Measurements of diffusivity of chlorinated VOCs in heavy absorption solvents using a laminar falling film contactor

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Chemical Engineering and Processing 47 (2008) 1478–1483

Measurements of diffusivity of chlorinated VOCs in heavy absorption solvents using a laminar falling film contactor Reda Hadjoudj, Hubert Monnier, Christine Roizard, Franc¸ois Lapicque ∗ Laboratoire des Sciences du G´enie Chimique, CNRS-ENSIC, B.P. 20451, F-54001 Nancy, France Received 28 April 2006; received in revised form 7 February 2007; accepted 5 April 2007 Available online 31 July 2007

Abstract The diffusivity of VOC in absorption solvents has been determined in view to modelling the performance of G/L processes for treatment of VOC-containing gases. The laminar falling film technique was used for the examples of dichloromethane (DCM) and tetrachloroethylene (TCE), to be absorbed in 2-di-ethylhexyl adipate (DEHA). The technique led to accurate results for DCM. In contrast, the moderate volatility of TCE corresponds to a low Henry’s law constant; the absorption was then largely controlled by gas-side transfer, the rate of which had to be accounted for in treatment of the absorption data. Nevertheless, as discussed in the paper, the well-known technique is poorly applicable in case of too soluble gases, i.e. exhibiting Henry’s constants below 1 Pa m3 mol−1 . © 2007 Elsevier B.V. All rights reserved. Keywords: Gas–liquid absorption; Laminar falling film; Volatile organic compounds; Heavy organic solvents; Transfer rates

1. Introduction Selective absorption of VOCs from waste gases can appear as an alternative route for their treatment, in addition to more conventional processes, e.g. condensation, incineration, thermal oxidation or adsorption on carbon materials. Some of the above processes, e.g. incineration, are not suitable for chlorinated VOCs because of the formation of hydrochloric acid or dioxins. Absorption processes represent a regenerative treatment route since upon separation of the liquid phase issued from the G/L contactor, VOCs could be recovered at far higher concentrations than in the waste gases, allowing their reuse in further applications to be considered. Previous investigations conducted with dichloromethane (DCM) and tetrachloroethylene (TCE) yielded selection of promising solvents for the absorption, in particular 2-diethylhexyl adipate (DEHA) [1]. Moreover, the potential of the process was recently investigated using conventional G/L column and a microstructured falling film contactor [2]. However, prediction of absorber performance √ of the absorption rate, often expressed in term of the group H D, according to the

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well-known theory of gas–liquid absorption [3]: Henry’s law constant, H, of the VOCs in DEHA was determined thoroughly using cross-linked techniques [1], and the present investigation deals with determination of VOC diffusivity in the above selected solvent, D. Various experimental techniques can be used for determination of the diffusivity of dissolved gases in liquids, as explained in [4] for instance; laminar flow apparatus have often been used for this purpose because of their known hydrodynamic behaviour. Amongst the various geometries offered, the cylindrical support configuration was used, since it has been widely used for a number of solute/solvent couples [3,5–7] The gas phase is usually circulated in the annular space defined by two co-axial cylinders, whereas the liquid flows in laminar regime on the wall of the inner cylinder. For sufficient gap between the cylinders, the gas phase can be assumed to be ideally mixed. For diluted gas phase, mass transfer phenomena are singlecomponent processes and transport is described using Higbie’s model [8], see [3]. For controlling liquid-side transfer rates, the transfer rate leads directly to the diffusivity. However, for soluble gases, gas-side transfer can exert partial control of the overall transfer, and gas phase transfer rates have to be accounted for in treatment of experimental data. We present here the results of determination of the diffusivity of two VOCs considered, namely DCM and TCE in DEHA and at 25 ◦ C, together with a

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Table 1 Physical properties of the chemicals used at 25 ◦ C, except for the molar volumes, which were taken at the boiling point [1,18] Chemical

Molecular weight (kg mol−1 )

Density (kg m−3 )

Viscosity (Pa s)

Molar volume, Vi (cm3 mol−1 )

DiG (m2 s−1 )

Hp (Pa m3 mol−1 )

Nitrogen SO2 DCM TCE DEHA

0.0280 0.0641 0.0849 0.1658 0.3706

1.15 2.62 1325 1623 980

1.75 × 10−5 – 0.0007 0.0018 0.0118

– 44 70 119 490

– 4.597 × 10−5 3.798 × 10−5 2.748 × 10−5 –

– – 7.64 0.721 –

critical discussion on the potential and the limits of the technique used. 2. Experimental 2.1. Chemicals and properties All chemicals were of analytical grade. DCM and TCE were purchased from Merck, and DEHA was obtained from Acros Organics. Nitrogen (R grade), carbon dioxide, and sulphur dioxide (all at 99.99%) were supplied from pressured cylinders. Physical properties of the various chemicals at 25 ◦ C are reported in Table 1. Henry’s law constant was deduced from vapour–liquid equilibria of the binary systems [1]: the published data were extrapolated to 25 ◦ C, and Henry’s law was written using the pressure-based constant, Hp , to relate the partial pressure of compound i, Pi , to its concentration in the solvent phase, Ci : P i = Hp C i

(1)

taking into account the solvent density and its molecular weight. The large difference in Hp exhibited by the two VOCs does not reflect the solvent capacity, but is mainly due to their very different vapour pressures, namely 57.8 kPa for DCM and 2.80 kPa for TCE at 25 ◦ C.

ranged from 100 to 500 Pa. Because of its high volatility, the primary DCM-nitrogen mixture was further diluted in pure nitrogen to reach the desired concentration range. The gas mixture was fed at the bottom of the G/L contactor, and left on top after VOC transfer to the flowing liquid. Due to the high dilution, the flow rate of the gas was not changed to a significant extent; moreover, the flow rate was measured and expressed at 1 atm and 25 ◦ C. The liquid leaving the contactor was collected in an intermediate vessel upstream of the storage tank. Depending on the flow rates of the two phases, the level of the intermediate vessel was adjusted in a way to allow their steady flow of the two phases, with minimal thickness of the liquid film remaining in the bottom of the annular space. Moreover, the liquid flow rate was selected to allow steady flow of the liquid on the cylinder, as controlled by visual observations prior to absorption experiments. Experiments were carried out at ambient temperature namely 25 ◦ C within 1 ◦ C, and the temperature of the two phases as well as the pressure were monitored during the runs. The outlet gas was driven to a CPG provided with FID for analysis using a six-way valve. Elution of the VOCs was carried out at 150 ◦ C in a 30 m × 0.542 mm capillary column provided with DB-624 liquid phase.

2.2. Experimental set-up Pure solvent was continuously fed to the contactor using a peristaltic pump: the solvent was admitted inside a stainless steel tube with an external diameter, d, of 15.65 mm, then through twelve 1.5-mm holes drilled around the cylinder surface, before flowing on the external surface of the cylinder. This surface was carefully polished with 1200 grit paper after machining. A conical piece (Fig. 1) around the upper part of the cylinder allowed establishment of the flow upstream of the gas chamber, formed by the above cylinder and an Altuglass tube with diameter dext = 64.0 mm. Wetted-wall contactors with the same dimensions were successfully used in previous determinations of gas diffusivity in liquids [9,10] and of kinetics of liquidphase reactions [11]. Displacement of the conical piece along the SS tube allowed the length of the G/L contactor, h, to be varied from a few millimetres to 200 mm. Nitrogen was partially saturated with the investigated COV by bubbling through a sintered tube in a thermostated G/L vessel: the temperature of the saturator was adjusted so that the inlet VOC pressure

Fig. 1. Schematic view of the falling film contactor.

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2.3. Preliminary investigations of the laminar film contactor Flow conditions were tested with both pure water and DEHA in view to determining the ranges of flow rates. The solvent rate had to be kept between 0.1 and 0.5 cm3 s−1 to allow formation of uniform, smooth film on the polished SS surface: corresponding Reynolds number of the liquid, ReL , was below 3.0, and the observations made were in perfect agreement with the results of Sherwood and Pigford investigations [11]. The gas flow rate measured at the ambient temperature was fixed at 22 cm3 s−1 for DCM and 20 cm3 s−1 for TCE. Moreover, the device was calibrated by carrying out absorption of pure carbon dioxide into pure water. The physical absorption is known to be controlled by liquid-side transfer rate, and the absorption flux of carbon dioxide then obeys the following relation:   1/3  πρL g 1/6 ∗ 1/2 QL JCO2 = dCCO2 (6hDCO2 ) (2) d 3μL ∗ where CCO is the solubility of the gas considered, QL the liquid 2 flow rate and h is the height of the falling film. Relation (2) was derived from Higbie’s penetration theory and the hydrodynamics of wetted-wall cylinders [3] Calibration was carried out at 25 ◦ C 1/3 varying the product (QL h1/2 ), and the linear variation was shown to hold for h over 50 mm. The slope  of the regression line ∗ yielded the transfer rate group CCO DCO2 , from which the 2 diffusivity of CO2 in water was found at 1.96 × 10−9 m2 s−1 , in perfect agreement with literature data [12,13].

Fig. 2. Overall absorption rate of TCE in DEHA at 25 ◦ C depending on the liquid flow rate and the film height. Gas flow rate = 20 cm3 s−1 .

responding to edge effects of the absorption device. The effective area involved in the G/L transfer, A, differed probably from the geometrical area of the laminar liquid film, Afilm , Afilm = πdh, in particular for low h values. The variation of the transfer rate with QL is shown in Fig. 3: the absorption rate obeyed a power-function of the liquid flow rate. For DCM absorption, the exponent ranged from 0.26 to 0.29 depending on h, in satisfactory agreement with relation

3. Results 3.1. Overall absorption rate Mass balance on VOC was written in the gas phase. For this purpose the absorption flux was expressed in terms of the difference of the partial pressure in the gas bulk and the corresponding pressure in the liquid bulk, (Hp Cb ): J = KG A(Pout − Hp Cb ) =

QG (Pin − Pout ) RT

(3)

assuming uniform pressure of VOC in the gas phase. The equivalent partial pressure in the liquid can be neglected taking into account gas and liquid flow rates and the value for Hp . The overall transfer rate, KG A, was therefore deduced: KG A =

QG Pin − Pout RT Pout

(4)

The transfer rate was measured depending on the liquid flow rate and the height of the falling film. As expected, higher transfer rates were allowed by increasing QL , as exemplified by Fig. 2 for the case of TCE absorption. However, KG A did not vary linearly with h and extrapolation of the data obtained with h larger than 20 mm to h = 0, led to a appreciable transfer rate: side absorption likely occurred by contact of the gas with the horizontal liquid film formed in the lower part of the device, cor-

Fig. 3. Overall absorption rate of VOC in DEHA at 25 ◦ C depending on the liquid flow rate and the film height. Theoretical law (2) with slope 1/3 is shown for comparison. Gas flow rate was 22 cm3 s−1 for DCM (a, top), and 20 cm3 s−1 for TCE (b, bottom).

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where an are the roots of the first zero-order Bessel function, J0 . However, use of this model was reported by Byers and King [16] to strongly underestimate gas absorption rate, and our results are also far larger the theoretical variation (Fig. 4), neglecting in the comparison the side absorption. Nevertheless, the theoretical law shows that exponent α increases regularly with ReG , attaining unity for ReG over 1000, in agreement with Crause and Nieuwoudt’s conclusions [14]. 3.3. Interpretation of the data

Fig. 4. Gas-side mass transfer coefficient vs. the Reynolds number in the gas  , defined as the ratio of the phase. Experimental data were in fact coefficient kG transfer rate, kG A, over the geometrical film area equal to (πdh) (relation (5)).

(2). For TCE, the exponent laid below 0.15 (Fig. 3b), indicating that TCE absorption was partly controlled by mass transfer in both sides of the interface, because of the higher Henry’s law constant. The gas-side transfer rate was thereafter accounted for in the expression of KG A.

Gas-side transfer rates were determined by absorption of sulphur dioxide in 1 M NaOH solutions. Sulfur dioxide was diluted in a nitrogen stream, so that the inlet pressure was of the order of 200 Pa. SO2 partial pressure was measured at the inlet and outlet of the contactor by means of a Binos® UV analyser. The data were confirmed by titration with iodine/dithionite system. The gas flow rate was varied from 20 to 330 cm3 s−1 , corresponding to Reynolds number ReG ranging from 21 to 350. Flow rate QL was fixed at 0.5 cm3 s−1 . Experiments were carried out with film heights of 52, 102, and 153 mm, corresponding to the three highest h values considered for VOC absorption. The gas trans , defined on the basis fer rate led to mass transfer coefficient, kG of the geometrical area of the film: kG A πdh

(5)

 was affected by h, and Because of the side-absorption, kG higher values were obtained for the lowest h value (52 mm).  varied with Re following a power function with Coefficient kG G exponent α. In the ReG range investigated, α was near 0.30, as shown in Fig. 4 for h = 102 mm. The present data are in good agreement with the results of previous measurements carried out with a similar film contactor [10]. Further comparison with literature data is difficult because of the moderate values of ReG considered here. For ReG higher than 1000, kG is proportional to ReG , in accordance with the predictions of models for this regime [14]. For low ReG , kG can be predicted by the expression given below, which is derived from coupling L´ev`eque’s laminar flow to penetration theory, as explained in [15]:   2 ∞  4 QG an πhD kG = − (6) exp − log πdh an2 QG n=1

The experimental data obtained for VOC absorption varying QL and h, were fitted to the theoretical expression (8) given below: 1 1 = KG A kG A

3.2. Estimation of gas-side transfer rates

 kG =

Gas-side transfer rates measured by absorption of sulphur dioxide were applied to VOC absorption, taking into account the change in diffusivity in the gas phase, and considering Higbie or Danckwerts surface-renewal models:  1/2 G DVOC kG AVOC = kG ASO2 (7) G DSO 2

+

Hp (6πd 2 (h + h

1/2

eq )DVOC )

(QL /πd)1/3 (ρL g/3ηL )1/6 (8)

In relation (8) kG A depends on h, as explained above. In addition, the side gas absorption observed was accounted for by the presence of height heq , and the global absorption was considered to occur at the interface of the cylindrical film with height (h + heq ). For each VOC, the set of data was fitted to relation (8) by minimization of the sum of squared deviations between theoretical and experimental KG A values. Only data for h = 102 and 1053 mm were considered to reduce significance of absorption edge effects. For both VOCs heq was of the order of 25 mm, corresponding to appreciable edge absorption. The diffusivity of DCM in DEHA at 25 ◦ C was found at 2.22 × 10−10 m2 s−1 , whereas that for TCE was determined at 0.45 × 10−10 m2 s−1 . Diffusivity of the VOCs was also estimated by means of various predictive relations summarised in Table 2, and using the values of the physical parameters reported in Table 1. For DCM, the experimental value is in perfect agreement with the predicted values, in particular with Siddiqui and Lucas’ relation [17]. For TCE, the experimental data lay significantly below the predicted values, and this point was further investigated. Because of its relatively high boiling point and its low activity coefficient in DEHA [1], TCE is largely soluble in this solvent, and gas-side mass transfer exerts a strong control on the overall absorption. The uncertainty in determination of kG A taking into account the change in chemical system was estimated at 20%. Fitting of the data was carried out with kG A taken at 80 and 120% of their nominal values, leading to a confidence interval for the diffusivity (Table 2). For DCM, gas absorption was mainly con-

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Table 2 Experimental and predicted values for the diffusivity of DCM and TCE in DEHA at 25 ◦ C Author

Correlation

Tyn and Calus [19] Hayduk and Minhas [20](*) Siddiqui and Lucas [17] Present work

D = 8.93 × 10−12 ηTL VVOC Vsolvent PaPasolvent VOC −0.23 −0.42 Vsolvent Pa0.50 D = 1.55 × 10−12 T 1.29 η−0.92 L solvent PaVOC −0.907 −0.45 0.265 −12 D = 9.89 × 10 TηL VVOC Vsolvent 1/6

1/3



0.6

DDCM (×10−10 m2 s−1 )

DTCE (×10−10 m2 s−1 )

1.79 2.14 2.32 2.07–2.42

1.42 1.71 1.82 0.31–0.65

The range given is the confidence interval established by fitting the data with (kG A) equal to 80 and 120% of its value. (*) Parachors were approximated by molar volumes.

trolled by liquid phase transport, resulting in a narrow confidence interval. In contrast, the large confidence interval obtained for TCE showed the moderate reliability of the technique for soluble compounds: the uncertainty in kG A determination had a great impact on the diffusivity value since D varies with the square of the absorption flux, according the surface-renewal model. 4. Conclusion The conventional laminar falling film technique has been used for determination of the diffusivity of DCM and TCE in an absorption solvent (DEHA). Experiments revealed the occurrence of side-absorption in the stagnant liquid at the bottom of the G/L contactor. For the case of DCM, absorption in the liquid film is controlled by liquid-side transfer, which allowed accurate estimation of its diffusivity, in good agreement with the prediction of published relations. In contrast, for low Henry’s law constant of the gaseous solute, absorption is largely controlled by gas-side transfer, whose kinetics had to be taken into account in treatment of the experimental data. Nevertheless, uncertainty in determination of gas-side transfer rates is detrimental to the accuracy of the technique in such cases, and the obtained value is to be underestimated. Although described in textbooks and used for decades, the laminar falling film technique has to be carried out with care for reasons given above, and its use should not be recommended for low Henry’s constants, say below 1 Pa m3 mol−1 . Acknowledgements This work was partly funded by ADEME within the research programme 02 74 037. Thanks are also due to R. Thielens for his experimental contribution. Appendix A. Nomenclature

A Cj d dext D g h heq

interface area (m2 ) concentration of species j (mol m−3 ) diameter of the wetted cylinder diameter of the external wall of the chamber diffusion coefficient (m2 s−1 ) gravity (m2 s−1 ) height of the film (m) equivalent height corresponding to side-absorption in the G/L contactor (m)

Hp J kG  kG kL KG P Psat Pa QG QL r R ReG T Vi

Henry’s law constant (Pa m3 mol−1 ) flux (mol s−1 ) Gas-side mass transfer coefficient (mol Pa−1 m−2 s−1 ) gas-side mass transfer coefficient expressed on the basis of the film area (πdh) (mol Pa−1 m−2 s−1 ) liquid-side mass transfer coefficient (m s−1 ) overall mass transfer coefficient (m s−1 ) pressure (Pa) vapour pressure of pure liquid at T (Pa) Parachor (cm3 g1/4 s1/2 mol−1 ) gas flow rate (m3 s−1 ) at 298 K and 1 atm liquid flow rate (m3 s−1 ) reaction rate (mol m−3 s−1 ) gas constant (J mol−1 K−1 ) G ρG Reynolds number in the stirred vessel = π(dQ ext +d)ηG temperature (K) molar volume of species i (cm3 mol−1 )

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