I
/ EC
Equipment
&
Design
HIGH PRESSURE HYDROGENby W. H. Campbell, Socony Mobil Oil Co., Inc.
I ODAY'S MODERN petroleum research
laboratory is a far cry from the laboratory of 20 or even 10 years ago. The search for new methods to increase the yield and quality of gasoline and light fuel from each barrel of crude petroleum has developed hydrocracking, reforming, hydrotreating, hydrodesulfurization, and many others. The catalytic cracking pilot unit of the late 1940's has given way to the hydrogénation unit of the 1960's. The change has created new problems for the petroleum research scientist. Operating pressures have gone from 1 or 2 atm. to several hundred, requiring the development of high pressure techniques in the design of pilot equipment. The processes also require a hydrogen atmosphere which complicates the design even further. Large quantities of gas are needed at pressures which create supply problems.
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High pressure design techniques have been discussed and presented at great length at such meetings as the American Chemical Society's 1955 Symposium, "Processing Under Extreme Conditions." The design problems are therefore well under control. One of the other problems facing the petroleum research engineer is a dependable, trouble-free source of high pressure hydrogen. The most common installation for hydrogen gas supply is the multiple cylinder manifold at each pilot unit or bank of units. T h e cylinders are usually the 1A size which is standard with most gas supply houses. They contain approximately 200 standard cubic feet of gas at a pressure of 2000 to 2200 p.s.i. Efficient utilization of the gas in the cylinders can be solved in several ways. One way, and unfortunately the most common, requires that the cylinders be used in a game of
INDUSTRIAL AND ENGINEERING CHEMISTRY
"musical chairs" where the full bottles are first connected to the highest pressure units which operate a some pressure less than 2000 p.s.i., and when the hydrogen has been drawn off down to the unit operating pressure, they are moved to a manifold serving lower pressure equipment and replaced by full cylinders. This constant juggling is expensive and usually results in handling costs greater than the cost of the gas itself. Another solution, necessary where units operate above cylinder supply pressure, requires a gas compressor which can take the gas from the cylinder at a low pressure and recompress it to the unit supply pressure or higher. This allows full utilization of the gas in the bottles, but may result in a large number of small gas compressors, spotted throughout the laboratory. Both of these solutions have their disadvantages. The case for switch-
LEFT:
Schematic flow shows h y d r o g e n sion system
diagram compres-
CENTER:
Tube trailer a n d reserve storage bank a r e connected in p a r a l l e l
RIGHT:
Two compressors a r e connected in p a r a l l e l so that they m a y b e operated alternately o r simultaneously
Central
A Laboratory Utility
hydrogen
compression
system offers safe, reliable
supply
at lower cost per unit
ing cylinders is poor, because of the h a n d l i n g necessary, the space required for a large manifold installation, a n d the large n u m b e r of piping connections which must be m a n i p u lated in changing cylinders. T h e individual unit compressor is better, because the space for cylinder supply is smaller, a n d handling is generally reduced, but the m a i n t e n a n c e of these pesky little machines is usually a very big problem to the user. T h e Socony Mobil Oil Co.'s research laboratories at Paulsboro, N. J., have solved the problem of hydrogen supply by a method superior to the two previously mentioned. This method uses a large central gas compression system with plant-wide distribution mains. T h e system operates automatically at 3500 p.s.i. and requires only periodic inspection by plant utility operating engineers. T h e pressure is sufficiently high to serve nearly all of
the laboratory's research programs. H a n d l i n g of supply containers has been eliminated by this a u t o m a t i c installation. T h e investment was paid off in the first three years of operation and the system has operated continuously without a shutdown since mid-1958. System Description Hydrogen is delivered in tube trailers which arc connected to the supply system in parallel with a reserve storage bank. T h e hydrogen is reduced in pressure a n d fed to the compressors through an oxygen converter where trace quantities of oxygen arc converted to water. T w o compressors, connected in parallel, compress the gas to 3500 p.s.i. T h e gas then passes through a conditioning train consisting of an oil separator a n d filter, desiccant dryer, oil removal tower, and final filter.
volume
It is then fed to storage cylinders a n d subsequently to the buildings in the laboratory t h r o u g h a n u n d e r g r o u n d piping system. T h e control unit reduces the pressure to 50 p.s.i.g. a n d regulates the gas flow so t h a t the compressors are fed by the t u b e trailer until the trailer pressure drops to 50 p.s.i. T h e feed is automatically switched to flow from the reserve storage bank, and a red signal light shows t h a t the trailer supply is depleted a n d a full unit must be ordered. A full t u b e trailer, containing 33,000 standard cubic feet of h y d r o gen at 2400 p.s.i., is delivered a n d connected to the supply manifold by the gas s u p p l i e r s employee, a n d the empty trailer is removed. W h e n the full trailer is connected to the supply manifold, the control unit allows the reserve storage bank to lie a u t o m a t i cally replenished to a full charge of 30,000 cubic feet at 1500 p.s.i.
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EQUIPMENT AND DESIGN The compressors are Norwalk, 4stage oil lubricated, piston ma chines, with a rating of 1500 standard cubic feet per hour at 3500 p.s.i. They are driven by 30-h.p. explo ionproof electric motors fitted with V belt drives and a special clutch for starting under load. The compressors are connected in parallel so that one machine may operate by itself, or in periods of high load on the system, both machines may be operated simultaneously. The use of two compressors also insures that maintenance can be carried on without interrupting the laboratory supply. The hydrogen supplied to the dis tribution mains must be of high quality, with oxygen, water, oil, or other foreign matter held to a mini mum. Research experiments can easily be upset by small traces of oil or oxygen in the hydrogen supply, so conditioning equipment must be an integral part of the system. Oxygen is converted to water by a Puregas Equipment Co. oxygen re moval unit, installed in the low pressure compressor inlet line. This unit, which contains a palladium catalyst, reduces oxygen to 1 p.p.m. in the hydrogen stream. A Pittsburgh Lectrofilter removes entrained oil from the compressor discharge before the gas enters a drying tower system. The dryers are Kemp Manufacturing Co. adsorptive dryers using Mobil Sovabeads and are semiautomatic in operation. The drying towers are alternated each 8 hours and reacti vated by a small flow of hydrogen which purges the electrically heated bead bed. The gas leaving the towers has a dew point of —10° F. at 3500 p.s.i. or approximately 35 p.p.m. water. Oil absorber towers containing charcoal are located after the drying towers as an additional precaution against contamination by oil vapor. A final filter containing felt pad units is last in the conditioning train. Storage capacity for the distribu tion system is provided by 5 20-cubic foot cylinders manifolded to the compressor discharge main. These cylinders are ASME code vessels of swaged seamless carbon-moly steel construction. Piping on the low pressure suction side of the compressors is standard weight brass pipe with Walworth Co. 44 A
Silbraz fittings and valves. The use of silver soldered pipe joints reduces the number of screwed pipe threads to a minimum, making for a more leakproof system. The high pressure piping around the filter, dryer, and absorber condi tioning train on the compressor dis charge, is constructed of Ά Χ 8 /s inch Type 304 stainless-steel tubing and Autoclave Engineers l / 2 inch "speed" fittings and valves. The distribution mains which carry the high pressure hydrogen from the storage cylinders to the pilot plant and laboratory buildings are, for the most part, underground. The mains are 3/s- and V-i-inch Schedule 80 brass pipe with Wal worth Sibraz high pressure valves and fittings. The underground mains are wrapped with a layer of polyethylene tape and are encased in a concrete protective envelope, ap proximately 6 X 4 inches. The con crete is colored yellow by a dye in the mix and was installed to provide protection from damage or rupture during later excavations for other utility repairs. The mains are above ground at pilot plant buildings and are supported 10 to 12 feet above grade on the outside of the buildings. In a typical supply station at a pilot plant building, the main shut-
off valve is clearly marked and is located low for easy access. A pressure regulator reduces the supply to a pressure high enough to satisfy the highest pressure unit or other user in the building. A rupture disk, rated at approximately 125% of this pressure, is located as shown discharging to vent. Excess surge valves on both the high pressure and reduced pressure side of the regulator will close in the event either the rupture disk bursts or a leak occurs in the building piping causing a high flow in the main. Instrumentation and Automatic Features
The instrumentation which has been applied to the compression sys tem has been chosen so that the plant may run unattended with safety. Each compressor is controlled by a pressure-indicating controller equipped with high and low set point switches. The instruments are Minneapolis-Honeywell Brown Electr-O-Vane controllers with a range of 2000 to 4000 p.s.i. The use of a separate controller on each compressor allows a maximum of flexibility in programming the operation of the machines. Either compressor may be set to carry the VENT
RUPTURE DISK "*
TO PILOT UNITS PURGf VALVE
This is α typical supply station piping layout
INDUSTRIAL AND ENGINEERING CHEMISTRY
EXCESS SURGEVAIVE
EXCESS SURGE VAIVE
PRESSURE ' REGULATOR
MAIN SHUTOFF. VALVE
FROM DISTRIBUTION MAINS
EQUIPMENT AND DESIGN load on the system with the other held in reserve and operated only periodically. For example, the controller for compressor No. 1 may be set to start the machine when the storage cylinder pressure drops to 3300 p.s.i. and to stop when the pressure reaches 3450 p.s.i. Controller No. 2 is set to start machine No. 2, if the cylinder pressure drops to 3250 p.s.i., and to stop at 3450. As the pilot units in the plant use hydrogen, the pressure drops to 3300 and compressor No. 1 starts, compressing 1500 standard cubic feet per hour. If the rate of usage exceeds 1500 standard cubic feet per hour, the pressure will continue to drop till 3250 p.s.i. when No. 2 will start. Both will stay on until the system pressure is back up to 3450 p.s.i. In this example, machine No. 2 is the standby unit, and will only operate during high load periods. Safe operation has been achieved by the use of many overriding control devices which shut the compressors down or relieve the gas to vent if some mechanical failure leads to a hazardous condition. These include : • A separate controller, used as a high pressure limit control, overrides the Brown Electr-O-Vane controllers in the event their high set point switches fail to operate. This control will shut down the compressors before the system pressure reaches a point where one of the frangible disks or relief valves will relieve the overpressure to vent. • Low pressure cutout controls protect against the possibility of drawing air into the suction lines. A positive pressure of 1 p.s.i.g. must be available at the first stage suction or the control interlock opens, preventing the compressors from operating. • A manual reset solenoid valve is installed in the line from the supply control unit to the pressure-reducing valve on the compressor suction line. This valve is pressure-switch operated, and closes if the pressure leaving the control unit drops to 25 p.s.i., such as would be the case if a major rupture in the compressor suction lines occurred. The solenoid would prevent the escape of the supply storage hydrogen to the atmosphere. • Overpressure relief devices are located at various points in the system. Relief valves protect the compressor interstage piping and
also protect each vessel in the conditioning train. The low pressure supply piping and vessels are also protected by relief valves. A combination relief valve and rupture disk assembly protects the high pressure storage. The frangible disk, which has a burst pressure of 40C0 p.s.i., is the primary protection; and the relief valve, also set at 4000 p.s.i., is secondary. If the disk fails because of fatigue or corrosion at a pressure less than 4000 p.s.i., the relief valve will not open, preventing a complete loss of the storage contents to vent. If, however, the disk fails because of overpressure, the relief will relieve the excess gas to vent. The valve will close at some pressure less than 4000 p.s.i., and even though it may leak slightly, it will prevent a complete loss of hydrogen and the resulting laboratory-wide unit shutdown. Failure of the disk will be indicated by a pressure gage which is connected between the disk and relief valve. The combination of disk and valve was chosen for the storage system overpressure protection because it provides the leak-tight feature of the frangible disk and the reseating feature of the relief valve. Economic Advantages
Several important economic advantages helped convince Paulsboro Laboratory management that the central hydrogen compression system is superior to the other methods of supply. They are : • Lower cost of hydrogen per unit volume • Reduced handling charges • Reduction in pilot plant operation costs A reduction in the basic cost of the gas is the major factor in the economic picture. Hydrogen delivered in bulk quantities by tube trailer costs 45 cents per 100 cubic feet less than hydrogen delivered in standard 1A cylinders. At an average monthly rate of 200,000 standard cubic feet, the savings are $900 per month. More efficient utilization of container contents by removing all of the hydrogen delivered also results in marked savings. Gas suppliers generally will not give credit for gas remaining in returned cylinders. If the average pressure is 200 p.s.i.,
the user is paying $1.00 for 90 cents worth of hydrogen. The savings here, of course, will vary from user to user, but using 200 p.s.i. as an average, it will be $200 per month for the 200,000 cubic feet usage. A central system allows these savings to be realized by depressuring the tube trailer supply to 50 p.s.i. We are billed only on the yield from 2400 to 50 p.s.i. Handling costs are also greatly reduced. The method used in handling gas cylinders will, of course, vary with each plant, but some handling is necessary in any system and this costs money. At Paulsboro, we estimate that handling costs for in-plant transportation, loading and unloading, connections to manifolds, storage of empty bottles, etc., were approximately 50 cents per cylinder or 25 cents per 100 cubic feet. Again, assuming an average monthly consumption of 200,000 cubic feet, this would cost $500 per month. Demurrage, or the charge for keeping cylinders over 30 days, averages 4 cents per cylinder, per day. This charge must also be considered in any economic comparison. The reduction in pilot plant operational costs is more difficult to estimate, but nevertheless it is a matter for consideration. How much do booster compressors cost, including connecting piping? How much maintenance will result from their installation? How much will it cost if a valuable run is cut short because the booster compressor failed? Will we have operators, paid for technical skills, "horsing" cylinders into place in the middle of the night? A central hydrogen compression system can make high pressure hydrogen a laboratory utility. Are you paying too much for your hydrogen? Division of Industrial and ? Engineering Chemistry, Symposium on H i g h Pressure Research, 139th Meeting, A C S , St. Louis, Mo., M a r c h 1961.
Our authors like to hear from readers. If you have questions or comments, or both, send them via The Editor, l/EC, 1155 16th Street N.W., Washington 6, D.C. Letters will be forwarded and answered promptly. VOL. 53, NO. 7 ·
JULY 1961
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