US3309287A

US3309287A - Controlling the operation of a train of distillation columns

Info

Publication number: US3309287A
Application number: US219606A
Authority: US
Inventors: Dale E Lupfer; Jr Minor W Oglesby
Original assignee: Phillips Petroleum Co
Current assignee: Phillips Petroleum Co
Priority date: 1962-08-27
Filing date: 1962-08-27
Publication date: 1967-03-14
Anticipated expiration: 1984-03-14

Description

3,309,287 CONTROLLING THE OPERATION OF A TRAIN OF DISTII'JLATION COLUMNS Filed Aug. 27, 1962 March 14, 1967 D. E. LUPFER ET AL 8 Sheets-Sheet 1 INVENTORS D E LUPF ER M.W. OGLESBY,JR.

mh wwm rllfillllllI/llllll'llllillluL March 14, 1967 D. E. LUPF'ER 'ET AL Filed Aug. 27. 1962 8 Sheets-Sheet 2 I N 23 v 2 B Bub A T TORNE KS March 14, 1937 PF R ETAL 3,309,287

CONTROLLING THE OPERATION OF A TRAIN OF DISTILLATION COLUMNS Filed Aug. 27, 1962 s Sheets-Sheet s COMPUTER? OPERATIONS O PERATIONS COMPUTER"! W, OGLESBY,JR.

March 14, 1967 D. E. LUPFER ET AL 3,309,287

CONTROLLING THE OPERATION OF A TRAIN OF DISTILLATION COLUMNS Filed Aug. 27, 1962 8 Sheets-Sheet 4 INVENTORS D E LUPF ER I I l L J M.W.VOGLESBY,JR.

A T TORNEYS March M, 1967 o. E. LUPFER ET AL 3,309,287

CONTROLLING THE OPERATION OF A TRAIN OF DISTILLATIQN COLUMNS Filed Aug. 27, 1962 8 Sheets-Sheet 5 SE /Gov 3 ATTORNEYS arch M, 1967 D. E. LUPFER ET AL 3,309,287

CONTROLLING THE-OPERATION OF A TRAIN OF DISTILLATION COLUMNS Filed Aug. 27, 1962 8 Sheets-Sheet 6 hb'" J ll 5 TERMINAL STREAM SPECIFICATIONS HQ ub .J

--}TO OPERATIONS COMPUTER FOR THIRD COLOMN TO OPERATIONS COMPUTER TO OPERATIONS COMPUTER COMPUTER FOR FIRST COLUMN hhhO' FLOW RATES OF COMPONENTS IN (fl dl FEED TO FIRST COLUMNIN TRAIN (F) (FEED FLOW RATE) INVENTORS D. E. LUP F ER MW. 06 LE SBY,JR.

A TTORNE VS areh 14, 1967 D. E. LUPFER ETAL I 3,309,287

CONTROLLING THE OPERATION OF A TRAIN OF DISTILLATION COLUMNS Filed Aug. 27, 1962 's Sheets-Sheet a l-flolll Bhb'" FIG. 8

TERMINAL STREAM SPECIFICATIONS TO OPERATIONS COMPUTER FOR FOURTH COLUMN --}TO OPERATIONS COMPUTER H FOR THIRD COLUMN To OPERATIONS- COMPUTER FOR SECOND COLUMN I A u SPECIFICATIONS COMPUTER I ESE AIET EO II N I lb '(FIAuw 1 (9 11 FLOW RATES OF COMPONENTS IN (F)C1;: FEED TO FIRST COLUMN IN TRAIN (ODE;

F(FEED FLOW RATE) MW. OGLESBY,JR.

A 7' TORNEVS United States Patent 3,309,287 CONTROLLING THE OPERATION OF A TRAIN OF DISTILLATION COLUMNS Dale E. Lupfer and Minor W. Oglesby, Jr., Bartlesville,

Okla, assignors to Phillips Petroleum Company, a corporation of Delaware Filed Aug. 27, 1962, Ser. No. 219,606 4 Claims. (Cl. 203-1) This invention relates to the operation of a train of distillation columns. In other aspects, it relates to a method and apparatus for controlling the operation of a train of distillation columns in order to produce terminal product streams of desired specifications.

The trend in automation in recent years has brought about many proposals for improving the art of controlling fractional distillation of multi-component mixtures. Many of these proposals have proven useful, but they generally have been limited to improving the control of the operation of a single distillation column. Little has been published or is known about automatically controlling the operation of a train of distillation columns as a unit, where control of each distillation column is integrated into an overall automatic control of the unit.

Although the fractionation or split of the feedstock of a particular distillation column can be improved by certain controls or instruments, notwithstanding some changes in the composition of the feedstock, there is no teaching in the art to our knowledge which is concerned with automatically controlling the composition of the feedstocks passed to different columns in a train of columns to insure production of terminal product streams with desired specifications. Rather, it has been the practice heretofore to make the column make the best split possible, i.e., make specification product as close as possible, of Whatever the column feedstock composition may be. In other words, the column must live with what ever feedstock is supplied thereto, and where the composition of the feedstock becomes such as to make it impossible for the column to make specification pro-duct, the operator trys to overcome this situation emperically by altering the operation of upstream columns in the train in an effort to insure production of a feedstock which can be split by downstream columns to produce specification products. Under such circumstances, control over the feedstock composition is difiicult and it is not always possible to make products with desired specifications; resort is often made to further fractionation of the product in yet another column in order to make specification product, with the consequent increase in utilities cost, equipment requirements, etc.

Accordingly, an object of this invention is to improve the operation of a train of distillation columns. Another object is to provide a method and apparatus for controlling the operation of a train of distillation columns so that terminal product streams of desired specifications can be produced with a low cost in utilities, equipment and other requirements. Another object is to provide a method and apparatus for controlling the operation of a train of distillation columns so that intermediate product streams can be produced having compositions such that downstream columns can effectively fractionate such intermediate product streams and make specification products.

Other objects and advantages of this invention will become apparent to those skilled in the art from the following description, appended claims, and accompanying drawing in which:

FIGURE 1 is a flow sheet of a two-column train of fractionators provided with the improved control system of this invention;

FIGURE 2 is a schematic view of circuitry which can be used in the control system of FIGURE 1;

FIGURE 3 is a flow sheet like that of FIGURE 1, but provided with another embodiment of the control system of this invention;

FIGURE 4 is a flow sheet of a three-column train of fractionators provided with a control system of this invention;

FIGURE 5 is a schematic view of circuitry which can be used in the control system of FIGURE 4;

FIGURE 6 is a schematic view of another control system of this invention;

FIGURE 7 is a schematic view of circuitry which canbe used in the control system of FIGURE 5; and

FIGURE 8 is a schematic view of another control system of this system.

We have discovered that the operation of a train of distillation columns can be improved by specifying terminal product streams, combining signals proportional to such specifications with other signals proportional to the flow of feedstock to the train and the amounts of com ponents in said feedstock in material balance equations for the feed streams in the train which are fractionated and for stream components in such feed streams, automatically solving such equations simultaneously for the amounts of components in intermediate feed streams which can be tolerated in such streams without affecting the fractionation of such streams into terminal product streams of desired specifications, and automatically controlling the operation of those columns within the train which pro duce a product stream employed as a feedstock by a down stream fractionator. The control system used in this invention to control the train of distillation columns can be called a predictive or feedforward control system.

Referring now to the drawing, we have illustrated in FIGURE 1 a two-column distillation train consisting of column 1 and column 2, each of which can be provided with a plurality of the usual vertically-spaced liquid-vapor contact trays (not shown). Feedstock comprising a mul ticomponent mixture, such as liquid petroleum gases, is supplied to the train via line 3 and introduced onto a feed tray located in the middle portion of column 1. This feedstock line 3 can be associated with the usual heat exchanger so as to preheat the feedstock, e.g., to vaporize or partially vaporize the same. Heat is supplied to the kettle of column 1, for example by circulation of steam or other heat exchange medium by supply line 4 through reboiler coil 6, the heat exchange medium being withdrawn from the kettle via line 7. Vapors are removed from the top of column 1 through an overhead line 8, cooled, and passed to an accumulator 9. Liquid distill-ate in accumulator 9 is withdrawn, and a portion of this withdrawn liquid is recycled as external reflux via line 11 to the top of column 1. The balance of the liquid distillate is removed from the system via line 12 as a terminal product stream. Bottom product is Withdrawn from the kettle of column 1, as an intermediate product, via line 13 and, after being heated, is passed to column 2 as feed for the latter. The light components of the feed will appear mainly in the overhead and the heavy components of the feed will appear mainly in the bottom product. The light components will comprise a light key component and components lighter than the light component, while the heavy components will comprise a heavy key component and components heavier than the heavy component. Since perfect separation between the light and heavy components is impossible, some of the heavy component will appear as an impurity in the overhead (and thus in the distillate product) and some of the light component will appear as an impurity in the bottom product.

Column 2 is provided with the usual appurtenances like that of column 1 and like elements have been designated with like primed reference numbers, this second column producing a distillate yielded as a terminal product stream 12' and a bottom product yielded as a terminal product stream 13. Other products can be removed from the columns at intermediate points though this is not shown in the drawing in the interest of brevity.

For pusposes of illustration, assume that the feedstock passed to the train of FIGURE 1 via line 3 comprises a mixture of components A, B, C and D, that column 1 is designated to produce overhead a terminal product stream 12 comprising components A, B and C and a bottom product 13 comprising components B, C and D; and assume column 2 is designed to produce overhead a terminal product stream 12 comprising components B, C and D and produce as a bottom product a terminal stream 13' comprising components C and D, For example, assume that the feedstock 3 comprises a mixture of ethane (A), propane (B), isobutane (C), and normal butane (D), and that column 1 is a depropanizer and column 2 is a butane splitter. The operation of the train illustrated in FIGURE 1 can further be illustrated by the following diagram, in which the vertical lines represent columns and the horizontal lines represent streams.

DIAGRAM I First Overhead O1 Composition:

llo'

B he (sp y) where:

F =fiow rate of feed to first column 0 :flow rate of overhead from first column B fiow rate of bottom product from first column O =fiow rate of overhead from second column B =flow rate of bottom product from second column B =decimal fraction of light key component B in feed A =decirnal fraction of component A in feed lighter than light key component B C =decimal fraction of heavy key component C in feed Dhhf= decimal fraction of component D in feed heavier than the heavy key component C B =decimal fraction of light component B in overhead from first column A =decimal fraction of component A in overhead from first column lighter than the light component B in the overhead from the first column C =decimal fraction of heavy component C to overhead from first column C =decimal fraction of light key component C in bottom product from first column B =decimal fraction of component B in bottom product from first column lighter than the light key component C in bottom product from the first column D =decimal fraction of heavy key component D in bottom product from first column C n decimal fraction of light component C in overhead from second column B -=decimal fraction of component B in overhead from second column lighter than the light component C from the overhead in the second column D "=decima1 fraction of heavy component D in overhead from second column C -=decimal fraction of light component C in bottom product from second column D 'r decimal fraction of heavy component D in bottom product from second column It should be evident from the Diagram I that the flow rate of any component in any flow line is equal to the product of the flow rate of material in that flow line and the decimal traction of said component; thus, (F)B is equal to the flow rate of the light component B in the feedstock passed to the first column.

An examination of Diagram I shows that all of component B in the feedstock passed to the second column ultimately ends up in a single terminal product stream, namely the second column overhead. The amount of B in this terminal product stream is (O )B which is equal to the amount of B in the bottom product from the first column, namely (BQB It the concentration of B in the second overhead (a terminal stream), is specified, and the operation of the second column is designed to make a split between components C and D, specification product will be made only if the concentration of component B in the feedstock to the second column is below certain limits. However, if the concentration of component B in the feedstock to the second column exceeds these limits, it is evident that the operation of the second column will not produce specification product since this second column must accept and pass overhead all of the B component in the feedstock passed thereto.

According to the subject invention, the two-column train of FIGURE 1, as schematically illustrated in Diagram I, is automatically controlled so that the concentration of component B in the feedstock to the second column, i.e., in the first bottom product, is controlled within the above-mentioned limits. This is done by automatically controlling the operation of the first column to assure its production of a bottom product stream which can be separated by the second column to meet specifications. I-Iow this control is accomplished in the case of FIGURE 1 will now be described.

The following material balance equations can be written covering the fractionation system illustrated in FIG- URE 1 and diagrammed in Diagram I:

Note that

Equations

1, 2, 3, 8 and 9 are on the column feed streams, and

Equations

4, 5, 6, 7, 10, 11 and 12 are on the components in said feed streams.

According to the subject invention, the terminal product streams 12, 12' and 13 of FIGURE 1 (namely, the first and second overheads and the second bottom product of Diagram I, respectively) are rigorously specified, and the material balance Equations 1 to 12 are solved simultaneously for B the decimal fraction or concentration of component B in the feedstock to the second column (i.e. the first bottom product). In Diagram I, the decimal fractions which are specified are followed by (specify), and the decimal fraction solved for is followed by (compute).

Referring again to FIGURE 1, there is illustrated a specifications computer, designated 15, which computer can be used to solve said Equations 1 to 12 simultaneously for B For the purpose of the computation performed by specifications computer 15, the composition of the feedstock in line 3 is analyzed by an analyzer 16, connected to line 3 by a sample line 17. Analyzer 16 comprises any suitable instrument which continuously or substantially continuously (i.e., rapid cycle) analyzes the feedstock in line 3 and determines the relative amounts of the components in the feedstock used in solution of said material balance Equations 1 to 12 for B and produces signals proportional thereto. A suitable analyzer for this purpose is described in I.S.A. Journal, Vol. 5, No. 10, p. 28, October 1958, and it preferably comprises a high speed chromatographic analyzer having a sample valve, motor, detector, chromatographic column,

programmer, and a peak reader, the latter functioning to read the peak height of the measured feed components, giving an equivalent output signal which is suitable for computing and control purposes. In operation, sample flows continuously through the analyzer. At a signal from the programmer, a measured volume of sample is flushed into the chromatographic column. When the component arrives at the detector, the resulting signal is measured, amplified and stored until the next signal when the sequence is repeated. The stored signal is a continuous output signal analogous to the amount of the component present. Signals proportional to the amounts of components can be transmitted to specifications computer 15 by a signal line 18.

In order to compute B it is also necessary to measure the flow of feedstock in line 3. For this purpose, a flow measurement device 19 can be disposed in the feedstock line 3 and by means of a suitable transmitter 21 the flow can be transmitted to specifications computer 15 by signal line 22.

It is also necessary, as evident from the above material balance Equations 1 to 12, that the specifications of terminal product streams be employed, and thus signals proportional to these specifications are also supplied to specifications computer 15, as shown in FIGURE 1.

FIGURE 2 shows schematically circuitry which can serve as specifications computer 15, in combination with analyzer 16. The boxes in FIGURE 2 containing an x are multipliers, boxes containing a are dividers, boxes containing or and are adders (or summers), and circles are also multipliers, e.g., potentiometers, the designated valves within the circles being multiplicands. Such circuitry elements are well known in the art and commercially available, in their electrical, pneumatic or mechanical embodiments, and those skilled in the art will readily understand the operation of such circuitry especially when read in view of the foregoing discussion.

The output signal 23 from specifications computer 15 is used according to this invention in the control of the operation of column 1 of FIGURE 1, this being a column which produces a product stream, namely bottom product stream 13, which is used as a feedstock for the downstream column 2 to produce terminal product streams, namely streams 12' and 13'. Such control of column 1 aids in manipulating the split effected by column 1 to insure the production of bottom product stream 13 having that concentration B of component B which can be readily split by column 2 to produce terminal product streams of desired specifications, notwithstanding changes in the concentration of component B in the feedstock passed to the train by line 3. Said output signal 23 from specifications computer 15 can be used as a setpoint adjusting signal for any fractionator column control system used to manipulate the split of the column. Two examples of such control systems and the adjustment of the same by such a setpoint adjusting signal, will now be described.

Referring again to FIGURE 1, the split effected by column 1 can be controlled by manipulation of the reboiler heat and reflux flow rate. T o accomplish this, the steam line 4 to the reboiler coil 6 can be provided with a conventional assembly comprising an orifice plate 24, flow transmitter 26, flow controller 27 and flow control valve 28. Since the bottom product in line 13 is the important product stream of column 1, so far as column 2 is concerned, the bottom product in line 13 is analyzed by an analyzer 29, connected to line 13 by a sample line 31, to determine the concentration of component B in line 13. This analyzer 29, which can be like that analyzer 16 used to determine the composition of feedstock in line 13, sends its output signal 32 to an analyzer recorder controller 33 which, ordinarily, would put out a signal 34 serving to adjust the setpoint of flow controller 27.

The setpoint of analyzer recorder controller 33 is manipulated by the output signal 23 (B from specifications computer 15. Overhead 8 can also be provided with an analyzer 35 and analyzer recorder controller 36, the setpoint of the latter, e.g., C being a predetermined specification supplied by signal line 38, to control the concentration of the overhead by manipulation of reflux flow controller 39. The split performed by column 1 is thus controlled to insure a desired concentration of key component B in the bottom product stream 13, so that column 2 can split this bottom product and produce terminal product streams with predetermined specifications.

The split performed by column 2 can be controlled by any conventional means, for example, it can be controlled by manipulation of reboiler heat and reflux flow rate, this manipulation being controlled by any feedback control system, such as that illustrated; however, the setpoint adjusting signals for the analyzer recorder controllers 33' and 36' of column 2 again will be predetermined desired specifications, for example, C and D n, respectively of Diagram I, these setpoint adjusting signals being designated 40, 41, respectively in FIGURE 1.

There has recently been discovered an improved distillation column control method and apparatus, namely that described and claimed in copending US. application Ser. No. 189,375, filed April 23, 1962, by Dale E. Lupfer. It is this distillation column control method and apparatus which we prefer to employ, rather than that illustrated in FIGURE 1, in controlling the operation of each distillation column in the train, such control of each column being integrated and dependent upon the specifications computer of this invention. We have illustrated in FIGURE 3 such an integrated control system for a twocolumn fractionator train, which in most respects is similar to that of FIGURE 1, and like reference numbers have been used where elements of FIGURE 3 are in com- .mon with those of FIGURE 1. FIGURE 3 will now be described.

In FIGURE 3, it will be noted that the feedstock in line 3 can be heated by indirect heat exchanger or economizer 46 and an indirect heat exchanger or preheater 47, the heat exchange medium of exchanger 46 being the bottom product 13 of column 1 and the heat exchange medium of exchanger 47 being a medium such as steam supplied via line 48, the flow rate of the latter being controlled by an assembly comprising flow control valve 49 and flow controller 50 and orifice plate 51. The external reflux in line 11 is controlled by an assembly comprising a flow control valve 52 and flow controlier 53, and the flow of bottom product in line 13 is controlled by an assembly comprising flow control valve 54 and flow controller 55. Heat is supplied to column 1 through a reboiler coil 6, the flow of heat exchange medium to this reboiler coil preferably being controlled by the liquid level in the kettle of column 1.

The above description of column 1 of FIGURE 3 applies similarly to column 2 of FIGURE 3.

As described and claimed in said copending application Serial No. 189,375, each of

columns

1 and 2 of FIGURE 3 can be controlled by measuring the flow rate of feedstock to the column and the concentrations of components in said feedstock, producing signals proportional to said measurements, combining said signals together with signals proportional to constants in statistically-derived equation to predict the internal reflux flow rate of such a column, producing a signal proportional to said predicted internal reflux flow rate, and contro1- ling the operation of the column in accordance with the latter signal to insure that the column produces distillate and bottom products of specified purities. Said predictive, statistically-derived equation for internal reflux flow rate is based on the expression:

R =predicted internal reflux flow rate (vol./ unit time) F =feed flow rate (vol./ unit time) F =generic symbol for components in feed, each expressed as a liquid volume fraction of feed E =average column tray efliciency F =feed enthalpy (Btu/lb.)

F =feed tray (numbering trays from top of column) H =specified liquid volume fraction of heavy key in distillate product L =specified liquid volume fraction of light key in bottom product Computers for such predicted internal reflux flow rate are shown in FIGURE 3 as operations computers, one such computer 57 for column 1 and another such computer 58 for column 2. The output signal from operations computer 57, proportional to the predicted internal reflux flow rate for column 1, i supplied by signal line 59 to adjust the setpoint of flow controller 53, so as to manipulate the external reflux in external reflux line 11 of column 1. Feed composition information for operations computer 57 can be supplied from analyzer 16 by signal line 61, or from another analyzer associated with feed line 3. Feed flow rate information for operations computer 57 can be supplied by signal line llt) which is connected with transmitter 21 described hereinabove. As described in said copending application Ser. No. 189,375, specifications proportional to the specified purity of distillate product and the specified purity of bottom product of a column are employed by such operations computers. In FIGURE 3 (which should also be read in the light of Diagram I for understanding), a signal proportional to the desired purity of distillate product 12, namely C is supplied to operations computer 57 by signal line 62, and a signal proportional to the computed concentration B of componentB in the bottom product line 13 is transmitted from the specifications computer'15 by signal line 23. In the case of column 2 of FIGURE 3, which produce terminal product streams "12 and 13, specifications D and C proportional to the heavy key component in the distillate product line 12' and the light key component in the product line 13 respectively, are thus transmitted to operations computer 58 by signal lines 63 and 64, respectively. The flow of bottoms product in line 13 from column 1 which becomes the feed for column 2 can be measured by a flow measurement device 112 which is disposed in line 13 and by means of a suitable transmitter 113 (similar in construction to transmitter 21) the flow rate of bottoms product in line 13 can be transmitted to specifications computer 58 by signal line 114.

Said copending application Ser. No. 189,375, in one of its other aspects, employs such operations computers to also manipulate the flow of bottom product from a column. The general equation for the flow rate of the bottom product can be expressed as:

.f( c1 F: HD: LB) Where:

B=predicted flow rate of bottom product (vol/unit time) F =generic symbol for the sum of the light key component and components lighter than the light key, each expressed as a liquid volume fraction of feed F=feed flow rate (vol./ unit time) H =specified fraction of heavy key in distillate (liquid volume decimal fraction) L =specified fraction of light key in bottoms product (liquid volume decimal fraction) For example, in FIGURE 3, operations computer 57 produces an output signal 66 proportional to a predicted bottom product flow rate, and this signal is transmitted to a biasing device 67, such a a conventional summing relay, where it is compared with a setpoint signal 68, the biasing device 67 accordingly producing an output signal 69 which serves as the setpoint for controller of bottom product line 13. The predicted bottom product flow is preferably overriden by a feedback control. To accomplish this feedback control, bottom product in line 13 is analyzed by an analyzer 60, connected to line 13 by a sample line 31, to determine the concentration B of the light key component B in the bottom product. The output signal from analyzer is transmitted to an analyzer recorder controller where it is compared with a setpoint ignal 70, according to this invention, proportional to the computed concentration B of component B, supplied by signal line 23 from specifications computer 15. Any ditference in the actual or measured con centration of component B in the bottom product and the computed concentration of component B is transmitted as signal 68 to bias relay 67.

In FIGURE 4, there is illustrated a three-column distillation train comprising columns 71, 72 and 73, each column provided with appurtenances similar to that of FIGURE 1. The split of each of these columns are similarly controlled by manipulating reboiler heat and reflux flow rate. The flow controllers controlling the flow of steam to the re'boiler coils of columns 71 and 73 and the flow controllers controlling the flow of reflux to columns 72 and 73 can be adjusted by feedback analyzer control systems. The setpoints for the analyzer recorder controllers 74 and 75 for columns 71 and 72, respectively, are supplied as output signals 76 and 77, respectively, from specification computer 78, these two columns producing product streams used as feed by downstream fractionators. The setpoiuts for

analyzer recorder controllers

79, 80, 81 and 82 are supplied by

signal lines

83, 84, 85 and 86, respectively, proportional to predetermined desired specifications.

In FIGURE 4, assume that the feedstock in line 87, supplied to the train and introduced to first column 71, comprises components A, B, C, D, E, F and G. Further assume that the first column 71 makes a split between components D and E and produces an overhead 88 containing components A, B, C, D and E anda bottom product 89 (a terminal product stream) comprising components D, E, F and G, that the second column 72 uses as feed the condensed overhead 88 produced by the first column 71 and splits the same between B and C to produce an overhead 90 (a terminal product stream) comprising components A, B and C and a bottom product 91 comprising components B, C, D and E, and that the bottom product stream 91 is split by the third column 73 to produce an overhead 92 (a terminal product stream) comprising components B, C and D and a bottom product 93 (a terminal bottom stream) comprising components C, D and E. The operation of the train of fractionators illustrated in FIGURE 4 can be diagrammed as follows:

DIAGRAM II Second Overhead O2 Composition:

B Cho" F=flow rate of feed to first column =fiow rate of overhead from first column B =fiow rate of bottom product from first column O =fiow rate of overhead from second column B =fiOW rate of bottom product from second column O =fiow rate of overhead from third column B =flow rate of bottom product from third column D =decimal fraction of light key component D in feed to first column C =decimal fraction of component C in feed to first column lighter than light key component D in feed to first column B =deci=mal fraction of component B in feed to first column lighter than component C in feed to first column A =decimal fraction component A in feed to first column lighter than component B in feed to first column E =decimal fraction of heavy key component B in feed to first column F =decimal fraction of component F in feed to first column heavier than heavy key component B in feed to first column G =decimal fraction of component G in feed to first column heavier than component F in feed to first column B =decimal fraction of light key component B in overhead from first column A =decimal fraction of component A in overhead from first column lighter than light key component B in overhead from first column C =decimal fraction of heavy key component C in overhead from first column D l=decimal fraction of component D in overhead from first column heavier than heavy key component C in overhead from first column E r=decimal fraction of component E in overhead from first column heavier than component D in overhead from first column E decimal fraction of light component E in bottom product from first column D =decimal fraction of component D in bottom product from first column lighter than light component B in bottom product from first column F '=decirnal fraction of heavy component in bottom product from first column G =decimal fraction of component G in bottom product from first column heavier than heavy component F in bottom product from first column B -=decimal fraction of light component in overhead from second column A -=decimal fraction of component A in overhead from second column lighter than light component B in overhead from second column C -=decimal fraction of heavy component C in overhead from second column C -=decimal fraction of light key component C in bottom product from second column B -=decimal fraction of component B in bottom product from second column lighter than light key component C in bottom product from second column D -=decimal fraction of heavy key component D in bottom product from second column E -=decimal fraction of component E in bottom product from second column heavier than heavy key component D and bottom product from second column C -=decimal fraction of light component C in overhead from third column B m=decimal fraction of component B in overhead from third column lighter than light component C in overhead from third column D m=decimal fraction of heavy component D in overhead from third column D -=decimal fraction of light component D in bottom product from third column C m=decimal fraction of component C in bottom product from third column lighter than light component D in bottom product from third column E m=decimal fraction of heavy component E in bottom product from third column It will be noted from FIGURE 4 (and Diagram II) that the first and second columns in the train produce product streams which are used as feed by downstream fractionators, the light component and heavy component in each of these streams being key components. It will further be noted that all of component E in the overhead 88 from the first column 71 ends up in the bottom product 93 (a terminal product stream) of the third column 73 and that all-of component B in the bottom product stream 91 of the second column 72 ends up in the overhead 92 (a terminal product stream) of the third column 73. Thus, in order to produce terminal stream products of desired specifications, notwithstanding significant changes in the concentration of components B and E in the feedstock supplied to the train via line 87, it will be necessary to control the manipulation of the splits performed by columns 71 and 72.

According to this invention, the following material balance equations, can be written covering the fractionation system of FIGURE 4:

11 1 1) llo'+( 1) lo+( 1) ho+( 1) hho'+ 1) hhho' 1) no'=( 2) uo" 0 10 2) io+( 3) ]lo"' 1) ho'=( 2) ho"+( 3) lo"'+( 3) 1ib"' 5 1) hho' 3) ho+( 3) 1b" 1) hho 3) hb" 2= a+ 3 2=( 2) ub"+( 2) ib"-l-( 2) hb"+( 2) hnb" 33 IO 2) 11b"=( 3) 1io" 2) lb" 3) io"'+( 3) Ilb z) nb"=( 3) hu"'+( 3) 1b"' 2) hhb" s) hb"' Material balance Equations 13 to 37 can be solved simultaneously for E (the decimal fraction of key component E in overhead 88) and B (the decimal fraction of component B in bottom product 91). A

computer which can be used for this purpose is illustrated in FIGURE 5.

The computer of FIGURE 5 can be used as the specifications computer 78 of FIGURE 4 to produce an output signal 76 proportional to E and an output signal 77 proportional to B the two setpoint adjusting signals employed by analyzer recorder controllers 74 and 75, respectively. Analyzer recorder 74 compares setpoint signal 76 with a signal from analyzer 94 proportional to the concentration of component E in the overhead from column 71, the difference being transmitted as a setpoint adjusting signal 95 to the flow controller 96 used in manipulating the flow of reflux to column 71. Analyzer recorder 75 compares its setpoint signal 77 with a signal from analyzer 97 proportional to the concentration of component B in the bottom product from column 72, the difference being transmitted as a setpoint adjusting signal 98 to the flow controller 99 used in manipulating the steam to the reboiler of column 72. Since column 73 produces an overhead 92 and a bottom product 93 which are both terminal product streams, the setpoint adjusting signals 85 and 86 transmitted to the analyzer recorder controllers 81 and 82, respectively, of 73 will be proportional to the desired specifications of a component in each of the terminal streams, for example C and D m, respectively. Analyzer recorder 81 compares signal 85 with a signal from analyzer 101 proportional to the measured concentration of component C in the overhead, the difference being transmitted as a setpoint adjusting signal to flow controller 102 so as to control the flow of reflux column 73. Also, analyzer recorder controller 82 compares signal 86 with a signal from analyzer 103 proportional to the measured concentration of component D in the bottom product, this difference being transmitted as a setpoint adjusting signal to flow controller 105 so as to control the steam to the reboiler of column 73.

Where the preferred control system and apparatus of said copending application Ser. No. 189,375 is used in this invention, the setpoints for the operations computer of each of such columns as illustrated in FIGURE 4 can be computed by specifications computer 106, such a system being illustrated in FIGURE 6.

The material balance Equations 1 to 12, pertaining to FIGURES 1 and 3 (and Diagram I) and the material balance Equations 13 to 37 pertaining to FIGURE 4 (and Diagram II) can both be expressed by a general set of simultaneous equations of the form:

for the x values, where the a and the k are constants. This set of equations can also be designated as follows:

In the equations of the Formula 38, x is a material flow, e.g., F of Equation 1, and x is the decimal fraction of a component in the flow, e.g., D of Equation 9. In all equations which contain only material flows b will be equal to zero.

The computers illustrated in FIGURES 2 and 5 are adapted to solve sets of simultaneous equations of the form 38. In addition, any other computers known in the art for solving simultaneous equations can be used, for example those disclosed in US. Patent No. 2,827,113, US. Patent No. 2,742,227, and US. Patent No. 2,808,989. In the practice of this invention, it is not actually necessary to write out the material balance equations for each particular train of fractionators, since the particular specification computer necessary to control a particular train of fractionators according to this invention can he arrived at implicitly.

In Diagram III below there is illustrated in diagrammatic fashion a four-column train of fractionators designed to fractionate a train feedstock comprising key components A, B, C, D and E to produce five terminal product streams having desired specifications.

DIAGRAJj III second Overhead 0:

Composition:

Biro (specify) F :flow rate of feed to first column 0 =flow rate of overhead from first column B =fiow rate of bottom product from first column 0 =flow rate of overhead from second column B =fiow rate of bottom product from second column O =fiow rate of overhead from third column B =flow rate of bottom product from third column O =flow rate of overhead from fourth column B =fiow rate of bottom product from fourth column C =decimal fraction of light key component C in feed to first column B decimal fraction of component B in feed to first column lighter than light key component C in feed to first column A =decimal fraction of component A in feed to first column lighter than component B in feed to first column D =decimal fraction of heavy key component D in feed to first column E =decimal fraction of component E in feed to first column heavier than heavy key component D in feed to first column A decimal fraction of light key component A in overhead from first column B decimal fraction of heavy key component B in overhead from first column c -decimal fraction of component C in overhead from first column heavier than heavy key component B in overhead from first column D l=decimal fraction of component D in overhead from first column heavier than component C in overhead from first column C =decimal fraction of light key component C in bottom product from first column D =decimal fraction of heavy key component D in bottom product from first column E =decimal fraction of component E in bottom product from first column heavier than heavy key component D in bottom product from first column A -=decimal fraction of light component A in overhead from second column B -=decirnal fraction of heavy component B in overhead from second column B decimal fraction of light key component B in bottom product from second column A -=decimal fraction of component A in bottom prod not from second column lighter than light key component C in bottom product from second column C -=decimal fraction of heavy key component C in bottom product from second column D "=decimal fraction of component D in bottom product from second column heavier than heavy key component C and bottom product from second column C m=decimal fraction of light component C in overhead from third column D w=decimal fraction of heavy component D in overhead from third column E m=decimal fraction of component E in overhead from third column heavier than heavy component D in overhead from third column D m=decimal fraction of light component D in bottom product from third column E m=decimal fraction of heavy component B in bottom 7 product from third column B M=decimal fraction of light component B in overhead from fourth column A w=decimal fraction of component A in overhead from fourth column lighter than light component B in overhead from fourth column C --=decimal fraction of heavy component C in overhead from fourth column C w=decimal fraction of light component C in bottom product from fourth column B --=decimal fraction of component B in bottom product from fourth column lighter than light component C in bottom product from fourth column D --=decimal fraction of heavy component D in bottom product from second column According to this invention, the terminal product streams are rigorously specified. Each of those columns in Diagram III which produce a product stream used as feed by a downstream distillation column is controlled by use of a specification computer to insure that such column produces a product stream having a composition such that it can be subsequently fractionated to meet desired terminal stream specifications. A specifications computer which can be used in control of such a train of fractionators is illustrated in FIGURE 7. Such a specification computer can be used to produce output signals utilized as adjusting signals for conventional control instruments used in controlling the split of a column, but again such output signals from the specifications computer are preferably used by the operations computers of said copending application Ser. No. 189,375 to control the operation of each column in the train. There is illustrated in FIGURE 8 the latter system, and it should be self-explanatory in view of the foregoing discussion.

In the control of a train of fractionators according to this invention, the setpoint adjusting signal computed for a column in the train which produces a product stream used by downstream fractionator will be a signal proportional to the concentration of a component in such a feed stream which is an undesirable component therein and which ends up in only one of the downstream terminal product streams, where it can be tolerated in said terminal stream in a concentration which does not prevent the specification of that terminal product stream from being met. Where the operations computer of said copending application Ser. No. 189,375 is used in control of each column in the train of columns, the two setpoint adjusting signals computed by the specifications computer of this invention and used in such operations computers will be dependent upon the nature of the product stream produced by each column. For example, each such operations computer will have two setpoint adjusting signals: one for the bottom product stream and one for the overhead stream. If the product stream is a terminal product stream, the setpoint chosen for this product stream will be the specified decimal fraction of a component present in this terminal stream. If the product stream is a feed stream for a subsequent fractionator, the setpoint chosen for this product stream will be the computed decimal fraction of an undesired component therein which ends up in only one of the downstream product streams.

An example of the use of the control system of this invention in controlling the operation of a train of fractionators will now be described, but it should be understood that such example is merely illustrative of a preferred embodiment of this invention and it should not be construed to limit unduly this invention.

Referring to FIGURE 4 and Diagram II, a feedstock comprising a mixture of liquid petroleum gases is supplied via line 87 to the first distillation column 71 of a train of distillation columns comprising columns 71, 72 and 73. This feed stock 87 has a flow rate of 1,000 bbls./hr., as measured by flow device 19, and has the following composition, as determined by analyzer 16:

Concentration of component Component in feed: in feed stock (v-ol. percent) According to this invention, the terminal product streams 89, 90, 92 and 93 are rigorously specified, these specifications being as follows:

Flow device 19 transmits via signal line 22 a signal proportional to the feedstock flow rate to specifications computer 78, and analyzer 16 transmits over signal line 18 to said specifications computer the concentrations of the ethylene, propane, isobutane and normal butane components in the feedstock. The abovementioned terminal stream specifications are dialed into specifications computer 78 as input signals proportional to such specifications. Specifications computer 78, such as that shown in FIGURE 5, accordingly, computes E the concentration of component E (isopentane) in the overhead 88 from column 71, and finds that it must be 0.327 vol. percent or less if columns 72 and 73 are to fractionate the first overhead and produce terminal products which meet the predetermined specifications. Also, the specifications computer, with the aforementioned information supplied thereto, computes B the concentration of component B (propane) in the bottom product 91 of column 72, and finds that it must be 0.212 vol. percent or less if column 73 is to be able to fractionate said stream 91 and produce said terminal product streams which meet the predetermined specifications.

Accordingly, specifications computer 78 produces output signal 76 (E and output signal 77 (E which signals can be used to adjust the setpoints of .analyzer recorder controllers 74 and 75, but are preferably used as the input signals for operations computers controlling the split of columns 71 and 72, respectively. Where the operations computers are thus used, said signal 76 will be supplied to such operations computer on column 71 together with a signal proportional to the predetermined specification E In the case of the operations computer on column 72, input signals to the latter will be B and a signal proportional to the predetermined specification Y. As far as the operations computer on column 73 is concerned, since this column does not produce a feed stream which is fractionated by a downstream column, the input signals sent to this operations computer on this column will be proportional to the output specification E and B thus controlling the operation of column 73 to assure production of product stream where D is 3 vol. percent and C is 1 vol. percent.

In the above example, the following rates for the various product streams are as follows:

Product stream:

First overhead product (0 624.5 First bottom product (B 375.5 Second overhead product (0 365.1 Second bottom product (B 259.4 Third overhead product (0 55.0 Third bottom product (B 204.2

Various modifications and alterations of this invention will become apparent to those skilled in the art from the foregoing discussion and accompanying drawing, and it should be understood that the preferred embodiments set forth herein are illustrative of the invention and should not be construed to limit unduly this invention.

We claim:

1. In a process wherein a multicomponent feed stock is separated in a train of fractional distillation columns to produce a plurality of terminal product streams each of which possesses a specified purity, an improved control method therefor comprising the steps of measuring a process variable indicative of the flow rate of said feed stock entering the first of said columns in said train; producing in response thereto a signal representative of the fiow rate of said feed stock; analyzing said feed stock to determine the concentration of key components present in the feed stock; producing in response thereto signals representative of concentrations of said key components; combining said signals in material balance equations with signals representative of predetermined concentrations of said key components desired in the terminal product streams of the individual distillation columns in said train; analyzing at least one stream exiting from said first column in said train which forms a feed to the next column located in said train to determine the concentration of specified key components therein; producing a feedback signal responsive to the concentration of said specified key component in said exit stream; biasing the signal representative of the concentration of said specified key component which can be tolerated in the said exit stream from said first column with said feedback signal; and controlling in response to said biased signal the flow rate of at least one stream selected from the group of streams comprising a bottoms product stream, a distillate product stream, an external reflux stream, a reboiler steam stream of each of said columns within said train so as to produce an exit stream used as a feed stream by a downstream column having a specified concentration of a predetermined key component therein; and repeating said control method for each of said columns in said train thereby assuring that each of the terminal product streams produced in the train of columns will possess predetermined purities.

2. In a process wherein a multicomponent feed stream is separated into fractions in a train of fractional distillation columns wherein each of said columns separates a feed stream into an overhead vapor stream and a liquid bottom product stream each of which possesses a specified purity, wherein said overhead stream is condensed and some of the condensed overhead is recycled to said columns as an external reflux stream and some of said condensed overhead is yielded as said distillate product stream, a control method therefor which ensures that each of said columns produces distillate and bottoms product streams possessing specified purities, said control method comprising the steps of measuring a process variable indicative of the flow rate of said feed stream to the first of said columns in said train; measuring the amounts of feed components in said feed stream; producing signals proportional to said measurements; combining in an operations computer for said first column said signals together with signals proportional to constants in a statisticallyderived equation based on the expression:

Ip=] or E: FT: e: HD: LB) Where:

R =predicted internal reflux flow rate for said column F =flow rate of said feed stream F =generic symbol for the amounts of said feed components expressed as fractions of said feed stream E=tray eificiency of said column F =feed tray of said column, numbering trays from the top of said column F =enthalpy of said feed stream H =specified liquid fraction of heavy key component in said distillate product stream L =specified liquid fraction of light key component in said bottom product stream; producing a signal proportional to said R combining said feed flow rate signals and said signals proportional to the concentration of said key components in said feedstock together with signals representative of predetermined specifications for said terminal product streams produced in each of the columns in said train in a material balance-computer; producing in said material balance computer a signal representative of the concentration of said specified key components which can be tolerated in said terminal product streams of each of the columns in said train; modifying said R signal with said signal representative of the concentration of said multicomponents which can be tolerated in the terminal product stream; controlling the flow rate of the external reflux to said first column in order to produce a specified purity of said key component in the distillate product stream and also in the bottoms stream which serves as a feed for a downstream column in the train of columns in response 17 to said modified R signal; and repeating said control method for each of said columns in said train.

3. A process according to claim 2 further comprising the steps of analyzing the bottoms stream to determine the concentration of a specified key component therein; producing a feedback signal responsive to said analyzation which is representative of the amount of said key component in the bottoms stream; comparing said predetermined calculated amount of key component in said bottoms stream with said feedback signal and biasing the predicted flow rate of said bottoms stream in response to the diiference between the predicted amount of key component in said bottoms stream and the measured amount of key component in said bottoms stream.

4. A process according to claim 2 further characterized in that the train of columns consists of two columns wherein the first column produces a distillate terminal product and the second column produces a distillate and a bottoms product wherein the bottoms from the first column is used as a feed to the second column.

References Cited by the Examiner UNITED STATES PATENTS 2,453,205 11/1948 Docksey 202160 X 2,459,404 1/ 1949 Anderson 202160 X 2,808,989 10/1957 Younkin 235-18O 2,972,446 2/1961 White 23646 2,972,447 2/ 1961 White 236-46 3,018,229 1/1962 Morgan 202-160 3,034,307 5/ 1962 Berger.

3,042,637 7/ 1962 Crouch.

3,108,929 10/ 1963 Tolin et al.

3,143,643 8/1964 Fluegel et a1. 235 3,150,064 9/1964 Dobson 202 3,151,237 9/1964 Hrabak 235151.13 3,230,154 1/1966 Walker 202160 FOREIGN PATENTS 1,177,743 12/1958 France.

OTHER REFERENCES G. H. Amber et 211.: Data Control, Automatic Control, May 1958, vol. 7-8, pages 4348.

Petroleum Refiner, I. F. Pink, March 1959, vol. 38, N0. 3, pages 215 220. Y

NORMAN YUDKOFF, Primary Examiner.

GEORGE D. MITCHELL, W. BASCOMB,

Assistant Examiners.

Claims

1. IN A PROCESS WHEREIN A MULTICOMPONENT FEED STOCK IS SEPARATED IN A TRAIN OF FRACTIONAL DISTILLATION COLUMNS TO PRODUCE A PLURALITY OF TERMINAL PRODUCT STREAMS EACH OF WHICH POSSESSES A SPECIFIED PURITY, AN IMPROVED CONTROL METHOD THEREFOR COMPRISING THE STEPS OF MEASURING A PROCESS VARIABLE INDICATIVE OF THE FLOW RATE OF SAID FEED STOCK ENTERING THE FIRST OF SAID COLUMNS IN SAID TRAIN; PRODUCING IN RESPONSE THERETO A SIGNAL REPRESENTATIVE OF THE FLOW RATE OF SAID FEED STOCK; ANALYZING SAID FEED STOCK TO DETERMINE THE CONCENTRATION OF KEY COMPONENTS PRESENT IN THE FEED STOCK; PRODUCING IN RESPONSE THERETO SIGNALS REPRESENTATIVE OF CONCENTRATIONS OF SAID KEY COMPONENTS; COMBINING SAID SIGNALS IN MATERIAL BALANCE EQUATION WITH SIGNALS REPRESENTATIVE OF PREDETERMINED CONCENTRATION OF SAID KEY COMPONENTS DESIRED IN THE TERMINAL PRODUCT STREAMS OF THE INDIVIDUAL DISTILLATION COLUMNS IN SAID TRAIN; ANALYZING AT LEAST ONE STREAM EXTING FROM SAID FIRST COLUMN IN SAID TRAIN WHICH FORMS A FEED TO THE NEXT COLUMN LOCATED IN SAID TRAIN TO DETERMINE THE CONCENTRATION OF SPECIFIED KEY CONPONENTS THEREIN, PRODUCING A FEEDBACK SIGNAL RESPONSIVE TO THE CONCENTRATION OF SAID SPECIFIED KEY CONPONENT IN SAID EXIT STREAM; BIASING THE SIGNAL REPRESENTATIVE OF THE CONCENTRATION OF SAID SPECIFIED KEY CONPONENT WHICH CAN BE TOLERATED IN THE SAID EXIT STREAM FROM SAID FIRST COLUMN WITH SAID FEEDBACK SIGNAL; AND CONTROLLING IN RESPONSE TO SAID BIASED SIGNAL THE FLOW RATE OF A LEAST ONE STREAM SELECTED FROM THE GROUP OF STREAMS CONPRISING A BOTTOMS PRODUCTS STREAMS, A DISTILLATE PRODUCT STREAM, AN EXTERNAL REFLUX STREAM, A REBOILER STREAM STREAM OF EACH OF SAID COLUMNS WITHIN SAID TRAIN SO AS TO PRODUCE AN EXIT STREAM USED AS A FEED STREAM BY A DOWNSTREAM COLUMN HAVING A SPECIFIED CONCENTRATION OF A PREDETERMINED KEY CONPONENT THEREIN; AND REPEATING SAID CONTROLL METHOD FOR EACH OF SAID COLUMNS IN SAID TRAIN THEREBY ASSURING THAT EACH OF THE TERMINAL PRODUCT STREAMS PRODUCED IN THE TRAIN OF COLUMNS WILL POSSESS PREDETERMINED PURITIES.