Abstract:
A refining unit treats light sweet charge oil with an N-methyl-2-pyrrolidone solvent, hereafter referred to as MP, in a refining tower to yield raffinate and extract mix. The MP is recovered from the raffinate and from the extract mix and returned to the refining tower. A system controlling the refining unit includes a gravity analyzer, a sulfur analyzer, and viscosity analyzers; all analyzing the light sweet charge oil and providing corresponding signals, sensors sense the flow rates of the charge oil and the MP flowing into the refining tower and the temperature of the extract mix and provide corresponding signals. One of the flow rates of the light sweet charge oil and the MP flow rates is controlled in accordance with the signals from all the analyzers and all the sensors, while the other flow rate of the light sweet charge oil and the MP flow rates is constant.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to control systems and methods in general and, more particularly, to control systems and methods for oil refining units. 
     SUMMARY OF THE INVENTION 
     A refining unit treats light sweet charge oil with an N-methyl-2-pyrrolidone solvent, hereafter referred to as MP, in a refining tower to yield raffinate and extract mix. The MP is recovered from the raffinate and from the extract mix and returned to the refining tower. A system controlling the refining unit includes a gravity analyzer, a sulfur analyzer, and viscosity analyzers. The analyzers sample the light sweet charge oil and provide corresponding signals. Sensors sense the flow rates of the charge oil and the MP flowing into the refining tower and the temperature of the extract-mix and provide corresponding signals. The flow rate of the light sweet charge oil or the MP is controlled in accordance with the signals provided by all the sensors, the refractometer and the analyzers while the other flow rate of the light sweet charge oil or the MP is constant. 
     The 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 embodiment of 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 
     FIG. 1 shows a refining unit in partial schematic form and a control system, constructed in accordance with the present invention, in simple block diagram form. 
     FIG. 2 is a detailed block diagram of the control means shown in FIG. 1. 
     FIGS. 3 through 13 are detailed block diagrams of the H computer, the K signal means, the H signal means, the KV computer, the VI signal means, the SUS computer, the SUS 210  computer, the VI DWC .sbsb.O computer, the VI DWC .sbsb.P computer, the ΔRI computer and the J computer, respectively, shown in FIG. 2. 
    
    
     DESCRIPTION OF THE INVENTION 
     An extractor 1 in a refining unit is receiving light sweet charge oil by way of a line 4 and N-methyl-2-pyrrolidone solvent, hereafter referred to as MP, by way of a line 7 and providing raffinate to recovery by way of a line 10, and an extract mix to recovery by way of a line 14. The raffinate is subsequently processed to yield refined oil. 
     Light sweet charge oil is a charge oil having a sulfur content equal to or less than a predetermined sulfur content and having a kinematic viscosity, corrected to a predetermined temperature, less than a predetermined kinematic viscosity. Preferably, the predetermined sulfur content is 1.0%, the predetermined temperature is 210° F., and the predetermined kinematic viscosity is 7.0. The temperature in extractor 1 is controlled by cooling water passing through a line 16. A gravity analyzer 20, viscosity analyzers 23 and 24, and a sulfur analyzer 28 sample the charge oil in line 4 and provide signals API, KV 210 , KV 150  and S, respectively, corresponding to the API gravity, the kinematic viscosities at 210° F. and 150° F., and sulfur content, respectively, of the light sweet charge oil. 
     A flow transmitter 30 in line 4 provides a signal CHG corresponding to the flow rate of the charge oil in line 4. Another flow transmitter 33 in line 7 provides a signal SOLV corresponding to the MP flow rate. A temperature sensor 38, sensing the temperature of the extract mix leaving extractor 1, provides a signal T corresponding to the sensed temperature. All signals hereinbefore mentioned are provided to control means 40. 
     Control means 40 provides signal C to a flow recorder controller 43. Recorder controller 43 receives signals CHG and C and provides a signal to a valve 48 to control the flow rate of the charge oil in line 4 in accordance with signals CHG and C so that the charge oil assumes a desired flow rate. Signal T is also provided to temperature controller 50. Temperature controller 50 provides a signal to a valve 51 to control the amount of cooling water entering extractor 1 and hence the temperature of the extract-mix in accordance with its set point position and signal T. 
     The following equations are used in practicing the present invention for light sweet charge oil: 
     
         H.sub.210 =1n1n(KV.sub.210 +C.sub.1)                       (1) 
    
     where H 210  is a viscosity H value for 210° F., KV 210  is the kinematic viscosity of the charge oil at 210° F. and C 1  is a constant having a preferred value of 0.7. 
     
         H.sub.150 =1n1n(KV.sub.150 +C.sub.1)                       (2) 
    
     where H 150  is a viscosity H value for 150° F., and KV 150  is the kinematic viscosity of the charge oil at 150° F. 
     
         K.sub.150 =[C.sub.2 -1n(T.sub.150 +C.sub.3 ]/C.sub.4       (3) 
    
     where K 150  is a constant needed for estimation of the kinematic viscosity at 100° F., T 150  is 150, and C 2  through C 4  are constants having preferred values of 6.5073, 460 and 0.17937, respectively. 
     
         H.sub.100 =H.sub.210 +(H.sub.150 -H.sub.210)/K.sub.150     (4) 
    
     where H 100  is a viscosity H value for 100° F. 
     
         KV.sub.100 =exp[exp(H.sub.100)]-C.sub.1                    (5) 
    
     where KV 100  is the kinematic viscosity of the charge oil at 100° F. 
     
         ______________________________________6. SUS =  C.sub.5 (KV.sub.210) + [C.sub.6 + C.sub.7 (KV.sub.210)]/[C.sub.8      + C.sub.9     (KV.sub.210) + C.sub.10 (KV.sub.210).sup.2 + C.sub.11 (KV.sub.21     0).sup.3 ](C.sub.12)______________________________________ 
    
     where SUS is the viscosity in Saybolt Universal Seconds and C 5  through C 12  are constants having preferred values of 4.6324, 1.0, 0.03264, 3930.2, 262.7, 23.97, 1.646 and 10 -5 , respectively. 
     
         SUS.sub.210 =[C.sub.13 +C.sub.14 (C.sub.15 -C.sub.16)]SUS  (7) 
    
     where SUS 210  is the viscosity in Saybolt Universal Seconds at 210° F. and C 13  through C 16  are constants having preferred values of 1.0, 0.000061, 210 and 100, respectively. 
     
         ______________________________________ 8. VI.sub.DWCO.sbsb.O =     --C.sub.17 --C.sub.18 (S) + C.sub.19 (KV.sub.210).sup.2 +     C.sub.20 (VI).sup.2,     +C.sub.21 (S).sup.2 + C.sub.22 (API)(KV.sub.210)--C.sub.23     (KV.sub.210)     (VI) + C.sub.24 (VI)(S)______________________________________ 
    
     where VI DWC .sbsb.O is the viscosity index of the dewaxed charge oil having a pour point of 0° F., and C 17  through C 24  are constants having preferred values of 18.067, 51.155, 1.0108, 0.0084733, 2.2188, 1.0299, 0.34233 and 0.67215, respectively. 
     
         ______________________________________ 9. VI.sub.DWC.sbsb.P =       VI.sub.DWC.sbsb.O + (Pour)[C.sub.25-C26 lnSUS.sub.210       +C.sub.27 (lnSUS.sub.210).sup.2 ]______________________________________ 
    
     where VI DWC .sbsb.P and Pour are the viscosity index of the dewaxed charge at a predetermined pour point temperature and the Pour Point of the dewaxed product, respectively, and C 25  through C 27  are constants having preferred values of 2.856, 1.18 and 0.126, respectively. 
     
         ΔVI=VI.sub.RO -VI.sub.DWC.sbsb.O =VI.sub.RP -VI.sub.DWC.sbsb.P (10) 
    
     where VI RO  and VI RP  are the VI of the refined oil at 0° F., and the predetermined temperature, respectively. 
     
         ______________________________________11. ΔRI =   [C.sub.28 + C.sub.29 (KV.sub.210) - C.sub.30 (S).sup.2 + C.sub.31   (ΔVI) (API)   -C.sub.32 (API).sup.2 + C.sub.33 (API) (KV.sub.210) + C.sub.34   (VI).sup.2   -C.sub.35 (KV.sub.210) (VI) + C.sub.36 (VI) (S) + C.sub.37   (ΔVI)    (KV.sub.210)]C.sub.38______________________________________ 
    
     where ΔRI is the change in refractive index between the light sweet charge oil and the raffinate and C 28  through C 38  are constants having preferred values of 99.848, 41.457, 32.735, 0.116, 0.37573, 23635, 0.03488, 1,3274, 1.2068, 0.25432 and 10 -4 , respectively. 
     
         ______________________________________12. J =  -C.sub.39 + C.sub.40 (ΔRI) + C.sub.41 (S).sup.2 - C.sub.42  (KV.sub.210) (T)  +C.sub.43 (VI) - C.sub.44 (S) + C.sub.45 (ΔRI) (ΔVI)  -C.sub.46 (ΔVI) (T)______________________________________ 
    
     where J is the methyl-2-pyrrolidone dosage and C 39  through C 46  are constants having preferred values of 31.022, 12315, 558.75, 0.08962, 2.9954, 860.35, 496.1 and 0.062708, respectively. 
     
         C=(SOLV)(100)/J                                            (13) 
    
     where C is the new charge oil flow rate. 
     Referring now to FIG. 2, signal KV 210  is provided to an H computer 50 in control means 40, while signal KV 150  is applied to an H computer 50A. It should be noted that elements having a number and a letter suffix are similar in construction and operation to those elements having the same numeric designation without a suffix. All elements in FIG. 2, except elements whose operation is obvious, will be disclosed in detail hereinafter. Computers 50 and 50A provide signals E 1  and E 2  corresponding to H 210  and H 150 , respectively, in equations 1 and 2, respectively, to H signal means 53. K signal means 55 provides a signal E 3  corresponding to the term K 150  in equation 3 to H signal means 53. H signal means 53 provides a signal E 4  corresponding to the term H 100  in equation 4 to a KV computer 60 which provides a signal E 5  corresponding to the term KV 100  in accordance with signal E 4  and equation 5 as hereinafter explained. 
     Signals E 5  and KV 210  are applied to VI signal means 63 which provides a signal E 6  corresponding to the viscosity index. 
     An SUS computer 65 receives signal KV 210  and provides a signal E 7  corresponding to the term SUS in accordance with the received signals and equation 6 as hereinafter explained. 
     An SUS 210 computer 68 receives signal E 7  and applies signal E 8  corresponding to the term SUS 210  in accordance with the received signal and equation 7 as hereinafter explained. 
     A VI DWC .sbsb.O computer 70 receives signal KV 210 , API, S and E 6  and provides a signal E 9  corresponding to the term VI DWC .sbsb.O in accordance with the received signals and and equation 8 as hereinafter explained. 
     A VI DWC .sbsb.P computer 72 receives signal E 8  and E 9  and provides a signal E 10  corresponding to the term VI DWC .sbsb.P in accordance with the received signals and equation 9. Subtracting means 76 performs the function of equation 10 by subtracting signal E 10  from a direct current voltage V 9  corresponding to the term VI RP , in equation 10, to provide a signal E 11  corresponding to the term ΔVI in equation 10. 
     A ΔRI computer 79 receives signals API, KV 210 , S, E 6  and E 11  and provides a signal ΔRI in accordance with the received signals and equation 11, as hereinafter explained. 
     A J computer 80 receives signals T, ΔRI, KV 210 , S, E 6  and E 11  and provides a signal E 13  corresponding to the term J in accordance with the received signals and equation 12 as hereinafter explained to a divider 83. 
     Signal SOLV is provided to a multiplier 82 where it is multiplied by a direct current voltage V 2  corresponding to a value of 100 to provide a signal corresponding to the term (SOLV)(100) in equation 13. The product signal is applied to divider 83 where it is divided by signal E 13  to provide signal C corresponding to the desired new charge oil flow rate. 
     It would be obvious to one skilled in the art that if the charge oil flow rate was maintained constant and the MP flow rate varied, equation 13 would be rewritten as 
     
         SO=(J)(CHG)/100                                            (14) 
    
     where SO is the new MP flow rate. Control means 40 would be modified accordingly. 
     Referring now to FIG. 3, H computer 50 includes summing means 112 receiving signal KV 210  and summing it with a direct current voltage C 1  to provide a signal corresponding to the term [KV 210  +C 1  ] shown in equation 1. The signal from summing means 112 is applied to a natural logarithm function generator 113 which provides a signal corresponding to the natural log function generator 113A which in turn provides signal E 1 . 
     Referring now to FIG. 4, K signal means 55 includes summing means 114 summing direct current voltage T 150  and C 3  to provide a signal corresponding to the term [T 150  +C 3  ] which is provided to a natural log function generator 113B which in turn provides a signal corresponding to the natural log of the sum signal from summing means 114. Subtracting means 115 subtracts the signal provided by function generator 113B from a direct current voltage C 2  to provide a signal corresponding to the numerator of equation 3. A divider 116 divides the signal from subtracting means 115 with a direct current voltage C 4  to provide signal E 3 . 
     Referring now to FIG. 5, H signal means 53 includes subtracting means 117 which subtracts signal E 1  from signal E 2  to provide a signal corresponding to the term H 150  -H 210 , in equation 4, to a divider 118. Divider 118 divides the signal from subtracting means 117 by signal E 3 . Divider 118 provides a signal which is summed with signal E 1  by summing means 119 to provide signal E 4  corresponding to H 100 . 
     Referring now to FIG. 6, a direct current voltage V 3  is applied to a logarithmic amplifier 120 in KV computer 60. Direct current voltage V 3  corresponds to the mathematical constant e. The output from amplifier 120 is applied to a multiplier 122 where it is multiplied with signal E 4 . The product signal from multiplier 122 is applied to an antilog circuit 125 which provides a signal corresponding to the term exp (H 100 ) in equation 5. The signal from circuit 125 is multiplied with the output from logarithmic amplifier 120 by a multiplier 127 which provides a signal to antilog circuit 125A. Circuit 125A is provided to subtracting means 128 which subtracts a direct current voltage C 1  from the signal from circuit 125A to provide signal E 5 . 
     Referring now to FIG. 7, VI signal means 63 is essentially memory means which is addressed by signals E 5 , corresponding to KV 100 , and signal KV 210 . In this regard, a comparator 130 and comparator 130A represent a plurality of comparators which receive signal E 5  and compare signal E 5  to reference voltages, represented by voltages R 1  and R 2 , so as to decode signal E 5 . Similarly, comparators 130B and 130C represent a plurality of comparators receiving signal KV 210  which compare signal KV 210  with reference voltages RA and RB so as to decode signal KV 210 . The outputs from comparators 130 and 130B are applied to an AND gate 133 whose output controls a switch 135. Thus, should comparators 130 and 130B provide a high output, AND gate 133 is enabled and causes switch 135 to be rendered conductive to pass a direct current voltage V A  corresponding to a predetermined value, as signal E 6  which corresponds to VI. Similarly, the outputs of comparators 130 and 130C control an AND gate 133A which in turn controls a switch 135A to pass or to block a direct current voltage V B . Similarly, another AND gate 133B is controlled by the outputs from comparators 130A and 130B to control a switch 135B so as to pass or block a direct current voltage V C . Again, an AND gate 133C is controlled by the outputs from comparators 130A and 130C to control a switch 135C to pass or to block a direct current voltage V D . The outputs of switches 135 through 135C are tied together so as to provide a common output. 
     Referring now to FIG. 8, the SUS computer 65 includes multipliers 136, 137 and 138 multiplying signal KV 210  with direct current voltages C 9 , C 7  and C 5 , respectively, to provide signals corresponding to the terms C 9  (KV 210 ), C 7  (KV 210 ) and C 5  (KV 210 ), respectively in equation 6. A multiplier 139 effectively squares signal KV 210  to provide a signal to multipliers 140, 141. Multiplier 140 multiplies the signal from multiplier 139 with a direct current voltage C 10  to provide a signal corresponding to the term C 10  (KV 210 ) 2  in equation 6. Multiplier 141 multiplies the signal from multiplier 139 with signal KV 210  to provide a signal corresponding to (KV 210 ) 3 . A multiplier 142 multiplies the signal from multiplier 141 with a direct current voltage C 11  to provide a signal corresponding to the term C 11  (KV 210 ) 3  in equation 6. Summing means 143 sums the signals from multipliers 136, 140 and 142 with a direct current voltage C 8   to provide a signal to a multiplier 144 where it is multiplied with a direct current voltage C 12 . The signal from multiplier 137 is summed with a direct current voltage C 6  by summing means 145 to provide a signal corresponding to the term [C 6  +C 7  (KV 210  ]. A divider 146 divides the signal provided by summing means 145 with the signal provided by multiplier 144 to provide a signal which is summed with the signal from multiplier 138 by summing means 147 to provide signal E 7 . 
     Referring now to FIG. 9, SUS 210  computer 68 includes subtracting means 148 which subtracts a direct current voltage C 16   from another direct current voltage C 15  to provide a signal corresponding to the term (C 15  -C 16 ) in equation 7. The signal from subtracting means 148 is multiplied with a direct current voltage C 14  by a multiplier 149 to provide a product signal which is summed with another direct current voltage C 13  by summing means 150. Summing means 150 provides a signal corresponding to the term [C 13  +C 14  (C 15  -C 16 )] in equation 7. The signal from summing means 150 is multiplied with signal E 7  by a multiplier 152 to provide signal E 8 . 
     Referring now to FIG. 10, VI DWC .sbsb.O computer 70 includes multipliers 160, 161 and 162 which effectively square signals S, E 6  and KV 210 , respectively, and provide corresponding signals. Multipliers 165, 166 multiply signal S with a direct current voltage C 1  and signal E 6 , respectively, to provide product signals. Multipliers 169, 170 multiply signal KV 210  with signals E 6  and API, respectively, to provide product signals. Multipliers 175 through 180 multiply the signals from multipliers 160, 166, 161, 169, 162 and 170, respectively, with direct current voltages C 21 , C 24 , C 20 , C 23 , C 19  and C 22 , respectively, to signals corresponding to the terms C 21  (S) 2 , C 24  (VI)(S), C 20  (VI) 2 , C 23  (KV 210 )(VI), C 19  (KV 210 ) 2  and C 22  (API)(KV 210 ), respectively, in equation 8. Summing means 182 sums the signals from multipliers 175, 176, 177, 179 and 180, to effectively sum the positive terms of equation 8, and provides a corresponding sum signal. The negative terms of equation  8 are effectively summed when summing means 185 sums the signals from multipliers 165, 178 with a direct current voltage C 17 . Subtracting means 187 subtracts the signal provided by summing means 185 from the signal provided by summing means 182 to provide signal E 9 . 
     VI DWC .sbsb.P computer 72 shown in FIG. 11, includes a natural logarithm function generator 190 receiving signal E 8  and providing a signal corresponding to the term 1nSUS 210  to multipliers 192 and 194. Multiplier 192 multiplies the signal from function generator 190 with a direct current voltage C 26  to provide a signal corresponding to the term C 26  1nSUS 210  in equation 9. Multiplier 194 effectively squares the signal from function generator 190 to provide a signal that is multiplied with the direct current voltage C 27  by a multiplier 196. Multiplier 196 provides a signal corresponding to the term C 27  (1nSUS 210 ) in equation 9. Subtracting means 198 subtracts the signals provided by multiplier 192 from the signal provided by multiplier 196. Summing means 200 sums the signal from subtracting means 198 with a direct current voltage C 25 . A multiplier 202 multiplies the sum signal from summing means 200 with a direct current voltage POUR to provide a signal which is summed with signal E 9  by summing means 204 which provides signal E 10 . 
     Referring to FIG. 12, multiplier 220 in ΔRI computer 79 effectively squares signal API while multipliers 222 and 224 multiply signal E 11  with signals API and KV 210 , respectively, to provide product signals. Multipliers 226, 228 and 230 multiply signal KV 210  with signal API, a direct current voltage C 29  and signal E 6 , respectively. Multipliers 235, 238 effectively square signals E 6  and S to provide product signals. Multiplier 239 multiplies signal E 6  with signal S. Multipliers 241 through 248 multiply the product signals from multipliers 220, 222, 224, 226, 230, 235, 238 and 239, respectively, with direct current voltages C 32 , C 31 , C 37 , C 33 , C 35 , C 34 , C 30  and C 36 , respectively, to provide signals corresponding to the terms C 32  (API) 2 , C 31  (ΔVI)(API), C 37  (ΔVI)(KV 210 ), C 33  (API)(KV 210 ), C 35  (VI)(KV 210 ), C 34  (VI) 2 , C 30  (S) 2  and C 36  (VI)(S), respectively, in equation 11. Summing means 250 effectively sums the positive terms of equation 11 when it sums a direct current voltage C 28  with the signals from multipliers 228, 242, 243, 244, 246 and 248 to provide a sum signal. Summing means 252 effectively sums the negative terms of equation 11 when it sums the signals from multipliers 241, 245 and 247 to provide a sum signal. Subtracting 255 subtracts the sum signal provided by summing means 252 from the sum signal provided by summing means 250 to provide a signal which is multiplied with a direct current voltage C 38  by a multiplier 256. Multiplier 256 provides signal ΔRI. 
     Referring now to FIG. 13, J computer 80 includes multipliers 260, 261 multiplying signal ΔRI with a direct current voltage C 40  and signal E 11 , respectively, to provide product signals. Multipliers 264, 265 multiply signal T with signals E 11  and KV 210 , respectively, to provide product signals. Multipliers 269, 270 multiply signals E 6  and S, respectively, with direct current voltage C 43  and C 44 , respectively, to provide signals corresponding to the terms C 43  (VI) and C 44  (S), respectively, in equation 12. A multiplier 273 effectively squares signal S to provide a signal which is multiplied with a direct current voltage C 41  by a multiplier 274 to develop a signal corresponding to the term C 41  (S) 2  in equation 12. Multipliers 276, 277 and 278 multiply the signals from multipliers 261, 264 and 265, respectively, with direct current voltages C 45 , C 46  and C 42 , respectively, to provide signals corresponding to the terms C 45  (ΔRI)(ΔVI), C 46  (ΔVI)(T) and C 42  (KV 210 )(T), respectively. 
     Summing means 280 sums the positive terms of equation 12 when it sums the signals from multipliers 260, 269, 274 and 276 to provide a corresponding signal. Summing means 284 effectively sums the negative terms of equation 12 when it sums a direct current voltage C 34  with the signals from multipliers 270, 277 and 278 to provide a sum signal. Subtracting means 288 subtracts the signal provided by summing means 284 from the signal provided by subtracting means 280 to provide signal E 13 . 
     The present invention as hereinbefore described controls an MP refining unit receiving light sweet charge oil to achieve a desired charge oil flow rate for a constant MP flow rate. It is also within the scope of the present invention, as hereinbefore described, to control the MP flow rate while the light sweet charge oil flow is maintained at a constant rate.