US2453205A - Controlled fractional distillation of petroleum - Google Patents

Description

Nb'v. 9; 1948. P. DOCKSEY 2,453,205

CONTROLLED FRACTIONAL DISTILLATION OF PETROLEUM F iled Jan. 26, 1945 2 Sheets-Sheet 1 l l 1 I f a b c Nov. 9, 1948. P. DOCKSEY 2,453,205

CONTROLLED FRACTIONAL DISTILLA'IION OF PETROLEUM Patented Nov. 9, 1948 CGNTROLLEDFIQACTIONAL DISTILLA'IION 0F PETROLEUM Patrick Docksey, 'Sunbury-on Ihames, England, assignor to Anglo- Iranian Oil Company Limited, London, England, a British joint-stock corporation Applie'ationJanuary'26, 1943, Serial No. 473,608- In Great Britain January 28, 1942 This invention relates to the production of closely out high grade fractions of petroleum distillates and in particular the production by continuous distillation of narrow boiling range fractions boiling in the gasoline boiling range from which high grade aviation or motor fuels may be producedby'blending. I

It is usual to control the operation in fractionating columns by varying the amount of reflux so as to maintain a constant temperature at the column head. Precision in fractionation is not however possible by such controlof column-head temperature where the boiling points or boiling ranges of the overhead and the other constituents of the material to be fractionated are separated by small temperature -diilerences.

It is nevertheless in some cases possible, as for example in the separation of n-pentane from isopentane, to effect a separation by temperature control if the control point be located in a selected tray in the column whereby small changes in composition of the products result in corresponding larger changes in tray composition, and hence in temperature. Thus in known manner change in tray composition may be used for temperature control of the overhead in such cases.

The invention has however among its objects to separate constituents of petroleum distillates closely related inboil-ing points, by greater preci'sion in fractionation than is possible by mere temperaturecontrol at the column head, to yield closely cut high grade fractions of high octane number.

Thus the principle upon which the invention is based is the control of the quantity of one of the product streams withdrawn from the fractionating column as a fixed proportion of the feed, the quantity of the product withdrawn being determined by analysisof the feed. Only one of the twoproduct streams may-be controlled in this way, that is to say, either the overhead product orthe bottoms product. The'other product is withdrawn from the column at such a rate as to :avoid the. accumulation of material in the column.

Where the composition of "the feed may vary from time to time as indicat'ed by an'alysis of the products, small adjustments of the controls applied-to the column may be made in the course 1 tained, the heat'supplied to the column, the reflux and the'liquid level in the 'base of the column are constant.

1 Claim. (Cl. 19671) The invention is diagrammatically illustrated in the accompanying drawings.

Figure 1 illustrates a 'fractionating column in which the overhead is the product stream which is volume-controlled.

Figure 2 illustrates a 'fractionating column in Y which the bottoms product is the stream which is volume-controlled.

Figures 3 to 10 are diagrams hereinafter re- 'ferred to. I

The production of a narrow cut usually involves two steps of continuous distillation, in one of which separation is made between low boiling material and the desired narrow boiling range cut, and in the other between high boiling material and the narrow boiling range'cut. The invention may be applied to one or both of these operations. I

The perfection of separation obtained in the fractionating column depends firstly on the characteristics of the column, namely the number of theoretical plates and the reflux ratio, and secondly on the control of operation. The effect of thesefactors is illustrated by the following brief analysis of the process, made with reference to Figures 3 to 10 of the accompanying drawings. Fractionation of a two-component mixture, the components being designated A and B, and

the mixture being such'th'at a: gal/hr. of A and y gal/hr. of 13 giving a total of zc+y gal/hr. of the two-component mixture is fed to the column, the following three cases may be considered.

Case 1.Pe1"fect operationPerfect column The conditions for this operation are shown in Figure -3 and it will be seen that the two components are produced in a state of absolute purity. It will be realised that in order to accomplish this result a column having an infinite number of theoretical plates and working at very high reflux ratio will be necessary. In practice neither of these requirements is obtainable, and it is necessary to accept the limitation in separation imposed by a practical column. This point with perfect operation gives us Case 2.

Case 2.-Perfect -operatio-fi--Practical column The conditions for this operation are shown in I "Figure'4. The'char'acteristics of the column are such that when feeding m gal/hr. of A+y.gal./hr. of B the distillate consists of (.'lI- oc) gal/hr. of A and a gal/hr. of B, and the residue consists of a gal/hr. of A and (1IJ-oc) gal/hr. of B. The degree of imperfection in the operation is measured by the quantity a.

' of Alirlthe residue.

Case 3.Imperfect operation-Perfect column half of the period of operation a:fi gal. are produced. The situation during the first half is illustrated in Figure 5 and during the second half in Figure 6. The total effect of the operation is obtained by adding together the distillates and residues produced in Figures .5 and 6, and the net effect is shown in Figure 7. It will beseenthat:

a measure of imperfect fractionation has been introduced which is indicated by the appearance gal. of Bin the distillate and the same quantity It will thus be seen that part Ofllhe im erfection in the separation isproduced by the practical characteristics of the column and part by the imperfect method of operation. In practice the actual degree of separation obtained is a combination of these two effects. n

The foregoing analysis has beenconfined to the simple case of a binary mixture. In practice close out fractions are produced from feed material containing such a wide range of components that it is not practicable to specify separation in terms of the component hydrocarbon and instead recourse is possible to the true boiling point-curve of the mixture Forexample Figure 8 shows the true boiling point curve of a mixture from which it is desired to separate a narrow boiling range .c'ut, containing substantially only that material (b) Required narrow boiling range fraction equivalentto the range P- Q gal. 1

(0) Heavy ends equivalent to the range Q-100 gal.

- To produce these three materials requires two operations, in one of which the cut is made to TP C. and the other at T C. I The cut point .aimed at in the first operation may beeither f these two, the choice being made from considerations which are immaterial to the present invention. Assuming that it is decided to cut first 'at To" C. so as to produce adistillate equivalentto the range 0-62 gaL, and residue equivalent to the range Q100 ga1.. With perfectoperation and a perfectcolumn-there will be no material boiling above T C. in the distillate, or below TQ C; in the residue. When using a practical column or imperfect operation or both, this will not be obtained, andthe quantity of material boiling above To" C. in the distillate and below T C. in the residue will be a measure of the degree of imperfection of the separation. These quantities are customarily referredto as the overlap and underlap respectively. Theresult of vcarrying out the operation in practice may be illustrated graphically by Figures 9 and 10. Figure 9 shows the true boiling point curve of the distillate and Figure 10 that of the residue. In both cases the true boiling point curve of the original feed in the cut point region is indicated by a dotted line. Figure 9 has been drawn to cover a range of 0-62 gal. and Figure 10 to cover a range of 62-100 gal. The imperfection of separation is measured on these diagrams by the amount of material boiling above "TQO C. in Figure 9 and the amount boilin below TQ C. in Figure 10, these amounts being QN gal. and RQ gal. respectively; These two quantities are the overlap and underlap respectively, and in a practical case their magnitude is due partly to the characteristics of the column and partly to the imperfection of operation.

.It is the purpose of the invention to minimise the efiect due to the imperfection of control, so that the degree ofseparation obtained with any given practical column will approach more closely to that which is theoretically obtainable.

As illustrated in Figures 1 and 2, the petroleum distillate to be fractionated may be fed tothe fractiorl'ating column e from any one ofthree feed tanks a, b, c that are used successivelyor alternatively. Thus for examplawhile the feed tank a is being charged withth'edistillate to be fractionated; the distillate in the feed tanlgb is being stirred to ensure uniform composition and analysed, and the tank 0 contains distillate which has already been analysed and isready to be fed to the column. Thus the feed is in'this case delivered by thefeed pump d to the fractionating column e from the tank c. Thus by the time the tank 0 is discharged, the distillate in the tank b will be ready for feedingto'the column e, and the tank 0 may then be re-cllarged with distillate; The distillate after analysis may however be otherwise fed to the fractionating column. I

Vapour is supplied at the bottomof thefractionating column e by a re-boiler m, and the bottoms productis delivered 'by the pump oto storlage, after passing throughthe cooler p.

"The overhead vapour from the fractionating column e passes to the condenser f whence the liquid condensate flows to the intermediate storage or reflux tank g. From this tankthat'part of the overhead productto be withdrawn is passed to storage by the overhead pump Z, while theremainder ispassedback to'thefractionating column by means of the reflux pump it.

It will be understood that the quantity of the reflux liquid is dependent upon heat supplied to the reboiler m; v

When the product st'ream to be volume-controlled is that of the overhead product, the plant conditions are as indicated in Figure :1 and as the amount of hereinafter described The feed pump 11 is set'to deliver feed fromthe feed tank at a constant rate; which r'nay bacontrolledby a fiow controller of conventional'type.

.The appropriate feed rate is dependent on the draw overhead product. from the tank g at a constant ratefand, may be controlled .by aLflow controller. The rate 'of withdrawal is determined from an" analysis of the feed, which determines .iiea 11 I :t he'proportion. of. theoverhead product it is desirodtoprnduce, and. the feed rate.

The quantity of steam to the re-boiler m. is maintained at a constant value determined to ivethemequired vapour. loading in the column.

The bottoms product withdrawal pump is operated under the. control of a level controller n coluninbase frornthe bottom tray of the column,

and-the. vapour. entering the bottom tray of the column from the re-boiler m. Boththese quan- .tities, are independent, of the bottoms pump 0,

the former being. determined by the quantity of reflu -r turned y. t e ump it, and the latt r by the steam admit ed. to the re-boiler m.

The levelv in. the reflux tank g must be controlled within narrow limits, since at any given mom nt the e overhead product mad yt column is. the difference between the overhead nanour, passing. to the condenser f, and the liquidlretunnedto the. column e by the pump it. If the level in thereflux tank is rising, this difference is increasing, while if the level in the reflux tanleis v falling. this difierence is decreasing, as comparedwith average difference between these two. q antities- Such va iations of level are t us an indication. of corresponding undesirable variations in the quality of the product. However, the presence oi? large quantities of material both the re u tank. a and on t e trays. in t e collieh-tsn lzt smooth o t e e r ations in q rity, and hence, the egree or accuracy with which the evelcqntroller frmust operate is not as great would. at first pear a d the. on of eve 2 onventi na pe o avera in level conl r set. to work. betwe n he ma est. pramils isdsqu t not. essential that, the. pumps or other ms Pr vi ed r uppl the ee st to th lwen. and Wither w nsth c n r l o sci iron th column ha l be n rul They may be operated to give a delivery at a constant rate under control by flow-controllers or meters or by. hand cont rol. It is however possible to la reg la e the. um th e for ample of a flow ratio controller. Alternatively there, maybe used positive displacement pumps operating at controlled speeds, displacement pumps. with variable stroke operated by a single meter, ora metermay be used for each stream in becontrolled and the flow maybe maintained constant by hand-control or by controllers applied to each stream. Or again a flow ratio controller may be provided that comprises two orifice meters adapted to be disposed one in each stream and so connected together that the ratio of the rates of flow in the .tWQst IQams is. maintained constant.

It is advantageous that the withdrawal of the product, the volume. of which is controlled, shall be. efiected froma body of the product, the liquid levelor head. of which is maintained constant.

"lhus where. the overhead product is controlled,

the quantity. thatiswithdrawn and is maintained constant may be taken from a tank, the liquid level in which may serve for the operation of a level controller determining the return of reflux to the column; while the bottoms product may be withdrawn under control of a level controller operating according to the liquid level to be maintained at the base. of the column.

When the controlled product is the bottoms product, the withdrawal of the overhead product from the tank may be effected by a level controller, while. the reflux may be supplied from the tank at a constant rate and the withdrawal of the bottoms product may be effected at a. constant rate from the base of the column, whereby the liquid level is determined by the heat supplied to the column which is controlled. by a level controller.

Before bringing the column on stream anappropriate heat and material balance is drawn up on the basis of the, analysis of the feed, the proportion itis desired to produce as distillate, the reflux ratio to be used, and the known permissible loading of the column. From this heat and material balance the feed and, overhead. rates. and steam to re-boiler m are determined.

To bring the column on stream the feed pump at is set to pump at the desired rate. As soon as an appropriate liquid level is. reached in the column lbase, steam is admitted at the determined rate to the re-boiler m. Further rise of level. in the column base is prevented by the controller n which determines the operation of;- the pump 0. The hot vapours from the re-boiler m gradually warm up the entire column, and condensate then appears in the reflux tank. The pump 1 is now operated at its deter-mined rate whereupon the level in the reflux tank g slowly rises. until; the pump it commences operation under the effect of the controller h. When thermal equilibrium has been attained, the pump it will be returning the determined amount of reflux to the column. During this period the products from the column withdrawn by the pumps 2 and 0 will pass either to waste or to whichever of the tanks a, b and c is being filled with the feed at the time.

On thermal equilibrium being attained in the column it is by no. means certain that material equilibrium will have been established, and it is necessary to. return the products to waste or to one of the feed tanks until analysis of the products indicates that material equilibrium has been reached. In many cases a good indication of approach to material equilibrium may be secured by a simple test applied to the two product streams such as for specific gravity or refractive index.

When the bottoms product is. the stream to be volume-controlled the plant conditions arelthose indicated in Figure 2 of the accompanying drawings and as hereinafter described.

The feed pump (1 is set to deliver feed from the feed tank at a steady. rate, controlled by a flow controller of conventional type. The appropriate rate for feed admission is determined in the manner described with reference to Figure 1-on the basis of the characteristics of the column, the percentage overhead product it is desiredv to make andthe reflux ratio.

The. bottoms product pump 0. is set to withdraw bottoms at a steady rate, which may be controlled by a flow controller or otherwise. The rate of withdrawal is determined from ananalysis of the feedwhich determines the percentage of bottoms product it is desiredto produce and the feed rate. r

mea e 1? ..'.1,"Ih.ieflux.pump k is set to return reflux at the desired constant rate to give the desired 'vapounloading in the column. The overhead product pump Z is controlled by the level controller h to maintain a constant level in the reflux tankg. The admission of steam to the re-boiler mis controlled by level controller n to maintain a. constant level in the column base by ensuring the-necessary evaporation of the liquid.

- A considerable variation is permissible in the level in the reflux tank g, but itis necessary that the level in the column'base should be closely controlled.

Volume control of the bottoms product is advantageous when that product is a small percentage of the feed, and when the column is operated under pressure controlled by a pressure controller in the vapour space of the reflux tank g, which operates by release of vapour and uncondensables from the reflux tank g through a valve. The vapours so released are part of the overhead product, and if their quantity is not negligible it is evident that the volume control of the overhead product by means of pump Z, as in themodification described with reference to Figure 1 does not operate successfully.

The preliminary calculations necessary to determine the feed and bottoms product rates and the reflux return rate made before bringing the column on stream are similar to those described with reference to the modification of Figure 1.

In order to bring the column on stream when volume control of the bottoms product is used, certain precautions are required inasmuch as at the commencement, when the column is cold, 'a 13 large proportion of the material fed by the pump vd appears as liquid in the column base, and since the rate of Withdrawal by the pump is controlled, the liquid will accumulate despite the operation of the level controller n, unless the reserve heating capacity of the re-b-oiler m is large. :This accumulation of liquid can be avoided by using a low feed rate when coming on stream, although this may be undesirable if it is desired to bring the column on the stream quickly. Furthermore, the overhead product initially accumu- .lating in the reflux tank 9 is all returned to the column, thus increasing the load on the re-boiler. .It is preferable therefore to bring the column on stream by breaking the connection between the level controller n and the steam inlet to the reboiler m, and between the level controller h and the pump Z. The feed pump (1 is started and feed introduced at the determined rate. When a determined level is attained in the column base, steam is admitted to the re-boiler m at about the rate indicated by the preliminary heat balance, and the pump 0 is started and hand controlled to maintain a proper level in the column base.

As soon as distillate accumulates in the reflux tank 9 the pump Z is started and the overhead product is withdrawn at approximately the rate determined by analysis of the feed and feed rate. As the overhead product continues to accumulate in the reflux tank g the level is maintained by starting the pump it, and gradually increasing the flow. When the rate of reflux return approaches that determined by the preliminary heat balance and the column is approximately in thermal equilibrium, and the level controllers are connected to the pump 1 and the steam pipe, the pump 0 is set to withdraw bottoms product at the desired rate, and the pump it to return reflux steadily at the desired rate. This method of bringing the column on stream is equivalent to control for a short time whilev approaching thermal equilibrium.

The following is a practical example of processing conditions in the application of the invention. 1 I

Taking the case of the removal of normalhexane and lighter constituents from a petroleum spirit of boiling range, determined in an efiicient laboratory column, of 50 C. to C., the'separation in this range is complicated by the fact that benzene forms with hexane a mixture-of anomalous boiling characteristics, so that even with very eflicient fractionation it is not possible to remove normal-hexane in a high degree of purity, and at the same time leave a negligible quantity in the residue. The separation by frac- 'tionation of benzene and n-hexane from mixtures of these substances or from mixtures in which they are present is ordinarily difficult because of the peculiar vapour/liquid relationship by reason of which benzene distills in association with n-hexane considerably before reaching its own boiling point, and actually between 65 C. and

; 78 0. Nevertheless a separation sufficiently accurate for practical purposes is possible.

r The feedstock was analysed by determining the true boiling point curve, using an efficient laboratory batch column equivalent to at least 20-theoretical plates. It is known that a satisfactory product can be obtained if an overhead corresponds in quantity to the percentage boiling below 72 C. by the boiling point analysis referredto.

The feed in the example was found to have 12% by volume 11.22% by weight) boiling to this temperature. The separation was carried out in a fractionating column with an operating vapour velocity of 150,000 lbs/hr. when taking-hexane overhead at atmospheric pressure. It was decided to operate at a re-boil ratio (lb. per hour of vapour from re-boiler: lb. per hour of bottoms product) of 3.8: 1. The temperature at the column head and base were approximately 180 F. and 280 F. respectively, and the temperature of the'returm'ng reflux F. Steam supply to the re-boiler was duction of 39.5001bs./hr. a detailed heat'balance taken over the column including the re-boiler and feed, indicated that the rate of flow of vapour at the top of the column was 117,500 lbs. per hour. The quantities calculated therefore required to be multiplied by the ratio The final balance was thus: 7

Feed gallons per hour 7,850 TOverheadproduct do 945 Bottoms product do 6,905 Steam to re-boi1er pounds per hour 27,600 .Internalreflux (column top) do 143,610 Reflux ratio 22.5 1:01

By a heat balance it was found that the provision of 143,570 lbs/hr. of reflux at its boiling point required 123,000 lbs/hr. 18,000 gallons per hour of cold reflux returned at 125 F.

The operation was conducted at atmospheric pressure and the overhead product was a small percentage of the feed. Volume control of this product was therefore adopted. The controls were connected as in Figure 1, and set to give the calculated flow rates. As soon as thermal equilibrium of the column was approximately attained, as indicated by the reflux return being approximately the calculated quantity, the accuracy of the feed and overhead product flow controllers was checked against tank dips. Snap samples of the overhead were taken at frequent intervals and the specific gravity and refractive index measured. The approach to material equilibrium was judged by the gradual approach of these physical constants to a steady value. The refractive index is a good guide here since the higher boiling material in the overlap region is rich in naphthenes and aromatics.

When material equilibrium was judged to have been reached a sample of the bottoms was taken nd analysed for normal hexane content, by distilling the sample in an eflicient laboratory fractionating column, bulking the distillate to 75 C. and analysing this for aromatic, naphthene and paraiiin content in known manner.

The content so determined, converted back to a percentage on the bottoms product, gives an indication as to whether the conditions are set correctly to give the desired normal hexane quantity in the bottoms, and allows a slight alteration to be made to the overhead product withdrawal rate, if necessary.

It will be understood that the process of the invention may be applied to the distillate in successive flow of material through a, number of columns in a manner such as described in the specification filed pursuant to the application Serial Number 428,530, dated January 28, 1942, (now abandoned) to yield for example isopentane. and isohexane fraction and an isoheptane fraction of high octane number, and for the segregation of low grade material, the fractiona tions being carried out in columns e, 1, g, h and i of such characteristics that fractionation may be carried out under superfractionation conditions in the use of a considerable number, such as 50 theoretical plates in the columns, whereby precision in fractionation is ensured. Alternatively the process may be applied to single or selected fractions, and the respective characteristics of the columns with respect to diameter and number of theoretical plates may vary according to the character of the crude oil from which the distillate is derived and its composition. The distillate feed may be first stabilised and then de-butanised in columns in which the fractionation may be carried out in known manner, before subjection to iii) 10 the process of the invention, and the pentane content may be similarly fractionated for the separation of isopentane from n-pentane, as hereinbefore described.

I claim:

A process of distillation for the production of closely out high grade fractions of a petroleum distillate in a fractionating column to yield an overhead product and a bottoms product comprising admitting a stream of the distillate to the column at a determined substantially constant rate by volume, withdrawing a stream of the overhead product from the column, condensing said overhead stream and introduicng the condensate into a reflux drum, withdrawing part of said condensate from the system as product at a substantially constant rate which is a determined substantially constant proportion of the distillate feed rate fixed by the cut point of the distillate as determined by analysis of the distillate from time to time, withdrawing a stream of the bottoms product from the column at such a rate as to maintain a substantially constant liquid level in the column base, maintaining a substantially constant determined vapour loading in the column by control of the heat applied to the column base, andv returning another part of the condensate from the reflux drum to the column as reflux at such a rate as to maintain a substantially constant liquid level in the reflux drum.

- PATRICK DOCKSEY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,930,166 Gard Oct. 10, 1933 1,940,802 Kallam Dec. 26, 1933 1,998,123 Dunham Apr. 16, 1935 2,029,501 Page Feb. 4, 1936 2,104,310 Roelfsema Jan. 4, 1938 2,109,201 Ragatz Feb. 22, 1938 2,246,934 Denny W June 24, 1941 2,254,387 Olcott Sept. 2, 1941 2,297,098 Carney Sept. 29, 1942 2,340,026 Storment Jan. 25, 1944 2,357,113 Houghland Aug. 29, 1944 FOREIGN PATENTS Number Country Date 198,804 Germany May 29, 1908 476,610 Great Britain Dec. 13, 1937 OTHER REFERENCES Fenske et al., article in Ind. and Eng. Chem, vol. 4; Apr. 1932, pages 408-418 (pages 414 and 415 pertinent), 196-150.

Dunstan et al., Science of Petroleum, vol 2 (1938), pages 1594, 1595 and 1613; Div. 31.

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