Scholarly article on topic 'Techno-economical evaluation of membrane based biogas upgrading system; a comparison between polymeric membrane and carbon membrane technology'

Techno-economical evaluation of membrane based biogas upgrading system; a comparison between polymeric membrane and carbon membrane technology Academic research paper on "Chemical engineering"

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Abstract of research paper on Chemical engineering, author of scientific article — Shamim Haider, Arne Lindbråthen, May-Britt Hägg

Abstract A shift to renewable energy sources will reduce emissions of greenhouse gases and secure future energy supplies. In this context, utilization of biogas will play a prominent role. Focus of this work is upgrading of biogas to fuel quality by membrane separation using a carbon hollow fibre (CHF) membrane and compare with a commercially available polymeric membrane (polyimide) through economical assessment. CHF membrane modules were prepared for pilot plant testing and performance measured using CO2, O2, N2. The CHF membrane was modified through oxidation, chemical vapour deposition (CVD) and reduction process thus tailoring pores for separation and increased performance. The post oxidized and reduced carbon hollow fibres (PORCHFs) significantly exceeded CHF performance showing higher CO2 permeance (0.021 m3(STP)/m2 h bar) and CO2/CH4 selectivity of 246 (5 bar feed vs 50 mbar permeate pressure). The highest performance recorded through experiments (CHF and PORCHF) was used as simulation basis. A membrane simulation model was used and interfaced to 8.6 V Aspen HYSYS. A 300 Nm3/h mixture of CO2/CH4 containing 30–50% CO2 at feed pressures 6, 8 and 10 bar, was simulated and process designed to recover 99.5% CH4 with 97.5% purity. Net present value (NPV) was calculated for base case and optimal pressure (50 bar for CHF and PORCHF). The results indicated that recycle ratio (recycle/feed) ranged from 0.2 to 10, specific energy from 0.15 to 0.8 ( kW/ Nm 3 feed ) and specific membrane area from 45 to 4700 ( m 2 / Nm 3 feed ). The high recycle ratio can create problems during start-up, as it would take long to adjust volumetric flow ratio towards 10. The best membrane separation system employs a three-stage system with polyimide at 10 bar, and a two-stage membrane system with PORCHF membranes at 50 bar with recycle. Considering biomethane price of 0.78 $/Nm3 and a lifetime of 15 years, the techno-economic analysis showed that payback time for the best cascade is 1.6 months.

Academic research paper on topic "Techno-economical evaluation of membrane based biogas upgrading system; a comparison between polymeric membrane and carbon membrane technology"

Accepted Manuscript

Techno-economical evaluation of membrane based biogas upgrading system; a comparison between polymeric membrane and carbon membrane technology

Shamim Haider, Arne Lindbrâthen, May-Britt Hàgg

PII: S2468-0257(16)30059-0

DOI: 10.1016/j.gee.2016.10.003

Reference: GEE 28

To appear in: Green Energy and Environment

Received Date: 1 September 2016

Revised Date: 21 October 2016

Accepted Date: 22 October 2016

Please cite this article as: S. Haider, A. Lindbrâthen, M.-B. Hàgg, Techno-economical evaluation of membrane based biogas upgrading system; a comparison between polymeric membrane and carbon membrane technology, Green Energy & Environment (2016), doi: 10.1016/j.gee.2016.10.003.

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.

Techno-economical evaluation of membrane based biogas upgrading system; a comparison between polymeric membrane and carbon

membrane technology

Shamim Haider, Arne Lindbrathen, May-Britt Hägg*

Norwegian University of Science and Technology, NTNU, Department of Chemical Engineering, 7491 Trondheim, Norway

*Corresponding author: May-Britt Hägg, Email: hagg@ntnu.no , Tel: +47 93080834

Key words:

Carbon membrane; Biogas upgrading; Techno-economical analysis; NPV calculations Highlights:

Biogas upgrading using CO2 selective membranes Multistage membrane system for CO2/CH4 separation Optimization of process conditions based on Hysys simulations

Techno-economical evaluation of multistage membrane system for 97.5% CH4 purity and 99.5% CH4 recovery

Comparison between polymeric membrane and carbon membrane technologies ABSTRACT

A shift to renewable energy sources will reduce emissions of greenhouse gases and secure future energy supplies. In this context, utilization of biogas will play a prominent role. Focus of this work is upgrading of biogas to fuel quality by membrane separation using a carbon hollow fiber (CHF) membrane and compare with a commercially available polymeric membrane (polyimide) through economical assessment. CHF membrane modules were prepared for pilot plant testing and performance measured using CO2, O2, N2. The CHF membrane was modified through oxidation, chemical vapor deposition (CVD) and reduction process thus tailoring pores for separation and increased performance. The post oxidized and reduced carbon hollow fibers (PORCHF) significantly exceeded CHF performance showing higher CO2 permeance (0.021 m3(STP)/m2.h.bar) and CO2/CH4 selectivity of 246 (5bar feed vs 50mbar permeate pressure). The highest performance recorded through experiments (CHF and PORCHF) was used as simulation basis. A membrane simulation model was used and interfaced to 8.6V Aspen HYSYS. A 300 Nm3/h mixture of CO2/CH4 containing 30-50% CO2 at feed pressures 6, 8 and 10bara, was simulated and process designed to recover 99.5%

CH4 with 97.5% purity. Net present value (NPV) was calculated for base case and optimal pressure (50bar for CHF and PORCHF). The results indicated that recycle ratio (recycle/feed) ranged from 0.2 to 10, specific energy from 0.15 to 0.8 (kW/Nm feed) and specific membrane

area from 45 to 4700 (m2/Nm3feed). The high recycle ratio can create problems during startup, as it would take long to adjust volumetric flow ratio towards 10. The best membrane separation system employs a three-stage system with polyimide at 10bar, and a two-stage membrane system with PORCHF membranes at 50bar with recycle. Considering biomethane price

of 0.78$/Nm3 and

a lifetime of 15 years, the techno-economic analysis showed that payback time for the best cascade is 1.6 months.

1. Introduction

Combustion of fossil fuel to meet the ever-increasing energy demand has resulted in depletion of natural resources[1]. At the same time, greenhouse gas emissions, especially CO2 produced by combustion of fossil fuel, is a major source of the contribution to global warming and climate change [2-4]. Development and use of renewable energy have become of major importance for long-term sustainability. By the year 2020, it is predicted in a Swedish case study that almost 20% of energy will be produced from renewable sources[5].

Biogas produced by microbial digestion of farm waste or sewage waste contains high concentrations of methane, which could be combusted to meet the energy and power demands. The biogas produced by microbial digestion of waste consists of several gases among which CH4 and CO2 account for most of the volume fraction. Other gases including H2S, N2, O2 and water vapours coexist in traces. Depending upon the raw material, digestion procedure and process condition, the CO2 concentration in biogas can reach up to 50%. This high concentration of CO2 significantly reduces the calorific value of biogas [6-9]. The produced biogas needs to be enriched in methane (CH4> 95%) by removing CO2 and other impurities from the gas [10, 11].

The CO2 can be removed from a gas stream by many different techniques. Some of the most investigated techniques for the upgrading of biogas involve CO2 capture by physical or chemical absorption in liquid, pressure swing adsorption, membrane technology or cryogenic separation [7, 12-16]. Among all these techniques, membrane technology offers several advantages like the compact modular design, small footprint, low capital and operational cost, simple operation and easy maintenance [17-19]. Due to these advantages, biogas upgrading by using membrane technology has gained a lot of attention. Much work has been done in the development of a competitive membrane material for different gas separation applications during the last two or three decades. The most important factor in membrane separation is the membrane material. Different materials have been suggested, here is only referred to a few representing both polymers and carbon membranes [20-23]. A novel Carbon hollow fibre (CHF) membrane was synthesized at NTNU using cellulose acetate as a precursor, which was de-acetylated to cellulose prior to the carbonization process. The CHF membrane showed high performance and potential to become an economically viable solution for biogas purification[24]. Their membrane showed high CO2/CH4 selectivity and good CO2 permeance. These CHF membranes were developed and tested on a pilot scale for biogas by MemfoACT AS a company which has now closed down.

The CHF membranes showed promising results and attractive properties for biogas upgrading under real test conditions at a biogas plant in Southern Norway. The modules of 2m area were made to test the membrane performance at the pilot plant. The plant was capable of processing 20 Nm /h of biogas. CHF membrane modules with feed on the shell side, showed weak mechanical properties when tested at high pressure, and many broken fibres inside the module were observed. Secondly, the potting which should bind that many fibres together at high pressure was not god enough and more research was needed. Modules with a small number of fibres (a few cm modules) tested in the laboratory showed good mechanical properties up to 70 bar. CHF membranes displayed a CO2/N2 selectivity of 30 with a CO2

permeance of 5.5E-3 m (STP)/m h bar. A further modification was done by applying oxidation, CVD, and reduction process on CHF membranes, which dramatically increased the CHF membrane performance (CO2/N2 selectivity of 82 and CO2 permeance of 2.1E-2 m3(STP)/m2 h bar).

The major separation in biogas upgrading process is CO2/CH4 separation. The membranes for CO2/CH4 separation are based on solution-diffusion mechanism, there is usually a trade-off between CO2 separation selectivity and permeability. The Robeson plot in Figure 1 shows that a membrane with high CO2 selectivity usually has low permeability [25, 26].

Figure 1: Robeson upper bounds for CO2/CH4 membrane separation [31]

The process to produce biomethane should be inexpensive and simple to control. Commercially available membranes for CO2/CH4 separation are mostly polymeric membranes, and these membranes do not have high enough separation factor (selectivity) to achieve a high recovery of CH4 in a single stage. The amount of energy required for biogas upgrading is a key factor when selecting a technology for this purpose. Due to low selectivity but high permeance of commercial membranes, big recycle stream has to be treated if high

recovery (99.5% of CH4) and pipeline spec (97.5% CH4) has to be reached in the two-stage system. This results in high costs due to the compressor price and compressor duty while in operation. The process operating cost can be reduced by optimization of feed pressure, inter stage pressure and recycle flow. A three-stage membrane separation could also be helpful to achieve high recovery and purity; however, operational complexity regarding intermediate pressure and biogas components concentration, suggest that adjustment of these variables is the key towards optimization. Plasticization inhibits the polymeric membranes to a threshold pressure[27], whereas CHF membrane has shown a stable performance and no plasticization up to 50 bar or even higher[28, 29].

Many researchers have been conducting Simulations and modelling of multi-stage membrane systems to evaluate and optimize membrane systems for CO2 capture [30-37]. Baker et al. [17] have provided a guideline for conducting simulations on commercially available membranes, which also shows the comparison between membrane system and amine absorption process. Baker suggested that membrane technology is suitable in small (less than 6000 Nm3/h) and medium scale (6000-50,000 Nm3/h) processes.

This study intends to demonstrate biogas upgrading with membrane separation technology. Permeance and selectivity of gases in polymeric membranes for biogas upgrading is abundantly available in the literature. However, similar data for carbon membranes has seldom been reported. Various modular configurations containing membranes with different CO2 selectivities, have been investigated in the current study, using HYSYS simulations for optimal performance and minimum energy consumption. Considering that the membrane separation system fulfils the German national standard for biogas as vehicle fuel, the evaluation focus to achieve 99.5% CH4 recovery and 97.5% CH4 purity. The important parameters such as compressor duty, recycle ratio (recycle/feed) and membrane area are discussed. In the end, techno-economical evaluation of the entire plant, including running costs and net present value (NPV) have been calculated. The purpose of this study is to present an economically viable scheme to upgrade biogas by using membranes with high CH4 recovery. Biogas composition depends on the source of the gas as shown in Table 1 [6].

Table 1: Biogas Composition from various sources [6]

Component Farm plant Sewage Digester Landfill

CH4 55-58 61-65 47-57

CO2 37-38 34-38 37-41

N2 Trace Trace 1-17

O2 Trace Trace 0-2

H2S <1 <1 <1

H2O 4-7 4-7 4-7

Aromatic Hydrocarbon Trace Trace Trace

2. Material and Methods

2.1. Materials:

Acros Organics (Belgium) supplied cellulose acetate (CA) and 1-Methyl-2-pyrrolidinone, while 99.5% (NMP) and PVP (Polyvinylpyrrolidone) were purchased from Sigma-Aldrich (Norway). Ionic exchanged water was used for coagulation.

2.2. Hollow fibre preparation

Using a dry-wet spinning process at commercial scale plant, delivered by Philos Korea, cellulose acetate hollow fibres (CAHF) were spun from a dope composed of CA / NMP/ PVP. CAHF were soaked in water and glycerol solution respectively after the spinning process. CAHF were deacetylated with NaOH / isopropanol / water solution and then dried in a humidity-controlled environment. Carbon hollow Fibres (CHF) were prepared by the carbonization of deacetylated CAHF in the presence of CO2 and N2. Details of this membrane preparation are given in the patent held by Hagg and Lie[24].

2.3. Modification

The pore size of CHF membrane, already mounted into a module with stainless steel casing, was tailored to enhance the membrane separation properties. The fig. 2 illustrate the steps followed in this work, as also reported in the patent held by Soffer et al. [38]. The durability, mechanical stability at operating conditions and separation efficiency of CHF membrane increased appreciably after oxidation and reduction process (PORCHF). The first oxidation step will produce quite large membrane pores with fairly low selectivity, then chemical vapour deposition (CVD) was performed using propylene to coat the membrane surface with a thin polymeric layer closing the pores again. The oxidation process was then repeated to open (tailoring) the pores in the newly formed layer to fit the molecular sieving of the gases in question (here CO2-CH4), finally followed by a reduction process, which reduced the aging of the membrane by stabilizing the pores.

Oxidation CVD Oxidation Reduction

(opening pores) (Surface coating) (opening pores) (Stabalization)

Figure 2: Steps followed for membrane pore tailoring

2.4. Transport Mechanism and membranes used in this work The mass transport properties of CHF and PORCHF were measured with the single pure gases CO2, O2, and N2 at different feed pressure and experiments were carried out without sweep gas on the permeate side. He et al. has performed the mixed gas experiments on the same type of carbon membrane (same protocol) and results indicated that the membrane performance for CO2 separation is the same or even higher in some cases for mixed gas as compare to single gas separation[39]. Due to fire hazard limitations, CH4 was not tested at membrane production facility, only in the lab. Therefore, the values for CH4 gas are estimated values (three times of N2 selectivity) in this work based on work done by He et al. (fig. 6 of the article)[39]. The performance of the membrane was evaluated by measuring the

CO2 permeance in m (STP)/ (m h bar) and CO2/N2 selectivities (a) using the equations 1 and 2.

Qpri _ QpYpri _ Jj ^^

Am(phxrpiyi) Am(phxrpiyJ (PhXrPiyi) I

« = ~Pj (2)

where J (m3(STP)/m2 h) is the flux of gas component i, qp is the volume of the permeating

gas (i) (m3(STP)/h), Pi is the permeability of gas component i ((m (STP)/m h bar), Ph and Pl are feed and permeate side pressures (bar), xi and yi are the mole fractions of component i on the feed and permeate sides and Am (m ) is the membrane area[36].

A benchmarking polymeric membrane is considered for comparison, as a future biogas upgrading membrane with a 1 pm thick selective layer, having a permeability of 100 barrer and a selectivity of 100 for CO2/CH4, which is above Robeson upper bond 2008. Gas permeation properties of CHF and PORCHF with other membranes used in the simulation of this work, are shown in table. 2.

Table 2: Gas permeation properties used in this work

Permeance, (m3(STP)/(m2 h bar)) Membrane Type [GPU], (m3(STP)/(m2 h bar)) S ingle gas selectivity Ref.

1 GPU = 2.736E-3

CO2 CH4 N2 CO2/CH4 CO2/N2

Polyimide hollow 5.6E-2 1.7E-3 6.0E-4 33 31 [40]

fibre [20.5] [0.62] [0.22]

CHF 5.5E-3 6.1E-5 1.8E-4 This work

[2.02] [0.0224] [0.0673]

2.1E-2 8.6E-5 2.6E-4

PORCHF 246 82 This work

[7.75] [0.0315] [0.0943]

Benchmarking 1.4E-2 1.4E-4 4.1E-4 100 33 See text

polymeric membrane [5.01] [0.0501] [0.15]

3. Process description and simulation Method 3.1. Pre-treatment of biogas

As shown in table 1, raw biogas contains several impurities, like water, dust, H2S, CO2, siloxanes, hydrocarbons, NH3, oxygen and several other components that must be removed in order to increase the membrane lifetime to avoid corrosion of the upgrading system and to comply for the biogas being approved as biomethane for vehicle fuel. Some membrane materials like polyimide have high permeability and can work in the presence of components which often are harmful to the membrane, like H2S and H2O[41]. As shown in table 1, Impurities like H2S and aromatic hydrocarbons need to be removed before biogas encounter the carbon membrane.

Biogas is usually saturated with water, and the amount of water which needs to be removed depend on how much water is allowed to enter into the compressor and membrane system. This water can be removed before entering the compressor or after membrane separation prior to high-pressure compression for vehicle fuelling or pipeline requirements. In the case of carbon membranes, less than 30% relative humidity (RH) is acceptable[29]. However, the presence of water may influence the separation of the other components, and it was documented that the permeability of N2 increased and CO2 decreased for CHF and PORCHF at 50% RH. When the fibres were again dried, it was observed that the permeability of CO2, N2 and selectivity of CO2/N2 slightly increased as compared to the initial values before the membrane was exposed to high humidity.

Biogas can contain up to 3000ppm H2S, which is a very high amount to expose the membranes to. Different techniques are used to remove sulphur from biogas: silica gel,

activated carbon, iron sponge and biological filtration. Polyimide membranes, unlike CHF and PORCHF, have high H2O and H2S permeability, which make it suitable for biogas upgrading process without special pre-treatment as these components will permeate with CO2[42]. However, biomethane as a vehicle fuel (German legislation) demands sulphur contents below 4ppm and water dew point of -10oC at 200bar, which require pre-treatment in both carbon and polyimide membrane separation process as considered in this work. It can be expected that biogas contains traces of organic components such as alkanes, halogenated hydrocarbons, ketones, aromatic compounds, siloxane, alkyl sulphides and alcohols depending on the substrate used for anaerobic digestion [42, 43]. Toluene, an aromatic hydrocarbon, is usually detrimental to the membrane, and it significantly decreased CO2 permeability and CO2/N2 selectivity when tested at 100ppm for CHF. The PORCHF membrane showed a significant increase in permeability of CO2, whereas selectivity of CO2/N2 decreased. Heptane showed the similar effect as toluene when tested 1000ppm for both CHF and PORCHF. However, the presence of methanol significantly lowered the permeability and CO2/N2 selectivity in both CHF and PORCHF. The presence of 300-ppm toluene on polyimide membranes decreased the CO2 permeability significantly and a slight decrease in CO2/CH4 selectivity was observed by Wind et al[44]. Many of these components have not yet been tested and will need further investigations. With the presently available membranes, it was concluded that pre-treatment is needed before the membrane is exposed to biogas.

3.2. Membrane Configurations

3.2.1. Three cases

Membrane processes may vary with respect to operational units, their arrangement and applied process conditions. Three different cases were evaluated in this study.

Single stage membrane process: A membrane system using only one stage to separate biogas for required methane recovery and purity has been simulated. Polyimide membrane is a commercially available membrane with modest selectivity, and in order to judge the separation performance of this membrane, using only one stage simulation at 10 bar feed pressure is evaluated (Fig. 3).

Figure 3: Single stage membrane process

Two-stage membrane process: To maximize the recovery of biomethane from biogas, a two-stage system has been simulated using all four membrane types with different selectivities. The permeate from the first stage enters into the second stage membrane to recover more CH4. Retentate from second stage mix with feed in the form of recycle stream prior to the compressor for better CH4 recovery (Fig. 2). There is no compression between the stages and the pressure of permeate 1 is adjusted with flow valve at Retentate 2. The pressure at permeate 1 was kept at a constant value for the specific feed pressure, obtained with formula as shown in equation (3). The basis of the formula is to maximize the pressure ratio (hence maximum perm purity) on both stages simultaneously by setting the interstage retentate. It was observed that the intermediate pressure value acquired, gave optimized membrane area for required purity and recovery of CH4.

interstage1

~^(Pfe ed ■ P pe rmea te2) (3)

Figure 4: Two stage membrane process

Three-stage membrane process: A three-stage membrane system may give better separation and reduce energy demand[45]. A three-stage configuration is shown in Fig. 3. Evonik Fibres Gmbh has applied for the patent of this configuration, and according to the patent, no one but Evonik can use a membrane with a CO2/CH4 selectivity of 30 or higher on the first stage[46]. Considering this patent is accepted, the energy demand of the process may increase by 0.027 kWh/Nm3 for other membrane providers. The three-stage system is simulated and economically evaluated here by using polyimide membranes.

Figure 5: Three stage membrane process

3.3. Simulation basis

• NTNU has an in-house membrane simulation model (Chembrane) which can easily be integrated into Aspen HYSYS. This model uses fourth-order Runge-Kutta method to calculate the flux along membrane length, and then iteration over permeate values to converge to a solution. ChemBrane model is integrated into 8.6V Aspen HYSYS for all simulations in this work.

• Countercurrent gas transportation without sweep on permeate side has been used in all hollow fibre membrane modules. Literature data shows that counter current flow exhibits the superior separation and uses lowest membrane area in hollow fiber module design. This module design is very efficient and has a high packing density

(can be up to 30,000 m /m for certain designs)[47].

• The Peng Robinson sour fluid package was used.

• In order to run the simulations smoothly, only separation of the main components CH4 ( 50 -70 %) and CO2 ( 50 - 30 %) is considered in the gas stream entering the membrane system. But techno-economic evaluation includes H2S removal, water removal and dust removal system.

• Intermediate pressure (inlet pressure for the stage 2 membranes in two-stage configuration and inlet pressure for the stage 3 membranes in three-stage configuration) is kept same for different membranes to balance the complexity of the system. Effect of intermediate pressure and sweet gas has already been studied by Deng et al[31].

• The adiabatic efficiency of the compressors is modelled as 75%.

3.4. Process Conditions

A water-saturated biogas stream of 300 Nm /h with 3000ppm of H2S is considered as a base-case. Biogas enters into biological H2S remover for bulk removal down to between 50-100ppm and passes further through activated charcoal, where H2S is taken down to below 1 ppm. The refrigeration process followed by zeolite molecular sieve is used to remove water from biogas in order to achieve a dew point of -10oC at 200bar. Biogas is compressed (single stage compression is used in simulations but multi stage compression with inter stage cooling is considered in cost calculation)_to required feed pressure (6, 8, 10 bars and all pressure values are absolute in this work) and then filtered to remove dust and oil droplets before entering the membranes. Biogas is fed to the membrane and the resulting biomethane is compressed up to 250 bars before it is stored for further usage as a vehicle fuel. A three-stage simulation using ChemBrane model in Aspen Hysys is shown in fig. 6.

Figure 6: A three-stage membrane system integrated into Aspen HYSYS

Table 3: Process conditions and feed composition used in this work

Process conditions used in simulation

30-50 % CO2, balance

Feed composition CH4

Feed flow rate (Nm3/h) 300

CH4 purity in product (%) 97.5

CH4 recovery (%) 99.5

S total <4ppm

Water dew point -10 oC at 200bar

Feed pressure (bar) 6, 8, 10

Intermediate pressure in two stage(bar) 2.45, 2.82, 3.16

Intermediate pressure in three stage(bar) 2.45, 2.82, 3.16

Flow pattern in membrane module Countercurrent

Biogas delivery pressure (bar) 250

Figure 7: Schematic diagram of biogas upgrading system

3.4.1. Effect of N

Using biological desulfurization may result in the addition of air components in the biogas stream. CH4 and N2 have close selectivity value for commercially available membranes, which makes it difficult to remove N2 from CH4 to achieve required CH4 purity. CHF membranes and PORCHF membranes have shown CH4/N2 selectivity of 3, making it less vulnerable to N2 in the gas stream. Only PORCHF membrane with the two-stage system has been simulated including N2. The intermediate pressure needed some extra optimization to get required results. N2 concentration in feed and operating conditions are shown in table 4.

Table 4: N2 concentration in feed and operating pressure

N2 in Feed Feed pressure Intermediate pressure

% bar bar

0 10 4.24

0.5 10 4.24

1 10 3.7

1.5 10 3.55

2 10 2.95

2.1 10 2.95

3.5. Cost estimation and Economic parameters

Accurate economic assessment of any process depends on available design details, a method of analysis used for calculation and accuracy of available cost data. Therefore, the economic calculations may differ considerably, as they are justified by the data available and also cost model. An economic evaluation was performed to assess the different membranes and their configurations, by taking capital cost, operating cost, pre-treatment cost and high-pressure compression (CBG) cost into account. A high recovery of biomethane is achieved, resulting in a very small fraction of CH4 loss with permeate stream (CO2). Thus CO2 obtained on permeate side is 99% pure and could be used for other applications. The price for the CO2 is not considered in this economical assessment. Table 5 is showing process parameters for economic assessment of a biogas upgrading process.

Table 5: Process parameters for economic assessment of biogas upgrading plant [51, 52] ($ used in this work is US

Total plant investment (TPI)

Polymeric membrane cost (PMC) Carbon membrane cost (CMCo) Installed compressors cost (CC) High pressure compressor cost (CBGC) Fixed cost (FC) Base plant cost (BPC) Project Contingency (PC) Total facility investment (TFI) Start-up cost (SC) TPI

Annual variable operating and maintenance cost (VOM) Contract and material maintenance cost (CMC) 0.05 x TFI Local taxes and insurance (LTI) Direct labour DL, cost based on 8hr/day Labour overhead cost (LOC) Utility cost (UC) ($/kWh) VOM

Other assumptions Membrane life for polyimide (t) Membrane life for carbon (t) Biomethane sales price ($) * Nominal interest rate (%) Depreciation (t)

LCC/LCI factor (Ordinary annuity factor) Plant availability (%) CO2/CH4 in feed (%)

$20/m2 [31, 33, 48, 49] $100/m2

$ 8700 x (HP/n)082

= 912 . (Wcomp)0 9315 . fm . fi . finst [50]

PMC/CMCo + CC + PTC + CBGC

1.12 x FC

0.2 x BPC

BPC + PC

0.10 x VOM

TFI + SC

0.015 x TFI $ 15/hr 1.15 x DL 0.07/kWh

CMC + LTI + DL + LOC + MRC + UC

7.5 years 5 years $0.8/Nm3 6%

15 years 9.7122 96% 40/60

4. Results and Discussion

4.1. Membrane configurations

A constant pressure of 10 bar was applied to the feed gas with different CO2 concentration to see the influence on recovery and membrane area. The result (Figure 8) shows that methane loss is high up to 17% when 30% CO2 is present in the feed at the applied operating conditions, and methane recovery decreases with increase in CO2 content in the feed as shown in fig. 7. The single stage system, as expected gives the lowest recovery; therefore, multiple stage system with recycle is discussed in the further results.

Figure 8: a single stage separation with polyimide membrane (10bar, 230C)

10 bar

200- A 6 bar 6 bar

"O d) JD Ci E 150- A... "A A A .

£ " A

OI ilj

< o 100-

Ô <D CL m •.......

CD c ro

J3 50- 1........

E m .......■

1 30 1 35 1 1 1 40 C02 in feed (%) 1 45 1 50

Figure 9: Two-stage membrane separation cascade (Polyimide membrane, T:23oC)

The feed pressure has a big effect on required membrane area for different CO2 concentrations in the feed as shown in fig. 9. The results show that higher the CO2 present in the feed, less area is required in this case to reach the targeted product (CH4) specifications. Area needed at low feed pressure (here 6 bar) is more sensitive to the feed concentration. Figure 9 demonstrates the required area for a two-stage separation system of polyimide membrane with varying CO2 loadings under a set of applied pressure. Results indicate that required area per Nm of feed gas under different concentrations of CO2 in the feed is three times higher when operating feed pressure is decreased from 10bar to 6bar. The effect of CO2 concentration in the feed gas is more sensitive at low pressure - this is as expected in this range when considering the basic equations.; here it shows the decline in required membrane

2 3 2 3

area from 175 m

2/Nm3 to 125m2/Nm3 when increasing from 30 - to 50% CO2, whereas, at 8 and 10bar pressure, the area is almost constant. The product purity is affected by selectivity limited region at 6bar (Separation factor vs permeate purity plot of Weller- Steiner equation[53]). When the pressure is 8bar or higher, the product purity is in the pressure ratio limited region. Two-stage membrane separation cascade using a polymeric membrane with CO2/CH4 selectivity of 100 is presented in fig. 10.

Required area per Nm of feed is lowest when feed pressure is 10bar and 50% CO2 is present in the feed - this is according to theory for solution-diffusion separation. Figure 11 shows the results for two-stage CHF membrane separation and a linear decline in required membrane area can be observed for different CO2 loadings. Maximum area is required when feed pressure is 6bar and the feed concentration of CO2 is 30%. A similar trend is observed in the case of two-stage PORCHF membrane cascade (fig. 12). Results presented in fig. 9-12 show

that change in required membrane area under a set of CO2 loadings is significant when feed pressure is low (6bar). However, when pressure is high (10bar) the trend line looks almost straight for fig. 9 and the variation in the area (fig. 10-12) is not very high. Figure 13 shows the results for a three stage membrane separation using polymeric membranes. In membranes, the performance is a trade-off between selectivity and permeability, and productivity is a function of material property and thickness of the membrane[54]. Assuming a selective wall thickness of 1^m for polyimide membrane and 20^m for carbon membranes, it was observed that carbon membrane requires >13 times larger area when same operational conditions are applied as for the polymeric membrane. A three-stage polyimide system requires the lowest area, which is three times less than the two-stage polyimide system. The area against different CO2 loadings shows a linear decline for CHF and PORCHF membranes when CO2 increases in the feed, whereas the curve is more visible in case of three stage polyimide membrane.

Figure 10: Two stage membrane separation cascade (selectivity 100, T:23oC)

Figure 11: Two stage membrane separation cascade (CHF membrane, T:23oC)

Figure 12: Two stage membrane separation cascade (PORCHF membrane, T:23oC)

Figure 13: Three stage membrane separation cascade (Polyimide membrane, T:23oC)

4.2. Recycle stream and compression duty

Operating cost of a biogas upgrading process depends largely on the compressor duty, and the recycle ratio (recycle/feed) higher than 1 can increase this compression energy requirement to a higher level. Results obtained by the simulation of different membranes with two and three stage configurations are plotted in fig. 14 and 15. The fig. 14 (A), demonstrates the recycle ratio at 6bar feed pressure, which is seven for the two stage polyimide membrane system and it would result in high operating cost; in the form of compression energy and also, a compressor with high capacity is required to treat the total volume of the gas which would increase capital cost as well. Whereas, the recycle ratio is below one (fig. 14 (A)) for PORCHF membrane system and the compression duty required for this system is one fourth of the amount required for two stage polyimide system as shown in fig. 15 (A). The efficiency of a membrane system increases with high selectivity as it can be seen from the recycle ratio and specific duty plots. The data shows that PORCHF having highest selectivity gives lowest recycle ratio and the required specific duty values. It was observed that the recycle ratio decreases in CHF and PORCHF membranes unlike polyimide membranes with high CO2 present in the feed. It is very important to choose an optimal point where capital investment and running cost are low and the system is efficient at the same time. The increasing of feed pressure to 8bar results in lower recycle ratio and energy demand. However, for two stage polyimide membranes, recycle ratio is still quite high especially when 50% CO2 is present in the feed as shown in fig. 14 (B). The figure 15 (B) indicates that the specific duty required is still four times higher for the polyimide membrane system as compared to PORCHF system. Plasticization effect inhibits polyimide membrane to go to

high pressures[27], so maximum pressure tested for polyimide membrane systems is 10bar in this section of work, which shows high recycle ratio in two-stage configuration as shown in fig. 14 (C). The three-stage system with polyimide shows recycle ratio about 1 for 8bar and 10bar simulations, and the specific energy demand for three stage polyimide is double as compare to PORCHF membrane system.

■ CHF membrane • Polyimide two stage A Polyimide three stage Y PORCHFmembrane

Polymer with 100 Selectivity

V ...........*............ * ••

"e •

"E z A .....A ...........A......... A ■ ■A

0 m .... B ...........m............. m- ■•

I « ........«...... < ■ <

0 ▼ •■ ...

(E ...........T......... ▼

30 35 40 45 50

CO., In feed (%)

Figure 14: Recycle ratio at different CO2 loadings in feed at 23 °C, (A) at 6bar, (B) at 8bar, (C) at 10bar

Figure 15: Specific Compression duty at different CO2 loadings in feed at 23 °C, (A) at 6bar, (B) at 8bar, (C) at 10bar

4.3. Effect of N2

It seems impossible to get fuel quality with this polyimide membrane if up to 2.5% N2 is present in the feed biogas, as infinite membrane area would be required to separate out 100% CO2, whereas PORCHF membrane can tolerate more N2 due to high CH4/N2 selectivity as compared to polyimide membrane as shown in fig. 16. The curve of area and duty in fig. 16 is expected to be asymptotical, which leads to infinite area and compression duty requirement in the presence of high N2 percentage in feed biogas. The presence of more N2 would result in increased recycle ratio to achieve required fuel (CH4) purity and recovery, which again leads to high membrane specific area and compression duty.

Figure 16: Effect of N2 concentration in feed gas (10bar, 23oC)

4.4. Cost calculation

4.4.1. Processing cost

Processing cost has been considered as the sum of capital and operating cost to calculate the price of compression duty and area for membrane systems over 15 years. The fig. 17 shows the processing cost of compression energy and area per Nm of upgraded biomethane for a two-stage polyimide membrane system. The effect of big recycle ratio can be seen in the form of high processing cost here for two-stage polyimide system. Even though the membrane cost is quite low, the cost of energy in capital investment and costs during running time is very high. The required processing cost is reduced to one-fourth by using a three-stage system for polyimide membrane as shown in fig. 18. The effect of membrane efficiency is more prominent in fig. 19 and 20 with CHF and PORCHF membranes, showing considerably low processing cost related to energy consumption, whereas the effective membrane area required is five-fold higher than for polymeric membranes. Considering $20/m for polymeric membranes and $100/m for carbon membrane, results show that membrane area can increase 30% to 80% of total investment if optimal pressure is not applied on the upgrading plant.

Figure 17: $ for duty & area in processing cost, (Two stage polyimide)

Figure 18: $ for duty & area in processing cost, (Three stage polyimide)

Feed pressure (bar)

Figure 19: $ for duty & area in processing cost, (Two stage CHF)

Figure 20: $ for duty & area in processing cost, (Two stage PORCHF)

4.4.2. Net Present Value (NPV) calculation

The NPV calculation includes pre-treatment, upgrading part and high-pressure compression on biomethane to the fuel standard. Considering that different material has specific advantages, an optimal pressure value has been applied on both carbon and polymeric materials to calculate the NPV in cost estimation. Fig. 21 shows NPV at 10bar pressure for polyimide two-stage and three-stage system. Carbon membranes have shown a stable performance under different CO2 loadings, with no plasticization up to 50bar[28]. It can be seen from the NPV results that optimal pressure for carbon membrane is 50bar or higher. For NPV calculations, 50% CO2 concentration in the feed is considered.

A three-stage polyimide membrane system has been calculated to have a maximum NPV of $ 9.3M at 10bar, whereas PORCHF membrane system looks competitive with an NPV of $ 7.4M. The resulting lower NPV value for PORCHF is due to high membrane cost and estimated for a lifetime of 5 years in comparison with 7.5 years of polyimide membranes. However, this price can be reduced by optimizing the membrane production process. (The production process was not fully optimized for the pilot scale production of CHF and PORCHF at the company MemfoACT).

PORCHF(50bar)

CHF(50bar)

Selectivity 100 ( as polymer and at 10bar) -H

PO^CHF(IObar)-

CHF(10bar)-

Three stage Polyimide(10bar)-M

I^Bnpv Two stage polyimide (10bar)-1

T—1—i—1—i—•—i—1—i—1—i—1—i—'—i—1—i—1—i—1—i—1—i—1——1—r

-96 0M -88.0M -80.0M -72.0M -64.0M -56.0M -48.0M -40.0M -32.0M -24 0M -16.0M -8.0M 0.0 8.0M

NPV (US$ for base case [300Nm3/hr]) Figure 21: NPV calculated for optimal pressures

4.4.3. Sensitivity analysis

Assuming an optimized process producing PORCHF at a price of $ 60/m instead of $ 100/m and a membrane lifetime of 7.5 years will give NPV for PORCHF of $ 8.8M. and applying 70bar pressure, then it can increase NPV for PORCHF over $ 9M as shown in fig.22.

Figure 22: NPV comparison of three stage polyimide and PORCHF membrane

5. Conclusions

In this study, it was found that the two-stage cascade process with recycle using a polyimide membrane was not economically viable for biogas upgrading due to high recycle ratio, and thus resulting in high operating cost, whereas the three stage polyimide membrane system is quite feasible in order to obtain fuel quality of biomethane. Carbon hollow fiber membrane and modified carbon hollow fibers produced on a pilot plant were tested to obtain the same fuel quality in two stage cascade, and these membranes consumed 22% less energy as compared to three-stage polyimide system. The drawback of these membranes is, however, the production cost, which is 5-fold higher than the assumed costs of a polyimide membrane. The optimization in the production process and choosing an optimal operating pressure can reduce the capital cost for CHF and PORCHF membranes, whereas the operating cost can be reduced by increasing the membrane life through regeneration of carbon fibers by applying CVD process if pore clogging is the problem. In the case of fiber breakage, the broken fibers

may be plugged in the module. Instant boosting with electrical regeneration applying low voltage and direct current (DC) has been documented as successful, but the effect of this regeneration procedure on the aging of the membrane is not sure.

Acknowledgements

The authors are very grateful to Ms Ingerid Caroline Tvenning Andersen for excellent laboratory work on the carbon hollow fiber membranes and Dr. Muhammad Saeed for fruitful discussion. The authors would also like to thank The Department of Chemical Engineering at NTNU for providing the possibility to work with this article.

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