Handling liquid Hydrogen Sulfide W. 0. BICE, FRED PRANGE,
AND
R. E. WElS
Philtex Experiment Sfution, Chemical Engineering Division, Phillips Pefroleum Co., Phillips, rex.
HE Phillips Petroleum Co. has been handling large volumes
T
of hydrogen sulfide since the erection of a large "sulfur compounds" plant in 1943 for the manufacture of tertiary alkyl mercaptans utilizing liquid hydrogen sulfide and olefins as feedstocks. The plant, shown schematically in Figure 1, is located a t Borger, Tex., where natural and refinery gases of high sulfur content provide an abundant supply of hydrogen sulfide (2, 3). From Girbotol treating units a water-saturated stream containing approximately 93% hydrogen sulfide, 6.5% carbon dioxide, and 0,5y0 hydrocarbons is passed through a bauxite drier which reduces the dew point of the stream to -20' F. or lower. The gaseous hydrogen sulfide is then pumped through a 3-inch carbon steel line at pressures of 20 to 40 pounds per square inch to the sulfur compounds plant, where liquefaction of the gas is effected by compression and cooling. The hydrogen sulfide from the Girbotol units, together with the recycle hydrogen sulfide from a high-pressure stripper, passes through a second bauxite drier and then enters the inlet scrubber of the compression system a t 20 pounds per square inch and at atmospheric temperature. The hydrogen sulfide passes from the inlet scrubber into the first stage of a two-stage compressor and is discharged through a cooler where the temperature is reduced from 150" F. to less than 100' F. The stream is then compressed by the second stage of the compressor and is discharged a t a pressure of 250 to 400 pounds per square inch and temperatures up to 250' F. through a mist extractor into the water-cooled condenser for liquefaction. The liquid hydrogen sulfide is stored in a 3600-gallon surge tank (Figure 2). The pressure in the surge tank is controlled by venting the noncondensed gases-methane, ethane, and carbon dioxide-to the flare and by maintaining the temperature in the tank below 85' F. The temperature is controlled by the rate a t which hydrogen sulfide is vaporized; the vapors may be vented to the flare or recirculated through the compression system and returned to the surge tank as a liquid. The composition of the liquid in the surge tank, as determined by mass spectrometer analysis, averages 90% hydrogen sulfide, 4% carbon dioxide, and 6% heavy material (hydrocarbons and sulfur chemicals). The vent gas contains 60 to 70% hydrogen sulfide, 20 to 34% carbon dioxide, and 6 to 10% light hydrocarbons. The liquid hydrogen sulfide is mixed with a tertiary olefin and passed through a catalyst bed at elevated temperature and sufficient pressure to maintain the mixture in the liquid phase. Follov'ing the synthesis step most of the unreacted hydrogen sulfide is removed for recycle in a high-pressure stripper; the remainder is removed in a low-pressure stripper and vented. The liquid from the strippers is further processed in two vacuum fractionation columns. The unreacted olefin is separated by the first
column for return to a recycle tank while the mercaptan is produced as overhead product in the second column.
A Sli on
-Type Plug Valve Is Being Tested tRe Mist Extractor Drain Line
The hydrogen sulfide is compressed by a standard gas enginedriven compressor, having two compression and two power cylinders. The compression cylinders are equipped with Ni-Resist linings, Inconel garter springs for the carbon steel metallic packing, and stainless steel valve assemblies fitted with Inconel springs. The life of the valve assemblies and metallic packing depends on proper lubrication. A medium-weight compressor oil (Phillips compressor oil No. 300) containing about 574 of a selected fatty oil has been found satisfactory for the lubrication of valve assemblies and packing. Conventional water-cooled tube and shell heat exchangers are used. The interstage cooler, which operates a t pressures up to 100 pounds per square inch, was originally fabricated with carbon steel shell and tubes. The steel tubes have been replaced with antimonial-admiralty metal, which has given better service on the water side. The condenser, which operates a t pressures up to 450 pounds per square inch, consists of a carbon-steel shell and 14gage seamless-steel tubes. The two scrubbers and the mist extractor are of conventional design and were fabricated of boiler plate. The inlet and interstage scrubbers are 3 X 10 foot vessels with '/S-inch thick walls and heads. The 3 X 6 foot mist extractor has 3/*-inch walls and 1-inch head. Each scrubber is equipped with a reflex-type liquid gage, a li'quid drain, a safety relief valve, and a Magnetrol controller that will stop the compressor in case the liquid level in the scrubber exceeds a safe operating height. The mist extractor is similarly equipped except that it does not have the Magnetrol controller. As operated, the mist extractor serves only as a surge in the line; any material that collects in the vessel is vaporized by means of a steam coil. The 5 X 24 foot liquid hydrogen sulfide surge tank is constructed of l'/g-inch fire-box quality steel, ASTM specification A-70, having a minimum tensile strength of 55,000 pounds per square inch. It was stress relieved but not x-rayed, static tested at 685 pounds per square inch, and hammer tested a t 570 pounds per square inch. It is equipped with a safety relief valve, a pressure controlled vent valve, a reflex-type liquid level gage, remotely controlled motor valves on both the inlet and outlet lines, and a Magnetrol-controlled alarm that sounds when the liquid level reaches a predetermined height. No allowance for corrosion was made in designing these vessels. The plant was originally constructed with Schedule 40 lap weld pipe for pressures up to 450 pounds per square inch. As an 2497
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4-STAGE S T E A M EVACTOR
( p y F q1 STORAGE TANK
Figure 1 ,
Schematic Flow Diagram of Sulfur Compounds Plant
added safety measure all lines in the hydrogen sulfide system operating a t pressures over 100 pounds per square inch have recently been replaced with Schedule 80 seamless steel pipe, although no deterioration was apparent in the old pipe, except at a few welded joints in the line between the high stage compressor and the mist extractor; this line is exposed to a pulsating flow of gaseous hydrogen sulfide a t 400 pounds per square inch and temperatures up to 250' F. Ih the mercaptan synthesis unit, piping operated a t pressures in excess of 400 pounds per square inch is made of Schedule 160 seamless steel pipe: The piping is of welded construction with a sufficient number of flanged joints to allow for inspection and equipment maintenance. The flanges are of raised-face type and are sealed with sheet aluminum- or asbestos-filled double shell steel gaskets. Several types of valves have been used successfully at various points throughout the plant. Both plug valves and gate valves have given good service in compression and liquefaction service and in the high-pressure synthesis portion of the plant. The majority of the valves in contact with liquid hydrogen sulfide are of the plug type with body and plug of carbon steel. Properly lubricated, these valves have given trouble-free performance; when not properly lubricated, the valves will not operate. Lack of proper lubrication has also caused embrittlement and failure of the thrust balls under the plugs. Although a number of different lubricants have been used in the plug valves, only one or two lubricants have proved suitable for hydrogen sulfide service. A number of lubricants recommended for this service by manufacturers have been tried; they may be divided roughly into three classes: water soluble; oil soluble; and oil insoluble and very
slightly water soluble. The latter type, which the manufacturer recommends for use with nearly any nonaqueous system, has proved satisfactory for both hydrocarbons and hydrogen sulfide services. The major trouble encountered with gate valves has been that of a suitable stem packing; aluminum-asbestos packing solved the problem. Neither the gate- nor the plug-type valve has been satisfactory for use in the drain line of the mist extractor. The high pressure drop (400 pounds per square inch) across the partially opened valve while draining a liquid which consists of hydrogen sulfide, mercaptans, hydroczrbons, and which also contains particles of rust and scale has resulted in rapid erosion and leakage. A slip-type plug valve that recently became available has been under test on the mist extractor drain line for about a year: this valve is trimmed with 316 stainless steel and has performed satisfactorily during the test period. A recent inspection of the valve showed it to be in good condition except for impressions on faces of the plug slips and on the seat; these impressions were evidently caused by closing the valve on pieces of rust or scale. The valve has been reinstalled for further testing. Safety relief valves installed in the plant were conventional cast steel valves trimmed with stainless steel and furnished with black steel springs. Embrittlement of the black steel springs as a result of exposure t o flare line gases containing hydrogen sulfide presented considerable operating difficulties during the initial period of operations. Such relief valve difficulties have been practically eliminated by the use of springs coated with aluminum and by the substitution of relief valves with outside springs. The use of outside spring relief valves has been limited because
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of mechanical difficulties with this type valve-e.g., misalignment of disk and seat can easily be brought about by an inadvertent blow. For satisfactory operation of either type of relief valve, ssmiannual cleaning has been necessary.
Safety Features in Plant include Cutoff to Stop Engine When Liquid Level in Scrubbers Becomes Too High Safety is of prime importance when handling hydrogen sulfide. Although extraordinary precautions are taken t o keep leaks a t a minimum, small amounts of hydrogen sulfide and low molecular weight mercaptans escape into the atmosphere and tend to collect in the plant area on still days. For this reason forced draft and natural ventilation features have been installed in the control room, compressor building, and other places where the operators work. A number of special safety features have also been incorporated in this plant. The compressors are equipped with evactors, which withdraw hydrogen sulfide that has leaked past the packing into the enclosed distance pieces and eject it into the flare line. The compressors are also equipped with a cutoff that will stop the engine under any of the following conditions: (I) The liquid level in the scrubbers becomes too high (Magnetrol controllers); (2) the compressor develops too great a speed; or (3) it is desired to stop the compressor from any one of three remote control stations located outside the plant area. These same stations contain controls that can close the motor valves on both ends of the surge tank in case of an emergency. All pressure gages operating a t pressures over 100 pounds per square inch are installed with an inverted steel ball check valve between the gage and the line. The check acts as an excess flow valve in case of a Bourdon tube failure. No failures have occurred since gages equipped with austenitic Type 316 stainless steel tubes were placed in service 4 years ago. Recording pressure control instruments that operate a t pressures of 400 pounds per square inch and over are separated from the hydrogen sulfide by Teflon or tantalum diaphragms. Fresh air masks, with 50 feet of attached hose for each mask, are located strategically throughout the plant area. Air is supplied to these masks from a compressed air system located upgrade a t a distance from the plant. A self-contained Chemox breathing apparatus is available for emergency conditions.
Worker Education AHords Protection during Plant Operation Continuous (24 hours per day, 7 days per week) plant operations are maintained. In addition to operating the hydrogen sulfide liquefaction system, the synthesis unit, and the fractionation system, the operators are responsible for minor maintenance of the equipment and for running the necessary control analyses. Very few operating difficulties are encountered as long as the system is kept free of water. Operators working in the plant are thoroughly schooled in the hazards involved in the handling of hydrogen sulfide and in the most practical methods of protecting themselves from these hazards. The operators learn and follow such common sense practices as holding the breath and seeking fresh air a t the first strong odor of hydrogen sulfide, spending spare moments on the upwind side of the plant, properly ventilating the buildings, and repairing all leaks as soon as possible. The operators learn that their olfactory nerves are likely to become partially paralyzed and that they may experience a slight headache, burning eyes, or clouded vision as a result of exposure to hydrogen sulfide.
Figure 2.
Liquid Hydrogen Sulfide Surge Tank
Fresh air masks are always worn by maintenance and opeiating personnel when opening lines or vessels that have contained hydrogen sulfide or when working in places where hydrogen sulfide may accumulate. Equipment is thoroughly purged of hydrogen sulfide before opening any line or vessel for maintenance or repair work. The purging is accomplished as follows: Sweet residue gas is passed through the system for several hours, all valves are closed and the vessels are opened and quickly cleared of gas by use of industrial-type blowers. The lines are then broken out in sections and purged by natural ventilation. The hydrogen sulfide surge tank is opened for inspection every 6 months; the vessel seams are Magnafluxed every 2 years. Only a few minor pits and one small hydrogen blister have been observed in the suyge tank during the past eight years. Other vessels are cleaned and inspected during the annuall turnaround. Selected portions of the high-pressure lines are inspected each year; a complete and thorough inspection of t h e entire plant is made every fourth year. During this inspection all valve8 are removed, cleaned, and repaired; all flanges are opened and inspected. Where the wall thickness of lines and vessels cannot be measured directly it is calculated from Penetron and Audigage readings. Inspections over the past 8 years have revealed surprisingly little corrosion, erosion, or embrittlement ; however, localizedpitting and embrittlement have occurred in lines and connections where there is little or no flow. This has made it necessary t o eliminate hollow bull plugs, to replace frequently or to eliminate small threaded nipples, and ts replace blinded tees with ells.
Age-Hardened lnconel Used for HighStressed Parts Reduced Embrittlement Failures I n general, corrosion problems with carbon steel in service with anhydrous hydrogen sulfide have not been severe. The corrosion rate seems to be of the order of 0.001-inch per year-so slight that it can hardly be measured by inspection calipers. A black sooty deposit that forms on the steel may be somewhat protective, and it is possible that this deposit lowers the corrosion rate from a rate that is initially fairly high. Pure copper is rapidly attacked by hydrogen sulfide, particularly in the presence of moisture or mercaptans. However, such copper alloys as admiralty brass and antimonial-admiralty metal show fairly good resistance to hydrogen sulfide corrosion, as do aluminum, .Stellite, Inconel, Ni-Resist, and 304 and 316 stainless steel.
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Aqueous hydrogen sulfide is much more corrosive than anhydrous hydrogen sulfide, and the various metals act differently in the two media. For instance, the corrosion rate on carbon steel in aqueous hydrogen sulfide may be as high as 0.10 inch per year, with some pitting of the steel. Consequently, it is desirable to remove all water that may be present in the hydrogen sulfide
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stainless steel. Soft materials can be used for valve trim, while Type 316 stainless makes satisfactory Bourdon tubes. Because hard steels are quite subject to embrittlement failures, steels of low hardenability should be used for welding so as to avoid a hard zone in the edge of the weld. Flame-hardened steels cracked badly because of high hardness and high residual stress. Embrittlement cracks on flame-hardened pump rods led to early fatigue cracks. Hard-facing overlays like Stellite substitute for flame-hardening and may be used to protect the packing area of soft rods. Even austenitic stainless coating, despite its softness, has been used satisfactorily.
Major Objective-Safe Operation of Plant-Has Been Maintained for 8 Years
Figure 3.
Stressed Steel Spring That Failed in Hydrogen Sulfide Atmosphere
and to design the equipment so that any water collecting in the system after shutdowns can be drained. Although every effort is made to keep the system dry, on several occasions moisture has inadvertently entered the system through heat exchanger tube leaks or by reason of gas-drying difficulties. A peculiar, though important, phenomenon occurring where hydrogen sulfide is present is hydrogen embrittlement. This phenomenon is caused by the action of hydrogen on steels, the hydrogen being generated by very slight corrosion of the steel ( 1 ) . A general rule, with no exceptions noted t o date, is that all hard magnetic steels, alloy or plain, are subject to hydrogen embrittlement if highly stressed. Steels having a hardness below Rockwell C-20 when exposed to anhydrous hydrogen sulfide have not been subject to hydrogen embrittlement. Since embrittlement on steel always occura to some degree, it is desirable to use only good ductile materials. Free-machining steels are notch sensitive and have poor transverse ductility. Even without further embrittlement they are hardly suited for such items as bull plugs and m a g e nipples. I n this plant only “refinery-type” fittings made of good ductile steels are used. Like free-machining steel, the steel used for most butt-weld and continuous-weld pipe is also of low transverse and notch ductility. I n addition, these types of pipe usually have a bad notch in the weld. To avoid inferior material, only seamless steel pipe is used to handle hydrogen sulfide. Embrittlement failures of highly stressed hard steel parts are quite common in sulfide environments. Plain steel relief valve springs fail in as little as two hours. Figure 3 illustrates such a failure caused by embrittlement. Compare this with the new spring of Figure 4. Relief valve springs made of age-hardened Type 18-8 stainless steel and ordinary springs coated with low temperature ceramic and with baked phenolic also failed. $luminizing, when of good quality, protects the springs. All internal relief valve springs used in the plant are of aluminized steel. Teflon coating, though easily damaged mechanically, protected a test spring for over a year. Steel garter springs for holding metallic packing and valve springs for reciprocating pumps had to be replaced with age-hardened Inconel. These replacement materials have given trouble-free service for over 5 years. Other embrittlement failures have been experienced in valve trim and in Bourdon tubes made of carbon steel and Type 304
The successful operation by the Phillips Petroleum Co. of a plant for the manufacture of mercaptans from olefins and hydrogen sulfide has demonstrated the feasibility of handling commercial quantities of liquid hydrogen sulfide. Adequate safety instruction, the practice of proper safety precautions, and the correct use of available safety equipment by the operators have made possible the attainment of a major objective, the safe operation of the plant. In 8 years of operation of this plant no serious injury has occurred as a result of contact with hydrogen sulfide.
Figure 4. Relief Valve Spring before Exposure to Hydrogen Sulfide
The handling of anhydrous hydrogen sulfide has presented some unusual problems; solutions have been found for the problems thus far encountered. Corrosion has been negligible on moat materials of construction; however, hydrogen embrittlement has necessitated the use of special metals in certain locations. The choice of appropriate materials of construction, together with the proper design of equipment, has been a large factor in the successful operation of the plant.
Acknowledgment The authors express their appreciation to Phillips Petroleum Co, for permission to publish this paper.
Literature Cited (1) Barta, M. H., and Rawlins, C . E., Corrosion, 4, No. 5 , 187 (1948). (2) Chem. Eng., 58, No. 3, 176 (1951). (3) Schulae, W. A,, Lyon, J. P., and Short, G. H., IXD. ENG.CHEM., 40,2308 (1948). RECXIVED for review June 16, 1952.
ACCEPTED ilugust 4 , 1952.
