An accurate control of the reaction zone in the product refluxer of 15N separation plant

Full text: In order to ensure the conditions for the 15N/14N isotopic exchange in NO, NO2 - HNO3 system on (packed) separation columns, one of the most important task is to maintain the same position of the chemical reaction zone between HNO3(s) and SO2(g) on the packing in the product refluxers: 2HNO3+3SO2+2H2O→2NO+3H2SO4, 2HNO3+SO2→2NO2+H2SO4. The flow rates of the nitric acid solution in the separation columns are maintained using metering pumps. The position of the reaction zone in the product refluxers was kept at an optimal height by controlling the flow of the sulphur dioxide by means of a fuzzy logic controller. The first action of the control procedure is the accurate localization of the chemical reaction zone in the product refluxers by sampling the temperature along the refluxer, with seven special temperature sensors. The temperature distribution in the refluxer is computed by spline interpolation. Cubic spline interpolation is a useful computing technique to interpolate between known sampled points, due to its smooth characteristics. Unfortunately it does not prevent overshoot at intermediate points, which is essential for many chemical engineering applications. The developed interpolation method prevents overshooting by reducing the smoothness. It combines the smooth curve characteristics of spline interpolation with the non-overshooting behaviour of linear interpolation. That was achieved by eliminating the requirement for equal second order derivatives at every point and replacing it with specified first order derivatives. The position and the maximum value are accurately obtained by computing the first derivative, the developed algorithm being fast enough to run in real time. (author)