DOI: 10.3303/CET2188130 
 

 
 

 
 

 

 
 
 

 
 
 

 

 

 
 
 

 
 
 

 
 
 

 
 
 

 
 
 

 
 

 
 
 

 
 
 

 
 
 

 

Paper Received: 26 April 2021; Revised: 21 September 2021; Accepted: 2 October 2021 
Please cite this article as: Sato Y., Kansha Y., 2021, Design of an Energy-Saving Membrane Separation Module for Algae Cultivation, Chemical 
Engineering Transactions, 88, 781-786  DOI:10.3303/CET2188130 

CHEMICAL ENGINEERING TRANSACTIONS 

VOL. 88, 2021 

A publication of 

The Italian Association 
of Chemical Engineering 
Online at www.cetjournal.it 

Guest Editors: Petar S. Varbanov, Yee Van Fan, Jiří J. Klemeš

Copyright © 2021, AIDIC Servizi S.r.l. 

ISBN 978-88-95608-86-0; ISSN 2283-9216 

Design of an Energy-Saving Membrane Separation Module 
for Algae Cultivation 

Yuki Sato, Yasuki Kansha* 
Organization for Programs on Environmental Sciences, Graduate School of Arts and Sciences, The University of Tokyo, 3-
8-1 Komaba, Meguro-ku Tokyo 153-8902, Japan 
kansha@global.c.u-tokyo.ac.jp 

In this study, focusing on a zeolite membrane separation method expected energy-saving, appropriate 
environments for algae cultivation were investigated from the CO2 capture aspects. In addition, the authors 
elucidated the membrane separation performance and its energy consumption through experiments and 
simulations for the integration of CO2-treated processes. The results suggested that only 2-15 % CO2 in air 
ventilation was required for several algae growth. To satisfy the CO2 concentration constraint, the membrane 
could separate CO2 to approximately 15 % with the energy required for 0.76 MJ/kg-CO2 and a recovery ratio of 
71 %. The combination of membrane separation and algae cultivation from low concentration CO2 sources is a 
promising method for carbon capture and utilisation. 

1. Introduction

Carbon Capture and Storage (CCS) and Carbon Capture and Utilisation (CCU) to suppress global warming 
have been attracting attention because carbon dioxide (CO2) causes global warming as a greenhouse gas. On 
the contrary, CO2 is an essential material for refrigerant (dry ice), carbonated drinks, laparoscopic surgery 
(Ilkben et al, 2021), welding as sealing gas (Math et al., 2021). Recently, large and high concentration CO2 
sources such as steel-works have been shutting down gradually due to world economics and environmental 
protections. Thus, it is required that CO2 emitted from low concentration CO2 sources such as thermal power 
generation plants or garbage incineration plants is separated to protect environments and to satisfy a variety of 
CO2 usage. 
There have been some methods such as chemical absorption, physical adsorption, membrane, and cryogenic 
separation methods to separate CO2 from a gaseous stream. The most commonly used method for CO2 capture 
in industries among the above-mentioned methods is a chemical absorption method with amine solutions 
(Teranishi et al., 2016). In this method, CO2 reacts chemically with amine solutions. Therefore, this process 
inherently produces pure CO2 and achieves a high CO2 recovery ratio. However, it is well-known that the process 
is energy intensive (Goto et al., 2009). Thus, energy-saving technology has been required and investigated 
(Goto et al., 2016). Nagumo et al. (2013) reported that membrane separation required the least energy-required 
method among the other CO2 capture methods. In addition, Basile et al. (2010) reported that a membrane 
separation method is an alternative solution for separating CO2 from off-gas of an integrated gasification 
combined cycle (IGCC). 
Some researchers have investigated several CO2 utilisation methods such as artificial photosynthesis to 
chemicals (Jia-Way et al., 2018), biological digestion (Alami et al., 2021) instead of CCS. Algae cultivation is 
one of the option for biological digestion. Environments including CO2 concentration are important for algae 
growth. However, there were few discussions about the methods to recover CO2 and to separate it before 
utilisation in the integrated process, and about the quantitative estimations in terms of energy consumption.  
In this study, suitable algae growth environments have been reviewed for CO2 capture in the first step. Then, 
the authors proposed a suitable CO2 separation method by algae and evaluated its performance and energy 
consumption quantitatively by experiments and simulations for the integration of low CO2 sources, the 
membrane separation process, and algae cultivations as CO2 capture and utilisation.  

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2. Review of algae cultivation environments for CO2 utilisation

To select suitable CO2 concentration methods, proper environment to grow some algae species was studied by 
literature review. 
Firstly, the authors searched the publications, which treated the appropriate pH of medium for algae growth. 
Komatsu et al. (1994) reported that the alga E. Prolifera grew well at pH 6.5 while ventilating pure CO2. In 
addition, the alga grew best at pH 8.5 and died at pH 4 or less as Figure 1 shows. Some studies report the best 
growth at near pH 8 (Takahashi et al., 2012). On the other hand, in the case of an oxyphilic alga such as 
Chlamydomonas eustigma (Guillermo E., 2018), the alkalisation process will not be required. Thus, a simpler 
process without an acidic gas removal unit is expected in this case. 

Figure 1: Dependence of cultivating pH for E. Prolifera with ventilating pure CO2 (Komatsu et al., 1994) 

Takahashi et al. (2012) reported that the concentration of nitrogen significantly affects to the cultivation of a blue 
alga and a diatom (Microcystis aeruginosa / Cyclitella sp.). In addition, they reported that the algae cell density 
was increasing with the feeding quantity of nitrate ion, NO3-. In this research, the authors concluded that it is 
essential to feed nitrate ions for some algae cultivation. 
Furthermore, there are several publications, which reported the effect of CO2 concentration to the algae 
cultivations. Some of them reported experiment with ventilating air containing CO2 to promote algae growth. 
These results are summarized in Table 1. In most of them, a low level of CO2 concentration (2-15 %) was used. 
Komatsu et al. (1994) reported CO2 fixation efficiency was only 9 % when they provide pure CO2 (99.9 %) to 
the alga shown in Figure 2. It seems that most of the feeding CO2 did not dissolve in the water and was released 
into the atmosphere. Therefore, in the case of adding CO2 directly to cultivate algae, it is not necessary to 
concentrate CO2 to high purity. 

Table 1: CO2 concentration in ventilating CO2-included air to some algae cultivation 

Alga species Conc. of CO2 in feed gas [%] Reference 
Scenedesmus dimorphus 2 % Iwai et al., 2017 
Botryococcus braunii 5 % Murakami et al., 1996 
Minami-aonori 15 % Kishimoto et al., 2012 
E.Prolifera 99.9 % Komatsu et al., 1994 

Figure 2: CO2 fixation efficiency as a function of CO2 aeration (Carbon-based flow rate) (Komatsu et al., 1994) 

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3. Membrane separation performance evaluation

A zeolite membrane separation was selected due to the low energy characteristics with algae cultivation 
environments. The membrane separation performance was experimentally examined, and energy performance 
was evaluated quantitatively by a commercial process simulator (PRO/II ver.10.2) with the experimental results. 

3.1 Separation performance of the zeolite membrane 

The schematic image of the experimental setup is shown in Figure 3. Two mass flow controllers (Fujikin Inc. 
FCS-PM1000A-SP) were set to the system to control the flow rate of each gas, CO2/N2. Unit number 2 in Figure 
3 is a separation unit. A tubular-type membrane is made of zeolite in this unit. High purity N2/CO2 (purity 99.99 %) 
gases were used.  
A needle valve was used to adjust feed pressure. In the steady-state of the gas flow, CO2 concentration in 
permeated gas was measured by FI-IR (Thermo Fisher Scientific K.K., Nicolet iS5 iD1 transmission). Flow-
meter (Ellutia 7000 series) to check the gas flow rate was set. The membrane performance (permeance, 𝑘𝑚) 
was calculated from these measured values (flow rate, CO2 concentration in non-permeated side, and each 
pressure gauge). Permeated gas concentration and flow rate were calculated from the material balance of each 
gas. The calculation equation is shown in Eq (1). Experimental conditions are summarized in Table 2.   

① Mass flow controller ➁ Zeolite membrane in the separation unit 

③ Pressure gage ④ Needle valve

⑤ FT-IR ⑥ Flow-meter
Figure 3: Experimental setup 

Table 2: Experimental condition for the membrane performance estimation 

CO2 concentration in feed gas [%] 
Separation 
temperature 

Differential pressure 

10 
30 °C 0.10 MPa 30 

50 

The results of CO2 concentration in permeated gas flow was summarized in Table 3. It was impossible to 
condense to high purity with a single step; however, it is possible to condense CO2 to 1.1-1.3 times higher 
concentration. In other words, it was confirmed that it is capable of a little separation of CO2 by the zeolite 
membrane unit. 

Table 3: CO2 and N2 concentration in permeated gas flow 

CO2 Conc. in feeding flow 
[mol%] 

Conc. in permeated flow [mol%] 
CO2 N2 

10.0 11.1±0.3 88. 9±0.3

30.0 35.9±0.9 64.1±0.9 

50.0 64.8±1.3 35.2±1.3 

𝑁𝑚 =
𝐹𝑝𝑒𝑟𝑚.,𝑚 ∙ 𝑥𝑝𝑒𝑟𝑚.,𝑚

𝐴
= 𝑘𝑚 (𝑝𝑓𝑒𝑒𝑑,𝑚 − 𝑝𝑝𝑒𝑟𝑚.,𝑚 )  (1) 

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3.2 Energy performance evaluation 

Installing the experimental data in section 3.1 into the simulator, the performance of membrane separation was 
examined. The simulation conditions were as follows: feeding concentration was 10 mol% assumed low CO2 
sources, differential pressure and temperature were the same as the experiment, adiabatic efficiency of the 
compressor to separate the feeding gaseous stream were assumed to be 80 %. Permeance area was assumed 
1000 m2, the value suit approximately 30-50 m3 scale plant by calculating from the area density 20-30 m2/m3 
(Strathman et al.,1985). The feed flow rate was changed between 0.01-10 kmol/s. The simulation result is shown 
in Figure 4 (a). In this figure, it can be seen that energy consumption for CO2 separation was less than 1 MJ/kg-
CO2 while feed rate was less than 0.03 kmol/s. Moreover, energy consumption was increasing with increasing 
feeding flow rate. In addition, the recovery ratio was decreasing with increasing of the feed rate. It can be seen 
that the more flow rate increases, the more it is disadvantageous from an energy consumption and a recovery 
ratio point of view. CO2 concentration in the permeated side as a function of feed flow rate is shown in Figure 4 
(b). The concentration was increasing but saturating at nearly 19 % over 0.1 kmol/s feeding. In other words, 
there is no reason to work under a feed rate that is too high. 

(a)  (b) 
Figure 4: Simulation result (a) energy consumption and recovery ratio (b) CO2 concentration in permeated gas 

as a function of feed rate 

4. Suggested process description for algae cultivation

Considering these experiment and simulation results of the appropriate algae growth environments investigation 
and the zeolite membrane performance, an integrated process with the membrane separation was developed. 
Generally, an exhausted stream from a thermal power generation plant or a garbage incinerating facility includes 
acidic gases such as NOx, SOx, and H2S. In addition, CO2 is also known as an acidic gas. Therefore, it is 
impossible to feed them directly into the medium to decrease its pH. However, some of them are also available 
for algae growth. Therefore, they should be neutralised by supplying a basic material while they are fed into the 
medium or be alkalised previously. For the latter method, the reaction equation in the case of feeding a calcium 
oxide (𝐶𝑎𝑂) as an absorbent is shown in Eq (2). When feeding calcium carbonate (𝐶𝑎𝐶𝑂3) into the medium, 
carbonate ion (𝐶𝑂3

2−) consumes hydrogen ion (𝐻+). As a result, the medium pH increase. In the hydrolysis
reaction shown in Eq (3), it is expected that alkalisation of the medium and supplying bicarbonate ion (𝐻𝐶𝑂3

−)
and a mineral (𝐶𝑎2+) occurs simultaneously.  

𝐶𝑂2 + 𝐶𝑎𝑂 → 𝐶𝑎𝐶𝑂3 (2) 

𝐶𝑎𝐶𝑂3 + 𝐻
+ ↔ 𝐶𝑎2+ + 𝐻𝐶𝑂3

− (3) 

However, about NOx exhausted gas from a thermal power generation plant or a garbage insincerity facility, there 
are possibilities to utilise for algae growth as a nitrogen source. Nitrate oxide (NO2) can dissolve into water. The 
NO2 decomposes into water and generates nitrate and nitrite ion. These ions are good for the algae. 
The zeolite membrane unit could separate CO2 to a low level of concentration with small energy consumptions. 
The separation conditions to work the unit should be decided in response to the different purposes in accordance 
with the utilisation. From investigation results, it was adequate with 15 % CO2 for algae cultivation. Therefore, 

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the appropriate feed rate was approximately 22 mol/s at this concentration from Figure 4 (b). Then, energy 
consumption was 0.76 MJ/kg-CO2, and the recovery ratio was 71 % at the feed rate of 22 mol/s from Figure 4 
(a). The result showed energy-saving separation compared with other methods such as 4.0 MJ/kg-CO2 by a 
chemical absorption (Goto et al., 2009), 3.8 MJ/kg-CO2 by a TPSA (Patricia A.P. M. et al., 2017), and a cryogenic 
method with its phase transition. The process of applying the membrane unit to algae cultivation is promising 
as one of the CCU technologies. However, water inhibits CO2 to permeate through the membrane because H2O 
molecular polarity is strong compared to CO2. There is a report to show the performance of each gas with H2O 
shown in Table 4 made by the report (Okamoto, 1998). Summarizing the above comprehensively, Figure 5 
shows the schematic image of the proposed process for CO2 capture using algae. The process is firstly removing 
acidic gases. At this time, NOx is removed as a nitrogen source for utilising the alga growth. Afterward, H2O is 
removed before separating CO2. Finally, the products are fed to the algae after alkalising the obtained CO2 and 
NOx. 

Table 4: Permeance of CO2/N2/H2 gas with/without H2O molecule 

Permeance, 𝑘𝑚 [cm3(STP) cm-2 s-1 Hg-1]
H2O CO2/N2/H2 etc. 

Wet (with) 8×10-3 ＜10-6 

Dry (without) - 2.5-3×10-5 

Figure 5: Suggested process considering proper environment for algae and zeolite membrane performance 

5. Conclusions

The authors searched the appropriate environment for algae from previous literatures in the first step. The 
appropriate environments for some algae were as follows; CO2 concentration was sufficient with 2-15 %. 
Moreover, the pH was at 8-8.5. However, some algae survive in an acidic environment for their growth. From 
the results, the authors considered that the membrane separation is suitable for CO2 separation for feeding 
algae. Thus, the authors evaluated zeolite membrane separation performance quantitatively by experiments 
and simulations for their integration and exhaust stream from low CO2 sources in the next step. The performance 
of zeolite membrane separation which has been expected as an energy-saving process was evaluated 
experimentally. It was possible to separate CO2 to 1.1-1.3 times at 10-50 % CO2 concentration in feeding 
streams. By using the experimental result, the energy consumption and the recovery ratio were simulated. As a 
result, at 22 mol/s feeding stream, the recovery ratio and energy consumption to separate CO2 were 71 % and 
0.76 MJ/kg-CO2, and this result is realistic and energy-saving compared with other separation methods such as 
chemical absorption with an amine, physical adsorption, and cryogenic method. Considering the above, the 
process after recovering low CO2 from a thermal generation plant or an incineration facility is recovered; 
separating by membrane unit, applying to algae cultivation is promising in terms of energy consumption. In 
addition, the overall suggested process description for CO2 fixation to algae was proposed. The authors 
recommend applying the membrane separation technology to CCU such as algae cultivation or CO2 adding 
agriculture, which requires low CO2 concentration for the future process design. 

Nomenclature

𝑝𝐻 – potential of hydrogen 
𝑁𝑚 – molar flux of a gas m (= 𝐶𝑂2/𝑁2), mol/(s·m2)
𝐹𝑝𝑒𝑟𝑚.,𝑚 – molar flowrate in permeated side, mol/s  
𝑥𝑝𝑒𝑟𝑚.,𝑚 – molar fraction in the permeated side, - 
𝐴 – permeation area, m2 

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𝑝𝑓𝑒𝑒𝑑,𝑚 – partial pressure in feed side, Pa  
𝑝𝑝𝑒𝑟𝑚.,𝑚 – partial pressure in permeated side, Pa 

Acknowledgements 

This project is financially supported by JST SICORP (JPMJSC18H5), Japan 

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