I
C. R. EBERLINE, R.
T. WILSON, and L. G. LARSON
Research Division, Phillips Petroleum Co., Bartlesville, Okla.
Hydrotreating Cracking Stocks Mild hydrogenation of cracking stocks can reduce carbon deposition 30%, sulfur and heavy metals content 90%, and
practically
crude petroleum is complex, and therefore fractions used as feed stocks will inevitably contain materials detrimental to the catalytic cracking reaction-e.g., compounds of sulfur, nitrogen, oxygen, and metals. Cracking stocks containing increased proportions of these compounds result from processing increasing quantities of high, sulfur and nitrogen crudes and recovering feed stocks from the heavier portions of crudes, These give good yields of high octane gasoline, but decrease efficiency of catalytic cracking. Increased catalyst contamination or poisoning by heavy metals, sulfur, and nitrogen causes poorer product distribution and requires higher catalyst replacement rates, However, using sulfurresistant catalysts, or additional steam in the case of natural clay catalysts, can minimize sulfur poisoning. Coke yields at a given conversion level increase and less gasoline is produced for a given carbon-burning capacity. Also, quality of the product is poorer because of higher sulfur and nitrogen content of gasoline and cracked distillate. Many catalytic cracking units are limited in conversion capacity by their ability to burn the coke deposited on the catalyst. Including marginal feed stocks may seriously decrease ;his conversion capacity and consequently, the gasolineproducing capacity. A suitable process for eliminating undesirable components from the feed stocks would improve product quality and distribution and also increase coke-limited capacity of the cracking unit for producing gasoline, T H E CHEMICAL COMPOSITION Of
-
eliminate
hydrogen sulfide
Experimental Hydrotreating tests were carried out in a small pilot plant unit. The hydrogen and oil streams’ are heated separately in electrically heated pre-
n
H2 PREHEATER
r
FLOW CONTROLLER
I CALlB R ATED
Hydrogenation Processes Hydrogenation processes for petroleum oils may be classified as destructive and nondestructive. In destructive hydrogenation (hydrocracking or hydrogenolysis) carbon-carbon bonds are cracked and the fragments saturated by hydrogen to produce lower boiling
ture (7-3). Essentially no information is given on operating conditions for hydrotreatment, but the production of appreciable quantities of gasoline-range naphthas indicates fairly severe treatment. Meager references to operating pressures indicate a range of 750 pounds per square inch gage or above. Studies made in this laboratory show that significant improvement in catalytic cracking characteristics can be obtained under mild hydrotreating conditions, at operating pressures as low as 500 pounds per square inch gage, for a variety of stocks such as wide-range highsulfur gas oils, visbreaker gas oils, and even wide-range virgin gas oils from lowsulfur crudes. Catalysts used for hydrotreating usually consist of heavy metal oxides or sulfides on carriers-e.g., molybdena, tungsten-nickel sulfide, and cobalt molybdate. Catalysts used in hydrotreating tests described here consisted of cobalt molybdate on alumina or bauxite.
products. Severe processing conditions are employed ; high hydrogen pressures are required to minimize polymerizations and condensations leading to coke formation. Essentially nondestructive hydrogenation or “hydrotreating,” is used to improve oil quality without appreciably altering boiling range. Milder processing conditions are employed so that only the more unstable materials are attacked, and production of lower boiling materials is minimized. Thus, bonds between carbon and such elements as sulfur and nitrogen are broken with the formation of hydrogen sulfide or ammonia; olefin bonds are saturated and unstable cokeformers are converted to more stable materials. Hydrogenation processes, although not new, have been little used in this country because of the lack of economical sources of hydrogen. Since by-product hydrogen from catalytic reforming has become available refiners are considering hydrotreating processes for improving various refinery streams. A few references to effects of hydrotreating on catalytic cracking characteristics of such stocks as high-sulfur virgin gas oils, coker gas oils, and catalytic cycle oils have appeared in the literaHYDROGEN
formation
+-
RESERVOIR
VALVE
OFF GAS
CONDENSER
The hydrotreating Pilot unit
HIGH PRESSURE GAS SEPARATOR FEED PUMP VOL. 49, NO. 4
APRIL 1957
661
a range of conversions. The processing period for each feed was determined by preliminary tests so that all stocks were cracked with catalyst having essentially the same average (mean value) carbon deposit. Three wide-range gas oils, recovered from their source material by a combination of atmospheric and vacuum flash distillations, were used in this study as representative of three different types of catalytic cracking feed stocks (Table I).
Typical Operation Conditions Temp., O F. Space rate (weight basis) Pressure, lb./sq. inch gage Steam diluent, 1b.f bbl. of oil Processing period, min. Carbon on catalyst (mean value), wt.
%
LOW S U L F U R VIRGINGAS OIL
m a U
V
I 30
40 50 60 CONVERSION, VOL. %
70
Figure 1. Carbon yields from catalytically cracking wide-range gas oils
0 Before hydrotreating After hydrotreating
heaters, combined, and charged downflow through the reactor which contains a fixed bed of catalyst pellets and is electrically heated to compensate for heat losses. The reactor effluent passes through a water-cooled condenser to a gas separator. The off-gas from the separator passes through a pressurecontrol valve and is metered and sampled. Liquid product is withdrawn from the separator and stabilized to remove hydrogen sulfide, hydrogen, and traces of light products. The stabilizer bottoms are then fractionated, if necessary, to remove gasoline-range naphtha. In large-scale operation a portion of the off-gas would be recycled. Recycle operation has been simulated by reducing hydrogen content of the feed gas. Hydrogen purities of 90% or even lower have no appreciable effect on the hydrotreating process. Raw and hydrotreated oils were catalytically cracked in fluidized confined-bed equipment developed in this laboratory (4, 5 ) . All variables were held constant, except space rate and processing period. Space rate variations obtained by varying the oil feed rate, allowed data to be obtained over
662
All three stocks were hydrotreated under similar conditions of temperature, pressure, and feed rate as shown in Table I. A higher hydrogen flow rate was used for the high-sulfur stock; however, varying this rate from 2000 to 5000 cubic feet per barrel has little effect on product properties. Hydrogen consumption was only 110 cubic feet per barrel for the low-sulfur virgin oil, but was considerably greater for the high-sulfur oil because of the greater quantity of sulfur converted to hydrogen sulfide. Olefin saturation accounts for part of the hydrogen consumption in hydrotreating the visbreaker gas oil. Properties of the three stocks which are considered important in catalytic cracking were improved by hydrotreating. Sulfur removal was from 78 to 92%, and for the high-sulfur gas oil, amounted to over 7 pounds per barrel of oil. Nitrogen removal was less complete-55 and 50% for the high-sulfur and low-sulfur virgin oils, respectively. In the visbreaker
Table I.
1 to 3
10
15 6 to 25
1.0 Plant equilibrium montmorillonite
Type of catalyst
ResuIts
900
Catalyst activity, Phillips activity index (6) Catalyst activity, (Jersey B L, approx.)
107
+
32.5
gas oil which had already been subjected to severe thermal conditions, the remaining nitrogen compounds were seemingly stable and no apparent reduction was effected. However, all of these oils were low in nitrogen. Improved catalytic cracking quality of hydrotreated oils was further indicated by decrease in carbon residue and heavy metals content. These metals, nickel and vanadium, which contaminate or poison cracking catalysts, are deposited on the hydrotreating catalyst but apparently have no deleterious effect on the process. However, in this work, large concentrations on the catalyst were not attained. The Bureau of Mines correlation indexes were lowered by hydrotreating, indicating a decrease in the
Hydrotreating Catalytic Cracking Feeds Gas Oil Wide Range Virgin From Visbreaking Wide Range Virgin from West Texas from High of West Texas Sweet Crudes Sulfur Crude Residuum Charge Product Charge Product Charge Product
Hydrotreating Pressure, lb./sq. in gage Temp., F. Hydrogen flow, cu. ft./bbl. of oil Liquidhourly space velocity Liquid recovery, vole Toof feed Gasoline produced, vol. % of feed Hydrogen consumed, cu. ft./bbl. % desulfurization Cracking feed inspection Gravity, API 24.8 Vacuum distillation, F. corr. to 760 mm. Hg IBP 440 10% 517 50% 741
INDUSTRIAL AND ENGINEERING CHEMISTRY
90% 95%
Sulfur, wt. % Nitrogen, wt. % Carbon residue, Ramsbottom, wt. % Bureau of Mines correlation index Heavy metals, p.p.m. Ni 0 v203
1006 1046 2.66 0.09 0.81 45
0.55 0.53
500
500
760
750 2000 1.0 100.0
5000 1.0 101.9
5.5
24.6
27.2
469 506 438 621 650 493 911 940 682 1098 1099 938 1117 1015 1120 0.72 0.16 0.24 0.16 0.16 0.04
0.13 34
