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Phase equilibria of refrigerant gas hydrate systems in the presence of sucrose.

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Date

2015

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Abstract

The South African sugar industry has the potential to be one of the largest suppliers of renewable energy into the national energy grid. The majority of the sugar mills obtain their fuel requirements by burning of final bagasse produced during sugar cane processing. In some cases, surplus energy is produced which can often go to waste. However, if this surplus energy could be injected into the national energy grid, it will not only increase the industry’s profitability, but also assist in meeting government’s renewable energy targets. One such area of the sugar milling process where excess energy could be recovered is the highly energy-intensive evaporation process. This study investigates gas hydrate technology as a possible aqueous solution separation process. It investigates gas hydrate technology as a feasible technique for increasing the solids content in aqueous carbohydrate systems. The aim of this study is to obtain hydrate dissociation conditions of various refrigerant hydrates in the presence of sucrose. The hydrate-vapour-liquid (HVL) phase equilibrium measurements were performed using the isochoric pressure search method with a newly developed high-pressure apparatus. In order to verify the temperature and pressure calibrations, vapour pressure measurements for 1,1,1,2-tetrafluoroethane (R134a), R410a and R507 were performed. The experimental apparatus and method were verified by conducting hydrate-vapour-liquid equilibrium measurements on known test systems. These include CO2 (1) + water (2) + {0 or 20} wt.% sucrose. New systems measured include 1,1,1,2-tetrafluoroethane (1) + water (2) + {12 or 15} wt.% sucrose (3), R410a (1) + water (2) + {12 or 15} wt.% sucrose (3) and R507 (1) + water (2) + {12 or 15} wt.% sucrose (3). In addition, a sucrose sample supplied by the Sugar Milling Research Institute (SMRI) was also investigated. From the measurements performed in this study, it is found that the presence of sucrose exhibits an inhibition effect on hydrate formation. This is seen by the shift of the H-V-L equilibrium phase boundary to either higher pressures or lower temperatures. Additionally, through measurements performed on the SMRI sample it can be seen that the presence of additional carbohydrates (glucose and fructose) in the system increases the effect of inhibition on hydrate formation. The van der Waals-Platteeuw solid solution theory is used to model the hydrate phase (Parrish and Prausnitz, 1972). For systems containing carbon dioxide, the vapour phase is modeled using the Peng-Robinson Equation of State (Peng, 1976), while for systems containing refrigerants, the vapour phase is assumed ideal based on the assumptions of Eslamimanesh et al. (2011). In all systems, the liquid phase is modeled using the UNIFAC model. The inhibition effect of sucrose is accounted for by using a purely empirical correction method proposed by Englezos and Hall (1994). The predicted results compare well to the experimental results. An approximate cost analysis, based on the findings of Heist and Barron (1983) was also performed. This analysis determined that the capital costs associated with the construction of a hydrate separation technology would amount to approximately R33 million with the operations and maintenance costs amounting to approximately R0.08 per kg of feed processed. These figures compare well to the costs associated with evaporation processes. In order for hydrate separation technologies to be implemented in the sugar industry, crystal size and quality need to be determined. Therefore, investigations regarding such properties should be conducted. Possible methods to be investigated include Spectroscopy, X-ray Diffraction and Laser Scattering. In addition, techniques to determine the concentration of the final solution, such as centrifugation of the hydrate slurry, need to be established.

Description

M. Sc. Eng. University of KwaZulu-Natal, Durban 2015.

Keywords

Sugar--Manufacture and refining--By-products., Refrigerants--Thermal properties., Sucrose., Hydrates., Natural gas--Hydrates., Chemical equilibrium., Theses--Chemical engineering.

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