tribution quotients for strontium. Dsr, and barium, DBa, were calculated for stages throughout the extractor. I n the scrub section, Ds, had values in the range of 0.7 to 1.1. while DBs had values, except for stage 3, in the range of 0.2 to 0.3. The separation factors, p values, for the scrub section were in the range of 3.0 to 4.0. I n the extract section, Dsr ranged from about 2.0 to 5.0, \vhile DBs ranged from 0.4 to 0.55, giving /3 values from about 5.0 up to about 10.0. The higher distribution quoticnts of strontium and barium in the extract section are in part due to the higher thiocyanate concentration in that section. Discussion
.A multistage extractor of the design described here could be set up to operate with more than 20 stages. The number 20 was chosen for convenience in separations work where fairly good separation factors may be developed. A larger number of stages would require more cycling before steadystate operation could be adequately satisfied. However, in the experiments on strontium and barium separation, a few more stages probably would have made it possible to prepare high purity strontium and high purity barium simultaneously from a mixture using the same liquids. Any extraction test requiring less than 20 stages could be operated on this extractor by merely changing interstage connections to bypass some of the stages. Since the analytical data indicated that only relatively small amounts of chloride and sodium were distributed to the organic phase, but that approximately 2 gram ions of thiocyanate per gram ion of combined strontium and barium were in the
organic, it appears that the species distributed to that phase contained (Sr, Ba) (SCN)z. Possible extension of the extraction system containing thiocyanate to separations involving calcium has been pointed out. Speculation based on known behaviors of the tested alkaline earth elements in a thiocyanate system leads one to predict that a system similar to that employed here for separating strontium and barium could be effective for separating barium and radium. Acknowledgment
The authors are indebted to Robert Heidel for supplying the x-ray fluorescence data used in the quantitative determination of strontium and barium, and to Bruce Raby for the colorimetric determinations of chloride and thiocyanate. literature Cited
(1) .4lderweireldt, F., Anal. Chem. 33, 1920 (1961). (2) Craig, L. C., Zbid., 22, 1346 (1950). (3) Lathe, G. H., Ruthven, C. R. J., Biochem. J . 49, 540 (1951). (4) Metzsch, F.-A. von, Chem. Zng. Tech. 31, 262 (1959). (5) Scheibel, E. G., “Technique of Organic Chemistry.” 2nd ed., Vol. 111, p. 332, Interscience, New York, 1956. (6) Wilhelm, H. A., U. S. At. Energy Comm.. Rept. IS-309 (June 1961). (7) Wilhelm, H. A., Foos, R. A,, IND.ENG.CHEkf. 51, 633 (1959). (8) IVilhelm, H. A., Foos, R. A., U. S. .4t. Energy Comm., Rept. ISC-458 (September 1954). RECEIVED for review October 13, 1961 . ~ C C E P T E D March 12, 1962 Contribution 1072. Work performed in the .4mes Laboratory of the U. S. .4tomic Energy Commission.
HYDROGEN DONOR DILUENT VISBREAKING OF RESIDUA A. W . L A N G E R , J O S E P H S T E W A R T , C. E. T H O M P S O N , H . T. W H I T E , AND R. M. H I L L Esso Research and Engineering Co., Linden, II’. J .
Thermal cracking of crude residuum mixed with a selected hydrogen donor diluent was investigated as a means for obtaining higher conversions and improved product quality. In visbreaking operations, an effective hydrogen donor can reduce residual fuel yield more than 20% when residuum conversion is limited b y asphaltene formation. The minimum residual fuel yields are obtained b y operating at the lowest severity which produces the desired viscosity.
to lower boiling products mild cracking in the presence of a hydrogen donor diluent has been described (2-4). I n this work, tetrahydronaphthalene and partially hydrogenated refinery streams containing a high proportion of condensed-ring aromatic compounds were used as the thermal hydrogen transfer agents. High conversion of residua to more valuable lower boiling products, with very low yields of coke and dry gas, was demonstrated. HE CONVERSION OF CRUDE RESIDUA
The present work is concerned with another application for this hydrogen transfer technique which is called hydrogen donor diluent visbreaking (HDDV). The use of hydrogen donor diluents in visbreaking reduces residual fuel yields by improving visbreaker tar quality. The need for such a process is evident wherever visbreaker operation is limited by fuel oil qualities other than viscosity. One of the most important of these limitations is the content of high molecular weight condensed-ring compounds commonly called asphaltenes. VOL.
1
NO. 4
OCTOBER 1962
309
Generally, the more asphaltenes present in the fuel, the poorer its burning qualities, although this depends somewhat on crude oil type. The concentration of these compounds can be measured by a test similar to the American i\rsociation of State Highway Officials test T-46-35 ( 7 ) . This test, in which the percentage of material insoluble in paraffin naphtha is determined, is referred to here as modified naphtha insolubles (MNI) , The chemical constitution of asphaltenes is not known. These compounds are formed readily in visbreaking or any liquid-phase pyrolysis or thermal cracking reaction of high boiling polycyclic aromatics. They may form by polymerization of large free radicals produced by dehydrogenation reactions. Hydrogen can be transferred from a sufficiently active donor ro satisfy these relatively stable radicals and, therefore, prevent polymerization. Experimental I n these studies, residuum was mixed with a hydrogen donor diluent in ratios between 2 to 1 and 5 to 1, and the mixture was cracked at 780' to 900' F. under 400 p.s.i.g. The cracking experiments were carried out in a continuous coil or coil and soaker unit (Figure 1). The product recovery system consisted of a liquid product receiver and a series of wet and dry ice traps. Noncondensable gas was measured with a dry test meter. The hydrogen donor diluent used was a 700' to 900' F. fraction of thermal tar obtained from thermal cracking of clarified oil from catalytic cracking of a LVest Texas gas oil. I t was partially hydrogenated by adding 400 standard cubic feet (SCF) of hydrogen per barrel of tar ( 3 ) . Properties of this hydrogen donor diluent and of the residua cracked are shown in Table I . In recycle operation, the donor diluent was cut from the products and rehydrogenated. A 95 to 5 blend of recycle to fresh diluent was used to maintain an adequate hydrogen donor concentration. Analyses indicated that the active donor materials originally present in the thermal tar were sufficiently
Table 1.
Feedstock Inspections
Hydrogenated 700-900' F. Thermal Tar
Carbon, wt. yo Hydrogen, wt. yo Sulfur, wt. Yo Specific gravitv, 60' F./60° F. Conradson carbon, wt. v n Modifikvd naphtha insolubles, wt. To yo Softening point, O r F.
90.24 9.46 0.81 1.014
27% 53% 93% Hawkins Bachaquero Bachaquero
84.87 9.91 4.54 1,046
84.81 10.44 3.36 1,030
24.6
20.3
...
2222 . 8
12.4
7.42
115.5
...
155
refractory to remain in the 700" to 900' F. boiling range but that dilution by cracked products from the residuum decreased the MNI-inhibiting properties of this fraction. Diluent makeup may range from zero to about loyo, depending mainly upon the composition of the residuum and the cracking severity. The plant flow plan of this continuous process is shown in Figure 2. Results and Discussion When Bachaquero residuum is visbroken in the presence of a hydrogen donor diluent to 10% yield of C? to 430' F. product, only a 127, increase in MNI is realized. I n the absence of a donor diluent, visbreaking to this severity causes a 557, increase in MSI. As a result, HDDV reduces fuel oil yield whenever M N I is limiting (Table 11). The residual fuel yield was reduced by over 28%, and a proportionate amount was upgraded to middle distillate as a result of reducing the volume of flux stock needed to blend to M N I specification. The added diluent would not decrease visbreaker capacity, since it is
P A R A L L E L MITY-MITE CONTROL VALVES
01 COLD FINGER CONTROL
I CONDENSER
FEED RES ERVOl R
'1VENT DRY TEST LIQUID PRODUCT RECEIVER
H,
f
-
MILTON ROY PUMP
c
LEAD BATH
-
-
LIGHT ENDS TRAPS
Figure 1. 310
l&EC
Flow plan of laboratory continuous HDDV unit
PROCESS D E S I G N AND DEVELOPMENT
85.26 11.08 2.63 0.978
GAS SAMPLE BALLOON
Table 111.
Table II. Yields from Bachaquero Residuum
Hydrogen Donor Diluent Visbreaking
53y0 Bachaquero Equal severity
Residuum length, vol. 93 yo in crudeb 10 Severity, vol. % .C4430' F. on resid. MNI increase, % on 55 feed MNI Yields, bb1./100 bbl. crudec Gasoline 16.3 -23.4d Middle distillate Residual fuel6 108f
Minimized fuel yields
53
93
10
5
55
28
12.3 11.6 - 17.8d 0 89.0 lO6f
Residuum/diluent wt. ratio SCF Hz/bbl. thermal tar Yields, wt. % on residuum Conversion to lighter products, wt. yo C 3-
HDDVa
53 9.3 45
53 10 12
No diluent
...
c 4
11.9
12.3 -1 O . P 1 3 , 4 99.3 75.8
Cj-430' F. 430-900' F. 900' F.
+
27% Hawkins
3/1 400
No diluent ..,
3/1 600
26.2
31.4
37.9
35.6
0.5 0.1 6.9 18.8 73.8
0.4 0.1 8.3 22.4" 68.6
1.6 0.8 9.3 22.2b
1 .0 1.3 10.0 21 .6'vb
A t 3/1 residuumldiluent ueight ratio. 7.0 M N I Bachaquero crude. c Includes distillate products from crude. d Blending stock needed for residual fuel i n excess of that available f r o m crude. e Blended with 700-900° F. virgin gas oil to same M N I and oiscosity ( 70 M N I and 175 Saybolt fur01 uiscosiv at 122'=F.). f Viscosity less than 175 Saybolt Jurol.
1000° F. Material balance, wt. Yo MN1 increase, 70c
possible to charge a much shorter residuum to HDDV than to visbreaking. In practice, one normally adjusts the residuum length (percentage on crude), cracking severity, and flux stock properties to obtain the lowest yields of residual fuel. For the particular Bachaquero crude used in these studies, the minimum fuel yields are given in Table 11. Cracking severity was the minimum needed to obtain a fuel of 10 M N I and a particular viscosity using the 700' to 900' F. virgin gas oil as flux stock. The most favorable visbreaking cases still produce 17.3 and 317, more residual fuel than HDDV. The comparison becomes increasingly more favorable for HDDV as one uses a lower biscosity: higher density flux stock or a lower M N I residuum. High M N I feedstocks such as Hawkins residuum also show a n advantage for HDDV with regard to suppression of M N I formation. Typical product yields obtained when visbreaking with and without a donor diluent a t comparable operating conditions are shown in Table 111. HDDV experimental data on Hawkins feed indicate that the extent of residuum conversion to distillate products, selectivity of distillate products to gasoline, and residual fuel
quality can be controlled through judicious manipulation of operating variables such as temperature, pressure, and feed rate. At a given level of residuum conversion, products of high temperature, high feed rate operation differ markedly from products of lower temperature, low feed rate operation with regard to selectivity to gasoline and quality of residual fuel. As operating temperature is increased, other factors remaining constant, corresponding increases occur in residuum conversion to distillate products, selectivity to gasoline, and modified naphtha insolubles content. A decrease in feed rate or longer residence time, other factors remaining constant, also results in a marked increase in residuum conversion. However, at the temperature employed. selectivity to gasoline and M N I content are definitely decreased. The effect of pressure is evidenced principally in conversion of residuum to distillate products. For example, as pressure is increased from 0 to 400 p.s.i.g., residuum conversion undergoes a corresponding increase from 13 to 45 wt. 70, while selectivity to gasoline and M N I content remain essentially unchanged. The lower conversion a t low pressure is due to partial vaporization decreasing the liquid residence time. Experimental data are tabulated in Table I V .
a
VACUUM TOWER
a
Ex-diluent.
98.8
62.1 96.0
64.4 98.4
45
12.9
15.8
0.7
F.
430-1000'
Based on MA'I of 4.30' F.
+.
ATMOSPHERIC TOWER I_ GAS GAS OIL
TOPPED CRUDE CRUDEFEED
100
HYDROGENATION
I
GASOLINE
DILUENT OIL
MA K'E- u -P' TAR
I i-
RESIDUAL F U E L
1
HYDROGEN DONOR DILUENT Figure 2.
HDDV visbreaking VOL. 1
NO. 4
OCTOBER 1 9 6 2
311
Table IV.
Effect of Operating Variables on HDDV L 4-
Val. 1VOI.1
F.
Hr.
P.S.I.G.
M N I on Resid. Conv.: Fresh Wt. 7 0 Resid.
Temperature Effect (-400 p.s.i.g.)* 3.38 49.0 18.8 3.36 50.4 20.0 3.17 54.3 21 . o
830 840 850
430' F. Selecticity" 25 29 -35
370 420 350
Feed Rate Effect(830' F.)h 5.24 35.2 3.38 49.0 2,60 59.4
20.0 18.8 17.5
26 25 21
0 200 400
Pressure Effect (830' F.)h 4.03 12.8 18.2 4.28 26.5 18.5 4.0 45(cal~d.)~ , . .
18 20
T-ol. % C4-430" F. on converted residuum ( 7000° F . - ) . Harekins rrsiduum, 2 I tesiduum-to-thermal far dillten! ratio. corrdation of JO0p.s.i.g. r i m r a[ other feed rates.
I
/
c..
/ I
When cracking rates and hydrogen transfer rates are In balance. polymerization will not occur. Higher concentrations of transferable hydrogen can be made available by decreasing the residuum to diluent ratio or by adding more hydrogen to the diluent. However, complete saturation of the condensed ring structures is to be avoided, as this decreases their hydrogen donor activity. The effect of this available hydrogen on MNI formation is shown in Figures 3 and 4. As severity is increased. the fuel oil yield becomes greater since M N I increases rapidly. The optimum operation is, therefore, one in which severity is the minimum consistent with necessary viscosity reduction. ,4t this low severity, hydrogen addition above 400 SCF per barrel of diluent has very little effect. Balance between the hydrogen transfer rate and cracking rate is very dependent upon temperature. Residuum cracking is initiated at about 700' F., whereas hydrogen transfer does not become appreciable below about 800" F. Consequently. in any thermal cracking operation, regardless of hydrogen donor content, the concentration of polymeric products undergoes an initial increase. The build-up of polymeric products continues in all cases until the activation temperature for hydrogen transfer is reached. Above this temperature, polymerization and hydrogen transfer become competitive reactions, the dominant factor being the relative concentrations of residuum radicals and hydrogen donor molecules. Therefore. the temperature range between 700' and 800' F.. should be spanned as rapidly as possible. The authors' experience indicates that, from the standpoint of donor diluent efficiency, the operating temperature should be in the 800" to 900' F. range, and the selectivity to naphtha should be kept low. Under these conditions. hydrogen consumption is less than 150 SCF per barrel of residuum. The gas oil can be cracked either thermally or catalytically to maximize naphtha production in cases where this is desirable. Conclusions
