STUDIES ON GAS-SOLID HEAT TRANSFER IN CYCLONE

Abstract

For more than a century, cyclones have carved out their place as a simple static device to separate gas-solid particles mixture in chemical process industry. However, their use as heat transfer equipment is recent one. Cyclones are suitable for heating solid particles where direct contact with hot gases is allowed. Their use as raw meal preheater in cement industry has already proved their worth in saving energy. They hold much promises to be used as gas-solids heat exchanger as well as separator in many allied industries like fertiliser, polymer powder, pharmaceutical and food stuffs in . near future. The above latent potential of cyclones merits study on gas to solids heat transfer in cyclone. Despite its considerable potential of being used as heat recovery equipment, research works leading to the sizing, design and performance prediction ofthe cyclones as heat transfer equipment are hardly available in open literature, mainly because of its high patent value. It appears that Yen et al. (1990) are the pioneer investigators in this field followed by Raju et al. (1994). Both the groups of researchers have done experimental work on heat transfer from air to sand inside acyclone and have proposed correlations for the prediction of heat transfer coefficient. Although both the investigators have addressed many aspects ofthis highly complex problem, the correlations proposed by them do not predict data ofother investigator within acceptable limit oferror. The above lacuna seems to be obvious, as during the above period hardly any substitutive experimental data were available neither to examine the validity of their correlations and nor to improve it. This calls for further refinement ofthe existing correlations to develop a more meaningful and widely applicable correlation. The cyclones are known for their complex flow fields. Addition of particles in to it and heat transfer between hot air to moving particles makes it all the more complex. Thus, the phenomena of gas to solid particles heat transfer inside the cyclone is highly complex and integrated/coupled. Since the movement of particles inside a cyclone may be approximated as a combination of fluidised bed and moving bed, it naturally calls for the study of associated phenomena taking place in fluidised, moving and packed beds to draw inferences and analogies. In the above backdrop, the present experimental studies have been planned to generate reliable data for gas to solids heat transfer in cyclone and to propose improved correlations for the estimation of heat transfer coefficient, holdup of solid particles in the cyclone, ratio of pressure drops for solid particles laden air to that of solid free air at same inlet air velocity across cyclone, and outlet temperatures of gas and solids. To achieve the above objectives, a well instrumented experimental setup has been designed, fabricated and commissioned to simulate the process of heating of solid particles by hot air inside a cyclone. A tangential entry reverse flow cyclone fabricated with mild steel sheet is used in the present study. The cyclone has internal diameter equal to 100 mm, length of straight cylindrical section equal to 200 mm and conical section equal to 200 mm with rectangular inlet of size 20 mm x 50 mm and a circular outlet of 50 mm diameter. Hot air at about 200 °C (higher than that used in earlier investigations) is used as gas phase whereas the locally available river sand at ambient temperature of about 30 °C is used as solids phase to exchange heat. Before the experiments for the present study are initiated, the reliability and integrity of the experimental setup has been established by conducting experiments to measure separation efficiencies for solids laden air and pressure drops for solids free air and comparing the measured values with those estimated from well-known correlations. The separation efficiencies as predicted from the models of Leith and Licht (1972), Mothes and Loffler (1988) and Clift et al. (1991) are n found in agreement with the separation efficiencies obtained experimentally in the present investigation. The pressure drops for dust free air obtained in the present work compares well with those predicted from the correlation proposed by Fie (1987). After the integrity is established, experimental runs have been planned to study the effect of variations ofinlet air velocity (9 to 23 ms*1), solids feed rate (0.5 - 8g s"1) and particles size (163 to 460 u.m) on the holdup of solid particles, heat transfer coefficient and pressure drop for solids laden air. In all one hundred experimental runs for 5 inlet air velocities, 5 solids feed rates and 4 particles sizes have been conducted. To gain an insight into the mechanism of heat transfer inside the cyclone, temperature profile inside the cyclone has been measured. A study of temperature profile along with air and particles flow profile inside the cyclone shows that the maximum temperature occurs at a certain radial distance from the wall towards the vertical axis of the cyclone and a temperature gradient exists on either sides of it. Air in the outer vortex moves co-currently with the solid particles from top to bottom of the cyclone. Also radially inward flow of air bypasses air from outer vortex to inner vortex. Thus, air transfers heat to solid particles confined to the wall as well as to the air in the inner vortex through air bypass. In addition, variation of exit solids temperature, LMTD, thermal effectiveness, heat transfer rate and heat transfer area with the operating parameters have also been studied. Heat transfer area in a cyclone is the total surface area of the particles inside the cyclone at a given instant, i.e., holdup of particles inside the cyclone. Since the holdup is a function of operating parameters, heat transfer area in a cyclone varies with operating parameters and particles size. This is unlike in conventional heat exchangers where area of heat transfer is constant for a given heat exchanger and is independent of operating parameters. Thus, the variation of the holdup of solid particles with different operating parameters has also been studied. It has been found in the present study that the holdup varies linearly with solids feed rate, increases with increasing inlet air velocity in and decreases slightly with increasing particles size. A correlation for the estimation of holdup has been developed using regression analysis based on Marquardt Levenberg algorithm for the present dataof solids to air loading ratio ranging from 0.05 to 0.87 and cyclone Froude number ranging from 82 to 552 as: ( Mh ^ VACADc/7sy f-\( 2 \0M -0.00096 m. VCi vmJUDcy The proposed correlation predicts the holdup of solid particles in the cyclone within an error band of+15 % to -10 % for the present data and within an error band of+15 % to -25 % for the data of other investigators. The proposed correlation seems to be better than existing correlations as the correlation given by Yen et al. (1990) predicts the present holdup in a error band of+200% to -50% and that given by Raju et al. (1994) within +75% to -40%. Following Gelperin and Einstein (1971), the heat transfer resistance resides in the air film surrounding the particles as the Biot number (< 0.1) for the present study is much less than 0.25. Therefore, overall heat transfer coefficient is considered as film heat transfer coefficient surrounding the particles. The average heat transfer coefficient between air and solid particles in the cyclone has been computed based on heat gained by solid particles, co-current log mean temperature difference based on inlet and outlet temperatures of air and solid particles and surface area of the solids holdup inside the cyclone. The heat transfer coefficient is found to be almost independent of air velocity, increases with increasing particles size and increases with increasing solids feed rate. The rate of increase of heat transfer coefficient with solids feed rate reduces progressively with increasing solids feed rate and approaches to almost zero at high values of solids feed rate. This leads to approaching of heat transfer coefficient to an asymptotic value, which depends upon particles size. Following the form of correlation proposed by Gnielinski (1981) for the prediction of heat transfer coefficient in the packed bed, the proposed correlation has been obtained after regression analysis using present data of IV solids to air loading ratio from 0.05 to 0.87, Fedorov number from 5.63 to 17.3, Reynolds number from 46.8 to 307, Prandtl number from 0.68 to 0.69 and Nusselt number from 0.16 to 2.72 as: Nup =0.0047Fe145Fm0375(2 +0.664Re0/ Pr033) The proposed correlation predicts the heat transfer coefficient for the composite data within an error band of+25 % to -15 % and appears to be better than existing correlations as the correlation proposed by Yen et al. (1990) predicts the present heat transfer coefficient in a error band of+100% to +20% and that proposed by Raju et al. (1994) within +10% to -70%. Computation of temperatures of air and solids at the cyclone outlets based on basic design equation is computationally intensive as it involves iteration. Therefore, a method for the computation of the outlet temperatures of air and the solids has also been proposed based on thermal effectiveness determined on the lines of conventional 1-1 co-current heat exchanger using proposed correlations for the heat transfer coefficient and the holdup, as given below: o _ _J_ai___[_ae_ . _Ti _ -NTU(l+R)| T.; - T.; 1+ R ai si where, R=-^-^- and NTU= hA ms Cps ma Cpa The method predicts the outlet air temperature within an error band of+2.5% to -1.5% and the outlet solids temperature within ±6%. The method seems to be useful for the practising engineers because it is computationally less strenuous. The pressure drop is a major performance parameter for a heat exchanger as it governs the operating cost. The pressure drop across cyclone for solids laden air is studied in terms of ratio of pressure drop for solids laden air to that of solids free air at same inlet air velocity. Within the range of parameters investigated, the pressure drop ratio increases with increasing particles size but reduces with increasing solids feed rate. The proposed correlation predicts the pressure drop ratio within an error band of ±10 %for the present data of solids feed rate from 0.5 to 8 g s"1, ratio of particles size to cyclone diameter from 0.00163 to 0.0046 and pressure drops ratio from 0 to 1.0. AP< 0.34 + °66 AP 1 + 3.4E-7 p vDcy m!64 The proposed correlation for the pressure drops ratio appears to be better than existing correlations given by Briggs (1946), Smolik et al. (1975) and Baskakov et al. (1988) for the data of the present investigation. Cyclone performance curves have been devised to help the plant engineers and the design engineers in understanding the effect of operating parameters on the heat recovery. The cyclone performance curves are plotted for different inlet air velocities between thermal effectiveness of air and pressure drop ratio for different solids feed rates and particles sizes as parameters. Cyclone performance curves suggest that to maximise heat recovery, solids to air loading ratio should be large and to minimise pressure drop across cyclone, small size particles should be used