Apr-2011

Small-scale gas to liquids

Microchannel reactor technology is on trial for the small-scale production of liquids from stranded gas

Andrew Holwell
Oxford Catalysts Group

Article Summary

Associated gas and stranded gas — gas reserves located far from existing pipeline infrastructure and markets — are potentially abundant sources of energy that are commonly squandered. Rather than being transported to refineries for processing, stranded gas is often just left in the ground. Associated gas produced along with oil is frequently disposed of by flaring — a wasteful and environmentally unfriendly process that is increasing subject to regulation — or by re-injection back into the reservoir at considerable expense.

According to the World Bank, 5.25 trillion cubic feet (tcf, approximately 140 billion m3) of associated gas — the equivalent of 27% of US gas consumption — was flared in 2008. The giant gas flares that light the night sky in Russia, Nigeria, Iran, Iraq, Algeria, Kazakhstan, Libya, Saudi Arabia, Angola and Qatar are a highly visible reminder of this waste. A further 12.5 tcf of gas was re-injected. In addition, there is thought to be as much as 3000–6000 tcf of “stranded” gas —unassociated natural gas already found but without cost-effective access to the world market and, therefore, not yet being produced.1,2  The reason? Cost-effective technologies for capturing these wasted resources are not available.

The available options for capturing the value of onshore stranded gas include liquifying or compressing the gas (to LNG or CNG), then shipping it in specially designed tankers. Both have serious drawbacks at small to medium scales, particularly in terms of cost. The economics dictate that new LNG projects are only economically viable for producing gas volumes greater than 500 mcfd over distances of 
4200 km (2500 miles) or more. Although CNG is a good option for transporting smaller volumes with throughputs as low as 100 mcfd, over shorter distances in the range of 1000–2500 km (600–1500 miles) it is too expensive to be used when reserves are more remote. 

A third way
For both stranded and associated gas,  gas to liquids (GTL) offers a potentially attractive alternative. Like LNG and CNG, GTL densifies the energy to make it cheaper to transport. In principle, GTL products can be transported in the existing petroleum infrastructure. But in order to work efficiently, GTL plants must be designed to work on a very large scale. Conventional GTL technology is only economically viable for large-scale plants producing around 30 0000 b/d of liquid fuel and this requires a very large capital investment.

This has proved to be a considerable barrier to the progress of the GTL industry. For example, although several larger-scale plants have been developed or announced in recent years, only three have made it off the drawing board:
• Sasol’s Oryx plant in Qatar was completed in 2006, but, due to an extended start-up period, did not achieve its nameplate production level of 34 000 b/d until late 2009. Costs rose from an initial estimate of $950 million to $1.5 billion
• Chevron’s 34 000 b/d plant at Escravos in Nigeria will cost an estimated $6 billion and is expected to start up in 2013
• Shell’s Pearl GTL plant in Qatar, the world’s largest GTL project, with an ultimate capacity of 140 000 b/d and an estimated price tag of $18–19 billion, is expected to start up in 2011.

But thanks to advances in the development of technology for distributed or small-scale GTL technology, a much more flexible and economical option for capturing associated gas, both on- and offshore — in the form of modular GTL technologies — is on the horizon. These systems are designed to operate efficiently and economically when producing just 500 b/d. When combined with petroleum crude, the synthetic crude produced from associated gas can be stored on-board or could be transported to shore along with the produced oil via existing tankers and pipelines, eliminating the need for a separate logistics system to transport the gas to market. Small-scale GTL could also prove useful for capturing shale gas resources now being exploited in the US.

Shrinking the hardware and scaling down the cost
The GTL process involves two operations: steam methane reforming (SMR), to convert natural gas into a mixture of carbon monoxide (CO) and hydrogen (H2), known as syngas, followed by Fischer-Tropsch (FT) synthesis to convert the syngas into a liquid fuel (see Figure 1). In SMR, the methane gas is mixed with steam and passed over a catalyst to produce a syngas consisting of H2 and CO. The reaction is highly endothermic, so requires the input of heat. This can be generated by the combustion of excess H2. The syngas is then converted into various forms of liquid hydrocarbons via the exothermic (heat-producing) FT process, using a catalyst at elevated temperatures.

For small-scale GTL, the challenge is to find ways to combine and scale down the size and cost of the SMR and FT reaction hardware while still maintaining sufficient capacity. And for offshore installations, whether they are drill ships or floating production storage and offloading units (FPSOs), the equipment also needs to be able to withstand high-intensity wave motion.

Fixed or slurry bed reactors — the two conventional reactor types currently used in FT plants — only function well and economically at capacities of 30 000/day or higher, and the technology does not scale down efficiently. However, new reactor designs, such as micro- and mini-channel reactors, offer a practical way forward.

Both types of reactor consist of compact, modular fixed-bed designs with process channels that are much smaller and provide a greater surface area than conventional FT reactors. Their small size, lighter weight and lower profile are advantages in an offshore environment (see Figure 2). 

Mini vs micro
Development of small-scale GTL depends on finding ways to intensify the SMR and FT processes. This relies on developing ways to enhance heat and mass transfer properties and increase their productivity. Since heat transfer is inversely related to the size of the channels, reducing the channel diameter is an effective way of increasing heat transfer and thus intensifying the process by enabling higher throughput.