US2475957A - Treatment of natural gas - Google Patents

Description

Tngl @N ORE ATTORNEY F. E. GILMORE TREATMENT OF NATURAL GAS Filed Aug. '7, 1944 July 12, 1949.

Patented July 12, 1949 4aiu-,951

TREATMENT OF NATURAL GAS Forrest E. Gihnore, Bartlesville, Okla., asslgnor to Phillips Petroleum Company, a corporation ot Delaware Application August 7, 1944, Serial No. V548.475

2 Claims. 1

This invention relates to the treatment of hydrocarbon gases. In one of its more specific aspects it relates to the treatment of hydrocarbon gases for the removal of nonhydrocarbon gaseous constituents. v

It has been found that some natural gases as produced contain appreciable amounts of gaseous impurities such as carbon dioxide, nitrogen or hydrogen sulfide. A gas containing as much as to 20 per cent impurity presents serious considerations when marketing is contemplated. The presence of carbon dioxide and nitrogen lowers the heating value of a gas merely by their presence while such a material as hydrogen sulflde causes the gas to be corrosive and to possess a foul odor. Combustion products of the latter possess a disagreeable odor as well as being corrosive especially when moist. In appreciable concentrations hydrogen sulfide is poisonous. In view of these and other considerationsthe removal of hydrogen sulde from natual gas to be used in domestic heating, process Work or metallurgical work is imperative.

Carbon dioxide and nitrogen affect a natural gas for the most part only by dilution since both these materials are substantially inert in a combustion zone. Transportation of such a natural gas by pipeline presents many difficulties and problems especially when the economics of the problem are considered. For example, when transporting, say 100,000,000 cubic feet per day of a gas containing 10% by volume of nitrogen through a long pipeline, the operation involves repeated compression of- 10,000,000 cubic feet of` an inert gaseous material and the construction of the pipeline with a capacity 10 per cent greater than would be necessary were the nitrogen removed. The cost of such compressions may amount to hundreds of thousands of dollars per year and the additional cost of the pipeline may be greater than the cost of a plant to remove the nitrogen. In addition, the diluting effect of such a gas materially lowers the heating value, as for example, a gas having a caloric value of say 1050 B. t. u. to approximately 945 B. t. u. Natural gas must usually be maintained as 1000 B. t. u. gas, in which case gas containing nitrogen must not be completely stripped of its condensible hydrocarbon content, or a naturally lean gas may have to be enriched.

It is suggested that a natural gas having a relatively high nitrogen content may be treated for removal of this inert nitrogen thereby upgrading the heating value. In case such a gas contains hydrocarbons of the gasoline boiling range or is wet as termed by the art, these hydrocarbons may at least in part be extracted from the gas in the form of natural gasoline, and upon substantial removal of the inert nitrogen still leave a natural gas of suiiiciently high or satisfactory caloriilc value.

(Cl. (i2-175.5)

It is one objectl of this invention to provide an economic process-for the removal of relatively large amounts of nitrogen from natural gas.

Another. object.of this invention is to provide a process for the simultaneous removal of inert nitrogen and extraction of condensible hydrocarbons from a natural gas containing these materials.

Still anotherv object -ot this invention is to provide a process for the simultaneous removal of inert nitrogen and extraction of condensible hydrocarbons from a. natural gas containing these materials and still leave a natural gas of sufciently high caloriflc value and without substantial loss of the main component of the treated gas. l

Still other objects and advantages will be apparent to those skilled in the art from a careful study of the following disclosure.

Figure 1 illustrates one form ofl apparatus in which the process of my invention may be practiced. and I do not wish to be limited thereby since this one embodiment is merely exemplary of the broad aspects of my invention.

Figure 1A is an illustration of a second method for reuxing of the fractionation tower.

Broadly speaking, my process involves removal of condensible hydrocarbons from a natural gas, dehydration ofthe remaining gas, and removal of a further quantity of condensible hydrocarbons. The resultant dry gas is compressed and chilled to such a temperature that methane condenses to a liquid. This liquifled methane contains some nitrogen and is fractionated to separate the nitrogen from the methane. Methane, as liquid, is used as a refrigerant to assist in liquefying the methane in process and is in turn evaporated and warmed. My unique application of heat exchange steps makes the necessary low temperatures economically feasible.

Referring now to the drawing, raw, impure natural gas containing appreciable amounts of nitrogen reaches my treating system through a main inlet gas line l. The impure gas in line l may come directly from a gas producing field or from an intermediate gas storage system, not shown. For purposes of illustration, I will assume that the gas from line I to be processed is maintained at a pressure ranging from pounds to 1000 pounds per square inch, as for example, about 250 pounds per square inch and at atmospheric temperature. Natural gas, as produced, in addition to condensible and noncondensible hydrocarbons usually contains hydrogen sulfide, carbon dioxide, moisture and some mercaptan sourness, as well as inert materials such as nitrogen or in exceptional cases helium. These impurities occur in natural gases in amounts varying from substantially none or traces in many gases to appreciable proportions in other gases; In exceptional gases the nitrogen or carbon difor purposes ci illustration I will discuss my invention as appliedy to the purification of `a natural hydrocarbon gas containing from say to 25% gaseous nitrogen, and in the speciflc case described the gas contained by volume of this inert gas. From the field line I the raw, impure gas ma 'be compressed, if necessary, by a compressor 2 to increase the pressure of the inlet gas to a favorable processing pressure. From the compressor the gas passes to a cooler 3, thence to a gas-to-gas heat exchanger 4 in which the gas is cooled to about 32 F. by indirect heat exchange with` previously cooled and processed gas. From this exchanger the cooled gas passes to a liquidgas separator 5 in which condensate formed in the exchanger may be separated from the uncondensed material. Much of the sourness, CO2 and moisture are dissolved in this hydrocarbon condensate or condensed simultaneously and may be removed from the gas treating system upon withdrawal of condensate from separator l through draw line 6 and passed to disposal, not shown. The uncondensed hydrocarbons and gaseous impurities pass from the separator 3 by a line 1 to a treater apparatus 3 in which such impurities as hydrogen sulde and carbon dioxide are removed. This treater may well be one utilizing a conventional process or processes, and the H2B and CO2 may be'removed together or separately as desired. From this treating apparatus 3 the C02 and HaS free gas passes by way of a line 9 to a dehydrator I0. This dehydrator may likewise, be apparatus of conventional design employing a conventional process and may consist of two or more vessels for alternate on-stream and regeneration as found necessary. The moisture must be eillciently removed from the gases to prevent hydrate formation in subsequent low temperaturel steps of the process.

The well dried hydrocarbon gases leave the de-I hydretor by a une n which une branches and permits a portion of the gas stream to pass through a gas cooled condenser I2 and the re- '4 f Condensed hydrocarbons which accumulate in the bottom of the condenser I4 are cold and may well serve as a cooling medium. Accordingly,

these liqueiled hydrocarbonsv are removed by a maining portion to pass through a liquid cooled condenser I3. After emerging from these con.

densers the two portions of the gas stream are combined and enter the lower portion lof a refrigerated condenser I4. This latter condenser may be substantially any type of condenser, provided, of course, it is of suitable design and construction for such low temperature service as herein contemplated.` The gas upward through the tube section II of thisl condenser which is cooled to the low temperature of from -30 to 1 100" F. by an independent refrigeration unit. In this unit such a low boiling refrigerant as ethylene, for example, may be used. In the ethylene cycle, the ethylene previously liqueed, acquires heat indirectly from the hydrocarbon gases being cooled, boils and evaporatesln the tube section I 5 of exchanger I4. The ethylene vapors are then withdrawn by a line Il and pass through a compressor Il, a condenser I8, from which latter the liquefied refrigerant passes into an accumulator or surge tank I3. From the surge tank the said liquefied refrigerant passes by a return line 20 to a cooler 2| and thence into the lower portion of the refrigerated condenser I4 to complete the ethylene cycle. Makeup ethylene may beyadded through a line 22 on the suction side of the compressor I l.

line 23 the flow being controlled by the liquid level apparatus 32 and the motor valve I3 into the liquid to gas exchanger I3 to chill a portion of the feed stock previous to entrance into the refrigerated condenser I4. This liquid material after giving up a portion of its coldness or after being somewhat'warmed in the liquid cooled condenser I3, passes by way of a line 24 into a stabilizer 2l which may be of conventional design but' suited for low boiling hydrocarbon stabilization. Overhead 'gases from this stabilizer exit by way of a line 2B, pass through a condenser 21, and the condensate in which at least some propane is condensed accumulates in reux accumulator 23. Liquid level is controlled by a level controller 29 which operates a motor valve 30 to permit return of liquid reflux to the top tray of the stabilizer 25.

Uncondensed hydrocarbons from the accumulator 23 pass by way of a line 3| to a main nitrogen free gas ,line 32 which conducts the finally treated and purified gas from my treating system to be further fully explained to a pipeline for transportation to a market or to a gas holder for storage, not shown, or to other disposal as desired.

Stabilized natural gasoline passes from the base of the stabilizer through a line 54 to storage or other disposal, as desired.

The uncondensed gases consisting of nitrogen and methane with substantially no ethane or heavier hydrocarbons issue from the refrigerated condenser I4 through a pipe 33 into an auxiliary chiller 43 and finally are passed through still another exchanger or condenser 34 wherein a major portion of the methane is condensed to liquid methane and most of the nitrogen, of course, remains as a gas. This mixture of gaseous nitrogen and liquid and gaseous methane passes from the said condener 34 by way of a pipe 35 into a fractionator tower 38 herein termed the denitrogenizen This vessel may usually be a conventional bubble cap fractionator. but so designed and constructed as to operate at low temperature and high pressure as required by such service as 'herein discussed.

The fractionator .or denitrogenizer 38 is equipped with conventional type bubble cap'trays,

. a reflux apparatus 3l and a reboiler coil 33. As

mentioned above, charge stock to this column contains largely liquid methane, some gaseous or uncondensed methane, nitrogen and any other diiliculty condensible material having a boiling point near that of methane or below which has not been 'previously removed. Liquid methane containing some dissolved nitrogen descends through the fractionator the nitrogenbeing fractionated out in the descent until the liquid accumulating in the reboiler section is almost pure methane. Reboiling, coil 33 furnishes heat to boil this methane to produce the desired rectication in the fractionator. Heat for said reboiling is furnished by passing a small portion of the partially chilled methane and nitrogen from the line 33 through abypass line 39 into the reboiler coil. The exhaust from the reboiler coil is passed through a pipe 4I and is discharged into condenser 34 with the gases from the chiller The upper portion of the denitrogenizer is for the most part constructed in a manner similar to that of conventional fractionators. A pipe 42 removes overhead vapors and gases from the fractionator, which material *n my case consists mainly of gaseous nitrogen and uncondensed methane. This material passes through the line 42 to a heat exchanger 43 and after being warmed it is compressed by a compressor 44 and cooled in a cooler 45. The stream issuing from cooler 45 is split. one portion passing through exchanger 43 and the other portion passing through an exchanger 46. These two stream portions issuing from said exchangers combine in a line 41 and arc passed to a reflux accumulator vessel 48. In this vessel methane which has been condensed in exchangers and 46 separates from the gaseous nitrogen and some uncondensed methane. The nitrogen and uncondensed methane are removed from the top of this accumulator by a pipe 49, expansion being permitted by an expansion valve 50 set to operate according to a desired pressure in accumulator 48. -The refrigeration available from this expansion absorbs heat in the exchanger 46 to chill and condense lsome methane from the methane-nitrogen mixture which passes in indirect heat exchange therewith. The expanded nitrogen-methane mixture then issues' from a line 5I for such disposal as desired. I prefer to operate the plant in such a manner that this off gas contains from 25 to 50% nitrogen, the remainder being methane, and in this manner sumcient methane is available for plant power purposes.

The liquid methane which has accumulated in the accumulator 48 is returned to the top of the denitrogenizer by a line 52 to serve as a wet reiiuxing material. The level of the liquid in accumulator 48 may be controlled by a motor operated valve 53 actuated by a liquid level mechanism on the accumulator tank. Upon opening of the motor controlled valve 53 the liquid methane passes therethrough from the pressure originating in the compressor 44.

A thermoregulator assembly 55 which controls the amount of flow of cold gas in line 39 to the reboiler 38 is actuated in response to the temperature of the liquid in the base of the column 36. This substantially nitrogen-free liquid methane is passed from the base of this column through a line 56 in which is placed a motor operated valve 6| actuated by a liquid level mechanism on the denitrogenator 36, to the space around the tubes of the heat exchanger in exchanger vessel 34. In the space around these tubes much or all of the liquid vaporizes under a lower pressure than the gas inside the tubes imparting its low temperature to the higher pressure methane inside the tubes by which operation the latter at least in part condenses. The Vaporized methane issues from the refrigerated condenser by a line 51 and is led to the exchanger 40. From this exchanger the methane passes by way of a line 58 to still another exchanger 2l, thence by a line 59 to yet another exchanger I2, previously termed the gas cooled condenser. From this condenser the methane gas passes by a line 69 into the first mentioned gas to gas exchanger 4, thence by line 12 into the main product line 32.

The liquid hydrocarbons, that is, those more easily condensed, are removed by line 23 and the flow in which is controlled by a motor valve 63 in the line 24 which is actuated by a liquid level apparatus 62. This motor valve serves as a back pressure regulator on the liquid cooled condenser I3 to prevent undue evaporation of the-cooling medium within the condenser. It also restricts or controls the flow of condensed hydrocarbons into the stabilizer tower 26. A pump 64 serves to assist in transfer of these condensed hydrocarbons from the condenser I3 into the stabilizer according to the level of the refrigerant in' accumulator I9 to control the rate of flow of said refrigerant by actuation of a motor valve 65 in a line 68. rThis refrigerant upon passage through valve 65 evaporates as heat is absorbed from gas passing through the condensing tubes i5. Refrigerant gas exits through the line I6 and is recompressed by compressor I1, as mentioned hereinbefore. Makeup refrigerant may be added through the line 22.

In the operation of the process according to my invention I contemplate treating iiD-100,000,000 cubic feet of a natural gas containing approximately 15% nitrogen by volume, a normal amount of sulfur sourness, carbon dioxide and some moisture.

'Ihe denitrogenizer or fractionator tower may be operated under dry reflux in place of the wet trim as mentioned above. In this case the overhead nitrogen-methane gas is expanded through a needle valve to almost atmospheric pressure the cold expandedgas passing through a closed reux coil in the top of the tower. This very cold coil is sufficiently cold to condense considerable of the methane which latter then flows down the tower as a liquid reflux. To assist further in the dry reiiuxing ofA this denitrogenizer tower, the apparatus and iiow illustrated in Figure 1A may be used. The above mentioned fractionator overhead nitrogen-methane gas line 42 connects to an expansion valve 15 which permits passage of expanded gas through the closed reflux coil 16. This cold expanded gas passes from the coil through an exchanger 11 as a refrigerant for cooling another refrigerating medium. This effluent nitrogen-methane gas may be-used as the refrigerant in a supplementary unit to assist in the very low temperature closed coil refluxing of this fractionator. However, if desired, another refrigerant may be used, as illustrated inthe Figure 1A. Referring to the figure this refrigerant is compressed in a compressor 18 and cooled in a cooler 19. The cooled stream passes through a line 8l)l and is divided into two portions as represented by lines' 8l and 82. Line 8l conducts a portion of this gaseous refrigerant to the above mentioned exchanger 11 while the remaining portion in line 82 passes through another exchanger 83. Both streams of the refrigerant are cooled and may be partially or wholly condensed on passing through these exchangers, and again join in a line 84, pass through an expansion Valve 85 and thence into an auxiliary closedrefiux coil 86. This coil is preferably disposed below the coil 16. The expanded eflluent from the coil 86 passes through the exchanger 83 in which it cools and may condense the refrigerant enroute to the reflux coil 86. If the top fractionator pressure is about '100 pounds per square inch or higher, the Joule-Thompson effect of the very cold expanding gases will refrigerate coil 16 to a low enough temperature to condense a large portion of the methane and the methane-nitrogen mixture leaving coil 16 may be utilized for fuel for power and heat needed for the operation.

The refrigerant, either the methane-nitrogen off gas or another refrigerant, .is compressed by compressor 18 to a pressure'of from 1000 to 5000 pounds per square inch. Upon being cooled in cooler 19 and further cooled in exchangers 11 and 83, and when expanded in valve 84 produces sufficiently low temperature in coil 86 to con-l dense methanes and'y even nitrogenif desired.

Since such an extraction plant as that discussed herein consumes considerable power, I believe it preferable and advisable to permit a relatively large concentration of methane to pass off the top of the fractionator with the nitrogen as mentioned hereinbefore. In this manner, the nitrogen can be more efliciently removed from the field gas and at the same time at less cost. Since it is a difficult and expansive operation to remove all or substantially all the methane from the nitrogen in the fractionator my proposed operation is relatively economical. contemplate to control the nitrogen containing off-product relative to its methane content in such a manner as to furnish all or substantially all the methane needed for heat and power purposes in the process.

For carrying out such a process as herein disclosed, it is needless to say' that all pipes, ex-

In fact, I

changers, valves, etc. which carry low temperature liquids or gases should be well insulated by the best thermal insulation obtainable. It might vbe advisable to install all the cooling that the individual pieces of equipment need not be so heavily insulated. All operating controls, indicators, and instruments should be outside this building, the temperature of which should preferably be maintained as low as 0 F.- or below.

The expansion valves, pipes, all vessels and all parts and members of this low temperature plant should be made of such materials and so designed as to withstand the very low temperaturesV of operation. Similarly, the equipment should be capable of withstanding all pressures necessary,

compressing and cooling the gas, dividing said compressed and cooled gas into two portions, chilling one portion by a rst indirect heat exchange with a refrigerated Vaporized methane, chilling the other portion by a second indirect heat exchange with refrigerated liquid methane, combining these two portions and further chilling the combined stream Aby a third indirect'heat exchange with vaporizing liquid methane to produce some liquid methane containing dissolved nitrogen and leaving uncondensed most of the nitro' gen and some gaseous methane, fractionating the chilled combined stream of liquid methane containing dissolved nitrogen and gaseous nitrogen'- and methane to produce a liquid methane bottoms product and an overhead gaseous product of nitrogen and methane, partially condensing this gaseous product and adding the condensate to the equipment within a well insulated building so 1 fractionating step as liquid reflux, and removing the nitrogen and some Auncondensed methane as a. product of the process; adding reboiling heat to the fractlonator liquid bottoms by the second indirect heat exchange thereby producing liquid methane free from dissolved nitrogen and termed refrigerated liquid methanevaporizing this refrigerated liquid methane inthe third heat ex` change thereby producing a refrigerated Vaporized methane,warming this refrigerated vaporized methanein the first heat exchange thereby producing warmed vaporized methane, and removing thlswarmed vaporized methane as the major product of the process.

2. A process for purifying natural gas containving gaseous nitrogen as an impurity, comprising, compressing and cooling the compressed gas, di- Viding the compressed and-cooled gas into two portions, chilling one lportion by a first indirect heat exchange with a chilled `gaseous methane, subsequently produced, chilling the other'portion by a second indirect heat exchange with al previously refrigerated liquid methane fraction ator bottoms, subsequently produced, combining these two chilled portions of compressed `gas into A one stream and further chilling this stream by a third indirect heat exchange with evaporating liquid methane, whereby some of the methane of the gas stream is condensed and dissolves some gaseous nitrogen, and passing as feed stock this stream of vliquid methane containing dissolved nitrogen and gaseous nitrogen and methane into a fractionation zone at a point intermediate the ends thereof and therein fractionating saidfeed stock to produce a liquid methane bottoms free from dissolved nitrogen and an overhead gaseous product containing nitrogen and some methane; compressing and chilling said overhead product to produce a liquid condensate and uncondensed nitrogen containing some methane, adding said condensate to the top of said fractionating zone as liquid reflux, and removing said uncondensed nitrogen containing some methane as a product of the process; adding reboiling heat to the bottom ofthe fractionation zone by said second indirect heat exchange, vaporizing said liquid methane in said third heat exchange, and warming said vaporized methane in said rst heat exchange, and removing the warmedV vaporized methane from saidiirst heat exchange, as the purified natural gas.

FORREST E. GILMORE;

- 'REFERENCES CITED The following referefnices are of record in the iile of this patent: i

y UNITED STATES PATENTS Number Baufre Apr.v 21,. 1942

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