The Corus steel plant in Scunthorpe is one of three integrated steel works within the UK. An integrated steelworks is a place where raw steel is manufactured from raw materials, which include iron ore, lime and coke. This metal undergoes subsequent processing to turn it into rolled products of a precise composition controlled to within partspermillion. These rolled products can be either ‘long products’, such as rail and steel bar used as concrete reinforcing, or ‘flat products’, such as steel strip and plate.
In an integrated steel plant you usually also find coating lines that apply a metal or organic coating to steel strip in order to make it more suitable to the demands of specific markets. Examples include tin-coated steel used in packaging, or zinc coated – galvanised – steel often used in building and automotive applications.
There are two coke making plants at Scunthorpe and their main purpose is to convert coal into metallurgical coke for use in the blast furnaces (Fig.1).
Tar is an important by-product of the coke making process and is transported via road tankers off site to tar distillation companies for processing. It is important to measure the moisture content of the tar to ensure the correct quality of tar is being transported to the distillation company.
The present method of despatch tar moisture analysis at both Dawes Lane and Appleby Coke ovens involves the operator taking a ‘spot’ sample during the tanker loading operation. The sample is then collected by the coke oven laboratory chemist and taken back to the laboratory and analysed using a Karl Fischer technique.
There were many disadvantages with this testing method causing possible inaccuracies in measurement and also time and cost impacts.
The spot sample only provides a snapshot of the tanker load and therefore may not be fully representative of the moisture content of the whole tanker load. In addition, the tanker driver has often left the site by the time the analysis is completed and may not be aware of the moisture content of the load.
There is also a time and cost impact due to the fact that on occasion the chemist has to travel to and from the plant for the purpose of picking up and analysing a single tar sample.
Analysing the sample using this method involves undesirable, indirect contact with the tar, both for the chemist and plant operator. There is an environmental impact and cost regarding the safe disposal of contaminated plastic sample pots, gloves, plastic syringes and rags.
A spokesman for Corus comments: “An advert in a magazine stated that Able Instruments & Controls marketed a Drexelbrook CM6 on-line water in oil analyser. The instrument consists of a sensing element and RF transmitter unit and operates on the principle that the capacitance of the oil is proportional to the water content.”
Previous attempts to measure the moisture content of coke oven tar using this technique had not been successful, Able instruments was confident however, that the CM6 analyser was proven technology on heavy oils and did not have any reservations with regard to a moisture in tar application.
It was agreed with Able that the instrument would be assessed for a trial period at Appleby Coke Ovens. The sensing probe was installed on 2nd October last year on a vertical down facing section of the tar delivery pipe just outside the pump house.
During the trial the on-line moisture analyser compared well with the laboratory results. The instrument has been on-line for several months, has performed very well and has not required any significant maintenance.
“This new method has ensured a number of benefits: more accurate analysis of the whole tanker load; improved control of tar loading operation – reduction of despatch tar tankers leaving with high moisture content; improvement to health and safety operation for plant operator and chemist – reduced chance of exposure to tar; reduced environmental impact and costs regarding disposal of contaminated plastic sample pots, gloves, pipettes and cleaning fluids/cloths; and, finally, time and fuel savings for the chemist having to pick up and analyse single tar samples,” the spokesman added.
The CM6 Cut Monitor is derived from years of experience in RF admittance technology enabling the electronics to ignore build up on the pipe and probe.
The method of using RF Admittance to measure water cut is widely successful because of the large difference between the dielectric constants of oil (k=2.3) and water (k=80). The sensing element and the pipe wall form the necessary two surfaces of the concentric capacitor. The system electronics transmit a radio frequency voltage to the sensing element that measures changes in capacitance. As the amount of water in the flowing oil increases, the net dielectric of the fluid increases which causes the capacitance to increase. The onboard electronics will compute the relationship between capacitance change and water cut. The method is straightforward, reliable and proven.
The insertion probe design enables it to analyse a large representative sample of the fluid that other manufacturers cannot match. The CM6 utilises a sensing element that is unique in its ability to be installed directly into the process without requiring spool pieces, side arms or slipstreams. The grey coloured sensing element will extend directly in to the main process line for a minimum of 380mm. The advantage of this is the capacitance of the fluid is taken over the entire length of the probe to create an averaging effect. The measurement is now taking a better sample of the fluid over a larger range to produce a smoother, more accurate response.
The patented Cote-Shield is designed into the CM6 series and enables the instrument to ignore a pre-determined length of the sensing element. The ability to ignore a pre-determined length allows the sensing element to extend into the fluid beyond the nozzle mounting, and possible pipe elbows, which can affect the measurement. The Cote-Shield puts the sensing area of the insertion probe directly into the process stream and guarantees a more representative sample of the emulsion.
