Design of Steam-Stripping Columns for Removal of Volatile Organic

Mass-transfer data at different feed and steam rates, using structured and random packings, have been collected to develop a model for the design or a...
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Ind. Eng. Chem. Res. 2000, 39, 731-739

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Design of Steam-Stripping Columns for Removal of Volatile Organic Compounds from Water Using Random and Structured Packings J. R. Ortiz-Del Castillo,† G. Guerrero-Medina,‡ J. Lopez-Toledo, and J. A. Rocha* Departamento de Ingenierı´a Quı´mica, Instituto Tecnolo´ gico de Celaya, Mexico Av. Tecnolo´ gico y Garcia Cubas, Celaya, Guanajuato, C.P. 38010, Mexico

Mass-transfer data at different feed and steam rates, using structured and random packings, have been collected to develop a model for the design or analysis of packed columns for stripping of volatile organic compounds (VOCs) from water. The steam-stripping experiments were conducted in a stainless steel column of 0.245-m internal diameter, with 2.8 m of packed section. The packings used were Sulzer BX gauze structured packing, Mellapak 250Y structured packing, 1-in. Flexirings, and 1-in. Fleximax random packing. The VOCs were chloroform (CH3Cl) and toluene (C7H8). The model considers the simultaneous occurrence of mass transfer and hydraulic phenomena with the same expressions for liquid and vapor mass-transfer coefficients and the same expression for effective interfacial area with one constant for each packing. The average deviation for the measured and calculated volumetric mass-transfer coefficient and effective height of packing is 0.29 and 0.23, respectively. Introduction Reduction of chemical discharge to the environment has become a major issue in the chemical and petroleum industries. For the removal of volatile organic compounds (VOCs) from water, several methods have been proposed; oxidation techniques offer a degree of flexibility in tailoring treatment to a specific water or wastewater at reasonable cost. Chemical oxidation is usually most effective as a complementary rather than a self-sufficient process; however, especially in wastewater applications, its use requires careful consideration of the chemical and biological integrity of effluent streams. One special type of chemical oxidation is the oxidation of organic compounds in supercritical water. At the moment this type of treatment is economically attractive only for special compounds. In the 1970s, adsorption by granular activated carbon was the most thoroughly proven technology for many organic pollution problems; further design and system refinements were desirable to make it more attractive economically and more practical operationally. In the 1980s, air stripping had the chance to be better economically than carbon adsorption1 for treatment of VOCs, but in order to prove it, several experimental studies of bench, pilot, and industrial scale2,3 needed to be performed. The results confirmed that air stripping was an economically better option to remove VOCs from groundwater. This decade steam stripping is being compared with air stripping for the removal of VOCs.4-7 The argument is that, because of the greater temperatures on steam stripping compared with air stripping, the organic contaminants will go easily with the vapor phase, using lower column volume and providing a rational use to many low-pressure steam streams, thus improving the * Phone: (524) 611 7802 and 7575, ext. 150. Fax: (524) 611 7744. E-mail: [email protected]. † Current address: Facultad de Ciencias Quı ´mico Biolo´gicas, Universidad Auto´noma de Sinaloa, Sinaloa, Mexico. ‡ Current address: ITESO, Guadalajara, Me ´ xico.

energy consumption indices. Another advantage cited is the possible recovery of the VOC when this forms a nonmiscible mixture with water, after condensing the steam-VOC stream that leaves the top of the stripping column. While air stripping merely transfers the VOC from water to air where it still must be dealt with, steam stripping will produce in some cases a recoverable phase, but as a minimum it concentrates the VOC in the condensate so that it can be dealt with more effectively. Overall, steam stripping is economically a better alternative than air stripping because of both capital and operational cost. For capital cost, the volume of a steam stripping column is in most cases lower than that for air stripping, and air blowers or compressors are expensive equipment. For operational cost, many plants have steam streams of low pressure that may be used as the stripping agent, improving the energy efficiency of the plant. Although absorption and stripping operation were well developed for some uses in the chemical and petrochemical industry, the low concentration ranges of VOCs involved in water stream purification provide uncertainty. The designers did not know if the design methods used for air stripping and for gas absorption would provide valid or confidence values. Again, experimental studies were needed. Experimental and Design Equations for Trays Fractionation Research, Inc. (FRI), provides largescale experimental data for a sieve-tray steam-stripping column of 1.22-m internal diameter and six perforated trays. The FRI data are considered to be of the highest quality and appeared first as a FRI Progress Report and later as a technical paper.7 Fair and Harvey6 used FRI Progress Report data to adapt equations from the Chan and Fair8 model for point efficiency prediction. They predicted the number of transfer units using the residence time for each phase and the volumetric mass-transfer coefficients (kLae,

10.1021/ie990432m CCC: $19.00 © 2000 American Chemical Society Published on Web 01/25/2000

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Ind. Eng. Chem. Res., Vol. 39, No. 3, 2000

Table 1. Characteristics of Column, Packing, and System for Steam Stripping of VOCs column diameter ) 0.245 m; average height of packing ) 2.80 m; atmospheric pressure structured packing (stainless steel)

random packing (stainless steel)

Sulzer BX superficial area ) 492 m2/m3 void fraction ) 0.90 characteristic length ) 0.0012 m Mellapak 250Y superficial area ) 250 m2/m3 void fraction ) 0.96 characteristic length ) 0.0010 m

Flexirings superficial area ) 215 m2/m3 void fraction ) 0.94 characteristic length ) 0.0017 m Fleximax superficial area ) 141 m2/m3 void fraction ) 0.98 characteristic length ) 0.0009 m

Physical Properties of VOCs VOC

MW

DL (m2/s)

DG (m2/s)

temp (°C)

m at 100 °C

chloroform CHCl3 toluene C6H5CH3

119.4 92.14

4.43 × 10-9 2.81 × 10-9

5.54 × 10-5 1.89 × 10-5

61.2 110.6

1795-1798 2098-2103

Physical Properties for Water and Steam FL H2O

(kg/m3)

FG (kg/m3)

µL [kg/(m s)]

µG [kg/(m s)]

σ (N/m)

0.596

0.000 28

0.000 012

0.059

958.3

Figure 2. Analytical system.

Sample Analysis Figure 1. Equipment setup for steam stripping.

kGae). From the individual number of transfer units, the overall number of transfer units is calculated and used to get the point efficiency; from this, tray and overall column efficiencies are obtained. Experimental Setup for the Stripping Column Filled with Packings At Instituto Tecno´logico de Celaya we tried to contribute to the design or analysis of the stripping column for the removal of VOCs from water streams. GuerreroMedina9 and Ortiz del Castillo10 first used a 0.10-mdiameter plastic-stripping column with the air-water system. Then a glass column with 0.076-m (3-in.) internal diameter and 1.5-cm Raschig rings was used. The final part of the experimental section was carried out in a stainless steel stripping column of 0.245-m internal diameter. The column was filled with structured packing Mellapak 250Y and Sulzer BX; the random packings tested were 1-in. Pall rings and 1-in. Fleximax rings. In the thesis of Ortiz del Castillo, several runs with trays are also reported. Figure 1 shows a scheme of the experimental setup for the 0.254-m metallic stripping column. Table 1 shows the characteristics of the column and the runs performed. To permit comparison with the tray stripping column, we used toluene as the VOC, but we also used chloroform to try to involve different physical properties. The experimental runs were performed at atmospheric pressure.

For the sample analysis, gas chromatography analysis was used with the method of purge and trap. The analytical equipment consists of a purge and trap (OI-Analytical model 4560) and a gas chromatograph Perkin-Elmer (Sigma 300), with an integrator (PerkinElmer 1020). Figure 2 shows the analytical system. Nitrogen was used as the carrier gas, while the makeup gas was compressed air and hydrogen. Table 2 shows the optimal operation parameters for the gas chromatograph and purge and trap equipment. Model Development For the design of stripping as well as distillation, absorption, or extraction columns, there are two parts: mass-transfer calculations that provide the effective height and hydraulic calculations that provide the diameter of the column. Traditionally these two steps are considered separately, but because in the operation inside the column the mass and momentum transport occur simultaneously, we believe that a more mechanistic modeling of these processes should involve the simultaneous consideration of both phenomena. In distillation operation the main resistance for mass transfer lies on the gas phase. The opposite is true for stripping operations, but the modeling of both processes should be the same, with the recognition that the stripping operation presents a lower efficiency or higher height of an equivalent theoretical plate (HETP) than distillation. As suggested by Rocha et al.11,12 and Gualito et al.,13 the liquid holdup (hdyn) is the parameter that links mass-transfer and momentum balances, through the use

Ind. Eng. Chem. Res., Vol. 39, No. 3, 2000 733 Table 2. Gas Chromatograph and Purge and Trap Operation Parameters

Table 3. Equations for Hydraulic Calculations in Packed Columns (Engel et al.14)

Gas Chromatograph capillary column temperatures (°C) FID injection port oven carrier gas chromatograph concentrator make up gas gas flow (mL/min) column make up gas

210 200 160

ReG ) ψ)

nitrogen nitrogen air hydrogen

(5)

(6)

( )( )( ) 0.66

uLap0.5

µLap1.5

g0.5

0.25

FL g0.5

σLap2 FL g

0.1

(7)

[ ( )]

5

hdyn ) hdyn0 1 + 6

20 180 100 180

x

6σL ∆Fg

dL ) CL

11 4 20

aL )

∆ptot FL g

2

(8)

(CL ) 0.4 for random packing; CL ) 0.8 for structured packing) (9)

6hdyn dL

(10)

(

∆ptot aL + ap  ) ∆pdry ap  - hdyn ∆ptot,flood ) FL g

)

4.65

(11)

x249hdyn0(xX - 60 - 558hdyn0 - 103dLap) 2988hdyn0

(12)

X ) 36002 + 186480hdyn0 + 32280dLap + 191844hdyn02 +

uG (1 - hdyn) uL hdyn

For structured packing uGe )

(4)

C1 C2 + + C3 ReG Re 1/2

hdyn0 ) 3.6

of the effective velocities. Equations 1a and 2a give the expressions used to calculate the effective velocities for random packings, and eqs 1b and 2b, those for structured packings. As observed, structured packings need sin θ as a correction factor to take into account the channel inclination.

For random packing uLe )

uGFGdp µG

FGuG2 1 ∆Pdry ) ψap 4.65 8 

Purge and Trap sample volume (mL) temperature (°C) purge desorption valve bake time (min) purge desorption bake

(3)

G

12 18

For random packing uGe )

6(1 - ) ap

dp )

30 m × 1.0 µm × 0.32 mm

(2a) uG

(1 - hdyn) sin θ

uL For structured packing uLe ) hdyn sin θ

95028dLaphdyn0 + 10609dL2ap2 (13)

(1a)

Table 4. Equations for Mass-Transfer Calculations in Packed Columns

Hydraulic operation and the mass-transfer process are simultaneously considered by using the Engel et al.14 model for irrigated pressure drop and flooding. This method is an extension of the Stichlmair et al.15 treatment for liquid holdup, irrigated, and flooding pressure drop predictions and is valid for both structured and random packings. Table 3 with eqs 3-13 provides the correlations used for hydraulic parameter estimation. The use of the equations proposed in the Engel et al. paper14 instead of the original paper of Stichlmair et al.15 is justified by the following: (1) The correlation for holdup is based on more data. (2) Engel’s equations are easier to apply in analytical form. (3) Engel’s equations are easier to apply in a computer program. For the mass-transfer process, the generalization obtained for structured packing in distillation presented by Gualito et al.13 was applied to the experimental data obtained in the stripping studies reported in this paper. The intent of providing a general method was successful in the sense that the individual mass-transfer coefficients are the same for the four packings tested and

] [ ] ] [ ]

0.1DG dp(µGe + µLe)FG dp µG

kL )

0.3415DL dp(uGe + uLe)FL dp µL

(1b) (2b)

[

kG )

[( [ ]

[ )( )]

0.2405

µG FGDG

0.2337

1/3

µL FLDL

(14) 1/2

(15)

uL2FLdp uL2 0.5 apdpC ae σ gdp ) ap uLFLdp 0.2 0.6  (1 - cos γ) µL

(16)

constant C depends on the packing: Sulzer BX 0.9772

[

Mellapak 250Y 0.7312

1 1 1 1 ) + KLa ae kL mkG HTU )

1-in. Flexirings 0.6298

(17)

uL K La

HETP ) NTU )

]

1-in. Fleximax 0.5005

(18)

HTU ln λ λ-1

[(

(19)

]

λ λ - 1 xin 1 ln + λ-1 λ xout λ

λ)

mG L

Z)

∫x*dx- x K a

)

(20) (21)

uL

L e

) NTU × HTU ≈ Nt × HETP

(22)

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Ind. Eng. Chem. Res., Vol. 39, No. 3, 2000

Table 5. Experimental Data Obtained with Mellapak 250Y Structured Packing xa (ppm)

xb (ppm)

% removal

54.88 20.29 4.80 81.14 85.64 27.41 9.72

0.28 0.12 0.04 0.32 0.56 0.29 0.17

99.5 99.4 99.2 99.6 99.4 99.0 98.2

25.74 8.30 31.56 10.37 41.57 31.57

0.50 0.28 2.20 1.10 1.34 0.90

98.0 96.6 93.0 89.4 96.8 97.2

UGS (m/s)

ULS (m/s)

NTU

HTU (m)

KLa (1/s)

Mellapak 250Y Structured Packing and Toluene 0.549 0.0019 0.171 358 0.549 0.0024 0.136 285 0.549 0.0029 0.113 237 0.794 0.0032 0.151 316 0.794 0.0025 0.193 406 0.794 0.0040 0.121 254 1.040 0.0030 0.212 446

5.38 5.24 4.90 5.64 5.15 4.68 4.13

0.52 0.54 0.57 0.50 0.54 0.60 0.70

0.003 72 0.004 56 0.005 13 0.006 40 0.004 56 0.006 60 0.004 36

Mellapak 250Y Structured Packing and Chloroform 0.549 0.0015 0.227 409 0.549 0.0019 0.170 305 0.549 0.0025 0.134 241 0.794 0.0024 0.196 352 0.794 0.0035 0.137 246 0.794 0.0040 0.120 216

4.03 3.48 2.77 2.34 3.56 3.69

0.70 0.80 1.01 1.20 0.79 0.76

0.002 09 0.002 42 0.002 44 0.002 05 0.004 44 0.005 24

the difference in performance lies on the effective interfacial area provided for each packing. Equations 14 and 15 proposed in this paper are different to those from Gualito et al.13 in three aspects: For the individual mass-transfer coefficient of the liquid phase, Gualito uses the individual effective velocity, whereas in this work, eq 15 uses the relative effective velocity (UGe + ULe). For the same liquid mass-transfer coefficient, Gualito et al. assume the Higbie expression with only the Schmidt number, whereas in this paper, Schmidt and Reynolds numbers are taken into account in eq 15, and finally the coefficient and exponents are different. For estimation of the effective interfacial area, the Shi and Mersmann16 correlation was proposed with the same dimensionless Reynolds, Froude, and Weber numbers and exponents, except the fraction of wetted to packed area was adjusted. The result is given by eq 16

[(

)( )]

uL2FLdp uL2 0.15 apdpC ae σ gdp ) ap uLFLdp 0.2 0.6  (1 - cos γ) µL

[

]

(16)

where ap is the packing area reported for each packing in Table 1, the exponent C is an adjustable parameter reported in Table 4, and The term cos γ according to Shi and Mersmann16 takes into account the wettability of the packing surface. For metallic packing

cos γ ) 0.90

for σ e 0.045N/m

cos γ ) 5.21 × 10-16.83σ

for σ > 0.045

(23) (24)

For ceramic and plastic, Gualito et al.13 provide similar equations that were obtained from a plot in the Shi and Mershman paper.16 The design or analysis of stripping columns with random and structured packings for mass-transfer calculation is modeled with the aid of eqs 14-22 shown in Table 4. The height of a transfer unit (HTU) or the HETP represents the efficiency of the packing for the stripping operation. The lower the numerical value, the more efficient the packing. Equation 22 provides the effective height of packing predicted. Some designers consider that a few extra lengths of packing is a lot less expensive than a column that fails to meet a regulatory requirement. Most of the

V/L

λ

correlations already have a safety factor included. If the designers want to add an extra length of packing, the additional length must not exceed the average deviation for the effective height, which for this work is 0.23. Results and Discussion In the experimental section, a minimum of five runs were performed for each packing-VOC combination, and inlet and outlet concentrations were measured and the volumetric flow rates for liquid and vapor were recorded. From these data the parameters shown in Tables 5-8 for structured and random packings were obtained and saved for each combination of packingVOC. With the equations for hydraulic and mass transfer proposed, the values for the coefficients and exponents in the expressions for mass-transfer coefficients were regressed. Also parameter C in eq 16 for the estimation of the effective interfacial area was regressed. The regression calculation was done with the minimization solver tool of an Excel spreadsheet from Microsoft. When all of the coefficients, exponents, and constant value (C) for specific area were obtained, the volumetric overall mass-transfer coefficient (KLa) given by eq 17 based on the liquid phase was calculated and compared with the measured value.

[

]

1 1 1 1 ) + KLa ae kL mkG

(17)

The deviation between experimental and calculated volumetric mass-transfer coefficients was defined with eq 25, and it provides a measure of the quality of the model.

dev )

abs(KLaexp - KLacalc) KLaexp

(25)

The average deviation for all combined 52 experimental points is 0.29, and although it is a relatively big number, it presents a safety factor for a conservative design or analysis of a packed column when used for the removal of VOCs from water streams. Figures 3-6 show the measured versus the estimated values for the overall volumetric mass-transfer coefficients for the four packings tested, and Figure 7 shows a parity plot for all of the data.

Ind. Eng. Chem. Res., Vol. 39, No. 3, 2000 735 Table 6. Experimental Data Obtained with Sulzer BX Structured Packing xa (ppm)

xb (ppm)

% removal

4.95 81.77 79.60 24.70 13.27 2.95 101.90 346.84

0.10 0.10 1.80 0.99 0.05 0.08 0.95 1.76

98.0 99.9 97.7 96.0 99.6 97.3 99.1 99.5

18.38 18.71 14.40 14.91 35.53 24.66 128.41 56.52

0.45 1.1 0.45 0.1 0.45 0.45 2.085 1.34

97.6 94.1 96.9 99.3 98.7 98.2 98.4 97.6

UGS (m/s)

ULS (m/s)

V/L

NTU

HTU (m)

KLa (1/s)

Sulzer BX Structured Packing and Toluene 0.549 0.0025 0.133 280 0.549 0.0005 0.695 1459 0.549 0.0010 0.338 710 0.549 0.0015 0.225 474 0.794 0.0010 0.498 1047 0.794 0.0019 0.249 522 1.040 0.0035 0.180 378 1.040 0.0030 0.210 441

4.02 6.78 3.89 3.32 5.66 3.69 4.79 5.40

0.70 0.41 0.72 0.85 0.50 0.76 0.58 0.52

0.003 55 0.001 15 0.001 36 0.001 74 0.001 94 0.002 54 0.005 96 0.005 76

Sulzer BX Structured Packing and Chloroform 0.549 0.0025 0.133 239 0.549 0.0010 0.338 608 0.794 0.0025 0.191 343 0.794 0.0010 0.494 888 0.794 0.0035 0.139 249 1.040 0.0040 0.155 278 1.040 0.0050 0.125 225 1.040 0.0045 0.140 252

3.83 2.93 3.59 5.09 4.49 4.14 4.25 3.87

0.73 0.96 0.78 0.55 0.62 0.68 0.66 0.72

0.003 40 0.001 02 0.003 22 0.001 76 0.005 53 0.005 99 0.007 62 0.006 18

λ

NTU

HTU (m)

KLa (1/s)

361 299 369 317 364

1.960 1.744 1.829 1.891 2.136

0.765 0.859 0.819 0.792 0.701

0.002 48 0.002 67 0.003 29 0.004 03 0.005 12

Pall Ring Random Packing and Chloroform 0.5507 0.0014 0.238 428 0.5507 0.0019 0.171 307 0.7972 0.0022 0.213 383 0.5507 0.0019 0.171 307 0.7972 0.0022 0.213 383 1.0437 0.0028 0.223 401 1.0437 0.0037 0.168 303

1.851 1.662 1.980 1.595 1.800 2.251 2.020

0.810 0.902 0.757 0.939 0.833 0.666 0.742

0.001 72 0.002 10 0.002 90 0.002 02 0.002 64 0.004 20 0.004 98

λ

NTU

HTU (m)

KLa (1/s)

364 286 369 315 324

4.385 5.612 4.838 3.307 2.637

0.342 0.267 0.309 0.453 0.568

0.005 54 0.008 98 0.008 71 0.007 06 0.007 21

Fleximax Random Packing and Chloroform 0.5507 0.0023 0.142 255 0.7972 0.0024 0.201 361 0.7972 0.0028 0.171 307 1.2902 0.0032 0.245 440 0.5507 0.0015 0.219 394 0.7972 0.0019 0.251 451

4.219 3.208 3.256 2.683 3.637 2.319

0.355 0.467 0.460 0.558 0.412 0.646

0.006 47 0.005 13 0.006 08 0.005 72 0.003 64 0.002 94

λ

Table 7. Experimental Data Obtained with Pall Ring Random Packing xa (ppm)

xb (ppm)

% removal

16.29 8.56 4.35 1.52 1.52

2.30 1.50 0.70 0.23 0.18

0.859 0.825 0.839 0.849 0.882

12.71 11.04 40.47 19.68 27.16 5.68 37.55

2.00 2.10 5.60 4.00 4.50 0.60 5.00

0.843 0.810 0.862 0.797 0.834 0.894 0.867

UGS (m/s)

ULS (m/s)

V/L

Pall Ring Random Packing and Toluene 0.5507 0.0019 0.172 0.5507 0.0023 0.142 0.7972 0.0027 0.176 0.7972 0.0032 0.151 1.0437 0.0036 0.173

Table 8. Experimental Data Obtained with Fleximax Random Packing xa (ppm)

xb (ppm)

% removal

36.58 16.17 8.75 4.34 1.53

0.46 0.06 0.07 0.16 0.11

0.987 0.996 0.992 0.963 0.928

55.06 12.3 40.47 19.68 27.16 5.68

0.82 0.5 1.57 1.35 0.72 0.56

0.985 0.959 0.961 0.931 0.973 0.901

UGS (m/s)

ULS (m/s)

V/L

Fleximax Random Packing and Toluene 0.5507 0.0019 0.173 0.5507 0.0024 0.136 0.7972 0.0027 0.176 0.7972 0.0032 0.15 1.0437 0.0041 0.154

It may be observed that the fit is not as good as we would like it to be. Probably, if we set specific coefficients, exponents, and C constants for each packing, we could get a lower averaged deviation. However, in that case we could lose the generalization we are looking for. In the four plots of Figures 3-6, it seems to be an effect of the operating conditions at KLa < 0.004 (1/s) for the method to overpredict, while at higher values the method underpredicts the measured values for the volumetric mass-transfer coefficient. Also, a system effect is observed because the points for toluene and chloroform tend to group in different parts of the plots. When the predicted value for KLa is used to predict the effective height of packing to get the desired outlet

concentration, the deviation between calculated versus experimental packing height is calculated with eq 26.

dev )

abs(Zexp - Zcalc) Zexp

(26)

The averaged deviation is about 0.23. This number is lower than the one obtained for the volumetric masstransfer coefficient, because the denominator Zexp is bigger than KLa,exp. Although cool liquid at 25 °C and live steam at 110 °C were used and the energy balance showed that the saturation temperature is reached in less than 1 cm of the packing, a uniform temperature of 100 °C was assumed over the entire length of the column.

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Figure 3. Experimental versus predicted volumetric masstransfer coefficient for Sulzer BX, using toluene (O) and chloroform (×) as VOCs.

Figure 4. Experimental versus predicted volumetric masstransfer coefficient for Mellapak 250Y, using toluene (O) and chloroform (×) as VOCs.

Figure 5. Experimental versus predicted volumetric masstransfer coefficient for 1-in. Fleximax, using toluene (O) and chloroform (×) as VOCs.

Application of the Developed Equations The obtained correlations have been implemented in a computer program presented by Lopez-Toledo et al.17

Figure 6. Experimental versus predicted volumetric masstransfer coefficient for 1-in. Pall rings, using toluene (O) and chloroform (×) as VOCs.

Figure 7. Experimental versus predicted volumetric masstransfer coefficient for all packings, using toluene (O) and chloroform (×) as VOCs.

to design three stripping columns for the removal of VOCs from aqueous streams using steam as the stripping agent. In the three cases, the original removal of VOC was performed using air stripping. The comparison of air stripping versus steam stripping indicates that steam stripping effectively provides for the same size of stripping column, a higher capacity mainly due to the higher value in the equilibrium constant. However, for steam stripping, metallic packing is required, and this is more expensive than the standard plastic packing used for air stripping. As in many other cases, the higher capital cost involved is only a small fraction of the total (capital + operational) cost. The capital cost of the packing must be paid only a few times for replacement, but the operational cost or better the capacity increase will be a continuous advantage for steam stripping. Also, the use of steam stripping may provide a good use for streams of low-pressure steam that may be available at the plants, improving the rational use of the energy. The application of the complete set of equations to one of the steam stripping columns covered by LopezToledo et al.17 is included in this work as Appendix A.

Ind. Eng. Chem. Res., Vol. 39, No. 3, 2000 737

Ratio of Liquid to Total Resistance Using the individual mass-transfer coefficients, the resistance of the liquid phase with respect to the total may be calculated with eq 27.

(

)

kL RL ) 1+ RT mkG

-1

and J. Carlos Cardenas-Guerra is truly appreciated for the experimental runs performed at the chemical engineering laboratory of Instituto Tecnolo´gico de Celaya, Celaya, Mexico. Nomenclature

(27)

The application of this equation to the data obtained in this study confirms that the stripping operation is clearly liquid-phase-controlled. The average value for the 52 runs was 0.97, with lowest and highest values of 0.88 and 0.99, respectively. Recovery of VOC at the Top of the Column The vapor that leaves the top of the column is condensed and stored in a tank for possible phase separation. It was found that the two-phase formation and then the recovery of the VOC were not found for all of the runs performed. Because the solubility varies with temperature, to predict the two-phase region, some stoichiometric and solubility calculations need to be performed at different temperatures. Conclusions Steam stripping is a good alternative for the removal of VOCs from groundwater to get concentrations of VOCs required by environmental regulations. Tray columns may be designed or analyzed using the papers of Fair and Harvey6 and Kunesh et al.7 The mass-transfer and hydraulic performances of steam stripping with structured and random packings are identical and may be modeled with the same masstransfer coefficients but a different effective interfacial area for each packing. The application of the combined hydraulic and mass-transfer correlations approximately explains the difference in VOC removal. The mass-transfer coefficients and the interfacial area obtained in this work allow the design or analysis of steam-stripping columns for the removal of VOCs from water streams, with an approximation of 29% for the overall volumetric mass-transfer coefficients based on the liquid phase. When the predicted overall volumetric mass transfer is used to calculate the effective height of the packing and this is compared with the experimental value used, the average deviation is 23%. The comparison of calculated and experimental volumetric mass-transfer coefficients shows a slight system effect that gives a higher calculated KLa for chloroform than for toluene. Both systems show that the stripping operation is liquid-phase-controlled with an average value of 0.97. The assumed recovery of the VOC using a decantor after the condensation of the VOC-steam vapors was possible only for a few runs; then this removal assumption needs additional study. Acknowledgment The kind support of COSNET, CONACYT, KockGlitsth, The Separations Research Program at the University of Texas at Austin, and Sulzer Chemtech is appreciated. Also the help of Don Lupe Jimenez, J. Carlos Cardenas-Rivera, Silvia G. Guerra-Velazquez,

ae ) effective specific area for packing, m2/m3 aL ) specific liquid surface area, m2/m3 ap ) specific surface area, m2/m3 C ) constant for effective specific area CL ) constant for particle diameter calculation: 0.4 for random packing and 0.8 for structured packing C1, C2, C3 ) Stichlmair constants dL ) diameter of liquid particles, m DG ) vapor molecular diffusion coefficient, m2/s DL ) liquid molecular diffusion coefficient, m2/s dp ) characteristic diameter, m ∆pdry ) specific dry pressure drop, Pa/m ∆ptot ) specific pressure drop, Pa/m ∆ptot,flood ) specific pressure drop at flooding conditions, Pa/m g ) gravity acceleration, m/s2 G ) molar flow rate of the gas phase, kmol/s G′ ) mass flow rate of the gas phase, kg/s HTU ) height of the transfer unit, m HETP ) height equivalent to a theoretical plate, m hdyn0 ) dynamic holdup below the loading point hdyn ) dynamic holdup KL ) overall mass-transfer coefficient, m/s KLa ) overall volumetric mass-transfer coefficient, 1/s kG ) gas-phase mass-transfer coefficient, m/s kL ) liquid-phase mass-transfer coefficient, m/s L ) molar flow rate for the liquid, kmol/s L′ ) mass flow rate for the liquid, kg/s m ) equilibrium ratio, mole fraction/mole fraction MW ) molecular weight, kg/kmol Nt ) number of theoretical stages or plates NTU ) number of transfer units uG ) superficial gas velocity, m/s uGe ) effective gas velocity, m/s uL ) superficial liquid velocity, m/s uLe ) effective liquid velocity, m/s QG ) volumetric flow rate for the gas phase, m3/s QL ) volumetric flow rate for the liquid phase, m3/s R ) resistance for mass transfer, s/m or 1/s Re ) Reynolds number X ) coefficient for calculation of the flooding pressure drop x ) concentration of VOC in the liquid phase, mole fraction xin ) inlet concentration, mole fraction, or other consistent unit xout ) outlet concentration, mole fraction, or other consistent unit Vc ) volume of the column, m3 Z ) height of the column, m Greek Symbols γ ) contact angle between the liquid and solid, deg  ) packing void fraction FG ) gas density, kg/m3 FL ) liquid density, kg/m3 σ ) surface tension, N/m µG ) vapor viscosity, kg/(m s) µL ) liquid viscosity, kg/(m s) λ ) stripping factor ) mG/L ψ ) friction factor

738

Ind. Eng. Chem. Res., Vol. 39, No. 3, 2000

θ ) angle (with horizontal) of channels in a structured packing

[ ( )]

hdyn ) hdyn0 1 + 6

∆ptot FLg

Example Calculation. Take as a base case the removal of trichloroethene (TCE) from water reported by Hand et al.3 Use steam (instead of air) stripping to calculate liquid holdup, pressure drop, mass-transfer coefficients, the interfacial area for a steam stripping column, HTU and NTU values, and the TCE concentration at the bottom of the stripper column. As reported by Lopez-Toledo et al.,17 the liquid capacity of the column using steam is increased 40% with respect to the air stripper. This corresponds to 456 277 kg/h of groundwater at ambient temperature; 11 000 kg/h of steam are used as the stripping agent. The column has 2.44-m internal diameter and 7.47-m height. For steam stripping the packing will be metallic Pall rings of 25 mm. The inlet concentration of TCE needs to be reduced from 72 to 3 µg/L. Solution. From Table 1 of Stichlmair et al.,15 ap ) 215 m2/m3;  ) 0.94; C1 ) 0.05, C2 ) 1.0, and C3 ) 3.0. The physical properties and some operational parameters for the system are as follows: L′ and G′ (kg/h)

F µ (kg/m3) [kg/(m s)]

u (m/s)

D (m2/s)

(f) Diameter of liquid particles and specific liquid surface:

dL ) CL

aL )

x

1. Hydraulic parameters. (a) Equivalent diameter or characteristic length of the packing with eq 3:

x

6σL ) 0.4 ∆Fg

260.161571 + 0.000104749∆Ptot2 (g) Total pressure drop:

(

∆ptot aL + ap  ) 79.047 ap  - hdyn

(b) Friction factor with eqs 4 and 5:

uGFGdp 1.08009 × 0.605 × 0.001674 ) ) µG 0.000017 64.362

C2 C1 0.05 1.0 + + C3 ) + 3.0 ) + ReG Re 1/2 64.362 x64.362 G

3.125

260.161 + 0.000104∆Ptot2 + 215 × 215

(

0.94 0.94 - 0.111764 - 0.000000045∆Ptot2

0.605 × 1.080092 ) 79.047 Pa/m 0.944.65 (d) Holdup below the loading point:

( )( )( )

hdyn0 ) 3.6

)

4.65

Solving this equation for the total pressure drop, ∆Ptot ) 331.52 Pa/m. (h) Holdup above the loading point:

[ ( )]

hdyn ) hdyn0 1 + 6

∆ptot FLg

2

)

(i) With this value for liquid holdup we may go to the mass-transfer calculations, but before doing that, we will calculate the pressure drop at flooding with eqs 12 and 13:

X ) 36002 + 186480hdyn0 + 32280dLap + 191844hdyn02 + 95028dLaphdyn0 + 10609dL2ap2 X ) 51128.26473 ∆ptot,flood ) FL g 2988hdyn0

FGuG2 1 1 ∆Pdry ) ψap 4.65 ) × 3.125 × 215 × 8 8 

µLap1.5

)

x249hdyn0(xX - 60 - 558hdyn0 - 103dLap) )

(c) Dry pressure drop:

0.66

)

4.65

0.111764 + 0.000000045∆Ptot2 ) 0.1167

6(1 - ) 6(1 - 0.94) ) 0.001674 m dp ) ) ap 215

uLap0.5

6 × 0.065 ) (958 - 0.605) × 9.81 0.002577

6hdyn 6(0.111764 + 0.000000045∆Ptot2) ) ) dL 0.002577

m σ (m.f./ (N/m) m.f.)

liquid 456 277 0.028 293 958 0.000 29 4.16 × 0.065 8765 (H2O) 10-9 vapor 11 000 1.080 09 0.605 0.000 017 1.24 × (steam) 10-5

ψ)

)

0.111764 + 0.000000045∆Ptot2

Appendix A

ReG )

2

0.25

σLap2 FL g

0.1

) g0.5 FLg0.5 3.6 × 0.263367 × 0.132118 × 0.892227 ) 0.111764 (e) Holdup above the loading point:

1052.587 Pa/m With this value the approach to flooding may be calculated; the result is 70%. 2. Mass-transfer parameters. (j) Effective velocities, from eqs 1a and 2a:

uGe )

uG (1 - hdyn)

uLe )

)

1.08009 ) 0.94(1 - 0.11671) 1.300854 m/s

uL 0.028293 ) 0.257895 m/s ) hdyn 0.94 × 0.11671

Ind. Eng. Chem. Res., Vol. 39, No. 3, 2000 739

(k) Individual mass-transfer coefficients with eqs 14 and 15:

[

] [ ]

0.1DG dp(uGe + uLe)FG 0.2405 µG 1/3 ) dp µG FGDG 0.000740 × 2.968096 × 1.313481 ) 0.002887 m/s

kG )

kL )

[

] [ ]

0.3415DL dp(uGe + uLe)FL 0.2337 µL 1/2 ) dp µL F L DL 0.000000847 × 8.312395 × 8.530403 ) 0.000060 m/s

(l) Effective interfacial area:

[(

)( )]

uL2FLdp uL2 0.15 apdPC σ gdp ae 1.354001 ) ) ) 0.881 ap uLFLdp 0.2 0.6 1.536891  (1 - cos γ) µL

[

]

ae ) 0.881ap ) 0.881 × 215 ) 189.415 m2/m3 (m) Global volumetric mass-transfer coefficient for the liquid phase with eq 17:

[

]

1 1 1 1 + ) ) KLa ae kL mkG 0.005279(16646.41353 + 0.039517) ) 87.88345 s KLa ) 0.011378 (1/s) (n) Height of a transfer unit, with eq 18:

HTU )

uL 0.028293 ) ) 2.486 m KLa 0.011378

(o) Number of transfer units, from eq 22, and the packing height equal to 7.47 m:

NTU )

7.47 Z ) ) 3.004 HTU 2.486

(p) Recovery of the VOC or concentration of TCE at the outlet of the liquid stream, using eqs 20 and 21:

λ)

mG 5558 × 11000 ) ) 133.993 L 456277

xin 1 λ ) 19.863 ) e[(λ-1)/λ]NTU xout λλ-1

[

]

xout )

µg 72 ) 3.62 19.863 l

Literature Cited (1) Gross, R. L.; Termaath, S. G. Packed Tower Aeration Strips Trichloroethylene from Groundwater. Environ. Prog. 1985, 4, 119.

(2) Byers, W. D.; Morton, C. M. Removing VOC from Groundwater; Pilot, Scale-up, and Operating Experience. Environ. Prog. 1885, 4 (2), 112. (3) Hand, D. W.; Crittenden, J. C.; Gehin, L. G.; Lykins, B. W., Jr. Design and Evaluation of an Air-Stripping Tower for Removing VOC’s From Groundwater. J.sAm. Water Works Assoc. 1986, 78 (9), 87. (4) Hwang, Y. L.; Olson, J. D.; Keller, G. E. Steam Stripping for Removal of Organic Pollutants from Water. 1. Stripping Effectiveness and Stripper Design. Ind. Eng. Chem. Res. 1992, 31 (7), 1753-1759. (5) Hwang, Y. L.; Olson, J. D.; Keller, G. E. Steam Stripping for Removal of Organic Pollutants from Water. 2. Vapor-Liquid Equilibrium Data. Ind. Eng. Chem. Res. 1992, 31 (7), 17591768. (6) Fair, J. R.; Harvey, R. L. Modeling of Tray-Type Steam Stripping Columns. AIChE Meeting, Atlanta, GA, Spring 1994. (7) Kunesh, G. J.; Ognisty, T. P.; Sakata, M.; Chen, G. X. Sieve Tray Performances for Steam Stripping Toluene from Water in a 4-ft Diameter Column. Ind. Eng. Chem. Res. 1996, 35, 2660. (8) Chan, H.; Fair, J. R. Prediction of Point Efficiencies on Sieve Trays. Ind. Eng. Chem. Process Des. Dev. 1984, 23, 814. (9) Guerrero-Medina, G. Desorcio´n con Vapor de Agua de COV (Steam Stripping of VOC). Reporte Final de Proyecto de Maestria en Ciencias (Final Report of Masters in Science), Instituto Tecnolo´gico de Celaya, Celaya, Me´xico, June, 1997. (10) Ortiz del Castillo, J. R. Desorcio´n de Compuestos Orga´nicos Vola´tiles de Corrientes de agua con Vapor, empleando Columnas Empacadas y de Platos (Steam Stripping of VOC Using Packed and Tray Columns). Tesis de Maestrı´a M.S. Thesis, Instituto Tecnolo´gico de Celaya, Celaya, Mexico, May, 1998. (11) Rocha, J. A.; Bravo, J. L.; Fair, J. R. Distillation Columns Containing Structured Packings: A Comprehensive Model for Their Performance. 1. Hydraulic Models. Ind. Eng. Chem. Res. 1993, 32, 641. (12) Rocha, J. A.; Bravo, J. L.; Fair, J. R. Distillation Columns Containing Structured Packings: A Comprehensive Model for Their Performance. 2. Mass-Transfer Model. Ind. Eng. Chem. Res. 1996, 35, 1660. (13) Gualito, J. J.; Cerino, F. J.; Cardenas, J. C.; Rocha, J. A. Design Method for Distillation Columns Filled with Metallic, Ceramic, or Plastic Structured Packings. Ind. Eng. Chem. Res. 1997, 36, 1747. (14) Engel, V.; Stichlmair, J.; Geipel, W. A New Correlation for Pressure Drop, Flooding and Holdup in Packed Columns. AIChE Annual Meeting, Miami, FL, Oct, 1998; Paper 132f. (15) Stichlmair, J.; Bravo, J. L.; Fair, J. R. General Model Generalization for Prediction of Pressure Drop and Capacity of Countercurrent Gas/Liquid Packed Columns. Gas Sep. Purif. 1989, 3, 19. (16) Shi, M.; Mersmann, G. Effective Interfacial areas in Packed Columns. Ger. Chem. Eng. 1985, 8, 87. (17) Lopez-Toledo, J.; Ortiz del Castillo, Rocha J. R.; J. A. A Computer Program for Steam Stripping Columns for VOC’s Removal. Distillation Horizon for the Next Millenium; Proceedings at the Spring AIChE Meeting, Houston, TX, Mar 14-18, 1999; Paper 1d.

Received for review June 11, 1999 Revised manuscript received September 30, 1999 Accepted October 13, 1999 IE990432M