Removal of 1-bromopropane from solvent degreasing processes emissions

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1 Removal of 1-bromopropane from solvent degreasing processes emissions Ana Beatriz Freitas 1*, Maria Filipa Gomes Ribeiro 1, Ana Paula Matos 2 1 Instituto Superior Técnico, Departamento de Engenharia Química - Avenida Rovisco Pais 1, Lisboa, Portugal 2 TAP Portugal, S.A., Departamento de Manutenção e Engenharia Edifício 25, Aeroporto de Lisboa, Lisboa, Portugal *ana.b.freitas@tecnico.ulisboa.pt November 215 ABSTRACT Nowadays, people are more aware of the consequences of VOCs emissions to the atmosphere. This led to an increase of studies that aim for the reduction of the amount of VOCs emitted and the fulfillment of the limiting emission values established by law. In the aerospace industry, the use of adsorption processes for this purpose has been gaining more importance over the last few years. However, the type of adsorbent used in this processes depends on the properties of the compound to be removed. In contrast to what happens with compounds like toluene and CFCs, there is a lack of studies focused in the removal of 1-bromopropane (1-BP). In this work, the removal of 1-BP from a gaseous industrial effluent was studied by adsorption in activated carbons with different characteristics. The characterization of these adsorbents was carried out by nitrogen physisorption and thermal programmed desorption (TPD). From the breakthrough curves obtained it was possible to conclude that ACC was the activated carbon that had a better adsorption performance as a result of a higher surface area and porous volume. It was also verified that the adsorbents particle size and 1-BP inlet concentration didn t had any influence in the amount of pollutant adsorbed. Keywords: 1- Bromopropane, Adsorption, Activated carbon, VOC, Breakthrough Curves. 1. INTRODUCTION The cleaning of metal pieces was done, during many years, with vapor degreasing processes that used organic solvents as cleaning agents, for instance 1,1,1 Trichloroethane and Trichloroethylene. Many of them were constituted by compounds that are nowadays designated as volatile organic compounds (VOCs) - any organic compound having at 293,15 K a vapour pressure of,1 kpa or more, or having a corresponding volatility under the particular conditions of use [1]. As a consequence of Montreal Protocol revision in the 9s, the use of those solvents became limited and was necessary to introduce new solvents like 1-bromopropane (1-BP), as possible substitutes [2-4]. In order to reduce VOCs emissions and assure the compliance of the limiting emission values established by law, it is common to introduce systems capable of removing these compounds. Depending on the type of system chosen it is possible or not to recuperate the solvent that is being used in the process [5,6]. Condensation of VOCs, absorption, membrane separation and adsorption are some of the processes that allow the recuperation of solvents. The condensation of VOCs is normally used in streams with high volumetric flows and with concentrations of pollutants above 1% [7-9]. The 1

2 application of the absorption process implies the use of a solvent to remove the pollutant from the gaseous stream. Since the ideal solvent to be used in these situations has to be stable, noncorrosive, non-volatile and with a high solubility to the compound to be removed, it can be very expensive [1]. The membrane separation process can be used as an alternative to absorption. In this process it will be also necessary the use of an extra compound, like silicone oils, so that the pollutant can be easily transferred from a gaseous stream to a liquid one [11]. The adsorption process is the most common to remove VOCs from gaseous streams. It is more efficient and economically viable than the other processes mentioned above. The main costs associated with the implementation of adsorption systems are related with the type of adsorbent chosen that will also influence the efficiency of the process [5,12]. The adsorbents used in the adsorption processes should have some of the characteristics mentioned in Figure 1. In this particular case, the adsorbents used specifically for VOCs removal should have high surface areas and great affinity to the adsorbed compound [13]. Figure 1 Main properties of adsorbent materials [14]. Activated carbons are the most common adsorbents since they have high surface areas and non-uniform pores that allow the adsorption of a large number of compounds simultaneously [4,13]. Due to their non-polar surface, the adsorption of non-polar compounds like organic solvents will be preferred to polar ones like water [4]. However, the presence of high quantities of humidity facilitates the occurrence of capillary condensation inside pores. This situation might have a negative impact on the process efficiency since the pollutants molecules will have to compete with water molecules to be adsorbed [5]. The activated carbon can be used in powder or granular form, fibers or cloths. If used adsorbents in powder form, the adsorption process will be quicker but the pressure loss will be higher if used in a fixed bed system. The granular activated carbons usually have higher internal surface areas and smaller pores. They are also easier to regenerate and for that reason it s commonly used when is necessary to recuperate the solvent adsorbed. The activated carbon in fiber and cloth form can be easily modeled to the adsorption system and have usually high surface areas than the forms presented above. They are also more expensive and for that reason they are only recommended for the removal of VOCs in low concentrations [15-17]. The removal of VOCs by zeolite adsorption systems is a viable alternative to the use of adsorbents since they have a great stability at high temperatures and are not flammable [18,19]. They are normally used for the treatment of effluents with high concentrations of pollutants [2]. For the removal of VOCs, the zeolites must have hydrophobic properties to prevent the adsorption of water, despite this particular type of zeolite bear an amount of humidity as high as 95% [21]. During the last years many were the studies related with the removal of VOCs like toluene, TCE or CFCs from effluents. However the 2

3 removal of 1-BP was never deeply studied. The most recent publication about 1-BP was in the US 28/ Patent in 28 [22]. In this work was studied the adsorption of 1-BP in activated carbons with different shapes and sizes. 2. MATERIALS AND METHODS 2.1 Materials In this experimental work was studied several commercial activated carbon samples presented in granular, fiber and cloth form. The samples used are indicated in Table 1. Granular D Table 1 Samples tested. ρ bulk, kg/m 3 p, mm Norit Norit RX3 Extra ,8 Comelt Carbosorb ,1 Merck Millipore CPL Carbon Link AC ,6 Filtracarb CC6 8x3 45 1,6 Cloth Kynol ACC Fiber Kynol ACF For the adsorption study was used an EnSolv 548 solvent containing around >9wt% of 1-BP and <3wt% of 2-butanol and 1-BP with 99% purity from Sigma-Aldrich. 2.2 Surface Characterization The characterization of the samples was carried out by nitrogen adsorption at 77K through an ASAP 21 (Analysis of Surface Area and Porosimetry) and by thermal programed decomposition (TPD). Before nitrogen adsorption, the samples were heated to 9ºC for 1h and heated again until 35ºC, staying at this temperature for 4h. This pre-treatment was essential to remove water that could exist in the samples tested. Through N 2 adsorption we obtained results for specific surface areas (S BET ), maximum quantities adsorbed (q m ), total pore (V pore ) and micropore volumes (V micro ) as well as pore distributions. The specific surface areas and maximum quantities adsorbed were obtained using BET equation in a range of p/p from,5 to,15 and pore distributions were determined through Density Functional Theory (DFT) method. The microporous volume and specific surface area (S micro ) were calculated using t-plot. The adsorption energy (E) was obtained from Dubinin-Radushkevich equation for p/p <,1 [23]. Through TPD we were able to identify the functional groups that exist in the carbons surface using,25g of carbon in a quartz reactor. The sample was heated 1ºC/min until 8ºC with 25mL/min of Argon. The quantities of CO and CO 2 emitted were measured in a gas analyzer ABB EL32. The identification of the different groups was done according to Figueiredo et al. in [24]. 2.3 Adsorption Studies The breakthrough curves were obtained for 1g of sample in a fixed bed reactor using an inlet concentration of 1-BP equal to 125 mg/l, according to Figure 2. N 2 solvent -15ºC Thermostatic Bath N 2 + solvent Figure 2 Scheme of the unit used for the adsorption study. Adsorbent Chromatograph The outlet stream from the column was analyzed every 4 minutes by gas chromatography (GC) in a Chrompack CP91 3

4 q (a.u.) chromatograph using a EC TM -5 column from Alltech (3 m x,32 mm x,25 µm) and a Grant Instruments thermostatic bath, model LTD- 6 at -15ºC. It was also used N 2 as a carrier gas at 49,7mL/min, measured in a Brooks Instruments flowmeter, model The analysis conditions are shown in Table 2. Table 2 Analysis Conditions. Detector 25 Injector 25 Oven 4 Column Pressure, kpa 1 The adsorption of 1-BP was also studied in,1g of Carbosorb64 in powder form and in 1g of Norit RX3 with 1-BP at different initial concentrations. The amount of 1-BP adsorbed by each adsorbent, q exp, was determined by the following equation, in which Q m is the molar flow, m is the mass of adsorbent used, C o is the initial concentration of 1-BP and C is the concentration of 1-BP at the outlet at time t. t E and t BP are the saturation and breakthrough point, respectively. can be described by equation 2 in which k TH is Thomas kinetic constant, q max is the maximum pollutant quantity adsorbed, Q is the total flowrate, V is the effluent volume and m the mass of adsorbent used. ln ( C C -1) = (k Th q máx m ) - ( k Th C V ) (2) Q Q Bohart-Adams model assumes that the adsorption rate is proportional to both the residual capacity of the activated carbon and the concentration of the adsorptive species [26] and it is described by equation 3. In this equation k BA is the Bohart-Adams kinetic constant, q BA is the maximum pollutant quantity adsorbed in mg/l, L is the bed depth and v is the linear flow velocity. ln ( C C ) =k BA C t- ( k BA q BA L ) (3) ν 3. RESULTS AND DISCUSSION 3.1 Surface Characterization The isotherms and results obtained from N 2 adsorption are presented in Figure 3 and Table 3. q exp = Q t=t E m m (1- C ) dt C t= (1) The breakthrough curves obtained were adjusted to different empirical models like Thomas and Bohart-Adams Models. Thomas model assumes that the adsorption process follows a Langmuir isotherm and a second order reversible kinetic reaction [25]. It,,2,4,6,8 1, p/p Figure 3 Adsorption Isotherms for Norit RX3 ( ), Carbosorb64 ( ), Filtracarb ( ), Merck ( ), ACC ( ) and ACF (Δ). Table 3 Results obtained from N 2 adsorption. Sample S BET, m 2 /g S micro, m 2 /g q m, mmol/g V pores, cm 3 /g V micro, cm 3 /g E, kj/mol Norit RX ,7,55,51 18,6 Carbosorb ,4,47,44 17,25 Filtracarb ,8,44,41 18,14 Merck ,6,51,43 17,15 ACC ,6,56,56 22,87 ACF ,5,71,69 2,45 4

5 Concentration, ppm Diferential pore volume (a.u.) Concentration, ppm All the tested activated carbon samples evidenced a type I isotherm, characteristic of microporous materials. The results shown in Table 3 allowed to verify that the superficial areas calculated through BET equation increase with micropore volume. Due to the high micropore volumes, it is expected that the samples with high superficial areas will be capable to adsorb higher quantities of 1-BP. The adsorption energies obtained are close to 2 kj/mol which is the amount of energy that is usually necessary in a physical adsorption process [27]. An increasing in pore width makes it easier for the pollutants molecules to adsorb in the adsorbent pores. For this reason, an increase in pore width will consequently decrease the adsorption energy required. Figure 4 shows that all the adsorbents present a bimodal pore distribution and that most of the pores have widths between 8 Å and 12 Å even though there is still a great amount of pores between 12 Å and 24 Å. 5 Pore width, Å 5 Figure 4 Adsorption Isotherms for Norit RX3 ( ), Carbosorb64 ( ), Filtracarb ( ), Merck ( ), ACC ( ) and ACF (Δ). It is important to have into account that N 2 adsorption is not the most advised method to use for the characterization of carbon samples molecules diffusion into micropores at 77K is very slow which results in the existence of diffusional limitations. This is why it is preferred to use CO 2 adsorption at 273K since CO 2 molecules have an easier access to ultramicropores [28]. From TPD profiles (Figure 5 to Figure 9) it is possible to notice a peak in the CO 2 concentration at low temperatures (<25ºC). According to Figueiredo [24], at these temperatures the only functional group that is decomposed in the form of CO 2 is the carboxylic one. Since this peak can be observed in all the samples tested, we can say that the surface of all the activated carbons contains carboxylic groups, except for Filtracarb. As for CO concentrations, contrary to what happens in the CO 2 case, it is not possible to identify a peak at temperatures below 8ºC. However, above this temperature, only carbonyl and quinone groups can be decomposed into CO Figure 5 CO (-----) and CO 2 ( ) concentrations from TPD tests for RX Figure 6 CO (-----) and CO 2 ( ) concentrations from TPD tests for Carbosorb 64. that might contain ultramicropores. The N 2 5

6 Concentração, ppm Concentration, ppm Concentration, ppm Concentration, ppm Carbosorb64 shows a peak at 6ºC that corresponds to the presence of an anhydride group in the sample surface since it is the only functional group to be decomposed both in CO and CO 2 at this temperature [24] Figure 7 CO (-----) and CO 2 ( ) concentrations from TPD tests for Filtracarb Figure 8 CO (-----) and CO 2 ( ) concentrations from TPD tests for Merck Adsorption Studies The adsorption of 1-BP was studied in several activated carbons and the outlet concentration of 1-BP was analyzed by gas chromatography. This analysis allowed to obtain a profile of 1-BP concentration evolution with time, known as Breakthrough curve (Figure 11). 1,,8 C/C,6,4,2, Time, min Figure 11 Breakthrough curves for 1g of Norit RX3 ( ), Carbosorb64 ( ), Filtracarb ( ), Merck ( ), ACC ( ) and ACF (Δ). The results presented for ACF sample (Figure 11 and Table 4) were corrected for 1g of adsorbent, since the adsorption study was performed in,4g of sample Figure 9 CO (-----) and CO 2 ( ) concentrations from TPD tests for ACC From Figure 11 and Table 4 it s possible to verify that ACF sample, the one with the highest surface area, didn t adsorb as many 1-BP as ACC and Norit RX3 samples did. This shows that not always the materials with high surface areas adsorbed the higher amounts of 1-BP. From all the samples studied, ACC was the one that adsorbed higher amounts of 1-BP and had higher breakthrough times. However, when compared with Norit RX3, the ACC activated carbon bed saturated more quickly Figure 1 CO (-----) and CO 2 ( ) concentrations from TPD tests for ACF. 6

7 Sample t BP, min t E, min Table 4 Results obtained from 1-BP adsorption breakthrough curves. q, mmol/g Thomas Model k TH, ml/mg.min q máx, mmol/g t BP, min Bohart-Adams Model k BA, ml/mg.min q máx, mmol/g Norit RX ,98,85 4,74 64,85 7,11 Carbosorb ,36 1,57 3, ,31 4,95 Filtracarb ,34 3,99 3, ,99 3,76 Merck 6 1 3,18 1,5 3,9 52 1,5 3,75 ACC ,42 2,36 5,28 1 2,36 7,92 ACF ,48 2,4 4, ,4 6,95 From Table 4 it is possible to observe that the results obtained from the models applied are similar to those obtained experimentally. Figure 15 Breakthrough curve ( ), Thomas Model ( ) and Bohart-Adams Model ( ) obtained for Merck. Figure 12 Breakthrough curve ( ), Thomas Model ( ) and Bohart-Adams Model ( ) obtained for Norit RX3. Figure 16 Breakthrough curve ( ), Thomas Model ( ) and Bohart-Adams Model ( ) obtained for ACC. Figure 13 Breakthrough curve ( ), Thomas Model ( ) and Bohart-Adams Model ( ) obtained for Carbosorb64. Figure 17 Breakthrough curve (Δ), Thomas Model ( ) and Bohart-Adams Model ( ) obtained for ACF. Figure 14 Breakthrough curve ( ), Thomas Model ( ) and Bohart-Adams Model ( ) obtained for Filtracarb. The Bohart-Adams model was only used for comparison of the breakthrough times, since the model can only describe the beginning of the breakthrough curve. The adsorbed quantities 7

8 obtained with this model cannot be used to describe the real amounts adsorbed. However, the breakthrough times were similar to the ones determined experimentally. In order to verify the influence of particle size in the adsorption capacities was performed an adsorption study using,1g of Carbosorb64 in powder form. The comparison of the breakthrough curves obtained is shown in Figure 18. The use of Carbosorb64 in powder form resulted in higher breakthrough times (6 minutes) and consequently a higher adsorption capacity of 1-BP (4,58 mmol/g). However, due to small particle sizes, the system pressure increased. 1,,8 C/C,6,4,2, Time, min Figure 18 Breakthrough curves obtained for,1g of Carbosorb64 in granular ( ) and powder form ( ). The difference of results can be due to the preferential paths existent in the fixed bed composed by activated carbon in granular form. This test also enabled to compare the results obtained with 1g and,1g of Carbosorb64 both in granular form. The adsorbed quantities were similar in both cases: 3,36 mmol/g and 3,5 mmol/g respectively This shows the reliability of the results obtained. In order to verify if the solvent impurities have any influence in the results it was done an adsorption test using 1-BP (9% purity) in a Carbosorb64 sample (Figure 19). C/C, 5 Time, min 1 15 Figure 19 Breakthrough curves obtained for pure 1- BP ( ) and for the solvent ( ). The impurities of the solvent don t have any influence on the results since the adsorption capacities obtained in both cases was the same (2,37 mmol/g) and the breakthrough curves were very similar. The influence of 1-BP inlet concentration in the amount of pollutant adsorbed is shown in Figure 2 and Table 5. The test was done using Norit RX3. C/C 1,,8,6,4,2 1,,8,6,4,2, Figure 2 Inlet concentration influence in 1-BP breakthrough curves. When 1-BP concentration increases, the saturation of the adsorbent bed occurs more quickly (decrease of the saturation times) and the breakthrough times will also be smaller. However, the amount of 1-BP adsorbed in each test can be considered constant with the increasing of inlet concentration (Table 5). Table 5 Inlet concentration influence in 1-BP adsorbed quantity. C = 125 mg/l C = 172 mg/l C = 238 mg/l C = 33 mg/l 5 Time, min 1 15 C, mg/l T, ºC q exp, mmol/g , , ,3 33 4,4 8

9 q exp, mmol/g Figure 21 shows the relationship between 1-BP adsorbed quantities and the adsorbents physical properties ,2,4,6,8 V micro, cm 3 /g Figure 21 Relationship between 1-BP adsorbed quantity and micropore volume: Norit RX3 ( ), Carbosorb64 ( ), Filtracarb ( ) and ACC ( ), ACF (Δ), Merck ( ). It is possible to observe that Filtracarb and Carbosorb64 have different micropore volumes but adsorb the same amount of 1-BP. This can be due to the preferential paths that were higher when used Carbosorb64 (pellet form) than Filtracarb (spherical form). ACF sample has a specific surface area and a micropore volume higher than ACC sample. However, ACF sample adsorbs lower quantities of 1-BP than the ACC, as mentioned before. This can be due to the fact that ACC sample contains a higher amount of fibers and a more compact structure than ACF. As for the other samples, the adsorbed quantity increases with the increase of micropore volume and consequently with the increase of specific surface area, as expected. 4. CONCLUSIONS The results obtained in this study show that the carbon samples with higher micropore volume had also higher specific surface areas. It was also confirmed that the samples with higher micropore volume adsorbed higher amounts of 1- BP, except when there are preferential paths. The adsorption capacities will be higher when used particles in powder form than in granular form and will remain constant with the increasing of inlet concentration, even though the adsorbent saturation will occur more quickly. The impurities of the solvent didn t have any effect on the adsorption of 1-BP. From the breakthrough curves it was possible to confirm that ACC sample was the one that adsorbed higher amounts of 1-BP and that the adsorption models used had a great fit to the breakthrough curves obtained experimentally. TPD analysis showed that the presence of carboxylic groups in the carbon surface doesn t have any influence in the amounts of 1- BP adsorbed by each sample. 5. REFERENCES [1] «Council Directive 1999/13/EC», Official Journal of the European Communities, n. 6, pp. L1 L85, European Union, [2] B. G. Travis and C. Boster, «Eliminating 1,1,1- Trichlor Vapor Degreasing In Aerospace Repair Applications», Plat. Surf. Finish., pp. 2 22, [3] R. L. Shubkin, «Making a Case for "Normal - Propyl Bromide», Met. Finish., vol. 13, pp , 25. [4] B. Kanegsberg and E. Kanegsberg, Handbook for Critical Cleaning, 1. a ed. Florida: CRC Press LLC, 21. [5] J. Rodríguez Mirasol, et al., «Influence of Water Vapor on the Adsorption of VOCs on Lignin Based Activated Carbons», Sep. Sci. Technol., vol. 4, pp , 25. [6] M. A. Campesi, et al., «Evaluation of an adsorption system to concentrate VOC in air streams prior to catalytic incineration», Journal of 9

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