US2134701A - Separation of hydrocarbons - Google Patents

Description

disclosed therein `comprises i i ing gas, from which stantial pressure and u feed gasV difliculties are `Patented Nov. 1, 1938` PAT-ENT ori-icsN SEPARATION OF HYDROCARBONS i Oswald C. Brewster,

Litchfield, Gann., assignmto Refinery Engineers. Inc., f v a `corporation of Missouri New York, N. Y.,

u u Application September Z9, 1936, Serial No. 103,123

` z claims. (01.' et+-115.5)

This invention relates to the recovery of liqueflable components from hydrocarbon gas and more particularly to improvemnts in the low i temperature recovery of such components whereinutrouble due tothe freezing of water and water vapor associated with the gas is avoided andat the same time more efficient `heat exchange is effected.

In a copending application, Serial No. 103,121

filed September` 29, `1936, a novel method for the recoveryof. valuable liquid components of hydrocarbon gas hasbeen fully disclosed.` The method the steps of expandthe liqueable i `components, `have been stripped, through an engine, thereby performing work and cooling the gas to a low temperature according to well-known thermodynamic principles. The cold dry gas Athen `serves as the cooling medium in heat exchangers for chilling the incoming wet feed gas at sub# u condensing therefrom its liqueiiable components. The unliqueiied re- -sidual gas then serves as the gas which is expanded andl chilled in the engine`-` The power generated converted into heat `and is utilized to the condensate recovered.` l l In another copending application, Serial No.

stabilize:`

103,122 led September `29, 1936, a` modification of the above described method is disclosed whereinthe power developed by the engine is utilized not` only toIuInish heat to the stabilizer butto compressthe feed gas or a portion thereof to a higher pressure than that at which it is originally available. u

In the practice of. the methods disclosed in the above named copending applications, temperatures Well below `the freezing point of water are attained and when, as often occurs, appreciable rquantities of `water `vapor are .associated with the encountered due to the formation of ice in the heat exchangers and pipes. `The formation of ice coatings onthe heat conducting surfaces of `heat exchangers results in lowered rates of. heat transfer and when the ac-` cumulation` `is sufficiently greatV may even cause a serious stoppage in thev `ow of gas.` u y u Furthermore, in the practice of the methods disclosed use is made of indirect heat exchange between warm gas `on one side and cold gas on the otherside for the entiretransfer of heat in cooling the feed gas. While entirely practical, the rate of `heat transfer from `gas to gas is low and as a result `large `heat exchange surface is re- 1. i

quired.

vapor form,

by the expanding gas inthe enginel is hydrocarbon gas; cooled by expansion are also used for coolingthe 45 "by passing the vapors It is an object of this invention to provide a method and apparatus for the recovery of lighter boiling liquefiablecomponents of hydrocarbon gas, and to provide a method and apparatus which minimizes the deleterious effects of water 5 presentin vapor or liquid form in a hydrocarbon mixture,` which mixture is in part or wholly in on the recovery of liquid hydrocarbons from such mixture by a procedure in which temperatures low enough to `freezelthe water 10 occur. i l

According to this invention, water that is so associated with a hydrocarbon mixture, `which mixture is partly' or wholly in vapor form and from which liquid hydrocarbons are recovered `by l5 a procedure in which temperatures low lenough to freeze water therein occur, is largely restrained from advancing to a'point in the recovery system at which occur temperatures low enough to A further feature of this in- 20 freeze the water. vention is that such a hydrocarbon mixture, which is extensively cooled in therecovery of liquid hydrocarbons therefrom, i-prfeliminarily cooled sufficiently to condense water therefrom i in liquid form tol facilitate separation of the water '25 from the hydrocarbons prior to such extensive cooling thereof. A further feature ofthis invention is that in such preliminary cooling the hydrocarbon mixture containing water is flowed countercurrently in contact with cold, liquid 30 hydrocarbons, the gas preferably being flowed upwardly while the liquid iiows downwardly, whereby any ice formed is washed back to a zone of higher temnera.tu`re.` Further features of this invention arefthatin such countercurrent 35 flow hydrocarbonsv are condensed hydrocarbons areutilized` as the liquid hydrocarbon`s--maintained in countercurrent flow with the gas; that the uncondensed gas is expanded to effect `a reduction of temperature 40 thereof; that the residual gases cooledbysuch expansion are tiiized in cooling the liquid'hydrocarbonsflowinginto countercurrent ow with the that the hydrocarbon gases hydrocarbon mixture fed` to the system; and thatthe condensed water is separated in liquid form from the hydrocarbons` and particularly from the liquid hydrocarbons resulting from the cooling caused by the countercurrent flow re1a` 50 tion of gas and cold liquid hydrocarbons. A further feature of this invention resides in` i the recovery of liquid hydrocarbons from mix- `tures thereof. with more volatile hydrocarbons of the hydrocarbonmix- 55 also condensed; that the i ,l l through pipe I. rises l rent to the descending cold condensate and is -lower of tower I for condensate and the separation therefrom of conture in countercurrent-contact with cold liquid ns to `condense hydrocarbons' from the mixture. expanding the uncondensed hydr carbons to reduce'the temperature thereof, absorbing in the cold expanded hydrocarbons heat rfrom theliquid hydrocarbons passing intov said countereurrentrilow. a-further feature-being the condensed use of hydrocarbons as the 'liquid employed in. the countercunent iiow. y

The invention resides in the separate use or conloint use'of any of the features thereof herein described, and is not limited to ,simultaneous use ofalloranyparticuiarnumberofsuchfeatures. The operation of this invention will be fully understood from the following description taken in connection with the which the single cally a preferred embodiment of the invention.

Referring to the drawing, wet

throughpipeiandpassesthroughheatexchanger! wherein itis cooled by indirect heat exchange` with c old dry gas to a temperature above the freesing point cf water. Depending on the :mount of tive dcgree'of saturation of the gas as it enters the system both with respect to liquefiable hydrocarbons and water vapor. a greater or lesser amount of condensate may be formed at this point, the condensate consisting of water or hydrocarbons or both. Prom heat exchanger 2, the .cooled gas andany resultant condensate pass through pipe 8 tocooling tower I. v l may be of any suitable type forthe performance of the required service. A preferred form for this service consists of avertical cylindrical shell I equipped with staggered bailes l around which gasmay risein the tower in a zigzag path countercurrent to a stream of descending condensate cascading from baille to baille andproviding intl'- mate contact between ascending gas and descendspace I is provided in the the accumulation of lcwmpanyingdrawingin densed water. s

Y condensate, free from water, is withdrawn from reservoir J through pipe I and is forced by pump I through pipe Il to heat exchanger Ii in which it is chilled to a low temperature by indirect heat exchange with cold dry gas. From heat 'exchanger II the chilled condensate is conducted through pipe Il. te the top of tower l throughv which it descends, cascading over baiiies I and `returning to reservoir l. 'Ihe gas entering tower in the tower countercur;

chilled thereby, lits higher boiling liquenable com- "ponents being liqueed and Joining the descendlngustream of condensate. By the time the gas has reached the top of tower! it has been chilled to a low temperature and its recoverable. liquenable components have been substantially condensed.so -thatthe gaspassing from tower I throughpipe il is dry gas. Water vapor contained in the gas as it enters the tower is almost entirely condensed therefrom as the gas is chilled in ascending the tower. Where the temperature becomes lower'than the freezing point of water ice crystals form in the condensate stream and are washed downv over the baiiles to reservoir 1. The condensate stream in descending the tower is progressively warmed by the ascending 'gas stream and the rate at which condensate is circulated through the tower is regulated so that ngure represents diagrammaticooling and also on the rela;

its temperature, when close to, but slightly above, the freezing point of water. Thus ice crystals formed in the conden in the bottom of reservoir l and may be decanted oi! through valve controlled pipe I4.

Dryl gas passesy from the top of tower l through pipe I8 toengine Il. In engine il the gas expands polytropically. performing mechanical work. In so expanding the gas is chilled to a relatively low temperature according to well lknown thermodynamic principles. The cold, expanded gas exhausts from engine Il into pipe il and passes through heat exchanger il, wherein it serves as the cooling medium Vfor' chilling the condensate pumped therethrough as already described. In absorbing heat from the condensate in heat exchanger ii the gas is partially warmed. 'lhe partially warmed gas passes from heat exchanger ii through pipe il to heat exchanger 2 wherein it serves as the cooling medium for cooling the wet feed gas passing therethrough. The dry tail gas passes from heat exchanger 2 through pipe il to any desired gas disposal system, as for example, a torch where it is burned.

condensate accumulating in reservoir 1, representing that currently condensed from the wet feed gas, is withdrawn therefrom and further processed as desired to produce the desired product. For example, a stable motor fuel and a lighter fraction consisting of material of lower boiling range than is desirable as motor fuel.

'I'he operation of the engine has been fully disclosed in the copending applications mentioned hereinbefore. Also the. fact that any type of engine utilizing the expansive energy of a ga's is suitable is fullyl explained therein, it being shown ing, or it may even be dissipated by means of a' brake or other device. In copendlng application,

`Serial No. 103,121, cited hereinbefore, the power developed by the engine is converted largely into heat which is used to stabilize the condensate. In copending application, Serial No. 103,122, also cited hereinbefore, the powerdeveloped is used to compress the feed gas, or aportion thereof, to aif'higher pressure than that at which it is originally available inaddition to its partialfconversion into heat for use in stabilizing the condensate. In the operation of this invention the power may be advantageously used as disclosed in the copending applications cited but its use is not limited thereto.V

In accordance with the invention, wet fe'ed gas,

it reaches reservoir 1, isy

reservoir 1. 'Ihe water it may be stabilized to produce containing from one third of a gallon to several gallons of recoverable motor fuel per thousand cubic feet of gas, is introduced into the system in pipe I at a substantial pressure. Preferably this pressure is of the order of one hundred pounds per square inch or higher although a pressure as low as ilfty pounds per square inch may be used. Depending on temperatures and pressure conditions, and on the degree of saturation, there may be associated with the wet gas as much as two percent by volume of water vapor, amounting to almost one pound of water per thousand cubic sl` InA passing is cooled to a temperature substantiallyabove the freezing point of water, preferablyfto va tempera-l ture higher than 40 `degrees Fahrenheit, for `ex ample to `about 60 degrees` Fahrenheit, The gas or water or both, enter the lower partiof cooling tower 4 at this temperature. Condensate being circulated therein is circulatedat such a rate that its temperature in reservoir vspace 1. is close to, but above, the freezing point of water, for example,- the temperature should not fall below about 35 degrees Fahrenheit and should preferably be around 40 degrees l Fahrenheit to insure the melting of all ice washed down to the bottom of the tower. The rate at which condensate is circulated by pump 9 through exchanger Il to the top of tower 4 may vary considerably depending upon conditions. Underl normal conditions the amount of condensate circulated varies `from as little as onehalf pound of condensate per pound of wet feedrgas introduced to'several pounds per pound of feed gas. Under preferred conditions in one specific operation the circulat-` ing rate is from one to two pounds of condensate per pound of feed gas. f The condensate in passing through heat `exchanger Il is chilled toa relatively low temperature, which, depending on conditions, may vary from around() degrees Fahrenheit to around `l00 degrees Fahrenheit. In onepreferred operation of the temperature of the i condensate is around 4--40 degrees "Fahrenheit to -50 degrees Fahrenheit and enters the top of cooling tower 4 at that gas in passing upv through tower 4 is cooled by the o descending condensate and stripped of its liqueable components. It is cooled to a temperature approaching, but not equalling, that of the cold condensate. `For example, in one case where the corndensate enters the tower at --40V degrees Fahrenheit to -50 degrees Fahrenheit the gas is cooled at the top of the tower to -5 degreesFahrenheit to 10 degrees Fahrenheit at which temperature, under the pressure conditions prevailing, it is substantially `stripped of its high boiling components. s Y

The dry gas on leaving. the top of tower 4 through pipe i3 enters at substantially the same' temperature as that at the top of tower 4. On expanding through engine Ii the temperature of the gas is lowered materially, the amount depending on the expansion ratio, i. e., the ratio of the pressure `before ex- `pansion `to that after expansion. The drop in temperature under practical conditions may vary from `eighty degrees orless to one hundred and fifty degrees or more. In a typical operation the ,gas enters the engine at about Fahrenheit and 110 pounds pressure absolute, and is expanded to about pounds absolute. The temperature of the gas exhausting from the en gine `is around -120 degrees degrees Fahrenheit andpasses to exchanger Irl at that temperature and serves to chill the condensate being circulated as described.`

`Substantially all, of the water `vapor contained in the gas is condensed in tower 4. Due to the vapor pressure characteristics `of water the great majority is condensed vbefore the gas` has been cooled `to 32 degrees Fahrenheit and the remainder forms as `small ice particles in the conr 2,184,701` measured atl standard. temperature through` the` indirect i liquid `to a gas, and any cdhdensate formed,eitherhydrocarbon (transfer per unit these conditions is-quite favorable, being almostl twice that obtained when the transfer'is made 15 temperature. The wet other bituminous material.

the intake `oi! engine I5V grees -Fahrenheit of' cold hydrocarbon liquid to a temperature -10 degrees Fahrenheit to -125 "drawing said condensed `water densate stream and is washed to the bottom oi the tower where it melts and is decantedthrough 'It will be observed that a large portion ofthe cooling load isborne by heat exchanger Il, the 5 r remainder being borne by heat exchanger 2. The

heat` exchangevinexchanger il is carried out transfer of heat from a i. e., `heat passesfrom the liquid to themetal surface confining it andthe heat is con" lo ducted* through this ``material and transferred tothe gas on the other side. The rate of heat of exchanger .surface under from gas to gas. This is due to the fact that the `-rate of heat transfer from a liquid to its confining surface is relatively very high compared with that from gas to its confining surface.

' For the purposes 4of control it is desirable to 42() `provide bypasses for a portion of the materials around the heat exchangers. `This control, coupled with the temperature control obtained by varying the rate at which condensate is` circulated permits the operation of the process to 2y;V

be carried out under steady conditions with the optimum recovery of the desired product..`

Other arrangements of the various elements used in Vcar'ryingout the invention are possible without departing from the scope of thisinvention which includes all embodiments thereof falling within themeaning of the appendedclaims. The process disclosed herein is applicable to the recovery of higherv boiling components fromany type of gas containing :such material andimay be applied to natural gas, petroleum gas associated with the production of crude petroleum, refinery still gas produced b y the distillation. cracking or polymerization of petroleum hydrocarbons, and gas produced in the processing of coal,` shale or 40 I'claim: 1. The method of recovering liquefiable higher boiling componentsfrom hydrocarbon gas containing water vapor which comprises cooling incoming hydrocarbon gas containing water vapor undersubstantial pressure in indirect heat exchange relationship with cool dry hydrocarbon gas at a` temperature substantially above 32 degrees Fahrenheit, passing said cooled incoming hydrocarbon gas. in countercurrent direct contaci'l relationship with a cyclic stream of cold hydrocarbon liquid to chill said cooled hydrocarbon gas to a temperature substantially below 32 deand to warm said cyclic stream above 32' degrees Fahrenheit whereby the higher boilingvliqueable components and the water vaporv of said hydrocarbon gas are substantially condensed and join, said cyclic stream of hydro- 00 carbon liquid, expanding the unliquefied dry gas portion of said chilled hydrocarbon gas to perform work and to lower the temperature of said dry gas, passing said warmed cyclic stream of hydrocarbon liquid and conjoined condensed water vapor to a separating zone, separately withvapor,i passing warmed water-free hydrocarbon liquid from said separating zone in indirect heat exchange rela- ,itionship with said cold expanded dry gas to chill 'I0 said hydrocarbon liquidsubstantially below 32 degrees Fahrenheit and `to partially warm said expanded dry gas, returning said chilled hydrocarbon liquid to said first mentioned cyclic stream of `cold hydrocarbon liquid, said partially il warmed expanded said tlrst mentioned dry gas being the dry ges in indirect heat relationship with incoming hydrocarbon gas, and withdrawing hydrocarbon liquid from said separating zone.

3. In apparatus f for ming gas, a prime mover ot -the type tor converting the expensive energy oi' r gu into mechanical work. a conduit leading from the intake thereof, a heat exchanger adapted to receive gas from the ex` haust outlet oi' 'said prime mover, a second heat exchanger adapted to receive gas from said. first named heat exchanger and incoming gas in 1ndlrect heat exchange relationship a Dump adapted to pass liquid from the collecting space of said tower to said rst named heat exchanger, liquid outlet of OSWALD. C. BREWSTER,

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