Residence Time Distribution Studies on Continuous Flow Pulping Digester Using Radiotracer

Abstract

Radiotracer technique has been used to measure the residence time distribution (RTD) of the industrial scale three-tube continuous pulping digester used for pulp production from wheat straw. RTD experiments were performed to observe the mean residence time (MRT), flow behavior and to optimize the operating conditions of the digester in M/s SATIA Industries Ltd. Muktsar, Punjab (India). The experiments were performed with different operating conditions using 82Br and 198Au radio-isotopes as radiotracers. The radiotracer was injected instantaneously in the form of impulse input in the liquid phase at the inlet of the first tube. Sodium iodide scintillation detectors were mounted at inlet and outlet of each tube to measure the concentration of radiotracer. Each experiment was repeated three times to check the repeatability of the results. The data recorded by detectors were plotted as a function of time. The obtained concentration data were treated for zero shifting, background subtraction, decay correction and data extrapolation. Mean residence time was calculated for each experiment and RTD curves were obtained. Analysis of the RTD curves showed the flow channeling or occurrence of parallel flow paths in the first and second tube of the digester. Axial dispersion model and tank-in-series with back-mixing model with a plug flow component in series were found suitable to describe the flow behavior of the liquid phase in the pulping digester. The results of the model fitting indicated plug flow behavior of the liquid phase in the digester. The degree of delignification and value of residual alkali were found optimum for a screw speed of 65 rpm (wheat straw feed rate of 6.3 m3/min) and white liquor (8.28 w/w % of NaOH) flow rate of 355 l/min. For second and third tubes, convolution procedure was used for simulating the experimental data due to non-ideal impulse input in tube two and three.