JPS6190701A - Control device for multiple-effect evaporator - Google Patents

Abstract

PURPOSE:To improve the controllability in the stage of changing the flow rate of heating steam by using a feed-forward control loop which adds the manipulated variables associated to the flow rate of the heating vapor to the operation outputs of a concn. controller and level controller. CONSTITUTION:Control valves 21, 16, 13, 26 are subjected to feed-forward operations in proportion to the increment of the flow rate of the heating steam in feed-forward arithmetic circuits 31-34 to which the measuring signal eS of the flow rate FS of the heating steam S is inputted. The discharge flow rate F0 of liquid and the supply flow rate F1 of the liquid to an evaporator 1 are preliminarily so operated in the evaporator 1 that F0.D1=F1.D2, F1-F0-V1=0 holds where D1, D2 are respectively designated as the supply liquid concn. of the evaporators 1, 2 and the evaporated vapor quantity of the evaporator 1. The same holds true of the evaporators 2, 3. As a result the concn. fluctuation and level fluctuation of the evaporators 1-3 are minimized.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to improvement of controllability when changing the flow rate of heated steam in a concentration control device using a multi-effect can.

<Prior Art> The configuration and problems of a conventional device will be explained using an example of instrumentation of a multi-effect can shown in FIG. 1.2.3 are first to third evaporators connected in cascade, 101, 201
．． 301 is the first. Second. heat exchanger of the third evaporator,
4 is a pipe line that supplies heated steam S to the heat exchanger 101 of the first evaporator 1; 102 is a pipe line that supplies evaporated steam V□ of the first evaporator to the heat exchanger 201 of the second evaporator 2; 202 is the second
The evaporated steam v2 of the steam can 2 is transferred to the heat exchanger 30 of the third evaporator 3.
1, 302 is the evaporated steam v3 of the third evaporator
This is a pipe line that leads the water to the condenser 5. In the condenser 5, 501 is a connection pipe to a vacuum pump, and 502 is a bleed pipe to the atmosphere. 103.203. 303 is the drain of the heat exchanger 101, 201, 301.

6 is a flow rate sensor that measures the flow rate F of the heated steam S, 7 is a control valve that controls the flow rate, and 8 is an output e of the flow rate sensor 6.
This is a controller that operates the control valve 7 in response to ll and a set value Y of the heating steam flow rate.

9 is a pipe line for supplying the feed liquid B to be concentrated into the third evaporator 3, 10 is a pump inserted into this pipe line, 11 is a control valve that controls the feed liquid flow rate F3, and 12 is an evaporator. 3 is a level sensor that measures the level L3 of the supply liquid B, and 13 is a level controller that receives the output et3 of the level sensor and the set value Yt3 and operates the control valve 11 to control the flow rate F3. 14 is a pipe line for transferring the feed liquid in the evaporator 3 to the evaporator 2; 15 is a pump inserted into this pipe; 16 is a control valve that controls the flow rate F2 of the feed liquid to the evaporator 2; 17 is a level sensor that measures the level L2 of the supply liquid B in the evaporator 2; 18 is the output 612 of the Hallepel sensor and the set value Yt2
This is a level controller that controls the flow rate F2 by operating the control valve 16 in response to the flow rate F2. 19 transfers the supply liquid in the evaporator 2 to the evaporator 1.
20 is a pump inserted into this pipe, 21 is a control valve that controls the flow rate of fluid to the evaporator 1, and 22 is the level L1 of the feed liquid B in the evaporator 1. 23 is a level controller that receives the output et□ of the level sensor and the set value Yt and operates the control valve 21 to control the flow rate F0.

24 is a discharge pipe for discharging the concentrated feed liquid B, 25 is a pump inserted into this pipe, and 26 is a discharge pipe for discharging the concentrated feed liquid B at a flow rate F. 27 is a concentration sensor that measures the concentration D1 of the feed liquid in the evaporator 1, 28 is the output e of the concentration sensor, and the set value Y, and the control valve 26 is operated to control the discharge flow rate F. . This is a concentration controller that controls the

29 is a pipe for bleeding air A into the pipe 302 to the condenser 5; 30 is a control valve inserted into this pipe; 31
3 is a pressure sensor that measures the pressure P3 inside the third evaporator 3;
2 receives the output e and set value Y of the pressure sensor p
This is a pressure regulator that controls the amount of bleed air by operating the control valve 30.

Tl, T2. T3 is the temperature inside the evaporator 1, 2.3, P
□.

F2. F3 is the pressure inside each evaporator, Ll, L2. L3 is the supply liquid level in each evaporator, D, F2. F3 is the feed liquid concentration for each evaporator.

In such a configuration, the pressure of each evaporator is Po〉1p
) The pressure gradient of p, the temperature is T 1 > T 2 > T 3
The temperature gradient and the concentration have a concentration gradient of D□>F2>F3. Therefore, when the temperature of the feed liquid B to be concentrated is low, a counterflow type configuration as in the embodiment is used, in which the heated steam S is supplied from the third evaporator 3 side in the opposite direction to the flow.

The concentration D of the discharged supply liquid B' is maintained at a set value Y by controlling the discharge flow rate F from the discharge evaporator 1.

And the level Ll, L2 of each evaporator. The Pel YY supply liquid flow rate F,, F2. F3 is controlled.

In the operation of such multiple effect cans, it may be important to maintain a constant concentration of concentrate within each evaporator without significantly varying the level of liquid within each evaporator. The control loop configuration shown in FIG.
It has become possible to effectively maintain concentration control and level control in a steady state where the set value Y of the flow rate F is constant and there are relatively small disturbances (supply liquid concentration, heating steam pressure, heating steam flow rate). There is.

However, when the flow rate of the discharged supply liquid is changed significantly, there is a drawback that good concentration control and level control are difficult. That is, if F is significantly increased in a short period of time (changing the discharged feed liquid is done by changing the vapor flow rate F), the calorific value of the first stage evaporator begins to increase. In order to maintain a constant level in the can, the level controller 23 operates to increase the feed flow rate F1 by an amount commensurate with the increase in evaporation. This keeps the level constant, but increases the concentration D1 in the can. This is the balance F for the can. D1=FID2 to D1=F0
・Do/F'.

holds true, and it is clear that as F increases, Dl also increases.

Therefore, in order to lower the concentration D1, the concentration controller 28 is increased. If Fo is increased, the level inside the can will drop, so Fo is increased again to keep the level constant. Repeating this process, D, Fo,
Fl fluctuates until it settles down to a steady value, and this fluctuation is the second.
This spreads to the level fluctuation in the third evaporator, and at the same time, the concentration D2 in each evaporator. F3 also fluctuates significantly, and it takes a long time for these to settle to new steady values.

<Problems to be Solved by the Invention> In order to solve the above-mentioned drawbacks, the present invention solves the above-mentioned drawbacks by providing a method in which the level and concentration of the liquid in each evaporator do not change significantly even if the flow rate of the discharged supply liquid is changed in a short period of time. The purpose of this invention is to realize a control device that can be stabilized in a short time.

<Means for Solving the Problems> The structural feature of the present invention is that a plurality of evaporators are connected in sequence and the steam generated in one evaporator is used to heat the next evaporator to heat the supply liquid. A control device for a multi-effect canister that sequentially condenses includes a steam flow rate controller that adjusts the heated steam flow rate based on a measured value and a set value of the heated steam flow rate supplied to the first evaporator, and a steam flow rate controller that controls the concentrated supply liquid. a concentration controller that adjusts the discharge flow rate of the exhaust vapor can based on the measured concentration value and setting value of the feed liquid in the evaporator; and a feedforward means for adding a manipulated variable related to the heated steam flow rate to the operational outputs of the concentration controller and the level controller. At the point.

Effect> A feedforward control loop that adds the manipulated variable related to the heating steam flow rate to the manipulated output of the concentration controller and level controller allows the discharge flow rate F to be adjusted when the heating steam flow rate setting is significantly changed. The supply flow rate F1 can be increased before the concentration controller output and the level controller output increase, and level fluctuations and concentration fluctuations in each evaporator can be suppressed to a small level.

<Example> An example of the apparatus of the present invention will be described with reference to FIG. Elements that are the same as those in FIG. 2 are given the same reference numerals and their explanations will be omitted.

Block C indicated by a dotted line is a feedforward control loop which is a characteristic part of the device of the present invention. Numerals 31 to 34 are feedforward calculation circuits which input the measurement signal e8 of the flow rate F8 of the heated steam S, and feedforwardly operate the control valves 21.16° 13 and 26 in proportion to the increment of the heated steam flow rate. The outputs efil 0f21 ef3T ef4 of the arithmetic circuits 31, 32, 33, and 34 are generally expressed by the following functions.

efi=fi(F8)=Ki(FB-,Fm'In)(
1) Here, Fm1n is the minimum flow rate of heating steam, i = 1 to 4
It is.

The town setting method is as follows. First, the flow rate of the supply liquid to each evaporator is Fl, F2. F3 and discharge flow rate F. is the opening degree θ1. of the control valves 21, 16, 11 and 26. θ2. θ3 and θ. Assume that it is proportional to . Only the method of setting 1 will be described below, but K2 to 4 can also be set using a similar method.

The opening degree of θ when the heating steam flow rate F3 is minimum and maximum is θ
θ 1m1n, 1max, maximum value of heating steam flow rate F, rr
When lax, K1-(θ1max-θ1.n, n)/(FIIn1a
x″″F, −, n) (2) Totoru.

Next, the operation by the feedforward control loop in the present invention will be explained.

First, if there is no feedforward control loop, the control valve 21°16 after changing Fll by changing the set value Y8.
, 11.26 settle down to a steady value, the fluctuations in concentration and level within each evaporator are large as described above.

In order to suppress fluctuations in the concentration D0 and level L1 in such a case, in the evaporator 1, where the amount of evaporated vapor is vl, Fo-D1= Fl・D2(3) Fl-Fo-V, = O(4+ It is sufficient to manipulate Fo and Ft in advance so that the following holds true.

The same applies to the evaporator 2.3. If the manipulated variables due to feedforward of Fo and F are set as (double equation), equation (31, (4)) will approximately hold true, making it possible to suppress concentration fluctuations and level fluctuations to the minimum limit. .

Although the above-described embodiment is an example in which the present invention is applied to a triple-effect can, the present invention is not limited to a triple-effect can, and can be applied to any multiplex configuration. Further, when the temperature of the feed liquid is high, similar feedforward control can be applied to a forward flow type where the feed liquid is fed from the first stage evaporator side.

The calculation of feedforward control has shown an example of proportional calculation, but considering the compensation of dynamic characteristics, the primary lead/lag element (TDg
+1/T8+1) may be added.

Here, the optimum value of the coefficient of the primary lead/lag element may be determined by a trial run of the control system.

In the above embodiment, the object of feedforward control was the operation amount to the control valves 21, 16, 11, 26, but it is limited to the control valves 21 and 26 that perform level control and concentration control of the evaporator 1 that discharges the supplied liquid. However, it is effective.

<Effects> As explained above, according to the present invention, the following effects can be expected.

(1) When the discharge flow rate of the feed liquid to be concentrated is significantly changed in a short period of time, the level and concentration of the feed liquid in each evaporator can be stabilized in a short time without greatly changing the level and concentration of the feed liquid.

(2) In steady state operation where the set value of the heating steam flow rate is not changed, the configuration is the same as the conventional control system, and the controllability of the conventional device is not lost.

(3) In conventional control systems, concentration control and level control interfere greatly when changing the amount of heating steam, and it is difficult to control both well. However, by adding feedforward control as in the present invention, Interference between the two is reduced, and good controllability can be expected for both.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of the present invention, and FIG. 2 is a block diagram showing an example of a conventional device. 1.2.3... Evaporator, 4... Heated steam supply pipe,
6...Heating steam flow rate sensor, 7.11.16.21.
26゜30...Control valve, 8, 13.18.23.2
8...Controller, 12°17, 22...Level sensor, 21...Concentration sensor? , 32...pressure sensor, S.
... Heating steam, B... Supply liquid, B... Discharge liquid.

Claims (1)

[Claims] In a control device for a multi-effect can which sequentially connects a plurality of evaporators and uses the steam generated in one evaporator to heat the next evaporator to sequentially concentrate the feed liquid, the liquid is supplied to the first evaporator. A steam flow rate controller that adjusts the heated steam flow rate based on the measured value and set value of the heated steam flow rate, and a discharge flow rate of the discharge evaporator that discharges the concentrated feed liquid to measure the concentration of the feed liquid in the evaporator. a concentration controller that adjusts the level of the feed liquid in the discharge evaporator based on the measured value and the set value; A multi-effect tank control device comprising feedforward means for adding a manipulated variable related to the steam flow rate to the manipulated outputs of the concentration controller and the level controller.