PROCESS AND APPARATUS FOR THE LOW-TEMPERATURE FRACTIONATION OF AIR

Abstract

The process and the apparatus serve for the low-temperature fractionation of air in a distillation column system, which has at least one separating column. Feed air is compressed in a main air compressor. Compressed feed air is cooled in a main heat exchanger. Cooled feed air is introduced into the distillation column system. At least one product stream is drawn off from the distillation column system, heated in the main heat exchanger and drawn off as a gaseous end product. At least one process parameter is set by a basic controller. The control of the process parameter is set by a combination of an ALC control and an MPC controller. This involves the ALC control outputting a first target value to the MPC controller. The MPC controller calculates from the first target value a setpoint value or a change to a setpoint value for a primary setpoint value output by the ALC control. The setpoint value determined by the MPC controller or a secondary setpoint value, which is calculated from the primary setpoint value output by the ALC control and the change to the setpoint value, is transmitted to the basic controller.

Claims

1. A process for the low-temperature fractionation of air in a distillation column system that has at least one separating column, in which feed air is compressed in a main air compressor, compressed feed air is cooled down in a main heat exchanger, cooled-down feed air is introduced into the distillation column system and at least one product stream is drawn from the distillation column system, warmed up in the main heat exchanger and drawn off as a gaseous end product, wherein at least one process parameter is set by a basic controller, characterized in that the control of the process parameter is performed by a combination of an ALC control and an MPC controller, wherein the ALC control contains a set of measured values of the parameter that have been recorded during trial operation of the system and correspond to the various load cases and the transitions between these load cases, wherein also the ALC control outputs a first target value to the MPC controller, the MPC controller calculates from the first target value a setpoint value or a change in the setpoint value for a primary setpoint value output by the ALC control, and the setpoint value determined by the MPC controller or a secondary setpoint value that is calculated from the primary setpoint value output by the ALC control and the change in the setpoint value is transferred to the basic controller.

2. The process as claimed in claim 1, characterized in that a first and a second process parameter are set, in that the ALC control outputs a first and a second target value to the MPC controller, the MPC controller calculates from the transferred target values the setpoint values for the process parameter and the MPC controller calculates from the second target value a change in the setpoint value for a primary setpoint value output by the ALC control for the second process parameter and the setpoint value determined by the MPC controller for the first process parameter and a secondary setpoint value for the second process parameter, which is calculated from the primary setpoint value output by the ALC control and the change in the setpoint value, is transferred to the basic controllers for the first process parameter and the second process parameter.

3. The process as claimed in claim 1, characterized in that a third process parameter is set, in that the ALC control transfers a setpoint value to the basic controller of the third process parameter directly without inclusion of the MPC controller.

4. The process as claimed in claim 1, characterized in that the combination of the ALC control and the MPC controller delivers setpoint values for a multiplicity of process parameters.

5. The process as claimed in claim 1, characterized in that the distillation column system is guided from a first load case to a second load case and the ALC control thereby specifies in discrete time increments setpoint values for one or more basic controllers or one or more primary setpoint values for the MPC controller.

6. An apparatus for the low-temperature fractionation of air comprising a distillation column system that has at least one separating column, a main air compressor for compressing feed air, a main heat exchanger for cooling down compressed feed air, a feed line for introducing cooled-down feed air into the distillation column system and means for drawing off a product stream from the distillation column system and for warming up the product stream drawn off in the main heat exchanger, a product line for drawing off the warmed-up product stream as a gaseous end product, and comprising at least one basic controller for setting a first process parameter, characterized by one or more open-loop and closed-loop control devices.

Description

[0031] The invention and further details of the invention are explained more specifically below on the basis of exemplary embodiments that are schematically represented in the drawings, in which:

[0032] FIG. 1 shows the most important elements of a low-temperature air fractionation process,

[0033] FIG. 2 shows a first exemplary embodiment of a combination of the first and second variants of the invention and

[0034] FIG. 3 shows an exemplary embodiment of the second variant of the invention.

[0035] In FIG. 1, feed air 1 is compressed in a main air compressor 2. The compressed feed air 3 is cooled down in a main heat exchanger 4. The cooled-down feed air 5 is introduced into a distillation column system 6. The distillation column system 6 has at least one separating column, for example a classic double column comprising a high-pressure column, a low-pressure column and a main condenser (not represented). From the distillation column system, at least one product stream 3 is drawn off, warmed up in the main heat exchanger 4 and as a gaseous end product 8.

[0036] Both exemplary embodiments of the invention relate to a system for the low-temperature fractionation of air. This system has basic controllers BR1 to BR3, which have a closed-loop control function, that is to say they set a specified setpoint value of a manipulated variable within a control loop. Further basic controllers BR4 to BR7 do not have a closed-loop control function, but set the transferred setpoint value of the corresponding manipulated variable directly and only change when there is a load change.

[0037] In FIG. 2, when there is a load change the changed product specifications for one or more products, for example of the gaseous oxygen product (GOX) and/or of the liquid nitrogen product (LIN), are input into the ALC. The ALC checks these inputs, calculates the core variables (states), which describe the aimed-for target state of the system, in particular the amount of air (AIR), the amount(s) to be expanded to produce work (TURBINE) and the proportion of air that is sent through a recompressor (BAC). The ALC then guides the transformation of these core variables and basic controller setpoint values on a predetermined ramp in each case from the initial state to the target state. This ramp is fixed for each parameter (core variables and basic controllers) by a relationship such as that represented in FIG. 1 under the heading Load change.

[0038] In the case of a first part of the manipulated variables (for the basic controllers BR1 and BR2, which are shown here as representative), an MPC controller LMPC calculates from the target values CVSP_i transmitted from the ALC a respective setpoint value PID_loop1.sp, PID_loop2.sp by using a linear model. Some of the target values CVSP_i are formed by the production target values, others by setpoint values for controlled variables such as temperatures or analyses. The setpoint values PID_loop1.sp, PID_loop2.sp are output as absolute values to the corresponding basic controller BR1, BR2. This realizes the first variant of the invention.

[0039] For a second part of the manipulated variables (for the basic controller BR3, which is shown here as representative), the MPC controller acts as a trimming controller, which calculates a correction value PID_loop3.sp. This correction value is added as a setpoint value change to the primary setpoint value PID_loop3.sp_avg calculated by the ALC and the sum is transferred as a secondary setpoint value sSW3 to the corresponding basic controller BR3. This realizes the second variant of the invention. Examples of corresponding setpoint variables are the return amounts for the columns of the distillation column system, parameters of gaseous products removed or streams for the production of cold or the distribution of the streams through heat exchangers.

[0040] Apart from the target values, limit variables and setpoint values that are constant or are specified by the operating personnel are possibly also entered into the calculations of the MPC controller. Examples of this are for instance product purities or energy consumptions of machines that may only vary within given limit values. In a realistic example, the MPC controller calculates for a total of around eight to ten basic controllers with a closed-loop control function absolute setpoint values or correction values.

[0041] A third part of the manipulated variables (for the basic controllers BR4 to BR7, shown here as representative), the ALC delivers the corresponding setpoint values directly in a classic way. These values are not influenced by the MPC controller. In a realistic example, the ALC delivers the setpoint values directly for a total of around 20 to 30 basic controllers without a closed-loop control function.

[0042] In FIG. 3, the second variant of the invention is used exclusively. As a difference from FIG. 1, here the MPC controller LMPC does not calculate any absolute values for manipulated variables, but instead just operates in the manner of a trimming controller according to the second variant for a specific number of setpoint variables, of which PID_loop1.sp_avg and PID_loop2.sp_avg are shown in the drawing by way of example for the basic controllers B1, B2 with a closed-loop control function. In practice, for example, three to six manipulated variables are determined in this way.

[0043] The other manipulated variables (for the basic controllers BR3 to BR7, which are shown here as representative), the ALC delivers the corresponding setpoint values directly in a classic way. These values are not influenced by the MPC controller. In a realistic example, the ALC delivers the setpoint values directly for a total of around 20 to 30 basic controllers without a closed-loop control function.

[0044] In the case of both exemplary embodiments, usually all of the basic controllers that are driven by ALC and LMPC are incorporated in an integrated process control system. The programs for ALC and LMPC are usually run on a dedicated process computer, which exchanges the data with the process control system by way of a network connection, and thus transmits the calculated setpoint values to the inputs of the process control system.