Maximising ethane in liquids crackers

Economics for energy and feedstock supply favour ethane feed cracking, but plant constraints must be examined rigorously before maximum ethane feeds are pursued

Technip Stone & Webster Process Technology

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Article Summary

Recent advances in the recovery of gas from shale reserves in the US have resulted in a shift in the economics of US ethylene plant feedstock. The American Chemistry Council estimates that US shale deposits contain 100 years of natural gas supply, a “game changer” that could rejuvenate America’s chemical industry. Strong ethane supplies are positioning the US as the most competitive, low-cost ethylene producer, resulting in increased investments in ethane recovery and pipelines. As a result, several companies have already announced their plans for major investments in the US ethylene sector.

At present, many ethylene producers currently cracking liquid feedstocks such as naphtha or gas oil are either maximising or considering maximising the cracking of lighter feeds such as ethane. Producers who designed plants to crack ethane and/or propane and butane feeds have inherent advantages since their plants require minimal and/or no modifications. However, plants that were configured for heavier liquid feedstocks such as naphtha and gas oil will be forced to consider the impact on their units if they are to process the ethane feed. With shale gas advances in other parts of the world, this trend may also propagate in other regions in the coming years.
The following factors need to be considered before deciding to shift to ethane feedstock.

Ethane availability and price
Shale gas recovery by horizontal drilling and the use of fracking 
technology has resulted in the US having an abundance of natural gas. Once natural gas is available, it is fractionated to separate ethane from the rest of the natural gas. The separated ethane is then fed into the pipeline. The US has a good pipeline infrastructure to supply ethane from the source to the ethylene producers. In addition, new pipelines are being constructed to meet the increased demand of consumers. Depending on the availability of shale gas reservoirs and the availability of fracking technology, ethane cracking may also become economically attractive in other parts of the world in the coming years. The desire to develop shale gas reserves outside of the US is strong. As per the International Energy Agency’s World Energy Outlook 2011,2 China has already auctioned shale gas exploration rights but with participation limited to Chinese companies. International companies willing to participate have therefore sought to enter into partnership with Chinese companies. Other countries becoming active or considering becoming active in shale gas exploration include India, Poland, Germany, Spain, the UK and Ukraine.

Liquids cracker configurations
Ethylene plants are typically designed with the demethaniser, deethaniser or depropaniser tower sequenced at the front end of the recovery section. Each scheme will present unique challenges for maximising ethane feed flexibility. Depending on the plant configuration, the cracked effluent’s flow rate and composition will vary in each section of the plant. Various sections of the ethylene plant may thus be more than adequate, with other sections requiring possible modifications.

Capacity increase in addition to flexibility
Some ethylene producers may well target increasing plant capacity in parallel to achieving maximum ethane cracking flexibility. Capacity increases will require additional capital spending, particularly in areas where existing equipment is already tight and flow rates and compositions are changing.

Available equipment margins in current operation
The extent of modifications required will depend on whether the existing equipment has any remaining margin. For maximum ethane cracking, flexibility will depend on the available margins in the existing equipment.

Products shift and market economics
In addition to ethylene, a 
typical liquids cracker produces 
byproducts including hydrogen, fuel gas, propylene, butadiene, pyrolysis gasoline, fuel oil, C4s and C5s. With ethane cracking, there is a significant shift in the overall product slate. Ethane cracking yields for ethylene are higher than those from liquid feed cracking; however, being a lighter feedstock, its cracking yields are significantly lower for the heavier products. Often, due to the small make, it becomes uneconomical to separate the heavier products formed by ethane cracking. This product shift can have an impact on the local market and can result in shortages, price hikes and possibly even result in the import of heavier products. There are technologies available for converting lower-chain olefins to higher-chain olefins, but these processes require additional capital, equipment and utility consumption. These processes can be used to offset the shortage of heavier co-products due to ethane feed cracking. Details of these processes are outside the scope of this article.

Ability to maintain liquids cracking capability
Ethane pricing and ethane cracking economics may change in the future. It is therefore important to maintain flexibility so that, where possible, owners can cost effectively shift back to liquids cracking when desirable.

Ethane cracking and products distribution
Figures 1, 2 and 3 explain the change in product make when gradually shifting the feedstock from 100% naphtha to 100% ethane. How much ethane can be cracked in a particular naphtha or gas oil liquids cracker depends on many factors: available design margins on equipment/piping, plant configurations, compressor performance curves and quench system design. The overall extent of the required modifications will have to be determined on a case-by-case basis. It should be noted that product yields are strongly dependent on cracking severity and ethane conversion per pass. Actual product yields will be dependent on the cracking conditions in a particular plant. Figures 1, 2 and 3 should be used only for explaining the effect of increasing ethane cracking on product distribution and should not be used for design work.

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