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. Highly dispersed flow was observed in the first tube and the dispersion decreased as the material passed through the first tube to the third tube of the digester. In another radiotracer experiment, at M/s Trident Industries Ltd. Barnala (India), 99mTc in the form of sodium pertechnetate was used to carry the RTD experiment in two identical two-tube continuous pulping digesters which have the same operating conditions. The data obtained after RTD experiments were pretreated and modeled using the same models that were used for the previous set of experiments. Flow behavior was analyzed using the obtained model parameters in terms of dispersion or mixing. It was observed that the old digester had very low MRT as compared to the theoretical MRT. The output parameters of the old digester in terms of Kappa number and residual alkali were found to be poor even at same operating parameters. In old digester, low MRT was probably due to the channeling and scaling. Operation of the new digester was found normal.