JPS587321B2 - Control method for distillation equipment, etc. - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for controlling a distillation apparatus, etc. 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 distillation equipment, etc., which has improved response characteristics and reduced energy loss.

Distillation equipment and the like used in this specification and claims refer to distillation equipment, strippers, and the like.

Conventionally, when looking at the control circuits around distillation equipment, for example strippers, the feed of raw materials to the stripper tower is a cascade control system that controls the liquid level in the upstream tank and controls the flow rate. It is said to be a cascade control system with flow rate control, and when looking at the distillation column, the raw material feed to the distillation column is a cascade control system of liquid level control and first flow rate control in the upstream tank, which affects product quality. The rectification section temperature is controlled by a cascade control system with reflux flow rate control, and the bottom temperature is controlled by a cascade control system with fixed value control or heat medium flow rate control to the reboiler.

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 or the like and the temperature inside the column fluctuates, the reflux flow rate and the heat medium flow rate 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 temperature inside the column when the amount of heat medium is increased or decreased is nonlinear and asymmetric, and the settling time becomes long in the feedback cascade system.

■In order to reduce the influence of disturbances such as raw material feed flow rate, concentration, temperature, etc., the minimum reflux ratio is approximately 1.2 in normal distillation.
The system is operated at ~2 times the speed, which slows down the corrective action 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 tower, which deteriorates the efficiency of the tower and deteriorates the quality of the liquid extracted from the top and bottom of the tower.

It has the following disadvantages.

SUMMARY OF THE INVENTION The present invention aims to eliminate the above-mentioned drawbacks and provides a feed forward control system with a simple configuration that can quickly correct disturbances that occur.

A further object of the present invention is to provide a distillation method with short settling time and low energy loss.

That is, the present invention detects the internal temperature of the column and the amount of energy supplied to the reboiler in the vicinity of the pinch point at stable time in the control system of a distillation apparatus, etc. where the pinch point at stable time is close to the raw material feed position, and It is an object of the present invention to provide a control method for a distillation apparatus, etc., characterized in that the control signal is used as a control signal for adjusting the feed amount.

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

Generally, distillation equipment is operated at a liquid-gas ratio. is controlled so that it is constant.

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

Breaking down the contents of L and V becomes.

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. As understood from equations (1) and (2), K and L are in a proportional relationship, and K and V 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 V is an asymmetric response, the average value is larger than the center of variation.

The fact that the average value is more critical than the center of fluctuation means that the distillation system is in an unstable state, which results in a long settling time and 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 the following.

In other words, F can be used as a control factor for 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 is true when V is constant, and L
In a system where both V and V fluctuate, it is necessary to apply feedforward control to F rather than V so that L and V cancel each other out.

In this way, when F was selected as a control factor 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 distillation equipment is usually designed to be located at the raw material feed position (it shifts slightly during actual operation). It turns out that this is due to the difference in distance to the point.

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

The term "near the pinch point" as used in this specification and claims refers to a region where the response speed is faster for changes in feed amount than for changes in reflux amount, and these can be determined based on preliminary experiments or design materials. more easily sought.

Of course, for systems without reflux, such as strippers, this means the entire tower.

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, both the concentration section operating line and the recovery section operating line can be stabilized, and by measuring the amount of energy supplied to the reboiler, the concentration section gas flow rate can be stabilized. The overall balance is controlled by using a combination of the temperature 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 V must be made as negligible as possible.

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

In the above, the distillation column has been mainly explained, but if the distillation column does not have a concentrating section, it corresponds to a stripper, and the method of the present invention is of course effective in this case as well.

The present invention is particularly effective in the case of strippers.

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

Figures 1 and 2 relate to the recovery of hydrogen chloride gas and azeotropic composition by stripping an aqueous hydrochloric acid solution.

FIG. 1 shows a conventional control method, and FIG. 2 shows a control method according to the present invention.

In Figures 1 and 2, 1 is a raw material feed pipe for the stripper tower 2, 3 is a steam supply pipe for the reboiler, 4 is a hydrogen chloride gas outlet, 5 is an azeotropic composition extraction port, 6 is a bottom reflux pipe, 7 is a reboiler 8 is a feed flow meter, 9 is a feed amount adjustment valve 10 is a steam flow meter, 11 is a steam amount adjustment valve, 12 is a controller, a, b, c, d are signal lines, and point A is near the pinch point. shows the temperature detection point.

An aqueous hydrochloric acid solution is supplied to the stripper tower 1 via a raw material feed pipe 2, and flows downward through the stripper tower while stripping hydrogen chloride gas.

As the stripper tower 1, a perforated plate tower, a packed tower, etc. can be used.

A bottom circulation line 6 is provided at the bottom of the stripper column 1, and a reboiler 7 is provided between the bottom circulation lines, which is heated by the steam supplied through the reboiler steam supply pipe 3. This provides heat to strip the hydrogen chloride.

The hydrogen chloride gas stripped from the aqueous hydrochloric acid solution is taken out from the hydrogen chloride gas outlet 4, and is absorbed into hydrochloric acid to be used to form concentrated hydrochloric acid, or is treated as appropriate for other purposes.

On the other hand, an aqueous hydrochloric acid solution having an azeotropic composition or close to an azeotropic composition from which hydrogen chloride has been stripped is taken out from the azeotropic composition extraction port 5.

This extracted azeotropic aqueous hydrochloric acid solution is generally used as an absorbent for hydrogen chloride gas in a reaction system that releases hydrogen chloride gas, and then treated with a stripper to remove newly absorbed hydrogen chloride. let

In FIGS. 1 and 2, the supply of raw materials and the extraction of each component are operated in the manner described above.

Figure 1 shows a conventional control method, in which the temperature at point A near the pinch point is detected and the supply amount of steam, which is the heat supply source, is controlled in cascade by the steam amount adjustment valve 11 using signal line a. ing.

In addition, the amount of raw material supplied to the stripper is determined by the feed flow meter 8.
The information is detected and the feed amount adjustment valve 9 is controlled by the signal line b, so that the flow rate is maintained at a substantially constant level.

According to such a conventional method, the Vs gas flow rate is controlled as a control factor, and as mentioned above, due to the asymmetric response, the average value becomes larger than the fluctuation center, resulting in a longer settling time and energy loss. This has the disadvantage of increasing.

Next, the control method of the present invention will be explained based on FIG. 2.

In the method of the present invention, the temperature detected at point A near the pinch point is sent to the controller 12 via the signal line a, and the flow rate detected by the steam flowmeter 10 is sent to the controller 12 via the signal line c, where it is sent to the controller 12 via the signal line a. Calculate as appropriate (calculate the flow rate ratio (feed amount/steam amount) from the feed flow rate from signal line b and the steam flow rate from signal line c, compare this with the stable L/V, and apply the signal line to this deviation. The steam flow rate signal from signal line a is multiplied to obtain the feed flow rate set output, while the temperature signal from signal line a is compared with the set value to find the deviation, feedback control calculation is performed on the feed flow rate, and the output is calculated as described above. Performed addition calculation with feed flow rate setting output.

), as a result, the raw material feed valve 9 is controlled by the signal line d in accordance with instructions from the controller 12, the raw material feed amount is adjusted, and the stripper is stably controlled.

However, if we detect the temperature near the pinch point and change the raw material feed amount based on that, the response will be proportional, and since the center of fluctuation and the average value will match, it will be more stable than the conventional asymmetric response. The benefits are reduced time and improved energy efficiency.

From the above relationship, it is clear that it is extremely effective to use the raw material feed amount as a control factor.

FIG. 3 shows an example in which the control method of the present invention is applied to a distillation apparatus.

In FIG. 3, 13 is a distillation column, 14 is a raw material feed pipe, 15 is a steam supply pipe for the reboiler, 16 is a low-boiling fraction outlet, 17 is a high-boiling fraction outlet, 18 is a bottom circulation flow pipe, 19 20 is a reboiler, 20 is a feed flow meter, 21 is a feed amount adjustment valve, 22 is a steam flow meter, 23 is a steam amount adjustment valve, 24 is a controller, 25 is a reflux pipe, 26 is a reflux amount adjustment valve, 27 is a reflux Flow meter, 28 is a condenser, 29 is a product outlet, a, b, c, d, e, f are signal lines, and point A is a temperature detection position near the pinch point.

The crude raw material to be distilled is supplied to the distillation column 13 via a pipe 14, and is fractionated into a low-boiling fraction and a high-boiling fraction within the column.

The distillation column 13 can be in the form of a sheep tray, a bubble cap, a packed column, or the like.

A bottom circulation line 18 is provided at the bottom of the distillation column 13, between which a reboiler 19 is heated by steam supplied via a pipe 15.
is provided to supply heat.

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

The low-boiling fraction is taken out from the low-boiling fraction outlet 16, condensed and liquefied in a condenser 28, and a part of it is recycled as a reflux fraction through a pipe 25 to the upper part of the distillation column.

On the other hand, the remainder passes through the pipe 29 and is recovered as a product.

The high-boiling fraction is also removed via pipe 17.

In the above configuration, conventionally, the temperature at point B at the top of the distillation column is detected and this and the reflux flow rate are controlled in cascade, and the temperature at point C at the bottom is detected and this and the reboiler flow rate are controlled in cascade. .

On the other hand, in the method of the present invention, the temperature detected at point A (near the pinch point) is sent to the controller 24 via the signal line a, and the raw material feed valve 21 is adjusted according to the instructions derived from the controller 24, thereby controlling the raw material feed amount. The purpose is to adjust the temperature inside the distillation column.

In Figure 3, B is used as a control system for the reflux flow rate.
We have shown an example in which the temperature at a point is detected and the reflux flow rate is controlled using the signal line e.
Alternatively, feedforward from the amount of steam supplied to the reboiler may be combined, or other control methods may be used.
For example, normal flow rate control may be used.

In carrying out the present invention, the raw feed amount is 19 reboilers.
The signal is sent to the controller 24 via the signal line c to perform feedforward control based on fluctuations in the amount of steam supplied to the Find the flow rate ratio (feed amount/steam amount) from the steam flow rate from signal line c, compare this with the stable L/V, and multiply this deviation by the steam flow rate signal from signal line c to determine the feed. On the other hand, the temperature signal from the signal line a was compared with the set value to determine the deviation, a feedback control calculation was performed on the feed flow rate, and the output was added to the above-mentioned feed flow rate setting output.

), the raw material feed valve 2 according to instructions from the controller 24.
1 to maintain overall balance.

In carrying out the method of the present invention, the amount of raw material feed can be controlled by a control method in which a branch pipe is provided to the raw material feed pipe, and the feed amount is controlled by this branch pipe, and the main pipe is kept fixed. .

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 benefit is that the settling time is significantly reduced and the energy efficiency is significantly improved compared to traditional asymmetric responses.

The method of the present invention has the advantage that it can be carried out using automatic control using a computer and can be easily applied to existing distillation apparatuses.

[Brief explanation of drawings]

Figure 1 shows a conventional control system for a stripper for a hydrochloric acid aqueous solution, Figure 2 shows a control system for a stripper for a hydrochloric acid aqueous solution according to the method of the present invention, and Figure 3 shows an example in which the control system for the method of the present invention is applied to a distillation column. It is. 1... Stripper tower, 2, 14...
Raw material feed pipe, 3, 15... Steam supply pipe for reboiler, 4... Hydrogen chloride discharge port, 5...
...Azeotrope composition extraction port, 6,18...Bottom circulation flow pipe, 7,19...Reboiler, 8,
20...Feed flow meter, 9,21...
・Feed amount adjustment valve, 10, 22... Steam flow meter, 11, 23... Steam amount adjustment valve, 12, 24... Controller, 13...・
・Distillation column, 16...Low boiling fraction outlet, 17.
...High boiling fraction outlet, 25... Reflux pipe, 26... Reflux amount adjustment valve, 27... Reflux flow meter, 28...・
...Condenser, 29...Product outlet,
a, b, c, d, e, f...signal line, A...
...Temperature detection point near the pinch point, B...Temperature detection point at the top of the distillation column, C...Temperature detection point at the bottom of the distillation column.

Claims (1)

[Claims] 1. In the control system of a distillation equipment, etc., where the pinch point when stable is close to the raw material feed position, the temperature inside the column near the pinch point when stable and the amount of energy supplied to the reboiler are detected, and these are used to adjust the amount of raw material feed. A method for controlling a distillation apparatus, etc., characterized in that the control signal is used as an adjustment control signal.