Valorisation of Stranded/Smaller Gas Field through GTL - Conceptual Design Studies
Megha Aggarwal, Engineer
Dr R N Maiti, Dy. General Manager
Ganesh Prasad, General Manager
Engineers India Ltd

The share of natural gas in world energy mix is steadily rising and is expected to grow from the present 23 per cent to 27 percent by the year 2020 . However, the distant location of the gas reserves from the market and costly transportation limit the economic utilisation of the gas. The method to avoid the need of cryogenic transportation, equipment and tankage and which would bring remote natural gas to the market is its conversion to higher hydrocarbons like diesel and wax through Fischer-Tropsch synthesis route. The present work provides the technical insights of a semi-commercial size GTL plant based on the composition of Tripura Gas, SMR+ATR for syngas generation, fixed bed reactor for F-T synthesis and utilisation of tail gas towards monetisation of smaller gas field/stranded gas.

About 50 per cent of the proven natural gas reserves are distant from any sizeable market, e.g., large reserves in Middle East. The costly transportation of this gas through pipelines or as LNG further restricts its economic utilization. The application of Gas to Liquid (GTL) is emerging as one of the most sought after technology options, not only to valorise stranded natural gas but also to provide additional hydrocarbon security. GTL provides the opportunity to convert natural gas to usable liquid products which are of high value and can easily be shipped, obviating the need for dedicated cryogenic transportation, equipment and tankage.

Gas to liquid (GTL) is a three step process in which the C1 fraction of natural gas is first converted to syngas followed by F-T synthesis to produce heavier liquid hydrocarbons (C5+) which are then fractionated into usable products such as naphtha, diesel, gas oils, paraffinic waxes and lube base oils. Since the gas is sulphur free, the products are of premium quality and there is no residue in the form of asphaltenes and resins. The Syn crude can also be blended with conventional crude oil. The blending of GTL products with conventional products helps refineries to meet future clean energy requirements and reduces the load on hydro-processing units in refineries.

The process is best suited for stranded gas fields where pipeline transfer of gas is not feasible. Current projects are being pursued in regions that are large gas producers and also have LNG facilities, so that GTL supplements the LNG operations rather than compete with them. However, the application of GTL on a stand alone basis appears to be a viable proposition given the benefits in terms of investment and operating costs as compared to LNG along with the premium that is possible for GTL liquids vis-a-vis conventional crude oil(s). Such GTL projects could come up at locations like Iran, Middle East, Nigeria and CIS countries where abundant natural gas resources are available at competitive price.

While India is not a major gas producer, GTL technology would help to provide additional hydrocarbon security since overseas natural gas can be transformed to usable products at site without the hassle of transferring natural gas through pipelines/LNG. This technology would also prove useful in valoriszing stranded gas fields in the North east, especially when the facility is integrated with an existing refinery. In the following section a design configuration of 6000 BPD GTL plant through Fischer Tropsch synthesis route along with technical details based on Tripura Gas composition (2.8 MMSCMD) is established, which could facilitate an investment decision to set up the project. The design would integrate the various steps of a GTL facility and would be based on inputs obtained from experienced licensors as well as indigenously developed information/know how, as explained in the following sections.

Key Technical Components of GTL Plant
The basic three steps in the GTL process are synthesis gas (a mixture of H2 and CO) production, Fischer-Tropsch synthesis and product upgradation. The various schemes are available for conversion of natural gas to syngas, which include steam methane reforming, partial oxidation and combination of both. The F-T synthesis is carried out in either a fixed bed reactor or slurry reactor with iron/cobalt catalyst. The synthesised liquid products are fractionated and mildly hydrocracked to meet product specifications.

Synthesis Gas Generation
Synthesis gas is produced from natural gas using one or combination of the available three technologies:
1)Steam Methane Reforming (SMR): It is an endothermic reaction which requires 800C to convert methane and steam to syngas in the presence of a catalyst.

2)Partial Oxidation (POX): It is an exothermic reaction in which oxygen is consumed to extinction in the temperature range of 900-1400C to form syngas .

3)Autothermal Reforming (ATR): It is a combination of SMR and POX which requires 0.3 kg of oxygen per kg of natural gas and produces syngas in the H2/CO molar ratio of 1.5-2.7. Reactions are carried out in various forms of exchange reformers and reactors filled with equilibrium catalysts.

4)Secondary Reactions: Many reactions take place in syngas reactor along with primary reactions, e.g., water gas shift reaction, CO2 reforming, carbon deposition, carbon conversion, etc.

EIL has designed and implemented SMR processes for hydrogen production. These reactors operate in high steam environment to maximise on hydrogen. The proposed application for GTL process involves operation at significantly lower steam rates. The partially steam reformed gas is routed to Autothermal Reforming (ATR) for further conversion of methane to carbon monoxide. Carbon dioxide recycle is also carried out to improve overall carbon efficiency of the GTL process.

The integration of SMR technology with ATR along with CO2 recycle stream is an important step with a view to reduce oxygen and steam requirement and to optimise on hydrogen export. The overall synthesis gas process configuration is also need to be optimized to produce desired hydrogen-CO ratio in syngas that is suitable for downstream FT process. The ratio itself is a function of the catalyst to be used in FT synthesis so that to maximise on overall carbon and thermal efficiency.

Fischer Tropsch (FT) Synthesis
The hydrogenation of carbon monoxide by FT process primarily produces saturated compounds of the homologous hydrocarbon series. Depending upon the catalyst, operating temperature and type of process employed, hydrocarbons ranging from methane to higher molecular weight paraffins and olefins are produced.

It is well known that group VIII transition metals, iron, cobalt, nickel and ruthenium are the most active as potential FT synthesis catalyst. Catalyst development activity has already centered on the preference for linear alkanes and diesel fuel production. Research has focused on achieving the highest chain growth possible to produce a product distribution of heavy waxes that can be hydrocracked to the desired products in the middle distillate range. It is reported that catalyst consisting only of Co or Fe are generally unsatisfactory since they produce mainly light hydrocarbons. The addition of promoter (typically noble metals) is essential to shift the selectivity towards higher molecular weight hydrocarbons. The FT synthesis can be summarized by the following reactions:

The first two reactions describe the formation of higher hydrocarbon and of methane respectively. The CH2- group in equation 1 indicates a link of a hydrocarbon molecule. Carbon monoxide can also react with water vapour, which is formed in reaction 1 & 2 or already present in syngas, to carbon dioxide and hydrogen. The yield of higher hydrocarbon can be maximized by suppressing reaction 2 & 3 to the maximum extent possible (catalyst specific ).

FT synthesis reaction is highly exothermic, thus heat removal is a very important factor in the selection and design of a commercial reactor. In principle three different types of reactor design are used for the FT synthesis.

1. Fluidised bed reactor
2. Fixed bed reactor
3. Slurry reactor

Fluidised bed reactors are not suitable for production of heavy waxes due to agglomeration of catalyst particles and de-fluidisation problems caused by deposition of heavy compounds present in the product on to the catalyst. The slurry and multi-tubular fixed bed reactors are preferable for production of high value linear FT waxes. Since fixed bed reactor has more advantages in wax separation, the flow-scheme is proposed to be developed with fixed bed reactor configuration only.

Concept Studies: Design Configuration of 6000 BPD GTL Plant
Concept design studies were carried out for 6000 BPD GTL plant based on the following:

  1. Tripura Gas composition is taken as the basis (Table 1)
  2. SMR+ATR simulations were carried out using FEM of Gibbs Energy algorithm in Aspen PLUS
  3. Fixed bed FT reactor ( Reactor Model; developed in-house)
  4. Utilization of tail gas

The simulation of syngas generation was carried out using Gibbs free energy algorithm which is considered reasonable in view of high temperature operation. The flow scheme given in Figure 1a envisages Steam Methane Reforming followed by Auto Thermal Reforming. The syngas from ATR is corrected with regard to H2/CO ratio with hydrogen - Pressure Swing Adsorption process already developed by EIL. The generated syngas is passed through FT reactors to produce liquid hydrocarbons and the flow scheme is shown in Figure 1b. The simplified mathematical model for multi-tubular fixed bed reactor was developed and used for simulation to predict conversion, yield, temperature and pressure profile along the reactor. The reaction rate constants were obtained from literature and were further extrapolated in the present operating range. The sensitivity analysis for water gas shift reaction was also carried out and it needs to be suppressed for meeting the present commercially achieved product yields. The products from FT synthesis were empirically fractionated in different carbon numbers. The overall material balance and enthalpy balance are shown in Figure 2 and Table 2.

The mass conversion comes out to be 64.9 per cent. The broad specifications of equipments (FT reactors, compressors and exchangers) are shown in Table 3 . Utilities like cooling water, steam, etc. are required for different purposes and their requirements are given in Table 4. No catalyst recovery from wax and catalyst regeneration scheme was considered. The downstream of FT synthesis was considered to be syn crude which can be easily blended with conventional crude.

Conversion of gas to liquid through FT synthesis is an important option in bringing remote natural gas to the market. The various technical components of GTL plant through FT route for natural gas utilisation are presented in this paper. Conceptual design configuration of 6000 bpd GTL plant is provided with recycle of CO2 and utilization of tail gas. Mass conversion came out to be 64.9 per cent while thermal conversion is 60.1 per cent for hydrocarbon liquids and 75.8 per cent when hydrogen production is considered. High pressure steam (450C, 42 atm) and excess hydrogen is available for export or integration with refinery. Overall an attempt is made to provide lots of insights, facts and figures, useful to the oil and gas industry for taking informed decision about GTL plants.