GAS-to-liquid process involves the conversion of natural gas into a clean source of energy mainly diesel and naphtha.
Currently, many giant oil and gas companies have or plan to have demonstration and commercial GTL plants. For examples, Sasol of South Africa installed and operated the first commercial GTL plant based on coal feedstock about half century ago. The South African company had already, jointly with Qatar Petroleum, constructed and commissioned a 34,000 BPD capacity GTL plant in Ras Laffan, in Qatar. Expansion of the plant capacity to 100,000 BPD is being evaluated by Qatar Petroleum and Sasol-Chevron. Shell has as well a 15,000 BPD commercial plant in Malaysia and is considering jointly with Qatar Petroleum a giant GTL integrated complex of 140,000 BPD capacity.
The list of companies includes also Exxon-Mobil which is running a 200 BPD demonstration unit in Baton Rouge, Louisiana and is considering jointly with Qatar Petroleum a commercial scale GTL project with a capacity exceeding 150,000 BPD. In addition, BP has a 300 BPD demonstration unit in Nikiski, Alaska. ConocoPhilips has constructed a 400 BPD demonstration unit in Ponca City, Oklahoma which was commissioned in mid 2003. MarathonOil and Syntroleum are constructing a 100 BPD demonstration unit in Catoosa, Tulsa, Oklahoma.
The question is why this flurry of gas-to-liquid projects? The answer is very simple: the market for GTL diesel is huge. As the sulfur and aromatics specification for Diesel oil becomes and will continue to be tighter to comply with exhaust mission requirements… Production of refinery diesel with ultra low sulfur content will be expensive to the point that makes production cost of GTL diesel oil which is practically contains zero sulfur and not more than 1% aromatics close to that of ultra law sulfur diesel recovered from crude oil.
At period of low crude oil prices, production of synthetic fuel by GTL route was found to be uneconomic as compared to the price of fuel derived from crude oil and the application of the process was frozen.
The interest in the process was regenerated mainly due to the price increase of crude oil and the realization that crude oil supplies are finite. As mentioned earlier, the other driving force for the present interest in the GTL process is the increasingly stringent legislation which entails more efficient (deep) desulphurization of diesel derived from crude oil and accordingly the increase in cost of the production of this ultra low sulfur diesel.
The GTL process consists of three main steps:
- Production of Synthesis gas
- Conversion of the Synthesis gas to waxy hydrocarbon material
- Hydrocracking the waxy material to the desired products
First, the Synthesis Gas Production Step:
Methane is steam reformed to produce syngas, required for synfuel and other petrochemical products production according to the equation.
CH4 + H2O = CO + 3 H2
It can be concluded from the above equation that Steam Reforming produces high Hydrogen: Carbon Monoxide ratio of about 3 which is not optimum for GTL production requiring a ratio of 2. However, the ratio from the steam reforming can be adjusted by removing the excess hydrogen by Membrane separation or Pressure Swing adsorption. The optimum H2:CO ratio for the GTL process can be achieved by “Partial Oxidation” of Natural Gas (Methane) in which natural gas is burned at high temperature according to the equation:
2CH4 + O2 = 2 CO +4 H2
Partial Oxidation of Methane requires an oxygen plant, using cryogenic air separation, for the production of Oxygen from air.
A process technology had been developed in which air is used in place of pure oxygen thus eliminating the cost of oxygen plant.
The third process to produce syngas with the required Hydrogen: Carbon Monoxide ratio of 2 is the Auto Thermal Reforming process. This process can be considered as a mixture of the other two processes namely Steam Reforming and Partial Oxidation. CO2 can be added to the blend through a recycle stream.
Second, Synthetic Fuel Production Step (F-T Process):
In this step, the Carbon Monoxide and Hydrogen produced in the first step is either passed through a Fixed Bed Catalyst of Cobalt / Iron or the mixture is bubbled through a Hydrocarbon slurry containing a catalyst. The product of the reaction is a waxy product send to the upgrading step. Hydrogen is reacted with Carbon Monoxide to give a long chain waxy product according to the equation:
nCO + (2n +1) H2 = CnH2n + 2 + nH2O
nCO + 2n H2 = n(- CH2 – ) + n H2O
In the above equation, the term – CH2 – represents basic building block of the paraffin molecule. Straight chain paraffins are main products of the F-T process with minor quantities of iso-paraffins and olefins also present in the products. Because of the paraffinic nature of the product, F-T diesel has high cetane number.
Third, The Hydro-cracking (Upgrading) Step:
In the upgrading step, the waxy paraffinic product is cracked in the presence of Hydrogen to any required molecular weight products.
The olefin molecules (CnH2n) become saturated with Hydrogen creation a range of paraffins. Thus, Naphtha is hydro treated and olefins are saturated to the corresponding parrafins.
By: Osama Abdul Rahman
General Manager of Orient Environmental Consultants