Creating Utility System Temperature Control for an Open Plate Reactor

Author(s):
Staffan Haugwitz - Department of Automatic Control, Lund Institute of Technology

Industry:
University/Education, Energy/Power

Products:
PXI/CompactPCI, LabVIEW

The Challenge:
Designing a data acquisition system and a temperature controller for a utility system that provides cooling water to an unstable chemical reactor.

The Solution:
Using a National Instruments PXI system and LabVIEW for flexible and efficient research and experiments on the cooling system and the reactor.

Intensifying the Process with More Effective Chemical Reactors

The synthesis of fine chemicals or pharmaceuticals, widely carried out in batch or semibatch reactors, is often strongly limited by constraints related to the dissipation of the heat generated by the reactions. A new concept in heat exchange reactors, the open plate reactor (OPR) developed by Alfa Laval AB, performs complex chemical reactions with very accurate thermal control. Thus, the OPR appears particularly suited to process intensification, as it simultaneously increases reactant concentration and reduces solvent consumption. One of the major benefits is the reduced need for down-stream separation, which leads to large time and money savings.

The Department of Automatic Control at Lund Institute of Technology has a long history in process control research. For this part of the project, we aimed to develop, in collaboration with Alfa Laval AB, a utility system with a corresponding control system that would deliver cooling water to the new plate reactor [1]. For exothermic reactions, if the temperature inside the reactor increases, it releases more heat, which further increases the temperature, releasing even more heat, creating a runaway reaction. To ensure safety, performance, and power durimg disturbances, it is critical to accurately and quickly control the temperature of the water flowing through the plate reactor. The control should be fast enough to prevent runaway reactions, if the temperature increases due to disturbance. The general reactor process control is described in [2].

Creating a Fast and Flexible Cooling System

The cooling system is a closed-flow circuit that transfers heat from the chemical reactor on the right to the cold water from a reservoir on the left. By recycling the water coming out of the reactor back to the control valve RV1, we significantly improve speed. The second recycle, around the heat exchanger HEX to the left, keeps the flow rate through the heat exchanger constant, regardless of the current RV1 valve position.

The objective is to control the cooling water TT5 inlet temperature. The main temperature controller signal of the temperature controller is the desired position of the control valve RV1, which determines how much warm recycled water that should be mix with cold water from the heat exchanger. The second control signal is the position of control valve RV2, which indirectly controls the heat exchanger TT3 water temperature. By combining the two control signals in a midranging control structure [2], one PI-controller manipulates RV1 to control the cooling temperature and another PI-controller manipulates RV2 so that the control valve RV1 works around some desired operating point, (for example, 50 percent to avoid valve saturation. The midranging control structure largely increases the existing hydraulic equipment flexibility and performance.

Implementing Control Algorithms with LabVIEW Quickly and Easily

For full process information, we used 16 temperature sensors, five flow sensors, five pressure sensors, and four analog outputs to control the pumps and valves. With a desired sampling frequency of 10 to 100 Hz, we used a National Instruments PXI-8176 embedded controller for stand-alone, high-performance, real-time control. We sampled the analog inputs using NI SCXI components, including thermocouple amplifiers. We developed a GUI in LabVIEW to carry out specific experimental procedures. We used PID-blocks from the LabVIEW PID Control Toolkit to implement control algorithms quickly and easily. Because all measurements were readily available in the LabVIEW program, we implemented standard process monitoring and error detection procedures. While developing a suitable chemical reactor control strategy, it was convenient to work with flexible control software such as LabVIEW, which is easy to learn.

For safety reasons, we used water instead of chemicals and superheated steam to provide a variable heat generation inside the reactor. The steam injections do not have the same dynamics as a chemical reaction, but still can be approximated as a very fast exothermic reaction, because we focused on verifying the cooling system temperature control.

We tested the control system in a series of experiments. We suddenly decreased the amount of steam injected, thus reducing the heat release inside the reactor by 50 percent. This causes the cooling water to be less heated, and the cooling outlet temperature TT6 decreases. Due to the recycle loops, the cooling inlet temperature TT5 is quickly affected, unless we made necessary control actions with the control valve RV1.

For more information, contact:

Staffan Haugwitz

Department of Automatic Control

Lund Institute of Technology

Box 118, SE-221 00 Lund

Web: www.control.lth.se/~staffan

References:

1. S. Haugwitz and P. Hagander ”Temperature control of a utility system for an open plate reactor”. In Proceedings of Reglermöte, May 2004.
2. S. Haugwitz and P. Hagander ”Process control of an Open Plate Reactor”, In Proceedings of IFAC World Congress, July 2005.

 

Browse All Case Studies »