Article pubs.acs.org/IECR
Foaming Prevention in Absorption Columns through Removal of Contaminants from Amine-Based Solutions Using a Solvent Resistant Nanofiltration (SRNF) Membrane Zahir Razzaz, Abdul Wahab Mohammad,* and Ebrahim Mahmoudi Department of Chemical and Process Engineering, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor Malaysia ABSTRACT: An environmentally friendly removal method based on a membrane process for the purification of aqueous amine solution for use in carbon dioxide absorption was analyzed. Two kinds of commercial solvent resistant nanofiltration (SRNF) membranes (Desal-DL and SR100, polymeric) were chosen, and their capabilities in removing three common contaminants from diethanolamine (DEA) solution were examined. These contaminants that caused foaming of DEA solution include sodium chloride, sodium sulfite, and acetic acid. From the two tested solvent resistant nanofiltration (SRNF) membranes, it had been found that Desal-DL membrane provided promising results and performance that can be an alternative to conventional separation, including distillation, for purification of amine solution. Experiments were also conducted to test the foaming tendency and stability of the untreated and treated amine solution. The untreated DEA with sodium sulfite had the highest foaming tendency. However, after NF treatment, there was substantial decrease of the foaming tendency. The results obtained confirmed the potential of using SRNF-membranes to reduce foaming tendency and stability in DEA solution.
1. INTRODUCTION The enhanced levels of anthropogenic CO2 emissions in the environment during the previous decades are one major contributor to global warming.1 For the last several years, aqueous amine solution treatments have the technology intended for the removal of acid gas produced by the gas sweetening industries. Therefore, they can be readily implemented on large-scale in existing industries for the capture of carbon dioxide.2 Aqueous diethanolamine (DEA) solutions (usually 25 wt %) are referral solvents intended for use in many processes.3 The industrial application of this solution has encountered several environmental and technical challenges. Significant solvent losses take place during the absorption and stripping processes. Amines are relatively volatile solvents that can change phase from liquid to gas, and subsequently react to yield hazardous compounds in the atmosphere.4 Furthermore, one key problem in using amine solutions is the accumulation of degradation products caused by irreversible reactions in solvent during the process, as well as the reduction in active DEA content.2 Surface active degradation products could cause an increase in the foaming problem.5 Foaming is widely encountered in gas sweetening industries and typically contributes to critical consequences, including decreased efficiency and area of mass transfer, carryover of solutions with high absorption further downstream in the plant, loss (amine solution) of absorption upon increased operation costs, and flooding before the proper time. Foaming takes place during plant start-up and over operation in the absorption and stripping processes.6,7 Foaming can be caused by numerous chemical contaminants that enter the processing plants with gas from feed, water in makeup, or a cooling water leak during the amine solution degradation reactions. These contaminants were dissolved in hydrocarbon and organic acid from solution© 2015 American Chemical Society
oxidized fragments (e.g., acetic acid and formic acid), amine degradation products, chemical additives, such as antifoaming agents and a corrosion inhibitor, and some inorganic compounds from water.8,9 Systematic studies indicating the foaming behavior of aqueous amine solution have been published. Pauley reported that organic acid and other contaminants increased the foaming tendency and stability of DEA, monoethanolamine (MEA), methyldiethanolamine (MDEA), and formulated methyldiethanol-amine in amine solution with all contaminants.10 McCarthy et al.11 observed that organic acid, antifoam, corrosion inhibitors, and other contaminants caused the foaming tendency of the DEA solution to increase with increased temperature and pressure. Thitakamol et al.7 systematically studied using MDEA, MEA, 2-amino-2-methyl-propanol-1 (AMP), and their combinations to examine the results of process parameters on their foaming tendency. Calculating the foaminess coefficient indicates the gas flow rate and solutions volumes. The gas flow rate and the concentration of contaminants used in this study were based on the recommendations of Thitakamol et al. The current approaches used to overcome the foaming phenomena include mechanical filtration, ion exchange, electrodialysis, distillation (solution reclamation), and addition of antifoaming agent.2 Filtration is normally performed through a bed of activated carbon, which requires a large amount of carbon for each ton of carbon dioxide. It also wastes a large amount of the amine solution.1 The use of ion exchange is suitable, since degradation product accumulation remains low, Received: Revised: Accepted: Published: 12135
July 20, 2015 November 11, 2015 November 16, 2015 November 16, 2015 DOI: 10.1021/acs.iecr.5b02642 Ind. Eng. Chem. Res. 2015, 54, 12135−12142
Article
Industrial & Engineering Chemistry Research Table 1. Solvent Resistant Nanofiltration Membranes and Membrane Characteristics Used in the Experiments Membrane
a
Manufacturer
Pore Size, MWCOa (Da)
Membrane property (contact angle, θ)
Desal-DL
GE Osmonics
150−300
Hydrophilic (42.8° ± 2.24)
TFC-SR100
Koch Membranes System
200
Hydrophilic (48.3° ± 3.49)
Type of polymer Polyamide TF (Thin Film) TF (Thin Film) composite polyamide
Membrane class NF, Dense NF, Dense
Aqueous solutions.
with concentrations staying in the ppm range. Higher flow rates and concentrations have increased the cost and scale of equipment.12 The energy consumption of ion exchange is lower than that of electrodialysis. Numerous electrodialysis membranes have been created with pH values between 2 and 9, which is problematic, since the normal pH of amine is between 9 and 11.13 Ion exchange and electrodialysis cannot eliminate nonionic impurities and require activated carbon to remove contaminants. More waste results from electrodialysis than from distillation. One significant drawback of distillation is that a large amount of waste generated means that it is not sustainable and is expensive.2 The addition of antifoam does not physically remove the contaminants and, therefore, does not permanently treat the foaming problem.10 Treatment technologies for amine solutions are generally not sustainable or economical. Therefore, there is potential for a membrane process to be an alternative to the conventional method. A significant decrease in energy consumption can be achieved by using a properly selected membrane, while simultaneously decreasing the waste volume through recovery of waste, fine particulates, and the final product. Another advantage of the membrane process is that it can be performed at mild temperatures, where conditions should be generally temperature mild, and the tendencies of acid gas and oxidative degradation paths to be scaled down to produce charged particles,14 decreasing greenhouse gas emission and increasing safety in comparison to traditional separation processes, including distillation. The solvent resistant nanofiltration (SRNF) membrane has interesting potential for waste solution reduction, recovery from the solution during the process,15 environmental and economic advantages, a wide pH range, and great separation performance.16 Researchers have been successful in introducing new developments in and generation of a nanofiltration (NF) process, as well as improving the performance and stability of NF membranes, which can be stable in the solvent throughout membrane manufacturing, thus solving certain problems (e.g., disintegration and dissolution of the membranes).17 Many studies have concentrated on the development of modifications about the thin layers of SRNF-membranes in order to develop and increase their morphology and performance. During the polymerization, the most effective approaches to increase the specific characteristics of the very thin layers would be to add suitable various additives in the very thin layer matrix.18 These membranes have numerous potentials for continuous operations during several months within refinery plants.19 SRNFmembranes have been developed and have various Molecular Weight Cut-Offs (MWCOs). For instance, Desal-DL and Desal-DK are used in organic solvent separation. Othman et al.20 reported the application of Desal-Dl, Desal-DK, and six types of SRNF-membranes to separate biodiesels. Darvishmanesh et al.21 reported the effect of organic solvents (methanol, acetic acid, acetone, n-hexane, and toluene) with different
polarities on the separation performance of the STARMEM membrane. The membrane showed a rejection rate of about 72% for the separation of acetic acid. Yang et al.22 studied the stability and retention of organic solvents (Safranine O, Orange II) and aqueous solvents on Desal-DK and Desal-DL membranes, and they showed that retention in aqueous solvent was higher than that in organic solvent. In this study, our goal was to analyze and select commercially available SRNFmembranes for the removal of three common contaminants (organic and ionic) from an amine solution. The separation performance associated with two SRNF-membranes has been evaluated to have potential for the purification of this solution. Furthermore, the acquired results were used to examine and compare the foaming tendency and foaming stability of treated and untreated amine solutions to further study the effect of foaming on capturing carbon dioxide.
2. MATERIALS AND METHODS 2.1. Membrane and Chemicals. Laboratory grade DEA (≥99% purity, Sigma-Aldrich Inc., Malaysia) was used for all experiments. Acetic acid (≥99.7% purity, Sigma-Aldrich Inc., Malaysia), sodium sulfite (≥98% purity, Sigma-Aldrich Inc., Malaysia), and sodium chloride (≥98% purity, Sigma-Aldrich Inc., Malaysia) were added in parts per million (ppm) concentrations to an aqueous DEA solution to simulate degraded DEA solutions. The aqueous amine solution was prepared by DEA in deionized water followed by sparging of 99.99% CO2 into the solutions. Two kinds of polymeric SRNFmembranes (GE Osmonics and Kosh Manufactures) were tested. The related properties of both membranes supplied in dried forms are listed in Table 1. 2.2. Permeation Experimental Setup. Permeability experiments were done in a stirred cell (Sterlitech HP4750, Sterlitech, USA) that containing a cylindrical stainless steel with detachable end plates (Figure 1). A membrane disk with a diameter of 0.049 m was placed at the end of this cell, supported by porous stainless steel. The active area of the
Figure 1. Dead-end filtration setup. 12136
DOI: 10.1021/acs.iecr.5b02642 Ind. Eng. Chem. Res. 2015, 54, 12135−12142
Article
Industrial & Engineering Chemistry Research membrane was 15.2 cm2. The maximum solution feed volume to the cell was 0.3 L, and the maximum operating pressure was 69 bar. The solutions were stirred using a Teflon-coated magnetic bar placed 3 mm above the sheet membranes. These membranes were pretreated/presoaked in the filtration solution overnight prior to the separation experiments. All permeation experiments were performed at ambient temperatures with pressures ranging from 2 to 14 bar. This stirred cell was pressurized by compressed N2 gas. The volume of the feed solutions was 0.2 L, and the volumes of permeates were measured using a graduated cylinder. The stirring speed was maintained at 1000 rpm by a Teflon-coated magnetic bar. The solvent fluxes were determined by measuring the filtration time difference t (in s) and collecting data on the permeate volume V (in mL). The flux of solvent is given as22
J=
Table 2. Concentration of Feed and Permeate Sulfite (mg L−1)
Chloride (mg L−1)
Acetic acid (mg L−1)
56.23 9.01
211 55.24
123 70
18.14
132.5
22
Feed Desal-DL (permeate) SR100 (permeate)
v AΔt
The effective area of the membrane is A (in m2). The SRNFmembrane was used for all of the experiments, and uncontaminated aqueous DEA solutions were used as the base case. The aqueous DEA solution was also added with the following contaminants: sodium chloride (1000 ppm), sodium sulfite (1000 ppm), and acetic acid (100 ppm). The permeate samples were collected using a 200 mL measuring cylinder until the filtration volume reached 100 mL. The permeates of the solutions were used in the foaming experiments. The concentrations of permeate and feed were analyzed by ion chromatography. The performance of the SRNF-membrane was reported as the rejection rate, which was calculated as follows:23
Figure 2. Schematic representation of foaming experimental setup.
diffuser (average pore diameter 80 μm), a drying column, and a gas flow meter. Commercial grade N2 was used instead of air as a distributed N2 to bubble solution and to avoid oxidative degradation that affects the foaming results. 2.5. Foaming Study. 2.5.1. Foaming Experimental Procedure. A 0.001 m3 graduated cylinder containing 0.0001 m3 of the test solution was placed in a water bath and heated to 40 °C prior to each experiment. The diffuser was submerged into the test solution, and the system thermal equilibrium was permitted to be attained for approximately 20 min. The initial volume of the solution prior to each test was recorded. Before entering into the flow meter (fixed flow rate of 2 × 10−3 m/s), the N2 gas was dried to remove wetting by the drying-column. This gas entered the graduated cylinder and traveled through the gas diffuser. The duration of bubbling was measured with a stopwatch. pH was determined before and after each experiment, as was the conductivity due to the alkanolamine degradation that changes the constituents of the solution. The interface between the foam and liquid was difficult to measure for the majority of the test solution cases during the foaming test. The volumes of the liquid and the foam, instead of just the foam volume, in the cylinder were measured every minute. Each foaming experiment was run for 25 min. A stable state after almost 5 min was reached in most foaming tests. Therefore, the data reading was reported as a steady state value. An uncontaminated amine solution was operated at a baseline prior to the foaming test of the contaminated amine solution. Considering that the result for uncontaminated solutions cannot be found to be similar each time, normalized foaminess was reported for comparing before and after SRNF filtration and also for different contaminants. 2.5.2. Data Analysis. The foaminess was calculated by subtracting the initial volume of the liquid from the total volume of the graduated cylinder that yields the volume of the total gas involved in the foam and dividing by superficial gas velocity (in m2·s):23,24
⎛ C ⎞ R = ⎜1 − P ⎟ × 100 (%) CF ⎠ ⎝
where CF is the feed concentration and CP is the permeate concentration. The standard deviation errors were up to 5% in all cases. 2.3. Chemical Analysis and FESEM. The surface morphologies and cross sections of the SRNF-membrane samples were analyzed before and after filtration by field emission scanning electron microscopy (FESEM) using a model Zeiss supra 55vp (Germany) at a voltage of 3 kV. The cross sections of the nanofiltration membrane were immersed in liquid nitrogen to break the dry membranes. The DEA solutions concentrations before and after filtration in both membranes were measured by UV absorption with the wavelength at 210 nm using UV−vis spectroscopy (with a UV1650 PC, Shimadzu instrument). The DEA concentration before filtration were 3.4775, and those after filtration were 3.4735 for the SR100 membrane and 3.4750 for the Desal-DL membrane. Anion concentrations were measured by ion chromatography (IC 850 Fessional, Metrohm) with a 1:50 dilution ratio and 0.2 mM NaOH, including formaldehyde (for stabilization of the sulfite). The acetic acid concentration was determined by acid−base titration using the standard solution (NaOH with phenolphthalein) as a reagent. Table 2 shows the concentrations of the permeates and the feeds. 2.4. Foaming Experimental Setup. Foaming tests were performed using a method modified from the standard ASTM D892 protocol by Thitakamol. 7 Figure 2 shows the experimental setup consisting of a 0.001 m3 graduated cylinder, a temperature controller immersed in a water bath, a gas
F= 12137
Vg Vt − Vi = G G DOI: 10.1021/acs.iecr.5b02642 Ind. Eng. Chem. Res. 2015, 54, 12135−12142
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Industrial & Engineering Chemistry Research where Vt is the total steady-state foam and liquid volume (in m3), Vi is the initial volume of liquid (in m3), Vg is the total steady-state foam volume (in m3), and G is the superficial gas velocity (in m/s). Dividing F by the foaminess of the uncontaminated amine solution (Fi) was calculated as the normalized foaminess (F*). It should be noted that the normalized foaminess of the uncontaminated amine solution for each step of the foaming experiment was different in this study, which implies that, after filtration to calculate F*, the filtered DEA solution with each contaminant was divided by the unfiltered neat DEA solution.
F* =
F Fi
Figure 4. Flux of DEA solution versus applied pressure for the DesalDL and SR100 membranes at ambient temperature.
The foam stability was estimated by break time (t, in sec). The time period in which foam breaks completely following the stop flow of gas was defined as the break time for this material. The standard deviation errors were up to 5% in all cases.
the DEA solution, in which the flux of the solution was enhanced by increasing the operating pressure. Based on Figure 4, the Desal-DL membrane presented the higher flux of the permeate in the uncontaminated aqueous DEA solution when compared with the SR100 membrane. The permeate fluxes of DEA were significantly higher for Desal-DL than for the SR100 membrane, thus indicating that SR100 was the less hydrophilic membrane and the hydrophilic membrane behavior can be described using the theory of hydrogen-bonding.22 Also, the variation of osmotic pressure and concentration polarization has a significant effect on the permeate flux. The feasible formation of a concentration polarization layer enhanced the osmotic pressure and the transport of retained compounds. Consequently, permeate flux is reduced substantially.17,26,27 The relaxation of polymer chains is caused by plasticization within organic solutions, which leads to swelling upon subsequent pore size reduction and decreased flux.27 Dijkstra et al.28 observed the nonlinear behavior of the flux of alcohol permeations through the laboratory-made dense SRNFmembranes (PDMS) with high pressure, which affected the solutions’ permeability due to the swelling of the structure of the membrane. Machado et al.29,30 reported the nonlinear behavior of the flux of alcohol permeations using the commercial polydimethylsiloxane membranes MPF60 and MPF50. Darvishmanesh et al.31 studied solvent fluxes using commercial SRNF-membranes (DuraMem150 and StarMem122) and reported a nonlinear behavior of the alcohol flux. Othman et al.20 showed the application of SRNFmembranes to produce biodiesel using eight commercial SRNF-membranes, including four hydrophilic and four hydrophobic SRNF-membranes, and they explained the nonlinear behavior of flux permeations. The flux permeate-pressure graph showed an increasing behavior that becomes more prominent with increasing hydrophilic membranes for a polar solvent. 3.1.2. Permeability Experiment on the Amine Solution with a Contaminant. The fluxes of aqueous DEA solution with sodium chloride, sodium sulfite, and acetic acid through the SR100 membrane as a function of time are shown in Figure 5a. The initial flux of the DEA solution with each contaminant obeys the following order: DEA with acetic acid (5.23 L/m2 h), DEA with sodium chloride (5.21 L/m2 h), DEA with sodium sulfite (4.02 L/m2 h). The DEA with sodium sulfite had the highest flux reduction when compared with the other solutions. In this case, the compaction factor was defined as the amount of time it takes for solution flux to become stable within a membrane during the hours of DEA solution permeation for different solutions. Dividing the final membrane permeability
3. RESULTS AND DISCUSSION 3.1. Membrane Experiments. 3.1.1. Permeability of the Uncontaminated Amine Solution. Preliminary permeation experiments for purification and separation were performed with an uncontaminated aqueous amine solution (uncontaminated 25 wt % DEA) and contaminated aqueous DEA solution (1000 ppm sodium chloride, 1000 ppm sodium sulfite, and 100 ppm acetic acid) for both membranes. Table 1 lists the hydrophilic membranes examined in this experiment. The effect of pressure on the flux of the uncontaminated aqueous amine solution was examined between 2 and 14 bar at ambient temperature (∼28 °C) in Desal-DL and SR100 membranes. Figure 3 shows the result obtained from the permeation
Figure 3. Flux of water versus different pressures for both membranes at ambient temperature. The standard deviation errors were up to 5% in all cases.
experiment for the water permeation flux, in which the flux of the water increased linearly upon increasing the applied pressure. This behavior can explain with the Hagen−Poiseuille equation in liquid and solvent cases.25,17 The result obtained indicates a behavior which was fitted by this equation. The highest water permeate flux was obtained for the Desal-DL membrane. The uncontaminated aqueous amine solution permeability experiment showed nonlinear behavior (as asymptotic action) on both membranes upon increased applied pressure at ambient temperature. Figure 4 shows the permeation flux of 12138
DOI: 10.1021/acs.iecr.5b02642 Ind. Eng. Chem. Res. 2015, 54, 12135−12142
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Figure 5. Flux of three contaminants in DEA solution versus time through (a) SR100 membrane or (b) Desal-DL membrane.
Figure 6. FESEM micrographs of the surface morphologies and cross section of the two membranes: (a) before Desal-DL filtration, (a′,c) after Desal-DL filtration, (b) before SR100 filtration, (b′,d) after SR100 filtration.
(Lf inal) by the initial permeability (Linitial) yielded the compaction factor (α); values of the compaction factor are 0
up to 1, and the membrane has compaction factor closest to 1. It is the highest level of resistance for the operating pressure.32 12139
DOI: 10.1021/acs.iecr.5b02642 Ind. Eng. Chem. Res. 2015, 54, 12135−12142
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α=
Lfinal
membrane has acceptable properties and performance to separate ionic and organic contaminants in amine solutions. 3.2. Foaming. 3.2.1. Sodium Chloride. Makeup water or leakage of cooling water are the main cause whereby sodium chloride enters into the amine solution system.2 Therefore, the effects of sodium chloride or chloride ion on foaming tendency need to be examined.6 As is presented in Figure 8, the effect of
Linitial
The respective membrane compaction factors for DEA with sodium sulfite, sodium chloride, and acetic acid were 0.57, 0.81, and 0.87. Figure 5b shows the DEA solution with different contaminants varied through the Desal-DL membrane. The initial flux decline has been reported previously.22 One explanation for the decreasing flux could include concentration polarization.33 The membrane compaction factor was 0.83, 0.91, and 0.99 for DEA with acetic acid, sodium chloride, and sodium sulphite, respectively. The DEA solution with sodium chloride had the highest initial flux, and the DEA with acetic acid acquired the highest flux falling. The molecular weights of contaminants are 58.44 g mol−1 for sodium chloride, 60.05 g mol−1 for acetic acid, and 126.043 g mol−1 for sodium sulfite. As is shown in Figure 5b, the increasing molecular weights of the contaminants in DEA were directly proportional to the increasing flux of each related contaminant solution. In real membrane process applications, the shown fluxes of the permeate and rejections are those following compaction.19 Figure 6 shows FESEM micrographs of the surface morphologies and cross sections of the two commercial membranes before and after filtration of DEA solution with sodium sulfite. Desal-DL and SR100 membranes have rigorous dense constructions that decrease upon membrane compaction.28 3.1.3. Separation Performance of the Membranes. The rejection of the amine solution for purification of each related contaminant is shown in Figure 7. The rejection of sodium
Figure 8. Effect of membrane separation of sodium chloride from DEA solution on break time and foaminess at 40 °C.
this contaminant within the aqueous DEA solution both before and after Desal-DL and SR100 membranes on foaming tendency was investigated with a 25 wt % DEA solution and 1000 ppm sodium chloride at 40 °C. Foaminess was significantly reduced after the Desal-DL membrane, and normalized foaminess (F*) became 1 after the Desal-DL membrane. The normalized foaminess became
