US1429175A - Process of treating natural gas and product thereof - Google Patents

Description

H. E. THOMPSON.

PROCESS 0F TREATING NATURAL GAS AND PRODUCT THEREOF.

APreLrcATloN FIL'ED AUG.V29,1921.

1,429p175- f mmeasepf. 12,1921

2 SHEETS-SHEET l.

COO/er N "l: 2 N. T h VII /reaoo/er cond Absorf H. E. THOMPSON.

PROCESS OF TREATING NATURAL GAS AND PRODUCT THEREOF.

APPLICATION FILED AUG.29. 1921.

Patented Sept. 12, 1922,

2 sHEETsSHEET 2.'

I M A n Pf x .N .mi M? Patented Sept. l2, i922..

narran er means HAROLD E. THOMPSON, OF CLENDENIN, WEST VIRGINIA, ASSIGNOR TO CARBIDE &v

CARBON CHEMICALS CORPORATION, .A CORPORATION F NEW YORK.

PROCESS 0F TREATING NATURAL. GAS AND PRODUCT THEREOF.

Application led August 29, 1921. Serial No. 496,321.

To all whom it may concern.'

Be it known that I, HAROLD E. THOMPSON,

a citizen of the \United States, residing at Clendenin, in the county of Kanawha and State of West Virginia, have invented certain new and useful Improvements in Processes of Treating Natural Gas and Product Thereof, of which the following is a specilication.

This invention relates to the manufacture, from natural gas, of liquid mixtures of hy# drocarbons adapted for use as motor fuel and for other purposes. An example is the liquid commonly known as natural gas gasolene, natural gasolene, casing-head gasolene,

etc. The object of the invention is to improve the processes for. maklng suchliquidsand to improve the products obtained.

Natural gases are essentially mixtures of paralin hydrocarbons. Only the lower members of the'paraftin series are present, andv the percentages of the individual hydrocarbons present usually bear an inverse relation to their molecular weights. Methane always predominates, and ,in some gases the content of other members of the series is negligible. Those natural gases which are `adapted for the production of gasolene, however, contain considerable quantities of ethane, propane, butane and the heavier hydrocarbons, and it is with such naturally occurring mixtures that the invention is concerned.

In the natural gas or casing-head gasolene industry, the desideratum isy to produce a liquid which can be transported and stored at ordinary temperatures and at or near barometric pressure,`in receptacles similar to those used for straight-run gasolene, without undue fire risk or loss by evaporation. In other words, the problem is to keep down the volatility of the product, as evidenced by the Vapor pressure of the liquid at certain temperatures. A high yield is of course desirable.

Partially because ,of ,lack of knowledgeof the compositionsof the natural gas used as raw material and of the products and byproducts obtained, the process asv now carried out is largely empirical. As the process is practiced, a preliminary product is obtained which is so volatile, or wild,7 that it cannot be safely stored 0r shipped. The volatility of this liquid is ordinarily reduced by the wasteful process known as weathering, which consists in storing the liquid in open tanks or tanks maintained at a low pressure and permitting spontaneous evaporation to go on until a sufficiently stable residue remains. This results in the loss in the aggregate ofenormous quantities of hydrocarbon material suitable for` gasolene.

By the application of improved ,methods of analysis, I have found that theexcessive volatility or wildness of freshly made casing-head gasolene is due to'the presence of ethane and propane. Even methane may be present in traces at times. Butane, which in pure form has a much greater volatility than is permissible in merchantable gasolene, can be present in casinghead gasolene in large proportions, up to the entire butane content of the gas treated, without excessive wildness ofthe product because of the solvent effect of the heavier hydrocarbons present upon it. I havefound that in the natural weathering process a large proportion of the butane passes off before the residue is suficiently stable. When this condition is reached, propane and even ethane are still present in the residue, while the butane may have been evaporated and lost in amount large in proportion to that of the residual liquid. 1

My inventioncomprises a rational treatment of material containing the less volatile portions of natural gas, whereby the material is separated into fractions which do not have more than one common ingredient in substantial quantity. As mentioned above, the natural weathering process pro.- duces a vapor containing ethane, propane, large amounts of butane, and even heavier hydrocarbons, while the residual liquid contains substantial quantities of all these compounds. By my process, I am able to expel substantially all the ethane and propane as vapor while volatilizing lonly a small quan; tity of butane, thus producing` a gasolene of as good or better grade than can be made by natural weathering and at the same timev very largely increasing the yield. In many cases vI prefer to evaporate 'a portion of the' less volatile gasolene, but in such cases butane -will be'the only hydrocarbon present in substantial quantity as a common ingredient in both the expelled vapor and the liquid residue. Even when a portion of the butane is expelled, the product is readily distinguishable from all petroleum distillates employed in the gasolene industry, as these latter contain a much smaller proportion of butane. All the butanemight be vapor- 'final vapor.

ized, or a portion of the propane might be retained in the residue if desired. Depending on the identity of the hydrocarbon which is the highest of those in the vaporized portion and the proportion of that hydrocarbon which is evaporated, the vapor pressure of the liquid residue may be adjusted to any desired value.

Certain procedure' in the process now to be described is referred to in the specification and claims as rectification The term rectification is used herein to describe a process wherein a liquid is boiled, the vapor produced is caused to contact with a second body of liquid of different composition in such manner that a second portion of vapor is evolved from the second body of liquid, the second portion Aor" vapor contacts with a third body of liquid which likewise evolves vapor, and so on through a sufficient number of vapor and liquid contacts. Liquid flows from one body to the next in a direction counter to that ot the vapor fiow, and the continuity of the process is maintained by adding liquid to the body from whichthe final portion ot vapor is evolved, the liquid so added being formed, for example, by partial condensation of the The uncondensed portion of the final vapor, and a continuously-withdrawn portion of the vboiling liquid first referred to, are ordinarily the products sought. It is only with such systematic processes involving repeated liquid and vapor contacts in a series of distinct phases, that the present invention is concerned.

A'complete separation of certain constituents of gas mixtures has sometimes been attempted by a fractional condensation. I

have found that complete separation cannot be accomplished in this way. In any fractional liquefaction process the initial con densation brings down some of the more volatile constituents which in my process l then removeby boiling and rectification.

For a better understanding of the invention, reference will be made to the accompanying drawings, in which Fig. 1 is a diagrammatic illustration of one method of applying my invention, and in which F ig. 2 illustrates a modified form of the invention.

-A typical natural gas gasolene process will now be described in connection With Fig. 1 of vthe drawings, and the manner in which the process of my invention may be applied thereto will be indicated. The process now to be described is ot' the type usually designated an absorption process.

Gras from one or more wells is subjected to a pressure of 250-300 lbs. per square inch by compressor 4 and cooled in the cooler 5 to approximately atmospheric temperature. The heavier hydrocarbons present, usually contaminated by lubricating oil and by petroleum. suspended in the gas, will condense and may be withdrawn at 6 for further treatment. The gas, still under pressure, is then passed to the water-cooled absorber 7, where it is brought into contact with an oil of the kind commonly employed for this purpose. The undissolved gas is passed trom the absorber to the main 8'while the oil with its dissolved hydrocarbons fiows to vent-tank 9 where the pressure on the oil is released by a suitable regulator`9a.

In tank 9, a pressure of about 6() lbs. per square inch is maintained and as the oil undei` high pressure flows into it, a large volume of gas comprising the lighter dissolved constituents is liberated. Using a natural gas of the following composition:

Methane 78.3% Ethane 13.3% Propane 5.6% Butane and less volatile bodies 2.8%

the composition of the gases so liberated may be, for example, as follows:

convenience, as high-pressure vent gas.

From tank 9 the oil with the substances dissolved therein passes to vent tank 10, in which 3 to 4 lbs. gage pressure may be maintained. rlhe pressure on the oil is released -by a suitable regulator 10a. The reduction in pressure causes an additional evolution 'ot' gases which comprise, in general, hea-vier hydrocarbons than those which constitute the high-pressure vent gas but which are in general of greater volatility than the constituents which remain dissolved in the oil. The analysis of the gas liberated in tank l0 may be, for example:

Methane 14.6% Ethane 36.0% Propane 36.4% Butane and less volatile bodies 13.0%

and this gas will be termed low-pressure vent gas.

' From tank l0 the oil passes to heat exchanger 11 and thence to still l2 where it is blown with live steam, the result being that practically all the substances remaining in the oil are expelled. The mixture of steam and hydrocarbons formed in the still passes to a condenser 13 maintained at a temperature somewhat below the boiling point of water. In this condenser most of the aqueous vapor is condensed andl is withdrawn through trap 14, but the hydrocarbons remain in vapor state as does also `a considerable amount of water. The oil fr om still 12, after being freed from dissolved substances, returns through heat-exchanger 11 to absorber 7 after being'cooled by water.

The hydrocarbon vapors pass from condenser 13v to the precocler 15, where they are cooled to substantially atmospheric temperature with the condensation of the heavier hydrocarbons present and most of the remaining water. rlhe liquid formed in the precooler is drawn off into storage tank 16, where the water is separated froniit in any suitable way. The liquid in tank 16 will be referred to as' the precooler liquid. lt may have, for example, the following `composition: v

By Weight. Ethane 0.`1% Propane 2.0% Butane 9.0% Pentane and less volatile bodies 88.9%

Vapore not condensed in the precooler pass to a compressor 17, where their pressure is raisedto about 50 lbs., and then to another cooling unit 18, where ,they are again brought to atmospheric temperature. r`lhis cooling unit is termed the .intercooler.7 The condensed liquid is drawn into the storage tank 19 and is termed the inrcooler liquid. It may have, for example, the following composition:

By weight. Methane 0.1% Ethane 0.7% Propane 10.0% Butane 52.3 Pentane and less volatile bodies 36.9%

By Weight. Methane 0.3% Ethane 2.9% Propane 17.4% Butane 71.0% Pentane and less volatile bodies 8.4%

A portion of the vapors still remain uncondensed by the aftercooler and they are herein termed, for convenience. fixed gases. Their composition may be as follows:

Absorption natural gas gasolene processes now in general use are similar in their general aspect to that outlined above. The low-pressure and high-pressure vent gases and the fixed gases are. ordinarily used only as fuel. The precooler, intercoolcr, and aftercooler liquids are combined to produce thegasolene, the product being highly volatile and requiring av further treatment in order to fit it for use.V This is accomplished by the ynatural weathering process already referred to. wherein the wild product is stored in tanks .at low pressure until a portion of it -has evaporated. By such procedure there is a total loss of the ethane, propane, butane and higher hydrocarbons which evaporate during weathering, while the butane inthe highand low-pressure vent gases and the fixed gases is not incorporated into the gasolene produced.

ln accordance with my invention, substantially the entire quantity of butane which dissolvesin the oil in the absorber 7 may be worked into gasolene, while the ethaneand propane which escape to the air in the natural weathering process may be recovered. The manner in which this may be accomplished is illustrated as follows, reference being had to Fig. 1 of the drawings: The precooler, intercooler and aftercooler liquids,

the highand low-pressurel vent gases, and the fixed gases,are introduced at appropriate points linto the rectifying column 23, ,the most volatile 'mixtures being 'introduced nearest the top and those most ments, means being provided for the downward flow of liquid and the upward passage of vapors, and the column is fitted with devices for drawing off water and for testing and regulation which need not be described here. A

Connected to the base of column 23 is a4 vessel 24 adapted to receive the liquid leaving the bottom of the column. The vessel 24 is provided with a heating coil 25 and a vapor pipe 26 is arranged to lead vapors evolved in vessel 24 into the column 23. The liquid from the column passes through trap 27 to the vessel 24.

As shown in Fig. 1, the various mixtures are introduced into column 23 in the following order, beginning at the top: High-pressure vent gas, low-pressure vent gas, fixed gas, vaftercooler liquid, intercooler liquid and precooler liquid. The last-named contains only small amounts of hydrocarbons more volatile than butane, and requires but little rectification; while the high-pressure vent. gas, having only 4.3% of butane, requires the most thorough treatment. The highand low-pressure vent gases are compressed'and cooled sothat they enter the column in liquid phase, as subsequently described. The expansion of the mixtures into the upper part of the column absorbs heat, while the temperature at the foot of the column is kept up by Warm vapors entering through pipey 26. The temperature gradient necessary for rectification is thus maintained. The gasolene is withdrawn from tank 24 at 29, continuously or at intervals.

The high-pressure vent gas and low-pressure vent gas pass through heat exchanger 31 on their way to the column, and therein are cooled, giving up their heat to the gases from pipe 28 which iiow through exchanger 31 in heat interchanging relation to the vent gases. The high-pressure vent gas may be compressed in compressor 32, before it enters the heat exchanger 31, to such a pressure that there will'be a partial or complete liquefaction of this gas as it passes through the expansion valve 34 and enters the column 23. The pressure in column 23 may advantageously be about 9 lbs. gage and may be maintained by the regulator 30. The pressure in vessel 24 is, of course, somewhat higher than at the top of the column because of the head of liquid on the trays 23a. The pressure before expansion may be 800 to 1000 lbs. per square inch, depending on the proportion of liquefaction desired and the eiiiciency of the heat exchanger 31.

In a similar manner the low-pressure vent gas is compressed in compressor 33 to a pressure which may be 600 to 800 lbs. per square inch before it enters the heat exchanger and is expanded from this pressure to column pressure through valve 35 wherein a partial or complete liquefaction may occur` The fixed gas from the aftercooler may likewise be further compressed before expansion but in general this Will be unnecessary to obtain the desired liquefaction at the expansion valve. The gases escaping at the top of column 23, consisting of constituents excluded from the gasolene, pass through the regulator 30 and may be used as fuel or for any other purpose.

The control of the process is very simple. If it-is desired to change the quality of the final product, this lmay be conveniently accomplished by varying the rate at which heat is supplied to vessel 24, the pressure in the system remaining unchanged. More heat supplied will obviously result in the evaporation of more material in unit time, and more vapors are released at 30 to keep down thepressure. Since a larger proportioai of material then enters the vapor fraction, the liquid fraction Will decrease in quantity, and since the hydrocarbons escaping at 30 are the most volatile of those entering the column, the volatility of the liquid fractionwill be decreased. Furthermore the vapor pressure of the liquid product can be accurately predetermined from the condi.- tions under which the column is working, namely, the pressure and the temperature ofv the vapors passing through line 20. These vapors may be maintained, for example, at a temperature of from to 90o F. In a particular instance when the vapors in pipe 26 had a tempera-ture of 83 F., the material produced had a vapor pressure of 8 lbs. at 70o F. and a vaporpressure of 14 lbs. at 100O F. By increasing the temperature of the vapor entering the column, the above figures can be considerably reduced, but With a decreased yield'of material. The composition of the vapor fraction can of course be regula-ted as easily as that of the liquid, and by the same means.

While the above description shows the most complete utilization of all the gasolene constituents removed from the natural gas by the absorbing oil, in many instances other embodiments of my invention will recommend themselves on the grounds of their simplicity. A preferred embodiment of a simplified form of the process in which the liquidl condensates alone are treated will now be described in connection with Fig. 2 of the accompanying diagrammatic drawing.

As shown in Fig. 2, the rectication treatment in column 23 is limited to the three condensates above described, namely, the liquids obtained from the precooler, the intercooler and the aftercooler. It Will be understood that the natural gas is preferably subjected to preliminary treatment similar to that heretofore described in' connection With Fig. 1 and that these condensates contain the major portion of the hydrocarbons less Volatile than propane originally present in the gas.

The hydrocarbon vapors pass through trap 14 into precooler 15, which is cooled to substantially atmospheric temperature. The heavier hydrocarbons and most of the aqueous vapor condense in this cooler and flow into storage tank 16. The Water may be Withdrawn from the tank in any suitable Way. The 'residual gases are then successively compressedA and cooled by pumps 17 and 20 and condensers 18 and 21, substantially as described in connection with Fig. 1.

The iixed gases from the aftercooler condenser 21 are Withdrawn through valve 22 Lasarte for any desired utilization. The condensates are introduced into the column 23 at points determined by their approximation to desirable gasolene mixtures, as previously de` scribed. v y This form of the process is characterized by the return of the vent gases from the column 23 to the circulation syste-m. rEhese ases pass through pipe28 with its reguator valve 30, and into pipe 38 which conducts them to the gas stream entering precooler l5. The purpose of the recirculation ofases leaving the column is to provide a su cient volume of liquid condensates, particularly in the intercooler 18 and the aftercooler 2l, to maintain .the desiredv tempera.- ture gradient in column 23. By this simplilied modification, the butane and less Volatile hydrocarbons contained in the highand low-pressure vent gases and those contained in the fixed gases leaving aitercooler 21, do not enter the gasolene, but this does not involve a serious loss as will be apparent from the analyses given above.

Besides the absorption process, there is another principal type of natural gas gasolene process, frequently styled the compression process, which is simpler in its general execution in that it does not involve an absorbing medium. in this process, the condensation is eected by compression and cooling. The compression is usually accomplished in several stages and the cooling takes place between stages by means oi2 suitable coils, condense'rs, etc. In this process the condensates are formed in the coolers after each compression stage and correspond very closely in theircomposition and further treatment to the condensates obtained in the absorption process described above, although in some cases there is no fraction corresponding to the precooler liquid. In

the simplest case in which there is but one stage of compression, the condensate corre-- sponds most closely to the intercooler liquid above. In the execution of my process there is no fundamental difference bei tween the treatment of the liquid fractions obtained in the absorption process and those obtained in the compression process.

The product of my cases, diEer from natural gas gasolene, prepared by prior process, in having relatively high gravity on the Baume scale, varying from 87 to 92 B., and at the same time having a vapor pressure of from 10 to '12 lbs. at 100 F., which pressure is characteristic of ordinary natural gas gasolene of gravity from 72 to 84 B. t t

While I have described'my invention in connection with certain particular ways of producing a series of mixtures containing the heavier hydrocarbons of natural gas, and in connectionwith certain series ofinixtures of particular composition, this is done invention may, in some ment of any fixed number of the fractions produced in .the preliminary treatment of the gas, but may treat only such of these as the circumstances in a particular case require. Any fraction which contains but little gasolene constituents may be discarded or utilized in other connections, while any fraction which is an acceptable material as a gasolene or otherwise, without further treatment, neednot go to the 'rectifying process. My invention is limited only by the appended claims.

I claim:

l. Process of forming a liquid of predetermined vapor pressure from a mixture of natural gas hydrocarbons, which comprises separating from the mixture a fraction enriched in the heavier hydrocarbons, 4compressing the fraction in a irst compression step to liquefy a portion thereof, compressing the unliqueied vapor to a higher pressure in a second compression step to liquefy a further portion, simultaneously -rec'tifying the two liquefied portions ina single rectiying column, whereby a liquid and a vapor are reduced, withdrawing vapor at the top of t le column, and adding the withdrawn vapor to the vapors passing to the first compression step.

2.' Process 'of'forming a liquid of predetermined vapor pressure from a mixture o intermediate its top and bottom, separating the ractionlby 'rectiiication into a liquid portion and a vaporous portion, withdrawing said vaporous portion at the top of the column, compressingat least a partpf the portion thus withdrawn, and expanding the same into the rectifying column.

3. Process of forming a liquid 'of predetermined vapor pressure from a mixture' of natural gas hydrocarbons, which comprises separating from the mixture a fraction having a higher vapor pressure than that desired in the final product, and rectifyingthe said fraction to produce therefrom a 4vaporous portion and a li uid portion, the rectication being solcon ucted that not more than one hydrocarbon of said lfraction will occur in substantial quantity in both the vaporous and liquidl portions.

4L. Process of forming a liquid of predebringing the mixture While under pressure into contact with a solvent tov absorb constituents of the mixture, passing the solvent to a region of lower pressure to liberate a first fraction of the absorbed constituents therefrom, liberating fromV the solvent a second fraction of the residual absorbed constituents as a material having a higher vapor pressure than that desired in the inal product, and simultaneously rectifying the two fractions in a single apparatus.

5. Process of forming a liquid of predetermined vapor pressure from a mixture of natural gas hydrocarbons, which comprises bringing the mixture into contact with a solvent to absorb constituents of the mixture, liberating a portion of the absorbed constituents from the solvent, separating from the liberated mixture a. plurality of fractions of different volatility, at least one of said fractions having a higher vapor pressure than that desired in the final product,

and simultaneously rectifying the said fractions in a single apparatus.

6. Process of forming a liquid of predetermined vapor pressure from a mixture of natural gas hydrocarbons, which comprises bringing the mixture into Contact with a solvent to absorb constituents of the mixture, separating absorbed material from the solvent as a fraction having a higher vapor pressure than that desired in the final product, and then rect-ifying the said fraction.

7. As a new material, a liquid having the characteristic composition and boiling curve of a natural gas gasolene, containing more butane than petroleum gasolenes, and being free from propane. y

8. A natural gas gasolene having a density not less than 87O B. and a vapor tension not substantially greater than 12 lbs. at lOO-O F.

l 9. As a new composition, a liquid such as can be prepared by the hereindescribed proeess comprising removing from wild natural gas gasolene a vaporous fraction by means of a rectification treatment so conducted that not more than one hydrocarbon occurs in substantial quantity in both the fraction removed and the residue.

ln testimony whereof, I affix my signature.

HAROLD E. THUMPSON.

Images