1636
I n d . Eng. C h e m . Res. 1994,33, 1636-1640
Experimental Investigations on the Thermal Effects in Packed Bed Liquid Desiccant Dehumidifiers M. Sadasivam and A. R. Balakrishnan’ Department of Chemical Engineering, Indian Institute of Technology Madras, Madras 600 036, India
This paper describes an experimental study to determine t ,a parameter defined in an earlier analytical study as the fraction of the heat of dilution in packed bed liquid desiccant dehumidifiers that goes into the air stream. 5 was determined at various operating conditions in a packed bed using calcium chloride solution as the liquid desiccant in countercurrent flow with air. [ has been correlated in terms of L/G ratios, gas Reynolds number, the ambient number, and a new parameter, the desiccant number. An extension of the CNTU model incorporating [ is presented, and comparison with data, both present and those reported in the literature, show good agreement.
Introduction Dehumidification of gases, an important operation, both in industry and in comfort conditioning of living space, can be accomplished by the vapor-compression cycle or by open-cycle desiccant systems. While the former requires high-grade energy such as electricity, the latter can use low-grade energy sources such as waste heat or solar energy which is required primarily for the regeneration of the desiccant. There have been many studies reported in the literature on liquid desiccant dehumidifiers since the early work of Lof (1955). Waugaman et al. (1993) have presented a review of the studies on open-cycle systems employing both solid and liquid desiccants. Gas and liquid can be effectively contacted in packed towers. Factor and Grossman (1980) and Gandhidasan et al. (1987)developed models for packed bed liquid desiccant dehumidifier design based on the earlier work of Olander (1961) and Treybal (1969). While Factor and Grossman (1980) neglected the transport resistances on the liquid side, Gandhidasan et al. (1987)neglected the heat-transfer resistances on the liquid side. Stevens et al. (1989) introduced an effectiveness (e)-NTU model which defines the number of transfer units (NTU) on the basis of the gas mass velocity. Sadasivam and Balakrishnan (1992) defined NTU on the basis of the minimum capacity fluid which is consistent with the definition of NTU used in heat exchanger design. Though the design procedure of Treybal(l969)included the thermal effects due to absorption, an important assumption in the model is that all the heat evolved during absorption is carried way by the liquid stream. Treybal (1984) subsequently noted that because of this essentially conservative approximation the liquid outlet temperature predicted would be higher than it should and this results in a taller column than required. Experimental data reported by Raal and Khurana (1973) for the ammoniaair water absorption system and by Stevens et al. (1989) for lithium bromide solution dehumidification of air in packed beds indicate that models based on the above assumption overpredict the liquid temperature profile and underpredict the outlet gas temperature. Sadasivam and Balakrishnan (1991)developed a model that incorporated a new parameter 5, defined as the fraction of the heat of absorption accounted for in the gas stream in the balance equations. Their study clearly brought out the importance of 5 in design where under certain operating conditions the outlet temperature of the gas predicted could be less than the true value by as much as 20 7%. The present study
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describes an experimental study to determine 5 for the calcium chloride solution-air system.
Model Equations The model description and derivation of the equations are available elsewhere (Sadasivam and Balakrishnan, 1991). The humidity gradient of the air in the axial direction of the packed column in which it is contacted with the liquid desiccant can be obtained from the masstransfer rate equations as
The temperature gradient of the air can be obtained from a gas-side enthalpy balance as
where 5 is the fraction of the heat of dilution, AHD,in the gas stream. The interfacial temperature can be obtained from an overall enthalpy balance using the gas-side and the liquid-side enthalpy balance as Ti = GB(dYA/dZ)(AHD - A,,
- CAT,)
+ h,a,T, + hG’a,TG (3)
These three equations obtained from the mass and enthalpy balances over a differential section of the packed bed have to be solved simultaneously. The equations are solved numerically over each differential section of the bed, the bed being divided into a large number of segments (see Sadasivam and Balakrishnan, 1991).
Experimental Section A schematic of the experimental setup used in the present study is shown in Figure 1. An acrylic tube of internal diameter 91mm which could be filled with packing to a maximum height of 1m was used as the column. The packing consisted of ll-mm ceramic Berl saddles, and packing heights of 0.4, 0.55, and 0.7 m were used. The column was insulated to ensure adiabatic conditions. Air is supplied to the bottom of the column at various flow rates, temperatures, and humidities using a blower, an on-line heater, a humidifier, and a bypass line. Liquid 0 1994 American Chemical Society
Ind. Eng. Chem. Res., Vol. 33, No. 6, 1994 1637 1 2 3 4 5 6 7
A i r blower Orifice meter Air h e o t e r Humidifrcotion chamber Spent solution tonk Solutlon t o n k w i t h heoter S o l u t i o n pump 8 Rotameter 9 Pocked column
H-Humidity T- Temperature C - Concentrotion M - Monometer I' o p p i n g s
T, In = 3'C G
XI, = 2 8 %
2.8
3.6 LIG
4.4
26
28 30 XI, 1%)
32
33
35 Tlj
i
0.89
37 I'C 1
39
Figure2. Performance characteristics of packed bed liquid desiccant dehumidifiers: bed height 0.7 m.
Figure 1. Schematic diagram of the experimental setup.
desiccant is pumped from a stainless steel tank through two rotameters in parallel (of different ranges) to the top of the column. The flow rate is controlled using a bypass valve. The liquid temperature is controlled by a thermostat connected to three electric heaters in the tank. The liquid is distributed uniformly over the packing material in the bed using a perforated plate distributor. Air dry bulb temperatures at the inlet and outlet were measured by Chromel-Alumel thermocouples. Similar thermocouples covered with moist wick are used to measure the wet bulb temperatures. A dripping arrangement (similar to the arrangement used in intravenous transfusions in medical practice) is employed to continuously moisten the wick. The concentration of liquid desiccant is obtained from its specific gravity measured at the reference temperature (20 "C in this study). Different concentrations of calcium chloride solution in water, the liquid desiccant used in the present study, were prepared, and the exact concentration was determined by titration with ethylenediaminetetraacetic acid (EDTA) using Erichrome Black T as indicator. Using this, a calibration chart was prepared showing concentration and specific gravity. Care is taken to see that the solution is always at 20 OC during density measurements as density is a function of both temperature and concentration. The reproducibility of concentration measurements by this method was reported by Patil et al. (1991) to be within 0.35 % . Experiments were performed with calcium chloride solution as the liquid desiccant drying the air stream. The data were obtained under different operating conditions such as fluid flow rates, inlet temperatures, liquid inlet concentrations, and air inlet humidities. The experimental results so obtained were used to evaluate the performance characteristics of the packed bed liquid desiccant dehumidifiers and to evaluate E, the fraction of the heat of dilution going into the gas stream, using the model equations described earlier.
Results Representative experimental results are presented in Figure 2 for a bed height of 0.7 m. They show the outlet humidity of the processed air as a function of LIG ratios, inlet liquid desiccant concentrations Xin, and inlet air temperatures Tcinfor a given inlet air humidity. Figure 2a shows the effect of Ll G ratio on the air outlet humidity. It may be seen from the figure that as L fG increases, the air outlet humidity comes down. Increase in liquid flow
rate results in more liquid being available for the transfer process. Mass-transfer coefficient and interfacial area increase with increase in liquid flow rate, and hence more mass transfer results. However, liquid flow rate cannot be increased beyond the floodingvelocity in countercurrent contactors. Figure 2b shows the effect of liquid inlet concentration on the air outlet humidity. The figure indicates that the air outlet humidity decreases with increase in liquid inlet concentration. That is, as the concentration of the desiccant increases, the ability of the desiccant to absorb moisture increases. This is because as the liquid concentration increases, the vapor pressure of the liquid decreases and therefore higher driving force between the phases for mass transfer results. Higher liquid concentrations favor increased dehumidification. In Figure 2c the effect of air inlet temperature on air outlet humidity is shown. It can be seen from the figure that the air outlet humidity increases with increase in air inlet temperature or, in other words, the dehumidification rate decreases with air inlet temperatures. The reason for this is that any increase in air inlet temperature increases the sensible heat-transfer rate thereby increasing the liquid temperature. This results in an increase in the vapor pressure of the liquid and hence lower mass-transfer rate. In fact, at higher air inlet temperatures, the sensible heat transfer and the resulting increase in liquid vapor pressure may cause mass transfer to take place in the opposite direction, Le., from liquid to air humidification. Lower air inlet temperatures are preferred to achieve high dehumidification rates.
Estimation and Correlation of 5 The experimental results obtained are used to evaluate
5, the fraction of the heat of absorption that is accounted for in the gas stream. The experimental outlet condition of the air is compared with the theoretically obtained values at different operating conditions with the model of Sadasivam and Balakrishnan (1991) described earlier. Values of E are adjusted until a match is achieved. This is obviously a trial and error procedure and is done on a computer as the model itself is an iterative calculation. It is obvious that the parameter 5 will depend on flow rates and inlet conditions of the two fluid streams. The height of the packing will also be a parameter, since the hydrodynamics of the packed bed along with the entrance effects may influence E. Flow rates of the gas and liquid streams can be expressed in nondimensional form as the LIG ratio. This is frequently used in gas absorption to determine the slope of the operating line and also in estimating the flooding conditions in countercurrent packed beds. The inlet conditions of the gas stream can be nondimensionalized by making use of the definition of enthalpy of the humid air which is given by (reference temperature is 0 "C)
1638 Ind. Eng. Chem. Res., Vol. 33, No. 6, 1994 HG
CSTG
+ AAoYA
(4)
1.z
The ratio of the second to first term, Le., enthalpy of the air due to its moisture content to enthalpy of the air because of its sensible heat, defines a dimensionless ambient number
.\./ "y
/
/
AAOYA Am='STG
Similarly, inlet conditions of the liquid stream also can be nondimensionalized. The enthalpy of the liquid is
where AHa,the integral heat of solution, is a function of the concentration of the liquid. It can be related to the heat of absorption (or dilution) by
AHa =
in
M H , 100 - xin
(7)
Using this, a dimensionless number similar to the ambient number called the desiccant number, is defined as
5 Prc Figure 3. Performance of the correlation.
Physical properties of the gas stream are included in the Reynolds number, which is defined as
where d, is the characteristic diameter of the packing which is the diameter of a sphere of the same surface area is the dynamic fractional void space in the packing, and q,,, i.e., void fraction under flow conditions. Physical properties of the liquid and packing geometry and materials are not separately considered as correlating parameters as these are included in ELO. However, the height of the packing may effect 5 and the ratio Zld, is used as a correlating parameter. This parameter also indicates the degree of axial dispersion within the packed column for a particular flow rate. d, is the nominal packing diameter. A correlation based on the above parameters and the values of 5 obtained from the experimental program is obtained by regression as
The standard error of estimate of the correlation is 0.13. The experimental ranges of the parameters are 1.6 5 L/G 5 4 . 0 ; 950 IReG 5 1550; 0.9 IAm I2.4; 1.1 IDe I1.5; 63. The performance of the correlation is 36 I Zld, .I shown in Figure 3. It may be seen from the figure that the experimental data lie within A14 5% of the values predicted by the correlation. Discussion In view of the fact that t depends on five different parameters, it may be useful to examine this dependence in some detail. It is apparent that 5 decreases with L / G ratio quite rapidly. This can be explained in terms of the mechanism that is proposed for this fraction of the heat going into the gas stream over and above sensible heat transfer due to temperature gradients by convective
transfer. The gas-liquid interface is probably not sharp but extends into both phases, but perhaps more into the gas side than liquid side. This is because once the vapor is in contact with the liquid it will condense completely. On the other hand, the liquid could be entrained on the gas side and vapor condensation in contact with the entrained liquid could take place before the interface is reached. This region may be called a "fuzzy region". As LIG increases, or the gas flow rate gets less, this fuzzy region is likely to get smaller and 5 falls. 4 increases with gas-side Reynolds number. This is as it should be for the same reason given above. As gas velocity increases, the thickness of the fuzzy region increases leading to larger values of 5. Wallis (1971) noted that entrainment of liquid increases with increasing gas velocity. The increase of 5 with Am can be explained by the physical significance of the ambient number. The ambient number is the ratio of the latent heat to the sensible heat of humid air. In other words, it is the ratio of the amount of energy available in the form of water vapor in the air to the amount of energy available by virtue of its temperature. The ambient number can be considered as a measure of the climatic conditions. A lower ambient number indicates hot and dry conditions. Higher ambient numbers indicate humid conditions. Therefore at high ambient numbers, i.e., the air being humid and therefore with higher partial pressure of water vapor in it, the driving force for mass transfer is high. Hence more water vapor can be expected to condense, resulting in the release of a corresponding amount of heat of dilution. With more of this process taking place in the fuzzy region, values of $, show an upward trend. 5 increases with De, and once again the reason for this can be explained by examining the physical significance of De. Analogous to the ambient number, the desiccant number is the ratio of the heat of solution to the sensible heat of the solution. In other words, it is the ratio of the enthalpy of the solution due to its concentration in the form of the heat of solution to the enthalpy of the liquid by virtue of its temperature. The desiccant number helps
Ind. Eng. Chem. Res., Vol. 33, No. 6,1994 1639 in choosing a suitable desiccant for a particular application. It indicates the ability of the desiccant used to remove the moisture. While selecting the desiccant from a number of candidate materials, it is better to choose one with as low a De as possible, since the heat release at the interface will be small, resulting in close to isothermal conditions and therefore more mass transfer. On the other hand, given a desiccant, a higher value of De is to be preferred during operation as it would lead to greater dehumidification capacity. This is possible by using higher concentrations or lower temperatures of the desiccant liquid. It is therefore obvious that higher De favors higher masstransfer rates caused by lower vapor pressure of the liquid and hence higher driving force. As more water vapor is absorbed, the amount of heat release at the interface is larger. As seen in the case of the ambient number, there must be a tendency toward more of the heat going toward the gas side as more vapor condensation will take place in the fuzzy region.
Table 1. Evaluation of the Integrals
11
and If
+
b 2cr I,=- 2 arctg All2 AI/'
A>O
1 In R - -I, b I, = c 2c 1 In R - -I, b I, = c 2c
I,=-
A
