Ind. Eng. Chem. Res. 2010, 49, 847–851
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Evaporative Cooling of Water in a Rotating Packed Bed (Split Packing) S. Bhattacharya, A. Mondal, A. Bhowal,* and S. Datta Department of Chemical Engineering, JadaVpur UniVersity, Kolkata 700032, India
Rotating packed beds (RPB) have been studied for intensifying mass-transfer processes. This work was undertaken to evaluate the thermal performance of an RPB with split packing for evaporative cooling of water involving simultaneous heat and mass transfer. Higher efficiency was achieved within a smaller contactor volume compared to mechanical draft cooling towers. At all values of the liquid-to-air flow rate ratio studied, efficiency was not significantly affected by rotor speed and decreased with increasing rotor speed beyond a certain limit at low air and water flow rates. A simple mathematical model was used to estimate process parameters from the experimental data based on the Merkel assumption. The hGa/kYaCs ratio varied between 0.85 and 1.18, and the tower characteristic of the RPB was between 1.4 and 4.0. Introduction Cooling towers are among the largest mass-transfer devices and are commonly used in various industries for treating recirculating water needed for condensers, heat exchangers, and other process equipment. In these towers, two phases, namely, ambient air and warm water, are in direct contact with each other. Water is cooled as a result of sensible heat exchange with the air and latent heat transfer due to evaporation of the water. The performance characteristics of mechanical draft cooling towers with different types of packings have been reported by various investigators.1-12 Bedekar et al.5 investigated the thermal performance of an induced-draft cooling tower filled with a vertically aligned filmtype packing. Milosavljevic and Heikkila¨7 reported the volumetric heat-transfer coefficients in a forced-draft cooling tower filled with packings such as fluted plates, smooth plates, and honeycomb filling. The volumetric mass-transfer coefficient in an induced-draft cooling with burned clay bricks as the packing9 was found to decrease when the water inlet temperature was increased beyond 38 °C. Gharagheizi et al.10 concluded from their experimental data that a vertical corrugated packing has a higher thermal efficiency than a horizontal corrugated packing. Lemouri et al.11 observed two operating regimes in a forceddraft counterflow cooling tower filled with vertical grid apparatus (VGA) packing depending on the liquid flow rate. The bubble dispersion regime (BDR) was found to be more efficient than the peculiar regime (PR). The design correlations suggest that the performance of cooling towers can be enhanced by increasing the volumetric gas-side mass-transfer coefficient. Ramshaw and Mallison13 invented a new concept, namely, the rotating packed bed (RPB), to intensify mass-transfer rates for gas-liquid systems. In this type of contactor, mass-transfer operations are performed in a high gravitational field produced by spinning the packing element. Liquid is sprayed on the inner surface of the rotating bed, whereas the gas is forced to flow inward from the outer radius countercurrent to the liquid flow. Operation in a high gravity field allows increased throughputs and attainable volumetric mass-transfer coefficients, thus reducing the physical size of the contactor compared to conventional ones. Sandilya et al.14 showed that the gas phase acquires the angular velocity of the packing within a short distance in the * To whom correspondence should be addressed. E-mail: avijit_bh@ yahoo.co.in. Tel.: +91 033 24146378. Fax: +91 033 24137121.
rotor. The observed enhancement in the volumetric gas-side mass-transfer coefficient is primarily due to the high packing surface areas used in RPBs. Chandra et al.15 determined from pressure drop studies that the use of a split packing instead of a single rotating packing element would promote the tangential slip velocity between the gas and the packing and, hence, enhance the gas-side mass-transfer coefficient. The volumetric gas-side mass-transfer coefficients in an RPB with a split packing (foam metal)16 was found to vary between 80 and 300 1/s. Mass-transfer performance characteristics of RPBs for absorption, stripping, and distillation operations have been reported by various investigators.17-23 In this work, an RPB was evaluated for the evaporative cooling of water, and simultaneous heat- and mass-transfer phenomena were examined therein. As the controlling mass-transfer resistance for this process is in the gas side, a split packing was used. To the best of our knowledge, no studies to determine the thermal performance of this type of contactor have been reported. Experimental Section A schematic of the experimental setup used in this study is shown in Figure 1a. Details of the dimensions of the rotor are presented in Figure 1b. The rotor unit consisted of a pair of 1.5-mm-thick stainless steel circular disks of 320-mm diameter. The axial distance between the two disks was 30 mm. The casing was cylindrical in shape, with a diameter of 350 mm and an axial length of 50 mm. There were three openings in the casing wall. Two of these were utilized for introducing air into the casing, and the third opening was either sealed or used to provide a means for inserting a pressure tap. Stainless steel wire mesh (30 mesh per linear inch) was used as the packing material. To obtain a counter-rotating split-packing RPB, a total of four annular spaces were created at different radial positions on each of the two disks by means of a thicker (1-mm) stainless steel wire mesh having an opening of diameter ∼5 mm. The packing wire mesh was wound spirally (from four to six windings) in each of the annular spaces. The contacting phases flowed through the openings in the wire mesh. The two disks were connected through a shaft to an ac motor so that they can be rotated in opposite directions about a horizontal axis. The average packing surface area in the rotor was calculated to be approximately 280 m2/m3. A magnetic pump was used to feed warm water from a storage tank (maintained at a constant temperature) to the RPB
10.1021/ie901207m 2010 American Chemical Society Published on Web 11/20/2009
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Figure 2. Variation of pressure drop with air flow rate.
rotor inside the casing and one of the air outlets were connected to a U-tube manometer filled with mercury. Mathematical Modeling
Figure 1. (a) Schematic diagram of the experimental setup. (b) Rotor dimensions.
unit. Water was sprayed onto the inner periphery of the innermost ring through a stationary distributor (consisting of 12 holes of 1-mm diameter) and flowed radially outward through the rotor under centrifugal force. The liquid that collected on the casing wall was expelled from the casing through an outlet at the bottom of the casing wall. Air used for cooling the warm water was fed to the RPB unit from a compressor. Between the compressor and the RPB unit, the air passed successively through a constant-temperature bath (through coils) and a moisture entrapment unit. The temperature of the bath was maintained constant with a chiller. The air travelled inward through the packing and left the rotor through perforated openings on disk 2 at a radial distance of 5.0-6.0 cm. It then entered the rotating shaft connecting disk 2 to the motor. The portion of this shaft inside the stationary housing was perforated. Air entered the stationary cylindrical housing through these perforations and was discharged to the atmosphere through two outlets (one at the top and the other at the side) in the housing wall. The temperatures of the water entering and leaving the RPB (as well as the dry-bulb temperature of air) were measured by PT-100 thermocouples connected to a digital display unit. A hygrometer (Zeal, London) was connected to the air outlet at the top of the stationary housing for measuring the drybulb and wet-bulb temperatures. (It was found that the entrainment of water in the air discharging through this outlet was negligible.) The corresponding values of the air inlet were measured offline at the beginning of each run. For the entire set of results presented, the wet-bulb and dry-bulb temperatures of inlet air varied in the ranges 15.0-17.2 and 24.3-25.8 °C, respectively, whereas the ambient temperature was in the range 24-28 °C. Each of the experimental runs was of approximately 45-min duration. The pressure drop across the rotor was measured in a separate study. The pressure taps located near the outer periphery of the
The differential equations for determining the bulk temperature (TG) and absolute humidity (YG) profiles of air along the length of a conventional cooling tower (at low mass-transfer rates) are given in Treybal.24 Applied to a differential volume in the rotor of 2πrtH dr, these expressions can be written as dTG ) hGaH(Ti - TG)2πrtH (1) dr dY ) kYaM(Yi - YG)2πrtH G (2) dr where Ti and Yi refer to the interfacial temperature and absolute humidity, respectively; G and Cs are the flow rate and humid heat, respectively, of air; and kYaM and hGaH are the volumetric air-phase mass- and heat-transfer coefficients, respectively. The variable tH is the distance between the counter-rotating disks, and r is the radial distance from the center of the disk. The overall enthalpy balance in a differential element of the rotor can be represented by GCs
dHG dTw dTG dY )G ) GCs + Gλ (3) dr dr dr dr The variables L, Tw, and Cw in this equation represent the flow rate, temperature, and specific heat, respectively, of water, and λ is the latent heat of vaporization. The Merkel assumption was assumed to be valid in the RPB. It supposes that the temperature at the interface is equal to the bulk water temperature. LCw
Results and Discussion Figure 2 shows the variation of the pressure drop with the air flow rate. The filled and unfilled symbols in the figure refer to the pressure drops at L ) 0 kg/s and L ) 0.017 kg/s, respectively. The pressure drop increases monotonically with the air flow rate at constant rotor speed and water flow rate. However, no significant variation in pressure drop was observed when the water flow was switched on. The pressure drop in an RPB with wire mesh packing14 stacked parallel to the flow direction was nearly an order lower than that obtained in this study. Chandra et al.15 also reported high values of pressure drop (up to 4 kPa) in a rotating packed bed with a split packing consisting of three rings with nylon net wound in each ring. Higher frictional losses result when air flows through the openings between wire strands in the wire mesh packing, as
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Figure 4. Variation of efficiency and evaporation rate with air flow rate.
Figure 3. Influence of rotor speed on efficiency.
also noted from the experimental pressure drop reported by Kołodzeij and Łojewska25 for wire gauge packing in a conventional packed bed. By opening the casing, it was possible to visually observe the liquid flow on disk 1 while it was rotating. The liquid phase issued as droplets from the outer periphery of the rotating packing in the direction of rotation into the annular space between the split packing. The packings helped to attach the water droplets and impart the centrifugal force necessary to throw the droplets radially outward. In the experimental studies, the air and water mass velocites varied in the ranges 0.32-0.57 and 0.75-1.5 kg/(m2 s) (based on the average packing radius), respectively. Entrainment of water with exiting air was observed at a rotor speed of 1250 rpm and G ) 0.01 kg/s for the entire range of water flow rates used. At rotor speeds of 900 and 1050 rpm, entrainment was appreciable at liquid flow rates of 0.02 kg/s and above at G ) 0.01 kg/s. The simultaneous heat- and mass-transfer performance of the contactor is presented in terms of the efficiency (η), defined as η)
Tw,in - Tw,out Tw,in - Twb,in
(4)
and the tower characteristic (kYaV/L) was determined based on the Merkel assumption. The numerator in the eq 4 represents the cooling range, Tw,in - Tw,out, and the denominator is the cooling range + approach, that is, (Tw,in - Tw,out) + (Tw,out Twb,in), where Twb,in is the inlet-air wet-bulb temperature. The variation of the efficiency with the rotor speed at a constant air flow rate of 0.01 kg/s is presented in Figure 3. It can be seen that, at a water flow rate of 0.0125 kg/s (Tw,in ) 55 °C), η remained nearly constant as the rotor speed was varied from 500 to 700 rpm and subsequently decreased with a further increase of the rotor speed. At L ) 0.017 and 0.025 kg/s (Tw,in ) 55 °C), the range of rotor speed within which η did not decline was wider (500-1050 rpm). The efficiency was observed to be lower at Tw,in ) 47 °C than at 55 °C for the same water flow rate (L ) 0.017 kg/s). Also, the fall in efficiency at Tw,in ) 47 °C sets in at lower rotor speed compared to Tw,in ) 55 °C. The mass of water evaporated into the air stream (m ˙ w) was calculated from the expression m ˙ w ) G(Yout - Yin)
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(5)
where Yout and Yin are the absolute humidities of air at the outlet and inlet, respectively. The evaporation rate was lower at Tw,in) 47 °C compared to 55 °C. This decreased the cooling range and, consequently, the efficiency as noted in Figure 3. The variations of the efficiency and evaporation rate with air flow at different rotor speeds are plotted in Figure 4. The filled
Figure 5. Variation of efficiency with L/G ratio.
symbols denote efficiency, whereas the unfilled symbols refer to m ˙ w at the same operating conditions. The evaporation rate increases with the gas flow rate and rotor speed. At a given rotor speed, the rise in η with increasing air flow rate is due to the increased uptake capacity of sensible heat and rate of evaporation. At an air flow rate of 0.005 kg/s, η declines continuously as the rotor speed is varied between 500 and 900 rpm. Similarly to the observation made regarding Figure 3, the decrease of η with increasing rotor speed is arrested at higher air flow rates. The variation of efficiency with L/G is illustrated in Figure 5. The filled and unfilled symbols shown in the figure refer to the efficiencies at Tw,in ) 55 and 47 °C, respectively. The efficiency (Tw,in ) 55 °C) decreased from 0.61 to 0.39 as the L/G ratio was varied from 1.2 to 3.1. At rotor speeds of 500 and 700 rpm, the efficiency-L/G profile appeared to level off at high liquid-to-air flow rate ratios. For a cooling tower with VCH packing, Gharagheizi et al.10 reported η values between approximately 0.35 and 0.2 for L/G ratios between 1.0 and 3.5 (V ) 0.375 m3). Foust et al.26 compiled some of the available correlations for kYa in cooling towers in terms of L′ and G′. Simulation studies were performed using these correlations with Lewis number ) 1.0 and Merkel assumption for the inlet conditions used in this study to determine efficiency of conventional cooling towers having the same volume as the RPB. The cross section of the tower was considered to the average radius of the rotor. With wooden slats,1 the maximum efficiency (Tw,in ) 55 °C) that would be attained was 0.15. The maximum efficiency within this contactor volume for a Berl saddle4 packing was estimated to be ∼0.03. The contribution of water evaporation to cooling water range was estimated from the magnitude of the term m ˙ wλ/[CwL(Tw.in - Tw,out)]. The minimum value obtained was 0.85, implying that mass transfer plays a major role in cooling the water in an RPB. However, the value was in excess of 1.0 for many of the
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operating conditions. This finding, along with observations made in Figures 3 and 4, suggests the existence of a phenomena that results in enhancing the sensible heat of water as it flows through the rotor. To identify the reason for this, water at Tw,in ) 45 °C was introduced into the rotating bed to the exclusion of air flow. The exit temperature of water was noted to be higher than the inlet temperature. For example, at 1250 rpm, the temperature rise was between 3.5 and 5 °C. It was less when the RPB was operated at lower rotor speed (2-3 °C at 900 rpm) or at a higher water flow rate. According to Keyvani and Gardener,27 the power input to be Higee is used to accelerate the liquid to the tip speed of the rotor and overcome the frictional forces as the liquid flows through the packing. For water flowing through the RPB, it follows from the first law of thermodynamics at steady-state operation that ˙s L(Hout - Hin) ) -W
dHG dTw ˙ Q )G + 2πrtH (7) dr dr V The profiles of the temperatures and absolute humidities of the phases were obtained by simulation of eqs 1, 2, and 7 with hGa and kYa as parameters. The hGa/kYaCs ratio was varied between 0.85 and 1.18. and kYa was varied between 11 and 24 kg/(m3 s). The latter term was used to determine kYaV/L for the operating conditions. The tower characteristics estimated at various rotor speeds are shown in Figure 6 as a function of the water-to-air flow rate ratio (Tw,in ) 55 °C). In cooling towers, kYaV/L is related to the L/G ratio through a correlation of the form LCw
()
m
Table 1. Comparison of kYa Values in an RPB and Cooling Towers type of packing
investigators
kYa [kg/(m3 s)]
RPB
(6)
assuming negligible kinetic and potential change of the fluid and heat exchange with the surroundings. The variables Hout, ˙ s represent the enthalpy of the water entering and Hin, and W leaving the RPB and the rate of work done on the fluid, respectively. As the liquid flows in contact with the packing through the rotor, viscous forces convert some of the input energy into thermal energy. Impingement of water droplets onto the stationary casing wall also contributes to the temperature increase to a small extent. As a result, the temperature and the enthalpy of the exiting water are raised. The decrease in efficiency (and hence cooling range) at low air flow rates noted in Figure 4 with an increase of the rotor speed from 600 to 900 rpm is due to the aforementioned reason. At low air flow rates, the decrease of the water temperature from the incremental change in evaporation rate as the rotor speed is increased is not sufficient to compensate for the additional imposed heat load due to the frictional forces. The same reason account for the observations made in Figure 3, that is, the efficiency remaining nearly constant or decreasing at higher rotor speed, and the decrease in η is initiated at lower rotor speed at Tw,in ) 47 °C . In the evaporative cooling of water in an RPB, the estimated enthalpy (air and water) associated with the exit streams expectedly exceeded the inlet enthalpy for the same reason. The ˙ can be attributed to difference in total enthalpy denoted by Q the thermal energy influx to water because of the said energy transformations. Assuming this term to be constant per unit volume of the contactor, the overall enthalpy balance given by eq 3 can be modified to
kYaV L )c L G
Figure 6. Tower characteristic of RPB (with Merkel assumption) and mechanical draft cooling towers.
(8)
wire mesh
present study
11-24
Cooling Tower wooden slats Berl saddle VCH VCH
Simpson and Sherwood1 Hensal and Treybal4 Gharagheizi et al.10 Lemouari et al.12
1.8-1.9 0.07-0.1 0.3-0.6 1.7-3.2
The values of the constants m and c in this equation were determined to be (-1.13, 5.62), (-1.13, 5.75), and (-1.17, 4.68) at 900, 700, and 500 rpm, respectively, for this contactor. The closeness of these fitting constants reflects the fact that the tower characteristic does not change significantly with rotor speed. The tower characteristics of conventional mechanical draft cooling towers are also plotted in Figure 6. The data were reported from the experimental investigations of Bedekar et al.5 and Lemouari et al.11 (bubble and dispersion regimes), which were carried out at Tw,in ) 52 and 50 °C, respectively. The tower characteristics for the packings used by Simpson and Sherwood1 and Garagheizi et al.10 (vertical corrigated packing) were determined using the constants c and m reported in the literature. Data for kYa of some of the packings used in cooling towers in the range of L′ and G′ varied in this study are listed in Table 1 along with that obtained in this RPB. It can be seen that both kYaV/L and kYa are higher in the RPB than in cooling towers. Conclusions The thermal performance of a rotating packed bed with a split packing was evaluated for the evaporative cooling of water. Cooling resulted primarily from the evaporation of water. The higher values of efficiency (0.39-0.61), volumetric gas-side mass-transfer coefficient [11-24 kg/(m3 s)], and tower characteristic (1.3-4.1) of the RPB suggest the suitability of this contactor as an alternative to cooling towers. Use of a rotatingpacked-bed contactor can contribute significantly to energy savings and economics. However, entrainment at high rotational speeds and an increase in the liquid temperature due to contact friction with the packing surface in the rotor negate some of the advantages of operation with this equipment. This study indicates the need to optimize the distribution of the packing surface in the rotor for further improvement of the thermal efficiency taking into account the need to maximize gas-liquid surface area and minimize contact friction and entrainment. Nomenclature c ) constant of eq 8 Cw ) heat capacity of water [kJ/(kg °C)]
Ind. Eng. Chem. Res., Vol. 49, No. 2, 2010 Cs ) humid heat of air [kJ/(kg °C)] G ) dry-air flow rate (kg/s) G′ ) air superficial mass velocity [kg/(m2 s)] HG ) enthalpy of air (kJ/kg) Hin ) enthalpy of water entering the contactor (kJ/kg) Hout ) enthalpy of water exiting the contactor (kJ/kg) hGaH ) volumetric air-side heat-transfer coefficient [kJ/(m3 s °C)] kYaM ) volumetric air-side mass-transfer coefficient [kg/(m3 s)] L ) liquid flow rate (kg/s) L′ ) water superficial mass velocity [kg/(m2 s)] m ) constant of eq 8 m ˙ w ) rate of evaporation of water (kg/s) ˙ ) difference between total inlet and outlet enthalpies of the Q streams (kJ/s) r ) radial distance (m) TG ) dry-bulb temperature of air (°C) tH ) distance between the two counter-rotating disks in the rotor (m) Ti ) temperature at the air-water interface (°C) Tw ) bulk temperature of water (°C) Tw,in ) temperature of water entering the distributor (°C) Tw,out ) temperature of water exiting through the casing (°C) Twb,in ) wet-bulb temperature of inlet air (°C) V ) volume of contactor/cooling tower (m3) YG ) absolute humidity of air (kg/kg of dry air) Yi ) absolute humidity at the air-water interface (kg/kg of dry air) Yin ) inlet absolute humidity (kg/kg of dry air) Yout ) outlet absolute humidity (kg/kg of dry air) Greek Letters R ) constant used to evaluate the enthalpy of saturated air β ) constant used to evaluate the enthalpy of saturated air η ) efficiency λ ) latent heat of vaporization of water (kJ/kg)
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ReceiVed for reView July 30, 2009 ReVised manuscript receiVed October 22, 2009 Accepted November 4, 2009 IE901207M
