I
CARLYLE S. HERRICK, NORMAN KIRK, THEODORE 1. ETHERINGTON, and A. EUGENE SCHUBERT' Research Laboratory, General Electric Co., Schenectady, N. Y.
Pilot Plants Diborane and pentaborane, two high-energy fuels for thrust engines from the energy-packed borane family, can be made by controllable, reproducible processes
thrust heat engine principle was described in ancient times, the ever increasing power requirements of modern aircraft and the recent availability of lightweight, high temperature materials have made it practical today. Thrust engines not only have greater fuel consumption and greater heat release capability than reciprocating engines the same size, but they also can use unconventional higher energy fuels. I n 1944 this laboratory began to explore the hydrides of boron as potential high energy fuels. Diborane, B2H6. the lowest borane homolog, has a calculated heat of combustion in excess of 33,000 B.t.u. per pound compared with 12,800 B.t.u. per pound for ethyl alcohol and 20,700 B.t.u. per pound for n-hexane. Thus diborane has a heat release greater than conventional fuels by a factor between 1.6 and 2.6. Henneberry ( 7 7 ) recently estimated that diborane has a 599& range advantage over hydrocarbon fuels in a ramjet at 70,000 feet and Mach 3.6. Measurement of the heat of formation of diborane has since been reported (27). The pilot plant operating experience has demonstrated that the preparation of diborane is a controllable and reproducible reaction which can be carried out at reasonable rates and good yields. The erratic performance reported by others as being characteristic of the lithium hydride-boron trifluoride etherate system was entirely eliminated.
reaction of boron trifluoride with alkali metal borohydrides (28) and with methoxyborohydrides ( 5 ) , the reduction of a boron halide in the presence of an active metal (12), an electric arc process ( 2 6 ) , and an electrolytic process ( 3 2 ) . Elliott, Boldebuck, and Roedel ( 9 ) studied the lithium hydride-boron trifluoride reaction and found that two distinct reaction patterns were possible when adding boron trifluoride to excess lithium hydride. 6 LiH f 8 BF3
+
ether B&
6 LiBFa (1) 6 LiH
+ 2 BF3 prz:Fr BZH6
+ 6 LiF
(2)
They also found that Reaction 2 proceeded in two steps. 6 LiH
+ 1.5 BF3 1.5 LiBH4
1.5 LiBHr
p
r
r
~
+ 4.5 LiF + heat
+ 0.5 BF3ether+ BZH6 + 1.5 LiF - heat
~
~
(3)
(4)
These conclusions were supported by Mikheeva and Fedneva (77). Shapiro (30) simultaneously reported that the reaction of lithium aluminum hydride with boron trifluoride to make diborane proceeded in two stages, with lithium borohydride as the product of the first step. Reaction 1 is to be avoided because the maximum yield is 25% based on boron. Reaction 2, which allows a 100% theoretical diborane yield, both with respect to boron and with respect to lithium hydride, is the desired reaction. Elliott, Boldebuck, and Roedel ( 9 ) list four ways of ensuring the occurrence of Reaction 3. TWOof them are used consecutively throughout this work. After the lithium hydride charge has been slurried with ether in the reactor, diborane is added in amounts of 5yo of the expected product (25). All ~ reactor exits remain closed as boron trifluoride addition begins so that any diborane resulting from Reaction 1 will be retained and used to promote Reac-
Theoretical The preparation of diborane is based on the reactions occurring between lithium hydride and the ethyl ether complex of boron trifluoride reported by Schlesinger and Brown (24, 27). Other methods of diborane preparation include Present address, Chemical Materials Department, General Electric Co., Pittsfield, Mass.
Flow scheme for diborane production shows equipment described in detail on page 108 VOL. 52, NO. 2
0
FEBRUARY 1960
105
11.5 Ib. B2H6
I
Table
I. Material Balance for Diborane Manufacture
B2H6
E t 2 0 118Ib.
= 20
Recovered Vent loss A s LiH in filter cake A s active H in filtrate Unaccounted for loss
got.
LiH 20lb.(95% LiHl
I2 REACTION KETTLE
FILTRATION
L
(For re-use)
62 I b . L i F I I b . Li IMPURITY 6 3 I b. T O T A L S O L I D S
591b.=IOgoL E t 2 0
116 lb.E$O (For re-use)
Production rate o f 10 pounds of diborane in an 8-hour period determines material balance
tion 3. The initial addition of diborane reacts with lithium hydride to form lithium borohydride according to Reaction 5. 2 LiH
+
B2H6 -+ 2
LiBH4
(5)
Once a minimum concentration of lithium borohydride is established in the reaction medium, reaction of boron trifluoride with lithium hydride will follow Reaction 3 to completion, then be supplanted by Reaction 4. Reaction 3 is exothermic, while Reaction 4 is endothermic, so the two are readily distinguished by following the reactor heat release rate.
it passes through a condenser to lower the ether vapor content, then through a compressor to raise the pressure. The high pressure diborane-ether mixture is separated into its components in a continuous low temperature distillation column. Liquid diborane product accumulates in a product receiver, then is transferred to a cylinder for storage. Spent slurry from the reactor is discharged through basket filters which retain lithium fluoride and lithium hydride solids. Filtrate ether is stored for reuse. The over-all reaction is indicated by Equation 6. 350
6 LiH
Diborane Process In general terms this scheme makes diborane by reacting an alkali metal borohydride with a boron halide, the borohydride being made in situ by a preceeding reaction in the same vessel. Lithium borohydride accumulates until all of the lithium hydride is consumed by boron trifluoride. Then lithium borohydride begins to react with boron trifluoride and diborane evolution begins. Diborane used to initiate or "spike" Reaction 3 is recovered without sacrifice as Reaction 4 proceeds to completion. Diborane withdrawal does not start until Reaction 3 is completed, otherwise the undesired Reaction 1 will occur. I n the flow diagram of the process (this page), ether and lithium hydride are charged to a batch reaction kettle, reaction is initiated by adding a small amount of diborane, then boron trifluoride etherate is added. When diborane gas begins to evolve by Reaction 4
106
c.
+ 2 BFs.O(C*Hs)z' o e ~ ~ ~ ~ ' e ' 6 LiF + B2H6 + 2(CzHs)20 ( 6 )
Lithium hydride and lithium fluoride are solids quite insoluble in ether, and the boron trifluoride etherate is a nonviscous liquid. The design production rate of 10 pounds of diborane per 8-hour period determines the material balance (see Chart).
Results Diborane generation started at a low rate after 5.7 gallons or 46'% of the stoichiometric amount of boron trifluoride-etherate had been adde d to the lithium hydride slurry. At 9.0 gallons or 72y0, the generation rate rose steeply to maximum value. I t was predicted that diborane generation should hit a steady state rate at 72.5y0 of boron trifluoride-etherate addition. At this same 72% rate, the heat of reaction changed abruptly from +40,000 B.t.u. per hour to -14,000 B.t.u. per
INDUSTRIAL AND ENGINEERING CHEMISTRY
100.0
Ib. Et-0
B F 3 * E t 2 0 113 I b . = 1 2 g a l .
i
% 82.5 4.5 1.0 2.5 9.5
hour, as required by Reactions 3 and 4. These data show clearly the transition from the borohydride formation reaction to the borohydride consumption reaction. As the stoichiometric end point was approached, the diborane evolution fell off sharply and all chemical activity ceased. The reaction temperature was closely controlled at constant value throughout the run. At the end the temperature was raised slightly, and the ether slurry was allowed to boil a few minutes to complete the removal of diborane. Temperature has a moderate effect on yield of diborane. At 35' C. the overall plant yield averaged 82.5y0, while a t 25" C. the yield averaged 797& A material balance around the diborane plant after a typical run at 35' C. shows the chemical yield to be a t least 87y0 (Table I). The principal cause of loss was diborane swept into the vent system by noncondensable gases. Because of the relatively low differential between diborane boiling point and distillation column condenser temperature, the diborane content of the vent gas was 28%. Minimizing the noncondensable gas formation therefore became an important problem. Ether used to form the boron trifluoride-ether complex was the chief source of noncondensable gas. Alcohols and other impurities present in the ether reacted in the slurry to liberate hydrogen. While C.P. grade ether was satisfactory from other aspects, it produced about three times more hydrogen than did ACS reagent grade ether. The unaccounted-for losses include the diborane inventory in the top portion of the distillation column and the column condenser at the end of a run, transfer losses in filling the product storage cylinder, and losses due to self-reaction of diborane. This self-reaction loss was observed at every point where diborane came in contact with ether. Filtrate from the reaction kettle left behind light brown brittle polymer deposits after pump packing gland leaks. The same kind of material was found in the reboiler of a n auxiliary column used only for filtrate ether distillation. Attempts to compress diborane in etherfilled elliptic rotary compressors were unsuccessful partly because of solubility and self-reaction in ether. The deposits accumulated slowly and necessitated only
BORANE PILOT PLANTS
Table II.
infrared spectroscopic examinations were used to characterize the samples (Table 11). Diborane content of the product is above 98%. Ethane occurs in the product because of a slow reaction between diborane and ether during shutdown periods of the diborane distillation column. Spot sampling revealed a small concentration of ethane in the diborane column at startup time. Ethane could not be detected anywhere in the system after the distillation was underway. The tetraborane content of the product appears to vary with age and to be a result of self-reaction of stored diborane. For example, one sample 16 months old had a tetraborane content of 4.9y0. Hydrogen also is a product of the selfreaction, as well as coming from etherate impurities. Lithium hydride particle size is an important plant parameter which limits the maximum rate for boron trifluorideetherate addition. Three sizes were explored, 3/~-inchlumps, 4 to 60 mesh, and 200 mesh. At the desired rate of 0.05 gallon/minute of boron trifluorideetherate, the 3/s-inch lump lithium hydride failed to react completely.
Diborane Product Analysis
Two different runs; stored for two months before sampling
Amount, Mole % Analytical Infrared distillation spectroscopy 98.3 0.8 0.7 0.0 0.0
Product BzHe c2HS (CzH6)zO B4HlO Hz Nz
...
... 0.9
+
Table 111.
0.4
...
Filter Cake Analysis %
LiF LiH LiBF, Volatile Unaccounted for
93.6 0.3 0.6 4.0
1.5 100.0
occasional cleaning of heat transfer surfaces. Product quality analyses were conducted on diborane samples removed from the product cylinder after the completion of the run. Analytical distillations at 300 mm. of mercury and
t40 +20-
,
0
l RATE'OF H k ' - \ EVOUrrlON BY REACTION (THOUSANDS BTU / HR 1
l
l
l
1
1
Both 4 to 60 mesh and 200 mesh lithium hydride did react completely with indications that 0.05 gallon/minute of borontrifluoride-etherate was near the maximum rate for the 4 to 60 mesh size. This size was used for all subsequent work because it was less difficult to prepare and handle than the 200 mesh size. A typical filter cake analysis (Table 111) shows that hydride utilization was nearly complete. Hydride particle size had a marked effect on the ease with which Reaction 3 could be initiated. Greater exposed hydride surface made the starting easier, with the 200 mesh size being significantly better than 4 to 60 mesh in this respect. Site Development The pilot plant facility was located at a previously undeveloped site. The facility consisted of the process building, boiler house, transformer vault, auxiliary generator building, chemical storage shed, and product storage magazine isolated by an earthen revetment. The process building contained office, laboratory, locker room, utility room, control room, and two storage rooms in addition to a 1600-sq. ft. high bay process area.
F
TEMPERATURE
o
150 -r
60
70
80
90
100
30
40
50
I I O L O
130
140
60
80
90
li L
Y
'
1.2
0.8 04
0
-%z---- -'IDE
5.7GAL.
9.0 GAL 12.5 GAL.
I
0
10
20
70
T E M P E R A T U R E 'F
A Decomposition pressure rise of 1 pound diborane showed a pressure increase of about 7 p.s.i.g. per month at 0" F. TEMPERATURE IN REACTION KETTLE --w
160 240 TIME, MINUTES
320
400
480
4 Here is a composite look at the behavior of the major variables during a typical run VOL. 52, NO. 2
0
FEBRUARY 1960
107
Diborane Pilot Plant PLANT EQUIPMENT
Reactor: 50-gal. steel kettle, designed for 200 p.s.i.g. working pressure; heating jacket and internal cooling coil each transferring 100,000 B.t.u./hr. ; hot, cold water streams with single temperature controller by overlapping inverse adjustment of two valve positions. Agitation by unshrouded turbine, 440 r.p.m.; lithium hydride charging from tote box throuqh top entering 4-in. line. Reflux Condenser: Connected to reaction kettle ; oversized, 36-sq. ft. cooling surface for safety; all-steel shell and double tube sheet construction, cooling water on shell side. Compressor: Diborane-ether gas stream leaving reflux condenser compressed from 10 to 65 p.s.i.g. ; two-cylinder, single-stage, refrigeration type; 1.35 cu. ft./min. displacement when belt driven at 320 r.p.m. Despite frequent oil changes, compressor bearings were highest maintenance item in plant. Self reaction of diborane in oil gave suspension of fine boron or boron polymer particles reducing bearing life. Distillation Column: For diborane purification, 6 ft. high 2-in. i.d., continuous low temperature, packed with 0.5-in. Berl saddles; vapor feed entered column midpoint. Steamheated 4.4-gal. reboiler; shell and double tube sheet vertical condenser, 3 sq. ft., cooled by boiling liquid ethane refrigerant on shell side. Float valve for refrigerant level control at midpoint of condenser shell; product take-off line jacketed with liquid ethane; 0.5-cu. ft. surge tank atop condenser provided inert gas capacity for smooth pressure control. Entire assembly made of Type 347 stainless steel; column, condenser, surge tank, product receiver assembled in insulated cold box. Process side design pressure 200 p.s.i.g. at -90' C . ; refrigerant side design pressure 625 p.s.i.g. at -90' C.; 1 to 1 reflux ratio gave satisfactory performance. Take-off control made by measuring column temperature just above feed point; controlled venting for pressure control. Product Receiver: 8.5-gal., stainless steel, fully jacketed for liquid ethane refrigeration, 800 p.s.i.g. process side design pressure at - 90 " C.; refrigerant level control by float valve in horizontal appendage to jacket. At conclusion of distillation, diborane warmed by external copper coil thermo-syphon to -20' C. to transfer diborane into product cylinder under its own vapor pressure; 400 p.s.i.g. receiver pressure transferred 10 lb. of diborane leaving 1-lb. inventory carried forward. By-product Removal: Spent slurry (30 gal. of ether, 63 lb. of solid lithium fluoride) dumped into two supported canvas bag basket filters, 5.5-cu. ft. capacity, holding three runs. Filter Vessel: 30-in. diameter, 50 p.s.i.g. design pressure, hot water-jacketed for cake drying; nitrogen sweep to vent condenser to assist ether removal; vacuum treatment to remove last of ether. Ether filtrate stored for reuse in 125-gal. tanks or distilled in 6-in. steel packed column. PRODUCT STORAGE
Cylinders: Standard steel gas cylinders, ICC-3A-2015, 2640-cu. in. volume, containing 10-lb. lots; equipped with forged brass hydrogen service valves; specially painted white, red, and blue for instant identification. Storage: 0" F. walk-in type freezer, total capacity 200 Ib. of diborane. Self reaction of diborane causes instability on long-term storage; pressure increase about 7 p.s.i.g./month at 0 O F. Semiannual surveillance of cylinder pressures; inert gas vented until 0 O F. vapor pressure of 330 p.s.i.g. approached. UTILITIES
Compressing-Drying: 50-cu. ft./min. system for pneumatic controllers, valve actuators, operating cylinders. Water: Hot and cold water provided as closed-loop recirculating systems treated to prevent corrosion (35); 1000gal. tanks with heat transfer tube bundles used in each system. Water heating by steam; cooling by 120,000-B.t.u./hr. Freon12 refrigeration system working into evaporative condenser. Total plant water consumption about 2.5 gal./min ; separate air-cooled Freon-12 system for product cylinder cooler. Refrigerant: Liquid ethane used in diborane distillation supplied by cascade arrangement-compressed ethane rejected heat to boiling Freon-22, then compressed Freoc-22 rejected heat t o cooling water. Heat transfer surfaces sized to reach steady state operation 1 hr. after start-up. Steady
1 08
INDUSTRIAL AND ENGINEERING CHEMISTRY
Pentaborane Pilot Plant PLANT EQUIPMENT
Four identical reactor units were installed and connected for simultaneous or individual use. Data here are for individual use of one reactor. Reactor: 40-in. length of 2-in. steel pipe with 3-in. hot oil jacket. Solid reaction products adhering to reactor walls loosened by cutter traversing reactor length; two bladed, tungsten carbide-tipped bits cut in either direction at 420 r.p.m.; loosened solids fell into solids filter beneath; cutter, shaft, drive motor mounted on carriage guided by vertical ways and moved by double-acting pneumatic cylinder; coating of solid reaction products on cutter shaft scraped off by cast iron bushing at bottom of stuffing box. Primary Filter: Beneath reactor, 10-cu. f t . pressure vessel with coolingjacket; &in. diameter, 3-ft. filter thimble mounted inside, covered with 60-mesh stainless steel screen. As hot gases leaving reactor cooled, most solid boron hydrides in gas stream removed by filter screen; bulk density of powder about 6 lb./cu. ft., 50 to 55 lb. of solid boron hydrides accumulated before emptying. Cooling water in filter jacket no lower than 35' C. ; lower temperatures condensed liquid product, caused formation of impervious mud on filter screen. Secondary Filter: Gas stream leaving primary filter filtered by cartridge of coarse steel wood in 2-cu. ft. vessel to remove cause of mechanical trouble during compression. Condenser: Gas mixture leaving secondarv filter contained maximum of 4y0 (vol.) condensable material (tetraborane, pentaborane, dihydropentaborane) ; balance diborane and hydrogen to be recycled. Large percentages of noncondensables necessitated relalively large heat transfer area (24 sq. ft.), long residence time in condensing unit. Unit was single pass, shell and tube, Type 304 stainless steel, double tube sheet construction to prevent mixing process gas with Freon-22. Crude product receiver connected to condenser bottom, also stainless steel, 2.3 gal. Condenser and receiver cooled to about -45' C. by Freon-22. Compressor: Gas stream leaving condenser (binary mixture of diborane and hydrogen) compressed from 50 p.s.i. up to 120 p.s.i. by small, two-cylinder, single-staqe reciprocating compressor. Similar compressor in parallel provided high circulation rates for initial mixing of charqe. Gas Storage: Two 15-cu. f t . steel tanks in series provided capacity within circulating system; desiqned for 150 p.s.i. working pressure; remainder of plant designed for 300 p.s.i. Preheater: For feed gas, 10-ft. length of 0.5-in. steel pipe, jacketed with hot oil at 175" C.; straight run of pipe on gas side for mechanical cleaning when required. Four separate preheaters accommodated within same oil jacket. Purification Condenser: Pentaborane purified by batch thermal soaking, degassing, simple distillation. Crude pentaborane container maintained at 65 C. by steam heated water bath; vapor from crude tank passed to 13-sq. ft. vertical shell and tube condenser cooled by 10" C. circulating water; condensed material returned to crude container through sight gage. Uncondensed gases vented to maintain system pressure at 800 mm. of mercury. Solids Removal: Back flushing with kerosine removed solids from primary filter; 1/,-hp. agitator mounted through tank top assisted formation of slurry; horizontal plate filter accumulated solid hydrides, 38 sq. ft. filtering area, 6.3 cu. ft. cake space ; kerosine recycled. PRODUCT STORAGE
Pentaborane of 99.9% purity is stable and can be stored for extended periods at room temperature. The crude mixture, however, contains unstable components which react to form higher hydrides and hydrogen when stored for any length of time. This tendency is dependent upon the quantity of im' F. purities present, Thus storage of crude pentaborane at 0 was desirable, and regular surveillance of storage container pressures was maintained. Containers: Original design approved by Interstate Commerce Commission for shipment by common carrier, holds 10 lb. of pentaborane; 3-gal, total volume with 1-gal. vapor space. Designed for 1000 p.s.i. at 600' F . ; hydraulic testing at 2000 p.s.i. Special flanged headings prevented casual con-
Diborane Pilot Plant
Pentaborane Pilot Plant
state capacity 14,000 B.t.u./hr. a t -85" C. with start-up load about three times steady stat? capacity. Nitrogen: Dry nitrogen under pressure (after vaporizing from liquid state) for purging, pressure control, inert gas blanketing where moisture or oxygen could cause process difficulties; continuous surveillance for traces of moisture and oxygen in nitrogen. Venting: Vents from process vessels connected to common manifold leading to vent scrubber-water descending 10 ft. through bed of 1-in. Raschig rings in 6-in. column hydrolyzed active boron compounds; remaining gases vented to atmosphere. Scrubber water recycled with 20y0 fresh make-up.
nections ; 0.5-in. bronze 3000-lb. globe valves adapted for these connections. Cover protected valves and connections during shipment.
ENGINEERING PROBLEMS
Extreme reactivity, moisture and oxygen sensitivity, and toxicity are three characteristics of boron hydrides which created engineering problems (3). Reactivity: Self-reaction has already been described. The other type of reactivity is initiation of Reaction 3. Variations in handling technique, brand changes, and particle size changes caused differences in amount and quality of lithium hydride surface available for reaction with diborane and boron trifluoride-etherate. Erratic starting performance occurred if initial boron trifluoride-etherate was added too rapidly. Satisfactory starting technique was to spike lithium hydride slurry with diborane, then commence boron trifluoride-etherate addition at low rate, increasing to normal rate over 5-min. period. Reaction heat release was convenient guide to proper startup. Incremental change in boron trifluoride-etherate rate caused proportional change in heat evolution. Then heat evolution was independent of etherate rate ; etherate accumulated in reactor resulting in reaction accelerating uncontrollably until excess boron trifluoride-etherate ivas used up. Oversized ether reflux condenser helped contain these surges during learning period. Hydrolysis: Diborane-water reaction helpful in vent system scrubber where diborane hydrolyzed completely ; hydride hydrolysis troublesome when opening filter vessels. Actual percentage of active hydrogen in filter cake small (Table 111), but reaction with atmospheric moisture generated enough heat to ignite ether-air mixtures. Both lithium hydride and boron trifluoride-etherate rapidly hydrolyzed by atmospheric water vapor; significant contact must be avoided. Dry nitrogen used. Reactivity with Oxygen: Measures to control extreme reactivity of diborane with oxygen included sturdy equipment designed for high pressures relative to operating pressures, allwelded construction to minimize leaks, closed vent system with rupture disks ahead of relief valves, explosionproof electrical equipment, electrically conducting flooring and drive belts, nonsparking tools at critical points. High-level ventilation provided 20 air changes per hour with no recycle, prevented accumulation of vapors sufficient to exceed explosive limits of diborane ( 7 8 , 79, 22, 36); combustible gas concentration continuously monitered a t potential trouble points. T o control effects of unexpected diborane-oxygen reaction, diborane storage was isolated from process building. Heavy reinforced concrete blast walls between process area and rest of building to control missiles and shock waves; explosion venting sash and light Transite panels on side walls and roof. Operation conducted from control room protected from process equipment. Continuous surveillance of process variables made by control and measuring instruments; audible monitoring system from contact microphones placed on important pieces of equipment. Instrument impulse lines had flow check devices to protect control room in case of instrument failure. Toxicity: Toxic properties of diborane and other boron hydrides unknown when program started. When highly toxic nature became evident, samples provided for studies of toxic properties ( 7 3 , 23, 33). Principal effect is pulmonary edema (74) ; recommended threshold limit value for diborane 0.1 p.p.m., ranking with stibene and ozone in toxicity (7). hTo exposures of personnel to toxic quantities of diborane during day-to-day operation. High ventilation level and lack of equipment leaks effective in maintaining working environment at below-toxic concentrations. Personnel exposures occurred during maintenance on process equipment. All purpose gas masks provided adequate protection.
UTILITIES
Pentaborane plant located in same process building as the diborane plant. Operation required hot water, cold water, dry compressed air, dry oxygen-free nitrogen, and vent system, all available from adjacent diborane installation. Two hot oil systems, Freon-22 system, and separate recirculating cooling water system also required. Oil heated electrically by immersion units in high velocity oil; low speed gear pumps circulated 10 gal./min. in each system. Water-cooled heat exchangers provided quick cooling at end of run. Vacuum System: Used for scavenging process equipment in preparing for start-up or maintenance work; pump capacity 9 cu. ft./min. at 1 mm. of mercury. Oil circulated through pump then discharged to steam-heated disengaging tank where condensable materials volatilized and vented. ENGINEERING PROBLEMS
Reactivity: Three special problems arose because of the extreme reactivity of pentaborane. The unstable self-reaction character of pentaborane mixtures has been described. Another difficulty relates to solid hydrides which accumulate in primary solids filter as pyrolysis products. Initially, solids from primary filter were slurried with carbon tetrachloride ; decaborane is soluble in carbon tetrachloride and could be separated from other solids in emptying operation. Higher hydrides removed on horizontal plate filter, and reasonably pure decaborane obtained by evaporating carbon tetrachloride. 1 his practice discontinued after rapid and extensive energy release observed during addition of carbon tetrachloride to primary solids filter. Concentrated solutions of decaborane in carbon tetrachloride since found to be shock-sensitive explosive (7). Kerosine, with no reactive tendencies toward solid hydrides, substituted as slurrying agent. Removal of solid hydrides from horizontal plate filter and their disposal also problems. Decaborane is stable in dry air, but mixtures of solids unpredictable especially when lower hydrides may be present on surfaces to react with atmospheric moisture. Occasionally enough heat generated to ignite solids. Unloading operations conducted with filter cake wet with slurrying agent. Rack of filter plates placed in closed carrier of light gage steel for removal to outdoor cleaning site; carrier purged with nitrogen. Solid waste buried in disposal pit. Reactivity with Oxygen: Measures to prevent reaction of pentaborane or solid hydrides with oxygen were the same as provided in diborane plant. High-level ventilation augmented by exhaust system providing spot ventilation at any point with flexible hoses. Lower explosive limit of pentaborane and oxygen (20) and explosive reaction of decaborane and oxygen ( 3 7 ) have been investigated. T o control possible missiles or shock waves, plant was located between heavy reinforced concrete blast walls complemented by explosion venting sash and light Transite panels for sidewalls and roof. Plant operation controlled from remote location except for gas charging, solids slurry, and pentaborane purification. Instrument impulse lines provided with flow check valves to prevent instrument failure from releasing process gas in control room. Toxicity. Pentaborane and decaborane both highly toxic materials attacking central nervous system (33, 34). Decaborane recommended threshold limit 0.05 p.p.m., ranks with arsine and phosphine in toxicity ( 7 ) . Pentaborane believed even more toxic (33). Clinical observations of individuals exposed to boron hydrides reported (23). Toxic levels of boron hydrides not present during normal operation because of high ventilation rate and leak tightness of heavy equipment; equipment maintenance and clean-up operations did result in toxic exposures. All-purpose gas masks and rubber gloves adequate protection with frequent canister changes ; improvements in canister materials reported ( 7 5 ) . Air line respirators along with clear plastic hoods and suits used in difficult situations. Detection of microgram quantities of airborne boron hydrides (70) adopted as guide for need of respiratory protection. VOL. 52, NO. 2
FEBRUARY 1960
109
Pentaborane
Table IV.
Pentaborane has been studied as a possible high energy fuel to help fullfil1 the greater heat release capability of thrust type heat engines. For some combinations of flight vehicle parameters denser pentaborane, which is a liquid at S.T.P., has advantages over diborane, which is a gas. Pentaborane can be formed from diborane or from the higher hydrides of boron (6). Pyrolysis of diborane yields a mixture of pentaborane and other hydrides, some of which can be further reacted to form pentaborane (29). Direct synthesis of pentaborane from simple compounds such as boron trichloride and hydrogen occurs only as a very minor by-product of diborane formation (26, 29). This report describes the results of a feasibility demonstration of pentaborane started in 1947. The pyrolysis of diborane has been studied by McCarty and DiGeorgio (76); Bragg, McCarty, and Norton ( 4 ) ; and Clarke and Pease (8). Reactions 6, 7, and 8 summarize their work very briefly. 5 B z H 6 k + 2B6H9
+ 6Hz ( A ) 5B2H6
(6)
Physical Properties of Boron Hydrides Boiling Point at
Compound Formula Borine BHa" Diborane BlHB Tetraborane &Hio Pentaborane BsHo Dihydropentaborane B5HII Decaborane BioHir Never isolated for measurement.
1 atm.,
Significant amounts of tetraborane and dihydropentaborane (Table IV) are formed as intermediates in this system. Reactions 1 and 2 account for 70 to 85% of the reaction product. Equation 3 indicates that most of the residual product is high molecular weight boron hydride of uncertain composition. Pentaborane Process
Approximately 10 pounds of pure diborane was charged into the 45 cubic foot pressure system (see flow sheet). Hydrogen was added to give a 1 to 1 volumetric hydrogen to diborane ratio.
225-290
O
...
C.
-92.5 18 58 63 213
Melting Point, ' C.
Density, Gram/Cc.
-165.5 120 46.6 123 99.7
0.447 (- 112' C . ) 0 . 5 6 ( - 3 5 ' C.) 0.63 (16' C.)
...
...
-
...
0.94 (25' C.)
This charge was mixed by circulating through the system without heating until analysis showed it to be homogeneous. Then the charge was passed through a vertical tubular reactor (see diagram) maintained between 240 ' and 290' C. by hot oil circulating through a jacket. Residence time in the reactor was controlled at 3 seconds. Reactor feed gas was preheated to 170' C. at 50 p.s.i. Reactor exit gas was cooled to 35" C., and solids were filtered out simultaneously in a 10 cubic foot filter. Liquid products were condensed at -45' C. and 50 p.s.i. in a vertical tubular condenser jacketed with Freon22. Crude pentaborane condensate accumulated in a receiver maintained at -40' C. Exit gas from the condenser
2B5H9+6H2
11"'
IN B @UT
PNEUMATIC CYLINDER
REACTOR P R E S S U R E RECORDER-CONTROLLER
VENT
-8
PURIFICATION PRESSURE RECORDERSYSTEM
-~
CONTROLLER
HEATING F INLET
PRI
E
PURIFICATION BATCH
STILL
Flow scheme for pentaborane production shows equipment described in detail on page 108
1 10
INDUSTRIAL AND ENGINEERING CHEMISTRY
B O R A N E PILOT PLANTS 100,
Typical relationships of major variables during pentaborane production are shown here
was compressed and returned to gas storage for recirculation through the reactor. During a run the reactor jacket temperature was progressively increased from 240" to 290" C. to maintain optimum reaction rate. As the diborane was depleted, pressure in the gas storage tanks increased from the initial 7 5 p.s.i. to about 100 p.s.i. because of the formation of by-product hydrogen. The batch charge was circulated through the reactor until a t least 80% of the original diborane had been converted. The batch, which initially contained 50% (volume) diborane, thus contained less than 10% at the end of the run. Further
Table V. Pentaborane Production Typical Reaction Conditions and Product Distribution Diborane charge, lb. Initial feed gas composition, % BZH6 Ha Final feed gas composition, yo BzHs Ha Reactor pressure, p.s.i. Reactor temp., O C. Initial Final Reactor residence time, sec. Product distribution, wt. yo of charge Tetraborane Pentaborane Dihydropentaborane Solid Hydrides Unreacted B9Hs
10-11
operation was not attractive because pentaborane was produced at progressively slower rates as hydrogen concentration increased. Progress of the run was monitored by continuous thermal conductivity analyses of the reactor feed gas and the condenser exit gas. The crude pentaborane product was purified in a separate step by a batch distillation apparatus. The reactor tube would become plugged with solid boron hydride products in a relatively short time if these were not removed periodically. Cleaning at suitable intervals (15 to 30 minutes) was done by mechanically scraping the solids from the reactor walls. At the start of a typical run (Table V) the system pressure was 67.5 p s i . The diborane-hydrogen charge was circulated by the process compressor for 20 minutes to mix the gases. Gas circulation was stopped while the hot oil systems for preheater and reactor were brought to temperatures of 175 O and 240" C., respectively. Circulation was started again with reactor pressure controlled a t 50 p.s.i. Reactor feed rate was set for a residence time of 3 seconds and periodic adjustments were made to compensate for the effect of changing gas composition on flowmeter calibration. Reactor temperature was increased during the run to oppose the decrease in conversion caused by hydrogen accumulation in the system. The run was concluded by stopping gas circula-
Table VI.
Pentaborane Purification Material Balance Wt. 7 0
Crude pentaborane charge BzHa B4HlO BsH9 BsHu
0.8
5.2 92.2 1.8
100.04 Pure pentaborane product BKHS (99.9%) Volatile residue Solid residue Vent loss a
68.8 2.0 19.2 10.0 100.05
50 50 8 I
240 290 3 ~
~~
.
A ~
2.4 41.6 1.8 36.9 17.3
I
c
c 5
I0
15
2 0
2 s 3 0 TIME, HFS
l
3 5
0
40
tion and cooling preheater and reactor. Final system pressure after equalizing was 86 p s i . Spent gas was vented, retaining in the system the correct amount of hydrogen for the succeeding run. Crude pentaborane was transferred to the purification step and distilled. As the feed gas is depleted in diborane, the conversion-per-pass falls off according to the prediction (8) of Equation 9
The steady increase in reactor temperature is not sufficient to offset the decreasing conversion. Clearly there is no practical benefit to continuing the pyrolysis beyond 8 hours when 41 .by0 of the diborane charge is present in the crude product as pentaborane. The solid hydrides are principally decaborane; however, the exact amount appears to vary with the method of separation. From the material balance, the over-all stoichiometric effect of the pyrolysis is estimated to be :
BzHs + 2 BHi.6
100% = 8.11 lb.
92 50
Solids formation during purification of crude pentaborane increased as ratio of diborane and pentaborane in the charge increased
1 4 5
Purification of crude pentaborane was improved by refluxing
+ 1.4 Hz
(10)
Calculation of conversion-per-pass was based on Equation 10. When reaction temperature is held constant a t 240" C. during a run, the conversion-per-pass had a high value of 3170 and decreased to an asymptotic value of about 5% because of the increasing hydrogen concentration. By using a program of regular temperature increases, the conversion-per-pass could be maintained a t about 23% as the end of the run approached. This gain reduced the processing time to one half the former value. Long recycling a t low conversions resulted in low pentaborane yields because recycled uncondensed pentaborane VOL. 52, NO. 2
FEBRUARY 1960
111
For a given cornposition, cylinder pressure increased when pentaborane was stored at room temperature
(2) Barkelew, C. H., Valentine, J. L., Hurd, C. O., Trans. Am. Inst. Chem. Engrs. 43, 25 (1947). (3) Barry, L. A., Chem. Eng. Progr. 54, 152 (1958). (4) Bragg, J. K., McCarty, L. V., Norton, F. J., J . Am. Chem. Soc. 73, 2134 (1951). (5) Brown, H. C., Schlesinger, H. I., Sheft, O., Ritter, D. M., Ibid., 75, 192 (1953). (6) Burg, A. B., Schlesinger, H. I., Ibid., 55, 4009 (1933). ( 7 ) Callery Chemical Co., Pittsburgh, Pa., Tech. Bull. C-070 (December 1957). (8) Clarke, R. P., Pease, R. N., J . Am. Chem. Soc. 73, 2132 (1951). (9) Elliott, J. R., Boldebuck, E. M., Roedel, G. F., Ibid., 74, 5047 (1952). (10) Etherington, T. L., McCarty, L. V., Arch. Ind. Hyg. Occupational Med. 5 , 447 ( 1952). (11) Henneberry, H. M., Natl. Advisory
Comm. Aeronaut., Research Memo. E51L21 (Feb. 25, 1952). (12) Hurd, D. T., J . Am. Chem. Soc. 71.
TIME, HRS
tends to be converted into solid hydrides. Moderate reaction conditions form higher percentages of tetraborane and dihydropentaborane. High reaction temperatures convert dihydropentaborane to pentaborane, and high hydrogen concentrations are desirable to dispose of unreacted dihydropentaborane and tetraborane by reconversion to diborane. As it was not desirable from a production standpoint to use more than 50% hydrogen in the batch charge, optimum initial hydrogen concentrations were not realized in this work. Also, at this low hydrogen concentration it was not desirable to use excessively high temperatures, as solid product formation increases greatly under these conditions. The over-all conversion reaction was exothermic. This was obvious when the reaction rate became uncontrollable during several runs, probably because of insufficient heat transfer from the gas stream to the reactor wall. The resulting runaway condition could be stopped by decreasing the feed rate, thereby limiting the amount of reaction possible per unit of reactor volume. Conditions for this uncontrolled reaction were most favorable at the start of a run when the feed was rich in diborane. Pentaborane purification was accomplished by using a combination reactor and distiller. Liquid impurities charged to the reboiler, tetraborane, dihydropentaborane, and diborane react to form either solids or volatile products. Refluxing crude pentaborane at the boiling point for 4 hours completed this rearrangement. Pentaborane was then distilled overhead in 99.9% purity (Table VI). Diborane, hydrogen, possibly some tetraborane, and traces of dihydro-
1 12
pentaborane wert removed as uncondensed vent gas saturated with pentaborane at the reflux condenser temperature of I O o C. This pentaborane loss in the vent gas constitutes one of the two main sources of loss in purification. The other loss occurred through the reaction of pentaborane to form some solid products during the purification process. The results suggest that any or all of the other hydrides may promote the reaction of pentaborane to form solids. Pure pentaborane can be refluxed indefinitely without chemical loss, which further supports the contention that impurities in the crude pentaborane catalyze a decomposition of pentaborane to solid products during the purification. Because these losses are considerably greater than those found working in glass equipment, it is suggested that steel surfaces in the large-scale apparatus have a significant catalytic effect. The over-all conversion of diborane to 99.9% pure pentaborane was 38Yo (weight). The operation of the plant was controlled and reproducible. Acknowledgment The advice and assistance received from many members of the General Electric Laboratory is gratefully acknowledged. L. V. McCarty supplied storage stability data and many analyses. Operating assistance was provided by R. L. Richard and members of the G. E. Chemical and Metallurgical Training Program. Literature Cited (1) American Conferenceof Governmental-
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Industrial Hygienists, Committee on Threshold Limits (Allen Coleman, secy., Bureau of Industrial Hygene, State Dept. of Health, Hartford, Conn.), “Threshold Limit Values for 1958.”
20 (1949). (13) Krackow, E. H., Arch. Ind. Hyg. Occupational Med. 8, 335 (1953). (14) Kunkel, A. M., Martha, E. H., Oikemus, A. H., Stabile, D. E. Saunders, J. P., ‘wills, J. H., A . M . A . Arch. Ind, Health 13, 346 (1956). (15) Long, J. E., Levinskas, G. J., Hill, W. H., Svirbely, J. L., A.M.A. Arch. Ind. Health 16, 393 (1957). (16) McCarty, L. V., DiGeorgio, P. A., J. Am. Chem. Soc. 73, 3188 (1951). (17) Mikheeva, V. I., Fedneva, E. M., Bull Acad. Sci. U.S.S.R., Diu. Chem. Sci. No. 8, 925 (1956). (18) Poling, E. L., Simons, H. P., IND. ENG.CHEM. 50, 1695 (1958). (19) Price, F. P., J . Am. Chem. Soc. 72, 5631 (1950). (20) Zbid., 73, 2141 (1951). (21) Prossen, E. J., Johnson, W. H., Pergiel, F. Y . , J . Research Natl. Bur. Standards 61, 247 (1958). (22) Roth, W., Bauer, W. H., J . Phys. Chem. 60, 639 (1956). (23) Rozendaal, H. M., Arch. Ind. Hyg. Occupational Med. 4, 257 (1951). (24) Schlesinger, H. I., Brown, H. C . (to U. S. Atomic Energy Comm.) U. S. Patent 2,543,511 (Feb. 27, 1951). (25) Ibid., 2,545,633 (March 20, 1951). (26) Schlesinger, H. I., Brown, H. C., Abraham, B., Davidson, N., Finholt, A. E., Lad, R. A., Knight, J., Schwartz, A . M., J . Am. Chem. Soc. 75, 191 (1953). (27) Schlesinger, H. I., Brown, H. C., Gilbreath, J. R., Katz, J. J., Ibid., 75, 195 (1953). (28) Schlesinger, H. I., Brown, H. C. Hoekstra, H. R., Rapp, L. R., Ibid., 75, 199 (1953). (29) Schlesinger, H. I., Burg, A. B., 16id., 53, 4321 (1931). (30) Shapiro, I., Weiss, H. G., Schmich, M., Skolnik, S.,and Smith, G. B. L., Ibid., 74, 901 (1952). (31) Snyder, A. D., Jr., Uniu. MicroJilms ( A p Arbor, Mich.), L. C. Card No. Mic. 58-2547; Dissertation Abstr. 19, 239 (1958). (32) Sundermeyer, W., Glemser, O., Angew. Chem. 70, 625 (1958). (33) Svirbely, J. L., Arch. Ind. Hyg. Occupational Med. 10, 298 (1953). (34) Ibid., 10, 305 (1953). (35) Van Brunt, C., Renscheid, E. J., Gen. Elec. Reo. 69, 128 (March 1936). (36) Whatley, N. T., Pease, R. N., J. Am. Chem. SOC.76, 1997 (1954).
RECEIVED for review February 20, 1959 ACCEPTEDSeptember 21, 1959 Work done under contract No. DA-30115-ORD-23.
