3.1. HBD feasibility zone in binary gas systems
The feasibility zone for using binary gas systems for hydrate-based desalinations were first evaluated and presenting in this section. Since a decade research in gas hydrate has profoundly focussed at the molecular level using solid state analytical instruments like X-ray diffraction and NMR spectroscopy and Raman spectroscopy to charactise the composition and structure of hydrate formed. Studies based on mixed hydrates using Raman spectroscopy and X-ray diffraction, suggesting that the reaction kinetics of each hydrate system is different and relies on the type of guest molecule and external pressure- temperature conditions [8-10]. 13C NMR studies performed for CH4+C3H8 during sII hydrate formation was presented by Kini et al., [11]. They observed that the large cages (51264) occupied with C3H8 form twice as fast as small cages (512) with CH4.Generally small molecules like CH4, CO2 tend to form s1 hydrates where the small cages are filled by CH4 and larger cages are filled by CO2 or C2H6, while larger molecules like C3H8, C4H10 form sII hydrates [10]. In the interim the gas molecules occupying different cavities of gas hydrates would also affect the stability of structure. Some of the properties of the various gas hydrate structures are listed below in Table 2[10].
Table 2: Structural and cage occupancy characteristics of gas hydrates
Properties of cage
|
sI
|
sII
|
sH
|
Cavity
|
Small
|
Large
|
Small
|
Large
|
Small
|
Medium
|
Large
|
Description
|
512
|
51262
|
512
|
51264
|
512
|
435663
|
51268
|
Number per unit cell
|
2
|
6
|
16
|
8
|
3
|
2
|
1
|
Average cavity radius(Å)
|
3.95
|
4.33
|
3.91
|
4.73
|
3.91
|
4.06
|
5.71
|
Coordination number a
|
20
|
24
|
20
|
28
|
20
|
20
|
36
|
Lattice type
|
Cubic
|
Face centered cubic
|
Hexagonal
|
Water molecules per unit cell
|
46
|
136
|
34
|
Ratio of diameter of guest molecule to diameter of cage for hydrate former
|
Guest
|
Diameter (Å)
|
|
|
|
|
|
|
|
CH4
|
4.36
|
0.886*
|
0.757*
|
0.889
|
0.675
|
|
|
|
CO2
|
5.12
|
1.041
|
0.889*
|
1.044
|
0.792
|
|
|
|
C3H8
|
6.28
|
1.276
|
1.090
|
1.280
|
0.971*
|
|
|
|
a number of oxygen atoms at the end of each cavity
*Indicate cage occupied by guest species
Zheng et al.,[12] presented a thermodynamic model to enhance the accuracy in the prediction of phase boundary of hydrates of pure components CH4, CO2 and binary mixture CH4+CO2 in presence of pure and saline water. They observed that the CH4+CO2 binary hydrates pressure phase boundaries decreased with increase in CO2 concentration. In another communication the clatharate hydrate phase equilibria of CH4+CO2 suggested the stable structure for the binary system to be s1 structure [13]. Identical perceptions have been addressed [14-15]. Propane molecule diameter is too large as listed in Table 2 to occupy the small 512 cages; so, it occupies the larger cages of 51264 leaving the smaller 512 cages empty. [10,16-18]. The small 512 cages of sII hydrate can possibly be occupied by the molecules having smaller diameter size like CO2 and CH4 at suitable pressure and temperature conditions. Essentially, these smaller guest molecules often stabilize the sII hydrates more than just the C3H8 molecule. Because there are usually no additional forces available between the host and the guest molecule, Van der Waal forces are thought to be responsible for this stability [10]. Based on the dissociation enthalpy value the mixed hydrate formed from the gas system CO2+H2+C3H8 was confirmed to be a sII hydrate [19]. With the help XRD and NMR spectroscopy it was communicated that the three gas system CO2+H2+C3H8 is composed of sII structure [20].
The three binary systems studied were CO2+CH4, CO2+C3H8 and C3H8+CH4. To evaluate the feasibility of forming suitable hydrates in the binary systems for the driving force and subcooling temperatures were estimated. The average driving force at 20 bar for 1 – 4 °C were reported, while the average subcooling temperatures for pressure ranging from 2.0 -4.0 MPa at 4 °C was reported. The selection of 2.0 MPa and 4 °C was to ensure the evaluation of the minimum conditions suitable to form more hydrates with less energy and pressure required.
Figure 1 shows the average driving force subcooling temperature for CH4+C3H8 system at 2 wt.%. This system is suitable for the utilization of natural gas constituents for desalination. In Figure 1, the subcooling temperature for pure CH4 and C3H8 at 4ºC are -4.7ºC and 0.28ºC, respectively. The system with 90CH4 + 10C3H8 rises the pure CH4 systems subcooling temperature by 12oC. Increasing the propane concentration up to 30% increases the subcooling temperature of pure CH4. Propane concentrations above 30% show a slight negligible impact on the subcooling temperature for CH4+C3H8 systems. The driving force for CH4+C3H8 mixtures behavior is similar as their subcooling temperature, however, pure C3H8 exhibits a high driving force than its mixture with CH4 as all concentrations (Figure 1). The pure C3H8 systems have poor subcooling temperature which is a limitation for its application [21-23,17] Therefore, 90CH4+10C3H8 or 10CH4+90C3H8 are suitable systems that could provide a significant driving force and subcooling temperature for hydrate-based desalination/water treatment at minimal/average energy intensity conditions of 4oC and 2.0 MPa This process is evident to show that the small addition of propane there is a pressure increase which is caused by the hydrate crystal change from s1 to sII as propane can only fit into larger 51264 cavity of sII so more pressure is required to fit into the cage as C3H8 is too large to occupy any other cavity as listed in Table 2. However, increasing the C3H8 composition in the C3H8+CH4 system increases the subcooling temperature and driving force. The system with 90C3H8+10CH4 exhibits the highest driving force and subcooling temperature of 1.69 MPa and 12.9°C, respectively. This is about 70% and 77.5% high than the driving force and subcooling temperature for 90CH4+10C3H8 system as shown in Figure 1.
On the other hand, CO2 and C3H8 mixtures also behavior similarly to CH4+C3H8 (Figures 1 and 2). The subcooling temperature for CO2-C3H8 is averagely about 0.39oC lower than CH4-C3H8, but about 0.05 MPa high than the CH4+C3H8 systems. This suggests that the hydrate formation behavior and the water recovery/metals removal in mixing C3H8 with CO2 is highly influenced by the pressure differential driving force. While the subcooling temperature highly controls the hydrate formation behaviour and the water recovery/metals removal efficiency in C3H8+CH4 systems. The binary mixtures of C3H8, CH4, and C3H8, CO2 at 70-80%:20-30% would averagely provide a suitable subcooling temperature and driving force for metals removal via hydrate-based desalination or water treatment methods at relatively moderate temperature and pressure conditions. Hence the process could occur and run efficiently with low energy intensity. Because CO2 and C3H8 can form hydrates at significantly lower pressures than methane, they have a wide range of potential applications [24-26]. However, increasing the C3H8 composition in the CO2+C3H8 system increases the subcooling temperature and driving force. 90CO2+10C3H8 to 50CO2+50C3H8 are suitable systems that could provide a significant driving force and subcooling temperature for hydrate-based desalination/water treatment at minimal/average energy intensity conditions of 4oC and 2.0 MPa. The system with 70-80%CO2+ 30-20%C3H8 exhibits the highest subcooling temperature of 1.495 MPa and 8.07 °C, respectively. This is about 29.45% and 34.1% high than the driving force and subcooling temperature for 90CO2+10C3H8 system as shown in Figure 2.
Figure 3 the CO2+CH4 gas composition exhibits a poor hydrate formation subcooling and driving force at low pressure and temperature conditions. Thus, using CO2+CH4 mixed gas systems for desalination or metal removal would require very high pressure and lower temperature conditions. These conditions would increase the energy demand for the process to occur. Increasing the concentration of CH4 in CO2+CH4 systems linearly reduces the subcooling and driving force of pure CO2 by 3 and 8 times, respectively (Figures 3). Generally, in the presence of electrolytes, the hydrate formation is delayed [27]. There is extensive literature [28-32] available with experimental data, models, and simulations of hydrate formation and dissociation in the presence of electrolytes. All of these studies show that the presence of salt in water produces an increase in hydrate equilibrium pressure and/or a drop in the hydrate equilibrium temp. As a result, the formation of water cages is impeded, and the stability of the hydrate structure is decreased [33]. In essence, using pure CO2 would yield suitable conditions to form hydrate that mixed CO2+CH4 systems, however, the driving force and subcooling for pure CO2 must be at lower temperature condition (< 4oC) and high pressures (< 2.0 MPa). This would be due to the double hydrate formation of CH4+CO2, where majority of the large cages might be accommodated by both guest species, though there is less occupancy of CH4 in the large cages because CO2 can only occupy the large cage, whereas CH4 can occupy both the large and small cages. This holds in good agreement with the study performed by few researcher [34-35] using NMR spectroscopy. In this case CH4+CO2 significantly higher pressure driving force is required which might not be economical for hydrate based desalination.
3.2. HBD feasibility zone in ternary gas systems
The hydrate formation driving force and subcooling behaviour of the ternary system for CH4+C3H8+CO2 was further investigated in this work. Figures 4-6 shows the results on the ternary systems. Generally, all the ternary systems exhibited high subcooling temperatures and driving forces that are suitable for high hydrate formation kinetics at low pressure and high temperature conditions (Figures 4-6). The presence of C3H8 in all the ternary systems assisted their increased driving forces, especially for the CH4+CO2 systems.
The driving force of the ternary systems in Figures 4-5 are similar to the binary systems in Figure 1 and 2, expect for CH4-CO2 systems (Figure 3). This implies that, using binary systems for desalination or metals removal purposes is preferable in terms of driving forces. The subcooling for the ternary systems varied significantly. This provides an added advantage to easy form hydrate when using ternary systems compared to the binary systems. However, the ternary system with constant C3H8 (10%) and varying CO2 and CH4 exhibited subcooling temperature conditions similar to the systems binary systems of CO2+C3H8 and CH4+C3H8. This might be due to the fact that small addition of C3H8 causes an increase in the subcooling temperature and a decrease in the driving force due to structural change from sI to sII and also follows literature [11]. Propane can only occupy larger cages of sII due to its large size as listed in Table 2 and CH4+CO2 forms s1 structure. Few researchers [14-15] have made similar observations. For constant 10 % CH4 and varying C3H8+CO2 the ratio of (10:80:10) exhibits highest subcooling temperature and driving force of 12.86ºC and 1.657 MPa at 4ºC and 2.0 MPa as shown in Figure 4. From figure 5 with constant 10 % CO2 and varying concentrations of CH4+C3H8 the ratio (10:30:60) exhibits the highest subcooling temperature of 13.22 ºC and at (10:10:80) provides a high driving force of 1.6575MPa at 4ºC and 2.0 MPa as shown in Figure 5. However, the ternary systems with either constant CO2 or CH4 and varying C3H8 composition exhibited higher subcooling temperatures up to 4oC high than binary systems. Instead of using binary system CH4+CO2 it is better to use a ternary system with small addition of propane as 10 % C3H8 addition to this system provides good driving force for hydrate-based desalination system as shown in Figure 6. For the ternary system C3H8+CH4+CO2 (10:40:50) or (10:50:40) provide a high subcooling of 8.25ºC which is about 96.5% increase from pure C3H8