membrane was used for the removal of water from nitric acid/water mixtures. The influences of the feed temper- ature, feed concentration, flow rate, and air gap on the permeate flux and the selectivity of water were studied.
The selectivity of water and flux were decreased with increasing feed concentration. With increasing feed flow rate and feed temperature, a decrement in the selectivity of water and an increment in flux were observed. The effect of the air gap thickness was also examined. Recirculation significantly changed the flux and nitric acid concentration in the permeate. This process share the best performance for the removal of water as vapor from nitric acid mixtures through a hydrophobic porous membrane.
Keywords: air gap membrane distillation (AGMD), total flux, nitric acid flux, selectivity of water, azeotropic
Introduction
1)
Membrane distillation (MD) is a separation process for the removal of liquid through a microporous hydro- phobic membrane based upon the vapor/liqiud equili- brium of the liquid mixture, concentration, and pressure.
The driving force of the process is supplied by the vapor pressure difference caused by the existing temperature difference at the liquid/vapor interface. The mass transfer occurs as a result of the vapor pressure gradient from the warm to the cold side [1].
The benefit of MD, in comparison with the other mem- brane and/or conventional distillation processes, is that it operates below the normal boiling point of the feed solution. The lower operating temperatures also allow the use of waste heat or alternative energy sources, such as solar and geothermal energies [2].
Membrane distillation is performed in various modes that differ in a mode of permeate collection, the mass transfer mechanism through the membrane, and the rea- son for driving force formation. The various types of MD include: direct-contact MD (DCMD), air gap MD
†To whom all correspondence should be addressed.
(e-mail: ismoon@sunchon.ac.kr)
(AGMD), sweeping gas MD (SGMD), vaccum MD (VMD), and osmotic MD (OMD) [3].
The main applications of the MD process are the pro- duction of high-purity water and the concentration of several non-volatile solutes in aqueous solutions (e.g., salt, sugar, fruit juices, blood, wastewater treatment). In recent years, MD has been applied to the separation of volatile compounds from aqueous mixtures, continuous removal of alcohol produced by fermentation [4], break- ing of azeotropic mixtures, [5] and concentrating various acids [1].
Tomaszeweska and coworkers [1] tested membrane dis- tillation as a means of concentrating different mineral acid using a PTFE-based capillary membrane. They found that the volume permeate flux decreased with an increase of the acid concentration in the feed. Gostoli and Sarti [6]
concentrated dilute aqueous solutions using a PTFE mem- brane and air gap membrane distillation. Udroit and cow- orkers [4] studied the effect of AGMD in the breaking of azeotropic mixtures of water and hydrochloric and pro- pionic acids. VMD was used to remove halogenated VOCs at very low concentration from water [7].
Sweep membrane distillation was used for ethanol re- moval from an 8 wt% ethanol/water solution using a tub- ular PTFE module. The authors found that the overall
(a) (b)
Figure 1. (a) Vapor liquid equilibrium of a nitric acid/water system. (b) Phase diagram of the nitric acid/water system.
mass transfer coefficient depended on the sweep gas flow rate [8]. Ames and coworkers [9] and Sportsman and coworkers [10] studied the dehydration of nitric acid us- ing pervaporation by means of a Nafion 90209 bilayer membrane. Thiruvenkatachari and coworkers [11] stud- ied the separation of nitric acid using air gap membrane distillation.
Figure 1(a) shows the vapor liquid equilibrium curve for nitric acid/water at atmospheric pressure; an azeo- tropic point occurs at a liquid concentration at 68 % [12].
The VLE strongly depends on the temperature, as shown in Figure 1(b). Thus, the azeotropic point also depends mainly on the temperature. The temperature difference is a deriving force of MD, which is supplied by the vapor difference due to the presence of the vapor-liquid in- terface. The separation of a liquid in MD not only de- pends on the VLE but also on thermodynamic and ki- netic effects [13].
Below the azeotropic point, the vapor phase contains more water. Thus, concentrating the nitric acid solution is easy. At an azeotropic point, the vapor composition is exactly the same as that of the liquid phase. Above the azeotropic point, the vapor contains more nitric acid;
during the distillation the permeate contains more nitric acid.
In this work, we studied the removal of water from ni- tric acid/water mixtures by using an air gap membrane distillation system. The process was performed using a hydrophobic PTFE (Teflon) porous membrane. The ef- fects of various operating conditions (flow rate of hot feed, temperature of feed solution, coolant temperature, feed concentration, and air gap thickness) were analyzed in terms of the selectivity of water and permeate flux of the nitric acid/water mixture. Our main aim was to apply this process for the removal of water from the mediated electrochemical oxidation process [14,15].
Figure 2. Schematic representation of membrane distillation.
Experimental
A schematic representation of the membrane distillation system is shown in Figure 2. The membrane distillation system consists of three compartments: the feed compart- ment, where the feed nitric acid solution was usually passed; the cooling compartment, in which water was passed on one side of the condensing plate; and the per- meate compartment, which was placed between the feed and coolant compartments. Over the feed plate, a stain- less-steel grid was used as a membrane support to avoid membrane bending and wrinkling. The effective area of the membrane was 13.68 cm2. The permeate vapor dif- fused through the membrane and condensed due to con- tact with the cooling plate. The permeate water was col- lected through two 5 mm circular channels on opposite sides of the permeate cell. The permeated liquid was col- lected in a graduated cylinder and the volume of the per-
ter flowed concurrently with the feed solution. Each ex- perimental result was obtained after the system reached the equilibrium time of ca. 4 h between the membrane and liquid mixture. The experiment was also conducted in the recycle mode of specific operating conditions.
Materials and Methods
A flat sheet porous membrane made of polytetrafluor- ethylene (PTFE; Teflon) supplied by Millipore FGLP was used for this experiment. A membrane having an average pore diameter of 0.22 µm, membrane diameter of 142 mm, thickness of 175 µm, porosity of 70 %, and tortuosity factor of 2 was tested for its permeation flux and separation of water.
The nitric acid/water mixtures were prepared from 60 % nitric acid (analytical-reagent grade) obtained from Dong- won Chemical Co. and ultrapure water (RO and UP System, P.NIX Power III, Human Corp., Korea).
The separation of water performance was usually dis- cussed in terms of the flux and selectivity. The flux was calculated by measuring the volume of the liquid col- lected in the permeate side during a fixed time:
where Q is the volume of permeate collected (L), A is the effectiveness area of membrane (m2), and
t is the operating time (h).
The selectivity can be defined as follows:
where yw and xw represent the mole fractions of water
Figure 3. Effect of feed concentration on the flux and se- lectivity of water (Th: 80 oC; q: 50 mL/min; Tc: 15 oC).
in the permeate (P) and feed (F), respectively.
Analysis
The samples were analyzed by titration against known concentrations of sodium hydroxide.
Results and Discussion
Effect of Feed Concentration
The feed concentration of nitric acid was varied and its effects on the permeate flux and the selectivity of water were examined (Figure 3). The permeate flux gradually decreased with increasing feed concentration of nitric acid. The decreasing trend of flux was observed up to the azeotropic point; a gradual increment of flux was ob- served beyond the azeotropic point. The increment of flux due to the higher concentration test was limited by the risk of membrane wetting. Above the azeotropic point, the vapor was rich in nitric acid, but in the case of dilute nitric acid solution the vapor was rich in water.
The selectivity of water increased with decreasing feed concentration. Above 25 wt% HNO3, the selectivity of water decreased with increasing feed concentration be- cause of the decreasing activity of water in the feed.
Therefore, at these concentrations, the effect of the de- creasing water activity in the feed was greater than the effect of decreasing the absolute pressure of the permeate. Due to wetting, the flux increased, and a drop in the selectivity occurred as the pores filled with liquid and the feed solution passed into the permeates. The se- lectivity of water and flux decreased with increasing concentration. A similar pattern of water separation was observed in the dehydration of nitric acid by pervapora- tion [10].
Figure 4. Effect of feed temperature on the flux and selectivity of water (CHNO3: 4 M; q: 50 mL/min; Tc: 15 oC).
Effect of Feed Temperature
We studied the effect of flux and selectivity of water by means of changing the hot feed temperature at a constant feed flow rate and concentration of the solution (Figure 4). The permeate flux increased with increasing tem- perature. The solid line represents the best-fit lines to the Arrhenius-type expression used in the literature when on- ly one component was transported through the membrane [16]. The selectivity of water decreased with increasing temperature until 80 oC; increasing the temperature fur- ther led to the selectivity of water decreasing marginally.
This behavior was due to the thermodynamic effect of the temperature difference, combined with the kinetic ef- fect and also effect of feed concentration, vapor pressure difference, and the activity coefficients on the partial pressures driving force.
Effect of Coolant Temperature
The effect of coolant temperature was studied by vary- ing the cold-side temperature between 10 and 25 oC at a constant hot feed temperature, flow rate, and concen- tration of feed solution. The results of permeate flux and selectivity of water are shown in Figure 5. The flux did not change significantly with the coolant temperature.
The selectivity of water decreased with increasing cool- ant temperature. This result can be attributed to the fact that decreasing the temperature difference between the hot and cold sides reduced the vapor pressure gradient, which was the driving force in this process. However, the changes in flux and selectivity were small.
Effect of Air Gap Thickness
The effect of the air gap thickness was studied at a con- stant flow rate (150 mL/min), feed solution concentration
Figure 5. Effect of coolant temperature on the flux and se- lectivity of water (Th: 80 oC, q: 50 mL/min; CHNO3: 4 M).
(4 M), feed temperature of the hot solution (80 oC), and coolant temperature (15 oC). The results are shown in Figure 6. The air gap thickness was varied using gaskets.
The flux was inversely proportional to the air gap thick- ness because of the higher mass transfer resistance for an increasing air gap thickness. Banat and coworkers [8] ob- tained a similar result for an ethanol/water system. The selectivity of water was inversely proportional to the air gap thickness, due to the reduced effect of flux on the temperature and concentration.
Effect of Feed Flow Rate
The effect of the feed flow rate was studied under the conditions of a constant initial concentration of the feed solution (4 M), feed temperature of the hot solution (80
oC), and coolant temperature (15 oC). Changes in the per- meate flux and selectivity of water with respect to the various feed flow rates are shown in Figure 7. The per- meate flux increased rapidly with increasing feed flow rate. The permeate flux increased rapidly and seemed to reach maximum values asymptotically for higher feed flow rates. A similar asymptotic increasing trend of per- meate flux with increasing feed flow rates was reported by Garcia-Payo and coworkers [13] for the separation of aqueous alcohol solutions. The selectivity of water de- creased with increasing the feed flow rate of the solution.
This behavior is due to wetting of the membrane at high- er flow rates.
Recirculation of Feed Solution
Two sets of experiments were performed to study the effect of the recirculation mode on the permeate flux, feed concentration, and permeate concentration. In the first set of experiments, the initial volume of the feed
Figure 6. Effect of air gap thicknesses on the flux and selec- tivity of water (Th: 80 oC; q: 50 mL/min; Tc: 15 oC; CHNO3: 4 M).
solution (2 L) was kept constant while varying the initial concentration of the feed solution (4, 2, and 0.5 M). The inlet temperature of the feed solution and the inlet cool- ant temperature were maintained constant throughout the experiment. The feed flow rate of the nitric acid solution was 150 mL/min. Under these specific conditions, Figure 8(a) shows the changes in the concentration of nitric acid in the permeate and feed solution during the experiment.
Both the permeate and feed solution concentrations in-
Figure 7. Effect of feed flow rate on the flux and selectivity of water (CHNO3: 4 M; Th: 80 oC; Tc: 15 oC).
reased during the experiment. At 2 M, the feed concen- tration change was higher than those at 4 and 0.5 M, and the permeate concentration was lower. The flux de- creased with an increase in the initial feed concentration of the solution (Figure 8(b)). In the case of an initial feed concentration of 0.5 M, the decline in the rate of the per- meate flux was greater than it was at the other con- centrations. This behavior is due to the change in the concentration of the feed solution.
(a) (b)
Figure 8. (a) Nitric acid concentration in the feed and permeate solutions at various initial concentrations, operated in the re- circulation mode (shaded symbols: feed; unshaded symbols: permeate; 4(■), 2(▲), and 0.5 M (●); q: 150 mL/min; Th: 80 oC; Tc: 15 oC; V: 2 L) (b) Permeate flux at various initial concentrations of feed solution; feed recirculation mode (q: 150 mL/min; Th: 80 oC;
Tc: 15 oC; V: 2 L).
Conclusion
By using AGMD, the effect of several operating param- eters on the separation of water from nitric acid/water mixtures and the permeate flux were studied. The results were based on the equilibrium conditions.
l) The separation of water and permeate flux were main- ly dependent on the concentration of nitric acid in the feed solution. The flux and selectivity of water decreased with increasing feed concentration. When the feed con- centration was above the azeotropic concentration, the flux increased as a result of wetting of the membrane.
2) The permeate flux increased with increasing flow rate; for high feed flow rates it seemed to reach its max- imum values asymptotically. The selectivity of water de- creased appreciably with flow rate.
3) The operating variable of temperature had a more significantly effect on the permeate flux, due to the rela- tionship between temperature and vapor pressure. The selectivity of water decreased with the feed temperature at a constant feed flow rate and concentration.
4) Flux and selectivity were more sensitive to the feed conditions such as the temperature and concentration, than to the coolant liquid conditions.
5) The selectivity of water and permeate flux increased with increasing the reciprocal of the air gap thickness.
6) When the feed solution was operated in a recircula- tion mode, the initial concentration and initial volume of the solution played an important role in determining the flux and nitric acid concentration in the permeate.
Acknowledgments
This study was supported by the Ministry of Commerce, Industry, and Energy (MOCIE), through a project of the Regional Innovation Center (RIC) and Core Environ- mental Technology Development Project for Next Generation (Eco-Technopia-21) of the Korea Institute of Environmental Science and Technology (KIEST), Repu- blic of Korea.
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