JPS5814802B2 - How to control distillation equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for controlling a distillation apparatus comprising at least two distillation columns whose pinch point during stability is close to the raw material feed position.

More specifically, the present invention relates to a new control method for a distillation apparatus with improved response characteristics and reduced energy loss.

Conventionally, the control circuit around a distillation device consisting of multiple columns is
The raw material feed to the first distillation column is a cascade control system that controls the liquid level in the upstream tank and controls the flow rate.
The raw material feed to the column or subsequent distillation columns is supplied by controlling the bottom liquid level of the previous column, and the bottom humidity in each column is controlled manually or by cascade control with the heating medium supplied to the boiler.

That is, the general principle of the control system is based on the liquid level on the upstream side, and any fluctuations occurring on the upstream side are corrected on the downstream side.

Therefore, when a disturbance occurs in the distillation apparatus and the humidity inside the column changes, the reflux flow rate and the amount of heat medium supplied to the reboiler are controlled.

Such conventional control methods have the following disadvantages.

■ When controlling the flow rate of heat medium to the reboiler by changing the temperature inside the tower, there is a delay in evaporation due to a delay in heat transfer in the reboiler,
After that, the equilibrium at each stage is shifted to a new state, and the disturbance is removed toward the top of the tower, so the correction operation is extremely slow.

In addition, the response of the mixture in the column when the amount of heat medium is increased or decreased is nonlinear and asymmetric, and the settling time is long in a feedback cascade system.

■ In order to reduce the influence of disturbances such as raw material feed flow rate, concentration, humidity, etc., the minimum reflux ratio is approximately 1.
The system is operated at 2 to 2 times the power, which slows down the correction operation and causes a large energy loss due to the asymmetric response.

■ The operating curve changes due to fluctuations in the liquid-gas flow rate ratio in the column.
The quality of the effluent from the top or bottom of the tower deteriorates due to the reduction in column efficiency.

As a method to solve this problem, the present inventors first determined the internal temperature of the column near the pinch point when stable and A method for controlling a distillation apparatus, etc. is provided, which is characterized by detecting the amount of energy supplied to a reboiler and using this as a control signal for adjusting the amount of raw material feed.

The method can be applied to either a single distillation column or a distillation apparatus having a series of distillation columns.

When applying the above control method to a distillation apparatus having a plurality of distillation columns connected in series, a method is generally adopted in which the effluent from the previous column is stored in an intermediate tank and sent to the next distillation column.

However, in existing plants, there are cases where there is no space to install an intermediate tank, so the application has been reluctant.

The present invention provides a novel control method for a distillation apparatus composed of at least two columns, which solves the above-mentioned disadvantages.

Another object of the present invention is to provide a control method with a simple feed forward control system capable of quickly correcting a generated disturbance, with a short settling time and with little energy loss.

Furthermore, the present invention provides a control method that can be easily applied to existing distillation equipment.

In other words, the present invention provides a control system for a distillation system composed of at least two distillation columns in which the pinch point during stabilization is close to the feed position, so that the amount of raw material feed to each distillation column is stabilized in each distillation column. The temperature inside the column near the pinch point and the amount of energy supplied to the reboiler are detected, and the amount of raw material feed supplied to each distillation column is adjusted accordingly.
The amount of energy supplied to the distillation column or the subsequent distillation column boiler is determined by the level fluctuation of the bottom liquid of the distillation column in front of each distillation column or the amount of energy supplied to the distillation column boiler installed after the top condenser of the distillation column in front of each distillation column. The present invention provides a method for controlling a distillation apparatus, which is characterized by detecting liquid level fluctuations in a liquid reservoir and adjusting the amount of energy supplied to each distillation column boiler accordingly.

The method of the present invention will be explained in more detail below.

In carrying out the method of the present invention, the amount of raw material feed to each distillation column is determined by detecting the internal temperature of each distillation column near the pinch point when it is stable and the amount of energy supplied to the reboiler. It is essential to adjust the amount of raw material feed supplied.

The reason is described below.

Generally, distillation equipment is operated at a liquid-gas ratio. K = liquid gas ratio K=L/V・・・・・・・・・(1) L = liquid flow rate ■＝Gas flow rate is controlled so that it is constant.

Therefore, there are L and ■ as control factors when the K value changes due to disturbance.

Breaking down the contents of L and
〃 Gas flow rate・・・・・・・・・(2) Ks=Recovery part liquid gas ratio Ks=Ls/Vs Ls= 〃Liquid flow rate Vs= 〃
Gas flow rate.

As mentioned earlier, the general principle of existing control systems is based on the liquid level on the upstream side, and the fluctuations that occur on the upstream side are corrected on the downstream side. It is controlled by either or both of Vs depending on the amount of heat supplied, and Ls is not used as a control factor.

As understood from equations (1) and (2), K and L are in a proportional relationship, and K and ■ are in an inversely proportional relationship.

Therefore, the average value coincides with the center of variation of the amount of change in L required to correct the amount of change in K, but on the other hand, since the change in ■ is an asymmetric response, the average value is larger than the center of variation.

The fact that the average value is larger than the center of fluctuation means that the distillation system is in an unstable state, which results in a long settling time until stabilization, resulting in an increase in energy loss.

From this, it is understood that it is desirable to use L as a control factor.

Although the adoption of Lr in known control systems is a preferable direction for the reasons mentioned above, in reality it does not exhibit sufficient effects.

If we further decompose Ls, which has not been used as a control factor in the past, we get Ls = Lr + qF ・-"(3) q=
Liquid ratio F in the feed fluid = feed flow rate.

In other words, F can be used as a control factor of Ls.

When F is used as a control factor, it responds in a proportional relationship to K and is desirable as a control factor.

This can be said when ■ is constant, and L
In a system where both and (2) fluctuate, it is necessary to apply feed forward control to F (2) so that L and (2) cancel each other out.

When F was selected as a control factor in this way and an experiment was conducted on an existing distillation apparatus, it was found that a control factor using F as a control factor could significantly improve the response characteristics and energy utilization rate.

The reason for this is that the pinch point of the distillation equipment is usually designed to be located at the raw material feed position (it may shift slightly during actual operation).
It was found that this is due to the difference in the distance from the reflux point of the reflux fraction to the pinch point and the distance from the raw material feed position to the pinch point.

Therefore, in a distillation apparatus where the pinch point during stability is closer to the raw material feed position than the reflux reflux position, it is effective to control the raw material feed flow rate.

The term "near the pinch point" used in this specification means a region where the response speed is faster for changes in the feed amount than for changes in the reflux amount, and these can be easily determined through preliminary experiments or from design materials. It will be done.

As used herein, "feed feed" refers to the crude product in the first column and the effluent from the column preceding each column as feed to the second or subsequent columns.

Therefore, the temperature near the pinch point and the amount of energy supplied to the reboiler are used as control factors for the raw material feed flow rate.

By measuring the temperature near the pinch point, it is possible to know the stability of the concentration section operating line and recovery section operating line, and by measuring the amount of energy supplied to the reboiler, the recovery section gas flow rate can be estimated. The overall balance is controlled by using a combination of the humidity near the pinch point and the amount of energy supplied to the reboiler.

In the method of the present invention, in order to utilize L for system control, changes in K due to fluctuations in (2) must be made as negligible as possible.

Therefore, it is necessary to change the raw material feed amount in response to changes in the amount of energy supplied to the reboiler.

From the above explanation, I hope you understand how to adjust the amount of raw material feed to each distillation column.

In carrying out the method of the present invention, the amount of energy supplied to the reboiler of the second distillation column or subsequent distillation columns is determined by detecting the liquid level fluctuation of the bottom liquid of the distillation column in front of each distillation column, or It is essential to detect fluctuations in the liquid level in the liquid reservoir provided after the top condenser of the column in front of each distillation column and adjust the amount of energy supplied to the reboiler of each distillation column accordingly.

The reason is detailed below. In order to adjust the amount of raw material feed to each of the distillation columns described above without cascading the distillation columns, it is necessary to provide an intermediate tank between the distillation columns.

This is because when the amount of heat medium supplied to the reboiler of the subsequent distillation column decreases, the amount of raw material feed decreases, as is clear from the above explanation, and in this case, the amount of heat medium supplied to the reboiler of the previous distillation column does not change. If so, the effluent bottom liquid or condensate from the previous stage distillation column will increase.

Conversely, if the amount of heat medium supplied to the reboiler of the subsequent distillation column increases, the amount of raw material feed increases.In this case, if the amount of heat medium supplied to the reboiler of the previous distillation column remains unchanged, effluent decreases.

It is assumed that the amount of heat medium supplied to the reboiler of the preceding distillation column does not change, but of course it does change in reality, so fluctuations in the effluent may occur frequently.

Therefore, if the amount of raw material fed to each distillation column is to be adjusted without cascading the columns, an intermediate tank must be provided between each distillation column.

In the case of such a method of providing an intermediate tank, extra equipment and site are required compared to the current process, and the method is particularly reluctant to be applied to existing equipment.

The present invention was made to solve the above-mentioned disadvantages, and the amount of energy supplied to the reboiler of the second distillation column or subsequent distillation columns is determined by the amount of energy supplied to the reboiler of the distillation column in front of each distillation column. Fluctuations in the liquid level of the residual liquid or fluctuations in the liquid level in the liquid reservoir provided after the top condenser of the column in front of each distillation column are detected, and the amount of energy supplied to each distillation column is adjusted accordingly.

The valve for adjusting the amount of energy supplied to the reboiler and detecting the fluctuation in the liquid level of the remaining liquid in the tank or the liquid reservoir part may be operated directly according to the detected value, or the valve for adjusting the amount of energy supplied to the reboiler may be operated directly according to the detected value, or A dead band may be provided in the terminal or controller so that the regulating valve is activated when the liquid level fluctuation is outside a predetermined range.

Therefore, by adjusting the amount of energy supplied to the reboiler of the second column or subsequent distillation columns as in the present invention, the amount of raw material feed can be adjusted to the temperature inside the column near the pinch point when each distillation column is stable. This method has the advantage that it is possible to solve problems such as intermediate tanks and accompanying site problems that arise when the amount of energy supplied to the reboiler is detected and the amount of raw material feed supplied to each distillation column is adjusted accordingly.

The method of the present invention will be explained in more detail below with reference to the drawings.

FIG. 1 is an explanatory diagram showing a method of controlling a distillation apparatus that feeds the bottom liquid to the next distillation column, and FIG. 2 is an explanatory diagram showing a method of controlling the distillation apparatus that feeds the top effluent to the next distillation column.

In FIG. 1, the raw material to be distilled is fed through line 28 to distillation column 1, where it is fractionated into a low-boiling fraction and a high-boiling fraction.

The amount of raw material supplied to the distillation column is maintained at a substantially constant flow rate by detecting information from a feed flow meter 12 and controlling a feed amount adjustment valve 3 via a signal line d.

The distillation column 1 can be in the form of a sieve tray, a bubble cap, a packed column, or the like.

A bottom circulation line 29 is provided at the bottom of the distillation column 1, and between this bottom circulation line there is a reboiler 4 which is heated by steam supplied via a line 34.
is provided to supply heat.

The amount of steam supplied is maintained at a constant flow rate by detecting information from a steam flow meter 13 and controlling a steam feed amount adjustment valve 5 using signal lines b and g.

In this example, a case is shown in which steam is used as a heat source, but of course other heat medium may also be used.

The low boiling point fraction is taken out from the low boiling point distillate extraction line 31, condensed and liquefied in the condenser 6, and stored in the liquid reservoir 7.
A part of it is recycled as a reflux fraction through a line 32 to the upper part of the distillation column by detecting information from a reflux flow meter 14 and controlling a reflux adjustment valve 8 via a signal line e.

On the other hand, the remaining liquid is recovered as a product via a line 33 by controlling the product take-out valve 9 using a signal line f to control the liquid level fluctuation in the liquid reservoir 7.

In the above configuration, the method of the present invention detects the temperature detected at point A (near the pinch point) through the signal line a, and detects the information from the steam flow meter 13 to determine the amount of energy supplied to the reboiler through the signal line b. is sent to the controller 10 by the controller 10
The raw material feed valve 3 is adjusted by the signal line C in accordance with the instructions derived from the signal line C, the raw material feed amount is adjusted, and the temperature inside the distillation column is corrected.

As an example of calculating this feed amount, for example, the flow rate ratio (feed amount/steam amount) is calculated from the feed flow rate from signal line d and the steam flow rate from signal line b, and this is converted into L/V at a stable state. This deviation is compared and multiplied by the steam flow rate signal from signal line b to obtain the feed flow rate setting output, and on the other hand, the temperature signal from signal line a is compared with the set value to find the deviation and perform feedback control calculation for the feed flow rate. , and add the output to the feed flow rate setting output described above.

By adjusting the raw material feed amount in this way, the response speed is fast, and since the fluctuation center and the average value match, the settling time is significantly shortened compared to the conventional asymmetric response, and energy efficiency is significantly improved. be done.

The high boiling point fraction is taken out via line 30 and supplied as a raw material to the next distillation column 2.

A bottom circulation line 35 is provided at the bottom of the second distillation column 2, and a reboiler 16, which is heated by steam supplied via a line 40, is provided between the bottom circulation lines. Heat is supplied by.

The amount of steam supplied is maintained at a constant flow rate by detecting information from the steam flow meter 25 and controlling the steam feed amount adjustment valve 17 using signal lines i and n.

As with the previous distillation column, the high boiling point fraction from the previous distillation column is controlled by a signal line h to control the temperature detected at point A' (near the pinch point) and by a signal line i to control the amount of energy supplied to the reboiler. 22, and adjust the raw material feed valve 15 through the signal line J according to instructions derived from the controller 22.
Adjust and supply raw material feed amount.

The amount of raw material supplied to the second distillation column is detected by the feed flow meter 24, and the feed amount adjustment valve 15 is controlled by the signal line k to maintain the feed amount at a substantially constant flow rate.

By adjusting the raw material feed amount using this method, settling time can be shortened and energy efficiency can be significantly improved.

In addition to the configuration described above, when implementing the method of the present invention, a liquid level fluctuation detector 11 for the bottom liquid of the first distillation column is provided,
The indication of this detector is cascaded by a signal line p to a control valve 17 for controlling the amount of steam supplied to the reboiler of the second distillation column 2.

By adopting such a control method, the liquid can flow smoothly between the first distillation column 1 and the second distillation column 2 without an intermediate tank, and can be easily applied to existing distillation equipment. There are advantages.

In the second distillation column 2, the low-boiling point fraction is taken out from the low-boiling point extraction line 37, condensed and liquefied in the condenser 18, and stored in the liquid reservoir section 19, where a part of it is passed through the line 38 as a reflux fraction to the distillation column. Recycled at the top.

The information from the reflux flowmeter 26 is detected and the reflux adjustment valve 20 is controlled by the signal line 1, whereby the reflux beam is recycled.

Although a case has been shown in which the flow rates of the reflux amounts in both the first and second distillation columns are controlled, feedforward from the raw material feed amount and/or the steam amount supplied to the reboiler may also be used.

The remaining 10,000 yen passes through line 39 and is recovered as a product.

In this case, it is preferable to detect the liquid level fluctuation in the liquid reservoir 19 and control the product take-out valve 21 using the signal line m to take out the product.

The high boiling fraction is removed via line 36 and used as product or as feedstock for the next distillation step.

In this case, the liquid level fluctuation in the remaining part of the distillation column 2 is detected by the liquid level detector 23, and the product take-out valve 27 is detected by the signal line q.
control and take out.

Therefore, the main point of the present invention is to adjust the raw material feed amount based on the temperature near the pinch point and the fluctuation of the amount of energy supplied to the reboiler, and to adjust the amount of raw material feed based on fluctuations in the temperature near the pinch point and the amount of energy supplied to the reboiler. Fluctuations in the liquid level of the bottom liquid in the first distillation column or fluctuations in the liquid level in the liquid reservoir provided after the top condenser of the first distillation column are detected, and the amount of energy supplied to the second distillation column is adjusted accordingly. By adjusting the method, it is realized that the distillation column can be stabilized and the conventional equipment can be applied by simple modification.

FIG. 2 shows an example of the control method of the present invention in which the low-boiling fraction from the first distillation column is used as raw material feed to the next distillation column.

In FIG. 2, the raw material to be distilled is fed through line 28 to distillation column 1, where it is fractionated into a low-boiling fraction and a high-boiling fraction.

A bottom circulation line 29 is provided at the bottom of the distillation column 1, and between this bottom circulation line there is a reboiler 4 which is heated by steam supplied via a line 34.
is provided.

The high boiling point fraction is taken out as a product from the high boiling point distillate extraction line 30 by controlling the product take-out valve 42 using the signal line r while controlling the liquid level of the can residual liquid using the liquid level detector 11. recycled as

The amount of raw material feed is determined by sending the humidity detected at point A (near the pinch point) to the controller 10 through the signal line a, and the amount of energy to be supplied to the reboiler through the signal line b, and sending instructions from the controller 10 to the signal line. C, the signal is transmitted to the raw material feed valve 3 to adjust the amount of raw material feed and correct the temperature inside the distillation column.

The low boiling point fraction is taken out via line 31, condensed and liquefied in condenser 6, and stored in liquid reservoir 7. A part of it is recycled as a reflux fraction to the upper part of the distillation column via line 32, while the remainder is It is sent as a raw material to the second distillation column 2 via a line 41.

As with the previous distillation column, the low boiling point fraction from the distillation column 1 in the previous stage uses the humidity detected at point A' (near the pinch point) as the signal line h.
The amount of energy supplied to the reboiler by the signal line i
The instruction from the controller 22 is transmitted to the raw material feed valve 15 via the signal line j to adjust and supply the raw material feed amount.

In the case of such a configuration, when carrying out the method of the present invention, a liquid level fluctuation detector 43 of the liquid reservoir section 7 provided after the top condenser 6 of the first distillation column 1 is provided, and the indication of this detector is connected to the signal line. S
It is cascaded with a valve 17 for regulating the amount of steam supplied to the reboiler of the second distillation column 2 through the .

In the second distillation column 2, the low-boiling point fraction is taken out from the low-boiling point extraction line 37, condensed and liquefied in the condenser 18, stored in the liquid reservoir section 19, and a part of it is passed through the line 38 as a reflux portion to the distillation column. Recycled at the top.

Meanwhile, the remainder is taken out from line 39 as a product or as a raw material for the next distillation process.

The high-boiling fraction is recovered as a product via line 36.

Note that the flow other than that described above with respect to FIG. 2 is substantially the same as that of FIG. 1.

In the figure, the explanation was given for the case where there are two distillation columns, but
Of course, the same structure as above can also be applied to a distillation apparatus consisting of two or more distillation columns.

Moreover, the control method of the present invention can be implemented in appropriate combination with other control methods.

According to the control method of the present invention as detailed above, since the raw material feed amount which acts proportionally to the K value is used as a control factor, the response speed is fast, and the center of fluctuation and the average value match. The benefits of significantly reduced settling time and significantly improved energy efficiency compared to traditional asymmetric responses are demonstrated.

Furthermore, according to the control method of the present invention, the liquid is supplied to the second distillation column based on the fluctuation in the liquid level of the bottom liquid in the former distillation column or the liquid level in the liquid reservoir provided after the top condenser of the former distillation column. Since we control the amount of energy used,
It also has the advantage of being able to flow smoothly without an intermediate tank and being easily applicable to existing equipment.

Furthermore, the present invention can be carried out using automatic control by a computer or the like.

[Brief explanation of drawings]

1 and 2 show an example of application of the control method of the present invention. 1, 2: Distillation column, 3, 15: Feed flow rate adjustment valve,
4, 16: Reboiler, 5.17: Steam flow rate adjustment valve, 6, 18: Condenser, 7, 19: Liquid reservoir, 8, 2
0: Reflux flow rate adjustment valve, 9, 21, 27, 4
2: Product take-out valve, 10, 22: Controller, 11, 2
3, 43: Liquid level fluctuation detector, 1 2, 24: Raw material flow meter, 13, 25: Steam flow meter, 14, 26: Reflux flow meter, 28: Raw material feed line, 29, 35
: Bottom circulation line, 30, 36, 41 : Product removal/raw material feed line, 31, 37: Low boiling fraction removal line, 32, 38: Reflux line, 33, 39: Product removal line, 34, 40: Steam line , a, b, c, d
, e, f, g, h, i, j , k, l, m, n, p, q
, r, s: signal line.

Claims (1)

[Claims] 1. In the control system of a distillation apparatus consisting of at least two distillation columns whose pinch point at stable time is close to the feed position, the amount of raw material feed to each distillation column is adjusted to the point at which each distillation column is stabilized. The humidity in the column near the pinch point and the amount of energy supplied to the reboiler are detected, and the amount of raw material feed supplied to each distillation column is adjusted accordingly, and the energy to the boiler of the second distillation column or subsequent distillation columns is detected. The supply amount is determined by detecting the liquid level fluctuation of the bottom liquid in the distillation column in front of each distillation column or the liquid level fluctuation in the liquid reservoir section installed after the top condenser of the column in front of each distillation column. A method for controlling a distillation apparatus, the method comprising adjusting the amount of energy supplied to each distillation column accordingly.