US3729624A

US3729624A - Means and method for controlling the strength of acid in an alkylation unit

Info

Publication number: US3729624A
Application number: US00169443A
Authority: US
Inventors: W Hopkins; L Chvatal; R Edwards
Original assignee: Texaco Inc
Current assignee: Texaco Inc
Priority date: 1971-08-05
Filing date: 1971-08-05
Publication date: 1973-04-24
Anticipated expiration: 1990-04-24
Also published as: CA982612A; SE396374B; AU466677B2; NL7210394A; BE787018A; IT963769B; NL159513B; JPS544638B2; BR7205158D0; DE2237983B2; GB1362894A; JPS4826704A; DE2237983A1; FR2149896A5; AU4457472A

Abstract

A system controls the strength of acid used as the catalyst in an alkylation unit, which reacts olefins with isoparaffin, in accordance with the average strength of the acid and the anticipated required strength of the acid. A signal corresponding to the average acid strength is developed by analog computers solving equations, hereinafter disclosed, using an output from an acid analyzer corresponding to the density of the acid and a signal corresponding to the volume of caustic used in analyzing the acid. An anticipated required acid strength signal is developed by an analog computer from signals, corresponding to the flow rate and composition of the olefin, from a flow rate sensor and chromatograph means, respectively, in accordance with equations hereinafter disclosed. Another analog computer provides signals for setting the set point of a flow recorder controller in accordance with the average acid strength signal, the anticipated required acid strength signal and equations hereinafter disclosed. The flow recorder controller controls the flow rate acid being discharged from the alkylation unit so as to control the quantity of fresh acid entering the alkylation unit thereby controlling the strength of the acid.

Description

[ MEANS AND METHOD FOR CONTROLLING THE STRENGTH OF ACID IN AN ALKYLATION UNIT [75] Inventors: Walker L. Hopkins, Houston; Le-

land A. Chvatal, Port Arthur; Richard R. Edwards, Groves, all of Tex.

[73] Assignee: Texaco, Inc., New York, NY.

22 Filed: Aug. 5, 1971 [21] Appl. No.: 169,443

[52] US. Cl. ..235/151.l2, 23/255 E, 208/D1G. 1, 235/150.l, 260/683.43

[51] Int. C1. ..C07e 3/52, 606g 7/58 [58] FieldofSearch .L ..235/15l.12, 150.1;

23/230 R, 232 E, 252R, 253 R, 254 E, 255 E; 260/683.4, 683.43, 683.46, 683.59, 683.62; 208/133,- 134, DIG. l

[56] References Cited UNITED STATES PATENTS 3,002,818 10/1961 Berger ..235/l5l,l2

3,173,969 3/1965 Kapf ....260/683.59X

3,325,391 6/1967 Waterman et a1. .,..260/683.43 X

3,442,972 5/1969 Massa ....260/683.62 X

3,513,220 5/1970 Brandel..... ..23/2 3O R X 3,625,655 12/1971 Culp et al. ..260/683.59 X

ACID HYDROCARBON Apr. 24, 1973 Primary Examiner-.1oseph F. Ruggiero Attorney-Thomas H. Whaley et a1.

[57] ABSTRACT A system controls the strength of acid used as the catalyst in an alkylation unit, which reacts olefins with isoparaffin, in accordance with the average strength of the acid and the anticipated required strength of the acid. A signal corresponding to the average acid strength is developed by analog computers solving equations, hereinafter disclosed, using an output from an acid analyzer corresponding to the density of the the set point of a flow recorder controller in accordance with the average acid strength signal, the anticipated required acid strength signal and equations hereinafter disclosed. The flow recorder controller controls the flow rate acid being discharged from the alkylation unit so as to control the quantity of fresh acid entering the alkylation unit thereby controlling the strength of-the acid.

13 Claims, 7 Drawing Figures 14 7 HYDROCARBON I l PRODUCT 40 RE E Q R D ER 4-4 ACID E I sETTLER CONTROLLER 12 BL v -v v PROGRAMMER ACID 27 E STRENGTH E SIGNAL E TE q b E E MEANS ./-35

M514 ACID STRENGTH 1 24 ANALYZER 4 CAUSTIC Em 25 SOURCE S 22J 25 2 SETPOINT 'C-E -E E SIGNAL 1 MEANS EIS E 2 J B- D H FLOW AND RECORDER [SOPARA'FIN CONTROLLER OLEFIN 7/36 v -v "2/- v V 1 E SIGNAL 1 4 24- 32 E6/I/' if MEANS V V v v, v r E 2 SOURCE OF 38 CHROMATOGRAPH Q S SEJ MEANS 37 voLTAGEs DISCHARGE 2 ACID E U6 I8 40 V,6

Patented April 24, 1973 7 Sheets-Sheet 4 M KE/w NON :DQrC m3 A 530m ONN F IL 1. Field of the Invention The present invention relates to alkylation units and, more particularly, to a control system for an alkylation unit.

2. Description of the Prior Art Heretofore, acid control systems, such as the system disclosed in U.S. application, Ser. No. 169,385 filed on Aug. 5, l97l by Child et al, and assignedto Texaco lnc., assignee of the present invention, uses a feedforward signal determined from the different inputs to the alkylation unit.

The system of the present invention is substantially different in that the composition of a single input is determined to develop the feedforward signal. The system of the present invention further distinguishes over the prior art by using the feedforward signal and a feedback signal corresponding to the acid strength to achieve a greater degree of control. It is not obvious, to one skilled in the art, from the aforementioned application that a feedback signal may be combined with a feedforward signal. Nor is it obvious how to combine the feedforward and feedback signals.

SUMMARY OF THE INVENTION A system controls the strength of acid in an alkylation unit. The alkylation unit reacts isoparaffin and olefins in the presence of acid to provide an acidhydrocarbon mixture to a settler where the hydrocarbon is separated from the acid to provide a hydrocarbon product and acid. A proportion of the acid from the settler is recycled while the remaining acid is discharged from the processing unit. Fresh acid is added to the recycle acid to replace the discharged acid thereby affecting the strength of the acid. A control device controls the quantity of the fresh acid entering the processing unit in accordance with the control signal. A sensing circuit provides a signal correspond- Another object of the present invention is to com-' bine acidity measurements with measurements of the olefin s composition and flow rate to provide a control signal for controlling the strength of an acid in an alkylation unit.

Another object of the present invention is to provide a signal corresponding to the strength of the acid in an alkylation unit.

Another object of the present invention is to provide strength as determined in accordance with the quantity of acid consuming constituents of olefin entering an alkylation unit.

The foregoing and other objects and advantages of the invention will appear more fully hereinafter from a consideration of the detailed description which follows,

a signal corresponding to an anticipated required acid I 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 is a block diagram of a system, constructed in accordance with the present invention, for controlling the strength of acid in an alkylation unit which is also partially shown in schematic form.

FIG. 2 is a diagrammatic representation of voltage wave forms occurring during a typical cycle of operation of the system shown in FIG. 1.

FIGS. 3 through 5 are detailed block diagrams of the programmer, the set point control signal means and the acid strength signal means shown in FIG. I, respectively.

FIGS. 6A and 68, when matched along line AA, is a detailed block diagram of the olefin signal means shown in FIG. 1.

DESCRIPTION OF THE INVENTION Referring to FIG. 1, there is shown a portion of an alkylation unit in which an olefin is reacted with isoparaffin in the presenceof a catalyst, such as sulfuric or hydrofluoric acid, to form a higher molecular weight isoparaffin. For purpose of explanation, the acid in the following description shall be sulfuric acid. The olefin may be butylene, propylene or a mixture of butylene and propylene, while the isoparaffin may be isobutane. The control system shown in FIG. 1 controls the strength of the acid during the reaction by determining the actual acid strength and comparing the actual acid strength with a desired acid strength. The control system also anticipates changes in the acid strength due to changes in the quantities of acid consuming constituents of the charge olefin to control the acid strength accordingly so as to speed up the control process. The charge olefin and isoparaffin enters a contactor 4, by way of a line 6, where the olefin and isoparaffin are contacted with acid entering by way of a line 7. Contactor 4 provides an acid-hydrocarbon mix by way of a line 8 to an acid settler 12. Settler 12 separates the hydrocarbon product from the acid and the hydrocarbon product is discharged through a line 14 while the acid is removed by way of a line 16. Acid settler 12 may be the only acid settler in the unit or it may be the last acid settler of a group of acid settlers. Fresh acid enters line 16 by way of a line 17 as needed to maintain a desired acid strength. A pump 20 pumps the acid from line 16 into line 7. A portion of the acid in line 7 is discharged by way of a line 21. The discharge acid may be provided to another alkylation unit or disposed of.

An acid strength analyzer 22 periodically samples the acid in line 16 and provides signals B, through E Acid strength analyzer 22 may be of the type fully disclosed in US. application Ser. No. 881,359 filed on Dec. 2, 1969 by R. A. Culp et al and assigned to Texaco lnc., assignee of the present invention. The acid sample is analyzed by titrating the sample with caustic from a source 24 entering analyzer by way of a line 25 at a constant rate. The start of each sampling cycle of the 'acid in line 16 is controlled by a reset signal E, from a programmer 27, as hereinafter described.

Signal 5;, shown in FIG. 2A, from analyzer 22 occurs at the time at which titration is initiated, while signal E shown in FIG. 2B, occurs at the time at which titration is completed. The occurrence of pulse signal E indicates that the amplitude of signal 13., corresponds to the density of the acid sample. Signals E E from analyzer 22 are applied to programmer 27, while signals E 13.; are applied to acid strength signal means 35, which is hereinafter described in detail.

Programmer 27 controls the timing sequence of the development of set point pulse train E and signal E provided by set point signal means 28, which are used to control the set point of a conventional flow recorder controller 30, as hereinafter explained. Controller 30 operates a valve 32, to control the flow rate of the discharge acid in line 21, in accordance with a signal E from a conventional type sensor 33, corresponding to the flow rate of the discharge acid in line 21, and its set points. It should be noted that since the acid system is essentially a closed loop system, the acid flowing in line 21 corresponds to the fresh acid flowing in line 17 so that the amount of fresh acid being added to the system is actually controlled by controlling the discharge acid in line 21.

Programmer 27 provides a signal E corresponding to the titration time AT of the acid sample, and pulse signals E through E to acid strength signal means 35. Acid strength signal means 35 is controlled by pulse signals E through E to provide a signal E corresponding to the average strength of the acid in accordance with signal 5., from analyzer 22 and direct current voltages V, through V, and V from a direct current voltage source 37, as hereinafter explained. Signal E is applied to set point signal means 28 which also receives a signal E corresponding to an anticipated required acid strength, from olefin signal means 36 as determined from the olefin and isoparaffin, as hereinafter explained. Set point signal means 28 is controlled by pulse signals E E through E from programmer 27 to provide pulse train E and signal E to controller 30 in accordance with signals E E and direct current voltages V,, through V from source 37. Set point signal means 28 also provides signal E to programmer 27 as hereinafter explained. Each pulse in pulse train E changes the set point of controller 30, while signal l5. controls the direction of the change of the set point. Pulse train E has no pulses when the set point of controller 30 is not required to be changed.

When the discharge acid flow rate in line 21 is changed, the acid level. in settler 12 changes accordingly. A level sensor 40 provides a signal corresponding to the acid level to a conventional type level recorder controller 42. Controller 42 provides a signal to a valve 45 in line 17 corresponding to the difference between the level signal from sensor 40 and the position of controller 42 set point which corresponds to a predetermined acid level for settler 12. Valve 45 controls the flow rate of the fresh acid in line 17 to restore the acid level in settler 12 to its predetermined level.

When the acid strength is to be increased, the flow rate of the discharge acid is increased, by an appropriate adjustment of the set point in controller 30, causing the acid level in settler 12 to decrease. Controller 42 operates valve 45 to increase the fresh acid flow rate thereby increasing the acid in settler 12 to the predetermined acid level. The increase in the fresh acid flow rate increases the strength of the acid in the system.

Similarly when the acid strength is to be decreased,

the discharge acid flow rate is decreased causing the acid level in settler 12 to increase. The increase in the acid level causes a decrease in the fresh acid flow rate to decrease the acid in settler 12 to the predetermined acid level. The decrease in the fresh acid flow rate decreases the strength of the acid in the system.

Olefin signal means 36 provides signal E to set point signal means 28 in accordance with a signal E from chromatograph means 38, corresponding to the constituents of the charge olefin, a signal E from a conventional sensor corresponding to the flow rate of the charge olefin in line 6, direct current voltages V through V V and V from source 37, and pulse signals E,,, E, through E from programmer 27, as hereinafter explained.

Chromatograph means 38 also provides a pulse train E to programmer 27. The pulses in pulse train 17A coincide with the peaks of signal E Referring to FIG. 3, programmer 27 includes an onoff switch 101 which receives a direct current voltage V from source 37. Switch 101 controls the operation of the control system. When activated, switch 101 applies voltage V to an inverter 103 and to an AND gate 106. Inverter 103 inverts voltage V to reset a flip-flop 104 to a clear state. Reset pulse E occurring at the end of each cycle of operation triggers flip-flop 104 to a set state in which it remains until again being reset by the closing of switch 101. In changing from the clear state to the set state, a 6 output of flip-flop 104 triggers a monostable multivibrator 1435. Multivibrator when triggered provides pulse signal E Pulse train E from chromatograph means 38 is applied to AND gate 166 which controls the development of pulse signals E, through E AND gate 106 is fully enabled by voltage V and a high level direct current output from a logic decoder 108. When enabled, pulse train E, passes through AND gate 106 into a conventional counter which counts the pulses in signal E corresponding to the different peaks in signal E Logic decoder 108 decodes the count in' counter 110 to provide a plurality of outputs to monostable multivibrators 114. Logic decoder 108 may be of the type that includes AND gates connected to each stage of counter 110 in a manner so that each AND gate provides an output for a different count in counter 110. Upon reaching a count of 10, the direct current output provided by logic decoder 108 to AND gate 106 goes to a low level thereby disabling AND gate 106 to block pulse train E, to prevent further counting by counter l 10.

Monostable multivibrators 114 represent a plurality of monostable multivibrators, each monostable multivibrator is connected to a different AND. gate in decoder 108 and is triggered by its output to provide a pulse signal. Monostable rnultivibrators 114 provide a plurality of pulse signals B, through E which coincide with the peaks of signal E corresponding to the various constituents of the charge olefin as determined by chromatograph means 38. FIG. 2C shows pulse E,

while FIG. 2D shows pulse E It should be noted that there is only one break associated with pulse E This is because pulse E, preceded pulse E in time and in this particular example, pulse E is used to start the timing sequence and is therefore time related to the pulses shown in FIGS. 2F through 2M.

Pulse signal E from monostable multivibrators 114 triggers yet another monostable multivibrator 116 which acts as a time delay and provides a pulse to a flipflop 117. The trailing edge of the pulse from multivibratgr 116 triggers flip-flop 117 to its set state. The Q and Q outputs of a flip-flop are high level and low level direct current voltages, respectively, when the flip-flop is in a set state. The levels of the Q and 6 outputs are reversed when the flip-flop is in a clear state. F lip-flop 117 provides its Q output as signal E shown in FIG. 2E, to acid strength signal means 35 and to an AND gate 118 partially enabling AND gate 118. The end of titration signal E triggers a flip-flop 132 to a set state causing flip-flop 132 to provide a high level direct current output to AND gate 118 thereby enabling AND gate 118. AND gate 118 controls the development of pulse signals E E E E,.-, E E and E so that they only occur after the acid strength analysis and the charge olefin analysis have been completed.

When fully enabled, AND gate 118 passes timing pulses from a clock 120 to a conventional counter 124. A logic decoder 125 decodes the count in counter 124 to provide a plurality of triggering inputs to a plurality of monostable multivibrators shown as monostable multivibrators 127. The decoder is arranged so that there is a time differential between the next to last count provided and the last count provided. Monostable multivibrators 127 provide signals E E E B E E and E as shown in FIGS. 2F through 2L, respectively. The time differential previously referred to occurs between signals E and E and should be of a sufficient duration to allow the set point in flow recorder control 30 to be moved through its entire range although usually it is not necessary to do so.

Upon termination of pulse train E set point signal means 28 provides a signal E to a flip-flop 134 triggering flip-flop to its set state. The Q output of flip-flop 134 enables AND gate 135 when flip-flop 134 is in a clear state and disables AND gate 135 when flip-flop 134 is in a set state. When enabled, AND gate 135 effectively passes timing pulses from a clock 137 as signal E and blocks the timing pulses when disabled. Pulse signal E, from multivibrators 127 triggers flip-flop 134 to a clear state so that the 6 output from flip-flop 134 enables AND gate 135.

The AT signal E is developed by a flip-flop 140, a clock 141, an AND gate 142, monostable multivibrators 144 and 145, a counter 148, a storage register 150 and a digital to-analog converter 151. The start of titration signal E from acid strength analyzer 22 triggers flip-flop 140 to a set state. The 0 output of flip-flop 140 enables AND gate 142 causing it to pass timing pulses to counter 148. Counter 148 counts the timing pulses passed by AND gate 142. The end of titration signal E triggers flip-flop 140 to a clear state causing the Q output to disable AND gate 140. When disabled, AND gate 142 blocks the timing pulses from clock 141 so that the count in counter 148 corresponds to the time interval between signals E E which is the titration time AT.

The change in the 6 output of flip-flop due to signal E, triggering flip-flop 140 to the clear state triggers time delay multivibrator 144 causing it to provide a pulse. The trailing edge of the pulse from multivibrator 144 triggers multivibrator 145 so that multivibrator 145 provides a pulse after counter 148 has completed a count. The pulse from multivibrator 145 controls storage register 150 to accept and store the count from counter 148. Digital-to-analog converter 151 converts the count stored in register 150 to signal E Pulse signal E; from monostable multivibrators 127 also triggers a delay monostable multivibrator 155 which provides a pulse to another monostable multivibrator 156 whose trailing edge triggers monostable multivibrator 156 causing it to provide reset pulse E shown in FIG. 2M. Reset pulse E is provided to chromatograph means 28, set point signal means 28, to

counters

110, 124 and 148, storage register 150,

flipflops

117, 104 and 132 resetting flip-flops 117 and 132 to the clear state to recycle the control operation.

Referring to FIG. 4, there is shown in detail set point signal means 28 which effectively combines signal E corresponding to feedback information pertaining to the acid strength, with signal E corresponding to the anticipated required acid strength, to provide pulse train E and signal E for changing the set point of flow recorder controller 30 as needed. Pulse train E and signal E are provided in accordance with the following equation:

where SP, is the set point value for the current cycle, SP,, is the set point value for the next previous cycle, F, is an error between the average acid strength and the desired acid strength for the current cycle, F,, is the error for the next previous cycle, U, is the anticipated required acid strength for the current cycle, and G through G are constants having the following values: 3.1, 3.0, 1.0 and 2.0, respectively.

A sample and hold circuit 201 is controlled by pulse signal E, from programmer 27 to sample and hold direct current voltage V, corresponding to a desired acid strength. Subtracting means 204 subtracts the acid strength signal E from signal means 35 from the output of sample and hold circuit 201 to provide an error signal to another sample and hold circuit 205. Sample and hold circuit 205 is controlled by pulse signal 13,; from programmer 27 and provides an output corresponding to the term F n in equation l.

Absolute value circuit 207 provides a signal corresponding to the absolute value of output F, from sample and hold circuit 205. Absolute value circuit 207 may be a squaring circuit, which squares the output from sample and hold circuit 205, and a square root circuit for taking the square root of the output from the squaring circuit to provide the absolute value signal lF l The absolute value signal IF,,| is multiplied by the F signal from sample and hold circuit 205 by a multiplier 209. The product signal from multiplier 209 is multiplied with direct current voltage V corresponding to the 6;, term in equation 1, by a multiplier 210.

The output from a sample and hold circuit 206, corresponding to the term F is multiplied with a direct current voltage V corresponding to the term G in equation 1, by a multiplier 214. Sample and hold circuit 206 is controlled by pulse signal E; to accept and store the F signal from sample and hold circuit 205 so that it may be used as the F signal during the next cycle.

A multiplier 217 multiplies the F, signal from sample and hold circuit 205 with direct current voltage V corresponding to the term G in equation 1. Summing means 218 sums the product signals from

multipliers

210, 214 and 217 to provide a sum signal to another multiplier 220.

Multiplier 220 multiplies the sum signal with direct 1 current voltage V corresponding to the term G in equation 1, to provide an output that corresponds to G (G F,,+ G tl G; l F, i-F

Summing means 225 sums the output from multiplier 220 with a signal, corresponding to the term SP in equation 1, from a' sample and hold circuit 226 and a signal from a multiplier 227 which corresponds to the term SP,, U Summing means 225 provides a sum signal, which corresponds to the term SP to sample and hold circuit 230 which is controlled by pulse signal E,- from programmer 27.

The SP,, and SP outputs from sample and hold

circuits

226 and 230, respectively, are compared with each other by a comparator 244 to provide signal E When the output from sample and hold circuit 226 is greater than the output from sample and hold circuit 230, signal E from comparator 244 is of one level to control the direction of change of the set point in flow recorder controller 30 so that the change, if any, will be in the one direction. When the output from sample and hold circuit 226 is equal to or less than the output from sample and hold circuit 230, signal E from comparator 244 is of another level so that'the set point in flow recorder controller 30 may be changed in an opposite direction.

where VOL is the volume of caustic required by the titration of the acid sample, N is the normality of the caustic, MEQ is milliequivalent weight of the acid, and W is the weight of the acid. Equation 2 may he rewritten as: I

3. AS=( C) e) QA)/(D,1)(VOL where AT is the titration time, FR is the flow rate of the caustic, D is the density of the acid and VOL, is the volume of the acid sample. The analog computer for solving equation 3 includes sample and hold circuit 300, multipliers 304 through 304C, a divider 306 and an electronic switch 308. Signal E, from analyzer 22 occurs when signal E corresponds to the density D of the acid sample and controls sample and hold circuit 300 to hold signal E, at that time. The output of sample and hold circuit 300 is applied to multiplier 304 where it is multiplied with a direct current voltage V which corresponds to the volume VOL, of the acid sample. Multiplier 304 provides a signal corresponding to the denominator in equation 3.

Multiplier 304A multiplies the AT signal E from programmer 27 with the caustic flow rate voltage V from source 37 to provide a product signal corresponding to (AT) F R The product signal from multiplier 304A is multiplied with direct current voltage V, corresponding to the normality of the caustic, which by way of example, may be 0.2 meq/ml, to provide a product signal to multiplier 304C where it is multiplied with direct current voltage V Direct current voltage V; corresponds to the milliequivalent weight MEQ, of the acid which for purpose of illustration may be sulphuric acid l-l SO Subtracting means 245 subtracts the SP,, output from sample and hold circuit 226 from the SP, output provided by sample and hold circuit 230 to provide an error signal, corresponding to the change in set point, to an analog-to-digital converter 248, which converts the error signal to a digital signal. Pulse signal E from programmer-27 controls a conventional type countdown counter 250 to enter the digital signal from converter 248. Counting pulses E; from programmer 27 enter countdown counter 250 causing counter 250 to count down. Counting pulses B are also provided as pulse train E Upon reaching a count of zero, a logic decoder 251 provides signal E to programmer 27, which terminates counting pulses E as heretofore explained, so that the number of pulses in signal E corresponds to the proper change in set point value.

Sample and hold circuit'226 is controlled by pulse whose milliequivalent weight is 0.04904 grams/meq.

The product signal from multiplier 304C corresponds to the numerator in equation 3. Divider 306 divides the numerator signal from multiplier 304C with the denominator signal from multiplier 304 to provide a signal, which corresponds to the term AS in equation 3, to electronic switch 308. Electronic switch 308 is controlled by pulse signal B, so as to pass the acid strength signal AS from divider 306 once every cycle.

The second analog computer in acid strength signal means 35 provides a signal in accordance with the following equation 4 and includes subtraction means 315; sample and hold

circuits

318, 318A; summing means 320 and a multiplier 325:

n n-1 i( n ;-i) where AS,, is theaverage acid strength during the current cycle, A S is the acid strength during the next previous cycle, AS, is an instantaneous acid strength obtained during the current cycle and C, is a constant having a value of 0.75. Subtracting means 315 subtracts an output from sample and hold circuit 318, which corresponds to the term 781, in equation 3, from the passed acid strength signal from switch 308 to provide a signal corresponding to the expression (AS, T,, A multiplier 325 multiplies the signal from subtracting means 315 with a direct current voltage V corresponding to the constant C, having a value of 0.75. The output from multiplier 325 is applied to summing means 320 along with the output from sample and hold circuit 318 to provide a signal corresponding to the average acid strength AS, to sample and hold circuit 318A. Pulse signal E controls sample and hold circuit 318A to hold the AS, signal to provide signal E Sample and hold circuit 318 is controlled by pulse signal E from programmer 27 to store the 2?, signal from sample and hold circuit 318A for use as the TS signal during the next subsequent cycle.

Referring to FIGS. 6A and 6B, the peaks of signal E from chromatograph means 38 correspond to the different constituents of the charge olefin in line 6. Sample and hold circuits 400 through 4001 are controlled by pulse signals E, through E; to hold the different peaks of signal E The following table relates a particular sample and hold circuit to a corresponding constituent.

Circuit Constituent Circuit Constituent 400 Ethane 4005 Normal Pentane 400A Propane 400F Propylene 400B iso- Butane 400G Butylene 400C Normal Butane 400H Pentylene 400D lso-Pentane 4001 All compounds with 6 or more carbon atoms The output from sample and hold circuits 400 through 4001 are applied to multipliers 403 through 4031 where they are multiplied with voltages V, through V respectively, corresponding to the various chromatograph means 38 scaling factors pertaining to the particular constituents. By way of example, the voltages V through V may correspond to 0.02, 0.2, L0, 0.2, 0.l5, 0.02, 0.2, 0.10, 0.02 and 0.10 volts, respectively. The product signals from multipliers 403 through 4031 are sampled and held by circuits 405 through 4051 respectively in response to pulse signal E from programmer 27. Sample and hold circuits 405 through 4051 are used so that outputs corresponding to the various constituents of the charge olefin may be presented simultaneously to summing means 406. Summing means 406 provides a normalizing signal.

Not all of the constituents in the charge olefin affect the acid strength. It has been determined that the propylene, butylene, pentylene and some compounds with six or more carbon atoms affect the acid strength. Therefore, the signals from sample and hold circuits 403F through 4031, corresponding to those constituents, are operated upon to provide signal E -Dividers 410 through 410C divide the signals corresponding to those four constituents from sample and hold circuits 405F to 4051 with the normalizing signal from summing means 406. The signals from dividers 410 through 410C are sampled and held each period by sample and hold circuits 412 through 412C in response to pulse signal E; from programmer 27.

Multipliers 415 through 415C, summing means 416 and 425,

multipliers

418, 422 and 423, sample and hold circuits 420 through 4208, anda divider 424 comprise an analog computer for solving the following equations:

A KR(C1Q1 zQz sQa C4Q4) where A is the load value on the acid system, K is a constant having a value of 1.0, R is the flow rate of the charge olefin, C through C are constants corresponding to the ratio of pounds of acid consumed per gallon of olefin for the acid consuming constituents and Q, through Q are percentages of the acid consuming constituents in the charge olefin.

n 8/ o l n 2 n-1) where U, is the anticipated required acid strength during the current cycle, A, is an initial load value on the acid system, while A, and A,, are the load values for the current cycle and the next previous cycle, respectively, and P P and P are dynamic constants of the system having the following values: 1.0, -1.0 and 1.0, respectively.

Multipliers 415 through 415C multiply the outputs from sample and hold circuits 412 through 412C with direct current voltages V through V corresponding to the constants C, through C in equation 5 Summing means 416 sums the outputs from multipliers 415 through 415C to provide a sum signal which is multiplied by the charge olefin flow rate signal E from sensor 40, by multiplier 418 to provide a signal to a multiplier 419. Multiplier 419 multiplies the signal from multiplier 418 with voltage V which corresponds to the term K in equation 5, to provide a signal corresponding to the term A in equation 5.

Sample and hold circuit 420 is controlled by pulse signal E from programmer 27 which occurs early in the processing to hold the signal from multiplier 418 as the A, signal. Divider 424 divides direct current voltage V which corresponds to the term P in equation 6, with the output from sample and hold circuit 420 to provide a signal corresponding to P /A Sample and hold circuit 420A is controlled each cycle by pulse signal E,, from programmer 27 to sample and hold the output from multiplier 419 thereby providing a signal to a multiplier 421 corresponding to the term A, in equation 6. Multiplier 421 multiplies the signal from circuit 420 with direct current voltage V which corresponds to the constant P, in equation 6, to provide an output to summing means 425. Sample and hold circuit 420B is controlled by pulse signal E from programmer 27 at the end of each cycle,'to sample and hold the output from multiplier 419 to provide a signal for the next subsequent cycle corresponding to A,, The signal from sample and hold circuit 42013 is multiplied with direct current voltage V corresponding to the term P Summing means 425 sums the output from multiplier 421 with the product signal from multiplier 422 to provide a signal, corresponding to the bracketed portion of equation 6, which is multiplied with the signal from divider 424 by multiplier 423 to provide signal E Although the system of the present invention has been shown as using analog computers, it may be used with a digital computer. Set point signal means 28, acid strength signal means 35 and olefin signal means 36 are replaced by the digital computer. Conventional analogto-digital converting techniques are used along with sample and hold circuits. Some of the sample and hold circuits are controlled by programmer 27 to hold the peaks of signal E as is done with the analog computers. Another sample and hold circuit is controlled by signal 13, from acid strength analyzer 22 to hold the density signal E The output of the sample and hold circuits and signals E and E are converted to digital signals.

The system of the present invention as heretofore described provides a rapid response control system for controlling the acid strength in an alkylation unit. Measurements of the acid strength are combined with the measurements of the olefin composition and flow rate to provide a control signal for controlling the strength of the acid in the alkylation unit. The system of the present invention, as heretofore described, provides a signal corresponding to the strength of the acid in the alkylation unit and another signal corresponding to an anticipated required acid strength as determined in accordance with a quantity of acid consuming constituents of olefin entering the alkylation unit.

What is claimed is:

1. A system for controlling the strength of an acid in an alkylation unit wherein an isoparaffin is reacted with olefin in the presence of the acid to provide an acidhydrocarbon mixture to a settler where the hydrocarbon is separated from the acid to provide a hydrocarbon product and a portion of the acid is recycled while the remaining acid is discharged from the alkylation unit and wherein fresh acid is added to the recycled acid to replace the discharged acid to affect the strength of the acid, comprising means for controlling the strength of the acid in the alkylation unit in accordance with a control signal, means for determining the strength of the acid and providing a signal corresponding thereto, means for determining the composition of the olefin and providing corresponding signals, means for sensing a condition of the olefin and providing a signal corresponding thereto, means for providing a reference signal corresponding to a desired acid strength, a circuit connected to the acid strength determining means and to the reference signal means for providing an error signal in accordance with the difference between the acid strength signal and the reference signal, and means for providing the control signal to the control means in accordance with the error signal and the signals corresponding to the composition and condition of the olefin.

2. A method for controlling the strength of an acid in an alkylation unit wherein an isoparaffin is reacted with olefin in the presence of the acid to provide an acid hydrocarbon mixture to a settler where the hydrocarbon is separated from the acid to provide a hydrocarbon product and a portion of the acid is recycled while the remaining acid is discharged from the alkylation unit, and wherein fresh acid is added to the recycled acid to' replace the discharged acid to affect the strength of the acid, which comprises controlling the strength of the acid in the alkylation unit in accordance with a control signal, determining the strength of the acid in the processing unit and providing a signal corresponding thereto, sensing the quantity and composition of the olefin entering the alkylation unit, providing signals corresponding to sensed quantity and composition of the olefin, providing a reference signal corresponding to a desired acid strength providing an-error signal in accordance with the difference between the acid strength signal and the reference signal, and providing a control signal in accordance with the error signal and the signals corresponding to the composition and quantity of the olefin.

3. A control system as described in claim 1 further comprising programming means connected to the determining means and to the control signal means for starting and stopping the operation of the control system and for periodically recycling the operation of the control system.

4. A control system as described in claim 3 in which the acid strength determining means includes acid strength analyzing means controlled by the programming means for periodically sampling the acid and titrating the sample acid with a caustic entering the analyzing means and providing signals corresponding to the elapsed time AT required for titration, and to the density 0,, of the acid; and means for providing a signal corresponding to the flow rate F of the caustic enter ing the analyzing means and providing a signal corresponding thereto.

5. A control system as described in claim 4 in which the acid strength determining means also provides a signal corresponding to the average acid strength AS, for the current cycle of operation as the acid strength signal in accordance with the following equations:

AIS AS,, C,,(AS,,AS, where AS is the instantaneous acid strength, N is the normality of the caustic, MEQ is the milliequivalent weight'of the acid, and VOL is the volume of the acid sample, TSL is the acid strength for the previous cycle, AS is the instantaneous acid strength during the current cycle, and C is a constant having the following value: 0.75.

6. A control system as described in claim 5 in which the composition signal means includes chromatograph means sampling the olefin and providing a signal corresponding to constituents of the charge olefin, as the composition signal, and the sensed condition of the olefin is the sensed flow rate of the olefin.

7. A control system as described in claim 6 in which the control signal means includes means for providing a signal corresponding to the instantaneous acid consumption A due to the acid consuming constituents of the olefin, and means connectedto the constituents signal means for providing a signal corresponding to an anticipated required acid strength U for the current cycle of operation in accordance with the following equations:

n 3/ o 1 n 2 nd) where K is a constant corresponding to volume of hydrocarbon product obtained from a unit volume of olefin, R is the flow rate of the olefin, C C C and C are pounds of acid per gallon of product produced for corresponding constituents of the olefin, Q Q Q and Q, are ratios of the acid consuming constituents to the whole olefin, A is the acid consumption a short time after the start of operation of the control method, A, is the acid consumption during the current cycle, A,, is the acid consumption during the previous cycle, and P P and P are constants pertaining to the dynamic characteristics of the control system.

8. A control system as described in claim 7 in which the control means includes a valve through which the discharge acid flows and the acid strength is controlled by controlling the flow rate of the discharge acid, a sensor sensing the flow rate of the discharge acid and providing a signal corresponding thereto, and the control signal means includes a flow controller, having a set point whose position corresponds to a flow rate,

connected to the discharge acid flow rate sensor and providing the control signal to the valve in accordance with a difierence between the flow rate determined by the position of the flow controllers set point and the flow rate signal from the discharge acid flow rate sensor, and means connected to the flow controller, to the anticipated required acid strength signal means, to the reference signal means and to the average acid strength signal means for providing a direction signal and a pulse train, each pulse in the pulse train changing the position of the set point in the flow controller by a predetermined amount in a direction determined by the direction signal, in accordance with the following equation:

where SP, is the number of pulses in the pulse train during the current cycle, SP is the number of pulses in the pulse train for the previous cycle, F is a difference between the desired acid strength, and the average acid strength for the current cycle, F is the absolute value of F,,, F is a difference between the desired acid strength and the average acid strength for the previous cycle, and G G and G are dynamic constants of the control system.

9. A method as described in claim 2 in which the steps are repeated periodically.

10. A method as described in claim 9 in which the acid strength determining step includes sampling the acid in the processing unit, titrating the sample acid with a steady flow of a caustic, and the acid strength signal step includes providing a signal corresponding to the titration time AT of the sample acid, a signal corresponding to the flow rate F of the caustic, and a signal corresponding to the density D,, of the acid sample, and providing the acid strength signal, which corresponds to an average acid strength TS for the current cycle, in accordance with the AT, F and D signals and following equations:

AS:(AT)(FC)(NC)(MEQA)/(DA)(VOLS) and E= H+CM H- MQ where AS is the instantaneous acid strength, N is the normality of the caustic, MEQ is the milliequivalent weight of the acid, VOL is the volume of the acid sample, ITS is the average acid strength for the next previous cycle, AS is an instantaneous acid strength occurring during the current cycle, and C is a constant having the following value: 0.75.

11. A method as described in claim 10 in which the olefin quantity and composition step includes sensing the quantities of acid consuming constituents of the olefin entering the processing unit during each cycle, and providing signals corresponding to the quantities of acid consuming constituents of the olefin.

12. A method as described in claim 11 in the control signal step includes providing a signal corresponding to an anticipated required acid strength U,, for the current cycle in accordance with the acid consuming constituents signal and the following equation:

n 3/ o 1 n 2 n-l) where A is the instantaneous acid consumption due to the acid consuming constituents of the olefin, K is a constant corresponding to volume of bycarbon product obtained from a unit volume of olefin, R is the flow rate of the olefin, C C C and C are pounds of acid per gallon of product produced for corresponding constituents of the olefin, Q Q Q and Q, are ratios of the acid consuming constituents to the whole olefin, A

is the acid consumption a short time after the start of operation of the control method, A, IS the acid consumption during the current cycle, A is the acid consumption during the previous cycle, and P P and P are relating to the dynamic constants of the control method.

13. A method as described in claim 12 in which the acid strength is controlled by controlling the flow rate of the discharge acid with a value responsive to the control signal, and the control signal is provided by a flow controller having set points in accordance with a difference between a signal corresponding to the flow rate of the discharge acid and the position of set points corresponding to a flow rate of the discharge acid, and the control signal step includes providing a reference signal corresponding to a desired acid strength, providing a direction signal and a pulse train to the flow controller, each pulse in the pulse train changes the position of the set points a predetermined amount in a direction controlled by the direction signal, in accordance with the average acid strength signal, the reference signal and the anticipated required acid strength signal and the following equation:

where SP corresponds to the number of pulses in the pulse train for the current cycle, SP,, corresponds to the number of pulses in the pulse train for the next previous cycle, F corresponds to the difference between the reference signal and the average acid strength signal for the current cycle, F is the absolute value of F F,, is the difference between the reference signal and the average acid strength signal for the next previous cycle.

Claims

2. A method for controlling the strength of an acid in an alkylation unit wherein an isoparaffin is reacted with olefin in the presence of the acid to provide an acid hydrocarbon mixture to a settler where the hydrocarbon is separated from the acid to provide a hydrocarbon product and a portion of the acid is recycled while the remaining acid is discharged from the alkylation unit, and wherein fresh acid is added to the recycled acid to replace the discharged acid to affect the strength of the acid, which comprises controlling the strength of the acid in the alkylation unit in accordance with a control signal, determining the strength of the acid in the processing unit and providing a signal corresponding thereto, sensing the quantity and composition of the olefin entering the alkylation unit, providing signals corresponding to sensed quantity and composition of the olefin, providing a reference signal corresponding to a desired acid strength providing an error signal in accordance with the difference between the acid strength signal and the reference signal, and providing a control signal in accordance with the error signal and the signals corresponding to the composition and quantity of the olefin.
3. A control system as described in claim 1 further comprising programming means connected to the determining means and to the control signal means for starting and stopping the operation of the control system and for periodically recycling the operation of the control system.
4. A control system as described in claim 3 in which the acid strength determining means includes acid strength analyzing means controlled by the programming means for periodically sampling the acid and titrating the sample acid with a caustic entering the analyzing means and providing signals corresponding to the elapsed time Delta T required for titration, and to the density DA of the acid; and means for providing a signal corresponding to the flow rate FC of the caustic entering the analyzing means and providing a signal corresponding thereto.
5. A control system as described in claim 4 in which the acid strength determining means also provides a signal corresponding to the average acid strength ASn for the current cycle of operation as the acid strength signal in accordance with the following equations: AS ( Delta T)(Fc(Nc)(MEQA)/(VOLS)(DA) ASn ASn 1 + C5(ASn-ASn 1) where AS is the instantaneous acid strength, Nc is the normality of the caustic, MEQA is the milliequivalent weight of the acid, and VOLS is the volume of the acid sample, ASn 1 is the acid strength for the previous cycle, ASn is the instantaneous acid strength during the current cycle, and C5 is a constant having the following value: 0.75.
6. A control system as described in claim 5 in which the composition signal means includes chromatograph means sampling the olefin and providing a signal corresponding to constituents of the charge olefin, as the composition signal, and the sensed condition of the olefin is the sensed flow rate of the olefin.
7. A control system as described in claim 6 in which the control signal means includes means for providing a signal corresponding to the instantaneous acid consumption A due to the acid consuming constituents of the olefin, and meanS connected to the constituents signal means for providing a signal corresponding to an anticipated required acid strength Un for the current cycle of operation in accordance with the following equations: A K R(C1 Q1 + C2 Q2 + C3 Q3 + C4 Q4), and Un P3/Ao (P1 An + P2 An 1) where K is a constant corresponding to volume of hydrocarbon product obtained from a unit volume of olefin, R is the flow rate of the olefin, C1, C2, C3 and C4 are pounds of acid per gallon of product produced for corresponding constituents of the olefin, Q1, Q2, Q3 and Q4 are ratios of the acid consuming constituents to the whole olefin, Ao is the acid consumption a short time after the start of operation of the control method, An is the acid consumption during the current cycle, An 1 is the acid consumption during the previous cycle, and P1, P2 and P3 are constants pertaining to the dynamic characteristics of the control system.
8. A control system as described in claim 7 in which the control means includes a valve through which the discharge acid flows and the acid strength is controlled by controlling the flow rate of the discharge acid, a sensor sensing the flow rate of the discharge acid and providing a signal corresponding thereto, and the control signal means includes a flow controller, having a set point whose position corresponds to a flow rate, connected to the discharge acid flow rate sensor and providing the control signal to the valve in accordance with a difference between the flow rate determined by the position of the flow controller''s set point and the flow rate signal from the discharge acid flow rate sensor, and means connected to the flow controller, to the anticipated required acid strength signal means, to the reference signal means and to the average acid strength signal means for providing a direction signal and a pulse train, each pulse in the pulse train changing the position of the set point in the flow controller by a predetermined amount in a direction determined by the direction signal, in accordance with the following equation: SPn SPn 1 +G4(G1Fn 1 + GFn 2 + G3 Fn Fn) + SPn 1(Un), where SPn is the number of pulses in the pulse train during the current cycle, SPn 1 is the number of pulses in the pulse train for the previous cycle, Fn is a difference between the desired acid strength, and the average acid strength for the current cycle, Fn is the absolute value of Fn, Fn 1 is a difference between the desired acid strength and the average acid strength for the previous cycle, and G1, G2 and G3 are dynamic constants of the control system.
9. A method as described in claim 2 in which the steps are repeated periodically.
10. A method as described in claim 9 in which the acid strength determining step includes sampling the acid in the processing unit, titrating the sample acid with a steady flow of a caustic, and the acid strength signal step includes providing a signal corresponding to the titration time Delta T of the sample acid, a signal corresponding to the flow rate Fc of the caustic, and a signal corresponding to the density DA of the acid sample, and providing the acid strength signal, which corresponds to an average acid strength AS for the current cycle, in accordance with the Delta T, FC and DA signals and following equations: AS ( Delta T)(FC)(NC)(MEQA)/(DA)(VOLS) and ASn ASn 1 + C5(ASn - ASn 1) where AS is the instantaneous acid strength, Nc is the normality of the caustic, MEQA is the milliequivalent weight of the acid, VOLS is the volume of the acid sample, ASn 1 is the average acid strength for the next previous cycle, ASn is an instantaneous acid strength occurring during the current cycle, and C5 is a constant having the following value: 0.75.
11. A method as described in claim 10 in which the olefin quantity and composition step includes sensing the quantities of acid consuming constituents of the olefin entering the processing unit during each cycle, and providing signals corresponding to the quantities of acid consuming constituents of the olefin.
12. A method as described in claim 11 in the control signal step includes providing a signal corresponding to an anticipated required acid strength Un for the current cycle in accordance with the acid consuming constituents signal and the following equation: A KR(C1Q1 + C2Q2 + C3Q3 + C4Q4) and Un P3/Ao (P1An + P2An 1) where A is the instantaneous acid consumption due to the acid consuming constituents of the olefin, K is a constant corresponding to volume of bycarbon product obtained from a unit volume of olefin, R is the flow rate of the olefin, C1, C2, C3 and C4 are pounds of acid per gallon of product produced for corresponding constituents of the olefin, Q1, Q2, Q3 and Q4 are ratios of the acid consuming constituents to the whole olefin, Ao is the acid consumption a short time after the start of operation of the control method, An is the acid consumption during the current cycle, An 1 is the acid consumption during the previous cycle, and P1, P2 and P3 are relating to the dynamic constants of the control method.
13. A method as described in claim 12 in which the acid strength is controlled by controlling the flow rate of the discharge acid with a value responsive to the control signal, and the control signal is provided by a flow controller having set points in accordance with a difference between a signal corresponding to the flow rate of the discharge acid and the position of set points corresponding to a flow rate of the discharge acid, and the control signal step includes providing a reference signal corresponding to a desired acid strength, providing a direction signal and a pulse train to the flow controller, each pulse in the pulse train changes the position of the set points a predetermined amount in a direction controlled by the direction signal, in accordance with the average acid strength signal, the reference signal and the anticipated required acid strength signal and the following equation: SPn SPn 1 + G4(G1Fn + G2Fn 1 + G3 Fn Fn) + SPn 1 (Un) where SPn corresponds to the number of pulses in the pulse train for the current cycle, SPn 1 corresponds to the number of pulses in the pulse train for the next previous cycle, Fn corresponds to the difference between the reference signal and the average acid strength signal for the current cycle, Fn is the absolute value of Fn, Fn 1 is the difference between the reference signal and the average acid strength signal for the next previous cycle.