CN1063214C - Method for real-time optimization control of cracking reaction depth in catalytic cracker - Google Patents

Abstract

The present invention relates to a method for the on-line real-time optimization control of cracking reaction depth in a petroleum catalytic cracker, which belongs to the field of the control and the optimization of production processes. In the present invention, dynamic accumulative quantity compensation is added to measured variables; for liquid products, measured discharge is corrected by the variation in the accumulated liquid level of a stripping tower, the liquid level of a gas-oil dissociation tank and the accumulative quantity in a main fractionating tower; for gas products, the measured gas discharge is corrected by the variation in the accumulative gas quantity in an accumulated tower and the gas-oil separation tank; influences of the time delay of raw material discharge and the measured discharge on the yield of products are accumulated, and the yield at the end of each period is corrected by the variance rate of the yield at the beginning of the period.

Description

Real-time optimal control method of cracking reaction depth in catalytic cracking unit

The invention relates to an online real-time optimization control method for cracking reaction depth of a petroleum catalytic cracking unit, belonging to the field of production process control and optimization.

The existing technologies all use steady-state mathematical models to predict (calculate) the yield of each product in the reaction product, optimize the optimization target based on the yield, and use changing the reaction temperature as the main optimization method; such as: BDL in my country Model (Petrochemical Research Institute, Daqing Petroleum Institute, Lanzhou Refinery and Chemical Plant), Thirteen Lumped Models (Luoyang Petrochemical Engineering Company); abroad: FCC-ONONPT of Profimatic Inc, RTOPT of SETPIONTInc, MPFCC of Maxprofit, [DMO] of DMC Corp. et al.

The shortcomings of the existing technology are: the calculation based on the steady-state model must wait for the production process to stabilize, which makes the optimization cycle longer, generally 2-8 hours, and is difficult to adapt to the production process that is often in dynamic changes; It is also difficult to obtain real-time measurement of the analytical data of raw material properties and catalyst activity; the optimization goal is often to increase the yield by about 1%. Due to the limited accuracy of the measured process variables and models used in the calculation, it is difficult to achieve accurate optimization; Using the reaction temperature as an optimization method, since the reaction depth is affected by many other factors besides the temperature, maintaining the optimized reaction temperature cannot maintain the optimized cracking reaction depth, and it is difficult to adapt to changes in production and on-site environments. Therefore, these methods can play a role in off-line optimization, but few examples of successful real-time applications have been seen.

The purpose of the present invention is, in prior art application number 90108193.0 (CN1060490A), on the basis of application number 95101183.9, a kind of need not raw material property and catalyst activity data is provided, does not require measuring instrument to be absolutely accurate, and the optimization cycle can be shortened to 10 Minutes, a real-time optimal control method that can adapt to dynamic changes.

The technical scheme adopted to achieve the above-mentioned purpose is as follows:

Real-time optimal control method for cracking reaction depth of catalytic cracking unit:

a) According to the feed flow rate, feed temperature, reaction temperature, catalyst temperature and circulation volume, calculate the required heat (reaction heat) of the unit feed in the reaction process, which represents the reaction depth, and optimize with the target yield or the highest economic benefit Target, adjust the heat of reaction to achieve optimal control;

b) Measure the flow of rich gas at the outlet of the oil-gas separation tank at the top of the main fractionation tower, the flow of crude gasoline, the flow of diesel oil out of the device, and the flow of oil slurry discharged outside; it is characterized in that,

c) When using the above-mentioned flow rates to calculate the various product yields in the reaction product online, add dynamic accumulation compensation to the measured variable;

c1) For liquid products, take into account the liquid level of the stripper, the liquid level of the oil-gas separation tank and the change of the accumulated volume in the main fractionating tower to correct the above-mentioned measured flow rate;

c2) Correct the above-mentioned measured gas flow rate for the gas product meter and the gas volume change in the tower and the oil-gas separation tank;

c3) Determine the actual boiling point range (dry point and initial boiling point) of the main fractionating tower distillate oil product with the temperature, pressure, flow (including the internal reflux of the tower) of the measured fractionating tower, and adjust the product by the standard boiling point range of the oil product The flow is corrected;

c4) calculate the productive rate of the product according to the above-mentioned corrected flow rate by taking into account the time lag from the raw material flow rate to the above-mentioned measured flow rate;

d) Use the average increase and decrease (change direction) of the yield and the change direction of the reaction depth to judge the optimization direction of the next cycle in the adjacent two weeks, and consider the pure lag time and dynamic relationship of the yield response to the reaction depth, and use each The rate of change of the production rate at the beginning of each period is corrected for the production rate at the end of the period.

The present invention relates to the following three aspects:

1. Real-time determination of optimization goals

The present invention is characterized in that: based on the gas distilled from the fractionation tower, liquid hydrocarbon (rich gas), gasoline, diesel oil, and oil slurry discharged outside, the dynamic mathematical model of the fractionation tower, cold exchange equipment, container and regenerator is utilized , online calculation of the yield of the corresponding product in the reaction product (at the entrance of the main fractionation column), as the basis for determining the optimization target (the highest yield of the target product or the highest economic benefit), avoiding a series of problems caused by using the steady-state model to predict the yield Problems (requires raw material properties and catalyst activity data, insufficient model accuracy, etc.).

When the present invention calculates the yields of various products in the reaction product on-line, it has the following characteristics:

1. Adding dynamic accumulation to the measured variables is compensation. For liquid products including: main fractionation tower and oil-gas separation tank bottom liquid level changes cause changes in accumulation, which can generally be calculated according to the following differential equation: $\mathrm{AdH}/\mathrm{dt}=Q_1^L-Q_2^L$

Or calculate according to the following time discretization equation: $Q_1^L\left(k\right)=\[h(k+1)-h\left(k\right)\]A/\mathrm{Ts}+Q_2^L\left(k\right)$

-流入容器的流量(通常是待计算的)-流出容器的流量(通常是可测的)Ts-计算周期Among them: H-liquid level A-container cross-sectional area t-time

- the flow into the container (usually to be calculated) - flow out of the container (usually measurable) Ts - calculation period

For the gas product, consider the change of the gas volume in the column, and use the following differential equation to calculate: $C_v\mathrm{dp}/\mathrm{dt}=Q_1^v-Q_2^v$

Or calculate according to the following time discretization equation: $Q_1^v\left(k\right)={\[P(k+1)-P\left(k\right)\]}^{C_v}/T_{\mathrm{the\; s}}=Q_2^v\left(k\right)$

in:

- column gas capacity;

P-column gas pressure;

-分别为流入、出塔器的流量

-respectively the flows into and out of the column

2. Correction of oil boiling point range

-分别为油品的标准初馏点和干点T_i，T_e-分别为油品的实际初馏点和干点Q₁-修正前的流量Q_e-修正后的流量Determine the actual boiling point range (dry point and initial boiling point) of the oil product distilled from the fractionation tower with the measured temperature, pressure, and flow rate (including the internal reflux of the tower), and correct the flow rate of the product according to the standard boiling point range of the oil product. The method described (see 95101183.9): $Q_S=Q_1\[1+K_i(T_i-T_i^{\mathrm{the\; s}})+k_e(T_e^{\mathrm{the\; s}}-T_e)\]$

-Standard initial boiling point and dry point T _i , T _e respectively of the oil product -Respectively the actual initial boiling point and dry point Q ₁ of the oil product -The flow rate before correction Q _e -The flow rate after correction

3. From the time lag from the introduction of raw materials to the above-mentioned measured flow rate, the yield is calculated according to the following formula:

Y _i =Q _si /Q _o (tT _i )

Where: Y _i is the yield of the i-th product

Q _o is the feed oil flow rate

Q _si is the flow rate of the i-th product after compensation and correction

T _i is the time lag of the i-th flow to the raw material

2. Dynamic self-optimization method, which is characterized by:

1. Use the average increase and decrease (change direction) of the yield in two adjacent periods and the change direction of the reaction depth to judge the optimization direction of the next cycle, and it is not required that the measuring instrument is absolutely correct:

2. Introduce dynamic correction and time lag. When judging the change of yield, consider the pure lag time and dynamic relationship between yield and reaction depth response, and use the rate of change of yield at the beginning of each cycle to correct the yield at the end of the cycle. Methods as below:

Y ^c _i (K)=Y _i (K)+β[Y _i (k-n+1)-Y _i (kn)]

Y ^c _i (K)-dynamically corrected yield

Y _i (K) - production rate at the end of the period before correction

Y _i (kn) - the yield at the beginning of the cycle

Y _i (k-n+1) - the yield of one calculation cycle after the start of the cycle

β-correction factor

3. Self-stop and automatic start

Measure the yield and the number of continuous optimizations at any time. When the yield does not change much, or the number of continuous optimizations reaches the specified value, the optimization will stop automatically and maintain stable operation.

Measure the yield, reaction depth and refining ratio at any time. When the properties of raw materials (including the refining ratio), yield or reaction depth change too much, the optimization will stop; when it is stable, it will automatically start the optimization at a certain time under the control of the timer. .

4. Do not exceed the constraints

Forecast the changes of reaction depth, reaction temperature, reaction pressure, regeneration temperature, regeneration oxygen content, liquid level of refinishing tank and tower bottom, air compressor and main fan load (main air flow), when it will exceed the limit, the reaction The depth can only be adjusted in the direction of not exceeding the limit.

5. With protective measures

When there is a fault in the measuring point, the calculation of the yield is unreasonable or contradictory, the optimization can be automatically stopped and an error message will be displayed.

3. Execution of optimization results, which is characterized by:

1. Measure and control the depth of reaction with the heat of reaction calculated online (existing patent application 90108193.0).

2. The main adjustment method is to change the reaction heat, with upper and lower limit protection.

3. When adjusting the reaction heat, the reaction temperature, preheating temperature, regeneration slide valve opening and pressure drop should not exceed the limit.

4. With protection measures: when the above-mentioned reaction heat calculation and reaction depth control system fails, the optimization can be automatically disconnected.

The present invention will be described in further detail below through the embodiments in conjunction with the accompanying drawings.

Fig. 1 is a partial schematic diagram of a catalytic cracking unit;

Fig. 2 is the real-time optimal control method embodiment of the cracking reaction depth realized by DCS (distributed computer system) in the present invention;

Fig. 3 is a block diagram of the reaction product yield calculation observer calculating the product yield;

Fig. 4 is a tuning block diagram of the response depth real-time optimizer;

Fig. 1 is a schematic diagram of a part of the catalytic cracking unit. The raw oil 11 is input into the riser reactor 13 through the pipeline 12, the catalyst 14 is heated to 700° C. and is input into the riser reactor 13 through the pipeline 15, and the reaction product is input into the main fractionation through the settler 16 and the pipeline 17. Tower 18, the rich gas outlet 20 and crude gasoline outlet 21 of the oil-gas separation tank 19 on the top of the main fractionating tower 18, and the diesel outlet 23 and 24 of the stripper 22 are outlets for discharging oil slurry. 25 is a return pipeline. The regenerator and air compressor are not shown in Fig. 1, and their working principle has been discussed in textbooks in the technical field of petroleum catalytic cracking. This schematic diagram is only for a better understanding of the real-time optimization control method of the present invention for a brief description.

The present invention can be realized on the catalytic cracking unit that has production process control computer; Can realize on DCS (distributed computer system), also can realize on the host computer of DCS, Fig. 2 is the embodiment on DCS, comprises The following content:

(1) Data collection and processing

(2) Conventional controllers and regulating valves and slide valves

(3) Reaction heat online calculation observer

(the above are prior art, represented by dotted frame)

(4) Response Depth Multivariate Coordinated Predictive Controller

(5) Reaction product yield calculation observer

(6) Real-time optimizer for response depth

(7) Setter

(The above is the technology related to the present invention, represented by a solid box)

(4) Response Depth Multivariate Coordinated Predictive Controller

This controller utilizes the existing technology of measuring reaction depth with reaction heat to realize reaction depth control; application number 90108193.0, publication number 1060490A, features of the present invention:

(a) While controlling the heat of reaction, take into account that the reaction temperature does not fluctuate too much or exceed the limit. For this reason, a floating upper and lower limit is set for the reaction temperature. The center value of the floating upper and lower limit is the reaction temperature controlled by using the reaction heat or Set by the operator. According to the weighted deviation of the reaction heat and the reaction temperature, the setting of the conventional reaction temperature controller is adjusted through the model predictive control algorithm for control.

Weighted deviation E=qE _t +(1-q)E _h

_Et is the deviation of the reaction temperature from the central value of the floating limit

E _h is the deviation between the heat of reaction and its given value when |E _t |<δ ₁ q=0 (only the heat of reaction is controlled)

(b) If conditions permit, when Et is not zero, the preheating temperature of the raw material, the flow rate of the stopper or the temperature of the regenerated catalyst can be adjusted so that the deviation between the reaction temperature and the floating center limit is small.

(c) Using multi-cycle control, the control cycle for reaction heat is T ₁ ; the control cycle for reaction temperature is T _{2 ,} and the control cycle for preheating temperature is T ₃ .

Generally, T ₁ <T ₂ <T ₃ is selected.

(d) Spool valve pressure drop, valve position, reaction pressure and other variables do not exceed constraints

(5) Reaction product yield calculation observer

The observer uses the following method to calculate the corresponding product yield in the reaction product from the actual measurement of the flow rate of rich gas, gasoline, diesel oil, and effluent oil slurry (clarified oil) at the outlet of the main fractionation tower; The coke yield was calculated based on the amount of carbon dioxide and the oxygen content of the flue gas. See Figure 3.

Dynamic Compensation:

(a) The correction of the flow rate due to the change of liquid level at the bottom of the oil-gas separation tank, the stripper and the main fractionation tower;

(b) Correction for changes in accumulation in the main fractionator

(Using the reflux flow rate in each section of the fractionation tower provided by the existing on-line calculation observer of the distillation range of petroleum products to compensate)

(c) Compensation for changes in gas volume caused by pressure changes

(d) Calculate the amount of coke burned (produced) from the oxygen content of the regenerator and the dynamic mathematical model

Petroleum product boiling point range correction:

(a) The standard boiling range of gasoline is calculated at 30-205°C; if the dry point Gep is calculated online during actual operation, the correction amount for gasoline and diesel (+, - respectively) is:

△F _g =a _g (205-G _op ) a _g is the adjustable correction coefficient

(b) The measured pressure P _g and temperature T _g of the oil-gas separation tank are corrected for gasoline and rich gas (respectively (+, -), and the correction amount is: F _w =a _wt (zT _g )-a _wp (P _g - P _go )P _go is rated pressure

(c) Diesel oil is corrected by online calculation of diesel 90% point (D ₉₀ ) $f_d=a_d({\mathrm{D.}}_{90}^0-{\mathrm{D.}}_{90})$ D ₉₀ is the standard diesel 90% point time lag compensation

Calculate the pure lag time of rich gas, gasoline, diesel, coke, and effluent oil slurry (clarified oil) to the change of raw oil flow rate from the model and measured data, and correct it when calculating the yield.

(6)Reaction depth real-time optimizer and (7)Setter

The optimizer calculates the observation value online according to the set optimization target, the yield calculation result and the reaction heat, and adjusts the reaction depth (heat) once at a certain time interval (called optimization period) as an optimization means. The steps are as follows: see Figure 4

Productivity Calculation Result Inspection

(a) The fluctuation range between the working condition and the calculated yield value cannot exceed a reasonable set value;

(b) The rate of change of production rate does not exceed the set value;

(c) (gasoline+diesel+rich gas+coke+exhaust) yield and 100% deviation is less than the set value;

(d) The deviation between (gasoline+diesel+rich gas) yield and (100%-outflow-coke) is smaller than the set value.

When the above conditions are not met, a signal of unqualified yield calculation is given

optimal calculation

(a) When the yield calculation is qualified, the optimization target value is calculated by the yield;

(b) Compare the target change (△J) and reaction heat change (△H) of the two optimization cycles, and determine the optimization direction according to the following logic:

If the two change directions are the same, increase the reaction depth;

If the direction of change of the two is not the same, then reduce the reaction depth.

When △J and △H are greater than the set value, it is considered that there is a change;

When △J and △H are less than the set value, it is considered that there is no change, and the tuning is stopped;

(c) Add dynamic compensation to the calculation of ΔJ, that is, calculate the change rate R ₁ of the target at the beginning of each optimization cycle. Added a correction value of -rR ₁ , where r is an adjustable parameter.

Constraint processing

Use the model to predict whether the reaction temperature, reaction pressure, regeneration temperature, oxygen content of regeneration flue gas, air compressor load, liquid level of refinishing tank, etc. exceed the limit. If it exceeds the limit, the tuning direction can only be adjusted towards the direction of releasing the constraint.

Optimal start and stop judgment

(a) When the target change is less than the set value or the number of optimization cycles reaches the set value, stop the optimization.

(b) When the change of the refining ratio (slag mixing ratio) is greater than the set value, or the calculation results of the yield and reaction heat are unqualified, stop the optimization; once the refining ratio (slag mixing ratio) does not change, or the calculated value is restored Normal, or when the heat of reaction does not change but the yield changes greatly, the optimization can be started automatically after two optimization cycles.

(c) After stopping the optimization reaching the time, start the optimization automatically.

(d) There is a manual operation switch, which can start or stop the optimization at any time.

(e) After using the real-time optimization system for the first time, start the optimization automatically.

Claims (8)

1. Real-time optimization control method for cracking reaction depth of catalytic cracking unit: a) According to the feed flow rate, feed temperature, reaction temperature, catalyst temperature and circulation volume, calculate the heat required by the unit feed in the reaction process to represent the reaction depth, and adjust the reaction with the target yield or the highest economic benefit as the optimization goal Thermal optimization control; b) Measure the flow of rich gas at the outlet of the oil-gas separation tank at the top of the main fractionation tower, the flow of crude gasoline, the flow of diesel oil out of the device, and the flow of oil slurry discharged outside; it is characterized in that, c) When using the above-mentioned flow rates to calculate the various product yields in the reaction product online, add dynamic accumulation compensation to the measured variable; c1) For liquid products, take into account the liquid level of the stripper, the liquid level of the oil-gas separation tank and the change of the accumulated volume in the main fractionating tower to correct the above-mentioned measured flow rate; c2) Correct the above-mentioned measured gas flow rate for the gas product meter and the gas volume change in the tower and the oil-gas separation tank; c3) determine the actual boiling point range of the oil product distilled from the main fractionating tower with the temperature, pressure and flow rate of the actually measured fractionating tower, and correct the flow rate of the product by the standard boiling point range of the oil product; c4) calculate the productive rate of the product according to the above-mentioned corrected flow rate by taking into account the time lag from the raw material flow rate to the above-mentioned measured flow rate; d) Judging the optimization direction of the next cycle by using the average increase and decrease of the yield and the change direction of the reaction depth in the adjacent two weeks, considering the pure lag time and dynamic relationship of the yield response to the reaction depth, using the time at the beginning of each cycle The rate of change of the production rate corrects the production rate at the end of the period. 2. The method according to claim 1, characterized in that, when the change in yield is not greater than a specified value, the optimization stops automatically and keeps running stably. 3. The method according to claim 1, characterized in that, when the number of continuous optimizations reaches a specified value, the optimization stops automatically and keeps running stably. 4. The method according to claim 1, characterized in that, when the change in the refining ratio, yield or reaction depth is greater than a specified value, the optimization is suspended, and the optimization is automatically started after stabilization. 5. The method according to claim 1, characterized in that the optimization is automatically stopped and an error message is displayed if the measuring point is faulty and the yield calculation is unreasonable. 6. The method according to claim 1, characterized in that after the optimization is stopped, the optimization is automatically started after a certain period of time under the control of a timer. 7. The method according to claim 1, characterized in that when the regeneration temperature, oxygen content, main air volume, reaction pressure, liquid level of the refinishing tank, liquid level at the bottom of the main fractionating tower, the load of the fractionating tower and the air compressor exceed the limit , adjust the given value of heat of reaction to release the constraint. 8. The method according to claim 1, characterized in that, by adjusting the given value of reaction heat, optimization is realized, and the reaction temperature, raw material preheating temperature, regeneration slide valve (26) opening and pressure drop do not exceed limit.

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