CO 2 desorption from activated DEA using membrane contactor with vacuum regeneration technology

Carbondioxide (CO2) content in natural gas must be removed because it inhibits liquefication process of natural gas. CO2 gas separation technology using membrane contactor has been developed, however solvent regeneration using membrane contactors are still rare because it requires a larger energy. The regeneration process by using membrane vacuum technology was put forward to reduce the regeneration energy consumption. In this work, arginine, piperazine (PZ), and potassium carbonate (K2CO3) as activators were added into diethanolamine (DEA) solution to form aqueous solutions of activated DEA. The experiment of CO2 desorption from activated DEA was carried out in hollow fibre membrane contactor (HFMC). The solvent with rich CO2 at 30-70°C was flowed in the lumen of the hydrophobic polypropylene HFMC, and the shell side was maintained at a reduced pressure by a vacuum pump at 20 kPa. The effect of solvent temperature and activators were investigated to get CO2 desorption flux and regeneration efficiency. Experimental result shows that increasing of solvent temperature could enhance CO2 desorption flux and regeneration efficiency. Instead of that, the activated DEA also give better result compared with non-activated DEA. Among three activators, K2CO3 give the best result for desorption flux and regeneration efficiency.


Introduction
Natural gas is the third largest natural resource in the world after coal and petroleum.Natural gas processing industry is an important sector in supplying energy or fuel.Compare to traditional coal, combustion of natural gas only produce half of CO2 and 33% compared to oil.The high potential of natural gas resources increasing efforts to develop more effective and efficient processing in terms of operations, economics, and environment.
In natural gas processing processes where the main component is a hydrocarbon must be separated from impurity components such as CO2, H2S, N2, and water vapor as they may affect the quality and processing of natural gas further.CO2 as an impurity in natural gas can cause corrosion of piping and equipment, otherwise CO2 is able to decrease the heating value of natural gas [15].In the process of Liquefied Natural Gas (LNG) processing, CO2 content must be removed to prevent icing during natural gas liquefaction process, because the process runs at very low temperatures of -161 ° C while the freezing point of CO2 around -78.4°C [14].
Contactor membrane technology is an improvement of CO2 removal technology combining gas absorption process with solvent and membrane technology in order to set the pattern of flow rate for avoiding flooding, loading, foaming, and entrainment that usually occurs in conventional absorption technology.The advantages of membrane contactors compared to conventional absorption technology are high surface area per unit contactor volume, independent control of gas and liquid flow rates without any operational problems, small size device, known gas-liquid interfacial area, modular and being easy to scale up or down.[9].
Feron et al. [3] conducted a study in CO2 gas separation process using a conventional hybrid membrane contactors for absorption and regeneration column for the desorption process with amine solvents.They compared the performance of the membrane absorber and packed column absorber.The results showed that performance CO2 absorption in membrane absorber was same compared to conventional absorber column, but the size of absorber can be reduced tentimes using a membrane absorber.Hollow fiber gasliquid membrane contactor as CO2 absorber has been comprehensively studied by several researchers.Those experimental and theoritical result have been reported [2,8,12,16].
Research and study about membrane technology for CO2 removal is increasing.However, it is important to MATEC Web of Conferences 156, 08012 (2018) https://doi.org/10.1051/matecconf/201815608012RSCE 2017 notice that there are only a few research on CO2 desorption through membrane contactors.The solvent regeneration operation unit requires greater energy, it was estimated that the energy requirement for CO2 desorption reaches 15-30% of coal burning power plant production [1].Therefore, it is necessary to do a lot of research on CO2 desorption through membrane contactors with low operating costs and capital costs.
CO2 desorption using membrane contactor can be done by vacuum or sweep gas method.Kosaraju et al. [9] conducted a study using polypropylene membrane contactor by combining CO2 absorption and desorption process.They performed a process of CO2 absorption and desorption on two continuously separated membrane contactor modules for 55 days using novel solvent and helium gas as stripping gas.The result showed that the performance of the membrane contactor was stable and the PP membrane material was not wetted by the solvent.
Mansourizadeh and Ismail [13] also conducted a similar study using a polivinildenefluorid membrane material (PVDF) with two separate modules for absorption and desorption using water as solvent and N2 as stripping gas.The results showed that the temperature of the solvent also affects the CO2 desorption efficiency, which higher by increasing the temperature of the solvent.While increasing the flow rate of solvent could decrease desorption flux.
CO2 desorption for solvent regeneration using membrane contactor vacuum was performed by Fang et al. [1].MEA with a lot of CO2 content was flowing to the membrane lumen and membrane shell was pressed by vacuum pump.Low temperature steam flowed in shell side of membrane module which acts as sweep gas improve desorption performance in order to avoid liquid loss.The results showed that CO2 desorption using membrane vacuum technology represented good performance and reduce energy consumption because it was operated at low temperatures.
Proper choice of solvent plays an important role in determining the performance of gas liquid membrane contactor for CO2 regeneration.Various solvent such as pure water, aqueous solution of NaOH, KOH, alkanolamines and ammonia have been studied experimentally.Many studies of CO2 absorptiondesorption with membrane contactors still use a single solvent without any mix of activators, such as Khaisri et al. [7], Lv, et al. [10], and Fang, et al. [1] who use MEA as a solvent.Then Mansourizadeh and Ismail [13], Karoor and Sirkar [6], used water, and several other studies using NaOH, MDEA (methildiethanolamine), or DEA [16].Some amines have some advantages, such as primary and secondary group amines whose reaction rate are faster than the tertiary amine group, but the tertiary amine group has a higher absorption capacity than the primary and secondary amines [11].
Recently, the blended alkanolamines or addition of activator have been presented by several researches in order to improve the performance of CO2 separation.Yeon, et al. [18] conducted CO2 separation using membrane contactor for absorption and stripper column for solvent regeneration by mixing PZ and triethanolamine (TEA) as solvent.The results showed that TEA with low absorption capacity could be increased by addition of PZ.TEA is a tertiary amine with high volatility can prevent hollow fiber membrane from wetting phenomenon.Then Hedayat, et al. [4] conducted a study for the absorption of CO2 and H2S gases from natural gas using membrane contactors by comparing MDEA solvents mixed with MEA and MDEA with DEA.The results showed that the addition of MEA into the MDEA improved the efficiency of CO2 separation, while the addition of DEA did not give significant results.Rahim, et al. [15] used four different amino acid salt (AAS) solvents and compared with aqueous solutions of MEA and NaOH.The results showed that AAS improved the performance of CO2 absorption-desorption when compared with MEA.
In this work, arginine, PZ, and K2CO3 as activators were added into diethanolamine (DEA) solution to form aqueous solutions of activated DEA.The activated mechanism were presented to explain the activation phenomenon.The experiment of CO2 desorption was carried out in polypropylene hollow fiber membrane contactor (HFMC).The use of such amine solvent mixture is expected to increase CO2 regeneration efficiency and reduce the cost of solvent usage so that it is more economical.

Characteristic of Activators
Three activators, arginine, PZ, and K2CO3 were chosen to form the three DEA-activated aqueous solutions : DEA-Arginine-H2O, DEA-PZ-H2O, and DEA-K2CO3-H2O.Arginine is a primary amino acid that can form alkaline amino acids salt.This amino acid salt is predicted as one of the proper substances in CO2 absorption because it has functional groups that are identical with alkanolamines.While PZ has a special molecular structure, the cyclic structure of a symmetrical diamino.PZ is an efficient and highly successful activator used for high CO2 separation capacity in industrial processes.Then K2CO3 is alkaline salt which has a high CO2 loading capacity and low heat of reaction.Therefore, it's to be a good activator for CO2 absorption.

Reaction Mechanism
The mass transfer of CO2 gas to the liquid phase is influenced by the diffusivity of the CO2 into the solvent.If the pore is hydrophobic and membrane is not wetted, then it is assumed that the pore membrane is only fully filled by gas.Diffusion begins in the bulk gas phase by the diffusion of the boundary layer and the chemical reactions on the fluid-membrane surface.The reaction between a CO2 and DEA is as follows : Where R1R2NH is the primary or the secondary amine.
The formation of carbamate proceeds as follows :  Formation of zwitterion (intermediate reaction)  Removal of the proton by a base B Where the base B, could be the amine itself, water molecules, or hydroxyl ions that reacted with DEA solution.The corresponding reactions are as follows : R1R2NH + COO -+ H2O → R1R2NCOO -+ H2O + (5) If there is an activator, the reaction that occured not only CO2-DEA but also CO2-activator take place in the system.Here is the reaction between DEA-Arginine-H2O with CO2 : HNCHR'COOHCOO -+ H2O For the system DEA-PZ-H2O, will give different reactions which will directly form carbamic ions without going through intermediate reactions, as follows : HN(C4H8)NH + CO2 ↔ HNC4H8NCOO -+ H + (10) Then the carbamate will convert CO2 into bicarbonate ion and PZ will regenerate as Eq. ( 11) HN(C4H8)NCOO -+ H2O ↔ HCO3 -+ HN(C4H8)NH (11) The overall reaction for the absorption of CO2 using DEA-K2CO3-H2O is described by reaction below : Since potassium carbonate and bicarbonate are strong electrolytes, it can be assumed that the metal is present only in the form of K + ions and Eq. ( 12) can be represented as : Reaction ( 13) proceeds according to the following sequence of elementary steps : H2O  H + + OH − (16) Reactions ( 14) and ( 15) are both followed by subsequent instantaneous reactions as follows : The reaction sequence ( 14), ( 16) and ( 17) are known as the acidic mechanism.The contribution of the acidic mechanism to the overall rate is negligible unless the pH of the liquid solution is very low.Almost all cases of industrial absorption is held at high pH (generally, pH > 8) and as such the acidic mechanism can be neglected.Therefore, reaction (15) is the rate controlling step for absorption of CO2 into potassium carbonate solution as reactions ( 16) and ( 18) are instantaneous reactions.

Data Analysis
In this study, CO2 desorption flux (JCO2) and CO2 loading (α) were used to analyze the CO2 desorption performance.They can be expressed by the following equations : CO2 concentration in the aqueous solution which written in Eq.20 was determined by the apparatus adapted from Chittick CO2 analyzer apparatus [15].From Eq.20 , Vgas is volume of displaced solution in the manometer tube (mL), VHCl is volume of titrant (mL), P is pressure (Pa), T is room temperature (K), C1 is solution concentration (mol/L), and V1 is sample volume (mL).
Regeneration efficiency was calculated to evaluate the regeneration performance of rich solvent by using membrane vacuum regeneration technology in this study.Regeneration efficiency is defined by the ratio of amount of CO2-rich solution, which can be calculated by the following equation :

Material
In this experiment, the material of membrane is a hydrophobic polypropylene membrane from GDP Filter Indonesia membrane industry with its characteristics shown in Table 1.

Experimental Procedure
The experimental set-up for CO2 desorption was shown in Fig. 1.The solvent containing CO2 gas from the absorber was prepared with CO2 loading 0,22 mole/mole and was added into the tank (1), then flows into coil heater water bath (3) by diaphragm pump (2) before entering the desorber (4).This aim of heating was to reduce the solubility of CO2 in the solvent so CO2 can easily released from the solvent.The temperature range in this study was 30-70 °C.After pre-heating, CO2-rich solution continuosly pumped into tube side of the hollow fiber membrane contactor with flow rate 100 mL/min.Liquid flow rate was controlled by liquid flow meter.During the regeneration operation, shell side of the module membrane contactor was kept at 20 kPa absolute by vacuum pump (8) that controlled with pressure gauge (5).Buffering tube (7) used to capture water vapor from output gas desorber before entering vacuum pump.The dissolved CO2 in the solvent in the tube section diffused outward through the membrane pores which was driven by the vacuum pressure.Finally, CO2 gas enriched and could be collected from the vacuum pump.The solvent exit desorber is called lean solvent (6) then analyzed with chittic titration.

Effect of Temperature on Desorption Flux
The effect of rich absorbent temperature on CO2 desorption flux was shown in Fig. 2. It is shown that desorption flux increased when temperature of rich absorbent was increased.This phenomenon due to the solubility of CO2 decreases very rapidly with the increasing of temperature.The increasing of absorbent's temperature increased the CO2 equilibrium partial pressure; thus also increased the driving force of CO2 desorption.Moreover, the reversible reaction equilibrium constant and CO2 diffusivity in DEA solution increased with the solution temperature, which leads to the improvement of desorption performance as well.The effect of temperature on these factor was studied by several researchers through experiments and mathematical modeling also which reported the similar result [2, 5, 13, 15, 17,].However, the membrane stability must be considered against high temperature operation due to susceptibility to chemical and thermal effect.

Effect of Temperature on CO2 Loading of Lean Absorbent
The effect of the rich absorbent temperature on CO2 loading was shown by Fig. 3.It is shown for each type of absorbent with the same CO2 loading of rich amine value of 0.22 mol CO2/mol DEA, CO2 loading of lean absorbent decrease with increasing absorbent temperature.In other words, the higher the temperature of the solvent, the more the amount of CO2 released from the solvent and cause the amount of CO2 contained in the lean amine solvent decreases.The parameter α is the content of CO2 dissolved in the solution from both physical and chemical absorption.It is expressed as moles of CO2 per mole of amine.

Effect of Activator on Desorption Efficiency
Value of desorption efficiency expresses the performance of CO2 regeneration process.It is shown in Fig. 4. At constant liquid flow rate of 100 mL/min and pressure of 20 kPa, efficiency increased with the increase of liquid temperature for any given absorbent.It was caused by the chemical reaction kinetics and CO2 diffusion coefficient.At higher temperatures, the dissolved CO2 concentration in the solvent moved to the low-concentration side and increased CO2 mass transfer driving force.Many researchers had conducted research about the effects of temperature on desorption through experiments and mathematical modeling which reported similiar results [2,13,15,17].
The experimental results showed that the DEA-K2CO3 solvent was the best desorption efficiency.The K2CO3 solution has an ionic structure, and bicarbonate and carbonate ions were formed by reacting with the CO2 in the solution.Therefore, bicarbonate and carbamate ions in reaction was unstable which caused the flux of desorption increase and the partial CO2 equilibrium pressure higher.

Conclusion
Desorption of CO2 from activated DEA using membrane contactor with vacuum regeneration technology was achieved in polypropylene hollow fiber membrane contactor using membrane vacuum regeneration technology.Arginine, PZ, and K2CO3 as activators were added into DEA solution to form aqueous solutions of activated DEA.The three activator are the liquid phase inlet and outlet CO2 concentration.Ql is volumetric flow rate of solvent.A represent the liquid-gas contact area in the membrane module.