Figure 4
Computed optimum temperature and bypass profiles of heated gas for cyclic operation of the ANG storage system. The main simulation parameters are: ( y tht , y tbm ) in = (0.4, 0.9) ppmv; F in / out = 100 L/min; P low , P high = (0.17,7.0) barg.
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The presence of high molecular weight hydrocarbons and sulphur-based odorants in natural gas has a negative impact on the storage of this fuel using adsorption technology. The reason for this is the deterioration of the adsorbent capacity on extended cyclic operation. Although a good adsorbent is key to the success of ANG, its potential will be lim...
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... optimization problem is solved using the Harwell software library package VE03. The T sp k values have been computed for various charge amounts in the range 75–125% of the nominal working value. This set of data allows the software to adjust the temperature profile to experimental conditions within that range by simple interpolation. An example of an optimized temperature profile is plotted in Fig. 4 for the operating conditions given in the caption. The actual experiments confirmed that the optimized temperature profile, coupled with on-line com- pensation by means of the bypass, can smoothly desorb the odorants at the required concentration, with very lit- tle undershoot and overshot, except for the very initial stage of the discharge. This problem can be avoided by slightly pre-heating the guard bed prior to discharge. We have briefly described a workable ANG storage process, incorporating a guard-bed for reversibly filtering out trace contaminants from the natural gas feed to the process. Based on experimental and mod- elling data, the system employs an advanced control scheme to allow controlled and progressive des- orption of the natural gas components from the guard bed in order to achieve the desired and uni- form gas composition at the outlet of the storage ...
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... After several charge and discharge cycles, storage capacity of the material is reduced by heavy hydrocarbons (C 3+ ) that were not discharged during desorption [6][7][8][9]. Guard-beds in storage tanks have been proposed [10,11] as a possible solution. In practical application, guard-bed solution means that we are required to give periodic maintenance in the car fuel tanks. ...
Adsorption processes with activated carbons can be used to remove heavy hydrocarbons from a natural gas flow. Using the Pressure Swing Adsorption (PSA) technology, one can introduce a flexible solution in pre-existing gas-processing units to deal with new marked demands, as for example a C3+ free gas composition to be used as adsorbed natural gas to vehicles fuel tanks. However, designing a PSA process is a laborious task because several cycle configurations and materials are available to perform the separation. To fulfill such task, a multiscale procedure is proposed. Molecular simulation, through Grand Canonical Monte Carlo (GCMC) method, was used to obtain adsorption isotherms for natural gas components in three different carbons: WV-1050, NORIT R1 and MAXSORB. Bed geometry was set based on the minimum fluidization velocity and on the working capacity of the adsorbents. Working capacities were calculated using Langmuir Isotherm applied to Ideal Adsorbed Solution Theory (IAST) to represent the mixture. Each PSA column was simulated in Aspen Adsorption® and operates according to a four steps cycle (Skarstrom cycle): pressurization, adsorption at 40 bar, blowdown, and purge at 1 bar. The operating conditions of the process (such as flowrates, bed geometry and step times) were optimized, seeking the maximization of the process performance parameters: purity, recovery and productivity. A preliminary design of the PSA unit indicates the carbon WV1050 as the best adsorbent to produce C3+ free gas fuel, ideal for storage by adsorption.
Porous adsorbents, including activated carbons, zeolites, silicas, and newer materials such as metal–organic frameworks, have been investigated extensively for gas storage and separation applications. A key consideration is the performance of a material in terms of both its pure gas and multicomponent adsorption behavior, and so measuring accurate gas adsorption data is essential both to assess the suitability of an adsorbent for a given application and for process design and optimization. This article therefore provides an overview of the information required for gas storage and separation applications, the most common laboratory techniques used to obtain such data, with a focus on multicomponent equilibria measurements, and the main challenges associated with this process. We cover challenges for high pressure pure gas adsorption, multicomponent gas adsorption equilibria, kinetic gas adsorption and diffusion measurements, and the difficulty of accurately defining and calculating adsorption capacities. Recent developments, including porous reference materials for high pressure gas adsorption, and new techniques and apparatus for multicomponent gas adsorption equilibria measurements, are also highlighted.
Adsorption of hydrocarbons by activated carbon (AC) has been the subject of recent development and advancement. One of the applications of this technology is in the capture of planned blowdown gases for the purpose of reducing greenhouse gas emissions. A conceptual design of a large AC-based adsorbed natural gas (ANG) storage system has been configured to capture at least 35–40% of the blowdown gases released from high pressure (60 bar) facilities. This design consists of 46 × 14″ diameter pipe modules mounted on a trailer. A smaller ANG unit consisting of a standard gas cylinder or a pressure vessel filled with AC has also been designed to capture blowdown gases from small-volume systems such as compressor casings. The performance of both storage units has been evaluated numerically, and important design parameters are quantified for a given range of operating conditions. Results of this evaluation and future work are highlighted.
Adsorption of each component of natural gas on adsorbent prepared from petroleum coke was studied. At 25 °C and 3.5 MPa, adsorption capacity of the components of natural gas are as follows: C3H8, H2S(0.980) > CO2(0.691) > C2H6(0.160) > CH4(0.136) > N2(0.096) (g/g). For natural gas, adsorption capacity is 145.2 (mL/mL) and delivery capacity is 105.7 (mL/mL). One equation between adsorption capacity and boiling point of adsorbed gas was firstly generalized. The adsorption capacity of different component like O2, N2, CH4, C2H6, CO2, H2S on adsorbents were predicted using the equation. The results fit well with the experimental data. The equation has significance in predicting the adsorption capacity for any component of natural gas. Charge-discharge tests were conducted 10 times, the result indicates that natural gas has significantly worse reversibility in adsorption and desorption in the adsorbent than that of CH4. The contents of the components after 10 charge-discharge show that the adsorption capacity drop of natural gas is due to the irreversible adsorption of heavy or polar components like C3H8, H2S.
The adsorption equilibrium properties of supercritical methane in the large-pore (lp) structure of the MIL-53(Al) metal organic framework were studied experimentally by gravimetric adsorption and theoretically by grand canonical Monte Carlo (GCMC) simulation. The adsorption experiments span a broad range of pressures (0.01–7 MPa) and temperatures (303–353 K). In our molecular simulation work, MIL-53lp(Al) is assumed to have a perfect, rigid lattice, and both fluid–fluid and solid–fluid interactions are modeled using the TraPPE-UA force field. The adsorption isotherms and isosteric heats of adsorption predicted by GCMC simulation, without any reparametrization of the TraPPE-UA force field parameters, are in good agreement with the experimental measurements. Our molecular simulations predict that the amount of methane adsorbed in the porous framework of MIL-53lp(Al) at 298.15 K and 3.5 MPa is 5.79 mol/kg, yielding a methane storage capacity of 132.6 v/v (volumes of stored gas, measured at standard conditions, per storage volume) for a monolithic block and 107.2 v/v for the theoretical limit of a close-packing of uniform spherical particles. For an isothermal (298.15 K) discharge cycle between 3.5 and 0.136 MPa, the predicted net deliverable capacity is 114.0 (v/v)net for a monolith and 93.1 (v/v)net for a close-packed bed. If, however, the storage system is operated at 253 K, the net storage capacity of a monolithic block of MIL-53(Al) increases to a value that is very close to the DOE target of 150 (v/v)net.
The current status of adsorbed natural gas technology for the vehicle fueling sector is reviewed. It is shown that there are
solutions to the all of the problems associated to adsorption storage, and that it is possible to build a light, compact,
and efficient system for storage, distribution, and dispensing of natural gas. The practical achievement of this objective
is essential for the natural gas vehicle to create a strong and sustained interest of the automotive market.
Key words: Natural gas; adsorption storage; gaseous fuels; natural gas vehicles
Experimental results are presented for the adsorption equilibria of methane, ethane, propane, butane, carbon dioxide, and nitrogen, as well as natural gas odorants tert-butyl mercaptan and tetrahydrothiophene, on an activated carbon with the desirable characteristics for use in a guard bed for adsorbed natural gas storage, but that can also be applied for separation of biogas components, such as carbon dioxide and nitrogen. The adsorption experiments were performed using both open- and closed-loop gravimetry over the pressure and temperature ranges of 0–9 MPa and 273–325 K, respectively. The two odorants were analyzed at the very low concentrations usually found in natural gas (0–25 mg/(N m3)). The experimental data were successfully correlated by the adsorption potential theory and collapsed into a single temperature-independent characteristic curve. This analysis allows for extrapolation of the adsorption data to higher alkanes, for which no experimental data are available, in order to span the global composition of a typical natural gas stream. The adsorption equilibrium data for methane, carbon dioxide and nitrogen were fitted to the Toth and Sips isotherm models and their isosteric heats of adsorption were determined. The preferential adsorption capacity for carbon dioxide indicates that the carbon can be used for methane purification from natural gas, carbon dioxide sequestration from flue gas, or biogas purification.
