The knife edge: a tailor made temperature measurement solution
06 April 2010
Accurately and automatically measuring the temperature of this polymerisation process led to a greater yield of the end product.
Kemira Water is the world’s third biggest producer of polymers and the leading producer of coagulants for chemical water treatment. The polymers produced at the Bradford plant in West Yorkshire, U.K., are used for effluent water treatment for mining, municipal, and industrial companies.
The process begins in a reactor where various chemicals and powders are mixed to form a wet gel. The gel comes out of the reactor and enters an enclosed conveyor system that is about 25m in length. Travelling on the conveyor, the gel goes through the polymerisation stage, which changes it from a wet gel to a dry solid. The rate of polymerisation — a critical factor — is controlled by the speed of the conveyor belt and the temperature of the process, which is held to the range of 50° to 55°C.
Kemira Water runs eight reactors of this type, and each reactor has its own polymerisation conveyor to produce the product.
Handheld temperature probes
The company was experiencing difficulty monitoring the temperature conditions along the conveyors, and because changes in the rate of polymerisation were not being detected accurately or quickly enough, this led to plant upsets and downtime. Besides losing time, these events were causing significant wastage of raw materials, which then needed to be disposed of as waste.
The costs incurred not only covered raw materials, but also plant downtime and throughput. On top of this, the manual temperature measurement method — an operator with a handheld probe — meant that operator working time also had to be factored in to the costs.
Kemira’s technical manager said he did not implement an automated temperature measuring solution because he doubted a supplier would be able to manufacture temperature probes robust enough to endure the forces involved in the application. As the gel passes down the conveyor, it gets thicker and more solid. At the wet in, near the reactor, the probe would not have a problem, but towards the dry end the more solid material, the technical manager feared, would break off a temperature probe from its mount.
They had to take action
Nevertheless, so much time and wastage was occurring with the manual procedure, Kemira Chemicals sought an alternative solution for reliable and repeatable measurement of the product’s temperature. They talked with engineers at Endress+Hauser.
E+H came up with a new idea for a temperature probe. They designed a stainless steel knife-edged polymer thermocouple assembly (see figure, which is a cross section looking down from the top of the probe) 30mm long and 8mm wide. There are two knife edges, and, as E+H engineer Phil Burns explains, they are ‘razor sharp, able to cut through the gel material.’ Down the centre of the assembly is a 3.3mm bore for the type J thermocouple element to be inserted, so the temperature element is embedded in the middle of the steel knife.
The entire temperature assembly is 500mm and the blade part of the assembly, which sticks into the moving gel, is 200mm. This is long enough to take the average temperature of the layer of gel, from top to bottom.
Five or six temperature probes are placed at various positions on top of each conveyor line, allowing real-time measurement of the polymerising gel during the reaction. The gel product passed smoothly around the sharp knife edges. In fact, said Mr. Burns, since the product is cut up by a macerator — a big chopping machine — and bagged at the end of the process, the knife-edge thermocouple assemblies helped with the cutting process by getting it started at an earlier point upstream.
Kemira Chemicals now relies on the probes for real-time and accurate measurement of the polymer’s temperature during reaction, ensuring the polymerisation process ultimately runs with less variation and also allows timely corrective actions to be taken when necessary. The 4-20 mA signals from the temperature probes are fed directly into the control system, which regulates the temperature and the speed of the conveyor. The temperature measurement has to be ‘reasonably’ accurate, to within two or three degrees. The process now works so automatically, operators only need to intervene when something goes wrong. The process has been optimised.
The Technical Manager at the Bradford site praised the Endress+Hauser solution. ‘The units surpassed all expectations in these areas and will help aid greater understanding of the variables involved in this process for the future,’ he said.
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