Sensitivity Analysis of the Separation Efficiency of a Vacuum Stripping Microchannel Device

The implementation of gas / vapor – liquid separation operations like absorption, stripping, and distillation at microscale constitutes an important milestone for the synthesis of multistep microchemical processes [1]. However, over the last two decades the development of separation microchannel devices for such operations has proven to be a challenging task as compared to the well-established microreactors, micromixers and micro heat exchangers. Indeed the high surface-to-volume-ratio prevailing at microscale results in (i) a poor control of the required thermodynamic vapor-liquid equilibrium [2] and in (ii) a high flow resistance particularly in liquid channel (s). Both phenomena result in the reduction of the separation driving force and thus in the drop of the separation efficiency.

Recent investigations showed that liquid repellent vapor-liquid contactors with hydrophobic surface properties may be successfully used in distillation [3], membrane distillation [4-5] and stripping [6-8] based separation microchannel devices. As compared to wetting vapor-liquid contactors [7-8], the liquid repellent vapor-liquid contactors, may have important potentials to reduce the energy consumption of such microchannel devices [9] as well as the flow resistance in the liquid channels [10]; contributing thereby to a better stabilization of the vapor-liquid interface and thus to the enhancement of the separation efficiency of the device.

In this work, the separation efficiency of a vacuum stripping microchannel device is investigated. The device is constituted of two micromachined rectangular straight channels separated by a flat sheet microporous hydrophobic membrane (contact area: 80 mm × 6 mm). The membrane serves as liquid-vapor contactor, which repels the liquid phase and constitutes therefore a support for the liquid-vapor interface at the entrance of the pores. The liquid feed (aqueous solution of 5 wt. % acetone) is circulated through one of the channels. The other channel called permeate channel is kept under vacuum. Acetone and water evaporate and diffuse through the membrane pores. The separation driving force in the device is created by the partial vapor pressure gradient for each component (acetone, water) across the membrane caused by the vacuum in the permeate channel. The permeated vapor phase is then stripped by vacuum outside the permeate channel. To avoid liquid intrusion into the membrane pores, the pressure difference across the membrane must be maintained below the liquid breakthrough pressure of the membrane [3-5, 11]. Furthermore the pressure in the permeate channel must be lower than the vapor pressure of the feed [11]. In this work, two commercial flat sheet microporous hydrophobic membranes provided by Pall (USA) were considered: polyethersulfone polymer cast on a non-woven polyester-support membrane (Supor®-450R, porosity: 66 vol %, mean pore size: 0.94 μm, thickness: 80 μm) and acrylcopolymermembrane on polyester-support (Versapor®-200TR, porosity: 39 vol %, mean pore size: 0.51 μm, thickness: 50 μm). The contact angles measured for the investigated liquid mixture on both membrane surfaces were found to be larger than 120 °. For the separation device presented previously, the following relevant design parameters were identified: (a) the liquid channel depth, (b) the geometric properties of the considered membrane contactor including porosity, mean pore size and thickness, (c) the feed flow rate and (d) the pressure in the permeate channel. The separation efficiency of the device was measured using the stripping degree. The latter is ranging between 0 and 1, which correspond to 0 and 100 % acetone removal from the liquid phase, respectively.

Theoretical investigations were carried out in order to get better understanding of the influence of the above mentioned design parameters on the separation performance of the vacuum stripping microchannel device at isothermal conditions (20 °C), and thus to elaborating an optimization approach for the design of the device. For this purpose, a two-dimensional mathematical model describing the transport mechanisms in the device including the liquid channel and the membrane was developed. Accordingly, the mass balance equation for acetone involving convection and diffusion was established in the liquid channel assuming laminar and hydrodynamically developed flow, whereas the Knudsen diffusion was identified as the prevailing mass transport mechanism for the vapor permeate in the membrane pores. Consequently, the boundary condition for mass transfer at the liquid-membrane interface was established based on the acetone permeate flux determined using Knudsen diffusion model, in addition to the liquid-vapor equilibrium relationship of the mixture at the liquid-membrane interface obtained by Aspen Plus®. The established model with all relevant boundary conditions has been implemented using Comsol Multiphysics®.

Based on the elaborated model, a sensitivity analysis of the separation efficiency of the device to the previously mentioned design parameters was conducted. Accordingly, a maximum stripping degree of 0.86 was achieved for a liquid channel depth of 50 μm, a feed flow rate of 0.103 ml / min, a permeate pressure of 10 mbar and with using Supor®-450R as liquid-vapor contactor. Furthermore, it was shown that the developed model allows to elucidating the impact of all investigated design parameters on the controlling resistance to mass transfer in the whole device including the membrane and the liquid channel, and thus constitutes a useful tool towards the design optimization of the device.

References

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