RECLAMATION OF DAMAGED NUCLEAR REACTOR COOLANT BY CATALYTIC HYDROCRACKING L
.
E, GA RDNER A
N D W
.
M
.
H U T C H I N S 0 N, Phillips Petroleum Go., Bartlesoille, Okla.
Organic liquids such as terphenyls have been proposed for use as neutron moderators and heat transfer media in production of electrical power from nuclear reactors. Theory and experience have shown that thermal and radiolytic effects produce higher molecular weight materials which are detrimental to heat transfer. Catalytic hydrocracking is recommended for reconstituting this undesirable high boiling waste into usable coolant. Active metal oxides and metals on low surface alumina are effective catalysts for selective hydrocracking of polyphenyls into smaller aromatic molecules. Among the better catalysts are NiO-AI203, CoMoO4-A1203, Pt-A1203, and CoO-V2Oj-A1203. Polyphenyls become more susceptible to hydrocracking with increasing chain length. Because of elimination of a vacuum distillation step, hydrocracking of total coolant is more efficient than that of undiluted high boiler. Reclaimed coolants from both methods were similar to terphenyl in viscosity, density, and vapor pressure. Although these products contained some alkylated aromatics, they were radiolytically stable a t 650' F. and dosages of 1O'O rads and thermally stable up to 750" F. Conservative economic evaluations indicated that hydrocracking coolant was commercially feasible for power plants producing 1000 megawatts (thermal). HE use of an organic liquid for neutron moderation and for Theat transfer in a nuclear reactor has been investigated ( 3 ) . Temperatures sufficient for steam generation can be utilized without the need of high pressures or corrosion-resistant materials of construction. Experience in the operation of an experimental organic moderated reactor has indicated that thermal and radiolytic processes convert the terphenyl coolantmoderator fluid into products of higher molecular weight called high boiler. A4ccumulation of this polymer in the circulating fluid increases its viscosity, decreases its heat transfer coefficient, and probably contributes to deposition of an insulating film which has been observed on fuel element surfaces. Replacement and disposal of the high boiler are expected to be major factors in the direct operating cost of an organic moderated reactor (25 to 100 pounds of high boiler per megawatt-day of thermal power). T h e present cost of terphenyl coolant, Santowax O M P (Monsanto Chemical Co. trade-mark for a mixture of 0-, m-,and p-terphenyl) is 17 cents per pound. The economic incentives for reclaiming damaged coolant from a large-scale reactor (1000-megawatt thermal power) are considerable. Make-up coolant cost has been calculated as equivalent to 0.66 mil per kilowatt (e)-hour (2). An additional expense is the removal and disposal of high boiler. Thus, successful reconstitution of damaged coolant could contribute to more efficient and economical operation of a n organic moderated nuclear power reactor. High boiler in terphenyl coolant damaged by irradiation in a reactor is a complex mixture of aromatic hydrocarbons. It is very soluble in low boiling aromatic solvents and completely soluble in molten terphenyl. Exact and complete characterization of this material has been difficult. In general, it consists mainly of polyphenyls (I) of six rings and higher, with some triphenylene (11), alkyl and aryl polyphenyls and triphenylenes (1111, aryl phenanthrenes (IV), and other fusedring aromatics and hydroaromatics. It appeared feasible to reconvert a portion of this high boiling waste by catalytic hydrocracking into terphenyls, or other material in the boiling range of terphenyls. of sufficient stability for use as a coolant. Since available analytical data indicated that the high boiler consisted primarily of hexaphenyls and
28
l&EC PRODUCT RESEARCH A N D DEVELOPMENT
higher multiples of the terphenyl molecule, proper selection of catalyst and conditions should result in cleavage between benzene rings. Thus the desired reaction is cracking of the carbon-carbon bonds between the rings with minimum ring saturation, polymerization, or condensation reactions. Cracking occurs thermally in the range 1000" to 1400' F. However, a t these high temperatures the terphenyls themselves undergo decomposition, and the condensed aromatics present tend to form more cokelike material. The catalyst requirements include cracking activity and sufficient hydrogenation activity to suppress coke formation from fragments formed in the cracking reaction. This investigation was divided into three phases. T h e first was a catalyst evaluation program in which a variety of compositions and supports were evaluated for hydrocracking polyphenyls. Exact catalyst requirements and necessary reaction conditions were determined. Several of the more promising catalysts were used in the second phase of the program in hydrocracking high boiler and coolant samples obtained from Core I1 and Core 111-A operations a t the Organic Moderated Reactor Experiment (OMRE) ( 3 ) . The final phase was to evaluate reclaimed coolant samples from bench-scale hydrocracking tests for radiolytic and thermal stabilities.
(I1
ini
imi
(3)" R=ALKYL GROUP
ARzAROMATIC GROUP
Experimental
The experimental hydrocracking units were continuous flow, pressurized systems employing downflow of both liquids and gases over a fixed bed of catalyst. Pressure, flow rates, and heaters were controlled automatically, and catalyst temperatures were recorded from thermocouples equally spaced in thermo\vells mounted concentrically in the reactors. Liquid and gaseous effluents were separated in heated, high pressure separators. Most of the 35 catalysts tested were prepared in the laboratory by standard methods. Active metals as oxides (except P t as the chloride) were incorporated into the support by impregnation followed by drying a t 240" F. and air-calcination a t 1000" F. Partial reduction presumably occurred when used for hydrocracking. Crushing and sizing to 10- to 20mesh particles gave a favorable ratio of particle size to reactor diameter in a n 80-cc. catalyst bed. All tests were made on catalysts which had been through a t least one coking-regeneration cycle. High boiler and spent coolant samples were obtained from the Atomics International operation of the Organic Moderated Reactor Experiment ( O M R E ) (2) a t the National Reactor Test Station near Idaho Falls? Idaho. The two high boiler samples differed in properties and appearance, as shown in Table I. Core 111-A high boiler was mainly first generation polymer (hexaphenyls) produced by maintaining its concentration in the coolant at about 570. Core I1 high boiler was mainly first and second generation polymer produced by maintaining its concentration in the coolant a t about 307,. High boiler samples were charged to the small-scale hydrocracker in aromatic solvents because the low pumping rates allowed freezing of undiluted charge. Most tests on total coolant samples were carried out using 40 to 50 weight 70 solutions in p-xylene. \$'hen it became apparent that the presence of the volatile solvent contributed to lower yields, solvent-free operation was demonstrated by using a liquid eutectic mixture of biphenyl, o-terphenyl, and rn-terphenyl (Santowax DOM. Monsanto Chemical Co.) to dissolve the high boiler. In an extensive series of tests product yields were found to be higher at all conversion levels in the absence of a volatile solvent. Analytical procedure for products from hydrocracking included : atmospheric distillation of solvent and products boiling below biphenyl ; microsublimation to remove terphenyl range product; and chromatographic analysis of sublimate and distillate. At high conversions in coolant hydrocracking only chromatographic analysis was employed. Quantitative calculation of conversion was possible by designation of the sublimation residue as unconverted high boiler. Sufficient separation by gas-solid chromatography was obtained for determination of the composition of reclaimed coolant. Definition of Terms and Calculations. HIGH BOILER. Usually described as material boiling above p-terphenyl. I n this research it has been arbitrarily but equally defined as
I
3ENZENE SOLVENT
P-TERPHENYL I8 %
ALKYLTERPHENYLS 4%
TRIPHENYLENE PUATERPHENYLS
ALKYLBIPHENYLS c I
160
I
I
180
I
I
I
1
I
200 220 240 ELUTION TEMPERATURE, 'C
Black solid
Appearance Molecular wt., no. av. C/H atomic ratio Sublimable at 660' F., 0 . 1 0 mm., wt. 70 Terphenyl content, wt. %
545 1.42
Reddish brown solid 450 1.35 84 5
43 9
residue remaining after sublimation at 240' C. (465' F.), 0.20 mm. of Hg, and 30 minutes' time. Samples were recovered from spent coolant by distillation. SPENTCOOLANT.Coolant damaged by irradiation in a pile
(OMRE). SYSTHETICCOOLAST. Coolant prepared by mixing high boiler with terphenyls. SPACEVELOCITY.Volumes of liquid polyphenyl charge per volume of catalyst per hour (volatile solvent not included). ALKYLBIPHESYLS. Components which eluted from the chromatographic column between biphenyl and o-terphenyl as shown in Figure 1. The first peak was believed to be 3,3'dimethylbiphenyl. ALKYLTERPHENYLS. Components which eluted from the chromatographic column between o-terphenyl and triphenylene (excluding rn-terphenyl and p-terphenyl) as shown in Figure 1. One component was al\vays found in these chromatograms, but not identified. COSVERSION.Disappearance of high boiler,
70conversion
=
% HB in --
charge - yo HB in product - x 100 % in charge
SELECTIVITY.Yield of biphenyl and benzene in hydrocracking of m-terphenyl, calculated as
76 selectivity
=
% benzene + 7,biphenyl - x 100 yo conversion
PRODUCTRECOVERY. Product includes biphenyl and heavier, or material remaining after distillation of solvent and products boiling lower than biphenyl. WEIGHTPERCENTYIELD. Based on coolant charged. wt.
G/c yield
=
wt. of component \vt. of coolant charged
x
100
KETYIELD.Based on high boiler in coolant charge converted.
Results
0-TERPHENYL 24 %
I
Properties of High Boiler Samples Obtained from OMRE Operation High Boilers Core 111-A Core 11
wt. yo net yield = wt. of component in effluent - wt. of component in charge ___ x 100 wt. of HB in charge - wt. of HB in effluent
M-TERPHENYL 41 %
BIPHENYL 4%
Table I.
I
1
260
I
,
280
,
8
300
Figure 1 . Typical product from hydrocracking OMRE Core Ill-A coolant containing 3070 high boiler
Terphenyl Hydrocracking. Catalysts low in surface area and essentially free from acidity were superior in selectively hydrocracking the inter-ring bonds in model polyphenyls such as o-terphenyl and rn-terphenyl. At a given set of hydrocracking conditions, conversion of rn-terphenyl varied between 5 and 95%, depending on the catalyst employed. A general picture of the differences in activities for various catalysts is shown in Table 11. Unpromoted gamma-alumina had very little activity, and even at higher reaction temperatures (1000 ' F.) only 77, conversion was obtained. The cobalt molybdateand platinum-containing catalysts which showed good selectivities Lvere both on low surface area supports (less than 100 sq. meters per gram). Catalysts containing acidic supports (boria-alumina, fluorided alumina, or silica-alumina) were nonselective and produced much more coke. VOL. 3
NO. 1
MARCH 1964
29
Table II.
Effects of Catalyst Type, Surface Area, and Acidity on Hydrocracking of m-Terphenyl Reaction conditions: 900' F., 500 p.s.i.g., 1.0 vol./vol. catalyst/
hr., 1570 m-terphenyl inp-xylene, and 20 moles hydrogen per mole terphenyl Surface Area, ConverSelecCoke,c Sq. M./G. sion,a % tiuity,b % Wt. % Catalyst Unpromoted alumina 21 1 2 57 0.1 CoMoOa-Al203 57 78 76 0.6 CoM004-F-.41203 1 81 52 2.0 CoM004-BzO~-A1203 103 88 12 4.4 Pt-Al203 80 90 73 1.2 Pt-F-Al203 180 86 43 3.5 a Calculated as disappearance of terphenyl. Based on terphenyl converted to benzene f biphenyl. Based on terphenyl charged.
The effect of surface area was demonstrated by preparation and testing of several cobalt molybdate on alumina catalysts in which areas were varied from 1 to 180 sq. meters per gram. Conversion was independent of surface area, but selectivity and coke yield were improved by decreasing the surface area as shown in Figure 2. Similar data were observed for both nickel oxide and platinum on alumina. The available data indicated that surface areas in the range of 20 to 100 were optimum. This effect was confirmed when several different catalysts were tested for hydrocracking OMRE Core I1 high boiler a t the same level of conversion. Selectivities were about 70% for catalysts of 25 to 100 sq. meters per gram compared to 30% for catalysts of 180 sq. meters per gram. Coke yields for the latter were usually four times those observed for the selective catalysts. T h e surface area effect may be explained by a combination of pore size and hydrogen diffusion. A correlation of average pore size with surface area for heat-treated alumina ( 7 ) indicated that pore diameter increased from 17.5 A. a t 200 sq. meters per gram to 130 A. a t 1.0 sq. meter per gram. Thus, pore diameters for alumina surfaces in excess of 100 sq. meters per gram might be small enough to slow down desorption of a molecule approaching hexaphenyl in size. If pore blockage then occurs, the diffusion rate of hydrogen is slowed down
500 PSIG, 0.4 V/V/HR, QOO'F. 20 MOLES H2/MOLE 03
100
z E
81
\* 4ot
I
20
L
* \ \*
I I
U
SURFACE AREA, M~IG (CoMoOq-A 1203 CATALYST)
Figure 2. Effect of catalyst surface area on hydrocracking of rn-terphenyl 30
I & E C P R O D U C T RESEARCH A N D D E V E L O P M E N T
Table 111.
Hydrocracking m-Terphenyl at 1000 P.S.I.G., 900' F., and 0.5 V./V./Hr. Selective, Nonselective, Pt-F-AI 20 3 Pt-AI 20 3 Conversion, wt. % 90 86
a
Net yields, wt. %a Alkyl benzenes Alkylbiphenyls Alkylterphenyls Benzene Biphenyl Selectivity, % Based on terphenyl convvted.
8
35
11 7 44 30 74
16 27 17 44
4
further, and active sites in the pore might be promoting polymerization and/or condensation reactions. This hypothesis might explain the disintegration observed in several of the nickel oxide-alumina preparations, since nickel is known to be a polymerization catalyst under proper conditions. Thus, at low rates of hydrogen diffusion and product desorption, polymerization in the pores could cause swelling and eventual disintegration. The only nickel catalysts that were stable physically had low surface area ( < l o 0 sq. meters per gram) and low nickel content (
