Experimental Study of a Hydrophobic Solvent for Natural Gas

Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

pubs.acs.org/jced

Experimental Study of a Hydrophobic Solvent for Natural Gas Sweetening Based on the Solubility and Selectivity for Light Hydrocarbons (CH4, C2H6) and Acid Gases (CO2 and H2S) at 298−353 K Jak Tanthana, Aravind V. Rayer, Vijay Gupta, Paul D. Mobley, Mustapha Soukri, Jim Zhou, and Marty Lail* Downloaded via IOWA STATE UNIV on January 25, 2019 at 19:07:21 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

RTI International, 3040 East Cornwallis Road, Durham, North Carolina 27709, United States S Supporting Information *

ABSTRACT: Henry’s constants of H2S, CO2, CH4, and C2H6 in a hydrophobic solvent (HPS) consisting of 2-fluorophenethylamine (2-FPEA)/4-methoxy phenol (MePhOH)/a mixture of polyethylene glycol dibutyl ether (Genosorb 1843) were determined to evaluate the potential use of the HPS for natural gas sweetening applications. In addition, the Henry’s constants of H2S, CO2, CH4, and C2H6 in an industrial solvent, promoted methyldiethanolamine (MDEA)/piperazine (PZ)/water, were also reported in this work. The gas solubilities were evaluated in the temperature range 298.15−353.15 K. The Henry’s constant of N2O in the HPS was also determined and used to differentiate the physical absorption of acid gases from the chemical absorption. The temperaturedependent Henry’s constant correlations of these gases were developed and used to determine the separation performance of the HPS in a simple absorber process simulation. The simulation results suggest that HPS can remove acid contaminants and achieve the targeted quality of natural gas for liquid natural gas (LNG) production. The HPS exhibits rather high CH4 and C2H6 absorption and, consequently, has low acid gas selectivity compared to the commercially available physical solvent. The acid gas selectivity can be optimized with the inclusion of a limited quantity of hydrophilic component to enhance acid gas solubility while still minimizing hydrocarbon solubility.

■

INTRODUCTION Gas sweetening is a critical process that removes the acid contaminants such as CO2, H2S, and COS from the natural gasa mixture of CH4, C2H6, and C3+to purify it for pipeline transportation requirements or to be used in chemical production.1−3 The absorption mechanism is classified as physical or chemical absorption depending on the intermolecular bonds formed during the absorption process. The physical absorption utilizes the weak intermolecular forces such as van der Waals and dipole−dipole interactions to hold the contaminants within the liquid phase and strongly depends on the partial pressure of these contaminants. The regeneration of physical solvents requires minimal energy and can be achieved through a pressure swing, a vacuum swing, or a mild temperature swing process. While relatively simple regeneration is possible for physical solvents, the absorption efficiency of physical solvents is reduced when removing the diluted species from the bulk gas, as the technique heavily relies on the large driving force of the absorbed species (i.e., high pressure or high concentration). On the contrary, the chemical absorption relies on strong intermolecular forces such as ionic and covalent bonds that form during the chemical reaction between acid gases and the absorption liquid.4−7 Thus, chemical solvents are much more effective in removing low concentration contaminants, as the efficiency is governed by reaction stoichiometry and © XXXX American Chemical Society

equilibrium. However, the regeneration process for chemical solvents requires significantly more energy than that for physical solvents and is typically achieved through a temperature swing process. Examples of commercially available physical solvents for natural gas sweetening are Selexol (mixture of polyethylene glycol dimethyl ethers) and Rectisol (refrigerated methanol), while chemical solvents include monoethanolamine (MEA), diethanolamine (DEA), diisopropanolamine (DIPA), and promoted methyl diethanolamine (promoted MDEA with piperazine). The absorption capacity (i.e., gas solubility) of the physical solvents is characterized by the quantity of the dissolved gas in a unit volume and defined by the Henry’s law solubility constant (H). The constant is determined under a given temperature and is specific to the gas−liquid components. The larger the Henry’s law constant, the lower the gas solubility in the liquid. The Henry’s law constant serves as a critical screening factor for physical solvents.8−10 In addition, the ratio between the Henry’s law constant of the desired product and contaminants is the gas selectivity (eq 1),11 and it also plays a crucial role in selecting Received: August 17, 2018 Accepted: January 7, 2019

A

DOI: 10.1021/acs.jced.8b00735 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

■

MATERIALS All of the chemicals used in this work are listed in Table 1 with their purities. They were purchased and used without further

physical solvents that can maximize the contaminant removal while minimizing the absorption of valuable products. Ideal Gas Selectivity Solubility of product = Solubility of contaminant H = contaminant Hproduct

Table 1. Chemical Names and Molecular Structures of the Studied Chemicals in This Work

(1)

Our lab has previously developed hydrophobic solvents (HPS) for postcombustion CO2 capture from flue gas; however, the solvents have both physical and chemical absorption properties which are applicable for the removal of acid contaminants present in natural gas. HPS is a hybrid solvent which is composed of both chemical (amine functionalized species) and physical (organic diluent) solvents. The main development goal of the HPS for CO2 capture applications is to reduce the energy penalty incurred during solvent regeneration. Significant advancement in the HPS technology has resulted in a solvent that exhibits a low solvent regeneration energy, high acid contaminant separation efficiency, low dynamic viscosity, low vapor pressure, and high resistance to thermal degradation.12,13 Fluorinated hydrophobic amines were found to have low Hazardous Materials Identification System (HMIS) health ratings14 (0 to 1) and National Fire Protection Association (NFPA) health ratings15 compared to other high-performing amines such as MEA and PZ (3 to 4). The physical properties and reactivity toward acid components in the gas stream not only allow HPS to be used for CO2 capture but also create an opportunity for gas sweetening and separation applications. This work presents an early development effort to evaluate the technical viability of HPS technology for gas sweetening applications. The solubilities of H2S, CO2, CH4, and C2H6 in HPS were evaluated, and the gas selectivities of the H2S and CO2 over the natural gas products, CH4 and C2H6, were determined in this study. The physical absorption contribution by the organic diluent is confounded with the chemical absorption by the amine species because of the hybrid nature of the HPS. In order to distinguish the contribution of the two absorption modes, N2O is used as a surrogate species to determine the physical absorption part of HPS. The method is described thoroughly by Versteeg and Van Swaaij.16 The physical absorption of the CO2 and H2S in a given solvent can be described using the N2O analogy proposed by Versteeg and Van Swaaij16 and Huttenhuis et al.17 and given as the following equations: ij HCO ,water yz 2 zz HCO2,Solvent = HN2O,Solventjjjj z j HN O,water zz k 2 { ij HH S,water yz 2 zz HH2S,Solvent = HN2O,Solventjjjj z j HN O,water zz { k 2

Article

purification. The hydrophobic solvent (HPS) was prepared by mixing 25−40 wt % amines (2FPEA) with a hydrophobic diluent (Genosorb 1843a mixture of polyethylene glycol dibutyl ethers: 1.9 wt % diethylene glycol dibutyl ether (112-732), 67.9 wt % diethylene glycol dibutyl ether (63512-36-7), 0.25−1 wt % mixture of N,N'-diaryl-p-phenylenediamine (68953-84-4), and 0.25−1 wt % phenylenediamines (273-2278) (remaining compositions are confidential with Clariant). Promoted MDEA was prepared by mixing 49.26 wt % MDEA (4.13 mol/kg) with 0.83 wt % PZ (0.1 mol/kg) and water, as reported by Xu et al.18 An analytical balance (Mettler-Toledo) with a precision of ±0.01 g was employed to prepare gravimetrically.

■

EXPERIMENTAL PROCEDURE The solubility was measured in a stirred-reactor vessel supplied by Chemisens AB, with an automated gas handling system. A diagram of the experimental setup is given in Figure 1. The reactor is a cylindrical, double-walled glass vessel with two, fourblade propeller stirrers to mix both liquid and gas phases. A detailed description of the experimental apparatus was presented in our previous work.12 During each experiment, gases are supplied by compressed gas cylinders through SV-1 (Figure 1) to the batch vessel (BV). The pressure of the BV was monitored by a pressure transducer (PBV), and it could equilibrate at the incubator temperature (Tincubator). A flow control valve (FCV) regulates the flow from the batch vessel to the feed manifold and reactor. The feed manifold between solenoid values (SV-5, SV-6, and SV-7) contains a pressure transducer (PMF) to monitor the increase in pressure of the manifold at Tincubator. The pressure in the reactor vessel (RV) is monitored by a pressure transducer (PRV), and the reactor temperature (TRV) is controlled and measured by a CPA-102 reaction calorimeter.

(2)

(3)

Solubilities of N2O and CO2 in water were taken from the work of Versteeg and Van Swaaij,16 while solubilities of H2S in water were taken from Kohl and Nielsen.5 A simplified HPS-based gas sweetening process using a gas−liquid absorber was performed in this work to validate the technical feasibility and project the acid removal performance of HPS. B



Article

Figure 1. Schematic diagram of the experimental setup used in this work.

Approximately 100 mL of solution (Vsolvent) was placed in the reactor in each experiment. The reactor was filled with N2, vented, and then evacuated by a vacuum several times before each experiment to remove atmospheric gases and degas the solvent. The reactor vessel was then isolated from the rest of the system, and the feed manifold was purged and then evacuated with the test gas several times to remove the N2. The feed manifold was then filled with the test gas to a pressure greater than the reactor vessel pressure to prevent back flow. The gas in the batch vessel and feed manifold was allowed to reach thermal equilibrium with the incubator temperature. The targeted reactor temperature was maintained by a circulation bath for at least 2 h. Once thermal equilibrium was reached, the test gas was introduced to the reactor from the batch vessel until the reactor pressure reached 170 kPa. The reactor was then allowed to reach vapor−liquid equilibrium, which was determined by monitoring the change in pressure in the reactor (PRV,final). The moles of gas absorbed in liquid and gas phase were calculated using eqs 4−9 below

Change in the number of moles of gas in the gas phase in the reactor vessel: Pgas,R =

ngas,R =

ngas,BV

PFM,initial yzz (VFM) ijj PFM,final jj zz − j RTincubator k Z FM,final Z FM,initial z{

(6)

Pgas,RV × (VRV − Vsolvent) RTRV

(7)

Moles of the gas absorbed in the liquid phase: ngasinLiquidphase = ngas , BV + ngas , FM − ngas , RV

(8)

Moles of the gas absorbed in the gas phase: ngasingasphase = ngas,RV

(9)

The Henry’s constant was calculated in kPa·m3·kmol−1 as

(4)

ÅÄÅ ÑÉ ÅÅ Cgas in gas phase ÑÑÑ Å ÑÑ × (R × T ) kPa·m 3·kmol−1 (Hgas) = ÅÅ ÅÅ Cgas in liquid phase ÑÑÑ ÅÇ ÑÖ

follows:

Change in the number of moles of the gas in the feed manifold: ngas,FM =

Z RV,final

where ZBV, ZFM, and ZRV are the compressibility factors calculated using the Peng−Robinson EOS at the desired temperature and pressure of each gas.

Change in the number of moles of the gas in the batch vessel: PBV,final yzz (VBV ) ijj PBV,initial jj zz = − j RTincubator k Z BV,initial Z BV,final z{

(PRV,final − PRV,initial)

(5)

(10) C


Journal of Chemical & Engineering Data ÅÄÅ ÑÉ ÅÅ ngas in gas phase (kmolgas) ÑÑÑ Å ÑÑ Hgas = ÅÅ ÅÅ ngas in liquid phase (kmolliquid) ÑÑÑ ÅÇ ÑÖ ÅÄÅ ÑÉ 3 3 ÅÅ Vliquid phase (m of liquid) ÑÑÑ Å ÑÑ × R ijjj kPa·m yzzz × ÅÅÅ Ñ j kmol ·K z ÅÅ Vgas phase (m 3 of gas) ÑÑÑ k { ÅÇ ÑÖ

Hgas

× T (K) ÄÅ É ÅÅ ngas ÑÑÑ ÅÅ ÑÑ Ñ× = ÅÅ ÅÅ nliquid ÑÑÑ ÑÖ ÅÇ

Article

ÄÅ ÉÑ 3 ÅÅ ÑÑ Vsolvent ÅÅ ÑÑ × R ijjj kPa·m yzzz ÅÅ ÑÑ j kmol ·K z ÅÅÇ (VR − Vsolvent − Δvex ) ÑÑÖ { k

(11)

× T (K)

(12)

Δvex (m of liquid) is the correction for volume expansion, which is calculated as below19 3

Δvex = βv0ΔT

Figure 3. Solubility of CH4 in HPS and promoted MDEA. Trend lines are calculated from eq 10. [* values calculated using selectivity of solvents (HCO 2/HCH 4) reported in the literature (Bucklin and Scheendel, 1984).30 HCO2 is taken from the literature as follows: for Selexol (Xu, 1990);31 for hydrophilic Genosorb 1753 (Rayer et al., 2012);32 for hydrophobic Genosorb 1843 (Clariant, 2002).33]

(13)

where β is the volume thermal expansion coefficient of solvent (assumed similar to water − 8.22 × (T − 273) − 2237 × 10−6/ K).19 v0 is the initial volume of the solvent (vsolvent). ΔT is the temperature difference between the reference room temperature, TR (295.7 K), and the experimental temperature, T (K). The selectivity of each gas component with respect to m kPa· m3·kmol−1 ethane (CH4) was calculated as Ideal Gas Selectivity =

HCH4 Solubility of gas = Solubility of CH4 Hgas

temperatures. The physical solubility trends of CH4 obtained in HPS and physical solvents are reversed compared to that of promoted MDEA. Physical solubility of CH4 increases with the increase in temperature for promoted MDEA. Partition coefficient (log P) values were used in this work to define hydrophobicity. Log P values less than absolute 0 are defined as “hydrophilic”, and those higher than absolute 0 are defined as “hydrophobic”. However, for HPS, Selexol, hydrophilic Genosorb 1753 (log P = −1.51) (a mixture of polyethylene glycol dimethyl ethers: 4 wt % triethylene glycol dimethyl ether (CAS No.: 112-49-2) and 3 wt % tetra ethylene glycol dimethyl ether (CAS No.: 143-24-8), remaining compositions are confidential with Clariant), and hydrophobic Genosorb 1843 (log P = 2.03), the solubility of CH4 decreases when the temperature increases. The different behavior observed for the acid-gas free promoted MDEA system under the given conditions can be attributed to the well-known salting in effect observed for solutions of low ionic strength. This trend is consistent with the general conclusion that the solubility of hydrocarbons in amine solutions increases with an increase in temperatures, as reported by Carroll and Mather.29 As observed in Figure 3, the Henry’s constant of hydrophobic Genosorb 1843 is consistently lower than hydrophilic Genosorb 1753 across the temperature range. This is likely due to a higher degree of van der Waals interactions between methane and butyl end-groups of the glycol ethers of Genosorb 1843 compared to methyl end-groups of Genosorb 1753. In the case of HPS, the addition of 2FPEA to Genosorb 1843 increases methane solubility over the temperature range (as observed in the decreased Henry’s constant). However, the Henry’s constant increases only slightly with temperature. Two explanations can be postulated to explain the HPS behavior. The first is that the increase in methane solubility upon addition of 2FPEA to Genosorb 1843 is due to added van der Waals effects from the aromatic amine. The second is the salting in effect. At this point, the explanations are purely speculative and evidence distinguishing the two has not been obtained. The results also indicate that the solubility of CH4 in HPS is 5−6 times higher than promoted MDEA; therefore, it will absorb more CH4 from natural gas streams compared to promoted MDEA at 300.15−400.15 K.

(14)

To validate the experimental procedure and setup, the solubility of nitrous oxide (N2O) in water was measured at temperatures of 313.15, 333.15, and 353.15 K (sample experimental calculations were given in the Supporting Information). The results agreed well and were found to be within the range of experimental values compared to available literature,16,20−28 as shown in Figure 2. The absolute average deviation of the experimental data was found to be ±4%.

■

RESULTS AND DISCUSSION Physical Solubility of CH4 and C2H6 in HPS and Promoted MDEA. Figure 3 shows the experimental and predicted data for the physical solubility of CH4 in HPS and promoted MDEA compared with commercial physical solvents (Selexol, Genosorb 1753, and Genosorb 1843) at different

Figure 2. Comparison of the solubility of N2O in water to literature experimental values.16,20−28 D



Article

HPS and promoted MDEA, the physical solubility of CO2 and H2S in HPS and promoted MDEA cannot be measured directly. Therefore, the Henry’s constants of CO2 in HPS and promoted MDEA were estimated using an N2O analogy using eq 2. Solubility of N2O was also measured in the pure amine species, 2FPEA in HPS, to determine the physical absorption contributed by the amine in the HPS. Experimentally obtained Henry’s law constants are shown in Tables 2−6. An Arrhenius type of equation (eq 15) was fitted to the experimental data obtained from this work to extrapolate the Henry’s law constant in concentration dependent units (kPa·m3·kmol−1) and concentration independent units (kPa used in the process simulation in this work, converted on the basis of eq 16: concentration the basis of defining the Henry volatility by dividing the partial pressure by the aqueous-phase molar concentration)34 at different temperatures where coefficients A and B are given in Table 7. It should be noted that this conversion includes the molecular weight of the solvent and becomes temperature dependent when molarity units are used, since the solvent density at the desired temperature is needed.35

Figure 4 shows the experimental and predicted data for the physical solubility of ethane (C2H6) obtained from the same

Figure 4. Solubility of C2H6 in HPS and promoted MDEA. Lines are calculated from eq 10. [* values calculated using selectivity of solvents (HCO2/HC2H6) reported in the literature (Bucklin and Scheendel, 1984).30 HC2H6 is taken from the literature as follows: for Selexol (Xu, 1990);31 for Genosorb 1753 (Rayer et al., 2012);32 for Genosorb 1843 (Clariant, 2002).33]

iBy Hgas,solvent = A expjjj zzz kT { Hgas,solvent (kPa) =

(15)

ρsolvent M. wt solvent × Hgas,solvent (kPa· m 3· kmol−1)

studied solvents. The Henry’s constants for ethane in these solvents are much lower than that of methane, indicating the absorption has a higher affinity toward larger hydrocarbons. Physical Solubility of Acidic Gases (CO2 and H2S) in HPS and Promoted MDEA through N2O Analogy. Solubilities of N2O, CO2, H2S, and CO in HPS and promoted MDEA (promoted MDEA) were measured at 298.15, 313.15, 333.15, and 353.15 K. Since CO2 and H2S react chemically with

(16)

The N2O solubilities of these solvents at different temperatures are shown in Figure 5. The comparison of the Henry’s constants of CO2 and H2S for the solvents in this work at 298.15 K is shown in Table 8 and Table 9. On the basis of the N2O data, it is estimated that the physical solubility of CO2 in 2FPEA is higher than that in other pure amines MEA,36 DIPA,36 and

Table 2. Henry’s Law Constant (HN2O) for Solubility of N2O in Solvents (mN2O) with Their Concentrations (CS) and Densities (ρ) at Different Temperatures (T) and Partial Pressures (PN2O) Obtained in This Worka solvent water water water HPS (2FPEA/MePhOH/Genosorb) HPS (2FPEA/MePhOH/Genosorb) HPS (2FPEA/MePhOH/Genosorb) HPS (2FPEA/MePhOH/Genosorb) HPS (2FPEA/MePhOH/Genosorb) 2FPEA 2FPEA 2FPEA 2FPEA promoted MDEA (MDEA/PZ/H2O) promoted MDEA (MDEA/PZ/H2O) promoted MDEA (MDEA/PZ/H2O) promoted MDEA (MDEA/PZ/H2O)

gas N2O N2O N2O N2O N2O N2O N2O N2O N2O N2O N2O N2O N2O N2O N2O N2O

T (K) 298.15 313.15 353.15 298.15 298.15 313.15 333.15 353.15 298.15 313.15 333.15 353.15 298.15 313.15 333.15 353.15

PN2O (kPa)

mN2O (mol/kg)

55.25 73.67 35.70 37.31 73.08 35.41 53.02 28.15 32.00 136.50 24.80 33.91 44.46 40.30 53.37 28.60

0.013 0.014 0.003 0.022 0.043 0.018 0.020 0.010 0.024 0.078 0.012 0.013 0.008 0.007 0.008 0.004

CS (mol/kg) 55.52 55.52 55.52 2.4/1.44/2.24 2.4/1.44/2.24 2.4/1.44/2.24 2.4/1.44/2.24 2.4/1.44/2.24 7.19 7.19 7.19 7.19 4.13/0.1/27.7 4.13/0.1/27.7 4.13/0.1/27.7 4.13/0.1/27.7

ρ (kg/m3) b

997.05 992.22b 971.79b 988.77c 988.77c 977.42c 960.03c 938.15c 997.00d 992.00d 983.00d 972.00d 1040.58e 1030.98e 1024.21e 1014.53e

HN2O (kPa·m3·kmol−1) 4114 ± 18.1 5453.7 ± 54.3 12762.4 ± 137.7 1672 ± 18.1 1692.06 ± 18.1 2012.5 ± 54.3 2651 ± 97.6 3036.3 ± 137.7 1354.7 ± 18.1 1759.9 ± 54.3 2168.6 ± 97.6 2725.8 ± 137.7 5233.9 ± 18.1 5804.1 ± 54.3 6577.7 ± 97.6 7985.0 ± 137.7

a

Standard uncertainties are u(T) = 0.01 K, u(Pgas) = 6.7 kPa, u(mN2O,water) = 0.003 mol/kg, u(mN2O,2FPEA/MePhOH/Genosorb) = 0.005 mol/kg, u(mN2O,2FPEA) = 0.0136 mol/kg, u(promoted MDEA) = 0.001 mol/kg. The expanded uncertainties (0.95 level of confidence) for Hgas are given in the table. The standard uncertainty in the concentration of solvent was estimated to be 0.0002 mol/kg for 2-FPEA, 0.0006 mol/kg for MDEA, 0.0557 mol/kg for MePhOH, 0.0660 mol/kg for Genosorb 1843, and 0.2145 mol/kg for PZ. The standard uncertainties in the concentration of HPS and promoted MDEA were estimated to be 0.0408 and 0.0870 mol/kg. bEngineering Tool box.37 cRayer et al.38 dRichner et al.39 eDerks et al.40 E



Article

Table 3. Henry’s Law Constant (HCO2) for Solubility of CO2 in Solvents (mCO2) with Their Concentrations (CS) and Densities (ρ) at Different Temperatures (T) and Partial Pressures (PCO2) Obtained in This Worka solvent HPS (2FPEA/MePhOH/Genosorb) HPS (2FPEA/MePhOH/Genosorb) HPS (2FPEA/MePhOH/Genosorb) HPS (2FPEA/MePhOH/Genosorb) 2FPEA 2FPEA 2FPEA 2FPEA promoted MDEA (MDEA/PZ/H2O) promoted MDEA (MDEA/PZ/H2O) promoted MDEA (MDEA/PZ/H2O) promoted MDEA (MDEA/PZ/H2O)

gas CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2

T (K) 298.15 313.15 333.15 353.15 298.15 313.15 333.15 353.15 298.15 313.15 333.15 353.15

PCO2 (kPa)

mCO2 (mol/kg)

73.08 35.41 53.02 28.15 32.00 136.50 24.80 33.91 44.46 40.30 53.37 28.60

0.022 0.018 0.020 0.010 0.024 0.078 0.012 0.013 0.008 0.007 0.008 0.004

CS (mol/kg) 2.4/1.44/2.24 2.4/1.44/2.24 2.4/1.44/2.24 2.4/1.44/2.24 7.19 7.19 7.19 7.19 4.13/0.1/27.7 4.13/0.1/27.7 4.13/0.1/27.7 4.13/0.1/27.7

ρ (kg/m3) b

988.77 977.42b 960.03b 938.15b 997.00c 992.00c 983.00c 972.00c 1040.58d 1030.98d 1024.21d 1014.53d

HCO2 (kPa·m3·kmol−1) 1236 ± 18.1 1431.4 ± 54.3 1800.8 ± 97.6 1980.1 ± 137.7 1001.5 ± 18.1 1251.8 ± 54.3 1473.1 ± 97.6 1777.6 ± 137.7 3869.1 ± 18.1 4128.3 ± 54.3 4468.2 ± 97.6 5207.3 ± 137.7

a

Standard uncertainties are u(T) = 0.01 K, u(Pgas) = 6.7 kPa, and u(mgas) = 0.005 mol/kg. The expanded uncertainties (0.95 level of confidence) for Hgas are given in the table. The standard uncertainty in the concentration of solvent was estimated to be 0.0002 mol/kg for 2-FPEA, 0.0006 mol/kg for MDEA, 0.0557 mol/kg for MePhOH, 0.0660 mol/kg for Genosorb 1843, and 0.2145 mol/kg for PZ. The standard uncertainties in the concentration of the HPS and promoted MDEA were estimated to be 0.0408 and 0.0870 mol/kg. bRayer et al.38 cRichner et al.39 dDerks et al.40

Table 4. Henry’s Law Constant (HCH4) for Solubility of CH4 in Solvents (mCH4) with Their Concentrations (CS) and Densities (ρ) at Different Temperatures (T) and Partial Pressures (PCH4) Obtained in This Worka solvent

gas

T (K)

PCH4 (kPa)

mCH4 (mol/kg)

CS (mol/kg)

ρ (kg/m3)

HCH4 (kPa·m3·kmol−1)

HPS (2FPEA/MePhOH/Genosorb) HPS (2FPEA/MePhOH/Genosorb) HPS (2FPEA/MePhOH/Genosorb) HPS (2FPEA/MePhOH/Genosorb) HPS (2FPEA/MePhOH/Genosorb) HPS (2FPEA/MePhOH/Genosorb) promoted MDEA (MDEA/PZ/H2O) promoted MDEA (MDEA/PZ/H2O) promoted MDEA (MDEA/PZ/H2O) promoted MDEA (MDEA/PZ/H2O) promoted MDEA (MDEA/PZ/H2O)

CH4 CH4 CH4 CH4 CH4 CH4 CH4 CH4 CH4 CH4 CH4

298.15 313.15 313.15 313.15 333.15 353.15 298.15 298.15 313.15 333.15 353.15

58.39 52.13 52.57 58.78 65.98 38.65 59.95 94.66 55.96 34.50 67.59

0.008 0.007 0.007 0.007 0.008 0.004 0.002 0.004 0.002 0.002 0.003

2.4/1.44/2.24 2.4/1.44/2.24 2.4/1.44/2.24 2.4/1.44/2.24 2.4/1.44/2.24 2.4/1.44/2.24 4.13/0.1/27.7 4.13/0.1/27.7 4.13/0.1/27.7 4.13/0.1/27.7 4.13/0.1/27.7

988.77b 977.42b 988.77b 988.77b 960.03b 938.15b 1040.58d 1030.98d 1030.98d 1024.21d 1014.53d

7347.2 ± 18.1 7934.6 ± 54.3 7983.5 ± 54.3 7953.4 ± 54.3 8870.5 ± 97.6 9084.6 ± 137.7 25509.4 ± 18.1 25424.5 ± 18.1 24611.0 ± 54.3 23240.2 ± 97.6 21522.1 ± 137.7

a


Table 5. Henry’s Law Constant (HC2H6) for the Solubility of C2H6 in Solvents (mC2H6) with Their Concentrations (CS) and Densities (ρ) at Different Temperatures (T) and Partial Pressures (PC2H6) Obtained in This Worka solvent

gas

T (K)

PC2H6 (kPa)

mC2H6 (mol/kg)

CS (mol/kg)

ρ (kg/m3)

HC2H6 (kPa·m3·kmol−1)

HPS (2FPEA/MePhOH/Genosorb) HPS (2FPEA/MePhOH/Genosorb) HPS (2FPEA/MePhOH/Genosorb) HPS (2FPEA/MePhOH/Genosorb) promoted MDEA (MDEA/PZ/H2O) promoted MDEA (MDEA/PZ/H2O) promoted MDEA (MDEA/PZ/H2O) promoted MDEA (MDEA/PZ/H2O)

C2H6 C2H6 C2H6 C2H6 C2H6 C2H6 C2H6 C2H6

298.15 313.15 333.15 353.15 298.15 313.15 333.15 353.15

78.22 60.68 57.11 49.01 88.20 83.02 68.30 39.17

0.031 0.022 0.019 0.014 0.005 0.005 0.005 0.003

2.4/1.44/2.24 2.4/1.44/2.24 2.4/1.44/2.24 2.4/1.44/2.24 4.13/0.1/27.7 4.13/0.1/27.7 4.13/0.1/27.7 4.13/0.1/27.7

988.77b 977.42b 960.03b 938.15b 1040.58d 1030.98d 1024.21d 1014.53d

2664.0 ± 18.1 2866.9 ± 54.3 3174.3 ± 97.6 3732.1 ± 137.7 16805.4 ± 18.1 15692.2 ± 54.3 15145.9 ± 97.6 14178.2 ± 137.7

a

Standard uncertainties are u(T) = 0.01 K, u(Pgas) = 6.7 kPa, and u(mgas) = 0.005 mol/kg. The expanded uncertainties (0.95 level of confidence) for Hgas are given in the table. The standard uncertainty in the concentration of solvent was estimated to be 0.0002 mol/kg for 2-FPEA, 0.0006 mol/kg for MDEA, 0.0557 mol/kg for MePhOH, 0.0660 mol/kg for Genosorb 1843, and 0.2145 mol/kg for PZ. The standard uncertainties in the concentration of the HPS and promoted MDEA were estimated to be 0.0408 and 0.0870 mol/kg. bRayer et al.,38 cRichner et al.,39 dDerks et al.40

MDEA.36 Furthermore, the physical solubility of CO2 in HPS is higher than the physical solubilities of CO2 in pure amines. Ideal Selectivity of Hydrocarbons in HPS and Promoted MDEA. Since HPS is a mixture of physical and

chemical solvents, the solubility of gases in HPS is due to both the physical and chemical absorption. In the absence of acid gas, high methane solubility is observed in HPA, but this could be reversed upon the absorption of acid gas in accordance with the F



Article

Table 6. Henry’s Law Constant (HH2S) for Solubility of H2S in Solvents (mH2S) with Their Concentrations (CS) and Densities (ρ) at Different Temperatures (T) and Partial Pressures (PC2H6) Obtained in This Worka solvent

gas

HPS (2FPEA/MePhOH/Genosorb) HPS (2FPEA/MePhOH/Genosorb) HPS (2FPEA/MePhOH/Genosorb) HPS (2FPEA/MePhOH/Genosorb) promoted MDEA (MDEA/PZ/H2O) promoted MDEA (MDEA/PZ/H2O) promoted MDEA (MDEA/PZ/H2O) promoted MDEA (MDEA/PZ/H2O)

H2S H2S H2S H2S H2S H2S H2S H2S

T (K)

PH2S (kPa)

298.15 313.15 333.15 353.15 298.15 313.15 333.15 353.15

mH2S (mol/kg)

73.08 35.41 53.02 28.15 44.46 40.30 53.37 28.60

0.022 0.018 0.020 0.010 0.008 0.007 0.008 0.004

CS (mol/kg)

ρ (kg/m3) b

2.4/1.44/2.24 2.4/1.44/2.24 2.4/1.44/2.24 2.4/1.44/2.24 4.13/0.1/27.7 4.13/0.1/27.7 4.13/0.1/27.7 4.13/0.1/27.7

988.77 977.42b 960.03b 938.15b 1040.58d 1030.98d 1024.21d 1014.53d

HH2S (kPa·m3·kmol−1) 362.8 ± 18.1 411.7 ± 54.3 475.6 ± 97.6 454.5 ± 137.7 1135.6 ± 18.1 1187.3 ± 54.3 1180.1 ± 97.6 1195.4 ± 137.7

a


Table 7. Coefficients for eq 15 (A and B) to Predict Henry’s Law Constant (Hgas) for Different Gases in Different Solvents concentration-dependent Henry’s constant35 (kPa·m3·kmol−1) eq 12

A

concentration-independent (kPa) B

A

B

−1072.7 −833.9 −949.9 −856.8 −1001.4 −529.1

19.9 20.3 19.4 18.8 19.2 20.6

−1072.7 −896.5 −963.8 −856.3 −1001.2 −529.0

−357.9 360.3

18.8 19.5

−357.7 360.1

−603.3 355.7

18.6 19.1

−603.2 355.7

−127.0 173.6

15.2 17.0

−136.4 191.8

CO2 MDEA MEA DIPA HPS (2FPEA/MePhOH/Genosorb) 2-FPEA promoted MDEA (MDEA/PZ/H2O)

50376.1 32840.0 32952.1 22293.0 29597.6 22345.5

HPS (2FPEA/MePhOH/Genosorb) promoted MDEA (MDEA/PZ/H2O)

24424.8 7616.0

HPS (2FPEA/MePhOH/Genosorb) promoted MDEA (MDEA/PZ/H2O)

19740.3 5076.7

HPS (2FPEA/MePhOH/Genosorb) promoted MDEA(MDEA/PZ/H2O)

612.6 666.3

CH4

C2H6

H2S

Table 8. Ranking of Henry’s Constant of CO2 in Increasing Order at 298.15 K (Inverse of Physical Solubility) solvent

HCO2 (kPa·m3·kmol−1)

2FPEA HPS (2FPEA/MePhOH/Genosorb) DIPA MDEA MEA promoted MDEA (MDEA/PZ/H2O)

1001.5 ± 18.1 1236.0 ± 18.1 1303.6 ± 65.2 1379.3 ± 69.0 1946.9 ± 97.3 3869.1 ± 18.1

Table 9. Ranking of Henry’s Constant of H2S in Increasing Order at 298.15 K (Inverse of Physical Solubility)

Figure 5. Physical solubility of N2O in HPS compared to pure solvents. Lines are calculated from eq 15.

salting out effect observed in aqueous amine solvents. Selectivities for various gases of HPS determined from this work are compared to the commercially available physical and chemical solvents30 in Table 10. Physical solvents compared with this work include diethyl ether glycol polyethylene glycol, DEPG (Selexol); N-methyl-2-pyrrolidone, NMP (Purisol); tri

solvent

HH2S (kPa·m3·kmol−1)

HPS (2FPEA/MePhOH/Genosorb) promoted MDEA (MDEA/PZ/H2O) 2FPEA DIPA MDEA MEA

362.8 ± 18.1 1135.6 ± 18.1 1315.4 ± 18.1 1712.8 ± 85.6 1812.2 ± 90.6 2558.0 ± 127.9

ethylene glycol dimethyl ether, TEGDBE (Genosorb 1753); tetra ethylene glycol dibutyl ether, TEGDBE (Genosorb 1843); and refrigerated methanol (Rectisol). Chemical solvents G



Article

Table 10. Comparison of the Selectivity of Gases (eq 1) in Commercial Solvents at 298.15 K physical solvent solvent

DEPG(Selexol)a

NMP (Purisol)a

CH4 C2H6 CO2 H2S

1.0 6.3 14.9 133.3

1.0 4.5 13.9 141.7

chemical solvent

TEGDBE (Genosorb 1843)b

TEGDME (Genosorb 1753)b

MeOH (Rectisol)c

MEAd

1.0 6.0 6.5 31.2

1.0 6.5 15.5 105.0

1.0 8.2 19.6 138.4

1.0 1.7 44.7 13.3

hybrid solvent

DEAd

promoted MDEAe

2FPEA/MePhOH/Genosorb (HPS)e

1.0 2.4 58.4 18.7

1.0 1.5 6.6 22.5

1.0 2.8 5.9 19.9

Bucklin and Scheendel.30 bClariant.33 cData at −25 °C from Burr, 2009.4 dCarroll and Mather.29 eThis work.

a

Table 11. Equilibrium Constants Used in the ENRTL-SR Model equation

A

B

C

D

reference

17 18 19 20 21 22

132.899 −32.95 −241.78 −0.7426 −13.798 −32

−13446 −17.284 −24.614 −681.5 −1874.4 −3338

−22.478 −0.0067 41.6654 −0.0807 0 0

0 0.06733 −0.0609 0.00369 0 0

Austgen (1989)46 this work this work this work this work Austgen (1989)46

Physical Solubility. The physical solubility of gases in the liquid phase is given by the Henry’s constant, and the vapor− liquid equilibrium between gases and dissolved gases given as

considered for comparison include monoethanolamine, MEA, and diethanolamine, DEA. Interestingly, in the absence of acid gas, the gas selectivity of HPS is remarkably close to Genosorb 1843, the major component in the HPS formulation. The apparent selectivity must be artificially low due to the small magnitude of Hcont used in eq 1. In the presence of acid gas, the actual selectivity for acid gases would be enhanced by the salting out effect, increasing the magnitude of Hcont substantially. In fact, according to this calculation, the selectivity of Genosorb 1843 for CO2 is reduced by the addition of 2FPEA, which cannot be the case in reality. Moreover, the CO2 and H2S selectivities of HPS which seem low, similar to both promoted MDEA and Genosorb 1843, calculated using Hcont of acid-gas lean solvents with low ionic strength and substantial quantity of hydrocarbons “salted in”, must be taken with caution. Therefore, solvent development is clearly needed to explain the selectivity for acid gases while minimizing the uptake of CH4 and C2H6 to use HPS for natural gas sweetening processes.

Pyi φi = Hixiγi*

where P is the system pressure, yi is the mole fraction of gases in the vapor phase, φi is the gas fugacity coefficient in the vapor phase, Hi is the Henry’s law constant in the mixed solvent, xi is the gas mole fraction in the liquid phase, and γi* is the symmetric activity coefficient normalized to the mixed solvent infinite dilution reference state. The Henry’s law constant of the mixed solvent can be calculated from the pure solvent properties as ij H yz lnjjjj ∞i zzzz = k γi {

■

wA =

MODELING OF SOLUBILITY OF GASES IN ASPEN PLUS Physical solubilities of gases (CO2, H2S, CH4, C2H6) measured in this work were used along with the chemical solubility of polar gases (CO2 and H2S) in our previous work to develop an Aspen Plus process model for HPS in gas purification processes using an electrolyte-non-random two liquid symmetric reference (ENRTL-SR) model41 as described in the following sections. Chemical Solubility. The ionic reactions considered in the liquid phase are K1

H 2O ⇔ H+ + OH− K2

K4

2(2FPEA) + CO2 ⇔ 2FPEACOO + 2FPEAH K5

−

H 2S ⇔ S + H K6

+

HS ⇔ S− + H+

+

A

(23)

xA(V iA∞)2/3 ∑B xB(V iB∞)2/3

∫p

P

0, l A

yz V iA∞ dpzzz z {

(24)

(25)

where HiA is the Henry’s constant of gas in pure solvent A, γ∞ i and γ∞ iA are the infinite dilution activity coefficients of gas in the solvent and mixed solvent, xA is the mole fraction of solvent A, V∞ iA is the partial molar volume of gas at infinite dilution in pure solvent A calculated from the Brevli−O’Connell model,42 and HiA(T, P) is the Henry’s constant of gas in solvent A at system temperature T and pressure P. The correlation used in the ENRTL-SR model for the Henry’s constant is given as

(18) −

ij H yz iA z z ∞z z γ k iA {

∑ wA lnjjjj

ij 1 HiA(T , P) = HiA(T , pA0, l ) expjjj j RT k

(17)

2FPEA + H+ ⇔ 2FPEAH+

(22)

ln Hij = aij +

(19)

bij T

+ cij ln T + dij

(26)

Experimental data measured for the chemical solubility of CO2 and Henry’s law constant measured in this work were used to regress the binary interaction parameters, ionic interaction parameters, and Henry’s law parameters for eq 26. The chemical solubility of H2S calculated as below was used in the regression:

(20) (21) H



Article

hydrocarbons greater than C2, all components, propane and above, were lumped into the C2H6 component. The absorber was modeled using a 15.24 m tall packed bed column with 75 mm IMTP random packing. The column had 10 equilibrium stages, and the column diameter was kept at 3.66 m. On the basis of the absorber size which is commercially feasible, two trains were needed to handle the large flow rate. The absorber operates at 5200 kPag, and the pressure drop across the absorber column was fixed at 20 kPa. It was assumed that the rich solvent, loaded with CO2 and H2S, can be regenerated using a combination of pressure and thermal regeneration to obtain an acid gas stream containing CO2 and H2S with some hydrocarbons that are absorbed by the solvent and a fully regenerated lean solvent. The lean solvent is pumped to 5000 kPa pressure and is recycled to the top of the absorber column. In reality, the regenerated solvent will have some residual acid gas components which will decrease the working capacity of the solvent, resulting in a slight increase in the required solvent flow rate. Thus, the simulation serves as a test case to evaluate the potential performance of the HPS for natural gas sweetening. A process flow diagram for the Aspen model is shown in Figure 6, and the results obtained from the model are given in Table 13.

Chemical SolubilityH S,HPS

ji HCO2,HPS zyz zz × Chemical Solubility = jjjj CO2 ,HPS j HH S,HPS) zz k 2 { 2

(27)

All Henry’s law constants in units of kPa·m3·kmol−1 were converted into units of kPa and used as an input to Aspen Plus to regress the Henry’s law constant for gases in the mixed solvent. The correlation for Henry’s law constant in kPa is given by eq 16. Coefficients for different solvents are given in Table 7. Equilibrium constants for eqs 17−22 used in this work are given in Table 11. The fitted experimental data and predicted data from the ENRTL-SR model developed in this work are given in the Supporting Information. The Henry’s law constants of some of the components in HPS are not available as of now; therefore, the Henry’s law constant of the closest component groups (amines and Genosorb 1843)43−45 were assumed and used to fit the model parameters.

■

PROCESS MODELING The ENRTL-SR equilibrium model was used in an Aspen Plus process model to evaluate the applicability of the HPS for a natural gas sweetening process. A test case from the literature was chosen for the evaluation. A preliminary process model was developed which focused on contacting the sour natural gas with the HPS solvent in an absorber with a packed bed configuration and evaluating the partitioning of the acid gas components into the treated gas and the HPS solvent. Abu Dhabi Gas Liquefaction Ltd. (ADGAS), part of the Abu Dhabi National Oil Company (ADNOC), uses natural gas to produce liquefied natural gas (LNG).47,48 ADGAS operates three LNG trains. Trains 1 and 2 have been operational since 1977, while train 3 was commissioned in 1994 and uses a “Benfield HiPure”49 process that uses two different solvents to remove CO2 and sulfur H2S from the sour gas. Sweetening of the sour gas in train 3 was chosen as a representative example for evaluating the applicability of the HPS for gas sweetening. The stream conditions from train 3 are as follows: (feed gas flow rate: 532,400 Nm3·h−1; feed gas temperature: 298.18 K; feed gas pressure: 5208 kPag). The composition of the sour gas is provided in Table 12. Process requirements call for the sweet gas with no more than 50 ppmv of CO 2 and 5 ppmv of H 2 S. The low CO 2 concentrations are needed to prevent CO2 precipitation during the downstream liquefaction process for removal of higher hydrocarbons. Due to lack of thermodynamic data for

Figure 6. Schematic of the simplified absorber configuration for the natural gas clean-up model.

Table 12. Composition of Natural Gas Considered in the Model component

xgas (mole fraction)

CO2 H2S N2 CH4 C2H6 propane i-butane n-butane i-pentane n-pentane

0.0467 0.0211 0.021 0.8141 0.0561 0.0271 0.0036 0.0060 0.0013 0.0024

Results from the process simulation, based on the regressed vapor liquid equilibria from the experimental data for the solubility of the acid gases in the HPS components, clearly show the feasibility of the HPS for natural gas sweetening. The sweet gas meets the product specifications of 50 ppmv max of CO2 and 5 ppmv max of H2S. The HPS solvent is able to remove the H2S concentration to

Experimental Study of a Hydrophobic Solvent for Natural Gas

Recommend Documents