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A Comparative Study of Alternate Flowsheets Using Orimulsion as Feedstock

This paper examines the feasibility of gasifying Orimulsion and converting the resulting synthesis gas to Fischer-Tropsch hydrocarbons. Four process flowsheets of increasing complexity, increasing hydrocarbon yield, and decreasing power production are compared. In addition, power generation via Orimulsion combustion in a boiler followed by power generation in a steam cycle and gasification of Orimulsion followed by power generation in a combined cycle are used as reference points for power production. Carbon dioxide emissions from all flowsheets are compared. INTRODUCTION Fischer-Tropsch (F-T) synthesis converts hydrogen and carbon monoxide into a wide boilingpoint range of hydrocarbons. The hydrogen and carbon monoxide (synthesis gas) can be produced from a variety of carbon-bearing feedstocks. The resulting F-T hydrocarbon stream can be further processed to specific boiling-point fractions and upgraded to high-value products. Of special interest is the diesel fuel fraction because it requires little processing from the F-T oil and it has desirable characteristics including very low sulfur and aromatic content, high cetane index, and it burns exceptionally cleanly in a compression-ignition engine. F-T technology was invented in Germany in the 1920’s and supplied that country with liquid fuels and other products during World War II. Since that time, interest in the technology has come and gone, generally in phase with increases in the cost of crude oil or supply restrictions. Two of the first plants in the U.S. were the Carthage Hydrocol Plant in Brownsville, TX in the late 1940’s (1) and the U. S. Bureau of Mines plant in Louisiana, TX in the early 1950’s (2, 3). A recent peak in F-T interest appears to have been stimulated by several factors including environmental issues and the resulting interest in clean-burning liquid fuels, a desire for fuels derived from secure, domestic feedstocks, interest in exploiting stranded or associated gas resources and heavy oil residues, among others. A recent investor research report (4) covers the history and background of F-T as well as current efforts in the field. Summarizing the current state of F-T technology is (5). Rentech, Inc. has been developing F-T technology since 1981. This work has involved catalyst development and manufacturing, pilot-plant operation, product characterization, flowsheet development, process modeling, and the design, construction, and operation of full-scale plants. Rentech focuses exclusively on three-phase slurry reactor technology and iron-based catalysts. In a slurry reactor, finely divided catalyst particles are suspended in a liquid hydrocarbon wax medium. Synthesis gas is introduced into the bottom of the reactor and bubbles up through the wax/catalyst slurry. Temperature control for the exothermic reaction is provided by cooling tubes suspended in the slurry and the resulting heat removed is usually used to generate steam for process or power requirements. The slurry reactor is attractive, especially for small plants, because of its low cost and excellent temperature control, catalyst inventory and catalyst consumption (6). Hydrodynamics plays a critical role in a slurry reactor via bubble diameter, catalyst distribution, and mass transfer (7). Therefore, the design of the synthesis gas distributor, reactor internals, and the reactor operating parameters are critical to successful operation. Typically, empirical correlations (8) have been used to estimate these design parameters although recently there have been successful attempts to predict gas holdup and backmixing by numerical methods (9). Rentech’s focus on iron-based F-T catalyst is based on a number of reasons including cost, availability, disposability/toxicity, and the ability to work with a wide range of synthesis gas hydrogen to carbon monoxide ratios. Because of this last advantage of iron-based F-T catalysts, a wide range of feedstocks can be considered including natural gas or other light hydrocarbon gases (10, 11), liquids including Orimulsion , naphtha, heavy oil, asphalt, refinery bottoms, and solids including coal (11), bitumen, petroleum coke and biomass (12).