Means and method for controlling a solvent refining unit for maximum yield

Abstract

A system for controlling a solvent refining unit so as to operate the refining unit at the maximum limit of its operating parameters. Three such operating parameters are the refining temperature which is limited by the miscibility of charge oil and the solvent, the flow rates of extract oil and refined oil, which are limited by the mechanical design of the refining unit, obtained from the refining of the charge oil. A plurality of computers determine the values of constants from equations, hereinafter disclosed, so that the operating parameter that is limiting may be determined. A plurality of analog computers generate control signals for the different limiting operating parameters. Switching means apply the proper control signals to the refining unit in accordance with the determination of which operating parameter is limiting.

Claims

1. A control system for a solvent refining unit which treats charge oil with a solvent in a refining tower to yield raffinate and extract-mix, strippers separate the solvent from the raffinate and from the extract-mix to provide refined waxy oil and extract oil, respectively, the solvent is returned to the tower and the refined waxy oil is subsequently dewaxed to provide refined oil, comprising means for controlling the operation of the refining unit, first means connected to the control means for providing control signals to the control means to operate the refining unit for a predetermined time period at a predetermined solvent dosage-temperature combination so as to provide refined oil of a desired quality, means for measuring at least one condition of the extract oil and one condition of the refined waxy oil anD providing signals corresponding thereto, means for measuring at least one property of the charge oil and providing a corresponding signal, signal means for providing signals corresponding to limitations of operating parameters of the refining unit and the refining operation, means connected to the condition measuring means, to the property measuring means and to the limitation signal means for determining which operating parameter is limiting and providing signals corresponding thereto, and second means connected to the determining means and to the control means for providing control signals to the control means after the predetermined time period in accordance with the signals from the determination means to control the operation of the refining unit so that the refining unit operates at a maximum capability while maintaining the quality of the refined oil. 2. A system of the kind described in claim 1 in which the solvent is N-methyl-2-pyrrolidone and the solvent flows at a maximum possible rate SOLLIM. 3. A system as described in claim 2 in which the first control signal means includes means for providing direct current control signals corresponding to a selected charge oil flow rate COSEL and to a selected refining temperature TSEL, and switching means connecting the COSEL, TSEL signal means to the control devices for momentarily applying the COSEL and the TSEL signals to the control devices to control the refining unit so that the refining unit operates with the selected charge oil flow rate and refining temperature until other control signals are applied to the control devices. 4. A system as described in claim 3 in which the signal means provides signals corresponding to the maximum possible flow rates EOLIM and ROLIM of the extract oil and the refined waxy oil, respectively. 5. A system as described in claim 4 in which the measured property of the charge oil is the viscosity, and the measured conditions of the extract oil and the refined waxy oil are the extract oil flow rate EOM and the refined waxy oil flow rate ROM; and the determining means includes means connected to the property measuring means for providing a signal corresponding to a correlation constant n in accordance with the viscosity signal from the property measuring means, means connected to the condition measuring means and to the n constant signal means for providing a signal corresponding to an a constant in accordance with the EOM and ROM signals from the condition signal means and the n constant signal from the n factor signal means, means connected to the limitation signal means for providing a signal corresponding to an aLIM constant in accordance with the EOLIM and ROLIM signals from the limitation signal means, and a comparator connected to the a constant signal means and to the aLIM constant signal means for comparing the a constant signal to the aLIM constant signal to determine whether the refined waxy oil flow rate is the limiting operating parameter, the extract oil flow rate is the limiting operating parameter, or the refining unit is balanced. 6. A system as described in claim 5 in which the n constant signal means includes memory means in various values of n have been stored, and selection means connected to the memory means and to the viscosity measuring means for selecting the proper n value from the memory means in accordance with the viscosity signal from viscosity measuring means; the a constant signal means is an a constant analog computer providing the a constant signal in accordance with the following first equation: a ROM/(EOM)n; and the aLIM constant signal means is an aLIM constant analog computeR providing the aLIM constant signal in accordance with the following second equation: aLIM ROLIM/(EOLIM)n. 7. A system as described in claim 6 in which the comparator provides a signal having one amplitude when the a constant signal is equal to or greater than the aLIM constant signal and another amplitude when the a constant signal is less than the aLIM constant signal; and the determining means includes a b constant analog computer receiving direct current voltages and being connected to the condition measuring means and to the first control means for providing a signal corresponding to the b constant in accordance with the EOM and ROM signals from the condition measuring means, the SSEL and TSEL signals from the COSEL, TSEL signal means, the direct current voltages and the following third equation: where m is a constant having a value within the range of 0.75 to 0.80; and the second control signal means includes means connected to the a constant computer, to the b constant computer and to the n constant signal means and receiving direct current voltages for providing signals corresponding to the charge oil flow rate COE and to the refining temperature TE for the condition where the extract oil flow rate is at its maximum possible rate EOLIM; means connected to the a and b constant computers and to the n constant signal means for providing signals corresponding to the charge oil flow rate COR and to the refining temperature TR for the condition where the refined waxy oil flow rate is at its maximum possible rate ROLIM; switching means connected to the COE, TE signal means, to the COR, TR signal means and to the comparator and controlled by the signal from the comparator to pass the COR and TR signals from the COR, TR signal means when the signal from the comparator is of the one amplitude and to pass the COE and the TE signals from the COE, TE signal means when the signal from the comparator is of the other amplitude; subtracting means receiving direct current voltages, corresponding to the miscible temperature TMISC of the charge oil and the N-methyl-2-pyrrolidone and to a temperature T1 occurring within the range of 10* to 20* F. for subtracting the T1 voltage from the TMISC voltage to provide a signal corresponding to the maximum permissable refining temperature TMAX; means connected to the a and b constant computers, to the n constant signal means and to the subtracting means and receiving direct current voltages for providing a signal corresponding to the charge oil flow rate COT for the condition where the refining temperature is at its maximum permissable level TMAX; a second comparator connected to the first switching means and to the subtracting means for comparing the TMAX signal with the TE or TR signal passed by the first switching means and providing a signal of one amplitude when the TR or TE signal is less than the TMAX signal and of another amplitude when the TR or TE signal is equal to or greater than the TMAX signal, and second switching means connected to the subtracting means, to the first switching means, to the COT signal means, to the control devices and to the second comparator for blocking the signals from the first switching means, from the subtracting means and from the COT signal means during the predetermined time period and for pAssing the COE, TE or the COR, TR signals from the first switching to the control devices as the control signals after the predetermined time period in response to the signal from the second comparator being of the one amplitude and for passing the COT signal from the COT signal means and the TMAX signal from the subtracting means to the control devices as the control signals after the predetermined time period in response to the signal from the second comparator being of the other amplitude. 8. A system as described in claim 7 in which the COE, TE signal means is an analog computer providing the COE and the TE signals in accordance with the following fourth through seventh equations: 9. A method for controlling a solvent refining unit in which charge oil is treated with solvent in a refining tower to yield raffinate and extract-mix, strippers separate the solvent from the raffinate and extract-mix to provide refined waxy oil and extract oil, respectively, the solvent is returned to the tower and the refined waxy oil is subsequently dewaxed to provide refined oil, which comprises refining the charge oil for a predetermined time period at a predetermined solvent dosage and a predetermined temperature to achieve a desired quality of refined oil, measuring at least one property of the charge oil, measuring at least one condition of the refined waxy oil and one condition of the extract oil, providing signals corresponding to the measurements, providing signals corresponding to limitations of the refining unit and the refining operation, utilizing the signals to determine which operating parameter is limiting, and controlling the operation of the refining unit after the predetermined time period in accordance with the determination. 10. A method as described in claim 9 in which the solvent is N-methyl-2-pyrrolidone. 11. A method as described in claim 10 in which the measured property of the charge oil is the viscosity, the limitation signals correspond to the maximum possible flow rates EOLIM and ROLIM of the extract oil and the refined waxy oil, respectively, and to the maximum permissable refining temperature TMAX, the measured conditions of the extract oil and the refined waxy oil are their flow rates EOM and ROM, respectively, and the charge oil flow rate and the refining temperature are controlled in accordance with the determination. 12. A method as described in claim 11 in which the determining step includes providing an n constant signal in accordance with the measured viscosity signal, providing an a constant signal in accordance with the measured extract oil flow rate EOM signal, the measured refined waxy oil flow rate ROM signal, the n constant signal and the following equation: a ROm/(EOM)n, providing an aLIM constant signal in accordance with the maximum possible extract oil and refined waxy oil flow rates EOLIM and ROLIM signals, respectively, the n factor signal and the following equation: aLIM ROLIM/(EOLIM)n, providing an m constant signal, having a value within the range of 0.75 to 0.80, and providing a b constant signal in accordance with the selected dosage SSEL signal, the selected refining temperature TSEL signal, the measured extract oil and refined waxy oil flow rates EOM and ROM signals, direct current voltages corresponding to a value of 1 and to an exponent m, respectively, and the following equation: 13. A method as described in claim 12 in which the determining step includes comparing the a constant and the aLIM constant signals, providing a comparison signal having one amplitude when the a constant signal is less than the aLIM constant signal and another amplitude when the a constant signal is equal to or greater than the aLIM constant signal, providing control signals COE and TE corresponding to a flow rate for the charge oil and a refining temperature, respectively, when the comparison signal is of the one amplitude, providing control signals COR and TR when the comparison signal is of the other amplitude, determining the miscible temperature of the charge oil and the solvent, providing a signal corresponding to a maximum refining temperature TMAX which is substantially less than the miscible temperature, comparing the TR or TE signal, whichever is provided, with the TMAX signal, providing a second comparison signal having one amplitude when the TR or TE signal is less than the TMAX signal and another amplitude when the TR or TE signal is equal or greater than the TMAX signal, providing a COT signal, corresponding to a flow rate for the charge oil, and the TMAX signal as control signals after the predetermined time period when the second comparison signal is of the other amplitude, and providing the COR, TR signals or the COE, TE signals as control signals after the predetermined time period when the second comparison signal is of the one amplitude. 14. A method as described in claim 13 in which the COR and TR signals are provided in accordance with the n constant signal, the ROLIM signal, the SOLLIM signal, the a constant signal, direct current voltages corresponding to values of 1, 100, and m and the following equations:
United States Paten 3,686,48 1 Aug. 22, 1972 Woodie [54] MEANS AND METHOD FOR CONTROLLING A SOLVENT REFINING UNIT FOR MAXI YIELD [72] Inventor: Robert Alan Woodie, Nederland, - Tex. [73] Assignee: Texaco Inc., New York, NY. [22] Filed: Dec. 29, 1970 [21] Appl. No.: 102,344 [52] US. Cl ..235/15L12, 196/132, 208/D1G. 1 [51] Int. Cl. ..G06g 7/58 [58] Field ofSearch ..235/l51.l2, 151.13, 151.1, 235/150; 208/36, 311, DIG. 1,33, 313; 196/145, 14.52, 132, 46 [56] References Cited UNITED STATES PATENTS 7 3,458,432 7/ 1969 Woodie et al ..208/D1G. 1 3,285,846 11/1966 King et a1 ..208/36 X 3,173,966 3/1965 Jones et a1. ..208/311 X 3,190,828 6/1965 Daniel et a1 ..196/132 X REFINING COOLING V WATER MEMORY CONV. Stewart ..208/DIG. 1 Graff et al ..208/DIG. 1 [ 57] ABSTRACT A system for controlling a solvent refining unit so as to operate the refining unit at the maximum limit of its operating parameters. Three such operating parameters are the refining temperature which is limited by the miscibility of charge oil and the solvent, the flow rates of extract oil and refined oil, which are limited by the mechanical design of the refining unit, obtained from the refining of the charge oil. A plurality of computers determine the values of constants from equations, hereinafter disclosed, so that the operating parameter that is limiting may be determined. A plurality of analog computers generate control signals for the difierent limiting operating parameters. Switching means apply the proper control signals to the refining unit in accordance with the determination of which operating parameter is limiting. 14 Claims, 6 Drawing Figures PPER DEWAXIN MEANS REFINED OIL 1 l 1 l l l l l I DIRECT CURRENT VOLTAGE SOURCE Patented Aug. 22, 1972 3,686,488 3 Sheets-Sheet 2 ID i Q 2 UPPER LIMIT .e I I z PREFERRED VALUE f n .6 g;/ f 4 //Z LOWER LIMIT o 30 40 so so 70 so 90 I HO I I I VISCOSITY OF CHARGE OIL AT 2|OF El? FIG. 3 I- 1" *fi 5B,] l E 33A -I4 41 7 V |E E3 I I I2 DIVIDER DIVIDER MULTIPLIER MULTIPLIER l I I 44 I 4Q/MULTIPLIER 42 I I b CONSTANT 46/ LcoMPuTER I ?I :EA r-Eg5 E 4 25 2o I I I 6| q l I l I I 335 MULTIPLE I, i DIVIDER MULTIPLIER T l I I as I I IE I I l DIVIDER 33c I i l l I I I I I K62 I I TEMPERATURE I SIGNAL CIRCUIT I :-5| l J L 4 E274 TE,OUM COMPUTER Patented Aug. 22, 1972 3,686,488 3 Sheets-Sheet 5 FIG. 5 E E I R.O.| |M-COMPUTER I l I |6)l I 22 l DIVIDER I l I E6] I75 [62A I I I 77 I Esvl TEMPERATURE I I DIVIDER' 350 SIGNAL I, CIRCUIT I 176 I I I I I I L .I FIG 6 E2 I 1 I I I02 I l M /AL I G l25 I i |-S-.- I l l ll8 I I03 I08 E us I 6| I E MULTIPLIER MULTIPLIER l I I I H5 l E6 I I "'I n4 I I I. MULTIPLIER MULTIPLIER 33F MULTIPUER I ls l hog I I I I I E I as 4 I E5|)| 33G E I IZI) I22] 2s I I l MULTIPLIER MULTIPLIER DIVIDER I E I L co COMPUTER MEANS AND METHOD FOR CONTROLLING A SOLVENT REFINING UNIT FOR MAXIMUM YIELD BACKGROUND OF THE INVENTION 1 Field of the Invention The present invention relates to control systems and, more particularly, to a control system for a, solvent refining unit. 2. Description of the Invention Heretofore, control systems for solvent refining units, such as disclosed in US. application Ser. No. 96,193, filed Dec. 8, 1970 by Robert A. Woodle, inventor of the present invention, and assigned to Texaco Inc., assignee of the present invention, provide optimum control of the refining unit. However, it may be profitable to operate a solvent refining unit at its maximum capability even though that operation is not an optimum operation. The control system of the present invention determines which operating parameter is limiting the yields of extract oil and refined oil from a solvent refining unit and provides control of the solvent refining unit accordingly to achieve a maximum operating capability while yielding refined oil of a desired quality. SUMMARY OF THE INVENTION A system controls a solvent refining 'unit which treats charge oil with a solvent in a refining tower to yield raffinate and extract mix. The solvent is separated from the raffinate and from the extract mix by strippers to provide refined waxy oil and extract oil, respectively. The solvent is then returned to the refining tower. This control system comprises control devices receiving control signals which cause the refining unit to operate for a predetermined time period at a predetermined solvent dosage-temperature combination for providing refined oil of a desired quality. At least one condition of the extract oil and the refined waxy oil is measured by a circuit which provides corresponding signals. Another circuit measures at least one property of the charge oil and provides a signal corresponding thereto. A signal source provide signals corresponding to the limitation of the refining unit and the refining operation. A network determines which operating parameter is limiting the refining of the charge oil from the measurement signals and the signals from the signal source and provides corresponding control signals to the control device. The operation of the refining unit is controlled after the predetermined time period by the last mentioned control signals. One object of the present invention is to provide a system controlling a refining unit so that the refining unit operates at a maximum capability. Another object of present invention is to operate a solvent refining unit at a refining temperature that is substantially lower than the temperature at which the solvent and charge oil becomes miscible. Another object of the present invention is to operate a refining unit at a maximum refined oil flow rate to yield refined oil of a desired quality. Another object of the present invention is to operate a refining unit at a maximum extract oil flow rate to yield refined oil of a desired quality. Another object of the present invention is to provide a system which determines which operating parameter DESCRIPTION OF THE DRAWINGS FIG. 1 is a simplified block diagram of a system, constructed in accordance with the present invention, for controlling a solvent refining unit so that solvent refining unit operates at a maximum capability. FIG. 2 is a graphic representation of a correlation of the viscosity of charge oil versus a mathematical constant n. FIGS. 3, 4, 5, and 6 are detailed block diagrams of the b constant computer, theEO RO and C0 computers. DESCRWI'ION OF THE INVENTION The aforementioned U.S-. application is for a system providing optimum control of a solvent refining unit. However, it may be desirable to operate the refining unit at its maximum capability even though the operation may not be optimum. There are at least three operating parameters that limit the capability of the refining unit and they are the refined oil flow rate, the extract oil flow rate and the miscible temperature. The miscible temperature is that temperature at which the charge oil dissolves completely in the solvent. The flow rates are limited by the physical design of the refining unit. Referring to FIG. I, there is shown a system for controlling a conventional type solvent refining unit to operate at its maximum capability where the solvent is N-methyl-Z-pyrrolidinone. The rate of the flow of the charge oil is controlled so as to regulate the flow rates of refined waxy oil and the extract oil. The temperature, which is also controlled, at which the refining of the crude oil takes place affects the yield of the refined oil and the extract oil. The rate of the charge oil entering a refuting tower 3 in a line 4 is sensed and controlled by conventional types sensing element 5, flow recorder controller 6 and valve 2. Sensing element 5 provides a signal to controller 6 corresponding to the flow rate of the charge oil. Controller 6 operates valve 2 to control the rate of flow of the charge oil to tower 3 in accordance with the signal from sensing element 5 and a signal E Signal E controls the set point of controller 6. Although not shown, for ease of explanation, the charge oil and refining solvent entering tower 3 through lines 4 and 7, respectively, have been heated to a predetermined temperature. Tower 3 contains packing 8 where the charge oil and solvent are contacted in counter current flow effecting the extraction of low viscosity index constituents of the crude oil. Raffinate including the refined waxy oil and a small amount of dissolved solvent is withdrawn through a line 10. A temperature gradient is maintained in tower 3 by means of a cooling coil 11 having cooling water flowing through it. The temperature in tower 3 is sensed by conventional type sensing means 12 which provides a corresponding signal to a temperature recorder controller 14. Temperature recorder controller 14, which may be of a type well known in the art, operates a valve 15 in accordance with the signal from temperature sensing means 12 and a signal E Signal E controls the set points of temperature recorder controller 14. Valve 15 controls the rate of flow of the cooling water to control the temperature in tower 3. Raffinate in line enters a stripper which strips the solvent from the raffinate to yield the refined waxy oil. The solvent is returned to tower 3 by line 7 while the refined waxy oil is provided to dewaxing means 16 through a line 17. Dewaxing means 16 removes the wax and provides refined oil for storage and blending with product lubricating oil. Elements having a numerical designation with a suffix are identical in operation and connection as elements having the same numerical designation without a suffix. Sensing means 5A and a conventional type flow transmitter 20 measures the rate of flow of the refined waxy oil from stripper 15 and provides a corresponding Extract-mix comprising solvent and dissolved low viscosity index constituents of the charge oil is withdrawn from tower 3 through a line 22 at a temperature controlled by cooling coil 11. The extract-mix in line 22 is passed to a stripper 23 where the solvent is stripped from the extract oil which is discharged through a line 25. The recovered solvent is withdrawn through line 7 for return to tower 3 and re-use. The flow rate of the solvent is maintained at a maximum and the solvent dosage is controlled by controlling the flow rate of the charge oil. Sensing means SE and a flow transmitter 20A senses the rate of flow of extract oil in line 25 and provides a corresponding signal E Initially the refining of the charge oil is done at a solvent dosage and temperature selected from various combinations of solvent dosage and temperature that yield the desired quality of refined oil. A source 26 of direct current voltage provides variable amplitude voltages E E corresponding to a selected flow rate CO for the charge oil and to the selected temperature T A switch 27 which may be of the momentary on double pole single throw type is activated to momentarily apply voltages E E as signals E and E respectively, to flow recorder controller 6 and to temperature recorder controller 14, respectively, to adjust the sets points so that the refining is initially performed at the selected solvent dosage-temperature combination. The selected dosage S is related to the selected flow rate by the following equation: SSEL=(SOLLIM/C0SEL) A sample of the charge oil'in line 4 is continuously applied to a viscosity meter 28, which may be of the type described by J. M. Jones, Jr. in US. Pat. Nos. 2,791,902 and 3,025,232. The effluent from meter 28 may be returned to tower 3 or discarded as slop. Viscosity meter 28 provides an analog signal E corresponding to the viscosity of the charge oil at 210F, to a conventional type analog-to-digital converter 29 which converts analog signal E to a digital signal. The digital signal from the analog-to-digital converter 29 controls a memory 30, which may be of a diode type with logic gating that is well known to one skilled in the art, to provide a digital signal corresponding to a characteristic constant n. The values of n as related to the viscosity of charge oil, shown in the form of a graph in FIG. 2, are stored in memory 30 and the digital signal from converter 29 controls the logic gating in memory 30 to pass the signal corresponding to the proper value of n. The digital signal from memory 30 is converted to an analog signal E, by a digital-to-analog converter 31. The n constant is used to calculate a constant a in accordance with the following equation: where RO is the measured flow rate of the refined waxy oil and EO is the measured flow rate of the extract oil. Signals E and E corresponding to the measured flow rates of RO EO of the refined waxy oil and the extract oil, respectively, and signal E are applied to an a constant computer 34, which provides a signal E corresponding to the a constant. Computer 34 raises the measured extract oil flow rate EO signal to the power n using an exponential circuit 33 which includes a conventional type logarithmic amplifier 35, a multiplier 36, an operational amplifier 37, and a feedack element 38. Signal E, from flow transmitter 20A is applied to logarithmic amplifier 35 which in turn provides an output to a multiplier 36 whose output is multiplied with signal E, by multiplier 36. The product signal from multiplier 36 is applied to operational amplifier 37 having feedback element 38 connecting to its input and output. Feedback element 38 is in essence a function generator, which may be of the PC-l2 type manufactured by Electronics Associates, that causes operational amplifier 37 to provide an output corresponding to (E0,,,)". Signal E from flow transmitter 20 is divided by the output from operation amplifier 37 by a divider 40 to provide signal E A computer 34A operates in a similar manner as a characteristic computer to provide a signal E corresponding to an a constant which is determined from the following empirically derived equation: LIM LIM K LIM)" (3) Direct current voltages E E from source 26 correspond to the maximum possible flow rates RO and E0, of the refined waxy oil and the extract oil within the physical constraints of the refining system. All direct current voltages from source 41 are with respect to a ground reference which is not shown. In the similar operation of computer 34A, as compared with computer 34, signal E replaces signal E while signal E replaces signal E Computer 34A also received signal E from converter 31. Another characteristic constant b is determined in accordance with the following empirically derived where S is a selected percent volume of solvent dosage, T is a selected refining temperature and m is a constant having a value in the range of 0.75 to 0.80, a preferred value being 0.775. It should be noted that although S SM and T are used, measured values of the charge oil flow rate and the refining temperature may be used in their place. A b constant computer 42 provides a signal E corresponding to the constant b, in accordance with equation (4), signals E E flow from transmitter and 20A, respectively, and direct current voltages E E E E E and E from source 26. Direct current voltages E E E E E and l5 correspond to flow rate CO the refining temperature T the term 1 in equation 4, the exponent m in equation 4, the maximum solvent flow rate SOL and the term 100 in equation 1 respectively. Referring to FIG. 3, computer 42 includes a divider 44 which divides signal E with signal Summing means 45 sums the resulting output from divider 44 with direct current voltage E and provides a corresponding sum signal to a divider 47. Divider 47 divides direct current voltage E by the sum signal from summing means 45 to provide an output. A divider 46 divides SOL signal E with CO voltage E to provide a signal to a multiplier 49 where it is multiplied with voltage E to provide a signal corresponding to S The output from divider 47 is multiplied with the S signal by a multiplier 48 to provide a product signal to another multiplier 50. Direct current voltage E is raised to the power m by an exponential circuit 33A receiving voltage E The output from exponential circuit 33A is applied to multiplier 50 where it is multiplied with the output from multiplier 48 to provide signal E The maximum refined oil and extract oil yields are constrained by the physical limitations of the refining unit. Thus the maximum refined oil yield occurs when the flow rate of the refined oil is at its physical limitation. The same is true for the extract oil. The determination of whether to operate the refining unit at the refined oil flow rate limitation R0,, or at the extract oil flow rate limitation EO is controlled by the constant a. When a is equal to or greater than a the refinery unit is operated for the maximum possible refined oil flow rate RO When a is less than a the refinery unit is operated for the maximum possible extract oil flow rate EO Referring to FIG. 1, an electronic switch 54 is controlled by a comparator 57, comparing signals E and E-,, to pass signals E E from an E0 computer 51, when signal E, from a constant computer 34 is equal to or greater than signal E, from a computer 34A and to block signals E E from E0 computer 51 when signal E is less than signal E Similarly, comparator 57, through an inverter 58, controls an electronic switch 54A to block signals E E from a RO computer 52 when signal E is equal to or greater than signal E and to pass signals E E from RO computer 52 when signal E is less than signal E Signals E and E corresponding to the charge oil flow rate C0,; and to the refining temperature T respectively, are developed by E0 computer 51 in accordance with the following equations: RO =a(EOLrM) 12 EOLIM+ I-l (6) (SO-LLIM where 8,; is the solvent dosage in percent volume and SOL is the maximum possible flow rate of the solvent within the physical constraint of the refining unit. Source 26 provides direct current voltages E E E E and E t0 EO computer 51 which correspond to the following terms, respectively: EO l, SOL 100 and l/m in equations 5 through 8. Computer 51 also receives signals E E and E from digital-toanalog converter 31, a constant computer 34 and b constant computer 42, respectively. Referring now to FIG. 4, an exponential circuit 338 provides a signal corresponding to (EO Y to a multiplier in accordance with signals E E Multiplier 60 multiplies the signal corresponding to (EO Y with a constant signal E to provide a signal corresponding to the flow rate RO of the refined oil. Summing means 61 sums the EO signal E with the R0,; signal from multiplier 60 to provide signal E corresponding to the flow rate co of the charge oil. A temperature signal circuit 62 includes a divider 64 which divides the SOL voltage E with signal E and the resultant signal is applied to a multiplier 65. The signal from divider 64 is multiplied with voltage E so that multiplier 65 provides an output corresponding to the term S in equations 7, 8. A divider 66 divides the b constant signal E with the output from multiplier 65 to provide a signal to a multiplier 69. The RO signal from multiplier 60 is divided by the EO signal E by a divider 70 in the temperature signal circuit 62 and the resulting output is summed CO EOR+ROLIM and Computer 52 receives direct current voltages E E E E and E from source 26, corresponding to the RO 1, SOL 100 and l/m terms in equations 9 through 12. The a, b, and n constant signals E E and E respectively, are also applied to RO computer 52. Referring to FIG. 5, a divider 75 in R0, computer 52 divides signal E with signal E and provides a resulting signal to an exponential circuit 33D. Another divider 76 divides the R0, signal 13,, with the a constant signal E to provide a signal to an exponential circuit 33D. Exponential circuit 33D provides a signal which corresponds to the extract oil flow rate EO to summing means 77 where it is summed with the RO signal E to provide signal E corresponding to the charge oil flow rate CO The EO signal from exponential circuit 33D is applied to a temperature signal circuit 62A along with signals E E and E and with voltages E E E and E Temperature signal circuit 62A provides T signal E in accordance with the applied signals and voltages and equation 12. Referring again to FIG. 1, electronic switches 54B, 54C control the refining temperature and the charge oil flow rate to assure that refining temperature does not exceed a maximum temperature. The maximum temperature T is set at a temperature to F less than miscibility temperature T When the value of the refining temperature, as determined by EO computer 51 or RO computer 52, exceeds T T is used as the refining temperature and the charge oil flow rate is changed to provide the correct solvent dosage associated with the T temperature so that the desired quality of refined oil may be maintained. A charge oil flow rate CO signal E for the T condition is developed by a C0 computer 90. Source 26 provides a variable amplitude direct current voltage E corresponding to T and another direct current voltage E corresponding to a temperature greater than 10 but less than 20 F. The value of TMISC may be determined experimentally from heating two volumes of N-methyl-2-pyrrolidone and one volume of charge oil until they become miscible. Subtracting means 91 subtracts voltage E from voltage E to provide a signal E corresponding to T to a comparator 57A, to CO computer 90 and to electronic switch 54C. Comparator 57A compares signal E with temperature signal E or E passed by switch 54 or 54A, respectively. Electronic switch 548 is controlled by comparator 57A, through an inverter 94, to pass the signals passed by electronic switch 54 or 54A when signal E is equal to or greater than signal E- or or E passed by switch 54 or 54A, respectively, and block the signals from switch 54 or 54A when signal E is less than signal E or E Electronic switch 54C is controlled by comparator 57A to block signals E E from C0 computer 90 and subtracting means 91, respectively, when signal E is equal to or greater than signal E or E and to pass signals E E when signal B is less than signal E or E Computer 90 develops CO signal E in accordance with the following equations: RO =G(EO where EO RO and CO are the flow rates for the extract oil, the refined oil and the charge oil when the refining temperature is T The C0 computer receives direct current voltages E E E E and E from source 26, corresponding to the l, SOL 100, 2 and m terms, respectively, in equation 13. Source 26 provides another direct current voltage E to computer 90 which does not correspond to a term in the aforementioned equations. Computer 90 also receives the n constant signal E the a constant signal E the b constant signal E and the T signal E Referring now to FIG. 6, voltage B is applied to a potentiometer connected to ground having a movable wiper arm 101 which is positioned by a direct current motor 102. It should be noted that all direct current voltages from source 26 are with respect to a ground reference which is not shown. A voltage E present at wiper arm 101 corresponds to the extract oil flow rate EO for the T condition. Voltage B is raised to the n power by an exponential circuit 33E receiving the n constant signal B A multiplier 103 multiplies the output from exponential circuit 33E with the a constant signal E to provide a signal E corresponding to the refined oil flow rate R0 for the T condition. Signal E is multiplied with voltage E by a multiplier 108 to provide a product signal corresponding to the term 2 a(E0 in equation 13. The a constant signal E is effectively squared by a multiplier 109 and the resulting signal is applied to another multiplier 110. The n constant signal E is multiplied with voltage E by a multiplier 1 11 and a resulting signal has voltage E subtracted from it by subtracting means 114. An exponential circuit 33F raises signal E to a power determined by the output from subtracting means 114 and the resulting signal is multiplied with the signal from multiplier 109 by multiplier 110. Multiplier provides a signal corresponding to the term a (E0 in equation 13. Summing means 115 sums signal E and the signals from multipliers 108, 110 to provide a signal, corresponding to the left side of equation 13, to subtracting means 1 16. Signal E is raised to the power 0.775 by an exponential circuit 336, receiving signal E and voltage E The SOL voltage E is multiplied with voltage E by a multiplier and the product signal is multiplied with the signal from exponential circuit 330 by another multiplier 121. The product signal from multiplier 121 is divided with the b constant signal E by a divider 122 to provide a signal to subtracting means 116 corresponding to the right side of equation 13. Subtracting means 116 subtracts the signal from divider 122 from the signal from summing means 115 to provide an output. When the output from subtracting means 116 is zero, signal E corresponds to the correct value of E0 The output from subtracting means 116 is amplified by an amplifier 125 to energize motor 102, when the output from subtracting means 116 is positive. Motor 102 moves wiper arm 101 of potentiometer 100 in a direction to reduce signal E until the output from subtracting means 116 is zero. Similarly, when the output from subtracting means 116 is negative, motor 102 moves wiper am 101 in a direction to increase signal E until the output from subtracting means 116 1s zero. Summing means 118 sum EO signal E with the R signal from multiplier 103 to provide the C0 signal E Referring back to FIG. 1, the signals passed by electronic switch 543 or 54C are applied to a double pole, single throw switch 130 which may be of the momentary on type. An operator closes switch 130 to provide the signals from electronic switch 543 or54C as signals E E to flow recorder controller 6 and to temperature recorder controller 14, respectively, to adjust their set points accordingly. As conditions change, the operator may change the amplitudes of the voltage from source 26 or the computed constant signals may change accordingly. The operator can then control the charge oilflow rate and the refining temperature by closing switch 130 to affect the refining of the charge oil in accordance with the new condition or conditions. The device of the present invention, as heretofore described, controls a solvent refining unit so that refining unit operates at a maximum capability. The solvent refining unit is operated at a temperature substantially less than the temperature at which the charge oil and the solvent become miscible. The solvent refining unit has been controlled to operate at the maximum refined oil flow rate possible when the refined oil flow rate is limiting and the computed refining temperature is substantially less than the miscibility temperature. The solvent refining unit has also been controlled to operate at the maximum extract oil flow rate possible when the extract oil flow rate is limiting and the computed refining temperature is substantially less than the miscibility temperature. ' Iclairn: 1. A control system for a solvent refining unit which treats charge oil with a solvent in a refining tower to yield raff'mate and extract-mix, strippers separate the solvent from the raffinate and from the extract-mix to provide refined waxy oil and extract oil, respectively, the solvent is returned to the tower and the refined waxy oil is subsequently dewaxed to provide refined oil, comprising means for controlling the operation of the refining unit, first means connected to the control means for providing control signals to the control means to operate the refining unit for a predetermined time period at a predetermined solvent dosage-temperature combination so as to provide refined oil of a desired quality, means for measuring at least one condition of the extract oil and one condition of the refined waxy oil and providing signals corresponding thereto, means for measuring at least one property of the charge oil and providing a corresponding signal, signal means for providing signals corresponding to limitations of operating parameters of the refining unit and the refining operation, means connected to the condition measuring means, to the property measuring means and to the limitation signal means for determining which operating parameter is limiting and providing signals corresponding thereto, and second means connected to the determining means and to the control means for providing control signals to the control means after the predetermined time period in accordance with the signals from the determination means to control the operation of the refining unit so that'the refining unit operates at a maximum capability while maintaining the quality of the refined oil. 2. A system of the kind described in claim 1 in which the solvent is N-methyl-2-pyrrolidone and the solvent flows at a maximum possible rate SOL 3. A system as described in claim 2 in which the first control signal means includes means for providing direct current control signals corresponding to a selected charge oil flow rate C0 and to a selected refining temperature T and switching means connecting the C0 T signal means to the control devices for momentarily applying the C0 and the T signals to the control devices to control the refining unit so that the refining unit operates with the selected charge oil flow rate and refining temperature until other control signals are applied to the control devices. 4. A system as described in claim 3 in which the signal means provides signals corresponding to the maximum possible flow rates E0, and RO of the extract oil and the refined waxy oil, respectively. 5. A system as described in claim 4 in which the measured property of the charge oil is the viscosity, and the measured conditions of the extract oil and the refined waxy oil are the extract oil flow rate EO and the refined waxy oil flow rate RO and the determining means includes means connected to the property measuring means for providing a signal corresponding to a correlation constant n in accordance with the viscosity signal from the property measuring means, means connected to the condition measuring means and to the n constant signal means for providing a signal corresponding to an a constant in accordance with the E0 and RO signals from the condition signal means and the n constant signal from the n factor signal means, means connected to the limitation signal means for providing a signal corresponding to an a constant in accordance with the E0 and RO signals from the limitation signals means, and a comparator connected to the a constant signal means and to the a constant signal means for comparing the a constant signal to the a constant signal to determine whether the refined waxy oil flow rate is the limiting operating parameter, the extract oil flow rate is the limiting operating parameter, or the refining unit is balanced. 6. A system as described in claim 5 in which the n constant signal means includes memory means in various values of n have been stored, and selection means connected to the memory means and to the viscosity measuring means for selecting the proper n value from the memory means in accordance with the viscosity signal from viscosity measuring means; the a constant signal means is an a constant analog computer providing the a constant signal in accordance with the following first equation: and the a constant signal means is an a constant analog computer providing the a constant signal in accordance with the following second equation: 7. A system as described in claim 6 in which the comparator provides a signal having one amplitude when the a constant signal is equal to or greater than the a constant signal and another amplitude when the a constant signal is less than the a constant signal; and the determining means includes a b constant analog computer receiving direct current voltages and being connected to the condition measuring means and to the first control means for providing a signal corresponding to the b constant in accordance with the EO and R0,, signals from the condition measuring means, the S and T signals from the CO T signal means, the direct current voltages and the following third equation: 1 b: ROM sEL) enn) where m is a constant having a value within the range of 0.75 to 0.80; and the second control signal means includes means connected to the a constant computer, to the b constant computer and to the n constant signal means and receiving direct current voltages for provid ing signals corresponding to the charge oil flow rate C0,; and to the refining temperature T for the condition where the extract oil flow rate is at its maximum possible rate EO means connected to the a and b constant computers and to the n constant signal means for providing signals corresponding to the charge oil flow rate CO and to the refining temperature T for the condition where the refined waxy oil flow rate is at its maximum possible rate RO switching means connected to the C T signal means, to the CO T signal means and to the comparator and controlled by the signal from the comparator to pass the CO and T signals from the CO T signal means when the signal from the comparator is of the one amplitude and to pass the C0,; and the T signals from the CO T signal means when the signal from the comparator is of the other amplitude; subtracting means receiving direct current voltages, corresponding to the miscible temperature T of the charge oil and the N-methyl-2- pyrrolidone and to a temperature T occurring within the range of to F. for subtracting the T voltage from the T voltage to provide a signal corresponding to the maximum permissable refining temperature T means connected to the a and b constant computers, to the n constant signal means and to the subtracting means and receiving direct current voltages for providing a signal corresponding to the charge oil flow rate CO for the condition where the refining temperature is at its maximum permissable level T a second comparator connected to the first switching means and to the subtracting means for comparing the T signal with the T or T signal passed by the first switching means and providing a signal of one amplitude when the T or T signal is less than the T signal and of another amplitude when the T or T signal is equal to or greater than the T signal, and second switching means connected to the subtracting means, to the first switching means, to the CO signal means, to the control devices and to the second comparator for blocking the signals from the first switching means, from the subtracting means and from the C0 signal means during the predetermined time period and for passing the CO T or the CO T signals from the first switching to the control devices as the control signals after the predetermined time period in response to the signal from the second comparator being of the one amplitude and for passing the CO signal from the CO signal means and the T signal from the subtracting means to the control devices as the control signals after the predetermined time period in response to the signal from the second comparator being of the other amplitude. 8. A system as described in claim 7 in which the CO T signal means is an analog computer providing the C0,; and the T signals in accordance with the following fourth through seventh equations: ROE: EOLIM) 00;; RO EO LrM S COE and where EO and S are the extract oil flow rate and the solvent dosage, respectively, for the condition where the refined waxy oil is flowing at its maximum possible flow rate RO and the received direct current voltages correspond to the term 100 in the 10th equation and the term 1 and the exponent m in the l lth equation; and the CO signal means is an analog computer providing the CO signal in accordance with the following 12th through 14th equations: RO -=a( EO and CO RO EO where EO and RO are the extract oil and refined waxy oil flow rates, respectively, for the condition where the refining temperature is at the maximum permissable level T and where the received direct current voltages correspond to the exponent m and the term 2 in the 12th equation. 9. A method for controlling a solvent refining unit in which charge oil is treated with solvent in a refining tower to yield raffinate and extract-mix, strippers the solvent is returned to the tower and the refined waxy oil is subsequently dewaxed to provide refined oil, which comprises refining the charge oil for a predetermined time period at a predetermined solvent dosage and a predetermined temperature to achieve a desired quality of refined oil, measuring at least one property of the charge oil, measuring at least one condition of the refined waxy oil and one condition of the extract oil, providing signals corresponding to the measurements, providing signals corresponding to limitations of the refining unit and the refining operation, utilizing the signals to determine which operating parameter is limiting, and controlling the operation of the refining unit after the predetermined time period in accordance with the determination. 10. A method as described in claim '9 in which the solvent is N-methyl-Z-pyrrolidone; 11. A method as described in claim 10 in which the measured property of thecharge oil is the viscosity, the limitation signals correspond to the maximum possible flow rates E0, and R0, of the extract oil and the refined'waxy oil, respectively, and to the maximum permissable refining temperature T the measured conditions of the extract oil and the refined waxy oil are their flow rates E and RO respectively, and the charge oil flow rate and the refining temperature are controlled in accordance with the determination. 12. A methodas described in claim 11 in which the determining step includes providing an n constant signal in accordance with the measured viscosity signal, providing an a constant signal in accordance with the measured extract oil flow rate EO signal, the measured refined waxy oil flow rate RO signal, the n constant signal and the following equation: providing an a constant signal in accordance with the maxirnum'possible extract oil and refined waxy oil flow rates E0 and RO signals, respectively, the n factor signal and the following equation: ' uu ROLIMK rly)", providing an m constant signal, having a value within the range of 0.75 to 0.80, and providing a b constant signal in accordance with the selected dosage S signal, the selected refining temperature T signal, the measured extract oil and refined waxy oil flow rates EO and RO signals, direct current voltages corresponding to a value of 1 and to an exponent m, respectively, and the following-equation: RO 1+EOM 13. A method as described in claim 12 in which the determining step includes comparing the aconstant and the an constant signals, providing a comparison signal having one amplitude when the a constant signal is less than the am, constant signal and another amplitude when theaconstant signal is equal to or greater than the a constant signal, providing control signals CO and T corresponding to a flow rate for the charge oil and a refining temperature, respectively, when the comparison signal is of the one amplitude, providing control signals C0,; and T when the comparison signal is of the other amplitude, determining the miscible temperature of the charge oil and the solvent, providing a signal corresponding to a maximum refining temperature T which is substantially less than the miscible temperature, comparing the T or T signal, whichever is provided, with the T signal, providing a second comparison signal having one amplitude when the T or T signal is less than the T signal and another amplitude when the T or T signal is equal or greater than the T signal,'providing a CO signal, corresponding to a flow rate for the charge oil, and the T signal as control signals after the predetermined time period when the second comparison signal is of the other amplitude, and providing the CO T signals or the C0 T signals as control signals after the predetermined time period when the second comparison signal is of the one amplitude. 14. A method as described in claim 13 in which the C0,; and T signals are provided in accordance with the n constant signal, the RO signal, the SOL signal, the a constant signal, direct current voltages corresponding to values of l, 100, and m and the following equations: SOLLIM s COR The C0,; and T signals are provided in accordance with the n factor signal, the EO signal, the SOL signal, the a constant signal, the direct current voltages corresponding to the values of l, 100 and m and the following equations: and and CO RO H0

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    US-3964975-AJune 22, 1976Texaco Inc.Means for controlling the temperature of a depropanizer tower
    US-4212070-AJuly 08, 1980Texaco Inc.Control system for a furfural refining unit receiving heavy sweet charge oil
    US-4224673-ASeptember 23, 1980Texaco Inc.Control system for an MP refining unit receiving heavy sour charge oil
    US-4224674-ASeptember 23, 1980Texaco Inc.Control system for an N-methyl-2-pyrrolidone refining unit receiving heavy sweet charge oil
    US-5402367-AMarch 28, 1995Texas Instruments, IncorporatedApparatus and method for model based process control
    US-6295485-B1September 25, 2001Mobil Oil CorporationControl of lubricant production by a method to predict a base stock's ultimate lubricant performance
    US-6317654-B1November 13, 2001James William Gleeson, William Francis Heaney, Eugenio Sanchez, Viswanathan VisweswaranControl of crude refining by a method to predict lubricant base stock's ultimate lubricant preformance