JPH09179603A - Device for determining product gas generation schedule for air separator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the cost of product gas by determining the generation quantity of product gas from plural air separators so that the inner pressure of oxygen gas is not lowered less than a lower limit and is reduced as less as possible. SOLUTION: In an oxygen plant provided with plural air separators 12 for separating and generating respective production gas components, i.e., oxygen gas, nitrogen gas and liquid algon, from air and an oxygen gas holder 18 for holding generated oxygen gas, the generation quantity of production gas in each air separator 12 is determined. In this case, the generation quantity of production gas in each air separator 12 is determined so that the predicting inner pressure of the holder 18 from current time up to prescribed time which is calculated from the detection value of the inner pressure of the holder 18 at current time, the predicted using quantity of oxygen gas and the changed quantity of oxygen gas generation so as to be included within prescribed upper and lower limits and reduced as much as possible while considering the limit of generation quantity change speed in each air separator 12.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a plurality of air separation devices for separating and generating product gas of oxygen gas, nitrogen gas and liquid argon from air, and an oxygen gas holder for storing oxygen gas generated from these air separation devices. The present invention relates to a method for determining a product gas generation schedule for an air separation device, which is suitable for application when determining the amount of product gas generated by each air separation device in an oxygen factory equipped with.

[0002]

2. Description of the Related Art Generally, an oxygen factory is attached to a steel mill, and as shown in FIG. 4, the oxygen factory usually has n compressors each having an air compressor 10. An (n plant) air separation device 12 and a plurality of oxygen gas compressors 14 for compressing and sending out oxygen gas generated from these air separation devices 12 are provided. Then, the air separation device 12 produces each product gas of oxygen gas, nitrogen gas and liquid argon by separating the compressed and liquefied air. However, in FIG. 4, only the oxygen production line is shown, and the nitrogen and argon lines are omitted.

When these product gases are produced in the above oxygen factory, most of the cost required therefor is the air compressor 1.
It is the amount of power used to drive 0. In a steel mill, the maximum use destination 16 of oxygen gas is a converter, and since this converter is a batch operation, the fluctuation of oxygen gas demand is large, and the amount of oxygen gas demanded is large. Since the generation capacity is smaller than that of nitrogen gas and liquid argon, the product gas generation amount by each air separation device is often determined mainly with oxygen gas.

Since the amount of oxygen gas used changes abruptly and irregularly, a large-capacity oxygen gas holder 18 is usually provided between the air separation device and the oxygen destination 16 such as a converter in order to mitigate the change. Is installed. Also, the air separation device 12
As described above, a plurality of oxygen gas compressors 14 are installed between the oxygen gas holder 18 and the oxygen gas holder 18. In these oxygen gas compressors 14, the discharge pressure by each compressor can be lowered by lowering the pressure in the oxygen gas holder 18, so that the power consumption can be reduced accordingly.

On the other hand, the internal pressure of the oxygen gas holder 18 is
There are upper and lower limits due to facility restrictions, and the upper limit is determined by the pressure resistance of the holder, and the lower limit is determined by the required pressure of the place of use 16. Therefore, when the oxygen gas generation amount becomes excessive and the internal pressure of the oxygen gas holder exceeds the upper limit, as shown in FIG. 5, the excess oxygen gas is automatically diffused into the atmosphere. There is. When the excess oxygen gas is diffused in this manner, the production cost of the oxygen gas is lost. On the contrary, when the oxygen generation amount is insufficient and the oxygen gas holder internal pressure is lower than the lower limit, the liquid acid tank 22 in which oxygen liquefied by the liquefaction device 20 (hereinafter, also referred to as liquid acid) is stored, The shortage is supplied to the oxygen gas holder through the evaporator 24. in this case,
Since it is necessary to always store the liquid acid in excess of the shortage, the production cost of the liquid acid is similarly lost.

Therefore, in the oxygen factory, it is important to determine the product gas generation amounts of the plurality of air separation devices so that the pressure inside the oxygen gas holder is kept within the upper and lower limits and the pressure becomes as low as possible. . However, although the operation of changing the product gas generation amount based on the determination is automated by some of the air separation devices, the change speed is limited.

Generally, in an oxygen factory equipped with a plurality of air separation devices, when determining the product gas generation amount of each air separation device, there is an upper limit and a lower limit of the generation capacity of each air separation device. , The oxygen gas generation amount of each air separation device is determined by mathematical programming so that the product gas generation amount meets the demand amount at each time and the electric power consumption of the air compressor is minimized. There is.

As the mathematical programming method used for this determination, as disclosed in Japanese Examined Patent Publication No. 1-28313, the power consumption and the nitrogen generation amount are given by a linear equation (or a non-linear equation) of the oxygen generation amount, and in the future. Under the constraint that the product gas generation amount at each time of the above satisfies the demand forecast at that time and the upper and lower limits exist in the capacity of each air separation device, the linear programming is performed with the power consumption of the air compressor as the objective function. There is a method of determining the oxygen generation amount by the method (or non-linear programming method). In this method, the oxygen demand amount is corrected using the oxygen gas holder internal pressure at the current time and the target pressure at the next time, so that the target pressure is approached.

Further, Japanese Patent Publication No. 58-3189 requires that the change in the holder pressure be predicted from the detected value of the oxygen gas holder pressure and the amount of oxygen gas used in the converter, and the average value should be brought close to the target value. A method is disclosed in which, after determining an appropriate oxygen gas generation amount, it is checked whether the capacity of each air separation device or the demand of other product gas is satisfied, and if not, the operator is notified.

[0010]

However, in Japanese Patent Publication No. 1-283313, when the total amount of oxygen gas generated is determined, the pressure in the holder at the next time is only taken into consideration, and the air separation device is used. The changeable rate of oxygen gas generation is not taken into consideration. Therefore, for example, as shown in FIG. 6, the next time t + after one time has elapsed from the current time t
Even if the holder internal pressure in 1 reaches the target value, if the oxygen demand amount is large thereafter, the increase rate of the air separation device becomes a bottleneck to restore the oxygen gas holder internal pressure. There is a problem that the supply of liquid acid may be necessary because the situation that cannot be done occurs and the pressure thereof eventually falls below the lower limit.

Further, in the method of controlling the average value of the oxygen gas holder internal pressure to a target value between the upper limit and the lower limit disclosed in Japanese Patent Publication No. 58-3189, the target pressure of the oxygen gas holder is set as shown in FIG. When trying to approach the lower limit as much as possible, the lower limit may be temporarily exceeded, and there is a problem that liquid acid supply similarly occurs. Also, in this method, after determining the amount of oxygen gas generated, it is checked whether the capacity of each air separation device or the demand of other product gas is satisfied, so that the optimum oxygen is taken into consideration in consideration of restrictions in advance. There is also a problem that an effective solution is rarely obtained as compared with the technique disclosed in Japanese Patent Publication No. 1-283313 for determining the generation amount.

The present invention has been made to solve the above-mentioned conventional problems, so that the amount of product gas generated by a plurality of air separation devices can be made as low as possible without the oxygen gas holder internal pressure falling below the lower limit. It is an object of the present invention to provide a method of determining a product gas generation schedule for an air separation device, which can determine the product gas production cost and reduce the production cost of the product gas.

[0013]

The present invention separates oxygen, nitrogen and liquid argon product gases from air,
In an oxygen factory equipped with a plurality of air separation devices and an oxygen gas holder that stores the oxygen gas generated from these air separation devices, the product gas of the air separation device that determines the amount of product gas generated by each air separation device In the generation schedule determination method, the oxygen gas holder from the current time to a predetermined time calculated from the detected value of the oxygen gas holder internal pressure at the current time, the predicted use amount of oxygen gas after the current time, and the oxygen gas generation change amount after the current time. Determine the amount of product gas generated by each air separation device in consideration of the limit of the rate of change of the amount generated by each air separation device so that the predicted pressure falls within the prescribed upper and lower limits and is as low as possible. By doing so, the above problems are solved.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described in detail below with reference to the drawings.

The oxygen factory to which this embodiment is applied is substantially the same as that shown in FIG. FIG. 1 shows an outline of a processing procedure in the schedule determination method of this embodiment, and various calculation processings described in detail below are executed in accordance with this procedure.

First, the meanings of symbols used in the following description will be defined.

T: current time n: number of air separation devices i: number of air separation devices, i = 1, 2, ..., N N: time range to be considered in optimization j: optimization Time intervals to be considered when j = 1, 2, ...
, N Xi (t + j-1): Oxygen gas generation amount of the i-th air separation device at time t + j-1 Y (t): Detected value of oxygen gas holder internal pressure at the current time V: Change in unit flow rate per unit time Coefficient Yp (t + j): Y (t) as a starting point, which is converted to the change in oxygen gas holder pressure afterward, at time t + when only the planned oxygen gas usage after the current time is fed forward
Predicted pressure in holder at j: Y (t + j): Predicted pressure in holder at time t + j in consideration of change of Xi (t + j-1) UDi: Upper limit of oxygen gas generation rate change rate of i-th air separation device Li: i-th Lower limit of oxygen gas generation amount of the air separation device Ui: upper limit of oxygen gas generation amount of the i-th air separation device LY: lower limit of oxygen gas holder internal pressure α (t + j): lower limit compensation value of oxygen gas holder internal pressure UY: oxygen gas holder the upper limit of Ni pressure (t + j-1): nitrogen gas generation amount of i-th air separation unit Ai (t + j-1) : liquid argon generation of i-th air separation unit _{a i, b i, c i} , d i , e _i, f _i: the model parameters of the i-th air separation unit LNi: i-th nitrogen gas generation amount limit UNi air separation device: i-th nitrogen gas generation amount upper limit LAi air separation device: i-th Liquid argon generation lower limit of gas separator UAi: Liquid argon generation upper limit of i-th air separator LYN (t): Total amount of nitrogen gas that must be generated within N hours after the present time t LYA (t ): Total amount of liquid argon that must be generated within N hours after the current time t Wi (t + j-1): Electric power consumption of the air compressor of the i-th air separation device Hereinafter, this embodiment will be described in detail. To do. As defined above, the number of air separators is n, the current time is t, the oxygen gas generation amount of the i-th air separator at time t + j-1 is Xi (t + j-1), and the oxygen gas holder at time t + j. The internal predicted pressure is Y (t + j) (i = 1,
2, ..., N, j = 1, 2, ..., N). Here, both the sampling period and the control period are set to 1 for simplicity.

First, the pressure Y in the holder at the current time
Observe (detect) (t). Next, using Y (t) as a starting point, the oxygen gas holder internal pressure when the oxygen gas generation amount Xi (t-1) of the previous cycle (one cycle before the current time) is not changed at all is Calculate using the estimated amount of oxygen gas used. That is, starting from Y (t), the time t at which only the planned future oxygen gas usage is fed forward
The predicted pressure in the holder at + j is Yp (t + j). Therefore, if the oxygen gas generation amount is actually constant from time t-1, the following equations (1) and (2) are established.

Y (t + j) = Yp (t + j) (1) Yp (t) = Y (t) (2)

In this equation (1), the oxygen gas generation amount after the present time for n plants to be determined: Xi (t), X
When the effect of the change in the generation amount at each time obtained from i (t + 1), ..., Xi (t + j-1) on Y (t + j) is added, the following equation (3) is obtained (k = 1, k = 1, 2, ...,
j). Here, V is a coefficient for converting a change in the unit flow rate into a pressure change after a unit time, and is determined by the capacity of the holder.

[0021]

[Equation 1]

When the change amount ΔXi (t + k) of the oxygen gas generation amount at each time is set as in the following equation (4), the above equation (3) can be expressed by the following equation (5). Then, this equation (5) is the optimization calculation shown in FIG.
It is used as a pressure model in the oxygen gas holder.

ΔXi (t + k) = Xi (t + k) −Xi (t; k−1) (4)

[0024]

[Equation 2]

In the optimization calculation, ΔX i (t + k) is calculated by using the equation (5) so that the optimum predicted pressure Y (t + j) in the holder can be obtained. From this, the oxygen gas generation amount at each time is calculated. However, at that time, as shown below,
(A) Oxygen gas generation rate change speed, (B) Oxygen gas generation rate, (C) Oxygen gas holder internal pressure, (D) Nitrogen gas generation rate, Liquid argon generation rate, (E) Nitrogen gas generation rate total,
Consider each constraint for each total liquid argon yield. These constraints are determined by the performance and efficiency of the device.

(A) Constraints by upper and lower limits of oxygen generation rate change rate (i × N): −UDi ≦ ΔXi (t + j−1) ≦ UDi (6) where UDi ≧ 0 (B) oxygen generation rate Upper and lower limit constraints (i × N): Li ≦ Xi (t + j−1) ≦ Ui (7)

This is done using ΔX i (t + j-1),
It can be rewritten into N constraints as follows. However, I _NN represents a unit matrix of size N × N.

[0028]

(Equation 3)

(C) Upper and lower limit constraints on predicted pressure in oxygen gas holder (N): LY + α (t + j) ≦ Y (t + j) ≦ VY (9)

In the equation (9), α is a lower limit compensation value for compensating the prediction error of Y (t + j). Y (t) feeds back the detected value, but Y (t + j) is always given as a prediction, and therefore always contains an error.
That is, the constraint expression that does not include the compensation value α is satisfied only when the predictions are completely matched. Therefore, by adding the compensation value α to the lower limit LY, the lower limit is not exceeded even if there is an error. Since the magnitude of the compensation value α is determined by the magnitude of the prediction error, it is small at the time when the prediction error is the closest to the current time and is large in the future several hours later. Here, such a compensation value is set in order to reduce the internal pressure of the oxygen gas holder as much as possible. On the contrary, when the pressure inside the holder exceeds the upper limit, it is automatically diffused to the outside without delay time, so the compensation value is not used.

(D) Generation capability (upper and lower limits) of nitrogen gas generation amount and liquid argon generation amount (2i × N pieces): Nitrogen gas generation amount and liquid argon generation amount and oxygen gas generation amount of each air separation device The relationship is represented by the following linear expressions of the nitrogen gas generation model of the equation (10) and the liquid argon generation model of the equation (11).

Ni (t + j-1) = _ai Xi (t + j-1) + b _i (10) Ai (t + j-1) = c _i Xi (t + j-1) + d _i (11)

The upper and lower limits of the nitrogen gas and liquid argon generation capacities are set by the following equations (12) and (13), respectively.

LNi ≤ Ni (t + j-1) ≤ UNi (12) LAi ≤ Ai (t + j-1) ≤ UAi (13)

(E) Constraints for satisfying demand for total nitrogen gas generation amount and total liquid argon generation amount (constraint of supply and demand balance) (two): The use amounts of nitrogen gas and liquid argon are stable as compared with oxygen gas. Therefore, it is not necessary to care about the capacity of the capacity system (gas holder and liquid tank) in a short cycle like oxygen gas. Therefore, the total amount of nitrogen gas that must be produced within N hours after the current time t is LYN.
(T), the demand for liquid argon is LYA (t),
The amount generated during that time may satisfy the following expressions (14) and (15). From the relationship of the expressions (10) and (11), the restrictions on the nitrogen gas and the liquid argon are the same as the upper and lower limit restrictions of Xi shown in the expression (8).
Each can be rewritten into N ΔX i constraints.

[0036]

(Equation 4)

Under the above constraints, ΔXi (t), ΔXi (t + 1), ..., Which minimize the following objective function:
Perform an optimization calculation to determine .DELTA.Xi (t + N-1).

As the objective function, a linear sum of "absolute value of pressure in oxygen gas holder" and "power consumption in air compressor" given by the following equation (16) is used. Therefore, the amount of electric power used in each air compressor is represented by the linear form of the oxygen gas generation amount (air compressor electric power consumption model) given by the following equation (17).

[0039]

(Equation 5)

[0040] Wi (t + j-1) = e i Xi (t + j-1) + f i ... (17)

The problem presented under the above conditions can be solved by linear programming. This optimization calculation process corresponds to determining the oxygen generation amount at each time when the objective function becomes the minimum under various constraints including the limit of the oxygen gas generation amount changing speed. Then, among the obtained solutions, .DELTA.Xi (t) (= Xi (t) -Xi (t-
Only 1)) is actually given as a change amount with respect to the oxygen gas generation amount of the i-th air separation device. The pressure in the oxygen gas holder is controlled by repeating the above calculation every cycle.

[Embodiment] In the present embodiment, the number of air separation devices i = 3 and the target time N =, according to the procedure described so far.
FIG. 2 shows the result of 30 (control cycle: 10 minutes).

In this embodiment, it can be solved as a mathematical calculation problem with 90 optimization variables and 392 constraints. As shown in FIG. 2, at the current time t, the air separation device 12
When the amount of oxygen gas generated by X is fixed at X1 (t-1), the oxygen gas holder internal pressure is predicted to exceed the upper limit UY near time t + N as shown by the curve of Yp (t + j). It can be seen that the curve representing Y (t + j) after optimization calculated at each time according to the invention falls within the upper and lower limits and can be controlled to a very low value. Further, it can be seen from FIG. 3 that after the application of the present invention, as shown by the curve (A), the internal pressure of the oxygen gas holder approaches the lower limit on average, and is not below the lower limit. Also, focusing on the point of time T, if the only goal is to approach the lower limit at the next time, the lower limit can be further approached as shown by the curve (B), but in this case, the time T +
Since the predicted pressure lowering speed due to the oxygen usage amount after 1 is fast, even if the oxygen generation amount is increased at the maximum speed, it is not in time, so that the pressure in the holder falls below the lower limit. On the other hand, in the case of the optimum control according to the present invention, the lower limit correction value α is set, and the restriction of the generation rate changing speed is taken into consideration. Since the temperature is increased, it can be seen that the occurrence of the above phenomenon that occurs when the lower limit is approached at time T is prevented.

As described in detail above, according to the present embodiment,
By considering the amount of product gas generated by multiple air separation devices, considering the capacity, efficiency, and generation rate change rate of each air separation device, the oxygen gas holder internal pressure stays within the prescribed upper and lower limits, and the oxygen content is as low as possible. It could be determined to be the pressure in the gas holder. As a result, the power consumption of the air compressor and the oxygen compressor can be reduced, and as a result, the cost reduction of the product gas can be achieved.

The present invention has been specifically described above, but the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the scope of the invention.

For example, the restrictions on the nitrogen gas and the liquid argon may be the restrictions considering the former gas holder internal pressure or the latter liquid tank level, like the oxygen gas.

[0047]

As described above, according to the present invention,
The amount of product gas generated by the plurality of air separation devices can be determined so that the internal pressure of the oxygen gas holder does not fall below the lower limit and is as low as possible, and the manufacturing cost of the product gas can be reduced.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing an outline of a processing procedure at the time of control according to an embodiment of the present invention.

FIG. 2 is a diagram schematically showing a control result according to an embodiment of the present invention.

FIG. 3 is a diagram showing the results of an example according to the present invention.

FIG. 4 is a block diagram showing an outline of the equipment configuration of an oxygen factory.

FIG. 5 is a diagram showing changes in the oxygen gas holder pressure when the upper and lower limits are exceeded.

FIG. 6 is a diagram showing the problems of the conventional method.

FIG. 7 is another diagram showing the problems of the conventional method.

[Explanation of symbols]

 10 ... Air compressor 12 ... Air separation device 14 ... Oxygen gas compressor 16 ... Where to use 18 ... Oxygen gas holder 20 ... Liquefaction device 22 ... Liquefied acid tank 24 ... Evaporator

Claims (2)

[Claims] 1. Oxygen comprising a plurality of air separation devices for separating and generating each product gas of oxygen gas, nitrogen gas and liquid argon from air, and an oxygen gas holder for storing oxygen gas generated from these air separation devices. In the method of determining the product gas generation schedule of the air separation device that determines the amount of product gas generated by each air separation device at the factory, the detected value of the oxygen gas holder internal pressure at the current time, the predicted amount of oxygen gas used after the current time, and the current The predicted pressure in the oxygen gas holder from the current time to the predetermined time calculated from the oxygen gas generation change amount after the time falls within the predetermined upper and lower limits,
In addition, the product gas generation amount of each air separation device is determined so as to be as low as possible in consideration of the limit of the generation amount change speed of each air separation device. Schedule decision method. 2. The absolute value of the internal pressure of the oxygen gas holder and the electric power consumption of the air compressor according to claim 1, under various restrictions including the limit of the oxygen gas generation rate changing speed, which are determined by the capacity and efficiency of the apparatus. A method for determining a product gas generation schedule for an air separation device, characterized in that the amount of oxygen gas generated at each time is determined so that the objective function given by a linear sum is minimized.