US2322635A - Method of stabilizing crude petroleum - Google Patents

Description

June 22, 1943. v P c. KEITH, JR 2,322,635

METHOD OF STABILIZING CRUDE PETROLEUM Filed July 21, 1939 zung/f M ATTORNEY Patented June 22, i943 2,322,635

UNITED VSTATES PATENT OFFICE iviii'riiop oF sTABILlzING CRUDE PETROLEUM Percival C. Keith, Jr., Peapack, N. J., assignor to The M. W. Kellogg Co., New York, N. Y., a corporation of Delaware Application July 21, 1939, Serial No. 285,680 In Great Britain July 27, 1938 6 Claims. (Cl. 19d- 11) This invention relates, in general, to the sta- The tendency of certain crudes to attack the bilization of crude petroleum for the removal equipment in which they are stabilized has been therefrom by fractional distillation of dissolved, found to be dependent upon the temperatures normally gaseous hydrocarbons, the presence oi to which they are heated. The tubes of directwhich causes excessive losses by evaporation Cil during the storage and shipment of the crude. of fractionating columns are thus the most rap- In one respect the invention relates, more idly corroded, While the upper portions of the particularly, to the stabilization of crudes concolumns and the reiluxing lines and condensers taining potentially corrosive impurities. are practically immune. By correlating tem- A general requirement for satisfactory stabi- 10 perature with rate oi corrosion, it has been iization of crude petrolum is that propane and found that little or no corrosion will occur in lighter hydrocarbons be removed substantially any part of a crude stabilizer providing the completely. The substantially complete removal crude is not subjected to a temperature above of propane is normally attended by the removal about 540 F. Various theories have been adcrude because of the closely related boiling postulates the existence of non-corrosive suli'ur points of these two compounds. For many purcompounds in the crude which decompose at poses, it is highly desirable to retain a substanelevated temperatures to produce corrosive ones. tial proportion of the butane originally present it is also known that many crudes contain in the crude, and this invention is particularly emulsified brine which hydrolyzes at high temconcerned with a stabilization process for acperatures to form hydrochloric acid. complishing this result, One of the objects of my invention is the pro- 'Ihe known methods of stabilizing crude with vision of a process for stabilizing crude at relasubstantial retention of butane employ a simple tively low temperatures and pressures wherein fractionating system, from which propane and the corrosion eliects are obviated.

head product. Some of the butane content is vision of a process for stabilizing crude at relaallowed to pass overhead also, as the retention tively low temperature and pressure Without exof more than 90% of the butane in the crude cessive loss of the butane content of the crude. makes for high equipment and operating costs, 3(

and is seldom economically justiiied. Stabilizer of my invention will be apparent from the ioloverhead vapors are passed to a water cooled lowing detailed description thereof. condenser, and a relatively high pressure is In proceeding in accordance'with my invenmaintained on the system to enable the contion, the crude to be stabilized is fiactionally densation of a liquid for reflux consisting largedistilled at relatively low temperatures and PreS- ly of propane at a temperature approaching sure whereby corrosion tendencies are minimized that of the cooling water. At the same time and under such conditions that the substantialthe stabilizer bottoms are reboiled at a tempera- 1y complete removal of propane and lighter hyture high enough to drive out all the propane drocarbons from the crude is effected without at the relatively high Ypressure maintained on 40 excessive loss of butane. These results are acthe system complished vby admixing butane with the crude Depending upon the quantities of light hydroto provide suicient reflux for the above fraccarbons present in the crude, previously protionating requirements, recovering the butane posed stabilizing systems have involved presthus added from the overhead vapois, and resures of 200 pounds per square inch gauge and 45 turning it to additional crudeV to be stabilized.

to 650 F. It has been observed, however, that covery of the added butane by `absorption in the stabilization of many crudes under these the Crude undergoing Stabilization Y01 a DOItOn conditions is attended by serious corrosion probthereof, or a portion of the crude which has lems, necessitating frequent replacement or exbeen previously stabilized or substantially depensive items of equipment. Such problems are butanized, and returning the butane to the fracespecially serious since stabilization units are tonating Z011@ in the absorbing oil.

normally located near the source of crude, and In stabilizing crude petroleum by fraction@ hence frequently at remote distances from suitdistillation, pursuant to my invention, I prefable repair and replacement facilities. Y erably employ a reboiler temperature of about 425 to 475 F. Within this temperature range, little if any corrosion will be experienced when stabilizing the majority of crudes, but some may require that slightly lower temperatures be used, while others remain substantially non-corrosive up to as high as 540 F. I maintain a pressure on the system just high enough to permit the condensation of butane from the overhead vapors for use as reflux. This pressure is dependent upon the percentage of butane, propane, and lighter hydrocarbons in the crude, and upon the cooling water temperature. When the preferred reboiler temperature range is adhered to, a pressure of about 100 to 150 pounds per square inch will ordinarily be required. I preferably recover butane from the uncondensed stabilizer overhead vapors by absorption, using as a medium one or more oils derived from the crude itself.

The volatility of these oils is a measure of their efficiency in absorbing butane, and, in accordance with my method I may control the volatility of these oils over a relatively wide range, suiting their properties to .the degree of recovery desired and to the quantity of gas to. be contacted. Specifically, my absorber oils are made up of one lor more of the following materials arranged in the order of decreasing volatilityV because of relative freedom from butane and lighter hydrocarbons and hence increasing efciency in absorbing butane, the proportions of each employed depending upon the properties desired: Y

1. Unstabilized crude.

2. Stabilized crude.

3. Stabilized crude after flashing at a reduced pressure suitable for debutanization.

After the absorber oils have been contacted with stabilizer overhead vapors and have absorbed butane therefrom, they are returned to the stabilizer, whereby the provision of an additional apparatus for stripping The elect of returning absorber rich oil to the stabilizer is to recycle or circulate constantly through the stabilizing zone a sufficient quantity of butane to provide reflux at the top of the stabilizer, whereby additional butane entering in the fresh crude is largely retained in the stabilizer bottoms and withdrawn with the stabilized crude.

When the butaneto be recovered by absorption has a relatively low value, so that a high percentage recovery is not demanded, I prefer to -use unstabilized crude alone as the absorber oil. With this material some butane will naturally be lost by vaporization on the top tray of the absorber, yet an overall retention of better than r75% of the butane fraction'can usually be obtained, and the heat load on the system is minimized.

When a higher butane retention is desired, I prefer to use stabilized crude as the absorbingv medium in the upper portion of the absorber tower, augmenting it, when desired, with a stream of unstabilized cru e introduced near the center. When the utmost yield of butane is required, I divert a stream of hot stabilized crude from the bottom of the stabilizer to a flash zone maintained at a pressure somewhat'below the pressure on the main system; at the lowered pressure, substantially complete removal of butane from the stream, that is, debutanization, can be eliected, and the debutanized oil is of the absorber to scavenge the last traces of butane unabsorbed by more volatile absorbing medium Yintroduced at awlower'point in the abpurposes is obviated.V

then passed to the top sorber. The vapors flashed from the stabilized crude to obtain clean-up oil are condensed and pumped to the base of the absorber, where they reunite with the crude from which they came.

It will be understood that an infinite variety of absorber oils can be made from the three components available, so as to satisfy to a nicety any required butane retention.

Referring now to the annexed drawing which illustrates one form of my invention, fresh unstabilized crude is withdrawn from any suitable source through line l by pump 2 and all or a portion thereof is passed through line 4 to heat exchanger 5 where heat is transferred to it from hot stabilizer bottoms. In the event economic considerations permit, a portion of the unstabilized crude may be diverted through line 3 for use as van absorbing medium, as will be hereinafter described.

Preheated unstabilized crude from exchanger 5 passes through line 6 and is joined by a stream of enriched absorber oil from line l. After joining, the two streams pass through line 8 to the center portion of the stabilizer 9 which is fitted with a plurality of vapor-liquid contacting elements such as conventional bubble trays. Heat is supplied to the base of the stabilizer 9 by means of a reboiler lil through which is passed a heating medium. Since it is important that no portion of the crude be heated above the chosen safe reboiler temperature, I prefer to use high pressure steam as the heating medium in reboler lil, since it is thereby possible to avoid excessive temperatures in the oil film immediately adjacent to the external heat transferring surface. By utilizing the latent heat of condensation of the steam, reboiler duty can be supplied at a precisely controlled constant temperature, which is not the case when direct-nre reboilers are used.

In passing down over the trays in the stabilizer, the crude is substantially freed of propane and lighter hydrocarbons, which pass upwardly together with other volatile constituents such as butane. A portion of the vapors reaching the top of the tower are condensed and return to the top tray as reux,v as for example, bymeans of the internal cooling coil Il through which a stream of cooling water isV passed. The temperature and pressure at the top of the stabilizer are such that all of the propane and lighter hydrocarbons present in the unstabilized crude remain uncondensed and pass overhead through line l2, together with a constant quantity of butane'which is recycled to the stabilizer in the enriched absorber oil. Sufficient reux is mai tained by the removal of heat to cooling coil H as to prevent the escape of more than the desired percentage of butane present in the unstabilized crude. The remaining butane is carried back down through the stabilizer as reflux condensate, and remains in the stabilized crude withdrawn from the bottom.V Stabilizer overhead vapors pass through line 2 to the base of an absorber i3 which is also fitted with bubble trays or the like.

The` three types of absorber oil which may be availed of for the purpose of recovering and recycling butane, are proportioned in accordance with the allowable loss of butane from the crude, and may be prepared and utilized as follows: Un-

stabilized crude is diverted from the charging Y stock stream through line 3 and passed to the absorber directly. Stabilized crude' is diverted from the main product stream leaving the process, and passed through line i8 to the absorber. Flashed stabilized. or debutanized crude is obtained by taking a portion of the hot stabilizer bottoms through line I9 to a low pres'surezone 20 where a vaporous fraction is separated from it. The material remaining unvaporized at the reduced pressure is Withdrawn through line 2l and cooled, for example, by heat exchange with enriched absorber oil in exchanger 22. It then `passes through line 23 to additional cooler 24,

and thence through line 25 to pump 26 which delivers it through line 2l to the absorber. For the sake of simplicity, the low pressure separator 20 has been shown as constructed integrally with the absorber I3. Obviously, however, a separate vessel could be used. The distillate Aflashed from the stabilized crude in the separator 20 is withdrawn therefrom through line 30 to condenser 3| and line 32 to accumulatorv33 whence it is Withdrawn through line 34 by pump 35 and delivered through line 36 to the base of the absorber.

Lines 3, 2l, and I8 carrying the three absorber oils are suitably maniolded, as shown, so that all or a portion of any one` of them may be delivered through lines 28 and 29 to either the middle or the top of the absorber respectively. The points of introduction of the various oils are determined as hereinbefore explained by the requirements as to butane retention which must be met. One or more intercoolers may be provided to remove heat from the absorber oil as it flows downwardly through the absorber, as exemplified by the intercooler 40.

In passing upwardly through the absorber I3, the stabilizer overhead vapors entering through line I2 are stripped to a relatively 10W butane content so that the gases leaving the top of the absorber through line 4I contain, in most instances, not more than approximately 5 to 25% of the butane originally present in the unstablized crude, according to the loss permitted by economic considerations. Enriched absorber oil containing butane to be recycled is withdrawn from the base of the absorber I3 through line 3'I together with condensed distillate from separator 20. This stream is passed by pump 38 through line 39 to exchanger 22 against flashed stabilized crude and thence through line 1 to join line 6, re-entering the stabilizer through linev 8.

Stabilized crude not set to the separator 20 for fiashing is passed through line I4 to heat exchanger 5 against incoming fresh crude and thence through line I5 to a final cooler I6. Continuing through line I1 a portion is diverted through line I8 for absorption purposes while the remainder is passed to storage or shipment through line 42.

It is to be understood that the arrangement of exchangers, coolers, condensers, and refluxing means, shown in the drawirm, is by Way of example only, for numerous other arrangements could be employed without departing from the essentials of the invention. For example, the cooling coil II could be replaced by an external condenser and separator interposed in the vapor line I2, with reflux return to the top tray of the stabilizer from the separator by means of a pump; also, the reboiler I0 could be supplied with the vapors of any substance having a suitable boiling point, or even with a liquid heating medium such as hot gas oil provided means for precise control of temperature Were used.

The following example will illustrate further the manner in which my invention may be practiced:

A 43 A. P. I. crude petroleum was to be stabl- 75 lized for the retention of approximately percent of its butane content, `and was known to become corrosive when heated above about 470o F. These circumstances, coupled with a summer cooling water temperature of F., made it desirable to use ashed stabilized crude as the absorbing medium in the top of the absorber, augmented by a stream of unstabilized crude introduced ata somewhat lower point.

The stabilizer tower, containing I25 bubble trays, was operated at a pressure of pounds per square inch, a bottom temperature of u F., and a top temperature of 105 F. The reboiler in the bottom of the stabilizer was heated with high pressure steam. About 121 mols `of butane entered the system per hour with the fresh crude, and 108 mols per hour left the system in the stabilized crude, leaving 13 mols per hour as loss in the absorber tail gas. In contrast with these quantities, approximately, 630 mols per hour of butane left the stabilizer as overhead vapors, to be returned thereto in the enriched absorber oil.

Stabilized crude was i'lashed at 400 F. and 40 pounds absolute pressure to provide a 3 A. P. I. lean or debutanized oil which contained less than 1.0 mol percent butane.- 4600 gallons per hour of this oil, supplied at the top of the absorber, plus 11,400 gallons per hour of unstabilized crude, supplied to the middle of the absorber, were sumcient to recover the recycled butane. Two intercoolers were used and the ab1 sorber was tted with 48 bubble trays.

From the foregoing it will be apparent that the process therein described accomplishes the objects of my invention of providing a process for the stabilization of crude with substantially complete removal of propane and lighter hydrocarbons at relatively low temperatures and pressure whereby corrosion eiTects are minimized and at the same time excessive loss of butane from the crude is obviated.

What I claim is:

l.. A process for stabilizing crude petroleum removal of propane and lighter hydrocarbons relatively low temperatures and pressures for by at lighter hydrocarbons under suitable conditions whereby the butane is largely absorbed in the crude and the lighter hydrocarbons separated, fractionating said butane-enriched crude into a bottoms product, constituting the desired stabilized crude, said product being substantially free of propane and containing butane in amount constituting a substantial proportion of the butane-content originally present in the crude, and an overhead vaporous product consisting of a mixture of hydrocarbon vapors including butane and lighter hydrocarbons, the quantity of butane therein being not less than that absorbed by the crude from said mixture of hydrocarbon vapors, enriching the butane-content of an additional poru tion of crude to be stabilized by contacting said portion with said overhead vaporous product under suitable conditions whereby the butane is largely absorbed in said portion and the lighter hydrocarbons separated, and continuously repeating said cycle of steps.

2. A process for stabilizing crude petroleum by removal of propane and lighter hydrocarbons at relatively low temperatures and pressures for the minimization of corrosionrand without excessive loss of butane, which comprises enriching the butane content of a portion of said crude by contacting said portion with a mixture of hydrocarbon vapors including butane and lighter hydrocarbons under suitable conditions Whereby the butane is largely absorbed in said portion and the lighter hydro'- carbons separated, mixing saidV butane-enriched portion with a second portion of said crude and fractionatingfsaid mixture into a 'bottoms product, constituting the desired stabilized crude, said product being substantially free oi` propane and containing butane in amount constituting a substantial proportionV of the butane l content originally present in the crude, and an overhead vaporous product consisting 'oi' a mixture of hydrocarbon vapors including lontane and lighter hydrocarbons, the quantity of blitane therein being not less than that absorbed by said first mentioned portion of crude from said mixture of hydrocarbon vapors, enriching the butane content of an additional portion of crude t0 be stabilized by contacting said portion with said overhead vaporous product under suitable conditions Whereby the butane content of said product is largely absorbed in said portion and its lighter hydrocarbons separated, and continuously repeating said cycle of steps.

3. A method of stabilizing potentially corrosive sulfur-bearing crude petroleum including the steps of introducing said crude and recycled butane into a fractionating column, operating said column at a relatively low 'bottom temperature and under such Va pressure that butane in amount equivalent to said recycled butane and a'minor proportion of the butane originally present in said crude, and substantially all hydrocarbons lighter than butane will distill oi as vapors; partially condensing said vapors and refluxing said column with the condensate, recovering butane from the uncondensed portion of said vapors, and recycling it to said column as aforesaid, whereby said crude is stabilized without the employment of corrosion-promoting temperatures and with retention therein of a major proportion of the original butane content thereof.

4. `A method as in claim 3 including the steps of recovering butane from said uncondensed vapors by absorbing it in crude petroleum to be stabilized, and recycling recovered butane by introducing the thus-enriched crude petroleum into said fractionating column.

5. A method as in claim 3 including the steps oi recovering butane from said uncondensed gases by absorbing it in stabilized crude petroleum, and recycling recovered butane by introducing the `thus-enriched stabilized crude into said fractionating column.

6. A method as in claim 3 including the steps of recovering butane from saidl uncondensed gases by absorbing it in debutanized crude petroleum, and recycling butane by introducing the thus-enriched absorbent medium into said fractionating column.

PERCIVAL C. KEITH, JR.

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