A Small-Scale, High Pressure Recycle Gas System - Industrial

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W. M. LANGDON and E. G. FOCHTMAN Armour Research Foundation, Illinois Institute of Technology, Technology Center, Chicago 16,111.

A Small-Scale, High Pressure Recycle Gas System This system can be used safely with dangerous materials at high pressures and temperatures

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PRESSURE processes have received extensive treatment by various investigators. However, studies with small scale equipment (2-4) almost invariably have been concerned with batch or single pass flow systems. The chemical factors have received detailed attention, while the mechanical features are considered to be of secondary importance. These mechanical features are usually of controlling importance in the operation pilot plants. The small-scale, high-pressure, recycle system described here, along with methods of installing and operating, presents an extremely reliable and flexible setup which is suitable for recycling gas streams from G to 1000 s.c.f.h. at pressures up to several thousand pounds per square inch. While numerous compressors are available for this purpose (7, 6) the control which must be built in is a major problem. Industrial regulation which is designed for specific operations, is usually not applicable, because it is either too costly or lacks the flexibility necessary for pilot plant studies. An atmospheric pressure off-gas scrubbing system is also described. This system employs simple liquid legs for control purposes and has many operational advantages over the more sophisticated control techniques.

General

engineering practice. However, more time and effort is usually expended on this point than can be justified. The present system employs piping disconnects, which are phased to eliminate errors, for switching from on-stream to regeneration. Valves are used only for throttling. Positive shut off is obtained by removing a section of pipe between valves and plugging the system ends. The important valves and meters are placed in bypass loops with disconnects so that they can be tested, and changed if necessary, during a run. Of the numerous types of connectors available it has been found that commercial O-ring unions (Clayton Mark, Evanston, Ill.) are the closest approach to a perfect and rapid disconnect for moderate temperatures. None of these several thousand unions leaks in the system, even after innumerable changes. The present system operates on sweet gas and uses standard equipment available in most laboratories. Wherever possible, black iron screwed piping, rated for 3500 p.s.i. cold working pressure is used. The uncoded equipment is tested hydraulically at four times the maximum operating pressure. Sour and corrosive gases can be handled similarly by using equipment resistant to the gases. (Corblin, American Instrument Go., Inc., Silver Spring, Md. Pressure Products Industries, Inc., Hatboro, Pa.)

pressor (1) capable of delivering 12 s.c.f.m. at 3500 p.s.i. is used. I n normal operation the upper pressure limit is set approximately by means of the manual bypass valve ( 2 ) . This valve also allows the entire system to be vented slowly and safely, when the main power switch is pulled in an emergency shutdown. Safety relief valves on each stage of the compressor prevent overpressure of the first and second stage of the compressor, An upper pressure limit of 1500 p.s.i. is controlled by the adjustable backpressure throttling regulator (6). The two pressure regulators in series (4, 5) maintain the suction pressure above 4 inches of water under all conditions of gas flow by bleeding gas from the high-pressure side to the suction side of the compressor. A maximum suction pressure of 12 inches of water is maintained by a simple water leg (16) to protect the overflow legs (*7 ft.) in the scrubbing system. The compressor is designed for SUCtion pressures close to atmospheric and can operate with a pressure of 15 p.s.i.g. on the suction side. The valve adjustment nuts for the first stage were supplied with caps to eliminate the slight leak at this point.

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The requirements of process equipment for pilot studies usually prevents or handicaps the use of explosionproof equipment in many locations ( 5 ) . Consequently, rather than utilize special equipment, a complete air change every 3 minutes is employed. This is sufficient to prevent explosions in all cases for the pilot plant used in these studies. Heated reactors, which could give rise to fire if a leak develops, are located in a tower where there is little occasion to enter during a run. If there is necessity to enter the tower, another exhaust fan is available which changes the air every half minute. The entire system, except for ventilating fan, can be shut down and vented automatically by pulling the main power switch. The construction of a leak-tight system in small scale operations is another primary factor which is subject to good

Gas Compressor System

This system is shown in the figure below. A three-stage water-cooled com-

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This high

pressure recycle gas system is flexible, reliable, stable, and uses standard controls readily available in most laboratories

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COMPRESSOR

VOL. 52, NO. 6

T JUNE 1960

479

Generally there is no technical advantage in maintaining the recycle gas at a high pressure during pilot plant work. However, if desired, the recycle gas pressure could be maintained at any pressure up to 3500 p.s.i. by introducing the recycle to the third stage suction. The first two stages would float on an independent loop. Because the discharge from the second stage operating with suction pressure close to atmospheric is normally limited to 495 p.s.i.g., the recycle pressure is limited by the leakage from the third stage to the low pressure loop. In some cases it is necessary to limit the degree of compression to prevent condensation of contaminants in the recycle gas. In these cases second- and third-stage bypass valves (3), in conjunction with the manual bypass valve, limit compression to 10 p.s.i. and up. The stability of the system enables continuous runs of over a month by pressure control through the manual bypass valve alone. The system is provided with low(12) and high-pressure (15) ballast tanks. A low-pressure tank of 8 cubic feet eliminates pressure pulsations between the compressor and the suction system. As the amount of gas in the system is normally held to a minimum, the oil-water separator (13) alone can serve as the high-pressure ballast. This separator is a 6-inch, Schedule 160 pipe, 24 inches high with l/Z-inch Berl saddles. This removes all but traces of the lubrication oil and the condensed water from the compressed gas. The elimination of trace quantities of oil and water would require a carbon absorber. Recycle-Gas Flow Control

A recycle regulator (7) adjusts the pressure of the gas upstream to a valve which is between the compressor and the reactor. The flow rate is indicated by a high-pressure rotameter (9) and is adjusted approximately by means of a valve. The actual flow rate is determined by timing a dry meter (8) placed on the suction side of the compressor.

(Dry meters rated for working pressures of 1 to 50 p.s.i. can be used.) The desired flow rate is set accurately by changing the pressure setting of the recycle regulator rather than the valve. By this means, the flow rate is essentially independent of pressure fluctuations in any part of the system and shuts off automatically in case of pressure build-up in the reactor. The system is also provided with rupture disk and 0ring check valves to protect the compressor in case of explosions. Off-Gas Scrubbing System

The off-gas scrubbing system is illustrated in the figure below. I t consists of three scrubbers, acid, base, and wash oil, which serve to sweeten the gas and remove the light ends from the recycle gas stream. The essential requirement of this facility is continuous, trouble-free performance without attention. The acid and base towers are operated batchwise with weekly make-up. The wash oil is continuously regenerated by steam stripping and provides a continuous material balance on the product stream. Experience has shown that the only reliable control in small scale operations is effected by simple overflow legs which are self compensating. Consequently the towers are mounted 14 feet above ground level and the sump tanks are 7 feet above ground level. This arrangement furnishes a 7-foot liquid head which, in relationship to the 4 to 12 inches suction pressure, provides ample safety for pressure surges. All tanks and piping rated for 50 p.s.i., are vented internally, and on rare occasions when liquid seals are blown they immediately re-establish themselves. The wash oil scrubber and steam stripper are operated with parallel feed streams in the ratio of 1 to 2. This is in contrast to industrial practice and requires about five times more steam. However, parallel operation affords the simplest and most reliable performance. The overhead and bottom streams from the stripper are provided

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with automatic decanters to eliminate condensed steam. These decanters operate at atmospheric pressure. Consequently, the bottom oil overflow from the ketcle is located 3 feet below the static level to compensate for pressure changes at the bottom of this tower. The bottom water leg is made 14 feet, also to compensate for kettle pressure, because the interface movement is roughly 10 times the pressure change. The stripped oil, recycled to the sump, is cooled to room temperature and the residual water drained from the sludge leg in the sump at 8-hour intervals. Vent and Make-up Gas Excess gas is vented through a simple water leg (16, figure on page 479) at 1 2 inches of water and is recorded on a dry meter (8). This water leg has enough capacity to take maximum discharge in case of emergency shutdown. Alternatively, the manual bypass (2) can be closed sufficiently so that suction pressure remains at 4 inches of water and gas accumulates in the high-pressure ballast tank (15). The accumulated gas can be removed periodically for the material balance, This alternative procedure is useful for small flows which can be measured only by visual observation of the test dial on the dry meter. Make-up gas is admitted to the suction line of the compressor and automatically displaces the equivalent volume of gas from the system and the make gas from the reaction. The make-up gas rate is controlled by a pressure drop across a long capillary (11) which, in turn, is controlled by a pressure regulator on the line leading from the gas supply tanks. When 5 s.c.f.h. of hydrogen are added, a 300-foot capillary of I / I s inch inside diameter tubing and a pressure drop of 20 p.s.i. are used. The rate is stable and independent of minor fluctuations in suction pressure. A major problem in operations of this type is water condensation. While traps are placed wherever possible, condensation often occurs due to pressure pulsations. The most critical locations are in the dry meters. Therefore, these meters are placed in a bypass loop so that they can be disconnected, checked, and replaced or drained while the system is running. literature Cited E. W., “High Pressure Technology,” p. 132 ff., McGraw-Hill, New York, 1956. (2) Ipatieff, V., Monroe, G. S., Fischer, L. E., IND.ENG.CHEM.40, 2060 (1948). ( 3 ) Lobo, P. A., Sliepcevich, C. M., White, R. R., Ibid., 48,907 (1956). (4) Mavity, J. M., Pines, H., Wackker, R. C., Brooks, J. A., Zbid., 40, 2375 (1948). (5) Porter, R. L., Lobo, P. A., Sliepcevich, C. M., Zbid., 48, 841 (1956). ( 6 ) Staff Rept., Zbid., p. 827. (1) Comings,

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PRODJCT

Off-gas scrubbing system, used to sweeten the gas and to remove the light ends from the recycle stream 480

INDUSTRIAL

AND ENGINEERING CHEMISTRY

RECEIVED for review September 17, 1959 A C C E P T E D March 14, 1960