JPH0326301A - Method for controlling composition of distillate - Google Patents

Abstract

PURPOSE:To obtain a high-purity material having a small b.p. difference by selectively controlling the heating of a liq. feed by a heater, the outflow of the material discharged from a bottoms line and the outflow of the material from a top vapor line. CONSTITUTION:A liq. feed is supplied to a distillation tower 1 and heated, a material having a high b.p. is drawn off from the bottom-out line 4 connected to the base of the tower 1, and a vaporized material having a low b.p. is discharged from the top vapor line 5 connected to the top of the tower 1. The material discharged from the line 4 or 5 is analyzed by an analyzer. When the composition does not reach the desired value, an element A for determining the temp. in the tower 1 is controlled by a means 34 until the desired composition is attained. The element A is at least the one selected from among the heating amount of the liq. feed by the heater 6, the outflow of the material discharged from the line 4 and the outflow of the material discharged from the line 5.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for controlling the composition of a distilled substance when the substance is distilled and fractionated in a distillation column.

[Conventional technology]

A conventional composition control method will be explained with reference to the system diagram shown in FIG.

A feed line 2 for supplying the feed liquid to be distilled is connected to the center of the vertically elongated distillation column 1, and a reboiler circulation path 3, a bottom outline 4 (waist), and a distillation column 1 are connected at the bottom of the distillation column 1. A top vapor line 5 is connected to the top.

A heat exchanger 6 (reboiler) that heats the feed liquid circulating in the circulation path is connected to the reboiler circulation path 3, and a control valve 7 controls the amount of heat medium (steam or hot water) supplied to the heat exchanger 6. to control the temperature of the feed liquid in the distillation column 1. This heating causes the liquid in the column to vaporize and rise upward within the distillation column 1.

Inside the distillation column, trays 8 with small holes are provided in several stages, and steam passes through the small holes. At that time, tray 8
The upper liquid and vapor come into contact, exchange heat, and undergo condensation, etc. Here, the higher the boiling point of the tray 8 is, the higher the boiling point is collected, and the lower the tray 8 is, the lower the boiling point is collected. As a result, within the distillation column 1, a composition gradient is formed in which the -5-16- ratio of substances with low boiling points increases from the bottom to the top. This cultivation ratio gradient appears as a temperature gradient corresponding to the condensation temperature.

The liquid accumulated in each tray 8 passes through the downcomer of each stage of the distillation column 1 and flows down to the lowest tray 8. The liquid on the lowermost tray 8 is taken into the reboiler, reheated, and distilled.

At the inner bottom of the distillation column 1, a liquid with a large proportion of high-boiling substances that has fallen from the lowest stage of the tray 8 (hereinafter referred to as the bottom liquid) is stored. Take it out. This bottom outline 4 is provided with a flow rate control valve 9 for controlling the outflow amount of the bottom liquid.

Additionally, the top vapor line 5 is connected to a condenser 1o that condenses and liquefies the vapor containing a large proportion of low-boiling substances taken out from the distillation column 1, and the resulting liquid (hereinafter referred to as the top liquid).
A receiver 11 is provided to store the
A reflux line 12 is connected to return the liquid in the distillation column 1 to the distillation column 1. A flow control valve 14 is provided at the top outline 13 through which night flows out. Reflux line 1
2, the top liquid is returned to the distillation column 1 from the receiver 11 and vaporized again, and the vapor is condensed again, so that high purity distillation can be performed.

As described above, the cultivation ratio gradient changes according to the temperature distribution within the distillation column 1, so in order to control the composition of the substance to be obtained, the temperature within the distillation column 1 may be controlled.

Conventionally, the temperature at one point in the distillation column 1 was detected, and if that temperature changed, it was assumed that the overall temperature gradient and, by extension, the cultivation ratio gradient had also changed, and the amount of steam used for heating in the reboiler was controlled by a temperature controller. 15 to control the control valve 7 to change the temperature of the supply liquid;
The formation of the material to be obtained was controlled.

For example, if the target temperature in the temperature controller 15 is set to 85°C and it reaches 80°C, this means that the composition ratio gradient has decreased, that is, the low boiling point -81 has become more dominant. means.

Therefore, in order to raise the temperature of the reboiler, the amount of steam is increased in the control well 7.

[Problem to be solved by the invention]

By the way, if the temperature difference between the upper and lower parts of the distillation column 1 is large, the above control is easy, but if the temperature difference is small, for example, 70°C at the upper part and 75°C at the lower part, the temperature controller 15 The temperature must be controlled by fluctuations below the decimal point, making control impossible.

Therefore, as shown in FIG. 6, there is an example in which an analyzer 16 (for example, gas chromatography) is provided on the bottom outline 4 or the top outline 13, and the composition is controlled based on the analysis results.

However, analysis using an analyzer 16 such as gas chromatography takes a long time, and the delay is too long to feed back the results.

In any case, in each tray 8 of the distillation column 1, the amount of liquid accumulated {referred to as hold up}
is determined by itself, and if something with a different composition is fed, it will not come out from the bottom outline 4 until after the stagnant amount has been replaced in each tray 8, so this delay will also affect the control.

By the way, in the distillation column 1, when the internal pressure is constant, the temperature rises as the high-boiling point components increase, and the temperature decreases as the low-boiling point components increase, but there are unexpected factors inside the distillation column 1 that cannot be read from the temperature. There are many cases where pressure changes occur.

Therefore, there is a problem in that a method in which the change in the temperature inside the column is linked to the amount of heating alone cannot efficiently separate a mixed substance with a particularly small boiling point difference to a high degree of purity.

The present invention takes into consideration the conventional problems as described above, and particularly,
It is an object of the present invention to provide a distillation chain control method that can obtain highly pure distilled substances even when distilling substances with small boiling point differences.

[Means to solve the problem]

The present invention heats the feed liquid supplied into the distillation column 1 to a bottom outline 4 connected to the lower part of the distillation column 1.
In the method of separating substances using the distillation column 1, in which substances with high boiling points are extracted from the top vapor line 5 connected to the top of the distillation column 1, and substances with low boiling points are extracted from the top vapor line 5 connected to the top of the distillation column 1, the following control methods are selected and determined. This is a combined method.

(2) A control method that monitors the temperature inside the distillation column 1 and controls the elements that determine the temperature inside the distillation column 1 according to the temperature change to cancel out the temperature change.

■The composition of the substance taken out from the bottom outline 4 or the top vapor line 5 (and by extension, the top outline 13) is analyzed with an analyzer (31, 54.61) to reach the set target composition ratio. If there is no
A control method that controls the elements that determine the temperature inside the distillation column 1 until this target introduction ratio is achieved.

■Monitor the pressure inside the distillation column 1 and respond to the pressure change.
A method for correcting the temperature measurement inside the distillation column 1.

■When supplying the feed liquid to the distillation column 1, measure the amount of the substance to be removed from the bottom outline line 4 or the top vapor line 5 in the feed liquid by distillation, and adjust the amount to the distillation column 1 according to the increase or decrease in the amount. A control method for controlling factors that determine the temperature in the distillation column
(It can be calculated from the ratio of the composition in the feed liquid and the flow rate of the entire feed liquid, which are measured by the analyzer (31, 54, 61) and the flow meter, respectively).

From the above, ■ Single, ■■, ■■, ■■, ■■■, ■
It is a combination of ■■, ■■■, ■■■■, and ■■.

In these combinations, the elements that should determine the temperature inside the distillation column 1, which is the object to be controlled, are (■) the amount of heating of the feed liquid by the heating device, and (2) the bottom outline 4.
and (3) the outflow amount of a substance taken out from the Tobu Baber line 5.

-11 12 - Gas chromatography is preferred as the analyzer (31, 54.61).

[Effect]

An example of control according to the present invention will be explained below.

(a) Control method based on the detection result of the tower internal temperature.

(a-1) When the temperature inside the column increases by Δt In this case, it means that the composition ratio gradient in the distillation column 1 has moved to the high boiling point side, and the bottom liquid and the top liquid have a boiling point higher than the target. It is a composition. Therefore, (1) lower the heating temperature by the reboiler.

(2) Decrease the amount of D in the distillation column 1 by reducing the amount of outflow of the top liquid.

(3) Decrease the amount of heat inside the distillation column 1 by increasing the amount of outflow of the bottom liquid.

Do at least one of the following:

Here, the control is preferably performed in proportion to the Δt (amount of change) or its time differential value.

(a-2) When the temperature inside the tower decreases Since it has the opposite meaning to (a-1), perform the opposite operation.

(b) Control method based on the analysis results of the analyzer (54.61).

(b-1) If the analysis of the bottom liquid or top liquid reveals a substance with a higher boiling point than the target component (1) Lower the heating temperature by the reboiler.

(2) Increase the amount of heat inside the distillation column 1 by increasing the amount of outflow of the top liquid.

(3) The amount of heat inside the distillation column 1 is lowered by reducing the amount of outflow of the bottom liquid.

Do at least one of the following:

Here, control is preferably performed in accordance with (in proportion to) the difference from the target value until the analysis result by the analyzer (54.61) reaches the target value.

(b-2) When the bottom liquid or top liquid is analyzed and the substance has a boiling point lower than the target component, the control is opposite to that in (b-1), so the control is reversed.

(c) Control based on the results of analyzing the supply liquid with the analyzer (31). For example, it is assumed that the composition of the feed liquid before distillation is high boiling point substance A = 60% and low boiling point substance MB = 40%, and this feed liquid is supplied at a rate of 5 Q per minute. Assume that as time passes, 50% of substance A has a high boiling point, and 50% has a substance B that has a low boiling point.

Anticipating that the temperature inside the distillation column 1 will drop due to an increase in the amount of low-boiling-point substances, the heating temperature by the reboiler is increased so as to increase the swamp content in the column.

(2) Increase the amount of heat inside the distillation column 1 by increasing the amount of outflow of the top liquid.

(3) To increase the amount of heat inside the distillation column 1 by reducing the amount of outflow of the bottom liquid.

Do at least one of the following: If the amount of high boiling point substances increases, the temperature of distillation column 1 will increase (1)
〜(3) Reverse operation.

(d) Correction due to changes in tower pressure When the tower internal pressure rises, the temperature rises and the ratio of components with high boiling points in the top liquid and bottom liquid increases, and when the pressure decreases, the temperature decreases and the proportion of components with low boiling points increases. The ratio appears to have increased. That is,
As the pressure changes, the temperature gradient in the column changes as shown in the graph of FIG. 4, making it appear as if the composition has changed. This does not allow for proper distillation, so this variation is corrected and controlled.

Correction based on changes in pressure is effective when there is a sudden change in the treatment within the column. In other words, even if the temperature inside the column changes, if the change is temporary due to a pressure change, if the above control is performed based on such temperature change, the necessary control will not be achieved. This results in improper temperature control.

Therefore, we decided to control the temperature by detecting pressure changes to determine whether such temperature changes were actually worthy of trust.

〔Example〕

Examples of the present invention will be described below.

<Example 1> As shown in FIG. 1, a vertically elongated distillation column 1 is provided. A plurality of trays 8 are provided in the distillation column 1, and each tray 8 is provided with a plurality of small holes.

Each tray 8 is connected by a downcomer along the inner wall surface of the distillation column 1, so that the liquid accumulated on the upper tray 8 flows down to the lower tray 8 through the downcomer.

A feed line 2 is connected to the center of the distillation column 1 for supplying the liquid to be distilled. This feed line 2 includes a first analyzer 31 (gas chromatography), a first flow rate detection end 32, and a first flow rate detection end 32.
A first flow meter 33 and a first flow control well 30 are provided, respectively. The outputs from the first analyzer 31 and the first flowmeter are manually input to the main controller 34.

In addition, a temperature sensor (not shown) is installed in the center of the distillation column 1.
A pressure sensor is attached, a temperature sensor is connected to the temperature controller 35, and a pressure sensor is connected to the auxiliary controller 3.
6. and a temperature controller 35,
The output from the auxiliary controller 36 is manually input to the main controller 34.

A reboiler circulation path 3 is provided at the bottom of the distillation column 1. This reboiler circulation path 3 is connected to the bottom of the distillation column 1, takes in the feed liquid that has been supplied into the distillation column 1, and has fallen through the downcomer, and is connected to a heat exchanger 6 provided in the circulation path. Heated steam is supplied to the heat exchanger 6, and the amount of the supplied steam is controlled by the heat medium control valve 42, thereby adjusting the amount of heating. It is becoming more and more popular. The opening degree of the heat medium control well 420 is adjusted based on a command from the first liquid level controller 43 that detects the amount of bottom liquid accumulated at the bottom of the distillation column 1. Opening adjustment is performed.

Further, a bottom outline 4 (waist) is connected to the lower part of the distillation column 1 to take out the bottom liquid accumulated at the bottom of the column. This bottom outline 4 includes the first
A second flow rate detection end 52 and a second flow rate control valve 53 whose flow rate is controlled by a flow rate controller 51 are provided, and a second analyzer 54 is further provided. Second analyzer 5
The output from 4 is manually input to the main controller 34, and the first flow rate controller 51 operates according to instructions from the main controller 34.

-17- -18 Furthermore, a top vapor line 5 is connected to the top of the distillation column 1. The top vapor line 5 is provided with a condenser 10 that condenses and liquefies the vapor containing a large proportion of low-boiling substances extracted from the distillation column 1, and a receiver 11 that stores the obtained column top liquid. A reflux line 12 is connected to return the liquid from the receiver 11 into the distillation column 1, while a top outline line 13 is connected to which the liquid from the receiver 11 flows out.
A third flow rate detection end 56 and a third flow rate control valve 57 are provided at. This third flow rate detection end 56 and third flow rate control valve 57 are connected to the second flow rate controller 5. The second flow rate controller 58 controls the flow rate of the top liquid flowing out from the top outline 13 in response to a command from the second level controller 62 that detects the amount of the top liquid accumulated in the receiver 11. has become,
The opening degree is adjusted to increase the amount of outflow when the receiver 11 overflows.

In addition, a receiver 1 is connected to the distillation column 1 from the reflux line 12.
A fourth flow rate detection end 5 is connected to the reflux line 12 so that the liquid that has been removed from the reflux line 12 flows into the reflux line 12.
9 and a fourth flow rate control valve 60 are provided, and the flow rate is controlled to a set constant flow rate.

Using the above apparatus, isobutane, butene-1, ph-tene-2, ■-phtane, and phtadiene are mixed.From night, isobutane is separated and taken out as the top liquid, and as the bottom liquid, butene-1, butene-2, n- An example of taking out a mixed liquid of butane and butadiene will be explained.

First, a mixture of isobutane, butene-1, butene-2, n-butane, and butadiene is fed from feed line 2 to distillation column 1.
supply to. This supply is carried out continuously. While the supply is being carried out, the first analyzer 31 (gas chromatograph)
The composition of the feed liquid is continuously analyzed at the first flow meter 33.
The flow rate of the feed liquid is measured, and the amount of isobutane contained in the mixed liquid supplied into the distillation column 1 is calculated, and the calculation result is manually input to the main controller 34 as first information. Ru. In other words, if the ratio of isobutane is 10% and the flow rate is 100 Q/min, then (10/100)xl00-20- = 10 Q/min of isobutane is being supplied.

The temperature of the supplied mixed liquid becomes about 55° C. due to the steam rising from the bottom of the tower due to the heat of the reboiler.

The vapor of the mixed liquid generated by heating comes into contact with the liquid on the tray 8 when passing through the small holes of the tray 8, and undergoes heat exchange and condensation. Here, the higher the boiling point of the tray 8 is, the higher the boiling point is collected, and the lower the tray 8 is, the lower the boiling point is collected.

As a result, within the distillation column 1, a plowing ratio gradient is formed in which the proportion of low-boiling substances increases from the bottom to the top. This composition ratio gradient appears as a temperature gradient corresponding to the condensation temperature.

The vapor vaporized from the top tray 8 is mainly isobutane, and this vapor is discharged from the top vapor line 5, condensed in the condenser 10, and stored in the receiver 11 as an overhead liquid containing mainly isobutane. will be retained.

In the receiver 11, the level sensor detects the storage amount, and when an overflow occurs, the second flow controller 5 measures the flow rate with the third flow rate detection end 56 in response to a command from the second level controller 62, and then the third flow control valve 5
7 and take out the isobutane liquid from the top outline 13.

On the other hand, the liquid accumulated at the bottom of the tower is taken out by opening the second flow control valve 53.

In the above process, as second information, the temperature inside the column is monitored by a temperature sensor, and the temperature inside the column is set to the standard temperature 5 for distillation.
When the temperature drops by Δt from 0° C., the temperature controller 35 manually outputs an output to the main controller 34 to notify that fact.

In addition, the pressure inside the distillation column 1 is detected as third information,
It is manually operated by the main controller 34.

Finally, as fourth information, the bottom liquid flowing out is analyzed by the second analyzer 54, and the analysis result is input to the main controller 34.

Then, in the main controller 34, so-called P・■・
D calculation is performed, and in response to the calculation result, the first flow rate controller 51 adjusts the opening degree of the second flow rate control valve 53, and the outflow amount from the bottom outline 4 is adjusted. Ru.

That is, based on the first information, when the amount of the composition in the supply liquid changes during operation, the second flow rate control valve 53 is controlled by the first flow rate controller 51 in response to the amount of change. , the flow rate of the bottom liquid from the bottom outline 4 is varied by P operation control (proportional operation control), so that isobutane mainly comes out from the top vapor line 5 and other compositions mainly come out from the bottom outline 4 at an appropriate ratio. The temperature inside the column is controlled in a feedforward manner.

On the other hand, in response to fluctuations in the temperature of the second temperature, the amount of outflow of the bottom liquid from the bottom outline 4 is varied by the P operation control here as well, in response to the fluctuations.

By the way, regarding temperature fluctuations, slow temperature fluctuations when the distillation column 1 continues stable operation are reliable information, but rapid temperature fluctuations are not reliable information. Therefore, temperature measurement values are always converted to temperature at standard pressure using pressure measurement values, and even if the temperature suddenly changes, if there is a corresponding pressure change, Care has been taken to avoid any accompanying controls.

In addition, based on the fourth information, it is determined whether the formation ratio of the tower bottom liquid has deviated from the target value, and if it has deviated, it operates (reset operation) until it reaches the target composition ratio. ) controls the amount of outflow from the bottom outline 4.

Note that since the first to fourth pieces of information are sequentially input to the main controller 34, the main controller 34 adds up the results of these calculations to determine the amount of operation, and determines the amount of outflow from the bottom outline 4.

52 is a second flow rate detection end.

Embodiment 2 As shown in FIG. 2, in this example, the second flow rate controller 58 controls the third flow rate control valve 57 in response to a command from the main controller 34, and controls the outflow of the top liquid. The composition is controlled by controlling the amount. The second liquid level controller 62 is adapted to control the heat medium control valve 42 of the reboiler. That is, when the receiver 11 overflows, the heat medium control valve 42 reduces the flow rate of the heat medium and lowers the heating temperature. The first liquid level controller 43 is adapted to control the second flow control valve 53. Therefore, when the bottom liquid overflows, the second flow rate control valve 53
The opening degree of the column becomes larger, and the amount of outflow of the bottom liquid increases. In addition, 5
6 is a third flow rate detection end. Other points are the same as in FIG.

<Example 3> As shown in FIG. 3, in this example, the top outline 1 following the top vapor line 5 in Example 2 is
The top liquid flowing out from the top vapor line 5 is analyzed by a third analyzer 61, and the analysis result is used as fifth information in place of the fourth information. A fourth flow rate detection end 59 is connected,
A fourth flow rate controller 71 connected to the fourth flow rate detection end 59 measures the amount of recycle top liquid from the reflux line 12, and operates the reboiler heat medium control valve 42 in accordance with the measured amount. It has become.

That is, the flow rate of the heat medium is controlled to be decreased when the amount of the top liquid to be recycled is large, and the flow rate of the heat medium is increased when the amount recycled is small. The other parts are the same as in the second embodiment.

<Example 4> Although not shown, a third analyzer 61 for analyzing the top liquid flowing out from the top vapor line 5 is provided in Example 1, and this result is treated as fifth information in the same manner as fourth information. or it can be replaced.

(Others) Note that although trays have been described in the examples, there is no difference in the effect even if a packed tower is used.

〔Effect of the invention〕

Since the present invention has the above configuration, the distillation column can be operated stably at all times, and there is little variation in the composition ratio of the distilled material obtained.
Even substances with small boiling point differences can be efficiently separated.

-251 -26-

[Brief explanation of drawings]

Figure 1 is a diagram showing the first embodiment, Figure 2 is a diagram showing the second embodiment, Figure 3 is a diagram showing the third embodiment, Figure 4 is the relationship between temperature, composition distribution, and pressure. 5 and 6 are graphs showing conventional examples. 1.◆Distillation column, 4.◆Bottom outline, 5.◆Top vapor line, 31, 54.61◆◆Analyzer.

Claims (6)

[Claims] (1) The feed liquid supplied into the distillation column is heated, and the substance with a high boiling point is extracted from the bottom outline line connected to the bottom of the distillation column, and the evaporated boiling point substance is extracted from the top vapor line connected to the top of the distillation column. In a method for separating substances using a distillation column that extracts low-quality substances, [1] The composition of the substances extracted from the bottom outline or top vapor line is analyzed with an analyzer, and if the composition does not reach the set target composition ratio, [2] The element that should determine the temperature in the distillation column is heated by a heating device to control an element that should determine the temperature in the distillation column until the target composition ratio is reached. amount, the amount of material extracted from the bottom outline,
A method for controlling the composition of a distilled substance, characterized in that the composition is controlled by at least one substance selected from the outflow amount of a substance taken out from a top vapor line. (2) The feed liquid supplied into the distillation column is heated, and the substance with a high boiling point is extracted from the bottom outline line connected to the bottom of the distillation column, and the evaporated boiling point substance is extracted from the top vapor line connected to the top of the distillation column. In a method for separating substances using a distillation column to extract low-temperature substances, [1] Monitoring the temperature inside the distillation column and controlling the elements that determine the temperature inside the distillation column according to the temperature change to cancel out the temperature change. [2] When supplying the feed liquid to the distillation column, the amount of the substance to be taken out from the bottom outline line or top vapor line by distillation in the feed liquid is measured, and the above-mentioned method is adjusted according to the increase or decrease in the amount. [3] A control method for controlling an element that should determine the temperature in the distillation column 1; and [3] The element that should determine the temperature in the distillation column is the amount of heating of the feed liquid by a heating device, the substance taken out from the bottom outline. A method for controlling the composition of a distilled substance, characterized in that the amount of the substance to be distilled is at least one selected from the following. (3) The feed liquid supplied into the distillation column is heated, and the substance with a high boiling point is extracted from the bottom outline line connected to the bottom of the distillation column, and the evaporated boiling point substance is extracted from the top vapor line connected to the top of the distillation column. In a method for separating substances using a distillation column to extract low-temperature substances, [1] Monitoring the temperature inside the distillation column and controlling the elements that determine the temperature inside the distillation column according to the temperature change to cancel out the temperature change. [2] Analyze the composition of the substance taken out from the bottom outline or top vapor line with an analyzer, and if it does not reach the set target composition ratio, distill it until the target composition ratio is reached. a control method for controlling an element that determines the temperature inside the column, [3] the element that determines the temperature inside the distillation column is the amount of heating of the feed liquid by the heating device, the substance taken out from the bottom outline; A method for controlling the composition of a distilled substance, characterized in that the amount of the substance to be distilled is at least one selected from the following. (4) The feed liquid supplied into the distillation column is heated, and the substance with a high boiling point is extracted from the bottom outline line connected to the bottom of the distillation column, and the evaporated boiling point substance is extracted from the top vapor line connected to the top of the distillation column. In a method for separating substances using a distillation column to extract low-temperature substances, [1] Monitoring the temperature inside the distillation column and controlling the elements that determine the temperature inside the distillation column according to the temperature change to cancel out the temperature change. a control method; [2] a method of monitoring the pressure within the distillation column and correcting a measured value of temperature within the distillation column controlled by the first control method according to a change in the pressure; [3] At least the factor that determines the temperature in the distillation column is selected from the amount of heating of the feed liquid by the heating device, the amount of outflow of the substance taken out from the bottom outline, and the amount of outflow of the substance taken out from the top vapor line. 1. A method for controlling the composition of a distilled substance, characterized in that: (5) According to claim 4, if the composition of the substance taken out from the bottom outline line or the top vapor line is analyzed with an analyzer and the composition ratio does not reach the set target composition ratio, until the composition ratio reaches the target composition ratio. A method for controlling the composition of distilled substances that includes a control method for controlling the elements that determine the temperature inside the distillation column. (6) According to any one of claims 1 and 3 to 5, when the feed liquid is supplied to the distillation column, the amount of the substance to be taken out from the bottom outline line or the top vapor line by distillation in the feed liquid is measured, A method for controlling the composition of a distilled substance, which includes a control method for controlling an element that determines the temperature in the distillation column according to an increase or decrease in the amount of the distilled substance.