Re: Bk Dutta Mass Transfer Solutions
Odina Conkright
In this research work, a coupled heat and mass transfer model was developed for salt recovery from concentrated brine water through an osmotic membrane distillation (OMD) process in a hollow fiber membrane contactor (HFMC).The model was built based on the resistance-in-series concept for water transport across the hydrophobic membrane. The model was adopted to incorporate the effects of polarization layers such as temperature and concentration polarization, as well as viscosity changes during concentration.
This study highlights the effect of key parameters for salt recovery from wastewater using the membrane distillation process. Further, the applicability of the OMD process for salt recovery on large scale was investigated. Sensitivity analysis was performed to identify the key parameters. From the results of this study, it is concluded that the OMD process can be promising in salt recovery from wastewater.
Industrial wastewater effluents contain a large variety of useful chemical compounds. For example, effluents from the textile, metal, and petrochemical industries typically contain inorganic salts such as carbonates, sulfates, and nitrates. The recovery of these chemicals from wastewaters has many advantages. Firstly, they can be reused, decreasing the stress generated by the intensive extraction of natural resources. The clean water obtained can then be recycled back to perform industrial operations. Moreover, a continuous increase in salt concentration in wastewaters can cause environmental issues such as eutrophication or increased soil alkalinity, hence affecting cultivation land [1].
The study on the efficient recovery of inorganic salts from wastewater effluents is considered as a focusing area during the past decade. Different conventional techniques have already been implemented for this purpose, such as electrolysis, reverse osmosis, nanofiltration, ion exchange membrane processes, and evaporation [2,3,4]. However, most of the mentioned treatment techniques are expensive and involve complex processes that lead to solids in sludge form, which can be difficult to reutilize.
For salt recovery, crystallization is commonly used because it provides a very high-quality product with a high recovery rate. In the literature, several crystallization techniques have already been implemented for salt recovery from wastewater [5, 6]. Among these, osmotic membrane distillation (OMD) is a promising technique because of the following advantages: (1) it can yield highly concentrated streams; (2) it can be operated at room temperature; and (3) a well-controlled supersaturation takes place [6,7,8,9]. Hollow fiber membrane contactors (HFMC) are commonly used to carry out OMD operations. HFMCs are the preferred choice over flat sheet membranes in OMD operation because they present high specific areas per unit volume, easy scale-up, and low manufacturing costs. HFMCs allow a non-dispersive contact between two phases due to the hydrophobic nature of the membrane without intimate mixing. The membrane used is hydrophobic to avoid any membrane wetting due to the presence of aqueous streams in boundary layers. During this process, water evaporates at the pore entry of the membrane from the feed side and is transported through its pores either by Knudsen or molecular diffusion and then condenses at the permeate side. The transport gradient between the two sides occurs due to the difference in chemical potential caused by water activity, pressure, or temperature gradients [10,11,12]. Because the transfer of water occurs from the feed to the permeate side, the feed stream is concentrated. In the OMD process, crystallization occurs when the feed solution reaches supersaturation. The hydrophobic membrane surface allows heterogeneous nucleation and crystals are continuously driven away by the flow of the feed solution. Crystals can further grow when coupled to a crystallizer [9, 13, 14].
Sodium carbonate (Na2CO3) is commonly found in wastewater effluents coming from the sodium hydroxide-based CO2 capture process, which is an essential step to control CO2 emission [15]. The recovery of Na2CO3 is necessary so that the process is economically viable; therefore, evaporation and crystallization are usually applied in the chemical industry to recover solid Na2CO3 from wastewater [16]. Recently, membrane contactors appear as an alternative technique that can replace both conventional evaporation and crystallization operations because of their adaptability and low energy consumption [17]. The current research work focuses on the concentration of Na2CO3 solution till the saturation point, using OMD through a modeling and simulation approach.
Modeling and simulation are relatively cheaper, robust, and fast techniques as compared to the experimental methods, and if the model parameters are carefully estimated, the validated model can be used to replicate the experimental data and predict the scaling up of the process. Mass and heat transfer modeling of HFMCs is inevitable to study the concentration and flux profiles, to investigate the solute transport, to categorize the controlling resistance, for the optimization of the process operating parameters, and to scale up the process [13]. Scale-up of HFMC has not been widely studied for OMD in salt recovery during crystallization. More theoretical and simulation investigation is required to encourage the entrepreneurs to utilize HFMCs for the selected application at a large scale.
Different approaches are used to model the OMD process, however, the resistance-in-series (RIS) model approach is a widely applied and acceptable technique to predict water transport flux through the membrane [18,19,20,21]. Most of the models used in literature based on the RIS model have not considered temperature/concentration polarization effects. Similarly, the effect of physical properties was considered constant during the concentration process. Ahmad et al. [13] studied the RIS model for osmotic membrane evaporation of fruit juices and found that physical properties like viscosity and density variations during the process had a significant effect on water flux. Ni et al. [22] studied the effect of temperature polarization in direct contact membrane distillation for NaCl concentration and observed that temperature polarization was enhanced with the increase of feed temperature and the decrease of feed and permeate velocity. Similarly, Lou et al. [23] investigated coupled temperature and concentration polarization in the membrane distillation treatment of brines with the help of the CFD heat and mass transfer model and observed the effect of coupled temperature and concentration polarization. Salmon et al. [24] experimentally applied the concept of OMD for the crystallization of inorganic salts using two HFMC in series. In another similar study, Salmon et al. [25] described the experimental and theoretical analysis of mass and heat transport of inorganic species during the OMD process.
In the current research work, an RIS model coupling heat and mass transfer resistances was adapted to model HFMC for the recovery of inorganic salts from the brine solution via OMD operation. The model incorporates the effect of concentration polarization in the feed side as well as the osmotic solution side. An aqueous stream of Na2CO3 was taken as an inorganic salt. The model was successfully validated with experimental data under the same operating and module geometrical conditions from the literature [25]. The effects of temperature/concentration polarization of water flux were studied through the simulation of the developed model. Finally, the feasibility of large-scale OMD operation for recovery of Na2CO3 was studied by estimating the required membrane area for industrial conditions which provides an insight into the mass and heat transport phenomena and their implications in scale-up OMD-based HFMCs.
The feed solution is passed through the lumen side of the HFMC module because the shell side is more prone to clogging at high concentrations [11, 18]. Mass transfer coefficient at the feed side (\(k_f )\), as described by Eq. (4), was calculated from Sherwood correlation considering the single fiber of the HFMC module [12, 25]:
Mass transfer of water vapors through the pores of the membrane was mainly due to Knudsen diffusion (Knudsen number (Kn) for this study was 2.5 which confirms Knudsen regime flow). Thus, the mass transfer coefficient was estimated by applying Eq. (12) [30]:
Heat transfer through the pores of the membrane takes place due to conduction in the form of sensible as well as latent heat when water vapors diffuse through the pores of the hydrophobic membrane [19]:
The simulation results for water transport flux across the porous membrane as a function of feed as well as osmotic solution were validated with the experimental results taken by Salmόn et al. [25] under the same operating and module geometrical conditions. In both cases (experimental as well as simulation), the transmembrane water transport flux was studied as a function of species concentration in the feed and osmotic solution streams. It can be seen from Fig. 4a and b, that in both cases (feed, as well as osmotic solution concentration, runs), simulation results are in good agreement with experimental data tested with the maximum deviation of 7%. The reason for this deviation could be attributed to the assumptions taken during model development, such as the single-fiber approach or the activity correlations used for the physical properties of the salt solutions. Experimental errors could also contribute to explain this deviation. Nevertheless, the simulated water transport fluxes were also in the range as studied by Luis et al. [42] for the similar inorganic salts recovery using brine solution in OMD processes. Later, the model was applied to study the effect of main parameters like feed and osmotic solution flow rates, feed temperature, the effect of the hydrodynamic boundary layer as well as temperature and concentration polarization on transmembrane water transport flux.
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