Jul-2015

Improved feedstocks for ethane crackers

Cryogenic recovery technology enables wet shale gas to become a valuable source of liquids for petrochemicals production.

RON KEY
Linde Process Plants

Article Summary

The petrochemical industry is booming in the US, largely fuelled by the low cost of shale gas and shale gas liquids as feedstocks. New drilling and exploration techniques enable natural gas and liquids to be extracted cost-effectively from shale rock and, when employing state-of-the-art cryogenic gases technology, even wet shale gas basins can now be developed efficiently to deliver valuable natural gas liquids for various industries.

While natural gas reserves occur in many other parts of the world, it is in the US where industry is most determined to develop unconventional sources of natural gas in order to reduce dependence on energy imports. But shale gas is a lot more than just a source of energy. Not only is it an ideal fuel for power plants and district heating, it also delivers valuable feedstock such as ethane for the petrochemical industry. Natural gas can be easily converted into base chemicals such as ethylene, which is a hydrocarbon compound and precursor of many common chemicals.

The best known are probably the plastics found in everything from packaging and cable insulation through car seats to toys, according to Thomas Rings, A.T. Kearney partner and expert for energy and utilities. The American Chemistry Council (ACC) estimates that, by 2020, the shale gas boom will fuel investments to the tune of almost $70 billion in the American petrochemical industry alone. Yet gas fields that will be of real interest to ethylene producers are still at the very early stages of development. Ethylene produced from shale gas has the potential to unleash a new industrial revolution in the US.

Before that can happen, however, there are hurdles to overcome. Processes for the recovery of shale gas liquids typically used thus far are not the most efficient. Efficient processing of these valuable raw materials calls for highly specialised know-how as the quality of shale gas reserves can vary significantly from one shale basin to another.

Wet shale gas
Until now, extraction efforts have mainly focused on dry gas, which is an excellent source of energy containing almost pure methane with a small share of longer chain hydrocarbons. Wet shale gas is a much more interesting prospect for industry, as these reserves contain extremely valuable raw materials. Their compositions can vary greatly. Wet shale gas contains less methane, but higher concentrations of ethane (C2) and longer-chain hydrocarbons such as propane (C3) and butane (C4) and a variety of higher molecular weight hydrocarbons. Referred to as natural gas liquids (NGL), they are the perfect feed for gas crackers, which produce ethylene. This translates into a huge market opportunity. Chemical insight and forecasting company IHS Chemical predicts that global demand for ethylene will rise to around 160 million tonnes a year by 2017.

Unconventional reserves present a number of challenges to chemical engineers, however. Wet shale gas is occasionally contaminated with impurities such as trace amounts of mercury and sometimes hydrogen sulphide (H2S) as well as carbon dioxide (CO2). These substances must be almost completely removed before the ethane and propane in the shale gas can be fed to an olefin plant. Cryogenic condensation technology that was originally designed for recovery of ethylene, propylene and heavier hydrocarbons for refinery and petrochemical plants has evolved for the recovery of ethylene, ethane, propylene, propane as well as other C2s and C3+ hydrocarbons from natural gas. Plants that  previously compressed and cryogenically cooled conventional raw natural gas only, in order to crack it into its constituent parts, can now also easily handle wet shale gas.

With so many refineries either being fully integrated or located next to petrochemical plants, existing cryogenic C2+ and C3+ recovery technology had to be adapted to meet evolving market demands – in other words, increase plant flexibility and recover more liquid hydrocarbons. This involved quite a few technical refinements. The higher concentration of hydrocarbons with a high molecular weight in wet shale gas made it also necessary to modify the feed treatment process.

The process
Both the original technology and its enhanced sister system are cryogenic recovery technologies utilising a turbo-expander to recover energy while cooling the feed gas. These cryogenic technologies are unique in their ability to process low-pressure hydrogen bearing refinery fuel gas streams and obtain high recoveries with less compressor and/or refrigeration horsepower than conventional or competing cryogenic processes.

To protect the unit against upset conditions, feeds may first pass through a coalescing filter/separator designed to remove solid particles and liquid droplets that may carry over from upstream processes. Although the cryogenic systems can tolerate small quantities of H2S and CO2 these compounds are not desirable. The use of an amine treating unit for removal of acid gas components removes these compounds in an absorption process as a feed conditioning step. The next phase is to compress the feed streams unless they are already at elevated pressures. Air coolers or cooling water reduces the temperature of the gas downstream of the compressor to remove the heat of compression.

To avoid ice and hydrate formation in the cryogenic section of the process, the water content of the gas is reduced to an acceptable level through adsorption in molecular sieve desiccant beds. The regeneration of the beds is a batch process, where two or even multiple adsorption beds are used. One or more of the adsorption beds is being regenerated to restore their capacity while the others are on-line and drying the feed gas. A recycle portion of the dry gas can be heated and used for regeneration of the beds to drive off the adsorbed water. Cooling of this stream condenses the removed water before it recycles and combines with the feed gas. A portion of the residue gas may also be used for regeneration on a once-through basis. Downstream of the adsorption beds, the gas passes through a dust filter to remove any particulate carry-over before subsequent processing.

After dehydration, the feed gas flows into the cold section of the process, where cooling by exchange of heat with the residue gas and cold separator liquids takes place using a brazed aluminium plate fin heat exchanger. Although it is not always a requirement, the gas may be further cooled using external refrigeration before it goes to the cryogenic portion of the process.

Following cooling, the feed gas is partially condensed and delivered to a vapour/liquid separator. The liquid then flows through the inlet exchanger to cool the feed gas before entering the deethaniser – or demethaniser for C2 recovery – for fractionation. The vapour flows to the inlet of the expander/compressor. As the gas expands, it provides the work/energy for the compression. The expansion and removal of energy cools the gas further and causes additional condensation. The expander discharges into the first tower of a two-stage fractionation process. The configuration and the combination of fractionation and heat transfer between these two columns is the proprietary, patented technology that gives this cryogenic technology its advantages – namely higher recovery at reduced horsepower – over competing technologies (see Figure 1).