Clay Desulfurization of Middle Distillates W. P. BALLARD, N. A. MERRITT, AND J. C. D. OOSTERHOUT The Texas Company, Port Arthur, Tex.
a
M O R phase desulfuripreliminary investigation of the feasibility of clay-dequent to being mined it wafi sulfurizing middle distillate stocks indicates that a highzation of cracked and dehydrated in a fired rotarj sulfur straight-run middle distillate can be partially destraight-run naphthas using kiln a t an average temperasulfurized by vapor phase contact with No. l Riverside natural clays is a well-known ture of 900 't o 950' F. process for improving clear earth at 750 O F. and 1 .O volume per hour per volume space X series of initial activity octane number and lead velocity. The degree of desulfurization is determined by desulfurization runs was the proportion of aliphatic sulfur to total sulfur in the susceptibility. The use of made to determine the effect fuller's earth for desulfurizastock. By following this step with clay vapor treating a t of space velocity in the range tion of straight-run naphthas the dew point plus rerunning or partial condensation, an from 0.5 to 12.5 volumes of has been practiced commeroil with a stable color above +I2 Saybolt can be produced. liquid charge per hour per volBy lowering the distillation range of the charge stock, the cially by The Texas Company ume of catalyst and the effect color and overhead yield can be improved. at several of its plants; h o a of reactor bed temperature in ever, no information on this the range from 600' to 900 O F , operation has been published. The desulfurization in each run was determined using the sulfur content of a composite samNormally the operation is conducted bj- passing vaporized naphple covering a catalyst age of about 0 to 70 barrels per ton, tha a t a temperature of about 700' to 750" F. over a fixed bed of Check runs were made in two instances and the desulfurizafuller's earth a t a liquid space velocity of about one volume of tion reproducibility was indicated t o be good, better than ~ 2 7 ' ~ . liquid charge per hour per volume of catalyst. Catalyst lives The results of this study are shown graphically in Figures 2 obtained in this operation are long enough t o obviate catalyst and 3. The same data were used to construct both curves. It regeneration. The pressure a t any point in the system is only can be readily seen from these figures that through the range that required to overcome subsequent pressure drops due to of conditions investigated the per cent desulfurization is (1) a log friction in the flow system. The sulfur susceptible t o removal function of the space velocity a t a constant temperature, decreasin this operation is removed as hydrogen sulfide. ing as space velocity increases, and ( 2 ) a linear function of temRecently there has risen at refineries charging primarilj- West perature a t a constant space velocity up t o a maximum desulTexas-New Mexico sour crudes the necessity for reducing the furization of about 60%. sulfur content of straight-run middle distillate stocks. In some Extrapolations of the curves in Figure 2 t o low space velocities instances this sulfur reduction is accomplished by conventional appear to be risky, inasmuch as the 900" and 760" F. curves acid treating at. plants having sulfuric acid recovery plants or will cross, indicating a probable discontinuity in the curves desuitable means of disposal of the used acid. Unfortunately, the veloped. The flatness of the 900" F. curve does indicate, howlocal situation is such a t some plants that acid treating cannot ever, that the effect of space velocity is much less pronounced a t be practiced without an undue acid disposal problem. I n an the higher temperature. effort to alleviate this situation, t,he feasibility of clay-desulfurizFrom Figure 3 it is evident that a maximum desulfurization ing these stocks over fuller's earth was investigated. The presof 60% is obtained a t 750" t o 850" F. when operating a t 0.5 and entation of the results of this work is the purpose of this paper. 1.0 volume/hour/volume space velocities, and that neither inEQUIPMENT creasing the temperature nor decreasing the space velocity ~ o u l d This work was conducted on fixed-bed bench model equipbe expected to yield a greater desulfurization. ment shown diagrammatically in Figure 1. This equipment, which operated slightly above atmospheric pressure, consisted essentially of a wound preheater (vaporizer) section controlled by a powerstat and a 400-ml. catalyst case (1.5-inch stainles3 steel OFF GAS tubing) which was placed in a wound heater block controlled by an automatic temperature device. Conventional charging assembly, condensers, receivers, absorption trains for hydrogen CONDENSER J, W 00 0 TRAP sulfide removal, gas meter, gas-collecting assembly, and purging lines for flue gas and air completed the equipment. I n operating CAUSTIC the equipment the liquid product was caustic-washed for hydroSCRUBBERS gen sulfide removal and the gas formed was caustic-washed for hydrogen sulfide removal prior to its passage through the gas meter.
J
STUDY OF TEMPERATURE AND SPACE VELOCITY The initial phase of this investigation was concerned with determining the effect of operating variables on the degree of desulfurization obtsined when charging a middle distillate from West Texas-Xew Mexico sour crude over 12- t o 30-mesh S o . 1 Riverside earth, a fuller's earth mined a t Riverside, Tex. Ident,ifying tests on the raw distillate are shown in Table I. The particular fuller's earth used is in commercial production and is used in various refining operations requiring fuller's earth. Subse-
2856
OIL
Figure 1.
Single Bed Unit
INDUSTRIAL AND ENGINEERING CHEMISTRY
December 1949 60
2857
-
50-
%
,E ix 3040-
3
1 1 1
;
20-
1
u
Q
l0I
0.2
1.0 2.0 4.0 LIQUID SPACE VELOCITY, VOL. / HR. /VOL.
0.4
10.0
01
I
620
580
20.0
22 3E
6o
-1
40
0
STUDY OF CATALYST AGE Having established that a satisfactory initial desulfurization could be obtained by this process, a catalyst age study was made. T h e catalyst age runs were made a t two extremes in operating conditions which gave initial desulfurizations in excess of about 50% as determined from the data previously obtained. One run was made a t 12.5 volume/hour/volume space velocity and 900" F. catalyst bed temperature and two runs were made at 1.0 volume/hour/volume space velocity and 750 ' F. catalyst bed temperature. The results obtained are shown in Figure 4. As is often the case in fixed-bed nonregenerative catalytic operations, the dependent variable, in this case desulfurization, is a
West Texas-New Mexico Sour Middle Distillate" Gravity, A.P.I. Color Saybolt Corrdsion, c u strip a t 122' F. Sulfur 76 Merciptan sulfur, g./l DiatiEIation
1.B.P.
~
5% 10% 20% 50%
a
37.5 Below-16 POS.
0.69 0.75 385 415 430 445 480 538
-
90 % E.P.
570
98.0 Recovery, % Second in series of four stocks covered b y work (see Table V).
Table 11. Middle Distillates from Sources Other Than West Texas-New Mexico Sour Crudes Middle Distillate Source Low-sulfur West Texas California California crude crude crudea 44.1 39.9 39.9 0.58 110 140 0.177 0.169 0.170 0.72 0.01 ... 308 356 373 356 370 380 .390 382 404 407 441 448 475 496 98.0 98.0 a Middle distillate stock treated with 10 pounds per barrel of furic acid. b 18-inch cell. ~
-
I
I
I
1
740
780
820
860
Figure 3.
I f time and temperature are considered to be interchangeable in these studies, and this does not seem unreasonable, this indicates that a ceiling desulfurization exists above which it is impossible to go regardless of the temperature or space velocity employed. Although data are lacking to prove this point conclusively under all conditions, a t least within the range of operating conditions of interest commercially this is a practical fact.
Table I.
I
700
CATALYST BED TEMPERATURE,
Figure 2. Effect of Space Velocity i
I
660
358 376 382 384 404 443 478 98.0 60% sul-
-
d
i
I
900
'E
Effect of Temperature
900 OF.,
12.5 V. /HR./ V.
I , 0 V./ --;R .ik
7500F
a "Ot
-
20
40
100 ZOO 400 CATALYST AGE. BBLS. / TON
Figure 4.
1000
2000
Effect of Catalyst Age
function of the log of the catalyst age. The run a t 900' F. catalyst bed temperature and 12.5 volume/hour/volume space velocity was superior from a catalyst age standpoint to t h e 750" F. and 1.0 volume/hour/volume space velocity runs. The latter runs checked throughout within 3% desulfurization. STUDY OF CHARGE STOCK TYPES I n order to extend the knowledge and application of this process, some additional catalyst age studies were conducted using different charge stocks. Two stocks of much lower sulfur content were chosen. Identifying tests are shown in Table 11. The middle distillate from the low-sulfur West Texas crude was a rather paraffinic kerosene range material, while the distillate from California crude was much less paraffinic and was somewhat similar t o the distillate from the West Texas-New Mexico sour crude. The catalyst age study on these stocks was conducted a t 750' F. and 1.0 volume/hour/volume space velocity. The results of this study are shown in Figure 5 along with the data previously shown on the distillate from West Texas-New Mexico sour crude. Because California crudes are known to be of high nitrogen content (3, 4) and published information is available on the adverse effect of nitrogen compounds in catalytic processes (.e, 6),one run was made on the California crude distillate which had been treated with 60% sulfuric acid for removal of possible nitrogen compounds. These data in Figure 5 show a wide variation in the degree of desulfurization which can be obtained on different stocks and also indicate that weak acid treating removes a very potent catalyst poison from the California middle distillate. The decline of catalyst activity with catalyst age appears t o be attributable t o impurities or components that exist in the charge stock rather than to any inherent decrease associated with the sulfur removal itself. This is indicated by the relatively
2858
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 41, No, 12
LOW SULFUR WEST TEXAS
CATALYST CASE
NO. I
CHARGE
HEATER
N0.2
i-l:
0 "0
BOTTOMS
I-
CATALYST AGE, BELS. / TON
5AUSTIC WASH
Figure 5. Catalyst Age Desulfurization Relationships
PRODUC'
Figure 6. Double Clay Bed U n i t with Partial Condensen minor effect of age on activity shown foi t h t distillate from the low-sulfur West Texas crude and the treated California distillate. I n an effort to determine the ieasan for the variation in initial desulfurization n i t h different charge stocks, the sulfur mmpounds in the stocks investigated were analyzed ( 1 ) ~ This scheme of analysis classified sulfur compounds as hydrogen sulfide, free sulfur, mercaptans (thiols), sulfides, disulfides (all the foregoing considered aliphatic), aromatic sulfides and thiophenes, and residual sulfur, Analyses of the stocks investigated indicated that in general the percentage of the sulfur compounds classified as aliphatic sulfur corresponds t o the percentage of sulfur removed by clay detxdhirization under conditions approaching the maximum desulfurization. This was also found to hold true for other stocks investigated. A compaiison of the aliphatic su!fur content with the actual desulfurization obtained is shown in Table I11 for the stocks under investigation and also for several other stocks studied in connection with other work. Although the agreement is not so good as desired in every case, i t gives a general idea of the range of desulfurization that can be expected. Although the number of stocks covered here is limited, work on other stocks has shonn in rather general terms that the bitial degree of desulfurization which can bp obtained is a direct function of the parafinicity of the stock, considering increasing 4.P.I. gravity on a straight-run stock to be a measure of increasing paraffinicity for a given boiling range. Stated another way, the more parafhic stocks tcnd to contain a greater Percentage of the total sulfur in the form susceptible (aliphatic) to r'emo\ a1 by this process.
DESULFURIZED O I L QUALITY Up to this point no information has been given as to the quality of the oil resulting from the clay-desulfurization step. Typical tests on a desulfull~edoil are shown in Table I V along with the charge tests. 'l'hc product oil had a poor color and flash and was still positive t o the doctor test although more than 98% of the mercaptan sulfur had been removed. The color of the oil from the desulfurization step, although rated the same (below -16 Saybolt) as the charge, was an entirely different color. The color bodies that appear in the product oil are thought to be polymerization products of the unsaturates resulting from the breakdown of the sulfur-bearing molecules. This is based on a consideration of the possible means whereby the color bodies could be formed and on the color (greenish yellow) which is imparted to the treated oil. The color is similar t o the characteristic color of polymer gasolines prior to the removal of the heavy polymers. Although the original color of the charge stocks used in this work was not good, the color which was so difficult t o remove from the oil was t h a t formed in the desulfurization step. The original color of the oil (brown) if present in the treated oil a t
any step in the subsequent color removal studies was masked b) the greenish-yellow color associated with the desulfurized oil. If an impiovement in the quality of clay-desulfurized oil it necessary, no difficulty would be expected in correcting the flash and doctor. Inasmuch as the color bodies should be high boiling, a rrrunning step would be expected to improve the color. FINISHING STUDY T o determine the degree of undercutting that would be re yuired to yield an oil with a given color, a sample of clay-desulfurized oil was rerun batchwise in glass equipment. Although a + 1 5 Saybolt color oil was obtained, the color stability of the oil nas poor. I n an effort to polymcrize these unstable color-prcsducing compounds so that they could be removed by a rerunning operation, clay vapor treating of the desulfurized oil a t the dew point oi t h e oil over fresh KO. 1 Riverside earth prior to batch
Table 111. C o m p a r i s o n of Aliphatic S u l f u r C o n t e n t a n d Actual Desulfurization (Conditions.
7.50' F., 1.0 vol./hour/vol., and 75 barrels per ton catalyst
awl
Actual AliDesulfurphatic Stock isation, % Sulfus.0 Low-s tilfur West Texas middle distillate 94 96 California middle distillate acid-treated 60 79 54 45 TVest Texas-Sew 3Iexico sour middle distillate (#) High-sulfur Diesel fuel 31 32 Naphtha produced by cracking California gss oil over fluidized catalyst 7 6 Kaphtha produced by cracking East Texas gas oil over fluidized catalystb 7 s a Usually 8.5% or higher aliphatic sulfides with remainder normally mer. ca t,ans and disulfides. f Cracked stocks included only for comparison of actual desulfurization with aliphatic sulfur percentage. All stocks covered in paper aro stmighs run.
Table IV. Clay Desulfurization C h a r g e a n d P r o d u c t Tests Using Middle Distillate" f r o m West Texas-New Mexico Sour Crude Stocl,
Gravity, A.P.I. Flash, Pensku-Martene, Color, Saybolt Pour. F.
F
Charge 37.5 160 16 Below 25
-
Product 38.1 Below 100 Below - If3 - 25 Pos.
-
PO&
0.39
0.69
Neg.
Pos.
0.07 0.75
360 389 426 430 482 480 537 538 570 571 Second in series of four stocks covered b y work (Table V).
-
4
0.06
