US3718809A

US3718809A - Means and method for controlling a solvent refining unit to provide optimum yields of refined oil and extract oil

Info

Publication number: US3718809A
Application number: US00136003A
Authority: US
Inventors: R Woodle
Original assignee: Texaco Inc
Current assignee: Texaco Inc
Priority date: 1971-04-21
Filing date: 1971-04-21
Publication date: 1973-02-27
Anticipated expiration: 1990-02-27

Abstract

A solvent refining unit is initially operated at a predetermined solvent dosage and a predetermined extract-mix temperature to yield refined oil of a desired quality from charge oil. A quality constant is determined from the initial operation of the refining unit. Limitations on a selectivity characteristic of the solvent, defining an area of feasible operation, are determined from various maximum operating characteristics and equations, hereinafter disclosed. The charge oil flow rate is repeatedly increased in a stepping fashion and an actual solvent selectivity characteristic for each step is calculated using the solvent dosage and the quality constant. An earnings position for the refining unit is determined for each step. When the earnings position for a current step is less than the earnings position for the previous step, or when the actual solvent selectivity characteristic for the current step does not lie within the area of feasible operation, the charge oil flow rate is decreased to the flow rate for the next previous step.

Description

United States Patent 1 Woodle 11 3,718,809 1 Feb. 27, 1973 1 MEANS AND METHOD FOR CONTROLLING A SOLVENT REFINING UNIT TO PROVIDE OPTIMUM YIELDS OF REFINED OIL AND EXTRACT OIL [75] Inventor: Robert A. Woodle, Port Arthur,

Tex.

[73] Assignee: Texaco Inc., New York, NY. [22] Filed: April 21, 1971 [21] Appl. No.: 136,003

[52] U.S. Cl. ..235/151.12, 208/33, 208/311 [51] Int. Cl ..G06g 7/58, C10g 21/00 [58] Field of Search ..235/151.l2, 151.35; 208/27-28, 33, 36, 311; 23/230 R, 203 A, 255 E [56] References Cited UNITED STATES PATENTS 3,458,431 7/1969 Nixon ..208/33 3,546,107 12/1970 Brown et al. ..235/151.12 X 3,549,514 12/1970 Brown et a1. ..235/l.l2 X 3,554,896 1/1971 Bozeman, Jr. et al. ..208/33 X 3,565,786 2/1971 Brown et a1. ..235/15l.l2 X 3,458,432 7/1969 Woodle ..208/36 REFINING 5 TOWER 4 came:

"- FRC SWITCH 6 5 COMPUTER COMPARATOR SWITCH SOURCE OF DC VOLTAGES Primary Examiner-Eugene G. Botz Assistant Examiner-Jerry Smith Attorney-Thomas H. Whaley and Carl G. Ries 57 ABSTRACT A solvent refining unit is initially operated at a predetermined solvent dosage and a predetermined extract-mix temperature to yield refined oil of a desired quality from charge oil. A quality constant is determined from the initial operation of the refining unit. Limitations on a selectivity characteristic of the solvent, defining an area of feasible operation, are determined from various maximum operating characteristics and equations, hereinafter disclosed. The charge oil flow rate is repeatedly increased in a stepping fashion and an actual solvent selectivity characteristic for each step is calculated using the solvent dosage and the quality constant. An earnings position for the refining unit is determined for each step. When the earnings position for a current step is less than the earnings position for the previous step, or when the actual solvent selectivity characteristic for the current step does not lie within the area of feasible operation, the charge oil flow rate is decreased to the flow rate for the next previous step.

12 Claims, 11 Drawing Figures EXTRACT on.

S COMPUTER an r COMPUTER COMPUTER EARNINGS COMPUTER V10 s s SOURCE OF DC VOLTAGES 35 MONOSYABLE SAMPLE MULTI AND 111 BR ATOR H'J L [I COMPARATOR PATEM'EDFEBZYIQH 7 718,809

SHEET 10F 6 FIG. IA' A REFINING TOWER WATER CHARGE OIL TRC B COMPUTER COMPARATOR SWITCH 6O SOURCE OF DC VOLTAGES 'PATENTED 3.718.809

SHEET 2 OF 6 E A FIG. IB

n STRIPPER L LEI I I6 DEWAXING =REFINED MEANS OIL I WAX E STR'PPER EXTRACT on. 5A b7 v 4 E, as) I I L CR0 CT S 2 COMPUTER E COMPUTER COMPUTER l0 Lv, A A v 4 S i L 32 )L EIO E EI2 l7 4 2 no I I I CA EARNINGS F v COMPUTER COMPUTER j p EI4 4 TEM 62 I COMPUTER ,1/ v

3 20 2l 22 2| I 7 7 L Io 6 vs s 1 SOURCE OF DC VOLTAGES TV I35 I22 L swwcTl?- 36 I44 FLIP MONOSTABLE SAMPLE MULTI AND FLOP VIBRATOR HOLD (I36 I27 i COMPARATOR- PATENTEUFEBZYIQIS 3 71 09 SHEET 30F 6 FIG. 2

MAXIMUM EXTRACT OIL FLOW RATE 8 MAXIMUM EXTRACT MIX TEMPERATURE FEASIBLE 0.05 REGION V 0.03 MAXIMUM REFINED- WAXY OIL FLOW RATE 1 l v l s I I I 60 I00 200 500 I000 2000 PATENTEB FEB2 7 I975 SHEET 0F 6 FIG. 4

vll

EIO

R F l D O B FIG.

I 0 COMPUTER 1 FIG. 6

C 0 COMPUTER PATENTEDFEBZYW 3,718,809

SHEET 5 BF 6 X l 81 E l0 9| l SQUARE cfisaJT I F 7 V L as EARNINGS COMPUTER-1 f u I {IMHO PATENTED FEB27I975 SHEET 8 BF 6 FIG. 10

' /REFERENCE TEMPERATURE C SOLVENT SELECTIVITY CHARACTERISTlC MEANS AND METHOD FOR CONTROLLING A SOLVENT REFINING UNIT TO PROVIDE OPTIMUM YIELDS OF REFINED OIL AND EXTRACT OIL BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to control systems and, more particularly, to an automatic control system for use in an oil refinery.

2. Description of the Prior Art Heretofore, optimum yield control systems for solvent refining units such as the one disclosed in U.S. application Ser. No. 96,193, filed Dec. 8, 1970, now U.S. Pat. No. 3,666,931 by R.A. Woodle, inventor of the present invention, and assigned to Texaco Inc., assignee of the present invention, use equations to determine the charge oil flow rate and refining temperature, i.e. the extract-mix temperature, that should be used for optimum yield and to control the refining unit accordingly.

The present invention provides a more simplified control system. The system of the present invention differs in concept from the aforementioned U.S. application by using an area of feasible operation as defined by limitations on a refining parameter and controlling the refining unit to provide optimum yields while operating within the area of feasible operation.

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. Strippers in the refining unit separate the solvent from the rafiinate 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 dewaxed oil. The control system includes a circuit which senses refining conditions and provides corresponding signals. A plurality of networks are connected to the sensing circuit. One network provides signals corresponding to limitations of a refining parameter in accordance with at least one sensed condition signal. Another network provides a signal corresponding to the actual value of the refining parameter in accordance with some of the sensed condition signals. Another network provides a signal corresponding to the relationship of a present earnings position of the refining unit with a previous earnings position in accordance with some of the sensed condition signals. A signal corresponding to a quality constant for a desired quality refined oil is applied to control means along with the signals from the networks. The control means control the refining unit in accordance with applied signals to provide optimum yields of the desired quality refined oil and the extract oil from the charge oil.

One object of the present invention is to control a solvent refining unit to provide optimum yields of refined oil and extract oil from charge oil.

Another object of the present invention is to use the relationship of a solvent selectivity characteristic and the solvent dosage along with the earnings position of the solvent refining unit to control the solvent refining unit to provide optimum yields of refined dewaxed oil and extract oil.

Another object of the present invention is to determine a quality constant for charge oil being refined in a solvent refining unit so that the quality constant may be used in controlling the refining operation.

The foregoing and other objects and advantages of the invention will appear more fully hereinafter from a consideration of the detailed description which follows, taken together with the accompanying drawings wherein one embodimentof the invention is illustrated by way of example. It is to be expressly understood, however, that the drawings are for illustration purposes only and are not to be construed as defining the limits of the invention.

DESCRIPTION OF THE DRAWINGS FIGS. 1A and 13, when matched along line A-A, provide a simplified block diagram of a system, constructed in accordance with the present invention, for controlling a solvent refining unit, which is also partially shown in schematic form.

FIG. 2 is a logrithmic graph of solvent selectivity characteristic versus solvent dosage, for different limiting refining condition, which define a feasible region of operation for the solvent refining unit partially shown in FIG. 1.

FIGS. 3 through 9 are detailed block diagrams of the S computer, the B computer, the C, computer, the C no computer, the C computer, the T computer and the earnings computer, respectively, shown in FIGS. 1A v& 18.

FIG. 10 is a graph of extract-mix temperature versus solvent selectivity characteristic for furfural solvent.

DESCRIPTION OF THE INVENTION Referring to FIG. 1, there is shown a system for controlling a conventional type solvent refining unit for optimum operation. The rate of flow of charge oil entering a refining tower 3 is controlled so as to regulate the flow rates of refined waxy oil and extract oil. The temperature of extract-mix, as the extract mix leaves the refining tower 3, is also controlled to affect the yield of refined dewaxed oil and the extract oil. The rate of the charge oil entering the refining tower 3 in a line 4 is sensed and controlled by conventional types sensing element 5, flow recorder 6 and valve 2. Sensing element 5 provides a signal to controller 6 corresponding to the flow rate of the charge oil in line 4. Controller 6 operates valve 2 to control the rate of flow of the charge oil to tower 3 in accordance with a difference between the signal from sensing element 5 and the position of its set point.

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 flow effecting the extraction of low viscosity index constituents of the charge oil. Raffinate including 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 controller 14.

Temperature recorder controller 14, which may be of a type well known in the art, operates a valve 15 in accordance with a difference between the signal from temperature sensing means 12 and its set point position. Valve 15 controls the rate of flow of the cooling water so as to control the temperature in tower 3.

Rafiinate 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 dewaxed oil for storage and blending with product lubricating oil.

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 signal E Elements having a numerical designation with a suffix are identical in operation as elements having the same numerical designation without a suffix.

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 T 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 reuse. The flow rate of the solvent in line 7 is maintained at a maximum and the solvent dosage is controlled by controlling the fiow rate of the charge oil in line 4. Sensing means 5B and a flow transmitter 20A senses the rate of flow of the 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 combination selected from various combinations of solvent dosages and temperatures that yield the desired quality of refined oil. A three pole, two position switch when in the position shown in FIG. 1 permits signals E E E, from an external signal source, which is not shown, to pass. Signal E adjusts the set point of temperature recorder controller 14 to correspond to a selected temperature during the initial operation. Signal E has pulses which change the set point of flow recorder controller 6 in a direction determined by signal E so that the flow rate of charge oil in line 4 is of the proper value for the solvent dosage required during the initial operation.

Yield alone does not define the quality of the refined oil. The quality can vary for a constant yield, depending upon the selectivity of the refining conditions. Selectivity is the relative ability of the refining solvent to discriminate between the undesirable or extractable molecules, and the desirable raffinate-quality ones. The less oil there is in the solution in the extract-mix, the more selective the separation since the solvent is more nearly pure solvent. Selectivity improves as the extraction temperature is reduced, thereby reducing the relative solubility of the oil in the solvent. However, since each volume of solvent contains less oil at the lower temperature, more volumes of solvent must be used to achieve the desired degree of refining. As the conditions are made more selective, by reducing the extraction temperature and increasing the solvent/oil ratio, the yield of refined oil having the specific quality improves, but the production rate is lowered. Thus there is an economic optimum selectivity.

Economic optimization involves the interrelationship between yield and production. Any given solvent refining unit has three main production limitations: solvent flow rate capacity, refined oil flow rate capacity and extract oil flow rate capacity. These limitations define an area of feasible operation for a solvent selectivity C =EO /(SOL+EO (l) where B0,, is the maximum possible flow rate of extract oil in line 25 as determined from the physical design of the solvent refining unit and SOL is the flow rate of the solvent in line 7. In developing FIG. 2, a maximum extract oil flow rate of 2,600 barrels per day (BPD) and a maximum solvent flow rate of 17,000 BPD are used. Since the solvent flow rate SOL is at a maximum, a voltage corresponding to that flow rate may be provided instead of sensing the actual flow rate in line 7.

The line corresponding to the limitation on the solvent selectivity characteristic C no due to the maximum possible refined waxy oil flow rate RO which for FIG. 2 is 6,000 BPD, is determined from the following equation:

(SOL) S(R0M) S(SOL) +100(SOL) -s(R0M CT: R Mrso) where T is a reference temperature, which is explained in detail hereinafter, for the solvent, T is the miscibility temperature, and for FIG. 2 is 230F., of the charge oil and A is a constant characteristic of the charge oil as determined by laboratory analysis which for FIG. 2 has a value of 186. The method of determining A is disclosed in US. Pat. No. 3,458,432. The exponent n is a characteristic constant related to the type of solvent and ranges in value from 0.42 to 0.67. An article entitled Figure Solvent Extract of Heavy Oils" written by the inventor and published in Hydrocarbon Processing, July 1966, Vol. 45, No. 7, page 133 relates different values of n for corresponding solvents. The miscibility temperature is that temperature at which the charge oil dissolves completely in the solvent. The miscibility temperature may be determined in the laboratory for a particular charge oil.

Referring to FIGS. 1 and 3, an S computer 32 provides a signal E corresponding to the actual solvent dosage S in accordance the refined waxy oil and the extract oil flow rate signals E and E respectively, and the following equation which by definition is the solvent dosage, i.e. the ratio of the solvent percent volume to the charge oil percent volume multiplied by 100:

SOL

where CO, R0 and E0 are the flow rates of the charge oil, the refined waxy oil and the extract oil, respectively. Computer 32 includes summing means 34 summing signals E E from

flow transmitters

20 and 20A, respectively. A direct current voltage V, from a source 36 of direct current voltages is divided by the sum signal from summing means 34 by a divider 38. A multiplier 41 multiplies the signal from divider 38 with a direct current voltage V, from source 36 to provide signal E The actual solvent dosage signal E is applied to a computer 44 which uses signal E along with temperature signal E, from temperature recorder controller 14 to provide a signal E corresponding to the quality constant B. Referring to FIGS. 1 and 4, the B constant signal B is provided after it has been determined by laboratory analysis that the refined oil is of the desired quality. Computer 44 provides signal 15, in accordance with the following equation:

B n TEM where m and n are constants hereinafter described in detail and T is the extract-mix temperature in tower 3. For furfural solvent, m and n have values of 1.33 and 0.5, respectively.

Signal E is applied to a logarithmic amplifier 45 which is part of an exponential circuit 46 that also includes a multiplier 47, an operational amplifier 50 and a feedback element 51. The output of logarithmic amplifier 45 is applied to multiplier 47 where it is multiplied with a direct current voltage V; from source 36 corresponding to the term (m-n) in equation 5. Feedback element 51, which may be a function generator of the type manufactured by Electronics Associates as their part PC-l2, cooperates with operational amplifier 50 to provide the antilog of the output of multiplier 47 so that the output of exponential circuit 46 corresponds to S""'". A multiplier 54 multiplies a direct current voltage V, from source 36, which corresponds to the constant A of the charge oil, with the output from exponential circuit 46. Subtracting means 55 subtracts temperature signal E, from a direct current voltage V from source 36, corresponding to the term T in equation 5. A divider 56 divides the output from multiplier 54 by the output from subtracting means 55 to provide a signal to a sample and hold circuit 58. When the refined oil is of the proper quality, a switch 60, which may be of the mometary on/off pushbutton type, is activated. When activated, switch 60 passes a direct current voltage V, from source 36 to sample and hold circuit 58 causing circuit 58 to sample and hold the output from divider 56. Circuit 58 provides signal E which remains constant for the remainder of the operation even though inputs to computer 44 may vary.

Referring to FIGS. 1 and 5, a signal E corresponding to the actual solvent selectivity factor C is provided by a C computer 62 in accordance with signals E E from

computers

32 and 44, respectively, a direct current voltage V from source 36, and the following equation:

u Sm (6) An exponential circuit 46A receives signal E and voltage V which correspond to the term m in equation 6, and provides a signal corresponding to the term S" to a divider 63. Divider 63 divides signal E from computer 44 with the output from exponential circuit 46A to provide signal E Referring to FIG. 1 summing means 66 and a divider 67 operate as a computer to provide a signal E in accordance with equation 1, which corresponds to the limiting value of the selectivity factor C due to the maximum extract oil flow rate line shown in FIG. 2. Summing means 66 sums direct current voltages V,, V from source 36. Direct current voltage V corresponds to the maximum extract oil flow rate as determined from the physical design of the refining unit. Divider 67 divides voltage V with the output from summing means 66 to provide signal E Referring to FIGS. 1 and 6, a C computer 70 provides a signal E corresponding to the value of the limitation on the solvent selectivity factor C for the maximum refined oil flow rate R0,, in accordance with E direct current voltages V V and V and equation 2.

Multipliers

72, 73 multiply direct current voltages V V with voltage V and signal E respectively. Voltage V corresponds to the maximum possible refined oil flow rate. Subtracting means 75 subtracts the output from multiplier 73 from the output from multiplier 72 to provide signal corresponding to l00(SOLS(ROM). Multiplier 76 multiplies signal Em with voltage V, to provide a signal to summing means 79, where the signal is summed with the output from subtracting means 75 to provide a signal corresponding to the denominator in equation 2. A divider 80 divides the output of subtracting means 75 with the output from summing means 79 to provide signal E Referring to FIGS. 1 and 7, a computer provides a signal E corresponding to the value of the limitation on the solvent selectivity factor C for the maximum operating temperature for the particular dosage rate. Computer 85 provides signal E in accordance with signal E from computer 32, direct current voltage V V and V and equation 3. A square root circuit 86 receives signal E from S computer 32 and provides a signal corresponding to S". Subtracting means 87 subtracts voltage V which correspond to the misciblity temperature, from voltage V to provide a signal corresponding to (T T The output from subtracting means 87 is multiplied with the output from square root circuit 86 by a multiplier 90. A divider 91 divides direct current voltage V.,, corresponding to the A constant, with the output from multiplier to provide signal E Signals E E and E from divider 67 and

computers

70 and 85, respectively, are applied to

comparators

94, 94A and 94B, respectively, where they are compared with signal E from C computer 62.

Comparators

94, 94A and 94B perform the function of ascertaining that the actual solvent selectivity characteristic C lies within the feasible operating area shown in Graph 2. When signal E is equal to or less than signals E E and E each comparator provides a high level direct current output. When signal E is greater than a signal, either E E or E the corresponding comparator provides a low level direct current output. The outputs from comparators 94 through 94B are applied to a NOR gate 125, which controls the changing of the set point in flow recorder controller 6, as hereinafter explained.

Referring to FIGS. 1 and 8, a computer 98 provides a temperature T signal E to switch 30 which is used to adjust the set point of temperature recorder controller 14 as hereinafter explained. Signal E is provided in accordance with signals E E from

computer

32 and 44, respectively, direct current voltage V V and V from source 36 and the following equation:

An exponential circuit 64B receives signal E from computer 32 and voltage V; from source 36 and provides a signal corresponding to S"""". A multiplier 100 multiplies the output from exponential circuit 46B with direct current voltage V, to provide a signal corresponding to the term AS" in equation 7. The signal from multiplier 100 is divided by signal E from computer 44 by a divider 102 to provide a signal, corresponding to AS""""/B, to subtracting means 103. Subtracting means 103 subtracts the signal from divider 102 from voltage V corresponding to the reference temperature T for the solvent, to provide signal E In determining the optimum yield, it is necessary to provide signals corresponding to an earnings position for the refining unit. Referring to FIGS. 1 and 9, earnings computer 110 provides a signal E corresponding to the current earnings position EP in accordance with signals E E from

flow transmitters

20 and 20A, respectively, direct current voltages V V and V from source 36 and the following equation:

EP (Value of refined oil)(RO)+Value of extract oil(E)-(Value of charge oil)(RO-i-EO). s

Multipliers 111, 112 multiply signals E and E respectively, with voltages V and V respectively, which correspond to the value of the refined oil and the extract oil, respectively. Summing means 114 sums the outputs from multiplier 111, 112, while summing means 115 sums signals E E to provide a signal corresponding to the charge oil flow rate to a multiplier 116. The output from summing means 1 is multiplied with voltage V which corresponds to the value of the charge oil. Subtracting means 120 subtracts the output from multiplier 1 16 from the output of summing means 114 to provide signal E Signal E is applied to a sample and hold circuit 122 and to a comparator 123. Sample and hold circuit 122 provides a signal corresponding to the earnings position for the next previous set of conditions, as hereinafter explained. Comparator 123 compares the current earnings position with the previous earnings position to determine whether the earning position has increased or decreased. When signal E from computer 110 is equal to or greater than the output from sample and hold circuit 122, comparator 123 provides a high level direct current output to NOR gate 125 which also receives the outputs from comparators 94 through 94B. When signal E from computer 1 10 is less than the output from sample and hold circuit 122, comparator 123 provides a low level direct current output. When a

comparator

94, 94A, 94B or 123 provide a low level direct current output, NOR gate 125 provides a high level direct current output. When

comparators

94, 94A, 94B and 123 provide high level direct current outputs, NOR gate 125 provides a low level direct current output.

The output from NOR gate 125 is applied to switch 30 which passes the output to flow recorder controller 6 when activated and blocks the output when not activated. When a low level direct current output from NOR gate 125 is applied to controller 6, the set point may be changed so as to increase the charge oil flow rate in line 4. When a high level direct current output from NOR gate 125 is applied to controller 6, the set point may be changed so as to decrease the charge oil flow rate in line 4.

The output from NOR gate 125 is also applied to AND gate 127 which is part of a circuit for changing the set point of flow recorder controller 6. The circuit further includes monostable multivibrators and 135A, a flip-flop 136, clock means 138 and 139, AND gates and 140A, and a switch 144, which may a momentary on pushbutton switch. Switch 144 is activated at the beginning of the refining operation to momentarily pass a direct current voltage V from source 36 to flip-flop 136. The direct current voltage triggers flip-flop 136 to a clear state causing its Q output to be a high level direct current voltage. The high level Q output from flip-flop 136 enables AND gate 140.

When disabled, AND gate 140 effectively blocks pulses from clock means 138. When enabled, AND gate 140 effectively passes the pulses from clock means 138. The pulse repetition rate of the pulses from clock means 138 is such that the refining unit reaches a steady state condition after a change in response to a pulse from clock means 138 before clock means 138 provides another pulse.

Each pulse passed by AND gate 140A controls circuit 122 to sample and hold the current earnings position signal E so as to provide a signal corresponding to the next previous earning position for the next step increase in the charge oil flow rate.

Each pulse passed by AND gate 140 triggers monostable multivibrator 135A to provide an enabling pulse to AND gate 140A. When disabled, AND gate 140A effectively blocks pulses provided by clock means 139. When enabled, AND gate 140A effectively passes the pulses from clock means 139. Each pulse passed by AND gate 140, when applied to flow recorder controller 6, changes the set point of controller 6 a predetermined amount in a direction controlled by the output from NOR gate 125. The width of the pulse from multivibrator 135A controls the number of pulses from clock means 139 that are effectively passed by AND gate 140A so as to control the amount of change of the charge oil flow rate. The passed pulses from AND gate 140A are applied to switch 130.

When the refining unit has reached a stabilized condition after being initially operated at the selected solvent dosage and temperature combination, switch 30 is activated so that the refining unit is controlled by the system of the present invention. The charge oil flow rate is periodically increased in a stepping fashion, due to the passed pulses from AND gate 140A and the output from NOR gate 125, so as to decrease the solvent dosage. Switch 30 also passes signal E from T M computer 98 to temperature recorder controller 14 so as to control the set point of temperature recorder controller 14. Temperature recorder controller 14 changes the temperature of the extract-mix leaving tower 3 so that the proper temperature is provided for each new solvent dosage.

The charge oil flow rate in line 4 increases in a stepping fashion until the actual solvent selectivity characteristic exceeds one of its limitations, or the current earnings position for the refining unit is less than the next previous earnings position. Thus, when signal E is greater than signal E E or E or signal E is less than the output from sample and hold circuit 122, a

comparator

94, 94A, 948 or 123 provides a low level direct current output to NOR gate 125 causing NOR gate 125 to provide a high level direct current output. The change in level of output from NOR gate 125 triggers multivibrator 135 which acts as a time delay. Multivibratorl35 provides a pulse whose trailing edge triggers flip-flop 136 to a set state. The Q output from flipflop 136 goes to a low level thereby disabling AND gate 140. Due to the time delay effect of the width of the pulse from multivibrator 135, one more pulse from clock means 138 is effectively passed by AND gate 140. That pulse causes a change in the set point of flow recorder controller 6, as heretofore explained. However, the change in the set point is in the opposite direction, since a high level direct current output from NOR gate 125 is being applied to flow recorder controller 6. The change in the set point of flow recorder controller 6 causes a step decrease in the charge oil flow rate, thereby increasing the solvent dosage, so that the refining unit has been returned to the next previous operating step.

Flip-flop 136 remains in the set state, until switch 144 is activated, so that the refining unit is maintained at an operating condition for optimum yields of refined oil and extract oil.

Although the system of the present invention has been shown using analog computers, a digital computer may be used. Analog-to-digital converters would convert signals E E E to digitals signals and apply them to the digital computer. Information relating to the maximum possible flow rates of the solvent, the refined waxy oil and the extract oil may be programmed into the digital computer. Digital-to-analog converters would convert digitals signals from the digital computer to analog signals so as to provide signals E through E-, and E to control the refining unit. The digital computer would use the aforementioned equations in providing signals B, through E, and E The system of the present invention, as heretofore described, controls a solvent refining unit to provide optimum yields of refined oil and extract oil from charge oil using the relationship of a solvent selectivity characteristic and the solvent dosage along with the earnings position of the solvent refining unit. The system of the present invention determines a quality constant for the charge oil being refined in a solvent refining unit and uses the quality constant to control the refining operation.

DERIVATIONS OF EQUATIONS 1 THROUGH 7 The following equation was disclosed in US Pat. No. 3,458,432 issued July 29, 1969 to R. A. Woodle et al. and assigned to Texaco Inc., assignee of the present invention.

e m (9) where S is the term F in the aforementioned patent, C, is the solvent selectivity characteristic which defined as the amount of extract oil in the extract mix and may be written in equation form as n EM1) el q n"' EM2) e2 q where q is a constant for a given solvent dosage, and C C are solvent selectivity characteristics for extract-mix temperatures T and T respectively. Equations 1 1, 12 may be rewritten as (ll)and R el am C21: R e2 am ez n er R e2 EMr er EM2 e2 solving for T equation 14 is written as T Trum n EM2 e2 R el e?! Using FIG. 10, which is the extract mix temperature versus solvent selectivity characteristic curve for furfural solvent and a particular charge oil, temperatures T T are 169F and 202F, respectively, and the selectivity characteristics C C are 0.0854 and 0.1087, respectively. From equation 15, the resulting reference temperature is 322.9F, which is rounded off to 323F. The dashed line is a reference line; the temperature value of the dashed line is the reference temperature T for the furfural solvent curve. Similarly curves may be drawn for other solvent types from experimental data and their reference temperatures may be determined accordingly.

As heretofore mentioned, the sumof the extract oil and refined oil flow rates is equal to the charge oil flow rate. It follows that E0 C0 R0 (16) where CO and R0 are the charge oil and refined oil flow rates. Substituting for E0 in equation 10, equation 10 may be rewritten as is done in the following equation:

Solving equation 4 for CO and substituting in equation 17, equation l2 may be rewritten as S =S(SOL) +100(SOL) S(RO) B C S 20 The exponent k may be written as l/m and equation 20 can then be written as (B)"'=B=C,,S"' 21 Equation 5 is obtained by substituting for C in equation 21 from equation 9. Equation 6 is a rewriting of equation where C, is designated as C for convenience. Equation 7 is a rewriting of equation 5, solving for the temperature.

What is claimed is:

l. 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 providing a signal corresponding to a quality constant for a desired quality refined oil, means for sensing some of the refining conditions and providing signals corresponding thereto, means connected to the sensing means for providing signals corresponding to limitations of a refining parameter in accordance with at least one sensed condition signal, means connected to the sensing means and to the quality constant signal means for providing a signal corresponding to the actual value of the refining parameter in accordance with the quality constant signal and some of the sensed condition signals, means connected to the sensing means for providing an earnings signal corresponding to the relationship of a present earnings position of the refining unit with a previous earnings position in accordance with some of the sensed condition signals and current values of the charge oil, the refined oil and the extract oil, and means connected to the limitation signal means, to the quality constant signal means, to the refining parameter signal means and to the earnings signal means for controlling the refining unit to provide optimum yields of the desired quality refined oil and extract oil from the charge oil in accordance with the quality constant signal, the refining parameter signal, the earnings signal and the limitation signals.

2. A system as described in claim 1 in which the sensed refining conditions are the flow rates E0, R0 of the extract oil and refined waxy oil, respectively, and the temperature T of the extract mix in the refining tower; the control means controls the refining unit by controlling the flow rate of the charge oil and the extract-mix temperature T and the refining parameter is a solvent selectivity characteristic C.

3. A system as described in claim 2 in which the quality constant signal means is connected to the sensing means and provides the quality constant B signal in accordance with the sensed extract-mix temperature T signal and the following equation:

where A is a characteristic of the charge oil S is the solvent dosage in percent volume, m and n are characteristic constants related to the type of solvent, and T is a reference temperature for the solvent.

4. A system as described in claim 3 in which the limitation signal means provides signals, corresponding to limitations C C and C on the solvent selectivity characteristic C, as the limitation signals in accordance with the following equations:

where EO RO and SOL are the maximum possible flow rates, for the refining unit, of the extract oil, the refined oil and the solvent, respectively, and T is the miscibility temperature for the solvent.

5. A system as described in claim 4 in which solvent selectivity characteristic signal means provides the solvent selectivity C signal in accordance with the following equation:

7. A method of controlling 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, which comprises determining the quality constant for a desired quality refined oil, sensing some of the refining conditions, determining the limitations of a refining parameter in accordance with at least one sensed refining condition, determining the actual value of the refining parameter in accordance with the quality constant and some of the sensed refining conditions, determining the relationship of a present earnings posi tion of the refining unit with a previous earnings position in accordance with some of the sensed refining conditions and current values of the charge oil, the refined oil and the extract oil, and controlling the refining unit to provide optimum yields of the desired quality refined oil and extract oil from the charge oil in accordance with the quality constant, the earnings relationship, the actual value of the refining parameter, and the limitations on the refining parameter.

8. A method as described in claim 7 in which the sensed refining conditions are the flow rates EO, R of the extract oil and the refined waxy oil, respectively, and the temperature T of the extract mix; the refining unit is controlled by controlling the flow rate of the charge oil and the temperature T and the refining parameter is a solvent selectivity characteristic C.

9. A method as described in claim 8 in which the quality constant is determined in accordance with the sensed extract-mix temperature T and the following equation:

AShn-n) TFTR where A is a characteristic of the charge oil, S is the solvent dosage, m and n are characteristic constants related to the type of solvent, and T is a reference temperature for the solvent.

10. A method as described in claim 9 in which there are three limitations C C and C on the solvent selectivity characteristic and which are determined in accordance with the following equations:

A CT(TR" TMIBC)SD 12. A method as described in claim 11 in which the controlling step includes increasing the flow rate of the charge oil in steps until the actual value of the solvent selectivity characteristic exceeds at least one of the limitations on the solvent selectivity characteristic, or the earnings relationship indicates that the present earnings position of the refining unit is less than the earnings position for the next previous step, decreasing the charge oil flow rate so as to avoid the limitations and the lesser earnings position, and controlling the temperature T of the extract-mix in accordance with the following equation:

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 providing a signal corresponding to a quality constant for a desired quality refined oil, means for sensing some of the refining conditions and providing signals corresponding thereto, means connected to the sensing means for providing signals corresponding to limitations of a refining parameter in accordance with at least one sensed condition signal, means connected to the sensing means and to the quality constant signal means for providing a signal corresponding to the actual value of the refining parameter in accordance with the quality constant signal and some of the sensed condition signals, means connected to the sensing means for providing an earnings signal corresponding to the relationship of a present earnings position of the refining unit with a previous earnings position in accordance with some of the sensed condition signals and current values of the charge oil, the refined oil and the extract oil, and means connected to the limitation signal means, to the quality constant signal means, to the refining parameter signal means and to the earnings signal means for controlling the refining unit to provide optimum yields of the desired quality refined oil and extract oil from the charge oil in accordance with the quality constant signal, the refining parameter signal, the earnings signal and the limitation signals.

2. A system as described in claim 1 in which the sensed refining conditions are the flow rates EO, RO of the extract oil and refined waxy oil, respectively, and the temperature TE.M. of the extract mix in the refining tower; the control means controls the refining unit by controlling the flow rate of the charge oil and the extract-mix temperature TE.M.; and the refining parameter is a solvent selectivity characteristic C.

3. A system as described in claim 2 in which the quality constant signal means is connected to the sensing means and provides the quality constant B signal in accordance with the sensed extract-mix temperature TE.M. signal and the following equation: where A is a characteristic of the charge oil S is the solvent dosage in percent volume, m and n are characteristic constants related to the type of solvent, and TR is a reference temperature for the solvent.

4. A system as described in claim 3 in which the limitation signal means provides signals, corresponding to limitations CEO, CRO and CT on the solvent selectivity characteristic C, as the limitation signals in accordance with the following equations:

5. A system as described in claim 4 in which solvent selectivity characteristic signal means provides the solvent selectivity C signal in accordance with the following equation: C B/Sm

6. A system as described in claim 5 in which the control means increases the flow rate of the charge oil in steps until the solvent selectivity characteristic signal is greater than at least one of the limitation signals, or the earnings signal indicates that the present earnings position of the refining unit is less than the earnings position for the next previous step, at which time the control means decreases the charge oil flow rate and controls the temperature T of the extract-mix in accordance with the following equation:

7. A method of controlling 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, which comprises determining the quality constant for a desired quality refined oil, sensing some of the refining conditions, determining the limitations of a refining parameter in accordance with at least one sensed refining condition, determining the actual value of the refining parameter in accordance with the quality constant and some of the sensed refining conditions, determining the relationship of a present earnings position of the refining unit with a previous earnings position in accordance with some of the sensed refining conditions and current values of the charge oil, the refined oil and the extract oil, and controlling the refining unit to provide optimum yields of the desired quality refined oil and extract oil from the charge oil in accordance with the quality constant, the earnings relationship, the actual value of the refining parameter, and the limitations on the refining parameter.

8. A method as described in claim 7 in which the sensed refining conditions are the flow rates EO, RO of the extract oil and the refined waxy oil, respectively, and the temperature TEM of the extract mix; the refining unit is controlled by controlling the flow rate of the charge oil and the temperature TEM; and the refining parameter is a solvent selectivity characteristic C.

9. A method as described in claim 8 in which the quality constant is determined in accordance with the sensed extract-mix temperature TEM and the following equation: where A is a characteristic of the charge oil, S is the solvent dosage, m and n are characteristic constants related to the type of solvent, and TR is a reference temperature for the solvent.

10. A method as described in claim 9 in which there are three limitations CEO, CRO and CT on the solvent selectivity characteristic and which are determined in accordance with the following equations:

11. A method as described in claim 10 in which the actual value of the solvent selectivity characteristic is determined in accordance with the equation: C B/Sm

12. A method as described in claim 11 in which the controlling step includes increasing the flow rate of the charge oil in steps until the actual value of the solvent selectivity characteristic exceeds at least one of the limitations on the solvent selectivity characteristic, or the earnings relatioNship indicates that the present earnings position of the refining unit is less than the earnings position for the next previous step, decreasing the charge oil flow rate so as to avoid the limitations and the lesser earnings position, and controlling the temperature TEM of the extract-mix in accordance with the following equation: