*3.2.1 Trialkyltin fluorides*

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applications [83].

molecular weight.

weight (Mw: 980,000 g.mol<sup>−</sup><sup>1</sup>

In propane and butane, PDMS was soluble at pressures close to the vapour pressures of propane and butane, while in ethane, it required high pressure (much greater than its vapour pressure) to attain solubility. For example, at 333 K and 2 wt% of PDMS in ethane, propane, and butane, the cloud point pressures were obtained to be equal to 18, 2.56, and 0.92 MPa, respectively. Furthermore, they found PDMS to be an effective thickener in propane and butane but an ineffective thickener in ethane. For example, at 333 K and at a concentration of 2 wt%, PDMS achieved viscosity increases of 1.2-fold in ethane, 2-fold in propane, and 4-fold in butane. It was also found to be a better thickener at high pressure (62 MPa). Overall, high molecular weight PDMS is not a viable thickener of NGL for EOR

In comparison to the results obtained by Heller et al. for 2.2 wt% of PAO (poly(1-pentene), poly (1-hexene), and P-1-D) in butane at 298 K, DRA increases butane viscosity substantially at even lower concentrations (0.5 wt%), because of the extremely high molecular weight of DRA, regardless the relatively high concentration of hexane added to the system. Although PDMS has a higher molecular

(fourfold) than the viscosity obtained by low molecular weight PAO (fivefold). These observations indicate that the increase in gas viscosity depends on several factors and not solely on the molecular weight of the additives. Additional factors that can influence the viscosity-enhancing ability of an additive include the nature of additives and the solvent, the concentration of additives, the molecular weight distribution of the additives, and the type of intermolecular interactions among the additives and the solvent [84, 85]. These chemical additives (PDMS and PAO) have different chemical structures. PAO has a carbon-carbon backbone with atactic molecular structure of mostly uniform head-to-tail connections with some headto-head-type connections in the structure [86]. On the other hand, PDMS has silicone-oxygen backbone and more flexible molecules than P-1-D molecules. Hence,

PDMS can have lower steric hindrance and greater bond angle (143<sup>o</sup>

mass. For example, the viscosity of PAO at 1000 g.mol<sup>−</sup><sup>1</sup>

**3.2 Small-molecule self-associating thickeners**

gases are discussed in the following section.

C-C-C) to rotate around the Si-O bond [87]. Furthermore, the effect of molecular structure and polymer molecular weight on viscosity has been studied by Zolper et al. [87] who found that similar viscosity can be obtained for different molecular

ity of PDMS at around 10,000 g/mol. This was attributed to the additional attractive intermolecular forces between the polymers with increasing branches, which leads PAO to having higher viscosity indices than PDMS. Therefore, the effect of PAO on butane viscosity could be attributed to the structure of the polymer. These effects are more pronounced in improving the solvent viscosity than the PAO

Similarly to studies for CO2, low molecular weight self-associating compounds have been studied as thickening agents of light alkane gases for gas mobility control and hydraulic fracturing purposes [54, 82, 88, 89]. Ideally, small-molecule compounds need two processes to attain dissolution and viscosity enhancement [88, 89]. The first is a high-pressure heating cycle, which disrupts the intermolecular association to enhance the dissolution. The second process is the cooling cycle to re-establish the intermolecular association necessary for viscosity enhancement. However, some small-molecule compounds do not require this two-step process to attain dissolution and viscosity enhancement in NGL [89]. Previous studies on the application of small-molecule compounds to increase the viscosity of light alkane

) than PAO, PDMS offers a lower relative viscosity

vs. 110° for

is equivalent to the viscos-

**80**

Dunn and Oldfield first reported on the use of tri-n-butyl tin fluoride (TBTF) as a direct thickener of non-polar solvents including carbon tetrachloride and n-propane [89]. **Figure 8** illustrates the association mechanism of tributyltin fluoride [90], where a linear polymeric chain of penta-coordinate tin atoms are linked by fluorine atoms. TBTF is a white powder with a melting point of 544 K [88, 89]. The three butyl arms attached to the tin atom enhances the solubility of TBTF in a hydrocarbon solvent, while the intermolecular association formed among the tin and fluorine atoms induces the viscosity-enhancing effect. It has been found that TBTF is soluble in organic liquids and light alkanes under stirring for several minutes without requiring a heating and/or a cooling cycle [89]. TBTF is an effective thickener for intermediate hydrocarbon components. Dandge et al. [54] found TBTF to be capable of improving the viscosity of propane and butane. For instance, at concentrations of 0.13–0.15 wt% at 298 K, it increased the viscosity of these components by 2–10-fold at 8.3 MPa. In addition, they also found that TBTF was only partially soluble in ethane and with no measurable viscosity change [79]. Later on, Enick and co-workers confirmed the ability of TBTF to thicken propane and butane liquids at 298 K at concentrations of 0.2–5 wt% [56]. Other trialkyltin fluorides have been tested in hydrocarbon solvents. Tripropyltin fluoride (TPTF) is not soluble in propane and butane, because the propyl arms are too short to induce the dissolution of TPTF in these solvents [91]. Therefore, it confirms that the solubility of trialkyltin fluoride in n-alkane increases as the number of carbon atoms in n-alkyl arms (R) increases [54]. However, at equivalent mass concentration, TBTF in n-hexane or n-butane has shown to outperform in viscosity enhancement compared with other solvents [54]. For example, at a concentration of 10 g/L and 310 K, TBTF increases the viscosity of n-hexane by 750-fold (from 0.265 cP to 196 cP), while the tetrachloroethylene viscosity is enhanced by 380-fold (124.45 cP) [54].

A recent study has tested the solubility and viscosity enhancement ability for dilute concentrations (>1 wt%) of TBTF in ethane, propane, and butane at high pressures (38–64 MPa) and high temperatures (298–373 K) [82, 88, 89]. TBTF is soluble in propane and butane above the corresponding vapour pressure of these components, while in ethane, TBTF is soluble at much higher pressures than the ethane vapour pressure. In addition, it was observed that the relative viscosity of TBTF in NGL components increases slightly with increasing pressure at all temperatures and TBTF concentrations. Increasing the pressure does not affect the self-assembly of the supramolecular structure; it only affects the solvent strength which has a less significant effect on the solution viscosity. Furthermore, as temperature increases, the intermolecular association between the tin and fluoride

**Figure 8.** *Association mechanism of tributyltin fluoride [89].*

molecules diminish, leading to a significant decrease in the viscosity enhancement in all light alkane components. For example, with 1 wt% concentration of TBTF in ethane at 298 K and 62 MPa, the achieved relative viscosity is 90, and it drops to 75 at 313 K. The relative viscosity significantly drops further to 20, 6, and 1.5 at 333, 353, and 373 K, respectively [82] (**Table 2**).

### *3.2.2 Hydroxyaluminum di-2-ethylhexanoate (HAD2EH)*

A mixture of aluminium disoap and gasoline liquid is heated to high temperatures (368–373 K) to promote its dissolution by dismantling the intermolecular associations between the aluminium disoaps. Then it is cooled down to allow self-assembly of the disoap molecules, whereby the viscosity of the solution is enhanced significantly [88, 89]. Enick and co-workers [62] studied a single aluminium salt, referred to as hydroxyaluminum di-2-ethylhexanoate (HAD2EH). **Figure 9** depicted the association mechanism of HAD2EH. They found HAD2EH to exhibit a remarkable solubility in light hydrocarbon gases such as propane and butane. It is also capable of thickening these components at dilute concentrations. For example, at 293 K, HAD2EH concentrations in the range of 0.2–1 wt% were capable of increasing the viscosity of the solution by 10–100-fold as tested in a high-pressure falling-ball cylinder viscometer. However, the solution formed was hazy, due to a portion of the HAD2EH molecules forming solid fibres in both liquid propane and butane at high pressures.

**Figure 9.** *Association mechanism of HAD2EH molecules [89].*

**Figure 10.** *Molecular structure of phosphate di-/monoester, phosphonic acid ester, and dialkyl phosphinic acid [89].*

**Figure 11.**

*Chelation mechanism and micellar structure of phosphate ester/metal ion complex [96].*

### *Direct Gas Thickener DOI: http://dx.doi.org/10.5772/intechopen.88083*

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353, and 373 K, respectively [82] (**Table 2**).

*3.2.2 Hydroxyaluminum di-2-ethylhexanoate (HAD2EH)*

molecules diminish, leading to a significant decrease in the viscosity enhancement in all light alkane components. For example, with 1 wt% concentration of TBTF in ethane at 298 K and 62 MPa, the achieved relative viscosity is 90, and it drops to 75 at 313 K. The relative viscosity significantly drops further to 20, 6, and 1.5 at 333,

A mixture of aluminium disoap and gasoline liquid is heated to high temperatures (368–373 K) to promote its dissolution by dismantling the intermolecular associations between the aluminium disoaps. Then it is cooled down to allow self-assembly of the disoap molecules, whereby the viscosity of the solution is enhanced significantly [88, 89]. Enick and co-workers [62] studied a single aluminium salt, referred to as hydroxyaluminum di-2-ethylhexanoate (HAD2EH). **Figure 9** depicted the association mechanism of HAD2EH. They found HAD2EH to exhibit a remarkable solubility in light hydrocarbon gases such as propane and butane. It is also capable of thickening these components at dilute concentrations. For example, at 293 K, HAD2EH concentrations in the range of 0.2–1 wt% were capable of increasing the viscosity of the solution by 10–100-fold as tested in a high-pressure falling-ball cylinder viscometer. However, the solution formed was hazy, due to a portion of the HAD2EH molecules

forming solid fibres in both liquid propane and butane at high pressures.

*Chelation mechanism and micellar structure of phosphate ester/metal ion complex [96].*

*Molecular structure of phosphate di-/monoester, phosphonic acid ester, and dialkyl phosphinic acid [89].*

**82**

**Figure 11.**

**Figure 10.**

**Figure 9.**

*Association mechanism of HAD2EH molecules [89].*

Dhuwe et al. [82, 89] examined blends of HAD2EH in several NGL (i.e. ethane, propane, and butane) individually under a range of pressures (34–62 MPa) and temperatures (298–273 K). HAD2EH was soluble in propane and butane while insoluble in ethane. At a temperature of 298 K, HAD2EH was insoluble in all light alkanes. Heating to 373 K, high pressure, and stirring were required to attain dissolution, followed by cooling to a temperature above 313 K to obtain a single phase solution. If the solution is cooled down to a temperature of 298 K, HAD2EH precipitated in both propane and ethane. Accordingly, it was observed that HAD2EH was an effective thickener in butane and propane at temperatures as low as 313 K. For example, at a concentration of 0.5 wt% HAD2EH and temperatures of 333–373 K, butane viscosity increases by 15–19-fold, while propane is thickened by 2–3-fold [82, 89].
