Pressure Swing Adsorption.pdf

Accepted Manuscript Pressure swing adsorption for biogas upgrading. A new process configuration for the separation of biomethane and carbon dioxide Rosaria Augelletti, Maria Conti, Maria Cristina Annesini PII:

S0959-6526(16)31611-0

DOI:

10.1016/j.jclepro.2016.10.013

Reference:

JCLP 8204

To appear in:

Journal of Cleaner Production

Received Date: 18 May 2016 Revised Date:

5 September 2016

Accepted Date: 5 October 2016

Please cite this article as: Augelletti R, Conti M, Annesini MC, Pressure swing adsorption for biogas upgrading. A new process configuration for the separation of biomethane and carbon dioxide, Journal of Cleaner Production (2016), doi: 10.1016/j.jclepro.2016.10.013. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Number of words: about 4750

Rosaria Augelletti, Maria Conti, Maria Cristina Annesini*

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Pressure swing adsorption for biogas upgrading. A new process configuration for the separation of biomethane and carbon dioxide Department of Chemical Engineering, Materials & Environment, University of Rome “La Sapienza”, Via Eudossiana 18, I-00184 Roma

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*Corresponding author. E-mail address: [email protected]

Abstract

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Pressure swing adsorption (PSA) is an interesting technology for biogas upgrading, due to compactness of the equipment, low energy requirements, low capital cost, and safety and simplicity of operation. Unfortunately, some shortcomings penalize its diffusion in comparison with other technologies; in particular, conventional PSA has a low methane recovery and cannot compete in this field with other processes such as amine scrubbing; furthermore, it produces an off gas stream with a significant methane content, which requires further treatment to avoid the emission of residual methane into the atmosphere. In this framework, this study focuses on the feasibility of a PSA based separation process able to obtain a biomethane stream suitable to be injected in the natural gas grid (CO2 <3% by volume) with a high methane recovery and an almost pure CO2 stream (CO2 > 99%). The proposed process uses Zeolite 5A as adsorbent material in two PSA units; the biogas is fed to the first unit which produces biomethane; the off gas of the first unit is sent to a second PSA unit which separates carbon dioxide from a residual gas stream, recycled to the first to enhance methane recovery. A dynamic non-isothermal model, based on the linear driving force approximation, is employed to demonstrate the technological feasibility of the separation units and to assess the performance of the whole process. In particular a methane recovery greater than 99% can be obtained with energy consumption of about 1250 kJ per kg of biomethane.

Introduction

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Biogas upgrading, pressure swing adsorption, biomethane, complete separation, methane recovery.

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In the last decades biogas has achieved a significant importance in the field of renewable energy, especially as a heat source; biogas is a gaseous mixture produced by methanogenic bacteria through anaerobic fermentation of organic matter. Biogas from anaerobic digesters contains mainly methane and carbon dioxide, while other contaminants (hydrogen sulphide, ammonia, oxygen, nitrogen, dragged solid particles, siloxane) almost always do not exceed the threshold of 4%; it is also saturated with water at the temperature at which it is produced. Biogas constitutes an important methane resource, especially for those countries that have to import natural gas and other fossil fuel; in 2013 in Italy the production of biogas was around 1815 kteq, while in Europe it reached 14400 kteq (1kteq=1000 tonnes of oil equivalent) (Biogas barometer, 2014).

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In addition to the purely economic benefit deriving from the use of biogas as a renewable fuel, the energy recovery from biogas leads to environmental benefits since methane has a warming potential 21 times higher than that of carbon dioxide (Grande and Rodrigues, 2007). Several paths can be undertaken in order to recover energy from raw biogas (Goossens, 1996): heat production from combustion in actual flares, electric energy production in internal combustion engines, combined heat and power production in cogeneration systems (CHP). A viable alternative that is gaining more and more attention is biogas valorization (upgrading) to obtain a gas, biomethane, comparable to natural gas (CO2 content lower than 3% in volume), which can be used for automotive applications or may be injected in the natural gas grid (Ravina and Genon, 2015). Upgrading requires processes to reduce the carbon dioxide content (initially 35% - 55%) and to remove other contaminants from biogas; with regard the CO2 removal, several and well established processes are commonly carried out: physical absorption with water or organic solvents, amine scrubbing, membrane-based processes, cryogenic processes and pressure swing adsorption (De Hullu et al., 2008; Kapdi et al., 2005; Petterson and Wellinger, 2009). Pressure swing adsorption (PSA) is one of the most known and established industrial processes for gas separation because of the compactness of the equipment, low energy requirements, low capital investment cost, and safety and simplicity of operation; all these advantages make the process suitable also for small installations (Delgrado et al., 2006). PSA technology uses an adsorbent material which is subjected to pressure changes to selectively adsorb and desorb the undesired gas components; the selective adsorption occurs because of different equilibrium capacities (equilibrium adsorbent) or by differences in uptake rates (kinetic adsorbent) (Ruthven, Farooq and Knaebel, 1994). The original PSA scheduling, designed by Skarstrom (Skarstrom, 1960), includes four steps (adsorption, blowdown, purge and pressurization) of equal or different duration, implemented in two or more columns in order to make the gas treatment continuous. What makes PSA a very versatile technology is the possibility to act on several variables, such as type of adsorbent material, type and sequence of cycle steps, steps duration, operative pressures, column size, and use of single or multi-bed process. PSA for biogas upgrading allows to obtain biomethane, a gas that has the same methane purity specifications of natural gas (CO2 <3% by volume), with low energy consumption. Both equilibrium and kinetic adsorbents, already commercially available for full scale applications, can be used for biogas upgrading; activated carbon, Zeolite 13X, Zeolite 5A among the kinetic adsorbent and carbon molecular sieve (CMS) among the kinetic adsorbent have been tested; furthermore, innovative materials like metal organic framework (MOF), silicalite or silicoaluminophosphate sorbents (SAPOs) are also under investigation at laboratory scale (Cavenati, Grande and E., 2004; Himeno, Komatsu and Fujitas, 2005; Jayaraman et al., 2002; Rivera-Ramos, Ruiz-Mercado and Hernandez-Maldonado, 2008). Unfortunately, if compared with other biogas upgrading technologies (such as amine scrubbing), PSA lacks in methane recovery, since a part of methane fed with biogas is lost with the off gas, i.e. with the residual stream mainly consisting of CO2 but with a significant methane content. Indeed, a methane recovery of 85-90% is obtained and an off-gas with a methane content of about 15-20% is produced (Cavenati, Grande and Rodrigues, 2005; Grande and Rodrigues, 2007; Santos, Grande and Rodrigues, 2011). Because of this high methane content, the off gas cannot be released into the atmosphere, but requires further treatment. Indeed, the off gas is often burned in flare or sent to a cogeneration unit with raw biogas, but in many cases the heat produced in the combustion exceeds the requirements of the anaerobic digester or of the heat consumers in the neighborhood of the upgrading plant, since other wastes, such as the digested biomass or agriculture residues, are also available as fuels. In order to overcome the problem of the off gas disposal and at the same time to increase the methane recovery, in this work we have studied the feasibility of employing a double PSA unit to separate the biogas in a stream of biomethane and an almost pure CO2 stream (CO2 content greater than 99% by volume); assuming that water and other contaminants were previously removed, we have considered a biogas mixture composed by CH4 and CO2 , with a methane content of 60%. The biogas is sent to a first PSA unit for the production of biomethane; the off gas here produced is fed to a second PSA unit which separates carbon dioxide from a residual stream, recycled to the first unit to enhance methane recovery. The high purity grade of carbon dioxide stream makes it suitable to be 2

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used directly, taking advantage of its physico-chemical properties or as a carbon source in chemical and biochemical processes; carbon dioxide can indeed be used as cooling fluid in refrigeration units for the transport of products at low temperature or for environmental purpose, such as in the pH control in waste water treatment or even in laboratories as supercritical fluid in extraction operations, etc. Here, a Zeolite 5A has been employed as adsorbent material in both PSA units; a dynamic non isothermal model based on the linear driving force approximation with a single lumped transport parameter to describe the adsorption/desorption kinetics has been developed; the model, implemented to assess the performance of the whole process, has proved that biomethane can be obtained with a methane recovery greater than 99% and low energy consumption.

Process definition

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Figure 1 shows the process diagram of the two PSA units studied in this work. A feed stream of 100 Nm3 /h of biogas (representative of a small-medium scale biogas plant) with a CH4 and CO2 content of 60% and 40%, respectively, is considered. The biogas is assumed available at 1 atmosphere and to have been previously treated to remove water (generally through refrigeration, adsorption on silica or allumina or absorption in triethylenglycol) and other harmful contaminants, such as hydrogen sulphide (i.e. through biological aerobic oxidation in the digester vessel or downstream the digester through adsorption on activated carbon or by means of membrane separation) or siloxane (i.e. through adsorption on silica gel or activated carbon or by means of cryogenic separation) (Ryckebosch, Drouillon and Vervaeren, 2011). The biogas pretreatment has not been studied in this work, but must be carried out in order to prevent the contaminants content from being harmful to the downstream separation units, natural gas grid, motors or end-users. Biomethane is produced during the high pressure adsorption phase of the first PSA unit; the off gas is instead sent to a second PSA unit to separate carbon dioxide from a residual stream which is recycled to the first PSA unit to enhance methane recovery.

diagram1.jpg

Figure 1: Process diagram for the biogas upgrading.

While the biomethane is produced continuously, the off gas is periodically extracted during the desorption phases of the first PSA. Due to the different cycle timing of the two PSA units and in order to guarantee a continuous feed and a homogeneous composition to the second unit, a storage system is included between the two units. For the 3

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same reason another storage is placed before the first unit in order to mix the raw biogas with the recycle stream produced discontinuously. Both PSA units use zeolite 5A as adsorbent material.

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Modeling and Simulation

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In order to simulate the PSA separation units and thus to evaluate the process performance in terms of purity, recovery and energy requirement, a dynamic non-isothermal model based on the linear driving force (LDF) approximation is used (Glueckauf and Coates, 1947); in this model, the adsorption/desorption kinetics is described by a single lumped transport parameter for each component, kLDF,i . Axially dispersed plug flow, perfect gas behavior, no radial concentration and temperature gradients, thermal equilibrium between gas and solid phase and no heat exchange with the external environment have been assumed in the development of the mathematical model. In particular, the assumption of adiabatic behavior becomes reasonable for large, industrial columns, for which heat exchange with the external environment is negligible. The equations of the model are summarized in table 1 together with the correlations used for estimations of mass and heat transfer parameters. In order to solve the system of partial differential equation, boundary and initial conditions are required. Since a PSA process is a sequence of different elementary steps, specific boundary conditions for each of these steps should be set. Table 2 reports the boundary conditions used in the in the various step constituting the PSA cycle, which will be described further below; in the same table the pressure trend adopted in each step of the cycle is reported. As for the initial conditions, the column filled with biogas at minimum pressure and with clean adsorbent solid has been considered at the beginning of the simulation. After a screening between several adsorbent materials, based on the analysis of the breakthrough curves, we have chosen zeolite 5A for both the PSA units (Ahn et al., 2012); zeolite 5A is an equilibrium-based adsorbent material, with selective adsorption towards CO2 . Adsorption equilibrium is described by the multicomponent Langmuir isotherm:

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bi pi qi P = qmax,i 1 + j bj pj

(1)

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where qi and pi are the adsorbed phase concentration and the partial pressure of each component, respectively and qmax,i and bi are the single component temperature-dependent parameters:

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qmax,i (T ) = qmax,i (T0 ) + kq,i (T − T0 )

bi (T ) = bi (T0 )exp(kb,i (

1 1 − )) T T0

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Adsorption isotherm parameters and the linear driving force mass transfer coefficient for CH4 and CO2 adsorption on zeolite 5A are reported in Table 3; in the same table the geometrical and physical-chemical properties of the zeolite beads are reported. Figure 2 shows the adsorption isotherm of the two pure components on the zeolite 3A at 25°C and at 50°C. It is clear that that zeolite 5A has a greater adsorption capacity for carbon dioxide, as shown in Figure 2, but the comparison of KLDF,i values, reported in Table 3, indicates that kinetics favor methane adsorption. It is worth noting that the nonlinearity of the adsorption isotherm of CO2 suggests to operate the solid regeneration under vacuum, thus realizing a vacuum PSA (VPSA). The performance of the PSA separation process has been evaluated in terms of: 4

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Table 1: Model equations used in PSA simulations. Mass balance equation for component i-th in the bed

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∂ q¯i ∂ ci i εb ∂c ∂t + (1 − εb ) ∂t = εb Dz ∂z 2 −

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Total mass balance equation

∂(uci ) ∂z

P ∂ q¯i ( ∂t ) = − ∂(uC) εb ∂C ∂t + (1 − εb ) ∂z nc

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Energy balance equation for solid and gas phases 2

∂(uC) −εb kz ∂∂zT2 + cp,g uC ∂T ∂z + cp,g T ∂z + uCT

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εb ∂P R (cp,g ∂t

(1 − εb )cp,s ρs ∂T ∂t − (1 − εb )

+P

∂cp,g ∂t )+

P (−∆Hi ) ∂∂tq¯i = 0 nc

Linear driving force equation for the adsorption rate

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∂ q¯i ∂t

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Langmuir adsorption isotherm

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qi qmax,i

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bP i pi 1+ bj pj nc

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Axial mass dispersion coefficient Ruthven D. M. (1984) εb Dz Dm

= 20 + 0.5ScRe

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ρg udp µg

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µg ρg Dm

Pr =

cp,g µg kg

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Axial heat dispersion coefficient Wakao and Funazkri (1978) kz kg

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= (7.0 + 0.5P rRe)

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Table 2: Boundary conditions and pressure time profile implemented in the PSA simulation model. z=0 z=L dP =0 Feed dt uC = uF E CF E i ucF E,i = uci − εb Dz ∂c ∂z

dP dt

α∆P0 e−t/τ τ (α+1)

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Blow-down

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Pressurization

dP dt

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∂ci ∂z

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∂T ∂z

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∂ci ∂z

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∂T ∂z

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∆P0 e−t/τ τ (α+1)

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Equalization pressurization

∂T ∂z

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∂T ∂z

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u=0 ∂ci ∂z

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dP dt

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∂ci ∂z

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Equalization depressurization

εb kz ∂T = −ucp,g (TF E − T ) ∂z

∂ci ∂z

u=0 ∂ci ∂z

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∂T ∂z

=0

uC = uP U CP U i ucP U,i = uci − εb Dz ∂c ∂z

εb kz ∂T = −ucp,g (TP U − T ) ∂z u=0

i ucEQ,i = uci − εb Dz ∂c ∂z

εb kz ∂T = −ucp,g (TEQ − T ) ∂z

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∂ci ∂z

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∂T ∂z

=0

= P P R (t) u=0 i ucP R,i = uci − εb Dz ∂c ∂z

εb kz ∂T ∂z

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= −ucp,g (TP R − T )

∂ci ∂z

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∂T ∂z

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Figure 2: Equilibrium isotherm on zeolite 5A at 25°C and 60°C.

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Table 3: Langmuir parameters, LDF coefficients and physical-chemical properties of the zeolite (Ahn et al., 2012) CH4 CO2 −1 qmax (298 K) mol kg 2.28 4.49 kq mol kg −1 K −1 -0.01192 -0.01858 b(298 K) bar−1 0.2 3.12 kb K 1731 207 −4H kcal mol−1 5.4 9.33 kLDF s−1 0.147 0.0135 −4 Average pellet radius m 6 · 10 Pellet density kg m−3 1160 Pellet porosity 0.64 Specific heat capacity cal kg −1 K −1 220

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• purity of the biomethane stream CH4 purity =

NCH4 NCH4 + NCO2

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where NCH4 and NCO2 are the molar flow rates of methane and carbon dioxide, respectively in the biomethane stream; • CH4 recovery CH4 recovery =

MCH4 FCH4

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where MCH4 is the molar flow rate of methane recovered with the biomethane stream (equal to NCH4 ) exiting the first PSA unit or recovered with the recycled stream exiting the second PSA unit; FCH4 is the molar flow rate of methane fed to each PSA unit.

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• purity in CO2 of the CO2 -rich stream CO2 purity =

LCO2 LCH4 + LCO2

where LCO2 and LCH4 are the molar flow rates of carbon dioxide and methane respectively in the CO2 -rich stream exiting the second PSA unit. • energy consumption, calculated by

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where tcycle is the total cycle time and P is the power consumption, evaluated assuming that the devices operate under adiabatic regime with a single stage compression   k RT . Pout k−1 k B ( ) −1 P= k−1 η Pin

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where k = cp /cv (assumed equal to 1.31), η is the mechanical efficiency, which typically assumes the value of . 0.8, R is the universal constant of gases, T is the gas temperature, B is the molar flow rate to be compressed, Pout and Pin are the pressures in the outlet and inlet section of compressor (or vacuum pump) respectively. As it will be shown in more detail later, we have considered four columns for both PSA units, which are interconnected with each other according to a proper timing cycle. In order to simplify the simulation and to reduce the CPU time required, simulations have been carried out considering a single column for each PSA unit and taking into account the recycles between the columns in the subsequent steps of the cycle; modeling a PSA unit with a single column, instead of the complete array of four columns, leads to the same results once the cyclic steady state is reached (for each unit an average of 40 cycles were needed to reach the steady state, corresponding to almost 2 h of simulation time with a i7 intel core processor). After a first simulation of the two units placed in succession, the loop has been closed and the simulation was repeated until convergence, which was fixed when two successive iterations differed in about 0.1%, calculated on the variation of the methane mole fraction in the biomethane stream. In the following, results obtained at the periodic steady state are discussed. 8

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First PSA unit for biomethane production

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The first PSA unit has been designed to produce the biomethane stream, that is a stream composed by methane and carbon dioxide with a methane content of at least 97%. This unit is composed by four interconnected column, each of them is subjected to the same phases sequence, as shown in Figure 3; in the same figure the sequence of the phases in bed 1 is also reported, with the details of the flux direction and the opening and closing conditions of the column sections during each step of the cycle. It is worth noting that this column is fed with the mixture of raw biogas and the gas recycled from the second PSA unit (see Figure 1); once periodic steady state conditions are achieved, a feed flow rate of 113.47 Nm3 /h with a CH4 content of 62.6% is obtained.

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Figure 3: Scheduling of four-columns PSA cycle and sequence of the phases in bed 1. FE: feed, EQ1 and EQ2: equalization, BD: blow-down, PU: purge, PR: pressurization. The phase sequence is composed by a) a feed step (FE) where the biogas is fed to the column at high pressure and the adsorption occurs; a CH4 -rich product is obtained from the column (biomethane); b) an equalization step (EQ2), where the high pressure gas present in the interstitial spaces of the column at the end of the feed phase is used to partially pressurize another column; c) a blow-down step (BD), where the column is depressurized until vacuum conditions in order to desorb carbon dioxide; d) a purge step (PU), where the regeneration is completed by feeding the column at low pressure with part of the product obtained during the feed phase; e) an equalization step (EQ1), where the column is partially pressurized with the high pressure gas extracted from another column; f) 9

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Table 4: Column characteristics, pressure operating conditions and cycle timing of the two PSA units. First PSA Second PSA Column length m 1.5 1 Column diameter m 0.4 0.31 Column density kg m−3 742.4 742.4 Column porosity 0.36 0.36 Pressure feed bar 6 6 Pressure purge bar 0.2 0.2 Feed Time s 190 130 Purge time s 190 130 Pressurization time s 170 110 Blow down time s 170 110 Equalization time s 20 20

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a pressurization phase (PR), where the column is pressurized with the biogas. As can be seen, the cycle scheduling ensures the continuous production of biomethane (during the feed step), while the off gas, withdrawn during BD and PU, is produced in a discontinuous manner. The cycle simulation has been carried out by implementing the model described in Table 1, with the boundary conditions and the pressure profiles reported in Table 2. In particular, a constant pressure value has been set during the feed and the purge steps at the maximum and the minimum pressure value of the cycle respectively, while linear trends of pressure vs. time have been assumed during pressurization and blow-down steps. In the equalization phases, instead, simultaneous exponential pressure variations have been set (pressure increase during EQ1 and pressure decrease during EQ2) with a time constant τ , as reported in Table 2. It is worth noting that the parameter α in the pressure variation equation accounts for the different capacities of the two columns that are connected during pressure equalization; its value is determined by ensuring that the amount of gas released during the equalization depressurization is equal to that fed into the column during the equalization pressurization.

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The column characteristics, pressure conditions, and the cycle timings used in the first PSA unit are summarized in Table 4, while in Figure 4 the simulation results are reported; in more detail, figure 4a) shows the pressure history during the PSA cycle, while figure 4b) and 4c) report the molar flow rate of the two component during each step of the cycle. For the convention adopted, the flows are positive if directed as the z axis: therefore a positive flow indicates that it is entering the column at z=0 and is exiting the column at z/L=1; flows equal to zero during EQ1, PR, EQ2 and BD indicate the closure of the column during these phases. As can be seen, the flow entering the column during the pressure equalization pressurization step (EQ1) corresponds to that exiting another column that is simultaneously in the pressure equalization depressurization step (EQ2); the connection between the two columns is reflected in the equivalence of the areas under the two equalization steps. Moreover it can be noted that the column regeneration occurs mainly at the end of the blow down step and the beginning of the purge step, as indicated by the carbon dioxide flow. Figure 4d) and 4e) report the adsorbed amount of the two components as a function of the axial position along the column; these plots show that methane adsorption occurs in the pressure equalization pressurization phase and in the pressurization phase, at the end of which methane is almost uniformly distributed along the column, except for the inlet section, where on the contrary carbon dioxide adsorption is greater. In the feed phase, methane is desorbed from the solid and replaced by carbon dioxide, as can be noted by the methane desorption front, shifted toward the outlet section. It is also evident that, as mentioned before, the CO2 removal takes place mainly during

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the blow down step, as is clear by the area between the EQ2 and BD curves. The comparison between the values of adsorbed amounts of the two components, together with the replacement of methane by the carbon dioxide, reflect the adsorption selectivity of zeolite 5 A towards carbon dioxide. Figure 4 f) shows the temperature profile during a cycle at three different positions: the figure shows that during the feed phase a significant temperature increase occurs along the column, especially around the position at z/L=0.35 at which corresponds the maximum adsorbed amount of carbon dioxide, with a temperature increase of about 35°C. During the regeneration steps the temperature decreases, with the maximum decrease during the BD phase, to which a maximum in the desorbed amount corresponds. As can be noted, during a cycle the bed experiences an appreciable temperature excursion, during which temperature reaches the maximum value of about 70° C corresponding to z/L=0.35. The significant temperature increase is due to the high heat of adsorption, as shown in Table 3; on the other hand it is worth recalling that the simulations have been carried out in adiabatic conditions and therefore overestimate the temperature changes and consequently underestimate the column performances, which are negatively affected by the heat effects connected with the adsorption process.

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The table in Figure 6 reports the mass balance of the first PSA and the cycle performances in terms of methane purity and recovery with the biomethane stream, together with the power consumption required for the separation. A product (biomethane) with a methane content of 98.9 % is obtained, but the methane recovery is rather low. The power consumption has been evaluated by dividing the energy required in a cycle by the time of the cycle; it is important to point out that here the energy is referred only to the energy required for biogas compression up to the maximum pressure of the cycle (6 atm) and for vacuum pump during the regeneration steps, without taking into account the additional energy required for the further compression of biomethane before its injection into the natural gas distribution grid.

Second PSA unit for off gas separation

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The off gas produced in the first PSA unit is sent to a second PSA unit for carbon dioxide separation; a storage at atmospheric pressure separates the two units in order to guarantee a constant feed to the second unit, which moreover works with a different cycle timing. The cycle scheduling is the same adopted for the first unit (see Figure 3) while the column characteristics,pressure conditions, and cycle timings are summarized in Table 4. Figure 5 shows the simulation results of this second PSA cycle.

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As can be seen from the adsorbed amount profiles, methane is adsorbed appreciably only during the pressure equalization step (EQ1) and especially during the pressurization step, at the end of which it only occupies the second half of the column, towards the outlet section. During the remaining steps of the cycle, the amount of methane adsorbed is practically close to zero; in particular during the feed phase, carbon dioxide replaces almost completely the methane that was adsorbed during the pressurization phases and now recovered with the product stream. The molar flow rates at the outlet section of the column (see figure 5c)) reflect the displacement of methane by carbon dioxide during the feed phase, since for about the first 90 seconds, the gas exiting the column is mainly composed by methane. It is worth noting that the column is now subjected to high temperature variations (see figure 5f), with a maximum of about 80 °C corresponding to z/L= 0.3; in this position the lower temperature value of about -13 °C is reached at the end of the purge step; the higher temperature value of about 97°C is instead reached at the end of the feed phase at z/L=1, where carbon dioxide is more adsorbed. The process performances of the second PSA unit are reported in the table of Figure 6. As can be seen, this second unit completes the separation by producing a CH4 -rich stream recycled to the first unit and an almost pure carbon dioxide stream (off-gas) withdrawn during the discharge phases; the high methane recovery with the 11

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Figure 4: Simulation of the first PSA unit: pressure history (a); molar flow rate of the two components at the top (b) and the bottom (c) of the column; adsorbed amount of CH4 (d) and CO2 (e) along the bed at the end of each step; temperature profile during a cycle at three different positions (f).

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Figure 5: Simulation of the second PSA unit: pressure history (a); molar flow rate of the two components at the top (b) and the bottom (c) of the column; adsorbed amount of CH4 (d) and CO2 (e) along the bed at the end of each step; temperature profile during a cycle at three different positions (f).

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recycle stream is a positive consequence of the displacement of methane by carbon dioxide during the feed phase, as observed from the simulation profiles. The power consumption is now referred to the power required to compress the off gas of the first unit from 1 (pressure of the storage tank) to 6 atmospheres (higher pressure of the second PSA unit) and to power the vacuum pump that operates during the regeneration steps. The further power required to store the carbon dioxide (for example in cylindrical tanks) was not taken into account in the power consumption assessment.

Overall process

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The overall process scheme is reported in Figure 6, together with the mass balances and the overall process performances. As can be seen, the complete separation of biomethane and almost pure carbon dioxide (CO2 > 99%) is obtained with a complete methane recovery (> 99%) in the biomethane. An energy consumption of about 1250 kJ per kg of biomethane, corresponding to 550 kJ per Nm3 of biogas, was evaluated; it includes the electrical energy required for biogas compression as well as blowdown and purge withdrown at low pressure; no recompression is required to recycle the F2 stream to the first PSA unit, because the two PSA columns operate at the same maximal pressure. No energy for biomethane compression for injection in the gas grid is considered (biomethane is then available at 4 atm). It is important to note that the specific energy consumption here evaluated seems to be very favorable if compared to values reported in other works, in which a single PSA unit is used for biogas upgrading, without operating a further separation step for the production of the carbon dioxide stream. Indeed, energy consumption between 700 and 1100 kJ per Nm3 of biogas are reported for a single PSA unit(Persson, Jonsson and Wellinger, 2006; Bauer et al., 2013). More specifically, Grande and Rodrigues (2007) studied the upgrading of a biogas stream with a composition very close to that considered in this work (55% CH4 and 45% CO2 ) in a single PSA unit with a working cycle including two depressurization steps in order to improve the methane recovery and evaluated energy consumption of about 4500 and 2680 kJ for kg of biomethane with Zeolite 13X or CMS-3K as adsorbent material, respectively. Wu et al. (2015) considered the upgrading of a biogas with a higher methane content (67%) in a PSA unit working with the four standard steps (PR,FE, BD, PU) and an equalization step, with a maximal pressure of 8 atm; these authors compared the performance of PSA process with the same adsorbent materials of Grande and Rodrigues (2007) and an innovative MOF and report energy consumption of about 2375, 1200 and 1040 kJ for kg of biomethane for Zeolite 13X, CMS-3K and MOF, respectively. The reason for the low specific energy consumption evaluated in this work with respect to other works is mainly due on the one hand to the better separation capability of the adsorbent material chosen for the separation process and, on the other, to the almost total methane recovery obtained from the whole process.

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F1 F2 F3 F4 F5 F6

Flow rate Composition kg/h Nm3 /h CO2 CH4 133.96 100 40% 60% 12.61 13.47 17.8% 82.2% 146.57 113.47 37.4% 62.6% 43.98 60.37 1.2% 98.8% 89.98 53.1 78.5% 21.5% 77.37 39.63 99.14% 0.86% First PSA Second PSA 83.9 97 10.2 kW 5.1 kW 99.4% 1250 kJ/kgbio

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CH4 recovery Power consumption Total CH4 recovery Energy consumption

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Figure 6: Overall process scheme of biogas upgrading for the separation of biomethane and carbon dioxide.

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The use of double PSA units for biogas upgrading seems to be a very interesting and feasible process to obtain an almost complete separation of biogas components. The configuration studied in this work, indeed, allows to obtain, on one side, biomethane with a CO2 content lower than 3%, that makes it suitable to be injected in the natural gas grid and, in addition, an almost pure carbon dioxide stream (>99%) that may be used for several applications as a carbon source in chemical and biochemical processes. The whole process yields an almost total methane recovery (>99%), which is reflected in the very low specific energy consumption (1250 kJ per kg of biomethane) if compared to other biogas upgrading processes based on a single PSA unit.

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bi equilibrium parameter of Langmuir model for component i, atm −1 ci gas phase concentration of component i, mol m−3 C total gas phase concentration, mol m−3 cp,i molar constant pressure specific heat of component i, cal mol−1 K −1 cp,g molar constant pressure specific heat of the gas mixture, cal mol−1 K −1 cp,s constant pressure specific heat of the adsorbent, cal kg −1 K −1 dp particle diameter, m Dz axial dispersion coefficient, m2 s−1 Dm molecular diffusivity of component i, m2 s−1 KLDF,i LDF coefficient of component i, s−1 kb equilibrium parameter of Langmuir model for component i, K kq equilibrium parameter of Langmuir model for component i, mol kg −1 K −1 kg gas thermal conductivity, W m−1 K kz axial gas phase thermal conductivity, cal m−1 s−1 K −1 nc number of components P pressure, atm pi partial pressure of component i, atm P r Prandtl number q i volume-averaged adsorbed phase concentration of component i, mol m−3 ∗ qi equilibrium adsorbed phase concentration of component i, mol m−3 qmax,i equilibrium parameter of Langmuir model for component i, mol m−3 Re Reynolds number Sc Schmidt number R universal gas constant t time, s tcycle total cycle time, s T temperature, K u superficial gas velocity, m s−1 yi molar fraction of component i in the gas phase z axial position in the bed, m εb bed porosity ∆Hi heat of adsorption of component i, cal mol−1 µg gas viscosity, P a s ρs gas density, kg m−3 ρs pellet density, kg m−3

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