Article pubs.acs.org/IECR
Desorption of Mercaptans and Water from a Silica−Alumina Gel Volkmar Chowanietz,*,† Christoph Pasel,† Michael Luckas,† Tobias Eckardt,‡ and Dieter Bathen† †
University of Duisburg-Essen, Lotharstraße 1, D-47057 Duisburg, Germany BASF Catalysts Germany GmbH, D-31582 Nienburg, Germany
‡
ABSTRACT: An industrially widely used technology for the removal of condensable components like water and mercaptans from natural gas is cyclic temperature swing adsorption. Optimized design of these coupled adsorption−desorption processes requires detailed knowledge of desorption properties of the used adsorbents. Therefore, the desorption behavior of water, methyl mercaptan, and ethyl mercaptan was investigated on a commercial silica−alumina gel. In dynamic experiments, a preloaded fixed bed was regenerated with hot purge gas (300 °C), and time-dependent concentration and temperature profiles were measured and discussed by means of equilibrium theory. Characteristic plateau temperatures were found between 43 and 63 °C, rising with the adsorption affinity. Experiments with competitive adsorption of water and mercaptans show that process dynamics are mainly controlled by water. The results are evaluated with regard to technical relevance.
1. INTRODUCTION Natural gas is a complex mixture of methane and other hydrocarbons with a large number of impurities like water and mercaptans which must be removed before technical utilization. A prevalent process in drying and purification of natural gas is adsorption. Depending on product specifications, process temperatures, and gas compositions, different polar adsorbents like zeolites, activated aluminas, silica gels,1,2 and silica−alumina gels3 are used. Silica−alumina gels are often employed in cyclic temperature swing adsorption (TSA) processes to remove condensable components like water, mercaptans, and higher hydrocarbons. The hydrocarbons may be recovered as a product. For the thermal regeneration of a loaded adsorber, hot raw gas is utilized. During this step, component-specific temperature plateaus are measured which can be used to monitor the progress of regeneration. During recent decades thermal regeneration of adsorbents was the subject of many studies. Basmadjian et al. examined experimentally nonisothermal desorption of CO2 by hot nitrogen on a 5A zeolite and carried out a thorough parameter analysis by a simplified process model, the equilibrium theory.4,5 They were able to analyze the desorption process qualitatively and in parts even quantitatively. The focus of their work is the dependence of concentration and temperature profiles on several process parameters. Because of the neglect of transport resistances in equilibrium theory, the theoretical profiles were much steeper than those found experimentally. Kumar and Dissinger6 modeled Basmadjian’s experimental data by mass and energy balances using a linear driving force (LDF) model with axial dispersion. They achieved a marked improvement in the quantitative description of the desorption process. They focused on the sensitivity of the profiles with © 2016 American Chemical Society
respect to axial heat and mass dispersion and the optimization of energy demand by proper choice of regeneration temperature. Because equilibrium theory provides analytical solutions for even complex systems, it is a popular tool for the mathematical modeling of chromatographic processes.7,8 A lot more research was done to develop desorption models and optimize TSA processes. However, the main focus lies on the modeling of sorption processes, and no qualitative discussion of experimental data referring to equilibrium theory occurs.9−15 In the case of mercaptan removal, profound modeling and simulation studies have been done on the application of PTSA16,17 and PVSA17 processes in commercial mercaptan removal units using zeolites as the main adsorbent. No data have yet been published on thermal regeneration of silica−alumina gels loaded with water and mercaptans. To deepen the understanding of thermal regeneration in adsorptive natural gas treatment, this work studies desorption of water, methyl mercaptan, and ethyl mercaptan from a commercial silica−alumina gel at 300 °C. The measured time-dependent concentration and temperature profiles of the different adsorptives are compared and qualitatively discussed in terms of equilibrium theory. To analyze the technical relevance of adsorptive interactions, experiments with adsorptive mixtures were performed. 1.1. Equilibrium Theory. The equilibrium theory is a simplified model to describe adsorptive processes in fixed bed reactors. Local adsorption equilibrium is postulated at each Received: Revised: Accepted: Published: 614
October 26, 2016 December 19, 2016 December 21, 2016 December 21, 2016 DOI: 10.1021/acs.iecr.6b04150 Ind. Eng. Chem. Res. 2017, 56, 614−621
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concentration. The plateau values are characteristic for the sorption system and must be experimentally determined. A quantitative prediction is possible only by simultaneous solution of the heat- and mass-transfer equations using realistic transport coefficients and considering heat loss effects. The incoming hot sweep gas desorbs molecules in the front part of the solid bed (temperature front) and transports them upstream to sections of lower temperature. There the molecules readsorb (concentration front), and a concentration plateau is built within the mass-transfer zone. The gas stream is cooled by the endothermal desorption step while the enthalpy released at readsorption raises the temperature of the gas. The value of the temperature plateau depends on the energy balance of the system. It rises with adsorption enthalpy and sweep gas temperature.18 In real sorption processes, mass- and heat-transfer resistances as well as heat loss and heterogeneous initial loadings result in deviations from ideal theory. The vertical fronts of temperature and concentration assume an s-shape caused by transport resistances. Heat losses mainly affect the characteristic level of the temperature plateau. Figure 2 shows an experimental timedependent profile of temperature at a fixed position in the adsorber bed. Similarly to the local profiles, the three zones can also be detected in the time profile. At the start of the desorption process, initial conditions prevail until the position in the bed is reached by the migrating concentration front where temperature and concentration rise. In Figure 2, the s-shaped course of temperature as a function of time can be seen. The inflection point can be easily determined by derivation and used to mark the arrival of the concentration front and the transfer zone, respectively. Then the bed position is in the transfer zone and holds the characteristic plateau temperature. After the arrival of the temperature front, marked by the inflection point behind the temperature plateau, the solid bed heats up and reaches the final desorption temperature. The transition to the final desorption equilibrium is strongly expanded by heat-transfer
position in the bed, i.e., mass- and heat-transfer resistances are neglected. Additionally, an adiabatic process and a uniform initial loading of the adsorbent are assumed. Figure 1 represents typical local profiles at desorption by hot sweep gas according to this theory.
Figure 1. Local profiles at desorption of a uniformly loaded fixed bed by hot sweep gas according to equilibrium theory, according to Basmadjian et al.4
Three zones can be distinguished in the fixed bed. The rear zone at the outlet is still in equilibrium with the state that prevailed after the previous adsorption step. In the front zone at the inlet, desorption is complete and the system is in equilibrium with the incoming hot sweep gas. Between these two equilibrium zones, two fronts are found, the temperature and the concentration front, bounding the zone of infinitely fast heat and mass transfer. Because of the infinitely fast kinetics, the fronts are vertical and temperature and concentration between the levels are uniform. During desorption, the fronts migrate through the solid bed. The value of the temperature plateau is between the initial and the desorption temperature while the concentration plateau is higher than the initial
Figure 2. Experimental time profile of temperature at a fixed position in the adsorber bed for desorption by hot sweep gas. 615
DOI: 10.1021/acs.iecr.6b04150 Ind. Eng. Chem. Res. 2017, 56, 614−621
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to particle diameter ranges from 10.9 to 15.2. This is above a technically acceptable limit of about 1019,20 where wall flow effects do not significantly contribute to flow profiles. Alumina content and specific surface area were specified by the manufacturer to 3 weight % and 750 m2/g, respectively. Table 1 lists the gases used in this work. Water (conductivity 99.999%, water concen-
resistances and the continuously decreasing temperature difference between the sweep gas and the bed.
2. EXPERIMENTAL SECTION 2.1. Apparatus. Figure 3 shows a flow sheet of the experimental apparatus. A gas mixing system with thermal mass
Table 1. Source and Purity of the Adsorptives chemical name
source
mole fraction purity
methyl mercaptan ethyl mercaptan
Sigma-Aldrich Fisher Scientific
≥98 ≥99
tration below 3 mol-ppm) were supplied by the university’s infrastructure. 2.3. Procedure. A desorption experiment consisted of two steps. In the first step, the fixed bed was loaded at adsorptive concentrations of 1000 ± 100 mol-ppm and 25 °C until equilibrium was reached. Equilibrium was assumed as soon as the outlet concentration had equaled inlet concentration and varied only within the statistical error range of the gas analytics for at least 30 min. In equilibrium, an average fixed bed temperature of 25 ± 1 °C without deviations larger than 1 °C was found at any measuring point. In the second step, the fixed bed was thermally regenerated by a hot gas stream. Therefore, a nitrogen stream (10 slm) was bypassed and heated to 300 °C by the electrical tube heating. In the case of water-containing systems, the stream was wetted to attain 1000 ppm of water to better simulate technical conditions in natural gas treatment where regeneration is performed with hot humid raw gas. When this temperature had been reached, the gas stream was piped through to the adsorber again. The temperatures were measured as a function of time at five positions in the bed. Simultaneously, the adsorptive concentrations were measured at the outlet of the fixed bed. Nitrogen was used as carrier gas at adsorption and as sweep gas at desorption. All experiments were performed at a pressure of 1.3 bar. Steuten et al.21 demonstrated that adsorption capacities of several sulfur compounds from methane and nitrogen were the same for the silica gel used in this work. Consequently, because nitrogen is cheaper and, particularly at high process temperatures, less hazardous than methane, in this work nitrogen was used as carrier gas. The adsorption and desorption of nitrogen is neglected so that experiments with one adsorptive and nitrogen as a carrier gas are designated as “single-component experiments”. The adsorbent was thermally prepared at 300 °C and ambient air for at least 12 h according to the manufacturer’s guideline. Then 140 ± 2 g of adsorbent were filled into the adsorber resulting in a fixed bed height of 18 cm. For determination of the adsorbed and desorbed amounts, the loadings q (mole-ppm adsorptive/kg adsorbent) were derived from the measured breakthrough curves using a mass balance according to eq 1:
Figure 3. Flow sheet of the experimental adsorption unit. 1, bubbler; 2, static mixer; 3, adsorber column; 4, air cooler; MFC, mass flow controller; EPC, electronic pressure controller.
flow controllers provides the raw gas. In the case of liquid adsorptives, water and ethyl mercaptan, part of the carrier gas stream is bubbled through the liquid. The gas stream is led to the vertical fixed bed adsorber through an electrically heated tube where it can be heated to 300 °C. The adsorber has a height of 18 cm and an inner diameter of 3.8 cm. Inside the adsorber, five thermocouples of type K with an accuracy of ±1 K are installed at a height of 3, 6, 9, 12, and 15 cm to monitor process temperatures. Additionally, temperature and pressure are measured at the inlet and outlet of the adsorber. Downstream of the adsorber the gas stream is cooled to ambient temperature by an air cooler and led to the gas analysis. Alternatively, the adsorber may be bypassed and the gas stream is directly conducted to analysis. To analyze the gas-phase concentrations, a micro gas chromatograph (Varian CP-4900 with PoraPLOT Q and CPSil 5 CB columns and a thermal conductivity detector) is used. In experiments with pure water, the water concentration is measured spectroscopically with a tunable diode laser absorption spectrometer (TDLAS; MultiPass II, Bernt Messtechnik GmbH). 2.2. Materials. The adsorbent Sorbead H was supplied by BASF Catalysts GmbH Germany. Sorbead H is a mesoporous silica−alumina gel in spherical particles with a diameter between 2.5 and 3.5 mm. This diameter distribution represents a narrow fraction of the material used in technical process which ranges from 2.5 to 5 mm. The resulting ratio of column
q=
ieq yin − yout, i n in ̇ ·Δti ·∑ mads i 1 − yout, i
(1)
where the index i refers to the measuring point; ṅin is the total inlet mole flow, mads [kg] the mass of adsorbent, yin [moleppm] the constant inlet concentration for the step, yout,i [mole616
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Figure 4. Adsorption isotherms from nitrogen on sorbead H at 25 °C.
Figure 5. Concentration profile at the end of the solid bed and temperature profiles at several heights for water desorption with nitrogen (10 slm) at 300 °C. Adsorbent loaded by a gas of 970 mol-ppm water at 25 °C.
ppm] the measured outlet concentration, and Δti the time span between two measurements. A comparison of the adsorbed and desorbed number of moles proves whether desorption was complete or some adsorbent load remains. 2.4. Experimental Error. The calculation of adsorbent loading according to eq 1 is afflicted with experimental errors contributed by the weighing of the adsorbent, the mass flow controllers, and the concentration measurement. The chromatographic concentration measurement has the largest contribution with a relative error of sσ = 0.02−1.2% depending on the concentration range. Taking into account Gaussian error propagation and a systematic error of 3% due to calibration gas uncertainties, a total relative uncertainty of 7% was found for the adsorbent loading. The spectroscopic concentration measurement of water using a TDLAS reduces the relative uncertainty to 5%.
3. RESULTS AND DISCUSSION 3.1. Adsorption Step. Figure 4 shows the adsorption isotherms on Sorbead H.22 Symbols represent measured data, and the lines are the Freundlich isotherms calculated from the measurements. It is obvious that water has the highest affinity to the surface. Among the mercaptans, the heavier ethyl mercaptan exhibits a higher loading than the methyl mercaptan. Adsorption isotherms at higher temperatures up to 300 °C were also measured and show the same order; therefore, it is likely to assume that the order of affinity does not change with rising temperature. 3.2. Desorption of Single-Component Systems. Figure 5 displays the concentration (black line) at the outlet of the adsorber and the temperature at several positions in the fixed bed (colored lines) for desorption of water. The scatter in concentration data at high water concentrations results from approaching the upper boundary of spectroscopic measurement. The data exhibit the characteristic pattern of a 3-zoneprofile as expected from equilibrium theory. 617
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Figure 6. Concentration profile at the end of the solid bed and temperature profiles at several heights for ethyl mercaptan desorption with nitrogen (10 slm) at 300 °C. Adsorbent loaded by a gas of 1009 mol-ppm ethyl mercaptan at 25 °C.
Figure 7. Concentration profile at the end of the solid bed and temperature profiles at several heights for ethyl mercaptan desorption with nitrogen (10 slm) at 300 °C. Adsorbent loaded by a gas of 1100 mol-ppm methyl mercaptan at 25 °C.
adsorbent particles. When temperature starts to rise, the section of the fixed bed is in the mass-transfer zone where desorption begins. The plateau temperature of water on Sorbead H is about 63 °C. With increasing bed length, the plateau is getting more extended because the concentration front migrates more quickly than the temperature front. The first inflection point of the concentration curve represents the concentration front, while the second inflection point coincides with the arrival of the temperature front at the end of the bed. At this point desorption is nearly complete, and the adsorber bed is further heated by the hot gas stream until thermal equilibrium is reached. The marked axial temperature gradient in the solid bed after desorption is caused by heat losses of the apparatus. The boundaries of the concentration curve are not as distinct as those of the temperature curve. The concentration peak of water is nearly symmetric with only a slight tailing. When the
Accordingly, the hot sweep gas desorbs water molecules in the front zone of the solid bed which readsorb in the upstream mass-transfer zone of the adsorber. The increased loading results in a concentration plateau in this zone. The boundary zone to the nonregenerated zone is the concentration front which migrates through the adsorber with constant velocity. The adsorption enthalpy of the readsorbing water molecules leads to a temperature rise in the mass-transfer zone. After approximately 260 s, the concentration front has completely passed the adsorber so that temperature and pressure rise at the end of the fixed bed. After a first steep rise, temperature shows an inflection point, and its further rise is reduced until a constant plateau is reached. As described in Equilibrium Theory, the inflection point reflects a deviation from equilibrium theory as the transition between the zones does not occur instantaneously. This is due to mass- and heat-transfer resistances inside the 618
DOI: 10.1021/acs.iecr.6b04150 Ind. Eng. Chem. Res. 2017, 56, 614−621
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Figure 8. Concentration profile at the end of the solid bed and temperature profiles at several heights for desorption of water and ethyl mercaptan with nitrogen (10 slm) at 300 °C. Adsorbent loaded by a gas of 1002 mol-ppm ethyl mercaptan and 950 mol-ppm water at 25 °C.
Figure 9. Concentration profile at the end of the solid bed and temperature profiles at several heights for desorption of water and methyl mercaptan with nitrogen (10 slm) at 300 °C. Adsorbent loaded by a gas of 1020 mol-ppm methyl mercaptan and 1010 mol-ppm water at 25 °C.
plateau value is 51 °C, which is lower than the value at water desorption. Simultaneous to the temperature rise at the end of the solid bed, also the concentration rises. The signal forms a symmetric peak with a slight tailing. After 2000 s, ethyl mercaptan desorption is complete. Figure 7 represents the course of temperature and concentration at desorption of methyl mercaptan. Owing to the still weaker affinity of methyl mercaptan to the adsorbent, the characteristic shape of the temperature profiles is less marked compared to ethyl mercaptan. Up to a height of 6 cm no significant inflection points are observed in the temperature profile. A discernible plateau appears only from a height of 9 cm. The plateau temperature is about 43 °C.
temperature front reaches the end of the solid bed after about 1500 s, evaluation of the breakthrough curve shows that desorption is completed by 90%. After 3000 s, water has been fully removed. Figure 6 shows the temperature and concentration profiles during desorption of ethyl mercaptan. The curves have a pattern similar to that in the case of water desorption. After a steep initial rise, temperature reaches a plateau which is less pronounced than at water desorption. This is due to the initial load of ethyl mercaptan being significantly lower than the load of water (see Figure 4). For that reason, ethyl mercaptan desorption consumes less energy, though the adsorption enthalpy of ethyl mercaptan is slightly higher than the adsorption enthalpy of water,22 and the temperature front migrates more quickly through the adsorber. The characteristic 619
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Figure 10. Migration velocities of temperature (diamond) and concentration (circle) fronts.
experiment, the initial load of methyl mercaptan is reduced by 50% because of competition with water. At the same time, the mass-transfer zones of both components overlap, resulting in a strong displacement of methyl mercaptan during desorption. The temperature profiles conform to the profiles observed at desorption of pure water with a plateau temperature of 63 °C. The dominance of water at desorption in the two-component systems may be illustrated by the migration velocities of the concentration and temperature fronts derived from the time profiles. The inflection points of the curves represent the temporal positions of the fronts which are plotted versus the local positions of temperature measurement in the bed. The resulting data points yield a curve for each of the fronts. The migration velocities are obtained from the slopes (see Figure 10). In all experiments, the curves are straight lines, and the fronts migrate through the adsorber with constant velocity. The concentration fronts have nearly the same velocity in all systems because it is determined by material and experimental parameters like gas velocity and the heat capacities of the fluid and the solid bed. These parameters were nearly identical in all experiments. The migration velocities of the temperature fronts are mainly determined by the energy demand for desorption. For these fronts, a conclusive tendency is apparent. In the case of the single-component systems, the migration velocity decreases with increasing surface affinity of the adsorptive. This corresponds to a higher load and with it a higher energy demand for desorption, provided the adsorption enthalpies are not very different. Given the same energy input from the desorption gas, the front migrates more slowly through the solid bed. In the two-component systems, the migration velocities are nearly identical and much slower than in the case of the pure sulfur components. Now water dominates desorption and defines the velocity of the temperature front.
The shape of the concentration curve is similar to the concentration signal of the ethyl mercaptan experiment. After 2000 s, methyl mercaptan is completely desorbed. 3.3. Desorption of Two-Component Systems. Because water features the highest affinity to the adsorbent, it is assumed to have a big influence on the desorption behavior of the other adsorptive in a two-component experiment. Figure 8 shows the concentration and temperature profiles of the simultaneous desorption of water and ethyl mercaptan. A comparison of the temperature curves and the concentration peak of ethyl mercaptan with the data from the singlecomponent experiment (see Figure 7) reveals two essential differences. There is no more temperature plateau at 51 °C, and the shape of the concentration peak has changed. The temperature plateau at 63 °C measured now corresponds to the value found for pure water desorption. There may be two reasons for this finding. The initial load of ethyl mercaptan is reduced by 30% compared to the singlecomponent experiment because now ethyl mercaptan competes for the adsorption with the more strongly bound water. Thus, the total energy demand for ethyl mercaptan desorption decreases and the influence on temperature in the adsorber is reduced. Additionally the concentration fronts of ethyl mercaptan and water reach the end of the solid bed at the same time, which indicates that the mass-transfer zones of both components overlap. Again the concentration signal of water is a symmetric peak. However, the signal of ethyl mercaptan is more like a steep front with an extended tailing. We assume that this effect is attributable to the displacement of ethyl mercaptan by water. Water has a high concentration in the gas phase and displaces the less affine ethyl mercaptan from the adsorbent surface. As a result, more ethyl mercaptan is desorbed over a short period in the beginning of desorption. Even so, the time elapsing until desorption is complete is not affected. Water and ethyl mercaptan are completely desorbed after 3000 and 2000 s, respectively. Figure 9 presents the concentration and temperature profiles of the simultaneous desorption of water and methyl mercaptan. The curves show the same pattern as in the experiment with water and ethyl mercaptan. Compared to the single-component
4. SUMMARY AND CONCLUSION The concentration profiles at the end of the solid bed and the temperature profiles at several positions of the solid bed were 620
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II. Experimental Verification of Equilibrium Theory. Ind. Eng. Chem. Process Des. Dev. 1975, 14, 340−347. (6) Kumar, R.; Dissinger, G. R. Nonequilibrium, nonisothermal desorption of single adsorbate by purge. Ind. Eng. Chem. Process Des. Dev. 1986, 25, 456−464. (7) Vu, T. D.; Seidel-Morgenstern, A.; Grüner, S.; Kienle, A. Analysis of Ester Hydrolysis Reactions in a Chromatographic Reactor Using Equilibrium Theory and a Rate Model. Ind. Eng. Chem. Res. 2005, 44, 9565−9574. (8) Butté, A.; Storti, G.; Mazzotti, M. Shock formation in binary systems with nonlinear characteristic curves. Chem. Eng. Sci. 2008, 63, 4159−4170. (9) Davis, M. M.; LeVan, M. D. Experiments on optimization of thermal swing adsorption. Ind. Eng. Chem. Res. 1989, 28, 778−785. (10) Ko, D.; Kim, M.; Moon, I.; Choi, D.-k. Analysis of purge gas temperature in cyclic TSA process. Chem. Eng. Sci. 2002, 57, 179−195. (11) Ko, D.; Moon, I.; Choi, D.-k. Analysis of the Contact Time in a Cyclic Thermal Swing Adsorption Process. Ind. Eng. Chem. Res. 2002, 41, 1603−1615. (12) Nastaj, J.; Ambrożek, B. Analysis of gas dehydration in TSA system with multi-layered bed of solid adsorbents. Chem. Eng. Process. 2015, 96, 44−53. (13) Schork, J. M.; Fair, J. R. Parametric analysis of thermal regeneration of adsorption beds. Ind. Eng. Chem. Res. 1988, 27, 457− 469. (14) Kim, Y.-M.; Suh, S.-S. A new mass transfer model for cyclic adsorption and desorption. Korean J. Chem. Eng. 1999, 16, 401−405. (15) Gholami, M.; Talaie, M. R. Investigation of Simplifying Assumptions in Mathematical Modeling of Natural Gas Dehydration Using Adsorption Process and Introduction of a New Accurate LDF Model. Ind. Eng. Chem. Res. 2010, 49, 838−846. (16) Qazvini, O. T.; Fatemi, S. Modeling and simulation pressure− temperature swing adsorption process to remove mercaptan from humid natural gas; a commercial case study. Sep. Purif. Technol. 2015, 139, 88−103. (17) Tohidi, Z.; Fatemi, S.; Qazvini, O. T. Mercaptan removal from natural gas by the efficient cyclic adsorption process; a simulation study. J. Nat. Gas Sci. Eng. 2015, 26, 758−769. (18) Weiß, S. Verfahrenstechnische Berechnungsmethoden, 2., völlig neu bearb. Aufl.; Dt. Verl. für Grundstoffind: Stuttgart, 1996. (19) Bathen, D.; Breitbach, M. Adsorptionstechnik; Springer: Berlin, 2001. (20) Bey, O.; Eigenberger, G. Fluid flow through catalyst filled tubes. Chem. Eng. Sci. 1997, 52, 1365−1376. (21) Steuten, B.; Pasel, C.; Luckas, M.; Bathen, D. Trace Level Adsorption of Toxic Sulfur Compounds, Carbon Dioxide, and Water from Methane. J. Chem. Eng. Data 2013, 58, 2465−2473. (22) Chowanietz, V.; Pasel, C.; Luckas, M.; Bathen, D. Temperature Dependent Adsorption of Sulfur Components, Water, and Carbon Dioxide on a Silica−Alumina Gel Used in Natural Gas Processing. J. Chem. Eng. Data 2016, 61, 3208−3216.
measured at desorption of water, methyl mercaptan, and ethyl mercaptan with nitrogen from a silica−alumina gel. The profiles of all systems exhibit the characteristic 3-zone pattern predicted by equilibrium theory for desorption of a cold solid bed by a hot sweep gas.4 The characteristic plateau temperature increases with the affinity of the adsorptive to the adsorbent surface. The highest plateau was found for water, 63 °C. Owing to heat losses from the lab-scale adsorber to the ambient and the smaller heat capacity of nitrogen in comparison to natural gas, the plateau temperatures should be lower than in the technical temperature swing adsorption process.5 In the experiments with two adsorptives, desorption of water, being the molecule with the highest affinity to the surface, dominates the process. Water desorbs in the front section of the adsorber and readsorbs in the rear, displacing the more weakly bound second adsorptive. Desorption of the mercaptans was always found to have completed earlier than water desorption. The temperature profiles look the same as in the case of pure water desorption. It is supposed that monitoring the position of the characteristic plateau temperature is useful to control the desorption step of TSA processes with concurrent adsorption and desorption.3 As soon as the plateau region of water arrives at the end of the solid bed it can be assumed that desorption of the other components has already been completed. This control measure would require only cheap temperature measurements. In a subsequent step to this work desorption experiments will be modeled using a linear driving force approach on the basis of mass and energy balances. Parameter studies will be performed to identify the main quantities affecting profile shapes and desorption efficiency and to investigate process optimization options.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Volkmar Chowanietz: 0000-0002-3352-8800 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors express their thanks to BASF Catalysts Germany GmbH for funding and support with adsorbent materials.
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REFERENCES
(1) Sircar, S. Basic Research Needs for Design of Adsorptive Gas Separation Processes. Ind. Eng. Chem. Res. 2006, 45, 5435−5448. (2) Kohl, A. L.; Nielsen, R. Gas Purification, 5th ed.; Gulf Pub.: Houston, TX, 1997. (3) Mitariten, M.; Lind, W. The Sorbead Quick-Cycle Process For Simultaneous Removal of Water, Heavy Hydrocarbons and Mecaptans from Natural Gas. Laurance Reid Gas Conditioning Conference, Norman, OK, 2007. (4) Basmadjian, D.; Ha, K. D.; Pan, C.-Y. Nonisothermal Desorption by Gas Purge of Single Solutes in Fixed-Bed Adsorbers. I. Equilibrium Theory. Ind. Eng. Chem. Process Des. Dev. 1975, 14, 328−340. (5) Basmadjian, D.; Ha, K. D.; Proulx, D. P. Nonisothermal Desorption by Gas Purge of Single Solutes from Fixed-Bed Adsorbers. 621
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