Optimal design issues of a gas-to-liquid process

Abstract

Interests in Fischer-Tropsch (FT) synthesis is increasing rapidly due to the recent improvements of the technology, clean-burning fuels (low sulphur, low aromatics) derived from the FT process and the realization that the process can be used to monetize stranded natural gas resources. The economy of GTL plants depends very much on the natural gas price and there is a strong incentive to reduce the investment cost and in addition there is a need to improve energy efficiency and carbon efficiency.

A model is constructed based on the available information in open literature. This model is used to simulate the GTL process with UNISIM DESIGN process simulator. In the FT reactor with cobalt based catalyst, CO2 is inert and will accumulate in the system. Five placements of CO2 removal unit in the GTL process are evaluated from an economical point of view. For each alternative, the process is optimized with respect to steam to carbon ratio, purge ratio of light ends, amount of tail gas recycled to syngas and FT units, reactor volume, and CO2 recovery. The results show that carbon and energy efficiencies and the annual net cash flow of the process with or without CO2 removal unit are not significantly different and there is not much to gain by removing CO2 from the process. It is optimal to recycle about 97 % of the light ends to the process (mainly to the FT unit) to obtain higher conversion of CO and H2 in the reactor.

Different syngas configurations in a gas-to-liquid (GTL) plant are studied including auto-thermal reformer (ATR), combined reformer, and series arrangement of Gas Heated Reformer (GHR) and ATR. The Fischer-Tropsch (FT) reactor is based on cobalt catalyst and the degrees of freedom are; steam to carbon ratio, purge ratio of light ends, amount of tail gas recycled to synthesis gas (syngas) and Fischer-Tropsch (FT) synthesis units, and reactor volume. The production rate of liquid hydrocarbons is maximized for each syngas configuration. Installing a steam methane reformer in front of ATR will reduce total oxygen consumption per barrel of product by 40 % compared to the process with a standalone ATR. The production rate of liquid hydrocarbons is increased by 25.3%. The process with a standalone ATR has the highest CO2 emission.

The Fischer-Tropsch (FT) reactor is sectioned into stages. The design functions (decision variables) are optimized to maximize an objective function. The decision variables are fluid mixing (dispersion), heat transfer area distribution, coolant temperature, and catalyst concentration. With the chosen kinetic models for iron and cobalt based catalysts, staging of the FT reactor will increase the production rate of C11+. By introducing the cost of heat transfer area in the objective function, the total heat transfer area requirement is reduced which will increase the annual profit. The optimal mixing structure for a two stage Fischer-Tropsch is completely mixed (CSTR) for the first stage and plug flow (PFR) for second stage. Having less heat transfer area, the purpose of the CSTR is to level out the temperature peak. If we set the CSTR to a PFR, the peak temperature will exceed the maximum temperature.