MTBE: A Carrier For Heavy Oil Transportation And Viscosity Mixing Rule Applicability

Abstract

The pipeline transportation of bitumens is made difficult by the excessive viscosities of the heavy oil. The current solution to this problem has been the addition of C ₄+ paraffinic liquid hydrocarbons. There are a number of drawbacks, however, to the use of this diluent. To overcome these difficulties the alternate use of MTBE (methyl tert-butyl ether), a gasoline additive, as a liquid solvent has been investigated. With the potential for Alberta to become a substantial exporter of both bitumen and MTBE, the prospect of transporting the two constituents simultaneously is promising.

A liquid viscosity model has been utilized to calculate the component viscosities as a function of temperature, and then use a mixing rule to determine blend viscosity as a function of composition. It was found that the addition of 25-30% MTBE (mass basis) was sufficient to reduce the viscosity of the Cold Lake bitumen to meet pipeline specifications. The viscosity model was found to be accurate over the entire range of compositions and temperatures examined. A simple distillation was performed on a 35% MTBE (mass basis) blend to study the ease of separation of the liquid diluent from the heavy oil phase at atmospheric conditions.

Introduction

The most detrimental factor in the production and transportation of bitumen is its excessive viscosity. A typical Cold Lake heavy oil will have a viscosity greater than 500,000 mPa-s at 4 °C, while the viscosity of a typical conventional oil may be 3-4 orders of magnitude lower ⁽¹⁾. An increase in temperature of the bitumen has been shown to greatly reduce its viscosity ⁽²⁾. This temperature-viscosity relationship is used in many enhanced oil recovery (EOR) techniques to produce heavy oil and bitumen at economical Rates ⁽²⁾. However, to adequately heat an entire pipeline for the length of the transportation is clearly unrealistic. The current solution to this problem has been the addition of liquid diluents to the bitumen to reduce its viscosity.

The most common diluent used in this manner is a C ₄+ paraffinic liquid hydrocarbon (or condensate) obtained from natural gas processing. This solvent generally achieves the needed viscosity reduction, but also has some inherent drawbacks. The availability of this liquid hydrocarbon is sometimes in question, and there may not be sufficient buyers to purchase the solvent at the end of the transportation. In fact, many suppliers of the bitumen consider the diluent to be a lost expense, and do not expect to be reimbursed. Also, the solubility of the solvent in the bitumen may pose some difficulties. The bitumen can be diluted with the condensate up to a critical dilution ratio, after which asphaltene deposition occurs ^(3,4). Therefore it can be seen that the ideal solvent choice is one which would achieve the required viscosity reduction, not cause the asphaltene fraction to precipitate, and also possess a substantial consumer market.

Selection of MTBE

The decision to investigate the bitumen-MTBE (methyl tert-butyl-ether) system was brought on by some favorable aspects. The demand for MTBE as an octane advancing additive for gasoline is growing rapidly as clean air legislation begins to take effect.