Observation of the heavy crude oil dissolution behavior under supercritical condition of water
Authors: Kishita Atsushi , Watanabe Noriaki , Vilcaez Perez Javier 
Published: December 2009

In development of on-site partial upgrading technology of the bitumen by using supercritical water as a reaction solvent, it is important to understand the role of water in the reaction field. Therefore experiments were carried out using a batch reactor system operating with varying reaction pressure at a temperature of 703 K, and reaction time of 15 min. From the results such as the viscosity and the boiling point distribution of oil product etc., it can be seen that higher density supercritical water inhibits the cracking of bitumen to lower molecular weight substance. In the case of water with alkali it is found that alkali also helps to inhibit the cracking to lower molecular weight substances. Furthermore, the amount of the hydrogen became higher. It was inferred that some mechanism of hydrogen generation takes place with the addition of alkali. In experiments using a continuous flow reactor system at a temperature of 703 K, it was confirmed that higher density supercritical water also inhibits the thermal cracking. However, that heavy ingredient of bitumen was collected in the reaction chamber bottom simultaneously. These findings suggest that bitumen and supercritical water did not become homogenous phase in reactor. To clarify the phase behavior of bitumen and supercritical water, visible type autoclave was used for in-situ observation. Water and bitumen or its fractions divided by solvents fractionation were put into the visible-type autoclave, dissolution of bitumen or its fractions to water was observed with increasing temperature above critical temperature of pure water. Under these conditions, there were two phases which were separated by an interface were observed. On the other hand, synthesis oil which has bitumen components expect of asphaltane (pentane insoluble) was dissolved to supercritical water. Result shows that heavier portion of asphaltane cannot be dissolved to supercritical water. These components had become coke in thermal cracking of bitumen was considered. Based on this observation result, the anti-coking effect of supercritical water was thought that it is necessary to consider the dispersion of the supercritical water molecular to the asphaltene like components of bitumen.

Introduction

When oil sand in Canada is developed by SAGD (steam assisted gravity drainage) methods, it is necessary to combine these methods with on-site technologies for both visbreaking and upgrading. It is expected that supercritical water treatment of bitumen combined with SAGD is suitable for on-site upgrading technology because hot water can be used circularly.

Supercritical water can prevent bitumen from coking. It has been suggested that the supercritical water treatment of organic polymers such as polyethylene can generate lower molecular weight substances due to too much inhibition of pyroysis. Our previous studies have shown that bitumen could be cracked to lower molecular weight substances to remove sulfur in supercritical water with an alkali[1].

However, the role of supercritical water as a solvent is not fully understood. The role of supercritical water in upgrading bitumen was therefore investigated by varying the experimental parameters, such as reaction pressure, that influence the density of water and hydroxyl ions in this study. Despite of the huge amount of reserves, heavy crude oil resources have not been developed extensively due to difficulties in the production, transportation and refining processes. Although the recent Steam Assisted Gravity Drainage (SAGD) technology has improved the productivity, additional technologies is required for the effective transportation and refining processes. The on-site upgrading process using water at supercritical conditions is one of the most favorable options when combined with the SAGD, particularly for improvement of the transport efficiency, since our recent studies on Canada oil sands bitumen showed considerable thermal decomposition and resulting viscosity reduction with less coke production, at the supercritical conditions. However, some expected phenomena such as dissolution of bitumen in water and resulting physical properties have not been revealed, which gives difficulties in understanding of the reaction mechanisms, and in design of effective reactors. The objectives of this study were therefore to examine the dissolution of bitumen in water and to discuss particularly the mechanism of the anti-coking effect, by means of in-situ observations of water-bitumen or -oil fractions system at supercritical conditions using a visible-type autoclave.