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.

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, the compressed feed air is cooled down in a main heat exchanger and introduced into the distillation column system wherein at least one product stream is drawn from the distillation column system, the at least one product stream is warmed up in the main heat exchanger and drawn off as a gaseous end product; and wherein at least one process parameter(s) of the distillation column system is set by a basic controller, characterized in that the control of the at least one process parameter(s) set by the basic controller is performed by a combination of an Automatic Load Control (ALC) control and an Model Predictive Control (MPC) controller; wherein the ALC control contains various load cases recorded during trial operation of the distillation column system corresponding to target values of the at least one process parameter(s) output by the basic controller as well as transitions between the various load cases; the ALC control outputs a first target value of one of the various load cases to the MPC controller, the MPC controller capable of calculating from the first target value, both a primary set point value and a change to the primary set point value forming a changed primary set point value, in which the primary set point value and/or changed primary set point value is then sent to the basic controller as a first process parameter of the at least one process parameter(s) output by the basic controller.

2. The process as claimed in claim 1, further characterized in that the ALC control outputs a second target value of the one of the various load cases and a secondary set point value to the MPC controller which calculates a change to the secondary set point value based on the second target value to form a changed secondary set point value which is then sent to the basic controller as the second process parameter of the at least one process parameter(s) output by the basic controller.

3. The process as claimed in claim 1, wherein the ALC control transfers a tertiary set point value directly to the basic controller as a third process parameter of the at least one process parameters output by the basic controller, without inclusion of the MPC controller.

4. The process as claimed in claim 1, wherein the ALC control and the MPC controller deliver set point values for a multiplicity of process parameters.

5. The process as claimed in claim 1, wherein the distillation column system is guided from a first load case of the various load cases to a second load case of the various load cases, the ALC control thereby specifying in discrete time increments, set point values for the basic controller or for a plurality of basic controllers or one or more primary set point values for the MPC controller.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) 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:

(2) FIG. 1 is a schematic of the elements of a low-temperature air fractionation process.

(3) FIG. 2 shows a first exemplary embodiment of a combination of the first and second variants of the invention.

(4) FIG. 3 shows an exemplary embodiment of the second variant of the invention.

DETAILED DESCRIPTION OF THE INVENTION

(5) 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.

(6) 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.

(7) 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.

(8) 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.

(9) 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.

(10) 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.

(11) 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.

(12) 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.

(13) 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.

(14) 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.