Ind. Eng. Chem. Res. 2000, 39, 2519-2524
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Evaluation of Process Variables in a Stripping/Regeneration Process Using the Experimental Design Methodology Tsair-Wang Chung,* Chun-Han Lai, and Honda Wu Chemical Engineering Department, Chung-Yuan Christian University, Chungli, Taiwan 32023, Taiwan, R.O.C.
A spray tower was used to strip the water vapor from the dilute solutions in this study. The role of each operating parameter and its relationship in this stripping/regeneration (ST/RE) process are still unknown. To improve the ST/RE process and elucidate the relationship between the process variables, an experimental design methodology was used to evaluate the process variables of the ST/RE process. A second-order polynomial model with a multiple linear regression to estimate the model coefficients of the five selected factors (process variables) was proposed to study the influence of the factors on the mass transfer coefficient response in the ST/RE process. Only 32 (or 25) experimental runs were necessary to assess these variables and the model satisfied the process very well since the coefficient of determination (R2) was very close to one. The analysis of the mass transfer performance with these experimental data was also reported in this study. Introduction There are many gas separation operations including absorption, stripping, adsorption, and desorption in the chemical engineering processes. Most of these operations have been discussed in the open literature for their applications and mass transfer performance. However, studies on the stripping operation are rare, especially on its mass transfer performance. Therefore, the purpose of this study was to understand the effects of the process parameters on the mass transfer performance of the air stripping process. A systematic study on the process parameters using the experimental design methodology was conducted in the stripping/regeneration (ST/RE) of desiccants in an absorption dehumidification process. As noted, the stripping operation is useful for regenerating aqueous working solutions in the absorption dehumidification process and to strip out the pollutants from wastewater. The regeneration of aqueous CaCl2, LiCl2, and LiBr solutions using the ST/RE process was reported by Gandihidasan (1994 and 1995), Lof et al. (1984), and Lenz et al. (1990). Johannsen et al. (1983), Peng and Howell (1984), and Park et al. (1994) studied the regeneration of aqueous triethylene glycol (TEG) solutions. These aqueous salt and glycol solutions are typical desiccant solutions in absorption dehumidification systems. On the other hand, Hwang et al. (1992), Djebbar and Narbaitz (1995), Meyer et al. (1995), and Gavaskar et al. (1995) studied the removal of volatile organic compounds (VOCs) from wastewater. It should be noted that some vapors or gases removed by the stripping process could be recovered simultaneously. Weiland et al. (1982) and Escobillana et al. (1991) reported on the recovery of acid gases from the stripping process. In some cases, the stripping process involves simultaneous physical absorption for continuous operation. This type of system is called an absorption-stripping system. The working solution is regenerated by air stripping and sent back to the absorber for continuous operation. Depending on the requirements, various * To whom correspondence should be addressed.
types of gas-liquid contact devices are used in absorption-stripping systems including spray towers, packed towers, and falling-film towers. These gas-liquid contact devices have certain advantages and disadvantages. A spray tower was used in this study of desiccant solution regeneration because studies on spray towers are rare in the literature (Chung and Wu, 1998). The working solutions of this absorption-stripping system for dehumidification were aqueous triethylene glycol (TEG) and propylene glycol (PG). Their versatility combined with good dehumidification performance makes glycols widely accepted by industry. The vent losses of glycols are minimal since their vapor pressures are relatively low. Glycols are also excellent solvents for many organic compounds and are completely watersoluble. Therefore, aqueous glycol solutions in the absorption dehumidification system have a dehumidification capacity and good solubility for some organic pollutants such as benzene, toluene, chloroform, etc. The gas-phase overall mass transfer coefficient, KGa, is one of the most important parameters to evaluate in the mass transfer performance of a stripper and to design the stripper in the ST/RE process. The experimental mass transfer coefficients were calculated in this study and discussed using the experimental design methodology to understand the influences of the process variables on the mass transfer coefficients. The experimental design methodology provides the means of building a statistically significant model of a phenomenon by performing a minimum set of well-chosen experiments (Oliveros et al. 1997). The analysis of variance (ANOVA) provides a way to understand or discuss the experimental data from the experimental design method. The factorial design of experimental design methodogy used in this study was able to elucidate the relationship between the experimental parameters and the system performance, such as the mass transfer coefficient. Experimental System The flow diagram of the ST/RE process is shown in Figure 1. The design of a “U-shaped” air tunnel with eliminators in the spray stripper to allow air and
10.1021/ie990663t CCC: $19.00 © 2000 American Chemical Society Published on Web 06/13/2000
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Figure 1. Stripping/regeneration system for this study. Table 1. Experimental Data of This Study (TEG) air liquid air air air inlet air outlet liquid liquid temp equilibrium spray rate of mass height flow flow inlet outlet humidity humidity inlet outlet below TEG humidity tower stripping transfer of a rate rate temp temp (g of H2O/kg (g of H2O/kg temp temp the fin concn (g of H2O/kg height (kg of coefficient transfer (kg/min) (kg/min) (°C) (°C) of dry air) of dry air) (°C) (°C) (°C) (wt %) of dry air) (cm) H2O/h) (kmol/m3 s) unit (m) 5.13 5.13 5.13 5.13 5.13 5.13 5.13 5.13 3.20 3.85 4.49 5.13 5.13 5.13 5.13 5.13 5.13 5.13 5.13 5.13 5.13
2.76 2.76 2.76 2.76 2.76 2.76 2.76 2.76 2.76 2.76 2.76 2.76 1.60 1.99 2.37 2.76 2.76 2.76 2.76 2.76 2.76
36.2 36.5 36.7 37.2 29.5 29.8 30.2 30.6 33.5 34.1 34.0 33.9 35.7 35.9 36.0 36.2 36.2 35.6 36.6 36.9 36.3
51.4 50.9 50.7 50.8 43.8 46.6 48.8 51.5 50.4 50.6 50.0 49.6 44.9 47.1 49.1 51.8 51.8 50.7 51.2 50.9 50.9
20.1 20.1 20.5 21.0 19.4 19.5 19.5 19.9 19.0 19.5 19.5 19.0 19.7 19.9 19.1 19.3 19.3 20.3 21.4 21.5 22.0
44.0 51.0 56.3 60.5 26.8 32.4 37.8 49.0 48.4 49.0 48.4 46.0 30.5 34.5 37.5 45.5 45.5 41.5 46.3 45.0 47.5
75.0 75.1 75.0 75.0 60.1 65.0 70.1 75.2 75.0 75.1 75.1 74.9 75.0 75.16 75.1 75.06 75.0 75.0 75.2 75.2 74.9
solution cocurrent contact neglects the carryover of the solution. The spray tower in this study was a stripper, which was used to regenerate the working solutions of TEG and PG. Full-cone spray nozzles were used in the spray tower. The flow rates of the nozzles varied from 1.5 to 3 L/min at different pressures. These corresponded to spray angles of 55-70°, respectively. The diameters of the liquid particles from the nozzles were 290-410 µm. The stripper managed air flow rates from 3.2 to 5.13 kg/min and liquid flow rates from 1.6 to 2.76 kg/min. The heat source for the solution regeneration was an 80-L insulated water tank with a 2-kW electric heater. TEG solutions of 91.8-95.8 wt % and PG solutions of 90.8-95.8 wt % were employed in this study. The concentration of the solution was measured using a refractometer. A Rotronic IDL 20K hygrometer with two humidity probes, which can measure the relative humidity from 0% RH to 100% RH at -20 to
64.6 60.2 59.1 58.7 50.6 53.8 56.8 60.3 63.0 61.3 60.9 60.4 61.7 60.2 62.5 63.7 63.7 60.6 61.3 62.7 61.2
67.9 66.0 64.7 63.9 56.0 59.4 62.4 65.5 66.4 65.3 65.1 64.9 67.1 67.5 67.0 66.9 66.9 67.2 68.1 67.6 66.4
95.8 94.0 92.5 91.8 93.7 93.7 94.0 94.5 95.0 95.3 95.0 94.5 95.8 95.6 95.5 95.5 95.5 95.3 95.0 95.5 95.5
46.3 64.0 86.0 95.0 34.8 43.5 52.0 58.5 54.0 51.0 54.0 59.0 46.3 46.5 48.5 48.5 48.5 51.0 54.0 48.5 48.0
70 70 70 70 70 70 70 70 70 70 70 70 70 70 70 70 70 70 70 70 70
7.35 9.51 11.01 12.15 2.28 3.97 5.63 8.95 5.65 6.81 7.78 8.31 3.32 4.49 5.66 8.06 8.06 6.50 7.70 8.31 7.85
0.124 0.148 0.164 0.169 0.053 0.083 0.107 0.143 0.093 0.110 0.127 0.142 0.072 0.090 0.109 0.136 0.136 0.114 0.123 0.129 0.124
0.96 0.80 0.73 0.71 2.30 1.46 1.12 0.83 0.80 0.81 0.82 0.84 1.69 1.35 1.10 0.87 0.87 1.04 0.97 0.93 0.96
+60 °C, was used in this study. The accuracy of this hygrometer was about (0.2% RH. The air flow rates were controlled by a transistor inverter on a 0.5 hp blower. The liquid flow rates were measured by a rotameter, and the air flow rates were measured by a hot-wire flowmeter. The flowmeter and flow controllers used in this study were calibrated using standard procedures. Results and Discussion The ST/RE process experiments were performed on a spray tower. Experimental results under different operating conditions such as the concentration and temperature of TEG and PG solutions, the air and liquid flow rates, and the humidity of the air are shown in Tables 1 and 2. The rate of stripping, the overall mass transfer coefficient, and the height of a transfer unit
Ind. Eng. Chem. Res., Vol. 39, No. 7, 2000 2521
Figure 2. Effect of various operating conditions on the rate of stripping. Table 2. Experimental Data of This Study (PG) air liquid air air air inlet air outlet liquid liquid temp equilibrium spray rate of mass height flow flow inlet outlet humidity humidity inlet outlet below PG humidity tower stripping transfer of a rate rate temp temp (g of H2O/kg (g of H2O/kg temp temp the fin concn (g of H2O/kg height (kg of coefficient transfer (kg/min) (kg/min) (°C) (°C) of dry air) of dry air) (°C) (°C) (°C) (wt %) of dry air) (cm) H2O/h) (kmol/m3 s) unit (m) 5.13 5.13 5.13 5.13 5.13 5.13 5.13 5.13 3.20 3.85 4.49 5.13 5.13 5.13 5.13 5.13 5.13 5.13 5.13 5.13 5.13 5.13 5.13
2.76 2.76 2.76 2.76 2.76 2.76 2.76 2.76 2.76 2.76 2.76 2.76 1.60 1.99 2.37 2.76 2.76 2.76 2.76 2.76 2.76 2.76 2.76
22.4 22.3 22.3 22.4 24.9 25.1 25.2 25.3 29.5 28.3 28.3 28.4 25.7 26.1 26.5 26.6 21.1 20.7 21.3 21.8 25.3 28.4 26.6
52.5 51.4 50.8 50.7 41.8 45.4 48.2 51.5 52.4 51.4 51.7 52.5 47.1 50.0 52.5 53.9 50.3 49.4 50.7 50.6 51.5 52.5 53.9
19.4 19.4 19.5 19.5 17.0 17.0 17.0 17.0 17.5 17.5 17.6 17.9 19.6 19.6 19.7 19.9 7.50 10.0 13.7 14.0 17.0 17.9 19.9
39.0 45.0 50.5 54.0 23.0 27.5 32.0 37.8 37.0 37.7 38.3 38.3 28.6 32.3 36.0 40.0 35.0 38.0 38.3 38.5 37.8 38.3 40.0
75.1 75.0 75.1 75.0 60.0 65.1 70.0 75.0 75.0 75.0 75.0 75.0 75.0 75.1 75.0 75.1 75.1 75.0 75.1 75.0 75.0 75.0 75.1
were calculated using the experimental data and listed in Tables 1 and 2. Analysis of Mass Transfer Performance. The rate of stripping was proportional to the difference between the outlet and inlet air humidities. Therefore, the rate of stripping, E, can be calculated using
E ) (Wout - Win)Gair
(1)
where Wout and Win are the water contents (kg of H2O/ kg of dry air) of the outlet and inlet air steams, respectively. Gair represents the air mass flow rate (kg of air/h). Figure 2 is a graphical presentation of the rate of stripping for each experimental condition. When the solution concentration and air humidity decrease, the rates of stripping are increased significantly in Figure 2, parts a and e. The lower the solution concentration, the larger the amount of water contained in the solution. Therefore, a large amount of water is able to evaporate from a solution of lower concentration. The rate of stripping was lightly influenced by the air humidity because the amount of water vapor removal during the
64.9 63.2 63.2 61.5 50.7 55.9 59.1 63.4 64.0 64.4 63.2 63.2 61.7 64.0 65.2 65.9 59.5 59.0 59.9 58.9 63.4 63.2 65.9
65.7 63.9 62.8 62.2 54.8 58.1 61.7 66.0 68.7 67.8 68.1 66.1 65.7 66.2 67.0 67.4 62.4 63.8 63.9 63.0 66.0 66.1 67.4
95.8 93.8 92.0 90.8 93.3 93.3 93.8 94.8 93.4 94.3 94.8 94.8 94.0 94.8 94.8 95.0 95.0 94.8 95.0 95.0 94.8 94.8 95.0
40.3 56.0 63.0 97.0 29.4 36.7 44.0 50.0 59.0 53.5 50.0 50.0 55.0 50.0 50.0 49.0 49.0 50.0 49.0 49.0 50.0 50.0 49.0
70 70 70 70 70 70 70 70 70 70 70 70 70 70 70 70 70 70 70 70 70 70 70
6.03 7.88 9.54 10.61 1.85 3.23 4.62 6.40 3.75 4.66 5.57 6.28 2.77 3.91 5.02 6.18 8.46 8.61 7.57 7.54 6.40 6.28 6.18
0.111 0.135 0.153 0.164 0.051 0.079 0.103 0.128 0.075 0.092 0.108 0.121 0.062 0.081 0.096 0.110 0.249 0.216 0.166 0.163 0.128 0.121 0.110
1.07 0.89 0.79 0.73 2.43 1.54 1.17 0.93 0.99 0.97 0.96 0.98 1.95 1.48 1.23 1.07 0.48 0.56 0.72 0.73 0.93 0.98 1.07
stripping process is much higher than that in the inlet air stream. This means that the effect of the humidity in the stripping air is not significant. In the stripping process, the growth of gas bubbles in the liquid can partially increase the turbulence or destroy the boundary layer and increase the diffusion mass transfer. In addition, a sufficiently large increase in liquid temperature, air flow rates, and liquid flow rates causes a significant decrease in gas solubility in the liquid phase. In this regard the stripping process reveals a higher mass transfer driving force. As shown in Figure 2, parts b-d, when the liquid flow rate is kept constant, the rate of stripping increases as the air flow rate increases. Similarly, when the air flow rate is kept constant, the rate of stripping increases as the liquid flow rate increases. An increase in the air and liquid flow rates are similar to increasing the amount of stripping air and the amount of water vapor evaporation in a certain operating time. Therefore, more stripping air and/or evaporating water causes more water vapor removal in the stripper. The theory of mass transfer in a gas-liquid contact system has been well developed in the textbooks of
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Figure 3. Effect of various operating conditions on the mass transfer coefficient.
Geankoplis (1993). The overall volumetric mass transfer coefficient for gas phase is obtained as follows:
(KGa)avg )
G Z
∫yy
A,out
(1 - yA)*M
dyA
(1 - yA) (yA - yA*)
A,in
(2)
where Z is the tower height (m), G is the molar flow rate of air (kmol/m2 s), and the gas-phase concentrations varied from yA,in to yA,out. yA* is the equilibrium total molar fraction of an air-water mixture, which is determined by the temperature and concentration of the TEG or PG solution entering the tower. The bulk flow concentration factor is determined using
(1 - yA)*M )
(1 - yA*) - (1 - yA) ln(1 - yA*/1 - yA)
(3)
The KGa is the overall volumetric mass transfer coefficient in the gas phase (kmol/m3 s). If the process is a dilute system, eq 2 can be simplified by mathematical manipulations.
(KGa)avg )
G Z
∫yy
A,out
A,in
dyA (yA - yA*)
(4)
Figure 3 shows the effect of various operating conditions on the overall mass transfer coefficient, which is similar to the effect on the rate of stripping. An increase in the overall mass transfer coefficient with an increase in the air or liquid flow rates is observed in Figure 3, parts c and d. As mentioned earlier, improvement in the gasliquid contact was observed with an increase in the gas and/or liquid flow rates. This will result in an increase of the overall mass transfer coefficient. An increase in the temperature of the TEG or PG solution increases the amount of water vapor evaporation from the solution. This also results in an increase in the overall mass transfer coefficient in Figure 3b. As shown in Figure 3a, water vapor removal from a solution of higher concentration is lower. Because the water content in a solution of higher concentration is lower, the amount of water evaporation is limited. In Figure 3e the overall mass transfer coefficient decreases as the inlet air humidity increases. Most experimental data on spray or packed towers are generally given in terms of the height of a transfer unit (HTUOG) rather than in terms of the mass transfer
coefficient, because the HTUOG is less dependent upon gas flow rates. This provides a means to evaluate system performance under different operating conditions. The HTUOG is defined as the molar velocity on total column cross section divided by the overall mass transfer coefficient.
HOG )
G (KGa)avg
(5)
From eq 5, the effect of various operating conditions on the height of a transfer unit shown in Figure 4 should be contrary to the effect on the overall mass transfer coefficient shown in Figure 3, since an increase in the overall mass transfer coefficient results in a decrease of the value of HTU. The lower the value of HTU, the higher the mass transfer performance expected in the stripper. As noted in Figure 4c, the HTU values are almost constant for different air flow rates because an increase in the air flow rate results in an increase in the overall mass transfer coefficient. The variation in the ratio of air flow rate to the overall mass transfer coefficient is minor. Therefore, the HTU values are almost constant in Figure 4c. Figures 2 and 3 show that the rate of stripping and the mass transfer coefficient in the TEG system are larger than those of the PG system. Therefore, smaller values in the height of a transfer unit in the TEG system were expected in Figure 4. Experimental Design Method In this study the factorial design model (Box et al., 1978) was used to identify the five selected process variables influencing the overall mass transfer coefficient and to understand the interaction between these process variables. The statistical analysis system (SAS) for factorial design of five factors and two levels applied in this analysis of the mass transfer performance in a spray tower were designed and tabulated in Table 3. Only 25 (five factors) experimental runs were enough for this type of analysis. The five process variables (or factors) chosen in this study were the ambient air humidity (AIH), the solution temperature (LIT) and concentration (CONC), the air flow rate (AFR), and the liquid flow rate (LFR). The response variable in the overall mass transfer coefficient was represented as MTC in Table 3. In accordance with the usual assump-
Ind. Eng. Chem. Res., Vol. 39, No. 7, 2000 2523
Figure 4. Effect of various operating conditions on the height of a transfer unit. Table 3. Representation of Factorial Design and Variables
Table 4. F Ratios and Probability for Main Effects and Interactions Effects
no.
pattern
AIH
LIT
CONC
AFR
LFR
MTC
factor
sum of squares
F ratios
prob > F
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
----+----+--++----+-+-+--++-+++----++--+-+-+++-+--+++-++-+++++++----+ +---+ -+--+ ++--+ --+-+ +-+-+ -++-+ +++-+ ---++ +--++ -+-++ ++-++ --+++ +-+++ -++++ +++++
13 18 13 18 13 18 13 18 13 18 13 18 13 18 13 18 13 18 13 18 13 18 13 18 13 18 13 18 13 18 13 18
60 60 75 75 60 60 75 75 60 60 75 75 60 60 75 75 60 60 75 75 60 60 75 75 60 60 75 75 60 60 75 75
91 91 91 91 95 95 95 95 91 91 91 91 95 95 95 95 91 91 91 91 95 95 95 95 91 91 91 91 95 95 95 95
3.2 3.2 3.2 3.2 3.2 3.2 3.2 3.2 5.13 5.13 5.13 5.13 5.13 5.13 5.13 5.13 3.2 3.2 3.2 3.2 3.2 3.2 3.2 3.2 5.13 5.13 5.13 5.13 5.13 5.13 5.13 5.13
1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6 2.76 2.76 2.76 2.76 2.76 2.76 2.76 2.76 2.76 2.76 2.76 2.76 2.76 2.76 2.76 2.76
0.047 0.022 0.1 0.09 0.035 0.022 0.065 0.049 0.061 0.026 0.153 0.131 0.049 0.021 0.087 0.075 0.064 0.047 0.126 0.117 0.052 0.037 0.096 0.08 0.092 0.069 0.196 0.178 0.077 0.045 0.153 0.13
AIH LIT CONC AFR LFR AIH × LIT AIH × CONC LIT × CONC AIH × AFR LIT × AFR CONC × AFR AIH × LFR LIT × LFR CONC × LFR AFR × LFR
0.000 008 29 0.001 824 89 0.001 069 93 0.000 069 99 0.000 037 80 0.000 120 13 0.000 000 50 0.002 211 12 0.000 162 00 0.002 211 13 0.000 264 50 0.000 002 00 0.000 496 12 0.000 002 00 0.000 684 50
0.2708 59.5882 34.9364 2.2855 1.2342 3.9224 0.0163 72.2000 5.2898 72.2000 8.6367 0.0653 16.2000 0.0653 22.3510
0.6099
