JPS6247406B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for controlling a distillation column equipped with a reboiler heated by a fuel gas containing two or more types of combustible gases.

Energy-saving control in the field of chemical processes is also becoming increasingly popular due to the recent spread of digital control systems. At the same time, there is a growing interest in advances in sensors and actuators, which can be considered control methods, and their application in actual processes.

An example of a conventional method for controlling a distillation column of this type will be explained with reference to FIG. 1. In the figure, 1 is a raw material tank, and the raw material is supplied to a distillation column 2.
Then, the raw material supplied into the distillation column 2 is separated into gas and liquid, and the liquid is sent to the bottom 3 of the column, and the gas is sent to the top 4 of the column. The top component from this column top 4 is condensed by a condenser 5, a part of which is refluxed to the distillation column 2 via a reflux tank 6, and the remainder is extracted to the outside.

In this manner, the distillate is separated into a distillate from the top 4 and a bottom liquid from the bottom 3 in the distillation column 2 and extracted to the outside.

The distillate from the column top 4 and a part of the bottom liquid from the column bottom 5 are each refluxed to the distillation column 2 for controlling the column temperature, and the bottom liquid is, for example, It is heated by a reboiler 7 in the form of a tube furnace. The heating source of this reboiler 7 is fuel gas 8, and the flow rate of this fuel gas 8 is measured and controlled by an orifice. Further, this fuel gas 8 is composed of a mixed gas containing two or more types of components.

However, in this method of controlling a distillation column, since the temperature at the top 4 of the distillation column 2 and the reflux flow rate are controlled in a cascade, and the heating furnace fuel is kept constant, mutual interference between the top and bottom of the column occurs. This makes stability difficult.

The fuel gas composition fluctuates, causing a large disturbance.

As a result, it was necessary to operate with excess reflux and excess fuel for stability and safety.

In view of the above-mentioned points, the present invention provides a method for controlling a distillation column that is designed to solve the above-mentioned problems.The present invention aims to eliminate calorie changes due to fluctuations in the composition of fuel gas supplied to the reboiler and measurement errors of flowmeters. This is a method of controlling a distillation column by analyzing fuel gas.
The fuel gas is corrected to a normal state based on the correlation between fluctuations in gas components, calorie changes, and flow rate changes.

Hereinafter, embodiments of the present invention will be described in detail based on the drawings.

First, before describing embodiments, the principle of the present invention will be described in order to facilitate understanding of the present invention. Changes in the fuel gas composition cause changes in the amount of heat supplied to the distillation column due to the two processes of calorie change and density change of the fuel gas, resulting in a disturbance. Therefore, in the present invention, the gas composition is detected and the disturbance in the amount of heat supplied due to these two influences is corrected.

First, the fuel gases supplied to the reboiler include hydrogen (H ₂ ) and methane (CH ₄ ), hydrogen (H ₂ ) and carbon monoxide (CO), and hydrogen (H ₂ ) and methane-based hydrocarbons (C _o H _2o+2 ), mixed gases such as hydrogen (H ₂ ) and ethylene hydrocarbons (C _o H _2o ), or methane (CH ₄ ) and ethane (C ₂ H ₆ ), methane (CH ₄ ) and propane (C ₃ An example of a fuel gas is methane (hereinafter referred to as _{CH 4 )} and hydrogen (H ₂ ) as the main components, with a small amount of carbon monoxide (CO),
Examining laboratory data for mixed gases containing nitrogen (N ₂ ), such as coke oven gas (COG), the calorific value H _s per unit of fuel gas is an explanatory diagram showing the relationship between CH ₄ concentration and heat input. As shown in Figure 2, it was found that it was a function of CH ₄ concentration. Furthermore, when measuring flow rate using an orifice, measurement errors occur due to changes in density due to changes in composition.

Here, fluctuations in composition often occur when byproduct gases such as coke oven gas (COG), converter gas (LDG), oil refinery exhaust gas, and petrochemical exhaust gas are used as fuel gas. Yes, this usually occurs in products with small buffer tank capacities. In such cases, it is possible to control the amount of heat supplied by measuring the total composition of the fuel gas and calculating the calorific value, or by conducting a calorific value test from time to time, but such measurements or tests are quite time-consuming. This poses a problem in that it is time-consuming and cannot handle fuel gas whose composition often changes.

Therefore, the present invention focuses on one combustible gas that accounts for more than half of the calorific value, and attempts to control it by measuring this. The component of interest is one whose calorific value accounts for the majority of the calorific value of the fuel gas, and preferably has a significantly high unit calorific value and has a high density. The concentration of this component of interest is measured directly or indirectly, and the density of the fuel gas is calculated from this. There are two ways to measure this component of interest: direct and indirect. Direct measurement is performed using infrared rays (IR), gas chromatography (GC), etc.; indirect measurement is In the case of a two-component system, other components may be measured, or if it is possible to measure only nonflammable gas, it is sufficient. For example, when the gas is composed of N ₂ , CH ₄ , and H ₂ and the CH ₄ /H ₂ ratio is constant.

Now, if a fluid with a density γ' is detected as a measured flow rate V in an orifice with a designed density γ, the corrected flow rate V' can be expressed as V'=V√' (1). Note that in this case, the temperature and pressure of the fuel gas supplied to the reboiler can be considered to be constant, and no correction for this is necessary.

In this way, CH ₄ of the fuel gas for reboiler heating
The concentration is measured, and the measured flow rate is corrected by the change in the density of the reboiler heating fuel gas that accompanies the change in the CH ₄ concentration using the above equation (1).

The calorific value H _s per unit volume of the above fuel gas is
This is a function of the CH ₄ concentration, and when measuring the flow rate using an orifice, measurement errors occur due to changes in density due to changes in the composition. Therefore, changes in the gas composition are a function of the control system's calorific value H _s → H
_s ′, density γ → γ′, the amount of heat supplied to the distillation column Q′ can be estimated as Q′=ηH _s ′V′=ηH _s ′V√′ ……(2). However, η is the efficiency of the heating furnace.

Then, from the concentration of the component of interest measured in the process of directly or indirectly measuring the concentration of the component of interest described above and calculating the density of the fuel gas to be supplied to the reboiler, Calculate the calorific value H _s ' from the following equation.

H _s ′=f×[C′]/[C]×H _s ……(3) However, [C] is the standard (design) concentration of the component of interest, [C′] is the measured concentration of the component of interest, H _s is the calorific value when [C] is present, and f is a constant. When the changes in the calorific value H _s and the density γ due to changes in the gas composition are plotted with respect to the CH ₄ concentration and H _s ′/√, a good linearity is obtained as shown in Figure 2. You can get sex. In addition, in this figure 2, the mark ・ is CH ₄
It shows the calorific value H _s with respect to the volume concentration (horizontal axis) of CH 4 , and the Γ mark shows H _s ′/√′ with respect to the volume concentration of CH ₄ (horizontal axis).

Therefore, H _s ′/√′=α[CH ₄ ]...(4) where α is a proportionality constant and [CH ₄ ] is the CH ₄ concentration.

Then, the fuel flow rate Vd converted to the standard composition is V _d = [CH ₄ ] / [CH ₄ ] _d V ... (5) where, V is the red correction flow rate, [CH ₄ ] _d is the standard CH ₄
It is concentration.

It is expressed as

In this way, the calorific value per unit volume of the fuel gas for heating the reboiler as the CH ₄ concentration changes, and therefore the amount of heat supplied to the distillation column changes based on the result of the correction using equation (1) above. In order to eliminate changes in the amount of heat supplied due to changes in CH ₄ concentration, the flow rate of the reboiler fluid is controlled according to equation (5) above so that it is the same as when a specified flow rate is given at a certain standard CH ₄ concentration. It is composed of

In addition, the relationship between CH ₄ concentration and gas density in the fuel gas actually supplied to the reboiler has an inverse characteristic, CH ₄
Since the relationship between concentration and calorific value is a positive characteristic, as shown in Figure 3, which is an explanatory diagram showing the error in heat input due to composition variation, two errors, area A of concentration change and area B of calorie change, are added. This may result in a large disturbance.

The control method of the present invention takes into account the fact that the above-mentioned calorific value H _s per unit volume of fuel gas is a function of the CH ₄ concentration, and that measurement errors occur due to density changes due to composition fluctuations in flow rate measurement using an orifice. Two corrections are made above using CH ₄ concentration (5)
Taking advantage of the fact that it has a simple form such as the formula,
The value V of the fuel gas flow rate is V _d =β [CH ₄ ]V (6) where β is a proportionality constant.

After correcting (V _d ), the flow rate is controlled (≒ heat supply amount control).

That is, the flow rate of the fuel gas supplied to the reboiler is corrected by the above-mentioned equations (1) and (3) to keep the amount of heat supplied constant.

To explain this in detail, first, the density of the fuel gas supplied to the reboiler changes as the composition changes.

On the other hand, when measuring the flow rate using an orifice, the measured value will change if the density of the fluid to be measured changes even if the flow rate of the fluid to be measured does not change. This is the measurement error.

Therefore, in an orifice with a design density γ, when a fluid with a density γ' is detected as a flow rate V, the corrected flow rate V' must be used as the true flow value. Then, this corrected flow rate V' is calculated from the above (1)
It is given by Eq.

Next, if the composition changes, the calorific value H _s per unit volume also changes. Therefore, when the composition changes, the amount of heat Q supplied to the distillation column changes.

Now, let Q _s be the amount of heat supplied when the design density is γ and the fuel flow rate is V _d , then Q _s = ηH _s V _d Here, H _s is the calorific value per unit volume at the design density γ. be.

On the other hand, if the density becomes γ', the amount of heat Q' supplied to the distillation column becomes Q'=ηH _s 'V _d .

However, in the case of an orifice with a design density γ, the measured value of the flow rate is not detected as the fuel flow rate V _d for the density γ' but as the uncorrected flow rate V. For this reason,
The measured value must be corrected and used to calculate the amount of heat supply. The corrected flow rate is is given by

becomes.

On the other hand, according to actual measurements, H _s ′/√′ is proportional to the concentration of CH ₄ (Vol%).

In other words, as in equation (4) above, H _s ′/√′
= α [CH ₄ ].

Therefore, when the reference composition = reference concentration [CH ₄ ] _d (density at this time = γ), H _s /√=α [CH ₄ ] _d , so, From this, H _s ′ is determined and the amount of heat supplied Q′ is determined as Q′=η·H _s ·[CH ₄ ]/[CH ₄ ] _d ·V.

From this, in order to maintain the amount of heat supplied to the same amount as when Q _s = ηH _s V _d regardless of the concentration change, ηH _s・[CH ₄ ]/[CH ₄ ] _d V=η・The flow rate may be controlled according to H _s ·V _d .

And the above-mentioned equation (5) is derived from this.
In addition, the above equation (6) is the standard CH ₄ concentration [CH ₄ ] _d of equation (5).
Since is a constant value, it is expressed by a proportionality constant β.

In short, the measured value V by the flowmeter is [CH ₄ ] _d / [CH ₄ ]・V _d = V _d /β [CH ₄
] The flow rate is controlled so that

Next, an example of a concrete implementation method of the present invention will be explained with reference to FIG. In the embodiment shown in FIG. 4, the above-mentioned heat supply amount is given at a constant value due to changes in the feed amount (raw material supply amount), and the bottom temperature changes, so such disturbances are avoided. This shows an example in which temperature changes are also controlled by controlling the fuel gas supplied to the reboiler.

In FIG. 4, which shows a configuration example of a composition compensation controller, solid lines indicate calculation blocks of hard wiring, and dotted lines indicate soft wiring. 9 is a flow rate transmitter, 10 is a concentration meter,
The flow rate transmitter 9 sends an air pressure signal X corresponding to the flow rate, and the concentration meter 10 sends its output signal Y to the calculation block of slot "1".
This calculation block calculates K ₁ √Y+K, and sends the fuel gas flow rate PV as an output to the calculation block of slot "2". On the other hand, the temperature information from the bottom temperature detection section 11 is sent to a control circuit that keeps the temperature of the calculation block in slot "3" constant, and is compared with the set value SP to perform bias control and set the required set value SP. The data is sent to the slot "2" calculation block as a result.

The calculation block of this slot “2” calculates the required
PID operation (proportional, integral, differential operation) is performed, and the flow control valve 12 is used as a control signal for the fuel gas valve.
and control it. In addition, in FIG. 4, 13 (FIC) indicates a flow rate indicator controller, and 14 (TIC) indicates a temperature indicator controller.

FIG. 5 is a block diagram showing an embodiment of the present invention in which a cross controller is introduced to reduce mutual interference between the top and bottom of the tower.

In FIG. 5 showing a configuration example of an improved distillation column control system, the same reference numerals as in FIGS. 1 and 4 indicate corresponding parts, and 15 indicates the signal As shown in the calculation block of slot “ ₁ ” in Fig. 4, the signal Y of
This is a multiplier that performs multiplication by +K, and these correspond to the calculation block of slot "1" in FIG. The temperature indicator controller 14a, which receives temperature information from the bottom temperature detection section 11 at the bottom 3 of the distillation column 2, corresponds to the calculation block of slot "3" in FIG. Also, the fuel gas flow rate PV from the multiplier 15 and the temperature indicator controller 14
The flow rate indicator controller 13a which receives the set value SP from a and controls the flow rate regulating valve 12 by its output corresponds to the calculation block of slot "2" in FIG.

Reference numeral 16 denotes a level indicator controller, which is configured to control a flow rate regulating valve 17 that adjusts the flow rate of the bottom liquid extracted from the bottom part 3 of the column according to its output. Reference numeral 18 denotes a level controller; 19 adds the output of a temperature indicator 14b which inputs temperature information from the tower top temperature detection section 20 in the tower top section 4, the output of the multiplier 15, and the output via the cross controller 21; The adder is configured such that its output is sent to the flow indicator controller 13b.
Note that 22 is a gas analyzer.

Next, the operation of the embodiment shown in FIG. 5 will be explained. First, in this embodiment, the column top 4 of the distillation column 2 that controls the distillate is mainly considered, and the change in fuel gas flow rate is dynamically compensated for and then added to the set value of the reflux flow rate. The cross controller 21 is of a first-order delay/progression type from the viewpoint of practical effect and ease of adjustment.

In the case of an actual process, complete non-interference is unlikely, so in order to avoid unnecessary changes in the fuel flow rate,
To control the tower bottom temperature, an algorithm is used to sample the tower bottom temperature at regular intervals, make a judgment based on the value and the rate of change, and finely adjust the flow rate of the fuel gas supplied to the reboiler 7.

Next, to explain this control method, first, to detect fuel gas composition fluctuations, as mentioned above, the compositions whose compositions fluctuate are coke oven gas (COG), converter gas (LDG), and petroleum refinery exhaust gas. The main components are secondary gases such as petrochemical exhaust gas, and the components of interest are those with a high calorific value,
Preferably, the calorific value is determined by this concentration,
That is, the method is focused on those that have a large amount of heat, a significantly high calorific value, and a high density difference.
There are two ways to measure this component of interest: direct and indirect. Direct measurement is performed using infrared rays (IR), gas chromatography (GC), etc.; indirect measurement is In the case of a two-component system, other components may be measured, or if it can be measured only by measuring a nonflammable gas, for example, a gas consisting of N ₂ , CH ₄ , H ₂ ,
For example, when the CH ₄ /H ₂ ratio is constant.

As shown in Figure 4 above, the control loop consists of three calculation blocks (referred to as slots).
The connection/separation between each slot is determined by the control mode of each slot (cascade,
It can be easily carried out by switching between automatic and manual.

The gas composition fluctuation correction was configured as shown in Fig. 4 above, but in terms of operation, a Normal time slot "1" (multiplication): Cascade, pre-correction flow rate V and CH ₄ concentration display, correction value ( correction flow rate)
Output _Vd .

Slot “2” (PID): Cascade, _Vd display, flow rate control.

b When the infrared analyzer is stopped (when the byproduct gas generation plant is stopped, during analyzer maintenance, etc.) Slot “1”: Auto, set to standard CH ₄ concentration [CH ₄ ] _d .

Slot “2”: Same as a.

Since correction can be easily executed/stopped simply by switching the control mode of the correction calculation slot, the operator can accept the advanced control without any discomfort. Furthermore, non-interference control can be added/disconnected in a similar manner depending on the start-up, shutdown, etc. of the plant. Additionally, each slot is equipped with alarms, output limiters, and other functions to ensure safety when testing new control methods.

6a to 6g are waveform diagrams shown in comparison with the conventional method to clarify the effects of the present invention, and show the results of implementing advanced control for energy saving as explained above. The left side (case 1) is the conventional control method, and the right side (case 2) is the control method according to the present invention.

First, a and b show the CH ₄ concentration and fuel gas flow rate, case 1 (conventional) on the left and case 1 on the right.
Similar changes in CH ₄ concentration are observed in case 2 (invention). And the fuel gas flow rate shown in b is case1
It can be seen that in case 2, it is constantly corrected according to the CH ₄ concentration (≒ gas composition).

Next, c and d indicate the heating furnace temperature and the exhaust gas O ₂ concentration, and compared to the conventional case 1, the variation width in case 2 is about 1/1 for both the heating furnace tube wall temperature and O ₂ concentration. 3, making it possible to operate at low O ₂ %.

Next, e, f, and g indicate the reflux flow rate and the temperature at the bottom/top of the column, and the operation of the distillation column is stabilized and controlled by heat supply in the heating furnace, stabilization of combustion, and non-interference control. sex has improved. This means that the variation in the reflux flow rate shown in e is about 1/6 in case 2, which shows the method of the present invention, compared to case 1, which shows the conventional method, and
This is clear from the fact that the temperature fluctuations at the top and bottom of the column shown in g and f are about 1/3.

Although the present invention has been described above using an example in which the flow rate of fuel gas supplied to the reboiler is measured using an orifice, other flow rate measurement methods may be used.

As explained above, the present invention has the following many advantageous features compared to conventional control methods of this type.

First, the control characteristics have been significantly improved by introducing new sensors and control systems. Second, the relationship between product specifications, motion costs, and controllability in the distillation column is such that improved controllability leads to energy saving effects and improved yields. Thirdly,
It has various advantages, such as improving the efficiency of the distillation column (η in equation (2)) due to the stability of the combustion system and achieving energy-saving effects.

As described above, the method of the present invention has greater effects than conventional methods and is unique as a method for controlling a distillation column.

[Brief explanation of the drawing]

Figure 1 is a block diagram to explain an example of a conventional distillation column control method, Figure 2 is an explanatory diagram showing the relationship between methane concentration and heat input, and Figure 3 shows errors in heat input due to composition changes. 4 is a block diagram for explaining an embodiment of the method for controlling a distillation column according to the present invention, FIG. 5 is a block diagram showing an embodiment of the present invention, and FIG. 6 is a block diagram showing the effects of the present invention. It is a waveform chart shown in comparison with a conventional method for clarity. 1... Raw material tank, 2... Distillation column, 13, 13a,
13b...Flow rate indicator controller, 14,
14a, 14b...temperature indicator controller, 15...multiplier, 19...adder.

Claims (1)

[Claims] 1. Measure the flow rate of fuel gas supplied to the reboiler of a distillation column equipped with a reboiler that heats with fuel gas containing two or more types of combustible gases, and based on the measured flow rate of fuel gas. In the method of controlling the amount of heat supplied to the reboiler, the focus is on the combustible gas in the fuel gas that accounts for more than half of the calorific value, and the concentration of this component of interest is directly or indirectly measured. When there is a difference between the first step of calculating the density of fuel gas and the density γ′ calculated in this first step and the design density γ when measuring the flow rate of fuel gas, V′=V√′ ( (where V is the measured flow rate and V' is the corrected flow rate); and a second step in which the measured flow rate is corrected using the formula; and a unit of fuel gas to be supplied to the reboiler from the concentration of the component of interest measured in the first step. Calorific value H _s ′ per volume is calculated as H _s ′=f×[C′]/[C]×H _s (where, [C] is the standard (design) concentration of the component of interest, and [C′] is the concentration of the component of interest. The measured concentration, H _s is the calorific value at [C], and f is a constant). A method for controlling a distillation column comprising adding a fourth step of making the amount of heat supplied constant by adding the correction obtained by the method to the flow rate control of fuel gas. 2. The method for controlling a distillation column according to claim 1, wherein the fuel gas supplied to the reboiler contains hydrogen and methane as main components, and the component of interest is methane. 3. A method for controlling a distillation column according to claim 1 or 2, wherein the flow rate of fuel gas supplied to the reboiler is measured using an orifice.