NT Low Tern erature Ethan; from R. G. HEITZ, C. F. OLDERSNAW, %-. E. BROWN. AND R . D. BARNARD T h e Dozu Chemical C o m p u n y , P i t t s b u r g , CuliJ.
T h e Joule-Thonipson effect was applied on a pilot plant scale to the separation of 995%pure ethane from a natural gas stream containing only 2,870 ethane. This paper deals chiefly with the problenis encountered in the plant operation such as water and carbon dioxide removal, steadyoperation of the expansion tubing, and measurement of liquid level. Water removal was accomplished by freezing, passing over alumina gel, and passing over calcium carbide. Carbon dioxide was removed by scrubbing with potassium hydroxide in a column of special design. Glycerol lubrica-
tion of the compressor prevented plugging of the exp.rnsioir tubing. The level of liquid a t low temperature was determined by means of a hot-wire electrical meter, which i q described. A description of a laboratory-sized nlethane unit to provide refrigeration a t -160" C. is presented.
T
HE: Joule-Thornsori esparisioii has beeii used cwinicrcialllas a means of obtaining low temperatures for air liquefactioii m d fractionation for inany years. Enough basic data are available in the literature for flow sheet design on many other gR:
July 1949
1541
INDUSTRIAL AND ENGINEERING CHEMISTRY
The plant was operated continuously 5 days per week for over
sepal ations and refrigeration cycles-for example, the separation of ethane and methane from natural gas, separation of ethylene from dilute gas streams, or producing -160" C. refrigeration for small scale use with methane (or natural gas) as the only working fluid. Calculations show t h a t such processes are especially attractive for small scale operation because the equipment is small and simple. However, the power required, in general, 15 larger than for alternate methods and the economics are not so attractive ior large scale plants except under special circumstances v here a large pressure drop is available at no cost. The operation of this type of process is very simple. Troubles from fouling, corrosion, repaii, and start-up time are negligible. However, there are several design requirements which must be met if the plant is to operate smoothly and continuously. This paper describes a pilot plant based on the application of the Joule-Thomson effect and emphasizes some of the sources of trouble and means of eliminat>ingthem.
1 year and required only part-time work for one operator.
Refrigeration Cycle. The process in its simplest terms is similar to a conventional high pressure small scale liquid oxygen plant. Thus, t o provide the refrigeration, about 66 t o 75% of the gas is compressed to 3000 pounds, cooled in a high pressure heat exchanger (built inside of a salvaged bomb) and expanded to 20 pounds gage. The resulting liquid methane is used to provide reflux cooling in the distilling columns. Figure 1 is a Mollier diagram showing the refrigeration cycle. The theoretical refrigeration available from methane by a throttling expansion under the operating conditions shown in Figure 1 is approximately 80 B.t.u. per pound. Since 320 pounds per hour were compressed to the high pressure, there were approximately 25,000 l3.t.u. per hour of refrigeration available. This is used approximately as follows: About 5500 B.t.u. are taken out in the product ethane; 4500 l3.t.u. are in the discarded liquid C3+; 11,000 l3.t.u. are lost by incomplete heat exchange when operating with a 25" C. At a t the warm end of the exchanger; and 4000 B.t.u. are lost by heat leaks. Flow Sheet. The essentials of the flow sheet are shown in Figure 2. Approximately one third of the high pressure stream flashed to a gas on expansion through the capillary tube, leaving a liquid containing all of the ethane and heaviers dissolved in liquid methane a t approximately - 140' C. and 20 pounds gage. T o this mixture then was added another stream of natural gas of about one third t o one half its quantity. This second stream, called the heating gas, was passed through coils in the bottom of the tower t o provide reboil heat before mixing it with the cold methane stream. About one third of the liquid methane way
DESCRIPTION OF PROCESS
Design Basis. The pilot plant was designed to produce 20 pounds per hour of 99% pure ethane from a natural gas stream. The analysis of this gas stream was: % N2
2
COe
0.3
94.2
CHI CaHe C3He+
2.8 0.7
Since this gas stream would not be a n economical source for a large scale supply of ethane, the plant was not operated to obtain design data.
S E PAR A T 0R
i t
CHq COLUMN
STARTING BY PA SSk
-
.
C2Hg
CONDENSER
*
T RYCOCKS REBOl L E R
t t t
HEATING
I
GAS 160 L B . / H R ,
8 AND H E A V I E R 2QLB./HR c TO k
Figure 2.
C
Flow Sheet of Ethane Recovery Process
H
BOILERS
q PRODUCT
1542
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
Vol. 41, No. 7
caused trouble by plugging the expansion tubing. This indicated t h a t some oil from the compressors was entrained or dissolved in the high presLlOUlD INLET sure methane and carried into the bomb where it apparently stayed in solution a t approximately -100" C. but was precipitated in the LIQUID LEVEL capillary as the temperature dropped towards PERFORATED PLATE -140" C. Although it might be possible t o BACK\G PLATE filter out entrained oil, the gas density was high enough so t h a t appreciable solubility of the oil in PERFORArlONS-O045 the gas phase could be expected (3). I n view of this possibility, glycerol was substituted for the lubricating oil. Diethylene glycol also was used successfully. Hydrate Formation. It is well known that natural gas under high pressure readily forms hydrates ( 3 ) . Thus it was necessary t o ensure that no section of the compressor aftercoolers or piping operated below about 23" C. This was 8ACWhG PLATE Gas done by recycling the cooling water. INLE? Water Removal. Substantially complete water removal is essential t o any low temperature procesf I n general, water can be removed by S i E V E PLATE COLUUY condensing or freezing, by absorption or by Figure 3. Sieve-Plate Carbon Dioxide Absorber chemical reaction. I n this case, all three of thePe methods were used and no icing conditions were encountered in the columns. HIGHPRESSURE STREAM.The high pressure gas was dehy-evaporated in cooling this heating gas and condensing the ethane drated by freezing the water out on the coils in the high pressure in it. The remaining liquid, about 90% methane, then was heat exchanger. The volume available for ice was approximately dumped into the top of a packed tower in which most of the 1 cubic foot. The water input a t 30" C. and 3000 pounds pressure methane was stripped out, leaving a liquid boiling between -90' was approximately 0.2 cubic foot per m-eek. No trouble was and -120' C. which is sufficiently cold to supply the refrigeraexperienced vith plugging, either during the normal 5-day operatt,ion for condensing t h e ethane reflux. This liquid contained ing cycle or with thawing out over the weekend. about 5 to 10% methane which had t o be removed. A small Since the exchanger would not remove water until it was cold, stripper heated by part of the heating gas stream was used for it was necessary to cool it before passing the gas through the this purpose, The resultant methane-free liquid then was fed capillary. This was done by by-passing the high pressure gas to the middle of the ethane column. The product ethane %-as through a hand-operated valve into the coils in the exchanger withdrawn about 2 feet below the condenser as either gas or When the high pressure gas n-as cooled to - 100" C., the capillary liquid, and the accumulated C3+ was dumped periodically when was opened. the level built up in the reboiler. Low PRESSURE GAS. The heating gas left the potassium hydroxide scrubber a t 70 pounds gage and 20" t o 30" C. with a PROBLEMS IN EQUIPMENT DESIGN AND OPERATION moisture content amounting to approximately 1 cubic foot of ice Carbon Dioxide Absorption. The carbon dioxide content of per week. the feed gas was about 0.37& Rough experiments indicated t h a t Although a volume of 4 cubic feet mas provided in the reboiler the solubility of carbon dioxide in liquid methane is so small that for this ice, the unit failed t o operate continuously more than 1 the gas entering the column had t o have less than 30 p.p.m. carbon dioxide to prevent plugging the packing. During initial operation it was found that 40% potassium hydroxide in a six-plate column removed only 60 to 70% of the carbon dioxide. Subsequent tests showed t h a t the absorption V O L T M E TE R increases with the degree of gas dispersion, liquid holdup in the 0 column, and the temperature. Taking these factors into conside1ation, a thirteen-plate section made up of perforated screens arid having a large liquid holdup, as shown in Figure 3, was added to the original six-bubble-plate section, and the feed gas was steam heated to maintain a n operating temperature of 25" to 35' C. This reduced the carbon dioxide content of the scrubbed gas t o between 30 and 60 p.p.m. The small perforations not only gave a high degree of gas dispersion b u t also made it possible to operate this column a t any Figure 4. Liquid Level gas rate below flooding. There was no weeping from the plates Gage-Hot-wire Type a t any gas rate. A very troublesome foaming problem vias solved by adding approximately 0.5 ml. of tincture of green soap per gallon of caustic. This is remarkably effective in curing foaming troubles with caustic solutions, but care must be taken t h a t too much is not added, since excess will increaee foaming. Compressor Lubrication. The initial use of oil as a lubricant
v
July
1949
INDUSTRIAL AND ENGINEERING CHEMISTRY CH4 A T M S . CH4 300 P S l G DRY
ICE
H E A T EXCHANGER SOLDER V4" TUBING V8"TUBlNG DEWAR
FLASK
SANTOCEL FIBER
INSULATION
SLEEVE
R E FR IG ER ATE D
__EX
Figure 5.
PAN SI ON
C H A M BE R
VALVE
Laboratory Methane Refrigerator
t o 2 days because t h e gas passages through the baffles were too small and plugged. Therefore, a pair of alumina gel traps were installed ahead of the reboiler. The gel units were regenerated with the dry vent gas from the plant. Even though the alumina gel units reduced the moisture content t o a very low value, it was found that a calcium carbide trap following the reboiler removed additional water. The carbide disintegrated t o a fine lime dust which tended to plug the trap. This was prevented by mixing the carbide with about an equal volume of granulated rock wool. The trap was changed every 3 to 6 months, at which time the bottom half was largely used up, but the top layer of carbide was covered with only a thin layer of lime. Throttling the High Pressure Stream. I n order t o obtain smooth operation in the distilling columns, it wsw necessary to have a uniform flow of both the high and low pressure streams. The low pressure stream was throttled with a valve in a conventional manner. However, the high pressure stream could not be controlled satisfactorily with a valve. It was found that a capillary gave uniform flow and trouble-free operation; 150 feet of 0.1875-inch, 20-gage copper tubing was used for this purpose. The superior performance of the capillary as compared with a valve is due to the much larger cross sectional area of the former. The diameter and approximate length of a capillary can be calculated readily (I), and the final adjustment in length can be made easily by field trial. High Pressure Heat Exchanger. The high pressure exchanger is a little unusual in t h a t the fluid enters as a gas above the critical temperature and pressure and changes without forming a definite phase boundary t o a liquid below the critical temperature. I n addition, it was necessary to provide considerable free space on the shell side t o allow for the accumulation of ice. T o ensure that the density increases would not mix the cooled fluid with the hot entering gas and thus destroy the countercurrent flow arrangement, the exchanger was mounted vertically with the flow entering the top and leaving at the bottom. Since the bomb was an alloy steel vessel formerly used in high temperature service, it was necessary to test a specimen of the metal for impact strength a t -100" C. Distilling Columns. The columns were constructed of copper with silver soldered joints except a t a few places where soft solder was used t o facilitate erection in the field. This construction was satisfactory except that of the soldered joints failed after months' Operation. This may have been due to high stresses developed during the cooling and thawing out. Failures
1543
were repaired either by brazing or resoldering with re-enforcing sleeves added. Low Temperature Insulation. All of the low temperature apparatus, except a copper storage tank located outside the building, was insulated with loose rock wool held in a sheet metal box. This proved to be satisfactory and is recommended especially for pilot plant work because changes and repairs can be made easily by digging out some of the loose rock wool and replacing it after the work is done. All of the piping and control valves operating at low temperatures were located as close to the columns as was convenient and buried inside the insulation. Connections for pressure taps coniing through the box were made of long spiraled lengths of 0.125inch tubing to minimize heat loss. The valves were operated by wooden handles about 12 to 18 inches long extending through the side of the box. Instrumentation. No difficulty was experienced in measuring pressures and temperatures by conventional means. However, measurement of liquid level was very difficult. Knowledge of two liquid levels (the reboiler level and the level in the reflux collector) was essential t o operation. Measurement of these liquid levels with a differential manometer was not satisfactory as slight surges in the system pressure would force liquid up through the connections where it would suddenly flash when it hit the warm tubing. This gave pressure surges on the manometers which made accurate readings impossible. The problem of level measurement in the reboiler was solved by installing try cocks. Since the reflux rate was determined by the head of liquid above a metering orifice in the collector pot, continuous indication of the liquid level in the collector was necessary. The problem of this level measurement was solved by the use of the electrical level indicator shown diagrammatically in Figure 4. I t s operation is based on the change of resistance with temperature of the heated tungsten wire. This resistance thus depends on the length of the portion covered by the liquid. METHANE REFRIGERATION
Pilot Plant Scale. This pilot plant also furnished -100" C. refrigeration t o a condenser for another pilot plant in the same area. The refrigeration was made available by withdrawing gaseous instead of liquid ethane. Occasionally liquid methane was drained from the ethane condenser during starting operations for use as a laboratory refrigerant. Laboratory Methane Refrigerator. Figure 5 shows a diagram of a laboratory refrigeration unit which operates on 300-pound natural gas. It is suitable for freezing-point determinations and other low capacity applications. ACKNOWLEDGMENT
The authors are happy to acknowledge the valuable advice given by William Giauque of the University of California on many of the practical design features. Much credit for the success of this plant is due to Paul LeRoy, foreman in charge of the research shop, who built many of the more critical parts of the apparatus and made several ingenious repairs under difficult circumstances. The authors sincerely thank The Dow Chemical Company for permission to present this paper. LITERATURE CITED
(1) Bolstad, M. M., and Jordan, R. C., Refrig. Eng., 56, 519 (1948). (2) Katz, D. L., and Kurata, F., IND.ENG. CHEW,32, 817 (1940). (3) Wiloox, W. I., Carson, D. B., and Katz, D. L., Ibid., 33, 662 (1941). RECEIVEDMarch 21, 1949.' Presented a8 a part of the Pilot Plant Symposium before the Division of Industrial and Engineering Chemistry a t the 119th Meeting of the AMERICAN CHEMICAL SOCIETY, San Francisco, Calif. Contribution from the Research Department of the Great Western Division. The DOW Chemical Company.
