Enhanced scrubbing of chlorinated compounds from air streams

Enhanced Scmbbing of Chlorinated Compounds from Air Streams Jodi Johnson and Wayne Parker

Kevin Kennedy

DepaItment of Civil and Environmental Engineering, Carleton University, 1125 Colonel By Drive, Ottawa, Ontario, KlS5B6, Canada

Department of Civil Engineering, University of Ottawa, 161 Louis Pasteur, Room A-412, Ottawa, Ontario K1N 6N5, Canada

This paper addresses an investigation o mass transfer

air streams containing chlorinated organics. Three target compoundr [dichloromethane (DCM), carbon tetrachhridt (CT) and tetrachloroethylene (PCE)] were evaluated to assess a ran e ofchemical and hysical roperties.

Kgetab i oil was found to i d e an ective scrubbingsolu- tion in removing the target compoundr )om the air streams and was employed in continuow flow tests of a bench-scale countercurrent packed column. Removal e tciencies a proached 90% or all three target compoun f w i t h gas-

A gas-liquid mass transfer model was developed and compared to the existing Onda correlations, to characterize mass trans er under various operatin conditions when

tions. It was found that the Onda correlations did notfit the experimental data of vegetable oil vey well. The existing Onda correlations were mod$ed by assuming that the gas phase resistance was controlling mass trans er.

an aqueous base a liquid-liquid contacting reactor was

that mass transfer could be achievedfor compoun s that were not high4 hydrophobic.

issues associated with an innovative hybri df process to treat

&id@. ratios f ss than 200.

water an df vegetable oil were employe ff as scrubbing solu-

In order to enhance mass transferfiom t d e oilphase to

proposed. T R e results of the liquid-liquid reactor su ested 7 INTRODUCTION

Air streams contaminated with chlorinated organic compounds arise from many different sources including industrial vents, contaminated soil venting off- gases and gas streams from wastewater treatment processes. These contaminated air streams have been a growing concern due to their negative impacts on stratospheric ozone and their carcinogenic effect on the human population.

Existing technologies such as incineration and carbon adsorption have proven to be ineffective or costly and hence a biological process that could overcome

these problems would be desirable. Conventional biofiltration processes are aerobic and are not suitable for the degradation of chlorinated organics. However, many chlorinated organics have been observed to be reductively dechlorinated under anaerobic conditions [3,4,101. Lee et a1 [41 investigated an anaerobic biofiltration process in which the chlorinated compounds were carried by oxygen-free helium gas. The work of Lee et al. [41, did not address the manner in which the target compounds would be isolated from oxygen. This project addresses an investigation of mass transfer issues associated with an innovative hybrid process designed to treat air streams containing chlorinated organics.

The proposed hybrid system involves the transfer of contaminants from the gas phase to a liquid phase in a countercurrent packed bed column. The liquid phase containing the chlorinated organics is then transferred to a separate high-rate upflow anaerobic sludge blanket (UASB) reactor where biodegradation of the chlorinated compounds will occur.

An obstacle to the design of such a system is that many of the chlorinated compounds are relatively insoluble in water as indicated by their high Henry’s law coefficients [l l . The high air-water partitioning coefficients would require large liquid flows, and hence, relatively large dimensions for the scrubbing process. In this study, an alternative non-aqueous phase scrubbing solution (vegetable oil) was investigated to assess its potential to enhance the scrubbing of chlorinated compounds. By employing this liquid in the scrubbing process the liquid flow can be reduced, thereby decreasing the dimensions of the scrubbing process. The use of vegetable oil as a scrubbing solution presented an additional challenge in transferring the hydrophobic compounds from the non-polar solvent into an aqueous phase for subse- quent biological treatment. However, biodegradation rates are often much slower than mass transfer rates and hence residence times required for biological processes are typically much greater than that

Environmental Progress (Vol. 19, No.3) Fall 2000 157

FIGURE 1. Experimental Set-up of Packed Column

required for mass transfer. The incremental increase in volume that is required for mass transfer in a biological process may therefore be small relative to the size of the biological process. To obtain more information on the mass transfer process a liquid-liquid mass transfer reactor was employed to determine mass transfer rate coefficients. The results from the experimental studies were employed to assess the feasibility of the hybrid process.

MATERIALS AND METHODS

Target Compounds The chlorinated compounds that were investigated

included dichloromethane (DCM), carbon tetrachlo- ride (CT), and tetrachloroethylene (PCE). These compounds were selected based on their wide range of Henry’s law coefficients and molecular make-up (number of chlorine atoms). Canola based vegetable oil and water were investigated as potential scrubbing solutions to be employed in the packed column.

Gas - Liquid Partitioning Batch Tests Gas-liquid partitioning tests were performed using

the EPICS (equilibrium partitioning in closed systems) technique [21. Gas-liquid partitioning coefficients were determined at a temperature of 2OoC for the three target compounds in water and vegetable oil.

Mass Transfer Packed Column Experiments A packed column with a length of 100 cm, an inner

diameter (1.D.) of 10 cm and constructed of stainless steel tubing was employed for continuous flow mass transfer experiments. The column had a total volume of 8.2 L and was divided into three sections that were connected by flanges for easy assembly. The bottom section was 25 cm in length, and contained a 1 cm

I.D. liquid effluent pipe that exited directly from the bottom. A 2 cm 1.D pipe supplied the contaminated air stream into the bottom of the column where it was distributed evenly over the packing by an air distributor. The middle section was 60 cm in length, with a sample port located halfway up the midsection. The middle section was packed with IntaloxTM 13 mm ceramic saddles. The top section was 15 cm in length with a 10 cm inner diameter. A liquid distributor was installed 5 cm down from the top of the section to provide an even distribution of flow over the entire packing surface area and to minimize channeling.

Experimental Description - Mass Transfer Packed COlUmn

A contaminated air stream was synthesized by injecting pure chlorinated compounds into the airstream with a Harvard Apparatus model # 55-2226 syringe pump, equipped with a 10 ml glass syringe. The mixture of chlorinated organics was introduced into the pipe at vol- umetric flowrates ranging from 25 pL/min to 100 pWmin depending on the desired concentration for the test. The pure phase compounds were immediately volatilized and carried off by the air stream into the packed column. A 30 cm loop of copper pipe was installed into the gas line to provide sufficient mixing prior to entering the column. A sample port was installed prior to the column entry to allow sampling of the contaminated air as it entered the column

The gas stream flowed upwards through the packed column and exited through its top. The scrubbing solution was pumped into the top of the column and removed from the bottom of the column by peristaltic pumps. Figure 1 describes the experimental set-up employed in the continuous flow packed column tests.

Prior to starting a run, the glass syringe was filled with a mixture of equal parts by volume of the pure phase target chlorinated compounds. The air supply valve was adjusted to the desired flow rate for the specific run. After obtaining three reproducible gas inlet samples with minimal changes in concentration, the liquid scrubbing solution was introduced into the column from the influent reservoir. Inlet and outlet gas samples were taken periodically to ensure steady state conditions were obtained within the packed column. After steady state conditions were reached with-

TABLE 1. Summary of Operating Conditions Employed in Continuous Flow Experiments

Run

-. __ 1

2

3 4

5

6

Initial Oil Volume

(ml)

2000

1000

2000

2000

2000

2000

-

Water Flow Rate Qw

(ml/min)

200

200

200

100

350

200

-

Oil Flow Rate Qo

(ml/min)

25

25

10

25

25

5

158 Fall 2000 Environmental Progress (Vol. 19, No.3)

FIGURE 2. Experimental Set-up of Liquid-Liquid Contacting Reactor in Continuous Mode

in the column, liquid effluent samples were collected in 40 ml amber bottles which were sealed with teflon lined septums. The inlet gas was sampled at the end of the run to ensure that the operating conditions within the column did not fluctuate through the run. A total of 43 runs (18 water and 25 vegetable oil 1 span- ning a range of gas to liquid flow ratios (QG/QL) ranging from 6-1221 were conducted.

Liquid-Liquid Mass Transfer Experiments Batch tests were performed to determine water-oil

partitioning coefficients for the three target compounds to assist in modeling mass transfer in the liquid-liquid contacting reactor. Three bottles with a volume of 40 mL were partially filled with 30 mL of vegetable oil containing 200 mg/L, 650 mg/L and 500 mg/L of DCM, CT and PCE respectively. The bottles were filled to the top with distilled water and allowed

to reach equilibrium. The water and oil concentrations were sampled and analyzed for the target compounds to determine their partitioning coefficients.

A 15 L glass column with a length of 150 cm and an outer diameter of 10 cm was utilized to characterize mass transfer from the vegetable oil to the water. Clean water was withdrawn from a reservoir and pumped into the top of the column by way of a peristaltic pump. A stainless steel liquid distributor was installed 20 cm down from the top of the column to provide adequate distribution of the clean water over the oil surface. Fig- ure 2 describes the experimental set-up employed in the continuous flow column experiments.

Prior to commencing a run in the continuous flow liquid-liquid contacting reactor a known volume of oil that was contaminated with VOCs, was pumped into the bottom of the column. Clean water was then allowed to trickle over the oil layer, and being more dense than the oil, passed downwards to form a separate phase below the oil layer. When the column was full, an effluent water stream was pumped out of the column to maintain a constant liquid level. At this time, fresh contaminated oil was introduced into the top of the existing oil and withdrawn out of the bottom of the oil layer to maintain a constant volume of oil in the reactor. The flowrate of clean water over the contaminated oil was evaluated over a range from 112 mVmin to 400 d m i n . The initial volume of contaminated oil pumped into the column was assessed over a range from 1000 ml to 4000 ml. Each test was run for approximately three hours including the fill time. Sampling of both the oil and water streams was performed periodically.

Analytical Methods The target chlorinated organics were analyzed on two

gas chromatographs, a Hewlett Packard model 5890 and a HNU model 311. The HP 5890 gas chromatograph was equipped with an electron capture detector (ECD) and a DB624 capillary column. The column was 30 m long with an internal diameter of 0.33 mm.

~~~~ ~~~ ~ ~

TABLE 2. Dimensionless Partitioning Coefficients Obtained from Previous Research and from Current Study at 20°C

___. DCM 1 0.189 1 0.256 Mackay et al., 1981 0.013 0.069

0.089 Gossett, 1987

0.121 Ashworth et al., 1988

1.590 Mackay et al., 1981 0.004 0.042

1.244 Gossett, 1987

_ _ _ ~ ._

CT

Ashworth et al., 1988

PCE 1.186 1 0.723 Gossett et al., 1987 0.001 0.005

I 0.697 Robbins et al., 1993

i 0.699 Ashworth et al., 1988

1 I

Environmental Progress (Vol. 19, No.3) Fall 2000 159

FIGURE 3. VOC Removal in Water as a Function of the Gas-Liquid Flow Ratio

The HNU Model 311 gas chromatograph was equipped with a photoionization detector and employed a 244 cm long column with an outer diameter of 3 mm, constructed of stainless steel, and filled with 60/80 Carbopack B 1% sp-1000.

Five mL gas samples generated from the packed column were directly injected into the HNU model 311 GC for quantification of the target compounds in the gas phase. The water samples were analyzed by injecting a 50 pL headspace sample into the HP 5890 GC. The oil samples were analyzed by injecting 0.125 mL of the oil sample containing the VOCs into a 160 mL serum bottle containing 100 ml of methanol. A 1 pL liquid sample was withdrawn from the methanol solution and was injected into the HP 5890 GC for analysis.

RESULTS AND DISCUSSION

Gas-Liquid Partitioning Batch Tests The gas-liquid partitioning coefficients that were

obtained from the EPICS tests are summarized in Table 2. The gas to liquid partition coefficients of the target compounds in water alone were comparable to previous research, thus indicating a high degree of confidence in the EPICS technique.

When vegetable oil was used as a medium the gas- liquid partitioning coefficients decreased for all three target compounds by approximately 1-3 orders of magnitude as compared to water alone. The most dra- matic decrease in the partitioning coefficients was observed with PCE in vegetable oil. PCE was the most hydrophobic of the three target compounds and exhibited the lowest gas-liquid partition coefficient in oil of the three target compounds. This observation was consistent with the work of Munz and Roberts [71, which suggested that the more hydrophobic the compound the greater the solubilization effect in vegetable oil. The results from the batch experiments clearly demonstrated that the target compounds strongly partition to vegetable oil and hence vegetable oil was deemed a viable scrubbing solution to be further researched within this study.

Gas-Liquid Mass Transfer Column The continuous flow mass transfer tests examined the

impact of gas and liquid flow rates as well as scrubbing solution composition on the removal of chlorinated contaminants from air streams. All tests employed either water or Canola vegetable oil, as the scrubbing solution.

The gas to liquid flow range of 1-40 was investigated to the greatest extent when water was employed as the scrubbing solution. Several tests examined the higher gas-liquid flow range to a limit of approximately 99, however under these conditions the removal efficiencies were relatively low, and hence the lower gas-liquid flow ratios were focused upon. When water was employed as the scrubbing solution within the packed column, the removal efficiencies varied for the three compounds. Figure 3 presents the steady state removal efficiencies of the three target compounds as a function of the gas-liquid flow ratios. For all compounds the removal efficiencies decreased with increasing gas-liquid flow ratios (Figure 3).

It was observed that CT was removed to the least extent of the three target compounds. The highest removal efficiency for CT approached only 15 % at the lowest gas-liquid flow ratios. The removal efficiencies for PCE rapidly decreased from 25% to 2% for gas-liquid flow ratios ranging from 8-40.

The trends exhibited in removal efficiencies were consistent with that which would be predicted by their Henry’s law coefficients. DCM had the lowest Henry’s law coefficient of the three compounds, which would indicate that it would have the highest partitioning to water as depicted in Figure 3 . The Henry’s law coefficient for CT was the highest of the three compounds, which explains the lowest removal efficiencies with water.

The results with water as the scrubbing solution con- firmed the hypothesis that water alone would not be desirable as a scrubbing solution for hydrophobic compounds such as PCE and CT because of the large quanti- ties of water that would be required to obtain substantial removals. From the batch tests it was found that the VOCs strongly partitioned to vegetable oil. These results suggested that vegetable oil may be a suitable medium for scrubbing VOCs and concentrating them into a liquid phase. Vegetable oil was therefore employed as an alternative scrubbing solution in the packed column under varying gas and liquid flowrates.

FIGURE4 VOC Removal in Oil as a Function of the Gas-Liquid Flow Ratio

160 Fall 2000

FIGURE 5. Continuous Flow Liquid-Liquid Reactor Experi- ments Testing for Steady State Conditions (Q, = 200 d m i n , Q, = 25 mWmin)

Figure 4 presents the steady state removal efficiencies of the target compounds as a function of the gas- liquid flow ratios when vegetable oil was employed as a scrubbing solution. The removal efficiencies when oil was employed as a scrubbing solution were consistent with theory. The least-polar compound (PCE) was the most readily removed and the removal efficiencies were relatively independent of the gas and liquid flows. Conversely the most polar compound (DCM) was removed to the least extent. High removals for all compounds were obtained when the gas to liquid flow ratio was less than 200. These results demonstrate that vegetable oil is a highly effective scrubbing agent for non-polar organic compounds.

The results of the continuous flow experiments demonstrate the importance of selecting a scrubbing agent that is appropriate for removing specific compounds from an air stream. Air streams such as the one synthesized for this study are especially challeng- ing when the compounds vary in hydrophobicity. However, under optimal process conditions, high removal efficiencies may be obtained.

Liquid - Liquid Partitioning Batch Tests Batch tests were performed to determine water-oil

partitioning coefficients for the three target compounds that could then be employed to assist in modeling the liquid-liquid mass transfer process. The experimental values for DCM were erratic and therefore an approximate water-oil partitioning coefficient was estimated from the gas-water partitioning coefficient and gas-oil partitioning coefficient previously obtained in the batch tests. Table 2 summarizes the results obtained for the water-oil partitioning coefficients for the three target compounds.

As expected, the most hydrophobic compound (PCE) exhibited the lowest water-oil partitioning coefficient. The least hydrophobic compound (DCM) was found to have the highest water-oil partitioning coefficient. These results suggest that PCE will partition to the least extent from the oil phase into the water phase in the liquid-liquid reactor.

Liquid-Liquid Mass Transfer Reactor Figure 5 presents a typical graph of the target com-

pound concentrations in the water phase as a function of time for the liquid-liquid mass transfer experiments. From the graph it was evident that DCM had not attained steady state by the end of the test. Converse- ly, the plateauing trend exhibited by CT and PCE suggested that these compounds had achieved steady state within the 3 hour time. These results were consistent with their water-oil partitioning coefficients, such that DCM exhibited the highest partitioning coefficient and would thus take longer to reach steady state conditions.

It was initially assumed that if the liquid-liquid column was operated for a minimum of 3 hours, steady state conditions were obtainable. The results from the continuous flow tests however indicated that not all the compounds were at steady state conditions. To accurately characterize liquid-liquid mass transfer within the reactor a dynamic liquid-liquid model was developed.

MODEL DEVELOPMENT

Model Development for Gas-Liquid Packed Column A model was developed to estimate mass transfer

coefficients from the continuous flow experiments in the gas-liquid mass transfer packed column. These observed mass transfer coefficients were determined to allow comparison with existing mass transfer models and to elucidate the mechanisms controlling mass transfer. Figure 6 presents a schematic of the packed column employed in the continuous flow tests and describes some of the nomenclature used in the modeling. The gas phase was passed upwards from the bottom of the column and exited at the top. The liquid phase was passed downwards over the column packing and exited at the bottom.

Steady-state gas and liquid phase mass balances assuming plug flow conditions were developed for

FIGURE 6. Packed Column Schematic

Environmental Progress (Vo1.19, No.3) Fall 2000 161

developed for the liquid-liquid column assuming the oil and water layers were completely mixed to yield equations 3 and 4 .

FIGURE 7. Liquid-Liquid Reactor Schematic

the packed column. Assuming a cross-sectional slice of thickness (dz) the following expressions were developed:

where:

Q1 =

Q, =

c, = c- =

I? A z dz

= dimensionless Henry's law coefficient = cross-sectional area of column (m2) = depth of column (m) = differential slice thickness (m)

K,a = mass transfer coefficient (sec-'1 (3)

liquid flow rate through packed column (m3/s) gas flow rate through packed column (m3/s> liquid concentration of VOC (mg/L) gas concentration (mn/L)

Equations 1 and 2 were integrated simultaneously to yield expressions that predicted outlet liquid and gas concentrations of the target compounds.

(4) Model Development for IJquid-Liquid Mass Transfer Reactor

A model was developed to describe liquid-liquid mass transfer within the liquid-liquid contacting reactor. The liquid-liquid model was developed as a dynamic model because some of the continuous flow data had not attained steady state conditions as previously assumed. Figure 7 presents a schematic of the liquid-liquid contacting reactor employed in the continuous flow tests and describes some of the nomenclature used in the modeling.

Water and oil phase mass balance equations were

= -QwCw + V,KLa (3) vw dt

V, - dC0 - - QoC," - Q,Co - V, KLa dt

where:

Q, = v, =

c, =

KP -

Q, = v, = c, =

-

K,a =

oil flow rate (mVmin) volume of oil (ml) concentration of VOC in the oil phase (mg/L) partition coefficient between water and oil phase water flow rate (ml/min) volume of water (ml) concentration of VOC in the water phase (mg/L) mass transfer coefficient (l/sec)

The liquid-liquid model was solved by writing equations 3 and 4 in terms of an explicit finite difference scheme and solving them simultaneously.

MODEL APPLICATION

Application of Gas-Liquid Mass Transfer Model The mass transfer coefficient in the gas-liquid mass

transfer model was calibrated with data obtained from the packed column under varying gas and liquid flow rates when vegetable oil was employed as a scrubbing solution. Figure 8 presents the observed mass transfer coefficients for PCE and CT as a function of the liquid velocity at a constant gas velocity of 1.05 m/min. From Figure 8 it is evident that as the liquid flow rate increased the mass transfer coefficients increased slightly. It was observed that the PCE mass transfer coefficient was less dependent on increases in liquid

FIGURE 8. Observed Mass Transfer Coefficients in Oil as a Function of Liquid Velocity (Constant Vg = 1.05 &mid

162 Fall 2000 Environmental Progress (Vol. 19, No.3)

TABLE 3. Modified Onda Correlations Gas and liquid Phase Resistances

I Original Onda

Modified Onda -DCM

omission of liquid phase resistance

Modified Onda - CT


Modified Onda - PCE


velocity (lowest gas-liquid partitioning coefficient in oil). The observed mass transfer rate coefficients for CT ranged from 5.33 x l/sec to 5.64 x lo-* l/sec for liquid flow rates ranging from 0.0012 m/min - 0.012 d m i n respectively. PCE mass transfer rate coefficients ranged from 6 . 8 9 ~ l/sec with liquid velocities ranging from 0.0012 d m i n - 0.012 m/min.

These results were consistent with the batch tests and the two-film resistance theory. All three target compounds were observed to partition to the greatest extent to the oil phase, which would result in low gas-oil partitioning coefficients. This would suggest that the gas phase was controlling mass transfer, hence there was only a slight dependence on the liquid velocities as compared to the increased dependence with the gas velocities on the I I ~ ; L ~ transfer rate cefficents.

l/sec to 8.87 x

Cur Vdaaly (m!nunl

FIGURE 9a. DCM Mass Transfer Coefficients vs Gas Velocity (Constant Liquid Velocity = 0.007 &mid

O a r V c I e c t t y r n : m i n ) Gas Velocity (m:min)

FIGURE 9b. CT Mass Transfer Coefficients vs Gas Velocity (Constant Liquid Velocity = 0.007).

FIGURE 9C. PCE Mass Transfer Coefficients vs Gas Velocity (Constant Liquid Velocity = 0.007).

Fall 2000 163 Environmental Progress (Vol. 19, No.3)

Figures 7a-7c present the observed mass transfer coefficients in vegetable oil as a function of the gas velocity at a constant liquid velocity of 0.007 m/min. From Figures 9a-9c, it can be seen that the observed inass transfer coefficients for all three target compounds increased with increasing gas velocity.

The observed mass transfer coefficients were compared to those estimated by the Onda correlations [Sl to assess the utility of these correlations for the current process. The Onda correlations predict liquid and gas mass transfer coefficients as well as the wetted surface area for packed columns. In general, the observed mass transfer coefficients in vegetable oil were not predicted very well by the Onda correlations (Figure 9). This observation was not surprising since the Onda correlations were not developed for scrubbing solutions such as vegetable oil, although the fluid properties of vegetable oil were employed in the correlations. The Onda correlations employ a number of empirical coefficients and it was not possible to cali- brate all of the coefficients with the existing data set within the scope of this study. It was however hypothesized that the empirically determined Onda correlations could be slightly modified to accommo- date other scrubbing solutions such as vegetable oil in modeling mass transfer within the packed column. From figures 8 and 9a-9c, it was observed that the gas phase resistance appeared to be dominating mass transfer. Hence it was elected to focus on the empirical constant in the equation for gas phase resistance, as it was believed that this would have the greatest impact on the correlation predictions.

When the overall gas and liquid phase resistances were calculated with the original Onda correlations it was found that the liquid phase resistance controlled mass transfer (>go% for all target compounds). This observation was not consistent with the previous results, which demonstrated that the mass transfer coefficients were relatively independent of the liquid velocity and that there was a strong dependence between the gas velocity and the mass transfer coefficients, suggesting gas phase resistance control. It was therefore suspected that the original liquid phase resistance was not correct when vegetable oil was employed as a scrubbing solu-

tion and was therefore discarded from the Onda correlations so that the main emphasis was put on the gas phase resistance. By recalibrating the model in this manner the observed mass transfer coefficients were better modeled by the modified Onda correlations.

Figures 7a-7c present the predicted mass transfer coefficients with the original Onda correlations, with the gas phase resistance assumed to be controlling (liquid phase resistance discarded), and with the empirical constant modified. Table 3 lists the modified equations for the Onda correlations for each compound corresponding to Figures 9a-9c.

It was observed that the compound with the lowest gas-liquid partitioning in oil (PCE) exhibited the smallest change from the original Onda correlations, aside from assuming gas phase control (Table 3). DCM, with the highest gas-liquid partitioning coefficient in oil, exhibited the most substantial modification from the original Onda correlations. The constant multiplying the gas phase resistance increased from 2 to 35.2. The results suggest that CT and PCE were likely gas phase controlled and hence the omission of the liquid phase resistance was probably valid. In contrast, D C M may have Some degree of liquid phase control and the liquid phase resistance may also have to be modified to accommo- date this observation.

As indicated by the Onda correlations, in aqueous packed bed systems that are employed to scrub the target compounds liquid phase resistance would be expected to dominate mass transfer. However, the air-oil partitioning coefficients in this study were one to three order of magnitude smaller than that of aqueous systems. This difference in the partitioning coefficients is therefore responsible for the change in the phase of mass transfer dominance.

Application of Liquid-Liquid Mass Transfer Model The mass transfer rate coefficients in the liquid-liquid

model were calibrated with the continuous flow data obtained in the liquid-liquid reactor. Table 5 summarizes the liquid-liquid mass transfer coefficients obtained under various process conditions for the continuous flow tests. It should be noted that the mass transfer coefficients are volumetrically based and are therefore a

TABLE 4: Summary of Mass Transfer Coefficients from the Continuous Flow Liquid-Liquid Contacting Reactor ------------p--~____-p----- _ _ _ _ ~ -~--p--- ------

I I I

CT I PCE I (l/sec) I (l/sec)

164 Fall 2000 Environmental Progress (Vol.19, No.3)

0, 01 cwiv

Q1 (1)

Q, (1)

FIGURE 10. Combined Process Schematic

function of the mixing and turbulence present in the experimental reactor. Further study would be required to determine the effect of scaling up the process on the mass transfer coefficients.

It was observed that the lowest mass transfer coefficients were exhibited by PCE under all process conditions (Table 4). The mass transfer coefficients for DCM and CT in the continuous flow experiments were similar for all the runs investigated. These results were in agreement with the water-oil partitioning coefficients obtained from the batch tests for DCM and CT being 0.07 and 0.04 respectively. PCE, the most hydrophobic compound, did not partition well from the oil phase to the water. The liquid-liquid contacting reactor process would require further improve- ment for highly hydrophobic compounds such as PCE. It was observed that, although there was a range in the mass transfer coefficients obtained for the three target compounds, no apparent trend was found to exist with respect to the liquid flowrates.

ml/min 50000

ml/min 1000000

Simulation of Hybrid Process The packed column and the liquid-liquid contact-

ing reactor mass transfer models were combined to form one joint steady state model. An attempt was made to characterize the impact of gas and liquid flows on the mass transfer coefficients for future scale up of the hybrid process.

This study focused primarily on the first phase of the hybrid process, in which contaminated VOCs were transferred from the gas phase to an oil phase by way of a packed column. The VOCs in the oil phase were then transferred into an aqueous solution by way of a liquid-liquid contacting reactor for further biological treatment in an anaerobic reactor. It is proposed to incorporate the second stage into a high rate anaerobic reactor that will maintain minimal aqueous- phase concentrations of the target compounds. The following combined model attempts to simulate the hybrid system for the target compound CT.

Figure 10 presents a schematic of the joint system in which the liquid-liquid reactor was incorporated directly into the anaerobic reactor. The packed column is referred to as system 1, and all nomenclature associated with the packed column employs the subscript 1. Similarly, the anaerobic reactor is labeled sys-

Qoa (2)

&a (1)

tem 2 and all nomenclature associated with the anaerobic reactor employed subscript 2.

Typical mass transfer coefficients calculated in the continuous flow tests of the packed column and liquid-liquid reactor were employed in the joint simulation. The mass transfer coefficients for the packed column (2 m deep with a surface area of 1 m2) were determined by employing the modified Onda correlations which assumed that the gas phase resistance controlled. A gas flow ( typical of full scale plant) of 1 m3/min was assumed to enter the packed column. A gas-liquid ratio of 20 was assumed, hence all liquid flows within the system could be determined. Assum-

d m i n 50000

l/sec 1.59E-04

TABLE 5: Joint Simulation for CT

k2

C ,

Constants Z(0) = Om Z(L) = 2 m

Area = 1 m2 K(CT)= 0.004 (aidoil) diameter = 1.1287 m

0.01

2.27E-03

G/L ratio I I 20

c,

I 1

0.01

v oil (2) I L 1 2929.4

l I

I

I

1. - - I I

K,a (2) I l/sec 1 2.00E+00 I I

I ,

YoRemoval in gas 95.000

Environmental Progress (Vol. 19, N0.3) Fall 2000 165

ing a typical inlet gas concentration into the packed column (Cgi, of 0.19 mg/L for CT, all other gas and liquid concentrations were calculated by the model for both columns assuming that CT was fully degrad- ed within the anaerobic reactor and, the water concentration was negligible.

Table 5 presents a typical simulation of the combined model for CT. The combined model assumed steady state conditions in both the packed column and the anaerobic reactor. The purpose of the simulation was to determine the volume of oil required in the anaerobic system so that the two processes could be sized appropriately. An initial inlet concentration of CT entering the packed column was assumed and the system of equations was solved iteratively to maintain a 95% removal efficiency within the packed column. It was determined in the simulation that a volume of 2929 L of oil would be required in the anaerobic reactor.

The joint model provides useful insight into the feasibility of operating the proposed hybrid system to remove VOCs from contaminated air streams. It was found that there is a high potential for the overall process to remove chlorinated compounds that vary in hydrophobicity and further research should contin- ue in this area.

CONCLUSIONS This research investigated the issues associated

with the mass transfer of chlorinated organics from a gas stream to a liquid stream that would be subse- quently treated in an anaerobic reactor. The results of batch tests provided conclusive evidence that the target compounds strongly partition to vegetable oil and hence, vegetable oil was further investigated as a scrubbing solution in the packed column experiments.

A bench-scale countercurrent packed column was designed and operated with various scrubbing liquids and gas-liquid flow ratios to characterize mass transfer and removal efficiencies. The results obtained from the continuous flow tests suggested that high removal efficiencies for all three compounds (90%) could be obtained when vegetable oil was employed as a scrubbing solution with gas-liquid flow ratios less than 200.

A scrubbing model was developed to allow the cal- ibration of mass transfer coefficients under various operating conditions. The observed mass transfer coefficients were then compared against those predicted by the Onda correlations It was found that the Onda correlations did not fit the experimental data for vegetable oil very well, hence the Onda correlations were modified by assuming that the gas phase resistance was controlling mass transfer. The assumption appeared to be valid for the compounds with lower gas-oil partitioning coefficients (CT and PCE). DCM appeared to have some component of liquid phase control, since it had the highest gas-oil partition coefficient of the three target compounds.

A liquid-liquid contacting reactor was operated to assess mass transfer from the oil phase to an aqueous phase. The results obtained from the liquid-liquid reactor suggested that mass transfer could be achieved for compounds such as DCM and CT, that were not

highly hydrophobic such as PCE. DCM and CT exhibited comparable mass transfer coefficients, as a result of their similar water-oil partitioning coefficients. PCE demonstrated the lowest mass transfer coefficients in the liquid-liquid reactor of the three target compounds. This result was consistent with its low partitioning coefficient from oil-water.

A combined mass transfer model was developed to simulate a hybrid process including an anaerobic reactor. From the combined model simulations, the major factor that influenced the overall oil volume in the anaerobic reactor was the degree of partitioning between the gas and oil phases within @e packed column. The hybrid system appears to be a feasible alternative for removing VOCs from air streams in industrial processes, and research should therefore be contin- ued in this area.

LITERATURE CITED 1.

2.

3.

4.

5.

6.

7.

8.

9.

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Enhanced scrubbing of chlorinated compounds from air streams