(437e) A CFD-Based Dynamic Model of Absorption/Adsorption Process to Simultaneously Purify Landfill Gas and Treat Leachate

Greenhouse gases (GHGs) have been increasing as the human population has increased. As the human population continues to grow, GHGs will become an even more serious problem. The culprit that most people associate with GHGs is the petroleum and cement industries. However, an often-overlooked producer of GHGs is the waste sector. Landfills contribute both methane and carbon dioxide to the atmosphere in roughly equal proportions (collective gas is landfill gas (LFG)). Over a 100 year span, methane has a global warming potential of 25 (meaning it traps 25 times more heat than carbon dioxide) (Omar and Rohani 2015). To lessen the greenhouse effect of methane, landfills typically flare the LFG. However, there is a lot of usable energy that is wasted with this method. A new paradigm in landfill treatment has emerged: bioreactor landfills. Bioreactors accelerate the biodegradation process, producing methane and carbon dioxide at a faster rate. With the accelerated production of LFG there is potential for using it as a fuel.

Many methods exist to separate CO₂ and CH₄ (e.g. absorption, adsorption, membranes and cryogenic treatment), each with its own advantages and disadvantages. In our work, we use absorption to separate CO₂ and CH₄. We use water and landfill leachate, as absorbents, to dissolve the CO₂ while keeping the CH₄in the gas phase. For the absorption packing, zeolite particles are used. The benefit of zeolite is that while leachate is being used as the absorbent, the zeolite can remove heavy metals as an adsorbent. The separation, therefore, will proceed in both an absorption and an adsorption column. In the paper written by (Omar and Rohani 2017), experiments were performed to compare the usage of leachate versus water as an absorbent and zeolite versus glass beads as absorption packing. They found that leachate was comparable to water as an absorbent and zeolite was only slightly less effective (between 5 and 10% different) compared to spherical glass beads as the column packing. They also developed a stationary (steady-state) computational fluid dynamics model of the absorption process. In this work, the model is extended to the dynamic case while including the adsorption process.

Results and discussion

The model consists of 3 parts: (1) the fluid flow of the 2 phases (liquid and gas), (2) the absorption of the carbon dioxide in leachate (taken as water in the CFD model) and (3) the adsorption of lead on the zeolite packing. The column geometry in the model was 6-cm diameter and 54.6 cm (21.5”) height. Since the geometry is cylindrical, a 2-D axisymmetric model approximation was enforced. The model equations were solved using the finite element method available in the COMSOL Multiphysics® Software package.

The effects of the different parameters on the efficiency of absorption and adsorption expressed in terms of the dynamic time (time to reach steady-state) of the absorption, the steady-state CH₄ purity, as well as the concentration of lead in the liquid effluent stream, and the breakthrough time, were examined. The objective of the study was to separate CH₄from the gas stream, and remove heavy metals (Pb considered in this work) from the landfill leachate. Parameters examined were: column dimensions (height and diameter), the type of absorbent, column pressure and liquid/gas flow rates.

As the height of the column increased, the time to reach steady-state increased. The steady-state CH₄ purity also increased. At larger column heights, the residence time for mass transfer (CO₂ dissolution) is longer, therefore, more CO₂ dissolves, increasing the purity of CH₄ and the time required to reach steady-state. As for the adsorption, increased height of the column decreased effluent liquid concentration of Pb and increased the breakthrough time. A longer residence time allowed more adsorption. The breakthrough time increased since the volume of the column was larger and the volume of zeolite packing was also larger. With more zeolite, there was more surface area for adsorption, which corresponds to more adsorbate removed and a longer breakthrough time. When the diameter of the column increased, the purity of CH₄ increased. This is due to a decrease in the liquid velocity since the volumetric flow is constant. The time to reach steady-state also increased. The liquid effluent concentration of Pb decreased since the velocity of the liquid decreased. This resulted in an increase in residence time. The breakthrough time increased as the diameter of the column increased. An increase in column diameter corresponds to a greater volume, and therefore an increase in the breakthrough time.

As the mass transfer constant increased (i.e. for a better solvent), the time to reach steady-state increased. However, the steady-state CH₄ purity in the effluent gas stream also increased. Increasing the pressure also increased the purity of CH₄produced. Also, like increasing the mass transfer constant, the time to reach steady-state also increased. The effect of pressure on liquid effluent concentration of Pb and breakthrough time was negligible. Pressure does not affect liquid adsorption. For an increase in both mass transfer constant and pressure, the breakthrough time and the operation time were not affected.

Liquid and gas flow rates are possibly the two most important parameters controlling both the absorption and the adsorption processes. In the case of a landfill, the gas flow rate is constant since it is controlled by the slow biodegradation process. Therefore, in most cases, liquid is the parameter used to control the purity of the effluent gas. When liquid flow rate is increased, the steady-state purity of CH₄ increased. However, when the liquid flow rate increased, the liquid effluent concentration increased and the breakthrough time decreased. However, the operation time was decreased due to a shorter breakthrough time. The flow rate of liquid must then be manipulated to optimize the production of CH₄ while ensuring that the zeolite is not saturated too quickly. Manipulating the gas flow rate also has advantages and disadvantages. Increasing the gas flow rate decreases the effluent CH₄ purity and the time to reach steady-state. Therefore, to increase the purity, gas flow rate must be decreased. However, decreasing the gas flow rate decreases the overall CH₄ production rate. Therefore, both liquid and gas flow rates must be optimized to produce the maximum amount of high purity CH₄.

Conclusions

An extension to the model developed by Omar and Rohani (2017) was presented to account for the transient behavior of a combined absorption/adsorption column. The column consists of zeolite particles used for column packing to increase mass transfer area. The gas being purified is LFG (50% CH₄/50% CO₂). The absorbent used is landfill leachate.

The model examines the effect of different parameters on the steady-state CH₄ purity produced from the LFG, the time to reach steady-state operation and the breakthrough time of Pb on the zeolite packing. Increasing the height of the column, increased both the purity and the breakthrough time of the column. Therefore, it is advantageous to increase column height since it increases purity of CH₄ and the time that absorption can occur due to an increase in breakthrough time. An increase in the column diameter increased the purity of the methane and increased the breakthrough time, meaning more gas can be produced before the operation is halted. However, the time to reach steady-state increased for both an increase in height and diameter of the column. Flooding must be avoided when calculating column diameter. By changing the solvent, the mass transfer coefficient can be increased to increase the CH₄ purity in the effluent gas stream. However, the goal of using a combined absorption/adsorption column is to remove heavy metals while purifying LFG. By increasing pressure, the purity of CH₄ increased. The mass transfer coefficient and pressure did not affect the breakthrough time. Therefore, higher purity of CH₄ can be achieved without sacrificing operation time. However, in both situations, the time to reach steady-state also increased. When liquid flow rate increased, the purity of the CH₄ increased. However, the breakthrough time decreased and thus the operation time decreased, meaning less CH₄ would be produced before operation is stopped. Therefore, there is a trade-off when increasing liquid flow rate. When the gas flow rate is decreased, the purity of CH₄ increased. However, less gas is purified. Therefore, there is also a trade-off when changing the gas flow rate. This model provides a method to optimize conditions for the CH₄production as well as treating leachate.

References

Omar H, Rohani S (2015) Treatment of landfill waste, leachate and landfill gas: A review. Front Chem Sci Eng 9:15–32. doi: 10.1007/s11705-015-1501-y

Omar HM, Rohani S (2017) Removal of CO2 from landfill gas with landfill leachate using absorption process. Int J Greenh Gas Control 58:159–168. doi: 10.1016/j.ijggc.2017.01.011

Tontiwachwuthikul P, Meisen A, Lim CJ (1992) CO2 absorption by NaOH, monoethanolamine and 2-amino-2-methyl-1-propanol solutions in a packed column. Chem Eng Sci 47:381–390. doi: 10.1016/0009-2509(92)80028-B