Fouling prevention with fluidized particles in evaporation of traditional Chinese medicine extract

CHINA PARTICUOLOGY Vol. 2, No. 2, 81-83, 2004z Research NoteFOULING PREVENTION WITH FLUIDIZED PARTICLES IN EVAPORATION OF TRADITIONAL CHINESE MEDICINE EXTRACT Mingyan Liu1,*, Xiulun Li1, Ruitai Lin1, Wanda Nie2, Rucheng Zhang2 and Ningsheng Ling2 1School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, P. R. China2Tianjin Zhong Xin Pharmaceutical Group Company Limited, Tianjin 300380, P. R. China*Author to whom correspondence should be addressed. E-mail: myliu@Abstract The present investigation shows that comparing with the evaporation of vapor-liquid two-phase flow boilingsystem, heat transfer is enhanced by adding proper inert solid particles into the traditional Chinese medicine liquid whichis under evaporation. As a result, fouling prevention effects are evident in such three-phase flow boiling evaporator.Keywords fouling prevention, fluidized particles, evaporation, traditional Chinese medicine liquidThe fouling of the inside wall of evaporator heating tubes used in concentrating traditional Chinese medicine (TCM) extraction liquid is solved by adding inert solid particles into the broth. While suspended in the boiling vapor-liquid two-phase fluid, these particles in motion continuously scour the heat transfer surface to prevent solid deposition as well as to remove scales that have already formed (Klaren, 1999; Zhang & Li, 2000; Liu et al., 2001). The presence of these added solid particles also increases the heat transfer coefficient, thus lowering the temperature of evaporating wall to effectively protect the TCM from over-heating.1. ExperimentalFig. 1 shows the experimental apparatus which consists of a quartz glass single-tube evaporator, operating with natural circulation, i.d. 0.038 m, o.d. 0.045 m, and 2.5 m in height, connected to a 1.2 m length stainless steel va-por-liquid separator followed by a stainless steel vapor condenser.The outside surface of the quartz glass tube is plated with a thin layer of transparent electrically conductive ma-terial for heating electrically the evaporating liquid and at the same time permitting viewing the flow and fouling phe-nomena. Except for the quartz glass evaporator tube, the apparatus is insulated with fiberglass.The inert solid particles are 0.0025x0.0025 m cylinders made of an inert material with a density of 2190 kg.m-3, that does not interact with the TCM extract being processed, Gengnian'an in ethanol and water in the present study. The selection of the material and size of the inert solid particles is based on previous research (Zhang & Li, 2000). The physi-cal parameters of the Gengnian'an extract are as follows: boiling point at atmospheric pressure 81.3 o C, vaporization latent heat 1040×103 J.kg-1, viscosity at 20 o C 1.54×10-3 Pa.s, density at 20 o C varying from 880 to 1030 kg.m-3 with con-centration. Inlet and outlet pressures are measured by pressure gauges, liquid temperatures at inlet and outlet by two armored 0.001 m diameter copper-constantan ther-mocouples, and the outside wall temperature by two cop-per-constantan thermocouples on the conductive film. After starting two-phase flow evaporation for a given time, solid particles are then added. The influence of solid holdup ε on the evaporation process is studied for ε=0%, 1%, 3% and 6%. The heat energy transferred through the heating tube, calculated from electric input for the 2.5 m length of the evaporating tube, is used as the basis for heat-transfer rate computation.Fig. 1 Schematic diagram of experimental setup of vapor-liquid-solid flow boiling of extraction liquid of traditional Chinese medicine,wherein, 1. test section of evaporator, 2. condenser, 3. tank ofcondensed liquid, 4. vacuum system, 5. electronic heatingsystem, 6. temperature measuring system.CHINA PARTICUOLOGY Vol. 2, No. 2, 200482 2. Results and DiscussionThe heat-transfer rate q is measured and calculated by either of the following equations:e l q q q =−, (1)q m λ=. (2)The heat-transfer coefficient h based on the outside tube area is determined as follows:o wo b o wo b ()()q qh A T t d L T t ==−π−. (3) 2.1 Effect of heat flux on heat-transfer coeffi-cientFig. 2 shows the relationship between heat flux q /A and heat-transfer coefficient h with different solid holdups, in-dicating that h first increases and then reaches a steady value with increasing q /A at given solid holdups. The h versus q /A curve can be basically divided into two parts: a linear part when q /A is low, and followed by a flat part when q /A is high. The transition between the two parts is con-sidered as the incipient point of boiling heat-transfer. The linear part results from the natural convection of sin-gle-phase liquid flow when ε=0% and liquid-solid two-phase flow for other values of ε. Heat transfer coeffi-cient h increases with solid holdup, sharply when the holdup is high, showing how solid particles can improve heat-transfer. The flat part of the curve for boiling heat-transfer involving vapor-liquid-solid three-phase flow shows reduced effect of the solid particles.Fig. 2 Variation of heat-transfer coefficient h with heat flux q/A.2.2 Effect of solid holdup on heat-transfer tem-perature differenceIt can be seen from Fig. 2 that the higher the solid holdup, the steeper the linear part of the curve, that is, the higher the slope of the linear part. Since the physical meaning of the slope is the reciprocal of the heat-transfer temperature difference, it can be said that the higher the solid holdup, the less the heat-transfer temperature difference at given heat flux in heat transfer for single or liquid-solid two-phase flow. For the flat part, vapor-liquid-solid three-phase boiling heat transfer is the main mechanism. The enhancement effects of solid particles on three-phase heat transfer can be revealed by studying the relationship between solid holdup and temperature difference for boiling heat transfer △T =T wo -t b at constant heat flux values, as shown in Fig. 3, that is, △T decreases with increasing solid holdup. A dis-tinct cause of heat-transfer enhancement is the boundary disturbance created by solid particle flow, especially when solid holdup ε increases up to 3%. Decreased △T meanslower wall temperature which is highly favorable for pro-tecting the quality of temperature-sensitive TCM products.Fig. 3 Temperature difference versus solid holdup in va-por-liquid-solid boiling of TCM extract.2.3 Effect of solid holdup on heat-transfer coef-ficientFig. 4 shows the heat-transfer coefficient for three-phase boiling evaporating system increases from about 600 to 1200 W .m -2.K -1 as solid holdup increases from zero toε=3%, for various heat flux values. With further increase insolid holdup, however, the increase in heat transfer coeffi-cient is far less significant.Fig. 4 Relations between heat transfer coefficients for TCM extractand solid holdup.Liu, Li, Lin, Nie, Zhang & Ling: Fouling Prevention with Fluidized Particles in Evaporation832.4 Relationship between vapor generating rate m and solid holdup εFig. 5 shows the variation of vapor generating rate of mwith solid holdup ε for vapor-liquid-solid three-phase flow boiling at given heat flux values. Theoretically, when heat flux is specified, the vapor generating rate for a given heat-transfer area is fixed. The fluctuating value of m in Fig. 5 may be largely due to experimental difficulty in con-trolling the heat loss and thus keeping the operationsteady.Fig. 5 Vapor generating rate versus solid holdup.2.5 Effectiveness of solid particles on foulingprevention or cleaningThe present experiments showed that, in the evapora-tion of Gengnian'an extract, without adding solid particles, in as few as 15 hours of operation the inside wall of thequartz evaporating tube would be coated with a brownfouling layer to make the tube no longer transparent. However, with the use of solid particles, evaporation could be carried out for as many as 60 hours without any such deposit. The latest experimental study on a pilot apparatuswith five heating tubes yielded further evidence that foulingcould be effectively avoided by adding inert solid particles.AcknowledgementThe authors wish to acknowledge the financial support of the Special Project for Modernization of Traditional Chinese Medicine of Tianjin (No. 0131086112).NomenclatureA o outside surface area of heating tube, πd o L , m 2d o outside diameter of heating tube, mh heat-transfer coefficient , W .m -2.K -1L heating tube length, mm mass flow rate of vapor, kg .s -1q heat transfer rate, Wq lheat loss rate, wo r wo r (9.40.052())()T t A T t +−−,Wq e electric heating power, Wt bboiling point of TCM extraction liquid, oC t r roomtemperature, oC T wiinside wall temperature of heating tube, oC T wooutside wall temperature of heating tube, oC △Theat-transfer temperature difference, T wo -t b , oC Greek lettersεvolumetric concentration of particle, %λvaporization latent heat, J .kg -1πcircumference ratio, dimensionlessSubscriptsb boiling point e electric heating i inside l heat loss o outside r room w wallReferences Klaren, D. G. (1999). Self-cleaning heat exchangers. Int. J. Hy-drocarbon Eng., 4, 42-48. Liu, M. Y., Jiang, F., Li, X. L. & Lin, R. T. (2001). Evaporation of extraction liquid of TCM with vapor-liquid-solid flow boiling.Proceedings of Annual Meeting of China Medicine Institute , China Medicine Institute, Beijing, Vol. 1, 207-208.Zhang, L. B. & Li, X. L. (2000). Study on boiling heat transfer inthree-phase circulating fluidized bed. Chem. Eng. J., 78, 217-223.Manuscript received February 6, 2004 and accepted March 7, 2004.。