Synthesis of pentaerythritol ester lubricants from palm oil methyl ester for refrigeration system
Polyol ester-based lubricants have been introduced in HFCs refrigeration system to replace conventional mineral oil-based lubricants due to compatibility issue of miscibility and solubility between lubricant and refrigerant. Polyol ester specifically the pentaerythr...
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Format: | Thesis |
Language: | English |
Published: |
2019
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Subjects: | |
Online Access: | http://psasir.upm.edu.my/id/eprint/85620/1/FK%202020%2033%20-%20ir.pdf |
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Summary: | Polyol ester-based lubricants have been introduced in HFCs refrigeration system to
replace conventional mineral oil-based lubricants due to compatibility issue of
miscibility and solubility between lubricant and refrigerant. Polyol ester specifically the
pentaerythritol (PE) ester that has high viscosity is known to have good miscibility with HFCs
refrigerant and can be synthesized from methyl ester of vegetable oils via
transesterification reaction. This research began with the optimization of
transesterification reaction between palm oil methyl ester with pentaerythritol to produce PE
ester at various reaction conditions. Kinetics study was performed on the
transesterification reaction using integral method. Subsequently, the physicochemical
properties also miscibility and solubility test with HFC refrigerant of PE ester base oil were
analyzed and compared with commercial polyol ester (POE) RL 68H for refrigeration
system. The reaction parameters selected for the optimization study were reaction temperature from
140°C to 170°C, molar ratio of POME to PE from 4:1 to 5:1, vacuum pressure from 5 to 50 mbar,
catalyst concentration from 0.5% to 1.5% w/w and stirring rate from 300 rpm to 900 rpm and the
reaction conducted for 2 hours in a stirred batch reactor. The optimum conditions of
transesterification reaction were found at 160°C, molar ratio reactants at 4.5:1, catalyst
concentration at 1.25%, vacuum pressure at 10 mbar and stirring speed at 900 rpm with the highest
composition of PE tetraester obtained at 36% w/w. The mechanism of transesterification reaction
was modelled as four-steps consecutive and irreversible reactions. The use of excess POME
and continuous removal of methanol via vacuum suppressed the backward reaction. The
reaction rate constants predicted follows the sequence of k1>k2>k3>k4. However, the
proposed reaction rate law model was only accurate for reaction of PE to PE monoester and reaction
of PE monoester to PE diester of reaction pathways. The activation energies for four stepwise
series reaction ranges from 49.02 kJ/mol to 181.59 kJ/mol and the highest activation energies
was obtained from the reaction of PE triester to PE tetraester. This explains the difficulty in
converting PE triester to tetraester. Finally, the PE ester was analyzed for its physicochemical
properties and was compared with commercial refrigeration polyol ester (POE) RL 68H. The
kinematic viscosity and density of PE ester were comparable with commercial POE. The pour point
obtained was at -24°C which is slightly higher than commercial POE. Nevertheless, the PE ester was not completely miscible
in refrigerant, HFC R134a. An appropriate surfact improve the
miscibility and solubility of PE ester with HFC refrigerant. |
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