US2977289A - Fractionation process control - Google Patents

Description

March 28, 1961 Q M KRQN FRACTIONATION PROCESS CONTROL Filed Jan. 20, 1958 INVENTOR. C M KRON HMLW. ZM?

A TTORNEYS INDICATOR OR RECORDER COMPARING CIRCUIT United States Patent i 2,917,289 y FRAc'noNArIoN PROCESS CONTROL Carl rM. Kron, Houston,l Texz, assignor to YPhillips Petroleum Company, a corporation of Delaware Filed Jan. 20, 1958, Ser. No. 710,159V Claims. (Clt 202e^40)' This invention relates toa method ofA controlling a fractionation process.

In many commercially important fractionations involving paranic and 'naphthenic hydrocarbons having 6 to 8 carbon atoms, especially 6 orY 7 carbon atoms, together with small quantities of an aromatic hydrocarbon having up to 8carbon atoms, especially benzene or toluene, l

it is important to accurately control the conditions at which the separation takesplace'.

For example, in the fractionation of straight run ygasolines, a light gasoline productis taken overhead and a bottom product is produced which is further fractionated-to provide a heavystraight rungasoline suitable as feed to a, platforming unit and a cyclohexane concentrate. The latter fraction can be processed in a hexane isomerization unit.

In-the rstfractionationabove'mentioned, it is desired that normal-hexane and less volatile-materials appear in the kettle product whileY isohexanes and more volatile materials are taken olf inthe overhead product. If normal hexane is taken overhead, it appears inthe light gasoline fraction and thus is lost to the feed to the hexane isomerization unit. Conversely, undercutting in the column-resultsin.A loss of-desired isohexane materials in the light gasolinefraction. Close control ofthe opera- Ition of this particular fractionation permits anv octane number improvement of 1.1 to Inumbers or even `more in the light gasoline product.-r

A similar problem exists-inmaking'analogous separations, forv example, in-fractionating mixedhexane fractionsor cyclohexane concentrates. Direct analysis for normall hexane in the overhead product of such separations is diicult-and unsatisfactory;

In accordance with thisinvention, asample from the upper part vof the column, or asarnple ofthe overhead product, is analyzed to determine its benzene content. The benzene is present only in small amountsinthe feed, and, surprisingly, it co-distills with the normal hexane despite a substantial difference in boiling point'of the two materials. Thus, by controlling the process to keep the benzene concentration at a predetermined low value in the sample, normal hexane and less volatile materials do not appear in the overhead product. Yet, the column can be operated under conditions of the greatest elliciency and throughput short of permitting normal hexane to appear in the overhead product.

The analysis can be made, according to the inventi'on, by an ultraviolet analyzer, and this instrument can be arranged to automatically controlone or more of the process variables' of the fractionation'column, for ex- 2,977,289 o AkPatented Mar. 28, 19461 ice .tion control 1 roce ss,utilizing` anultraviolet analyzerl It is a still further object to provide a close'fractio'nation control-yielding substantial. economies in operation. Various otherv objects,. advantages and features offthe invention will` becomefapparent from the following detailed descriptiontaken in conjunction with the accompanying -drawings in which:v n Y o Figure 1 is a flow diagram of a fractionation process utilizing `the improved control systemyand. Y f l Figure 2 isa top View of a suitable ultraviolet analyzer'. Referring-now` to Figure 1,1 have shown afractionation= zone or column 10 to which feed is suppliedby'a line 11 under the control of an automatic valve 12. Heat can besuppliedtothe bottomof the column in various Ways. in the example-illustrated,- steam is vfed .through av coil 13 under the control ofan automatic valve.14.v4 Overhead product is withdrawn'through a line 15 and passedthrough'a condenserV` 16 to an accumulator 17. Aportion of'the condensed material is withdrawn .through a line 18and is passed back to the column-as reflux inan arnountvcontrolledA by an automaticv valve 19.` Theremainder ofthe material from the accumulator 17 is -passed through an overheadproduct withdrawalline 20 at a rate determined by an automatic valve 21 normallycontrolled by!` a liquid level controllerv 22y responsive tothe material in the accumula-tory 17.

In accordancefwith the invention,.anultraviolet.spec-V line- 24 is..connected directly to the` overhead line 15,

' rate oroverhead product withdrawal.` Where'theil fer the line 25 is connected to the third tray below thetop of. the column-and the line 26 is connectedl tothe sixth trai/behavvY th'e top of the column. Thus', a Vsam'plecan bev withdrawn a's`desired fromany of these threel locai tions. 'Broadly speaking, a sample can be withdrawn fromany tray in the top region ofthe column, i.e., one more than five trays above the feed tray with theV usual fractionati'ng" devices for this type of service Iwhich have i so to iootrays.

The output of the spectrometer 23, which is'shownas k i' i a pneumatic output, is selectively applied throu'gh.`the valved lines 27, 28, 29 and 30 to the'controlfunits of the respective Valves 12, 14, 19 and 21. IIt will be noted Y `that the valve in each of the lines Z7 to 30 permits the control impulse to be applied'toany desiredone or` combin'atio'nV of thevalves 12, 14,19 and 21. In this mahrler, one or'more process variablesl of the systemv are controlled, the ones specificallyA illustrated being,` of

course, the rate at which heat issupplied to the bottoni` `of the column, the feed rate, the rellux ratLadthe variable is controlled, the liquid level controller 22 is disconnected from the valve 21.

It is oftentimes desirable to incorporate a time delay in the correction made to the process variables corresponding to the time taken for the process to respond to the correction. For this purpose, I have shown a time delay unit 31 in the control line 29.

In many commercial embodiments of the invention, the bottom product from the column is further fractionated. To this end, I have shown a fractionation column 32 to which the bottom product from the column 10 is fed by a line 33. The column 32 is provided with a suitable source of heat at the lower end thereof, this being a steam coil 34 in the example illustrated. The column 32 also has a bottom product withdrawal line 35, and an overhead product withdrawal line 36 which extends to a condenser 37 connected to an accumulator 38. A portion of the material from the accumulator 38 is passed back to the top of the column as reux through a valved line 39 while the remainder of the material from the accumulator is passed to an overhead product withdrawal line 40 under the control of an automatic valve 41 and a liquid level controller 42 responsive to the level in the accumulator 38. In operation, a stock to be fractionated containing 6 or 7 carbon atom parainic and naphthenic hydrocarbons together with a small amount of benzene or toluene is fed to the fractionator 10 through the line 11. The operating variables of the process are so controlled that the benzene or toluene co-distills with the most volatile parafnic or naphthenic component of the kettle product to be obtained in the fractionation. For example, where the feed contains isohexanes, normal hexane and benzene, the normal hexane is the most volatile component of the bottom product, and this material distills with the benzene. As another example, where the separation is made between n-heptane and lower boiling branched chain Cqs, in the presence of toluene, the toluene co-distills with the n-heptane, which is the more volatile component of the kettle product. Therefore, by analyzing for toluene in the overhead product or at a lower tray of the column, the amount of n-heptane at that part of the column can be controlled. Similarly, in the separation of n-octane from lower boiling branched chain Cs in the presence of ethyl benzene, analysis is made of the overhead product for ethyl benzene. As still another example, where the separation is made between n-octane and Z-methyl heptane in the presence of toluene, the latter compound co-distills with the toluene, and the analysis detects toluene in the kettle product or at a lower tray of the colhmn. Assuming that the sample is fed to the spectrometer through the line 24, the presence of aromatic material, as determined by the spectrometer, is indicative of the presence of the undesirable parainic or naphthenic component in the overhead product. Responsive to an output representative of aromatics, one of the variables of the cmolumn is adjusted to increase the eiciency of the separation to such an extent that the aromatic hydrocarbon, and thus the undesirable paraflinic or naphthenic component, is eliminated from the overhead product.

Where the analyzer sample is withdrawn through one of the lines 25 or 26, the analyzer is set to maintain a fixed predetermined concentration of aromatic hydrocarbon at the tray of the column from which the sample is withdrawn. This amount of aromatic, for example, 0.01 percent at the third tray from the top, produces an overhead product which is free from aromatic materials and thus of the undesired paranic or naphthenic component. Conversely, if the aromatic concentration falls below its pre-set value, the process is adjusted to increase the amount of material processed so that the eiciency of separation remains at its highest value. Even more accurate control canbe obtained, in many cases, by providing a time delay unit 31 in the output ofthe analyzer soV Kettle Product Feed Overhead Composition, Liquid Volume Percent Product S-Methylpentane. n-Hexane Methylcyclopcntane Benzene 2,3-Dlmethylpentane 2-Mcthylhexane C is and trans 1,3-Dimethylcyclopentane. Cts and trans 1,2-Dimethylcyelopentane. 3Mtthylhexane 3Ethylpentanc n-Heptane Heavier than heptane Typical operating conditions for the column charging Panhandle crude were as follows:

No. 1 tower (column 10):

Charge, barrels/hour 900 Overhead, barrels/hour 168 Bottoms, barrels/hour 732 Reux, barrels/hour 215 Charge, temperature, F 250 Top tower, temperature, F 165 Bottom tower, temperature, F-; 322 Accumulator, temperature, F 102 Top tower, pressure, p.s.i.g 48 No. 2 tower (column 32):

Charge, barrels/hour 732 Overhead, barrels/hour 352 Bottoms, barrels/hour 380 Reux, barrels/hour Charge, temperature F 218 Top tower, temperature, F 205 Bottom tower, temperature, F 299 Accumulator, temperature, F 99 Top tower, pressure, p.s.i.g 4

On applying the control system of the present invention, with the sample taken from the line 15, the amount of benzene in the overhead product is reduced from 0.5 liquid volume percent to a trace with a consequent reduction in the amount of normal hexane appearing in the overhead product to 0.2 liquid volume percent.

By controlling the bezene content within the range of 0.00 to 0.10 liquid volume percent, the normal hexane content can be controlled within the range of 0.0 to 2.0 percent.

In the described system, the kettle product is further fractionated in the column 32 to provide a cyclohexane concentrate for feed to a hexane isomerization unit, and a feed to a heavy gasoline platforming unit. On applying the present invention, the normal hexane is substantially eliminated from the light gasoline overhead prod- Bottom Reflux/Feed, Volume/volume uct, and yet the isohexanes are. effectively concentrated in thisove'rheadi'product. This results iii-an octane num'- ber improvement of from 1,'1 to 1.3 octane numbers in the light gasoline fraction. Also, since the benzene all goes to the kettle in the column 10, this material is available for conversion into the more valuable cyclohexane by hydrogenation in an ensuing step of the process.

Similar results are'obtained on charging a Mex-Tex crude fraction of essentially the same boiling range.

A test was made to show how the normal hexane content of the overhead product of column varies directly with the benzene concentration. The data obtained were asvfollows:

Normal Hexane Benzene Liquid Liquid Volume Volume Percent Percent Volume Percent Distilled Thus, it is evident that the benzene and normal hexane co-distill in the aboveseparation, so that the column can be effectively controlled by determination of the benzene content; directanalysis for thernormal hexaney in the above stream, cannot at present be made with suicient accuracy for successfulA control of a commercial operation.

In another specific example, a cyclohexane concentrate boiling at 130 to 176 F. is separated in a 70 tray fractionation column, the feed entering at the thirty-first tray, into an overhead fraction of isohexanes and more volatile materials together with a bottom product of normal hexane and less volatile materials. The operating conditions are -asfollows:

Kettle Stream Feed" Product` Temperature. F

VolumeyThousand Gallon per Day-..

Composition, Liquid Volume Percent:

Isopentane n-Pentane Cyclopentane. 2,2-Dimethylbutan 2-Methylhexane Pressure,v p.s.i.g.:

Top

By maintaining the benzene percentage at the sixth tray below the top of the column at a preselected value, for example, 1.0 percent, the amount of normal hexane in the overhead product is reduced from 1.9 percent to less than 0.2 percent, thus effecting substantial economie. in operation.

ln still another specific embodiment, a mixed hexane stream boiling at 130 to 160 F. is fractionfted in a fifty tray fractionation column, the feed entering at the and more volatile materials together with a bottom prod uct of normal hexane and less volatile materials. The'V operating conditions are as follows:

Over- Kettle v Stream Feed head Product Product Temperature, F. 210 l Volume, Thousand Gallon per Day- 111. 2 42, Composition, Liquid Volume Percent:

Isopentane 0. 3 0. n-Pentane. 7. 8 17. Cyclopentane 4. 3 9. 2,2-Dimethylbutane-- l. 5 3. Y' 2,3-Di.methylbutane- 3. 2 6. 8 0. 1 2-Methylpentane 23. 8 46. 0 5. 0 13. 6 14. 3 13.0 32. 4 2. 1 58.10 8. 5 0.3 15.3 0. 2 014 1. 6 Trace 3.0 A O. 2 0. 4 Cyelohexane 2. 6 4." 8'

Pressure .s.i.g.:

To 1.1i a5 Bottom 42 Redux/Feed, Volume/volume 3.90

below the top ofthe tower at 0.1 percent, the amount ofy normal hexane in the overhead product is reducedfrom 2.1 percent to less than 0.2 percent and the column'is operated at maximum output.

The described analysis for benzene or toluene can be eectively made by an ultraviolet analyzer ofthe type shown by Figure 2. In this analyzer, a beam of ultraviolet radiation from a source, such as a hydrogen lamp 50, is trained upon a detector unit 51 which can bey a photomultiplier tube. The tube 51, in turn, isiconnected to a comparing circuit 52 and an indicator or recorder 53 which can have an electrical or pneumatic output.

In its passage from the source to the detector, the ultraviolet energy is collected by a mirrorv 54 and focused on an aperture assembly 55, which serves as a point source of ultraviolet radiationl for the rest of the apparatus.I From the aperture 55, the ultraviolet radiation traverses` a iilter 56',v and a lensr57 which focusesV the radiation upon the detector 51. Disposed in the path of the beam between the lens 57 and the detector 51 are a gas-tilter 58', a sample cell 59, and a rotatable chopper 6i!l xed on a shaft 61 which is driven by a motor 62.

The cell 59 is adapted to cont-ain a liquid sampleV of the`- material to be analyzed which is fed thereto from one 0f the lines 24, 25, or 26, Figure 1, and it is of adjustable length.

The chopper 60 4comprises a pluralityof sector=shaped elements formed from iilter material which alternatev with sector-shaped sections of material transparent to, ultraviolet radiation.

In one specific embodiment of theiinvention, the. lter material attenuates radiation having a wavelength lower than about 250 to 260 millimicrons. Where@ theV cut-off frequency is 250 millimicrons, the filter material can ybe a liquid thickness of 1/2 centimeter of carbon tetrachloride; Where aliquid iilterl material s used, it is incorporated Within interior chambers, not shown, of the chopper'disc which is formed from a radiation-transparent material such `as quartz. Alternatively, the filter material can be a type 9700 lter manufactured by the Corning Glass Works having color specification 9-53, cut to a thickness of 1 millimeter.

Where the cut-oit wavelength is 260 millimicrons, a thickness of 1A centimeter of carbon tetrachloride canr The filter 56 is of the band pass type which passes -v i.

wavelengths of about 230 millimicrons to an upper wavetwentieth tray, into an overhead product of isohexanes length of greater than 275 millimicrons, for example, 420

7 V millimicrons. Where a liquid-type filter is desired, thickness of centimeters of an aqueous solution containing 240 grams per liter nickel sulfate (NiSO4-6H2O) and 45 grams per liter cobalt sulfate (CoSO4-7H2O) can be utilized. The liquid is contained within a quartz liquidcontaining cell transparent to ultraviolet radiation. Alternatvely, the lter 56 can be a type 9863 filter produced by the Corning Glass Works having a color specication 7-54 and a thickness of 3 millimeters.

The auxiliary filter 58 which cuts off ultraviolet radiation of wavelength greater than 270 millimicrons, can be a 5 centimeter thickness of chlorine gas. As will become evident hereafter, this is an optional feature of the invention. Where used, the gas is contained within a cylindrical body 63 of opaque material having transparent end plates 64 and 65.

In the operation of one specific embodiment of the invention, Figure 2, the overhead product fed to the cell S9 contains a trace to 2.0 percent of benzene which is to be quantitatively analyzed. Benzene has strong absorption bands at wavelengths of 225 to 270 millimicrons. The filter 56 has a pass band extending from a lower wavelength of about 250 millimicrons to an upper wavelength of greater than 275 millimicrons. The filter 58 transmits wavelengths of less than 270 millimicrons and attenuates radiation of longer wavelength.

Accordingly, the filters 56, 58 in combination define a band pass filter extending from 250 to 270 millirnicrons so that the ultraviolet radiation passing through the sample and the filter system varies in intensity in accordance with the amount of benzene in the sample.

The lilters in the chopper disc 60 are formed from material which attenuates radiation having a wavelength lower than a value of about 250 millimicrons and which passes radiation of higher wavelengths. Therefore, when one of these lters is positioned in the path of the radiation beam, the amount of radiation incident upon the detector 51 is not responsive to the benzene concentration of the sample. However, when the transparent portions of the disc 60 are in the path of the beam, the output is responsive to the benzene concentration. Thus, by electrically comparing these two outputs in the circuit 52, a resultant signal is produced which is insensitive to absorption caused by miscellaneous components which may be in the sample and absorb at other wavelengths. The filter 58 aids by cutting out interference caused by absorption at ultraviolet wavelengths of greater than 270 millimicrons. Accordingly, the resultant output at unit 53 is responsive solely to the benzene concentration in the sample.

'I'he described arrangement can also be used for analysis of toluene which has strong absorption bands at wavelengths of 225 to 275 millimicrons. However, when analyzing for small concentrations of toluene, somewhat greater sensitivity can be obtained by eliminating the chlorine gas lter 58, and changing the cut-oft wavelength of the filter material in the chopper 60 to 260 millimicrons. In this manner, the absorption bands existing between the chlorine cut-off wavelength of 270 millimicrons and the upper wavelength limit of the toluene band, 275 millimicrons, contribute to the signal produced when the beam passes between the sectors of the chopper disc. By increasing the cut-oi wavelength of the chopper disc material itself to 260 millimicrons, the toluene bands are suiiiciently screened out of the detector output so that adequate compensation for interfering components is made when the output produced with the filter sections of the disc 60 in the path of the beam is compared with the output produced when the transparent sections are in the path of the beam.

The present analyzer permits accurate and close control of the normal hexane or other key component concentration in the overhead product of the column 10, Figure 1, through analysis for benzene or toluene. The column is thus controlled to obtain significant advantages in commercial operation.

While the invention has been described in connection with a present, preferred embodiment thereof, it is to be understood that this description is illustrative only and is not intended to limit the invention.

I claim:

1. In the art of fractonating a stream containing isohexane, normal hexane and a small amount of benzene, the steps which comprise introducing said stream into a fractionation zone; controlling heat supplied to the bottom of said zone, heat withdrawn from the top of said zone, the feed rate, the rate of withdrawal of overhead product and bottom product and the reflux rate as operating variables to separate said stream into an overhead product containing isohexane and more volatile materials and a bottom product containing normal hexane and less volatile materials; withdrawing a sample from a region of said zone located substantially above the region of feed introduction; analyzing said sample to produce an output signal representative of the benzene content thereof by passing a beam of ultraviolet radiation through said sample and measuring the intensity of the beam after it has passed through said sample, the measured intensity being representative of said output signal; and controlling one of said operating variables in accordance with said output signal to maintain said benzene content at a predetermined value less than the benzene content of said stream so that substantially no normal hexane appears in said overhead product.

2. The process of claim 1 wherein the operating variable is the reux rate.

3. The process of claim 1 wherein the operating variable is the product withdrawal rate.

4. The process of claim 1 wherein the operating variable is the feed rate.

5. The process of-claim 1 wherein the operating variable is the supply of heat to the kettle of said fractionation zone.

References Cited in the file of this patent UNITED STATES PATENTS 2,386,831 Wright Oct. 16, 1945 2,459,404 Anderson Ian. 18, 1949 2,529,030 Latchum Nov. 7, 1950 2,709,678 Berger May 31, 1955 2,764,536 Hutchins Sept. 25, 1956

Claims (1)

1. IN THE ART OF FRACTIONATING A STREAM CONTAINING ISOHEXANE, NORMAL HEXANE AND A SMALL AMOUNT OF BENZENE, THE STEPS WHICH COMPRISE INTRODUCING SAID STREAM INTO A FRACTIONATION ZONE; CONTROLLING HEAT SUPPLIED TO THE BOTTOM OF SAID ZONE, HEAT WITHDRAWN FROM THE TOP OF SAID ZONE, THE FEED RATE, THE RATE OF WITHDRAWAL OF OVERHEAD PRODUCT AND BOTTOM PRODUCT AND THE REFLUX RATE AS OPERATING VARIABLES TO SEPARATE SAID STREAM INTO AN OVERHEAD PRODUCT CONTAINING ISOHEXANE AND MORE VOLATILE MATERIALS AND A BOTTOM PRODUCT CONTAINING NORMAL HEXANE AND LESS VOLATILE MATERIALS; WITHDRAWING A SAMPLE FROM A REGION OF SAID ZONE LOCATED SUBSTANTIALLY ABOVE THE REGION OF FEED

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