cloth filters, an insulated, agitated slurry trough resembling that for a rotary drum filter was employed. A high capacity vacuum control panel was used to maintain the filter vacuum constant throughout the filtration period. Details of the apparatus and procedures are available (Passut, 1970). Results Dewaxing Products. The dewaxing feed stocks included a Pennsylvania wax distillate, a paraffinic neutral oil, and super-refined paraffinic white oils. The neutral and white oils were previously processed in conventional dewaxing to 0 to 10 OF. The boiling point distributions of the stocks were determined by temperature-programmed gas chromatography. The hydrocarbons present include C17-C33 for the wax distillate, C20-C22 for the neutral oil cut, C22-C27 for an intermediate range white oil, and Cl+& for a wide range white oil. The oils were dewaxed to low temperatures using blends of methyl ethyl ketone and methyl isobutyl ketone as solvents. Based on the premise that the oil occluded in the wax cake contains the same concentration of solvent as the dewaxed oil in the filtrate, it was possible to calculate the theoretical oil and wax contents of the charge oil as a function of temperature. This calculation is valid when the cloud and pour points of the dewaxed oil equal the dewaxing temperature, with optimum dewaxed oil yield indicating a proper match of oil/ wax solubility and selectivity in the solvent. The limiting dewaxed oil yields, presuming the absence of losses by oil occlusion in the wax, are compared in Figure 1for each type of oil. Dewaxing of Pennsylvania wax distillate from 70 to -40 O F resulted in a dewaxed oil yield of 63%, compared to a theoretical oil yield of 84%. Properties of the recovered fractions are listed in Table I. Stepwise dewaxing of this type of stock, first to 10 O F , then to -40 O F , would provide more desirable wax cuts. The first wax cut provides the usual paraffin wax. The wax cut from the second dewaxing would be comparable
100
8
,
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L
I
Table 11. Dewaxing of Paraffinic Neutral Oil
I .
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Charge
Oil 5-9596 boiling range, O Yield, % Pour point, O F Viscosity, cSt at 100 O F cSt at 210 O F Viscosity index
685- 795'F WHITE OIL
635-695'F NEUTRAL OIL
c I 40
100
0 DEWAXING TEMPERATURE,
5-95%, boiling range, O Yield, % Pour point, O F Viscosity, cSt at 100 O F cSt at 210 O F Viscosity index
F
Dewaxed oil Wax
560-900 70 14.2 3.24 104
635-695 82 -40
10
18 50
ca. 13 ca. 2.9 78
9.32 2.57 118
Dewaxed oil
Wax
12.1
2.85 87
Table 111. Dewaxing of Paraffinic White Oil
Table I. Dewaxing of Pennsylvania Wax Distillate Charge
Wax
-100 O F
Figure 1. Theoretical dewaxed oil yields for paraffinic mineral oils.
Oil
F
Dewaxed oil
63 -40
37 85
17.1 3.50 86
to that shown for the neutral oil in Table 11. This wax cut is fluid at indoor temperatures and has a pour point of 50 O F and a viscosity index of 118. The yield of -40 O F pour oil from this second dewaxing step is 82% of the neutral oil charge. The properties of the products of dewaxing a super-refined paraffinic white oil to -75 O F are shown in Table 111. A 60% yield of low pour oil is obtained, compared to a theoretical oil yield of 80%.This oil, with its high viscosity index, is of the type defined by specification MIL-H-27601. The wax cut is fluid a t indoor temperatures and has a pour point of 45 O F and a viscosity index of 137. Solvent Selection. Solvents are used in deep dewaxing of mineral oils to lower the viscosity level by dissolving the oil phase to provide a better environment for wax crystal growth and removal. Ketone solvents have been shown to exhibit good solubility for the oil phase and good selectivity for oil over wax a t low dewaxing temperatures (Klaus et al., 1962). Blends of ketones are preferable to blends of ketone and aromatic hydrocarbons. The latter solvents produce pour points more than 10 O F higher than the dewaxing temperatures (Marple and Landry, 1965) in dewaxing to conventional temperatures. In accordance with the proven technology of conventional dewaxing, it is necessary to optimize the ketone blend for each specific charge stock and dewaxing temperature in order to obtain an optimum yield of oil with minimum pour point. Solvent-to-oilweight ratios of approximately 5 and 8 were found to provide workable viscosity levels a t -40 and -80 O F , respectively. Solvent compositions which gave high yields of dewaxed oils with cloud and pour points essentially identical with the dewaxing temperatures are shown in Table IV for oils of two different molecular weight ranges. Some adjustments in solvent treat and composition may be necessary for successful dewaxings of oils from other sources, with different refining steps, and with different molecular weight ranges. Knowledge of the solubilities of oils in ketone solvents a t low temperatures can be used to understand and optimize dewaxings to very low pour points, in the manner described by Marple and Landry (1965) for conventional dewaxing.
Oil 5-95% boiling range, O F Yield, % Pour point, "F Viscosity, cSt a t 100 O F cSt a t 210 O F Viscosity index
Charge 685-795
60 -75
5
15.2 3.31 94
12.7 3.08
112
40 45 11.0 2.95 137
Table IV. Solvent Treats for Low Temperature Dewaxing with Methyl Ethyl Ketone (MEK) and Methyl Isobutyl Ketone (MIBK) Oil range
c,;-c2
Dewaxing temp, O F IWt of solvent/wt of oil1 Solvent (by weight) .
-40 5
Cr,-C29
j
-40 -
-80
5
8 -
The solubilities of two paraffinic white oils in several ketone solvents are shown as a function of temperature in Figure 2 . Solubility increases .with increasing carbon number of the solvent, with increasing temperature, and with decreasing boiling point spread of the oil. A 3/1 blend of methyl isobutyl ketone-methyl ethyl ketone provides the same solubility as methyl propyl ketone. The solvent blend is preferred for dewaxing in that solvent composition adjustments provide a degree of flexibility in optimizing yields. The usefulness of solubility data in understanding results of dewaxing demonstrations is illustrated by the data in Table V. Use of the 3.4:l solvent-to-oil ratio provides the proper pour point but a very low yield of dewaxed oil. This can be attributed to insufficient solubility of oil in the solvent and the inability to separate the undissolved oil from the wax. This conclusion is substantiated by the discrepancies between the actual solvent content of the filtrate and the calculated solvent content of the filtrate based on complete solution of dewaxed oil. For tests in which all of the oil is dissolved a t the 7.7:l solvent-to-oil feed ratio, the actual solvent-to-oilratio in the filtrate has approximately the same value as the calculated value. At the 3.4:l solvent-to-oil feed ratio, the measured solvent-to-dewaxed oil ratio is higher than the calculated value. In fact, the solvent content of the filtrate is the same as that estimated from the cloud point-solubility data of dewaxed oil-ketone blends in Figure 2. The solvent passing through the filter carries with it only the amount of oil it can hold in solution; the remainder of the oil stays with the wax cake. The pour points (16 to 30 O F ) of the recovered waxes were 20 OF lower than usual due to the presence of this oil. Ind. Eng. Chem., Process Des. Dev., Vol. 16, No. 1, 1977
121
Table V. Effect of Oil solubility on -70 OF Dewaxinn of 685-795 "FParaffinic White Oil Solvent to oil wt ratio
Dewaxed oil
Filtrate Solvent
Charge
[3 MIBKh MEK] [3 MIBK/1 MEK] MIBK
Actual
3.4 7.7
5.8
8.7 11 6.8
Pour point,
Calcda
Solubility
Yield, wt %
4.2 9.5
9
-71
36
9
62
7.1
2.5
-71 -55
O F
Assuming all of the dewaxed oil (theoretical yield = 81 wt %) is dissolved in the solvent.
,
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,
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OIL
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l
MPK
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'
MlBK
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( 3 MlBK/IMEK)
20
I
-20
0
WITH REFRIGERANT
25 C 3 0 L I N G RATE, ' F / m i n
50
Figure 3. Effect of cooling rate and method on dewaxed oil yield from 685-795 "F paraffinic white oil at -75 O F .
I
-60
-I00
With pure methyl isobutyl ketone at a solvent-to-oil ratio of 5.8, the pour point of the dewaxed oil was about 15 O F higher than the dewaxing temperature, indicating that too much of the oil charge was dissolved in the solvent a t the dewaxing temperature. It is evident that the solvent dissolving power and solvent-to-oil ratio must be sufficiently high t o dissolve all of the dewaxed oil. Due t o solvent selectivity for oil over wax, some additional solvent can be used without raising the cloud and pour point of the dewaxed oil. Less than twice as much solvent as that required for minimum solubility is recommended. Crystallization Techniques. The methods investigated for cooling ketone-oil blends to produce dewaxing slurries included indirect cooling, direct injection of chilled ketone solvent into the oil, and direct contact evaporation of miscible and immiscible refrigerants. Providing that the viscosity, solubility, and selectivity characteristics of dewaxing slurries have been reasonably optimized, the prime factor in governing the success of the dewaxing process was found to be the cooling rate. Based on general principles of crystal nucleation and growth, it is expected that what is important is the cooling rate on a microscale rather than a macroscale. This is a function of both temperature of the fluid elements and the intensity of mixing of the fluid elements. Nearly comparable dewaxing results can be achieved with the different refrigeration techniques, providing that the cooling rates are on the same order of magnitude. Cooling rate affects the rate and nature of crystal growth and consequently has a direct effect on size distribution of the wax crystals and the amount of dewaxed oil (and the ketone solvent associated with it) occluded with the wax crystals. These properties are manifested in the filtration characteristics of the slurries and in product yields. The effect of bulk cooling rate on the yield of -75 O F de122
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THEORETICAL
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-
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Ind. Eng. Chem., Process Des. Dev., Vol. 16, No. 1, 1977
i !
:
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,
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IO COOLING RATE,"F/min
, I 100
Figure 4. Effect of cooling rate on filtration rate for -80 O F dewaxing by chilled solvent addition to 685-795 O F paraffinic white oil. waxed oil from a paraffinic white oil is shown in Figure 3. The yield is seen to decrease from 65% to 48% upon increasing the cooling rate from 10 "F/min to 45 OF/min. The slowest cooling rate of 2 OF/min was attained by indirect cooling. The intermediate cooling rates were attained by direct evaporation of miscible ethylene, direct contact evaporation of immiscible liquid hexafluoroethane, and chilled solvent addition. The highest cooling rate was obtained by direct injection of cold gaseous ethylene. In all cases the cloud and pour points matched the dewaxing temperature. In other tests using miscible refrigerants such as bromotrifluoromethane or propane that have the capability to dissolve some wax, the cloud and pour points were higher than the dewaxing temperature and the dewaxed oil yields decreased, even with compensating decreases in ketone-to-oil ratio. The effect of bulk cooling rate on filtration rates for -80 O F dewaxing slurries produced by chilled solvent addition to paraffinic white oil is shown in Figure 4.The initial filtration rate with clean 50-pm porosity nylon filter cloth is seen to fall sharply as the cooling rate is increased from 3 "F/min to 10 OF/min. These initial filtration rates nevertheless can be
I
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- HIGH SHEAR
\
._
655-845'F 575-815'F
---+
LOW SHEAR
E 20
I-
FILTRATE
FILTRATE VOLUME, l i t e r s
Figure 5 . Effect of feed stock on wax cake resistance; 0.1 ft2, 5-10-wm cloth filter.
0 _.
Figure 7. Effect of sheared crystals on filtration resistance with 0.1 ft2 cloth filter.
COOLING RATE
L
COOLING RATE: 9 ° F / m i n .
E
w
' g oi lilil 0
IO 20 PRESSURE DROP, inches Hg
VOLUME, l i t e r s
CLOTH POROSITY: 2 0 - 3 5 p m
Po
i
O
TIME, minutes
Figure 6. Effect of pressure drop on filtration rate of gas-free dewaxing slurry.
Figure 8. Continuous rotary drum filtration with scraper: gas blowback discharge for -75 O F dewaxing by chilled solvent addition to 655-845 O F paraffinic white oil.
considered high to medium at 9 to 1.5 gal of filtrate/(min ft2) of filter cloth area, respectively. Filtration Characteristics. The nature of the oil feed stock affects the filtration resistance of wax cakes. In Figure 5, filtration rates for several dewaxing slurries with clean cloth filter media are compared. These gas-free dewaxing slurries were prepared by chilled solvent addition with cooling rates of 9-12 OF/min to -70 O F . The super-refined paraffinic white oils exhibited higher wax cake resistance than the more naphthenic neutral oils in these tests as well as in all other dewaxings a t similar conditions. The effect of filtration pressure drop on filtration rate for gas-free dewaxing slurries is shown in Figure 6. The slurries were prepared by chilled solvent addition to 685-795 OF boiling range paraffinic white oil to dewaxing temperatures of -50 to -68 O F . Since the initial filtration rate is almost proportional to pressure driving force, the gas-free wax cakes can be considered fairly incompressible. With wax cakes saturated with carbon dioxide gas from dry ice coolant, this pressure dependence was not shown, and such cakes are considered compressible. Dewaxing tests were conducted to show that crystal sizes could be reduced by mechanical action during the cooling period. Batches of -80 O F dewaxing slurries were prepared by chilled solvent addition to 655-845 O F boiling range paraffinic white oil with cooling rates of 16 to 19 OF/min. The mixing speed of the 3-in. propeller was varied from 300 rpm (low shear) to lo00 rpm (high shear). In Figure 7, it is seen that increasing the shear rate increased the wax cake resistance (slope), which indicates that the crystal sizes were reduced. The increase in filter cloth resistance (intercept) shows that the high shear rate has decreased the wax crystal sizes enough to pass through and partially plug a 50-pm porosity filter. The filtration results obtained in the low temperature dewaxings in this study are largely in direct accord with previous works
described for conventional dewaxings (Marple and Landry, 1965; Reeves and Pattillo, 1948). Continuous Filtration. One goal of this work was to demonstrate that low temperature dewaxings could be performed using continuous filters without completely blinding the filter cloth with wax crystals during successive filtration cycles. For this purpose, a special rotary drum filter was constructed, adaptable for scraper discharge, gas blow-back discharge assist, string discharge, and belt discharge. With the last one, the filter cloth can be washed with chilled solvent between pickup cycles. The filter had a diameter of 6 in. and an effective face of 4 in. The filter was installed in a refrigerator controlled a t the dewaxing temperature. Batches of dewaxing slurries were prepared in insulated barrels by chilled solvent addition to 655-845 O F boiling range paraffinic white oil using bulk cooling rates of 5 to 9 OF/min. The solvent consisted of 3/1 methyl isobutyl ketone-methyl ethyl ketone employed at an overall solvent-to-oil ratio of 7.6. The chilled solvent a t approximately -100 O F was sprayed into a 1:l by volume blend of solvent and oil at 13 O F . The spray rate was adjusted to keep the cooling rate constant. Mixing was done in a barrel using a 5-in. marine propeller at 700 to 900 rpm. The final slurry temperature was about -80 O F . The slurry was then continuously fed to the filtration pilot plant, which was maintained a t -75 O F . The blow-back gas was prechilled dry air. The filtrate receivers were maintained at a constant vacuum with a pressure control on the vacuum system. Details of this dewaxing pilot plant are available in a government report (Barton et al., 1969). Filtration rate is shown as a function of time in Figure 8, using scraper discharge with gas blow-back assist. The rate is based on submerged pickup area of filter cloth, the fraction submergence being 58%. The rate of rotation was 1.2 rpm. The filter vacuum averaged 6 to 7 in.Hg. The filter cloth gradually plugged during the first half-hour of operation and thereafter Ind. Eng. Chem., Process Des. Dev., Vol. 16, No. 1, 1977
123
the filtration rate remained a t an average of 0.3 gal of filtrate/(min ft2) of immersed filter cloth. This is a moderate rate. The cake thickness was y& in. The filtration rate was increased to 0.7 gal/(min ft2)by using belt discharge with solvent wash. The cake thickness averaged :y8 in. with 57% submergence a t 0.75 rpm and a 7 in.Hg pressure. The above data were obtained with slurries saturated with carbon dioxide gas, and increasing the filter vacuum from 7 to 23 in. of mercury did not increase the filtration rate. With gas-free slurries a t a filter pressure of 20 in.Hg, the filtration rate should be maintainable a t twice the above values. In conclusion, this study has provided data useful for designing plants to dewax petroleum-based oils to temperatures in the range of -40 to -80 O F .
Klaus, E. E., Tewksbury, E. J., Fenske, M. R., ASLE Trans., 5 ( l ) , 115 (1962). Lestz, S. J.. Bowen, T. C., "Army Experience with Synthetic Engine Oils in Mixed Fleet Arctic Service", S A € Fuels and Lubricants Meeting, Houston, Texas, June 3-5, 1975. Livingston, J. G., Moreton. A. G., Tiedje, J. L., (to Exxon Research and Engineering Co.), U S Patent 2 880 159 (Mar 31, 1959). Marpie, S., Jr., Landry, L. J., "Modern Dewaxing Technology," in Vol. 10 of "Advances in Petroleum Chemistry and Refining," J. J. McKetta, Ed., Interscience, New York, N.Y., 1965. Miller. 8. J.. Rogers, T. W.. Smith, D. B., Trautwein, W. P., "Synthetic Engine Oils-A New Concept", SAE Automotive Engineering Congress, Detroit, Mich., Feb 25-Mar 1, 1974. Passut. C. A . , M.S. Thesis, The Pennsylvania State University, University Park, Pa., 1970. Microfilm of the thesis referenced may be obtained, free of charge for specified periods, on interlibrary loan from Pattee Library, The Pennsylvania State University, University Park, Pa. 16802. Reeves, E. J., Pattillo. I. E., Petrol. Refiner, 27 (3), 80 (1948).
Received f o r review M a r c h 1, 1976 Accepted A u g u s t 27,1976
Literature Cited Barton, P., Klaus, E. E., Tewksbury, E. J., "Processes for Low Temperature Deep Dewaxing of Mineral Oil", AFML-TR-69-128, Air Force Materials Laboratory, Wright Patterson Air Force Base, Ohio, May 1969. Gudeiis, D. A,, Eagen, J. F., Bushneli, J. B., Hydrocarbon Process., 52 (9), 141 (1973).
T h i s research was s u p p o r t e d in p a r t by t h e U n i t e d States Air Force u n d e r C o n t r a c t F33615-67-C-1251 m o n i t o r e d by t h e M a n u f a c t u r i n g Technology D i v i s i o n , Air F o r c e M a t e r i a l s L a b o r a t o r y , W r i g h t - P a t terson Air Force Base. O h i o 45433.
A Novel Continuous Pyrolyzer. The TTU Retort1 Harry W. Parker Department of Chemical Engineering, Texas Tech University, Lubbock, Texas 79409
A variation of the Bureau of Mines gas-combustion retort for oil shale has been developed which omits the troublesome internal gas distributor. This is accomplished by intermittent addition of air to the gas being recycled through the retort or by continuous small additions of air to the recycle gas. The 16-cm diameter pilot retort has been operated at processing rates of 940 kg/m2 h on 0.64 to 1.9-cm manure particles containing 12.6% water and 55% ash. Higher capacities would be expected retorting oil shale since it has a higher bulk density and mechanical strength.
Introduction Simple high-capacity pyrolyzers are essential if pyrolysis is to be economically utilized in the processing of organic containing solid wastes and solid fossil fuels. This paper describes the development of a moving bed pyrolyzer which is exceedingly simple, an open cylinder with discharge grate and solids feeder. Experimental data are presented for processing of feedlot waste, although the pyrolyzer is also adapted to many other organic solids which retain their physical integrity during pyrolysis. Origin of the TTU Retort-The Gas-Combustion Retort The T T U retort is a variation of the Bureau of Mines gascombustion retort developed for oil shale shown in Figure 1. Oil shale enters a t the top of the retort, descends through the retort, and is finally discharged through a grate a t the bottom. The spent shale is partially cooled by injection of recycle gas a t the bottom of the retort. Air injected by means of an internal gas distributor in the central portion of the retort permits combustion to generate heat for the retorting process. Recycle gas is mixed with the air to control the maximum
'
Presented a t t h e Rocky M o u n t a i n R e g i o n a l M e e t i n g of t h e A m e r i c a n C h e m i c a l Society, L a r a m i e , Wyo., J u n e 17-19, 1976.
124
Ind. Eng. Chem., Process Des. Dev., Vol. 16, No. 1, 1977
temperature in the retort. The hot combustion gases flow upward in the retort to retort the oil shale and preheat the entering shale. The gases are cooled by the cold shale within the retort causing the product shale oil to condense as a fog which exits from the top of the retort. This successful technique of removing the shale oil as a fog in the produced gases is the key to the thermally efficient, simple operation of t,he gas-combustion retort. Design of the internal gas distributor has been a very difficult task in scaling up the gas-combustion retort. Air and recycle gas must be injected uniformly across the entire retort diameter so the rate of heat release via combustion will be uniform within the retort. In addition to the requirement of uniform gas injection across the entire retort this same gas distributor must not impede the downward flow of shale through the retort. Difficulties with gas distributors and design modifications to improve gas distributors occwpy many pages in reports regarding the gas-combustion retort (Matzick et al., 1966; Ruark et al., 1969,1971) The TTU retort described in the following section completely circumvents the problem of an internal gas distributor hy omitting it.
Description of the TTU Retort The means by which the YI'U retort operates are the same as employed in the Bureau of Mines gas-combustion retort.
