Catalytic-CO2-Desorption Studies of DEA and DEA–MEA Blended

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Catalytic-CO2‑Desorption Studies of DEA and DEA−MEA Blended Solutions with the Aid of Lewis and Brønsted Acids Huancong Shi,*,† Linna Zheng,† Min Huang,† Yuanhui Zuo,† Shifei Kang,† Yuandong Huang,*,† Raphael Idem,*,‡ and Paitoon Tontiwachwuthikul‡ †

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Department of Environmental Science and Engineering, University of Shanghai for Science and Technology, Shanghai 200093, PR China ‡ Clean Energy Technology Research Institute (CETRI), Faculty of Engineering and Applied Science, University of Regina, 3737 Wascana Parkway, Regina, Saskatchewan S4S 0A2, Canada S Supporting Information *

ABSTRACT: Heat-duty reduction is the major challenge in CO2 desorption and amine regeneration. The use of a combination of heterogeneous catalytic desorption with improved amine solvents is a novel approach to address this issue. We studied CO2-desorption tests of noncatalytic diethylamine (DEA) solvents as a benchmark and focused on five blended amines (DEA−monoethanolamine, MEA; 4.5:0.5 to 2.5:2.5 M) with three types of catalysts (γ-Al2O3, H-ZSM-5, and 2:1 blended γAl2O3−H-ZSM-5) to explore the synergy effects of DEA-based amine blends with solid catalysts. The heat duty and CO2 production of each case scenario were tested for six sets of solutions with initial loading of 0.5 mol of CO2 per mole of amine at 363−378 K and were compared with those of 5 M DEA solvents. The results showed that the three catalyst conditions (blended catalyst, H-ZSM-5, and γ-Al2O3) followed different trends at rich and lean loadings. Finally, both 5 M DEA and 4.5:0.5 M DEA−MEA with blended catalysts exhibited very low heat duties of 151.2 and 168.0 kJ per mole of CO2 at loadings of 0.50− 0.20 mol per mole of amine at 378 K among the six solutions. Both approaches proved to be the most-energy-efficient amine solutions whereas the blended amine with blended catalysts was the best strategy that was applicable in the CO2 desorber.

1. INTRODUCTION Increasing CO2 emissions have been known to create serious global warming and climate issues. Carbon capture, utilization, and storage (CCUS) is one of the most effective and efficient ways to control CO2 emissions.1 One of the capture technologies is the postcombustion-CO2-capture (PCCC) technology, which aims to absorb CO2 from flue gases of power plants via an amine-scrubbing process. The major challenge is the huge energy costs of CO2 desorption. Briefly, it is known that the solvent-regeneration process accounts for ∼70% of the total energy cost of a CO2-capture process.1,2 A review has recently discussed the specific area of CO2 desorption with chemical solvents.1 Among these conventional and novel technologies, the target is to develop promising energy-efficient approaches to reduce energy costs.1 Since 2005, many researchers have conducted CO2desorption studies to analyze and reduce energy contributions.3−6 As is widely accepted, there are three useful methods for heat-duty reduction: process intensification,3 heterogeneous catalysis,1 and solvent improvement.3 For process intensification,7−11 the focus is to improve process configurations and optimize operation parameters in order to make full use of external heat.1 The intention is also to utilize a combination of heterogeneous catalysts with newly designed © XXXX American Chemical Society

amine solutions for optimization. The new research trend is to develop the combination of catalysts and solvents with minimum energy costs. Idem et al. proposed the idea of heterogeneous catalysis in 2008,12 and Liang conducted thorough studies of the catalytic CO2 desorption of 5 M monoethanolamine (MEA) with γAl2O3, H-ZSM-5, and blended catalysts at 378 K in a batch process.13 It was shown that 60 g of γ-Al2O3, H-ZSM-5, or blended catalysts reduced the heat duties for 2 L solvents from 5.9 to 4.4, 4.4, and 4.2 MJ per kilogram of CO2, respectively.13 Srisang et al. performed full-cycle studies of CO2 capture with γ-Al2O3, HY, H-ZSM-5, and silica-alumna at 358 K14 and discovered that both γ-Al2O3 and H-ZSM-5 decreased heat duty and that the Brønsted−Lewis acid site ratio had the most influence on CO2 desorption.14 For solvent improvement, most researchers have blended primary amines with tertiary amines and have observed that an MEA−R3N solvents (for instance, MDEA) can reduce heat duties by 12−27% as compared with MEA alone as the Received: Revised: Accepted: Published: A

March 6, 2018 July 25, 2018 August 3, 2018 August 3, 2018 DOI: 10.1021/acs.iecr.8b00961 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research benchmark under various conditions.7,15−17 Such mixing is reasonable, because MEA has fast absorption rates but a small capacity that is limited to 0.50 mol/mol as well as a large reboiler heat duty.15 Tertiary amines (R3N) have slow absorption rates but large cyclic capacities of 0.7−0.8 mol/ mol and very low heat duties.15 Blending MEA with tertiary amines offsets the disadvantages and lowers the heat duty. Recently, Liu et al.18 developed CO2 desorption with MEA− DEEA and MEA−1DMA2P with and without solid catalysts. The improvement of the noncatalytic heating process is indicated by the following order: 5:1 M MEA−MDEA > 5:1 M MEA−1DMA2P > 5:1 M MEA−DEEA > 5 M MEA.18 Other researchers have studied MEA with AMP and shown that the combination of these two chemicals also facilitated CO2 desorption.19 Furthermore, some researches have also focused on the synergetic effects of heterogeneous catalysts with amine blends. Shi et al. studied the catalytic CO2 desorption of MEA−R3N, and generated an energy-surface diagram in 2014.20 Liu et al.18 conducted catalytic-CO2-desorption studies of several MEA− R3N with solid acid catalysts (H-ZSM-5, MCM-41, and SO42−/ZrO2) in 2017. The combinations 5:1 M MEA−MDEA and 5:1 M MEA−1DMA2P with H-ZSM-5 have minimized heat-duty ratios of 49−52% compared with that of MEA.18 Srisang et al.21 performed catalytic desorption of 5:2 M MEA− MDEA and 5:1.25 M MEA−DEAB with γ-Al2O3 and H-ZSM5 in 2018. The combination 5:1.25 M MEA−DEAB with HZSM-5 had the lowest heat duties.21 In summary, blending MEA with R3N in combination with solid acid catalysts has been shown to be an effective and promising method of improving the heat duties of 5 M MEA. On the basis of the advantages of catalysts with blended solvents, this study focuses on the catalytic CO2 desorption of blended diethyamine (DEA, dominant)−MEA. Other than 5 M MEA solvents, 5 M DEA solvents were compared as standards. DEA was underestimated because of its slow CO2 absorption. However, it has much lower energy costs than MEA. On the basis of a previous study in a capture plant, MEA requires 9000 kJ per kilogram of CO2 to achieve 0.22 mol/mol, whereas DEA consumes only 1500 kJ per kilogram of CO2.15 Despite the slow absorption rate, our previous study has shown that CO2−DEA absorption can be accelerated with the aid of CaCO3 and MgCO3. Moreover, blending DEA with a small amount of MEA can accelerate CO2 absorption as well. Because the disadvantage of the absorption can be offset, the combination of catalytic CO2 desorption with blended DEA− MEA solvents is worth studying to explore the potential of novel, energy-efficient amine blends in CO2 absorption− desorption processes. For this study, we aim to optimize the combinations of six DEA (dominant)−MEA−H2O solvents with five catalytic conditions from a series of catalytic-CO2-desorption tests with recirculation13,18,20 in order to discover the best ratio of amine blend and solid catalysts. The heat-duty reduction and enhanced CO2 production was the research focus. The noncatalytic 5 M DEA solvents were tested as benchmarks. The MEA (dominant)−DEA−H2O solvents were not studied because they are MEA-based solutions with MEA as the benchmark. This study included three parts: (1) Six sets of DEA (dominant)−MEA−CO2−H2O solutions with five catalytic conditions were investigated to discover different trends of heterogeneous catalysis. (2) Heat-duty calculations were conducted for 30 catalytic-desorption cases to discover

the combination that minimized the heat duty compared with that of 5 M DEA. (3) To determine the synergy effect, 5 M DEA and 4.5:0.5 M DEA−MEA with blended catalysts were exacted, and CO2-desorption tests were performed with the optimized catalysts at 378 K in an oil bath. The aim of this study was to find out the optimized combination of solid catalysts and amine blends and exhibit their synergistic effects of CO2-desorption-heat-duty reduction.

2. THEORY 2.1. Catalytic Roles of Lewis and Brønsted acids. The catalytic roles of Lewis and Brønsted acids have been previously discussed in the literature.13,20 The authors showed that the role of γ-Al2O3 was to replace part of HCO3− at the lean CO2 loading. Rosynek et al.22 performed an infrared study of CO2 desorption at 0.34 mol/mol):25 DEAH+ + HCO3− ↔ DEA + H 2O + CO2 (DEA regeneration)

(4)

MEAH+ + DEA ↔ MEA + DEAH+ (MEA regeneration) B

(5) DOI: 10.1021/acs.iecr.8b00961 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research MEA + HCO3− ↔ MEAH+ + CO32 −

(6)

CO32 − + DEAH+ ↔ DEA + HCO3−

(7)

MEA converts about 10% of HCO3− to the stronger base CO32−, and CO32− reacts directly with DEAH+ via eq 7. Carbonate can facilitate DEA regeneration, because the pKa of HCO3−/CO32− is 10.33 and the pKa of DEAH+/DEA is 8.95.23 The overall amineH+ deprotonation is given in eq 9, where the conjugate acid−base pair HCO3−/CO32− facilitates the conversion of DEAH+/DEA.

Lean region with negligible HCO3− (α < 0.34 mol/mol): DEA−COO− + H 2O ↔ DEA + HCO3−

(8)

DEAH+ + HCO3− ↔ DEA + H 2O + CO2

(4)

MEAH+ + DEA ↔ MEA + DEAH+

(5)

MEA +

HCO3−

+

↔ MEAH + CO3

CO32 − + DEAH+ ↔ DEA + HCO3− K° =

2−

(DEA−MEA blended solution)

(6)

CO32 − + DEAH+ ↔ DEA + HCO3−

[CO3 ][DEAH ]

=

K a(DEA) K a(HCO3−)

↔ 3DEA + MEAH+ + CO2

= 1010.33 − 8.95

(9)

In conclusion, bicarbonate (HCO3−) reduces the reaction energies of both CO2 desorption and the proton transfer of amineH+ deprotonation.20 Carbonate (CO32−) is a short-lived intermediate, and it facilitates the amine deprotonation of DEAH+ on the basis of pKa analyses.

DEA−COO− + MEA + 2DEAH+ (9)

(HCO3−

2.3. Roles of Bicarbonate and Carbonate and CO32−). HCO3− is a very important ion for CO2 desorption, as discussed in the literature.13,18,20 From experiments, CO2 desorption is faster with the existence of bicarbonate and slow without it. HCO3− plays two roles in CO2 desorption.20 The first role is to accept a proton from amineH+ (amine = MEA, DEA, R3N, etc.) to quickly release CO2. This process can reduce the required reaction energy of carbamate breakdown from 85.6 to 21.2 kJ.20 The second role of HCO3− is to act like a catalyst to divide one step of proton transfer from MEAH+ to water into two steps, as H2CO3 can release a proton to water to generate HCO3− and H3O+ again. Such breakup can reduce the energy of proton transfer that would occur from a strong base directly to water.13,18,20

3. MATERIALS AND METHODS 3.1. Chemicals. Pure CO2 gas and amines MEA and DEA were purchased from Guoyao Chemical Ltd. HCl, methyl orange (indicator for titration), and heating oil are commercially available from Klamar Chemical Ltd. The solid catalysts, γ-Al2O3 and H-ZSM-5, were purchased from Shandong Yinghe Catalysts Company Ltd. All the chemicals were of high purity (>99%). 3.2. Experimental Setup and Procedures. CO 2 desorption was conducted in a recirculation-process vessel equipped with an electrometer (schematic diagram in Figure S3)18,20 to extract the energy-efficient combinations from 30 sets of solvents and catalysts. The amine concentrations used were 5 M DEA and 4.5:0.5, 4:1, 3.5:1.5, 3:2, and 2.5:2.5 M DEA−MEA. The five catalytic conditions used were 8 and 15 g of γ-Al2O3, 8 and 15 g of H-ZSM-5, and 15 g of blended solids (2:1 γ-Al2O3−H-ZSM-5).13 This process was similar to that used in the literature.17,20 The 1000 mL volume of the flask was filled with 500 mL of the amine solvent. The 5.0 mol/L amine solvents were preloaded with CO2 at 0.50 mol/mol, ready for desorption. Various catalysts were placed into the solvents. For the desorption of each solvent blend, we tested one noncatalytic and five catalytic conditions.13 The selection of catalysts is the same as that of Liang,13 which were proven to have a catalytic effects for MEA solutions. These catalysts represent Lewis acids, Brønsted acids, and blended catalysts with combined advantages.12,13 This ratio of mixed catalysts was the optimized catalyst mixture for MEA among the different ratios tested (3:1, 2:2, 2:1, 1:2, and 1:3).13 The experimental procedures were similar to those in our previous study.20 The process was stirred and heated to 363 K. The catalytic effects on CO2 desorption were evaluated by analyzing the CO2 loading of samples at 0−6 h. The samples were pipetted into a vial and allowed to cool down in a cold water bath to maintain CO2 loading. CO2 loading was tested right after sample collection by titration, with results obtained and recorded within 3−5 min.26 We collected 8 samples in the first hour and 4 samples for the second hour, and then 1 sample every 30 min, with 16−18 samples in total. DEA is

Releases CO2 directly: MEAH+ + HCO3− ↔ MEA + H 2CO3 (3)

Acts as a catalyst: Two steps with HCO3− : MEAH+ + HCO3− ↔ MEA + H 2CO3 H 2CO3 + H 2O ↔ HCO3− + H3O+ One step without HCO3− : MEAH+ + H 2O ↔ MEA + H3O+

+

DEA−COO− + MEA + 2DEAH+

The overall reaction of eqs 4−8 for DEA blended with (0− 20%) MEA is the following:

↔ MEA + H 2O + CO2

2−

= 24.00 (298 K)

(7)

↔ 3DEA + MEAH+ + CO2

[HCO3−][DEA]

(7)

(1)

The role of CO32− has recently been discovered in our study for solvents of DEA blended with 0−20% MEA. Briefly, CO32− is a short-lived intermediate that can be generated by MEA and HCO 3 − , as in eq 6. From ion-speciation plots, the concentration of CO32− is negligible in MEA solvents.24 The pKa values of MEAH+/MEA and HCO3−/CO32− are close 9.51 and 10.33, respectively.24 However, the concentration of HCO3− is large in DEA solvents at rich loading (Figure S2)24 and is generated from a carbamate-hydrolysis reaction eq 8 at the lean stage because DEA−COO− is less stable. Free C

DOI: 10.1021/acs.iecr.8b00961 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research much easier to desorb than MEA, and thus only six samples18 were enough. The temperature was increased to 378 K after CO2 loading 2.5:2.5 M DEA−MEA, which is normal. Another trend is quite unique for the noncatalytic desorption, which follows the order of 4.5:0.5 M DEA−MEA ≈ 4:1 M DEA−MEA > 5.0

M DEA, which is 0.20 > 0.19 > 0.17 mol/mol. From the experiments, DEA blended with 10−20% MEA releases CO2 quicker than DEA. The reason is as explained earlier in Section 2.3. Briefly, the acceleration of CO2 production resulted from conjugated CO32−/HCO3−, which facilitated DEAH+ regenerE

DOI: 10.1021/acs.iecr.8b00961 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Table 1. CO2-Loading Changes after 30, 60, and 120 min CO2-loading change (mol of CO2 per mole of amine per hour) γ-Al2O3

noncatalyst

2:1 γ-Al2O3−HZSM-5

HZSM-5

amine (mol/L)

time (min)

0g

8g

15 g

8g

15 g

15 g

5.0 DEA (benchmark)

0−30 30−60 60−120 0−30 30−60 60−120 0−30 30−60 60−120 0−30 30−60 60−120 0−30 30−60 60−120 0−30 30−60 60−120

0.167 0.053 0.057 0.201 0.035 0.103 0.190 0.034 0.097 0.167 0.031 0.083 0.145 0.030 0.086 0.146 0.033 0.067

0.218 0.028 0.102 0.217 0.034 0.093 0.224 0.037 0.054 0.202 0.044 0.080 0.170 0.041 0.052 0.172 0.026 0.052

0.241 0.024 0.085 0.242 0.058 0.047 0.240 0.033 0.054 0.206 0.050 0.066 0.172 0.060 0.051 0.172 0.020 0.064

0.248 0.033 0.074 0.228 0.046 0.087 0.208 0.064 0.059 0.194 0.043 0.057 0.183 0.049 0.043 0.175 0.018 0.077

0.257 0.050 0.065 0.245 0.055 0.056 0.226 0.057 0.072 0.191 0.050 0.057 0.193 0.046 0.068 0.197 0.035 0.046

0.264 0.076 0.040 0.261 0.060 0.041 0.237 0.080 0.036 0.188 0.067 0.092 0.209 0.059 0.071 0.189 0.034 0.076

4.5:0.5 DEA−MEA

4.0:1.0 DEA−MEA

3.5:1.5 DEA−MEA

3.0:2.0 DEA−MEA

2.5:2.5 DEA−MEA

Table 2a. CO2 Production (nCO2) for the First 60 min CO2 generation (mol of CO2) γ-Al2O3

noncatalyst

2:1 γ-Al2O3−HZSM-5

HZSM-5

amine solvents (mol/L)

0g

8g

15 g

8g

15 g

15 g

5.0 DEA 4.5:0.5 DEA−MEA 4.0:1.0 DEA−MEA 3.5:1.5 DEA−MEA 3.0:2.0 DEA−MEA 2.5:2.5 DEA−MEA

0.549 0.590 0.559 0.494 0.437 0.449

0.615 0.628 0.653 0.617 0.526 0.495

0.662 0.749 0.682 0.641 0.580 0.479

0.703 0.684 0.678 0.593 0.579 0.483

0.767 0.749 0.682 0.604 0.597 0.580

0.852 0.803 0.795 0.638 0.671 0.555

Table 2b. CO2 Production (nCO2) for the First 120 min CO2 generation (mol of CO2) γ-Al2O3

noncatalyst

2:1 γ-Al2O3−HZSM-5

HZSM-5

amine solvents (mol/L)

0g

8g

15 g

8g

15 g

15 g

5.0 DEA 4.5:0.5 DEA−MEA 4.0:1.0 DEA−MEA 3.5:1.5 DEA−MEA 3.0:2.0 DEA−MEA 2.5:2.5 DEA−MEA

0.692 0.848 0.802 0.701 0.652 0.617

0.870 0.860 0.812 0.817 0.657 0.624

0.875 0.870 0.817 0.806 0.708 0.639

0.889 0.871 0.824 0.736 0.686 0.675

0.925 0.880 0.820 0.746 0.767 0.694

0.942 0.889 0.885 0.868 0.848 0.744

after 120 min. The CO2 production of 4.5:0.5 M DEA−MEA was comparable to that of 5 M DEA without catalysts and with γ-Al 2 O 3 (relatively weak catalysis). However, the CO2 production of DEA was highest for the six solutions with HZSM-5 and blended solids. This trend reflects the potential of Brønsted acids and blended catalysts, which speed up CO2 release. DEA as a benchmark has better CO2 desorption under catalytic conditions, with the orders H-ZSM-5 > γ-Al2O3 > blended catalyst at rich loading and blended catalyst > H-ZSM5 > γ-Al2O3 at lean loading. These orders are different from those of MEA,13 which are H-ZSM-5 > blended catalyst > γAl2O3 at rich loading and blended catalyst > γ-Al2O3 > HZSM-5 at lean loading.13

ation. The 0−20% MEA helped to convert a small portion of HCO3− into CO32−, with Keq = 0.16. The carbonate (CO32−) converted DEAH+ to DEA and boosted proton transfer as the initial, rate-determining step of CO2 desorption (eq 9). With the aid of the catalysts, the CO2 desorption of 5 M DEA was accelerated and comparable to that of 4.5:0.5 M DEA−MEA (Table 1). Furthermore, the catalytic process at 30 min followed the order 5.0 M DEA ≈ 4.5:0.5 M DEA−MEA > 4:1 M DEA−MEA for the three catalysts, which is 0.264 ≈ 0.261 > 0.237 mol/mol. 4.2. CO2 Production for the First 2 h. CO2 production was calculated in eq 12 and is shown in Table 2a for 60 min at a loading of 0.50−0.20 mol/mol. For Table 2b, about 55−75% of CO2 was produced out of 1.25 mol of CO2 in the solvents F

DOI: 10.1021/acs.iecr.8b00961 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research HCO2 = =

heat input/time amount of CO2 /time

Q total (or E) kJ of electricity/h kJ = nCO2 mol of CO2 /h mol of CO2

(11)

From Table 3a, it is apparent that catalytic desorption requires much less heat duty than noncatalytic desorption, because it requires less desorption time. From Table 3b, catalytic desorption has less or comparable heat duty than the noncatalytic process for most of the time, except for several cases: For 1:4 MEA−DEA, the noncatalytic desorption is 1248 kJ/mol, comparable to that with 8 g of γ-Al2O3 (1300 kJ/mol), and both of these are related to the different heat inputs of 432 and 450 kJ per 2 h, respectively. For the case of 2:3 MEA− DEA, the noncatalytic desorption is comparable to that with γAl2O3 but is 6% lower than that with H-ZSM-5. Such abnormal deviation may result from measurement error. The special case is for 0.5:4.5 M MEA−DEA, which has a relatively low heat duty in the lean region. This is because the solvent blend is at the optimized ratio with minimized energy costs.28 After a detailed comparison of catalytic CO2 desorption with different blended catalysts, the trends of catalysis were shown in Table 3c. For the rich loading of 0.50−0.30 mol/mol, the catalytic order of DEA is H-ZSM-5 > γ-Al2O3 > blended catalyst, which is different from that of MEA.13 This is because DEA has abundant HCO3− at that region. Equation 3 is the main reaction, and γ-Al2O3 is favored. For the blended solutions, 0.5:4.5 and 1:4 M MEA−DEA has the same trend, because over 80% of the amine in the solution is DEA. On the other hand, the 2:3 and 2.5:2.5 M MEA−DEA blends have trends similar to that of MEA as follows: H-ZSM-5 > blended catalyst > γ-Al2O3. There is less HCO3− in the blended solutions of 2:3 and 2.5:2.5 M MEA−DEA than in those of 0.5:4.5 and 1:4 M MEA−DEA. Consequently, carbamate breakdown is important, as such a high ratio of H-ZSM-5 is better. The blended catalysts contain 33% H-ZSM-5, which makes them have better catalysis than γ-Al2O3. For the lean loading of 0.30−0.10 mol/mol, the catalytic order of DEA is blended catalyst > H-ZSM-5 > γ-Al2O3, which is different from that of MEA.13 This is because of the low carbamate stability of DEA. Bicarbonate (HCO3−) can be generated by carbamate hydrolysis, and the effect of γ-Al2O3 is least important. For blended solutions, 0.5:4.5 and 1:4 M MEA−DEA have the same trend, because over 80% DEA is in the amine solution. On the other hand, 1.5:3.5, 2:3, and 2.5:2.5

Figure 3. Heat duties of three amine solutions (5 M) with different catalysts (15 g) at 120 min.

nCO2 = (αrich − αlean) × Camine × Vamine (mol of CO2 ) (12)

4.3. Heat-Duty Analysis with Optimized Catalysis and Amine Blends. As an important parameter, heat duties are calculated in eq 11 and given in Table 3a for rich loading (0.50−0.30 mol/mol) at 363 K and Table 3b for lean loading of (0.30−0.15 mol/mol) at 378 K. Under such conditions, all solvents have the same concentration (5.0 M) and volume (0.5 L), which means that they are compared at similar αrich and α lean values. The operation temperature, T, and CO 2 production, nCO2, are the same on the basis of eq 12. This consistency kept the heat-duty calculations similar. The difference was the time of desorption of each solvent to reach the loading of 0.30 and 0.15 mol/mol. These times were measured from the desorption curves in Figures 1 and 2, and the heat input (E) is calculated with the desorption time. For rich loading at 363 K, the heat input (E) is 180 kJ/h (3 kJ/ min) for noncatalytic desorption but 216 kJ/h (3.6 kJ/min) for catalytic desorption. For lean loading at 378 K, the heat input (E) is 432 kJ per 2 h (3.6 kJ/min) for noncatalytic desorption but 450 kJ per 2 h (3.75 kJ/min) for catalytic desorption. The heat duties of both regions are given in Tables 3a and 3b.

Table 3a. Heat Duties of Six Solvents for α = 0.50−0.30 mol/mol at 363 K T = 363 K, H (kJ per mole of CO2) γ-Al2O3

noncatalysta

2:1 γ-Al2O3−HZSM-5

HZSM-5b

amine concentration (mol/L)

0g

8g

15 g

8g

15 g

15 g

5.0 DEA 0.5:4.5 MEA−DEA 1.0:4.0 MEA−DEA 1.5:3.5 MEA−DEA 2.0:3.0 MEA−DEA 2.5:2.5 MEA−DEA

270 168 210 306 420 480

86 108 180 209 374 426

65 94 122 180 302 414

50 94 194 180 346 408

36 65 115 180 230 281

86 108 130 184 274 302

The energy cost of noncatalytic desorption is 3 kJ/min multiplied by the time of desorption, because E = 180 kJ/h = 3 kJ/min, nCO2 = CV(αrich − αlean) = 0.50 mol, and H = E/nCO2. bThe energy cost of catalytic desorption is 3.6 kJ/min multiplied by the time of desorption, because E = 216 kJ/ h = 3.6 kJ/min and nCO2 = 0.50 mol as well. a

G

DOI: 10.1021/acs.iecr.8b00961 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Table 3b. Heat Duties of Six Solvents for α = 0.30−0.15 mol/mol at 378 K T = 378 K, H (kJ per mole of CO2) γ-Al2O3b

noncatalysta

2:1 γ-Al2O3−HZSM-5

HZSM-5

amine concentration (mol/L)

0g

8g

15 g

8g

15 g

15 g

5.0 DEA 0.5:4.5 MEA−DEA 1.0:4.0 MEA−DEA 1.5:3.5 MEA−DEA 2.0:3.0 MEA−DEA 2.5:2.5 MEA−DEAc

2189 960 1248 1776 1901 2112

1090 1100 1300 1360 1910 1811

1110 1110 1270 1400 1970 1832

1030 1090 1210 1650 2184 1843

860 1080 1060 1550 2050 1886

530 690 710 1035 1150 1800

a

The energy cost of noncatalytic desorption is 3.6 kJ/min multiplied by the time of desorption, because E = 432 kJ/h = 3.6 kJ/min and nCO2 = CV(αrich − αlean) = 0.375 mol bThe energy cost of noncatalytic desorption is 3.75 kJ/min multiplied by the time of desorption, because E = 450 kJ/ h = 3.75 kJ/min. cFor this case, the end CO2 loading is 0.160 mol/mol, nCO2 = CV(αrich − αlean) = 0.35 mol.

results of Figure 3 reflect the fact that the mesopore surface area (MSA), Brønsted−Lewis active sites ratio (B/L), and carbamate stability each play an important role in catalysis.13,18 Moreover, Srisang et al.14,21 conducted studies of catalytic CO2 desorption using H-ZSM-5 and γ-Al2O3 with 5 M MEA and 5:2 M MEA−MDEA and 5:1.25 M MEA−DEAB in a benchscale unit. Both results indicated that H-ZSM-5 > γ-Al2O3 at an average temperature of 85 °C,14,21 which is similar to the results from this work. Finally, Figure 3 demonstrates that the solvents with low heat duties are the blended solids with 5.0 M DEA (477.9 kJ per mole of CO2) and 4.5:0.5 M DEA−MEA (485.7 kJ per mole of CO2). Table 3d indicates that blended catalysts cut the

Table 3c. Order of Catalysis under Different Cases of Blended Solvents T = 363 K 5.0 DEA 0.5:4.5 MEA−DEA 1.0:4.0 MEA−DEA 1.5:3.5 MEA−DEA 2.0:3.0 MEA−DEA 2.5:2.5 MEA−DEA MEA13 T = 378 K 5.0 DEA 0.5:4.5 MEA−DEA 1.0:4.0 MEA−DEA 1.5:3.5 MEA−DEA 2.0:3.0 MEA−DEA 2.5:2.5 MEA−DEA MEA13

rich-loading range (0.50−0.30 mol/mol) H-ZSM-5 > γ-Al2O3 > blended catalyst H-ZSM-5 > γ-Al2O3 > blended catalyst H-ZSM-5 > γ-Al2O3 > blended catalyst H-ZSM-5 ≈ γ-Al2O3 ≈ blended catalyst H-ZSM-5 > blended catalyst > γ-Al2O3 H-ZSM-5 > blended catalyst > γ-Al2O3 H-ZSM-5 > blended catalyst > γ-Al2O3 lean-loading range 0.30−0.15 blended catalyst > γ-Al2O3 blended catalyst > γ-Al2O3 blended catalyst > γ-Al2O3 blended catalyst > H-ZSM-5 blended catalyst > H-ZSM-5 blended catalyst > H-ZSM-5 blended catalyst > H-ZSM-5

similar to DEA similar to DEA transition similar to MEA similar to MEA

Table 3d. Relative Heat Duties (%) of Three Solvents with Catalysts for 120 min

mol/mol

heat duty (%)

> H-ZSM-5 > H-ZSM-5

similar to DEA

> H-ZSM-5

similar to DEA

> γ-Al2O3

similar to MEA

> γ-Al2O3

similar to MEA

> γ-Al2O3

similar to MEA

amine concentration (mol/L) 5.0 DEA 4.5:0.5 DEA−MEA 4.0:1.0 DEA−MEA

noncatalyst (0 g)

γ-Al2O3 (15 g)

HZSM-5 (15 g)

2/1 γ-Al2O3−HZSM-5 (15 g)

100 89.1

93.6 89.4

85.0 85.9

83.6 84.9

94.1

92.5

93.7

88.8

heat duty by about 15−16.5% if 5 M noncatalytic DEA (571.8 kJ per mole of CO2) is considered as the benchmark (Hbaseline) of 100%. This ratio is smaller than 30% for 5 M MEA with and without catalysts13 because DEA has a very low heat duty already.15 From Table 3d, the blended 4.5:0.5 M DEA−MEA has a lower heat duty (−10%) than noncatalytic 5 M DEA for both the noncatalytic and γ-Al2O3 conditions; however, it is generally 1−2% higher than 5 M DEA with both H-ZSM-5 and blended catalysts. 4.4. Catalytic Desorption of 5 M DEA versus 4.5:0.5 M DEA−MEA under Optimized Conditions. On the basis of eq 10,27 we aimed to exploit the potential of the 5 M DEA and 4.5:0.5 M DEA−MEA solutions with blended catalysts, with their higher desorption rates and lower heat duties. Both amine blends were energy-efficient tactics. Then, we managed to conduct catalytic-CO2-desorption tests at a higher temperature of 378 K. For the tests, heat duties were calculated with eq 13 instead of eq 11; the heat loss needed to be deducted for the 60 min heating of the oil from 363 to 378 K. Other studies neglect the heat loss because the overall process takes 8−9 h for MEA with 2 L solutions.13,18 However, it is not negligible

> γ-Al2O3

M MEA−DEA have trends similar to that of MEA, which is blended catalyst > γ-Al2O3 > H-ZSM-5.13 This is because DEA is easier to regenerate than MEA because a large fraction of DEA was regenerated at loading H-ZSM-5 > γ-Al2O3, which is the same as the catalytic CO2 desorption of DEA at lean loading in Table 3c. This trend is reasonable, as the rich loading takes only 30 min, and the lean loading takes an additional 90 min. Therefore, blended catalysts become the best option. The H

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trations, volumes of solution, operation temperatures, and desorption time periods. Most comparisons were completed within a closed system with fixed conditions.13,17,18 However, this limitation can be mitigated partly with the desorption parameter,27 which is suitable for different amine blends. At a CO2 loading of 0.50−0.10 mol/mol for overall desorption, the cyclic capacity is 0.40 mol of CO2 per mole of amine multiplied by 5.0 mol of amine per liter of solvent, which is equal to 2.0 mol/L CO2 for both desorption processes at 363 and 378 K. The process takes only 40 min at 378 K. The nCO2 is 1.00 mol for DEA and 0.96 mol for 4.5:0.5 M DEA− MEA, as given in Table 4. The heat duty is 336.8 kJ per mole of CO2 for DEA and 349.5 kJ per mole of CO2 for 4.5:0.5 M DEA−MEA with a final loading of 0.10−0.11 mol/mol. Therefore, the desorption parameters are 1.485 × 10−4 (mol of ij 1.00 mol of CO × 2.0 mol/L CO yz 2 2z zz for DEA and CO2)3/kJ·L·min jjj j 40 min × 336.8 mol ofkJCO z 2 k { −4 3 1.373 × 10 (mol of CO2) /kJ·L·min for 4.5:0.5 M DEA− MEA. Compared with the results of 363 K in Figure 3, nCO2 is 0.94 and 0.89 mol for a 120 min desorption. The heat duties are 477.9 and 485.7 kJ per mole of CO2 for DEA and 4.5:0.5 M DEA−MEA. The desorption parameters are about 0.328 × ji 0.94 mol of CO2 × 2.0 mol/L CO2 zyz zz for 10−4 (mol of CO2)3/kJ·L·min jjj j 120 min × 477.9 mol ofkJCO z 2 k { −4 3 DEA and 0.305 × 10 (mol of CO2) /kJ·L·min for 4.5:0.5 M DEA−MEA. For the same catalysts and solvents with similar cyclic capacities and nCO2 values, the desorption at 378 K has larger desorption parameters than that at 363 K, which is 4.53 times bigger for DEA and 4.50 times bigger for 4.5:0.5 M DEA−MEA. At the operation CO2 loading of 0.50−0.20 mol/mol, the cyclic capacity is 1.5 mol/L CO2. The desorption process takes 18 and 20 min for both solvents, with nCO2 = 0.75 mol in Table 4. The heat duties of DEA and DEA−MEA are 151.2 and 168.0 kJ per mole of CO2, smaller than that calculated in Table 2b (274.6 and 282.6 kJ per mole of CO2) with a ΔCO2-loading of 0.33 mol/mol (nCO2 = 0.825 mol) for 60 min. The desorption parameters are 4.134 × 10−4 (mol of CO2)3/kJ·L· ij 0.75 mol of CO × 1.5 mol/L CO yz 2 2z zz for DEA and 3.35 × 10−4 (mol min jjj j 18 min × 151.2 mol ofkJCO z 2 k { 3 of CO2) /kJ·L·min for 4.5:0.5 M DEA−MEA at 378 K. However, they are only 0.826 × 10−4 (mol of CO2)3/kJ·L·min ij 0.825 mol of CO × 1.65 mol/L CO yz 2 2z jj zz for DEA and 0.803 × 10−4 (mol kJ jj z 60 min × 274.6 mol of CO 2 k { 3 of CO2) /kJ·L·min for 4.5:0.5 M DEA−MEA at 363 K. The values are 5.00 times bigger for DEA and 4.17 times bigger for 4.5:0.5 M DEA−MEA. Desorption is favored at 378 K. Furthermore, the results for this solvent are better than those observed by Liu et al.18 with H-ZSM-5 and 5:1 M MEA−MDEA (10.1 MJ per kilogram of CO2 = 444.4 kJ per mole of CO2) at 371 K, where CO2 loading reaches 0.35 mol/ mol from 0.50 mol/mol within 90 min, with a desorption parameter of 0.338 × 10−4 (mol of CO 2) 3/kJ·L·min ij 1.80 mol of CO × 0.75 mol/L CO yz 2 2z jj zz. The desorption parameters of kJ jj z 90 min × 444.4 mol of CO 2 k { this studies at 378 K are much bigger than those of Liu,18 as a result of bigger cyclic capacities and smaller heat duties.

for this case of 1 h with 0.5 L solutions. The overall heat input is 828 kJ/h (0.23 kWh), and the heat loss is 324 kJ/h (0.09 kWh). Thus, the heat input is 504 kJ/h. This is 2.25 times larger than 450 kJ per 2 h for the earlier processes without heat-loss calculations. Such an increase is due to the higher operation temperature of 378 K, which made Qvap increase significantly above 373 K because of the latent heat of vaporization. desorption parameter initial desorption rate × cyclic capacity = heat duty nCO2 × cyclic capacity ≈ time × heat duty

(heat input − heat loss)/time amount of CO2 /time Q total(E) − Q loss(E) kJ/h

(10)

HCO2 = =

nCO2

mol of CO2 /h

=

kJ mol of CO2 (13)

The CO2-desorption curves are plotted in Figure 4, with the heat duties presented in Table 4. From Figure 4, the overall

Figure 4. CO2 desorption of 5 M DEA and 4.5:0.5 M DEA−MEA solvents with optimized catalysts (15 g of 2:1 Al2O3−H-ZSM-5) at 378 K.

process was reduced from 240 to 45 min with 0.50−0.10 mol/ mol as compared with the earlier desorption curves in Figure 1. The two solvents desorb CO2 at similar rates, with DEA being a little faster. This time is much shorter than that with MEA− R3N.18 Technically, it is difficult to compare the heat duties of different amines systems because of the different concenTable 4. Heat Duties of the Catalytic CO2 Desorption of DEA and DEA−MEA Solvents at 378 K heat dutya (kJ per mole of CO2) amine concentration (mol/L)

40 min

0.50−0.20 mol/mol

5.0 DEA with blended 0.5:4.5 MEA−DEA with blended difference

336.8 349.5 +3.7%

151.2 (18 min) 168.0 (20 min) +16.8 kJ

a The heat input is 828 kJ/h, and the heat loss is 324 kJ/h at 105 °C. The heat input is Qin − Qloss = 504 kJ/h. bH = (Qin − Qloss)/nCO2.

I

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Industrial & Engineering Chemistry Research Effect of Operation Temperature on Heat Duty: 363 versus 378 K. On the basis of desorption-parameter analyses, it became apparent from eqs 10 and 11 that there was some effect from the operating temperature. Generally, a lower temperature resulted in a smaller heat input (Q) and less CO2 production (nCO2). In contrast, a higher temperature resulted in a larger heat input (Q) and greater CO2 production (nCO2) and saved time (t). Both cases could result in lower heat duties. Therefore, careful analysis is needed in order to determine the conditions under which each scenario is best and the special cases. (1) If CO2 production is not enhanced significantly with increased Qinput, then a temperature increase to 378 K is not favored. For this condition, a lower temperature is better. (2) If a higher temperature facilitates massive CO 2 production (nCO2), such as that observed in this study, 378 K is favored as compared with 363 K. A faster CO2desorption rate reduces the desorption time, which reduces the heat duty. (3) For catalytic CO2 desorption with DEA−MEA−H2O systems, a higher temperature is preferred on the basis of the results in Tables S1 and 4. (4) The temperature swing needs to balance the heat input (Q) and desorption time (t), because a lower temperature saves energy costs (Q), whereas a higher temperature increases Q, enhances CO2 production (nCO2), and saves time (t). This requires a comprehensive analysis based on specific desorption conditions. 4.5. Comparison of Single and Blended Amine Solvents in Absorber−Desorber. In summary, 5 M DEA with the blended catalyst has minimum heat costs in this study. The blended 4.5:0.5 M DEA−MEA solutions have lower heat costs (∼10%) than 5 M DEA without catalysts and with γAl2O3; however, they have +1−4% higher heat costs than 5 M DEA with solid catalysts on the basis of the results in Table 3d. The 4.5:0.5 M blend is the most suitable in the desorber, where the solid catalysts were either placed into the layer between the inert structured-packing materials or blended with inert beads with random packing,14,21 It is difficult to make all the solvent release CO2 with assistance of catalysts. In reality, part of the amine solvents will desorb CO2 with the catalysts but the rest will desorb CO2 without the catalysts. The blended DEA−MEA (4.5:0.5 M) is suitable for both catalytic and noncatalytic conditions. Furthermore, blended 0.5:4.5 M DEA−MEA solutions have much faster rates of CO2 absorption than DEA. The 10% MEA is a perfect promoter. Researchers have repeatedly conducted studies on the kinetics of DEA−MEA blended solvents,28,29 and the overall reaction rate using blended solutions is 131% higher than that with DEA alone.29 At 298 K, the overall reaction rate constant, k0, of 0.16 M DEA is 29.31 s−1, but the k0 is 67.34 s−1 for 0.16:0.01 M DEA−MEA.29 We also performed CO2-absorption tests with 5 M DEA and 4.5:0.5 M DEA−MEA for comparison (Figure 5). The experiments were conducted in a batch reactor with 300 mL of amine solvents and a CO2 flow rate of 1.5 L/min.30 The absorption of blended solvents was faster than that of DEA alone, with a large difference of 0.35 mol/mol where HCO3− initially appeared. Ultimately, the blended 4.5:0.5 M solvent is more suitable than 5 M DEA in a CO2-absorption−desorption process. It can

Figure 5. CO2-absorption curves of DEA and 4.5:0.5 M DEA−MEA solvents.

significantly facilitate CO2 absorption, with a comparable heat duty to that of 5 M DEA under optimized catalyst conditions (blended 2:1 γ-Al2O3−H-ZSM-5). This provides an energyefficient tactic of using blended amines and catalysts for the amine-scrubbing process, because a promotor has been added which improves absorption.

5. CONCLUSION (1) Catalytic CO2-desorption curves were generated for the DEA−MEA−CO2−H2O system, from 5:0 to 4.5:0.5 to 2.5:2.5 M DEA−MEA. The order of catalysis was carefully categorized. The 0.5:4.5 and 1:4 M MEA−DEA solvents have similar trends to DEA, but the 2:3 and 2.5:2.5 M MEA−DEA solvents are similar to MEA. Under blended catalytic conditions, the heat duties of the amine solutions are in the order 5.0 < 4.5:0.5 < 4:1 M DEA−MEA for the 2 h periods. (2) After studies of 30 catalytic CO2-desorption curves with heat-duty analyses, two energy-efficient approaches were observed: 5 and 4.5:0.5 M DEA−MEA with blended catalysts (γ-Al2O3−H-ZSM-5). Under optimized catalysis conditions at 378 K, blended 4.5:0.5 M solvents can facilitate CO2 absorption, but this approach has only a 4% higher heat duty than that of 5 M DEA with catalysts. (3) On the basis of using a desorber with different packing methods, blended 4.5:0.5 M DEA−MEA was more suitable than 5 M DEA under both catalytic and noncatalytic conditions. The blended solution also has better absorption performance with the aid of MEA as a promoter.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.8b00961. Ion-speciation plots of the MEA−CO2−H2O and DEA− CO2−H2O systems at 298 K, ion-speciation plots of the 2 M MEA−DEA−CO 2 −H 2 O systems, schematic J

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(11) Li, X.; Wang, S.; Chen, C. Experimental Study of Energy Requirement of CO2 Desorption from Rich Solvent. Energy Procedia 2013, 37, 1836−1843. (12) Idem, R.; Shi, H.; Gelowitz, D.; Tontiwachwuthikul, P. Catalytic method and apparatus for separating a gaseous component from an incoming gas stream. U.S. Patent US20130108532A1, May 2, 2013. (13) Liang, Z. W.; Idem, R.; Tontiwachwuthikul, P.; Yu, F.; Liu, H.; Rongwong, W. Experimental study on the solvent regeneration of a CO2-loaded MEA solution using single and hybrid solid acid catalysts. AIChE J. 2016, 62 (3), 753−765. (14) Srisang, W.; Pouryousefi, F.; Osei, P. A.; Decardi-Nelson, B.; Akachuku, A.; Tontiwachwuthikul, P.; Idem, R. Evaluation of the heat duty of catalyst-aided amine based post combustion CO2 capture. Chem. Eng. Sci. 2017, 170, 48−57. (15) Idem, R.; Wilson, M.; Tontiwachwuthikul, P.; Chakma, A.; Veawab, A.; Aroonwilas, A.; Gelowitz, D. Pilot Plant Studies of the CO2 Capture Performance of Aqueous MEA and Mixed MEA/ MDEA Solvents at the University of Regina CO2 Capture Technology Development Plant and the Boundary Dam CO 2 Capture Demonstration Plant. Ind. Eng. Chem. Res. 2006, 45, 2414−2420. (16) Sakwattanapong, R.; Aroonwilas, A.; Veawab, A. Behavior of Reboiler Heat Duty for CO2 Capture Plants Using Regenerable Single and Blended Alkanolamines. Ind. Eng. Chem. Res. 2005, 44 (12), 4465−4473. (17) Shi, H. NMR Analysis of Various Amine-CO2-H2O Systems Interactions for Studies of Vapour-Liquid Equilibrium and Catalyst Aided and Unaided Solvent Regeneration. Ph.D. Thesis, University of Regina, Regina, Saskatchewan, Canada, 2013. (18) Liu, H.; Zhang, X.; Gao, H.; Liang, Z.; Idem, R.; Tontiwachwuthikul, P. Investigation of CO2 Regeneration in Single and Blended Amine Solvents with and without Catalyst. Ind. Eng. Chem. Res. 2017, 56 (27), 7656−7664. (19) Choi, W. J.; Seo, J. B.; Jang, S. Y.; Jung, J. H.; Oh, K. J. Removal characteristics of CO2 using aqueous MEA/AMP solutions in the absorption and regeneration process. J. Environ. Sci. 2009, 21 (7), 907−913. (20) Shi, H.; Naami, A.; Idem, R.; Tontiwachwuthikul, P. Catalytic and non-catalytic solvent regeneration during absorption-based CO2 capture with single and blended reactive amine solvents. Int. J. Greenhouse Gas Control 2014, 26, 39−50. (21) Srisang, W.; Pouryousefi, F.; Osei, P. A.; Decardi-Nelson, B.; Akachuku, A.; Tontiwachwuthikul, P.; Idem, R. CO2 capture efficiency and heat duty of solid acid catalyst-aided CO2 desorption using blends of primary-tertiary amines. Int. J. Greenhouse Gas Control 2018, 69, 52−59. (22) Rosynek, M. Isotherms and energetics of carbon dioxide adsorption on alumina. J. Phys. Chem. 1975, 79, 1280−1284. (23) Shi, H.; Liang, Z.; Sema, T.; Naami, A.; Usubharatana, P.; Saiwan, C.; Idem, R. O.; Tontiwachwuthikul, P. Part 5a: Solvent chemistry: NMR analysis and studies for amine−CO2−H2O systems with vapor−liquid equilibrium modeling for CO2 capture processes. Carbon Manage. 2012, 3 (2), 185−200. (24) Nasrifar, K.; Tafazzol, A. H. Vapor-Liquid Equilibria of Acid Gas-Aqueous Ethanolamine Solutions Using the PC-SAFT Equation of State. Ind. Eng. Chem. Res. 2010, 49 (16), 7620−7630. (25) Li, M.; Liu, H.; Liang, Z.; Tontiwachwuthikul, P. The research of coordinative and competitive relationship of CO2 absorption into MEA and DEA in blended aqueous amines. Proceedings of the American Institute of Chemical Engineers (AIChE) Annual Meeting, Salt Lake City, UT, Nov 12, 2015. Available at http://www3.aiche.org/ proceedings/Abstract.aspx?PaperID=426569. (26) Horwitz, W., Ed. Association of Official Analytical Chemists Methods, 12th ed.; George Bant: Gaithersburg, MD, 1975. (27) Narku-Tetteh, J.; Muchan, P.; Saiwan, C.; Supap, T.; Idem, R. Selection of components for formulation of amine blends for post combustion CO2 capture based on the side chain structure of primary, secondary and tertiary amines. Chem. Chem. Eng. Sci. 2017, 170, 542−560.

diagram of the CO2-desorption process, and heat duties

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of all the solvents for the first 120 min (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.S.). *E-mail: [email protected] (Y.H.). *E-mail: [email protected] (R.I.). ORCID

Huancong Shi: 0000-0003-4333-4118 Raphael Idem: 0000-0002-2708-0608 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support was provided by the Youth Teachers Support Program (ZZsl15042) and Young Eastern Scholar (QD 2016011 and QD 2016011-005-11). Funding from the National Natural Science Foundation of China (NSFC nos. 21606150 and 61775139) and the Natural Sciences and Engineering Research Council of Canada (NSERC) is gratefully acknowledged. Financial support provided by Innovation Saskatchewan (Government of Saskatchewan, Canada) is also gratefully acknowledged.

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L

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