(124a) Energy Improvements – Past, Present, Future

The ethylene production process has been one of the most energy intensive due to its high heat of reaction and complicated product recovery scheme. Early units were relatively small, unselective and inefficient. However when both feedstocks and fuels were low-cost, overall ethylene production cost was not significantly impacted by design efficiency and related incentives were relatively low.

That situation changed significantly in 1973 when the first energy crisis came and energy costs increased four-fold. Ethylene designs eventually reacted to this energy cost impact and in this same era plant sizes increased significantly up to 600 kta capacity. The focus on higher efficiency (and heavier lower cost feedstocks) was further promoted following the second energy crisis in 1979, where costs increased further by a similar ratio. It is interesting that following the recent fuel price hike we can anticipate another round of developments focusing on efficiency and lower cost feedstocks.

Today's specific energy consumptions are in the range of 3500-4500 kcal/kg C2H4 for gas crackers and 4500-6500 kcal/kg C2H4 for liquids crackers. These values are approximately half of the pre-energy crisis values.

When we addressed the energy challenge in 1970's/1980's many schemes were identified and incorporated. (our list was up to 100 such major/minor improvements). Key design changes which were introduced by the late 1980's included the following:

? Upgrade natural draft cracking furnaces to induced draft. ? Integral SHP steam superheater ? Gas turbine integration ? Higher temperature quench oil tower for higher level heat recovery ? Higher efficiency rotating equipment ? More extensive process-process heat recovery schemes ? Steam system analysis and optimum configuration ? Simultaneous cryogenic heat and mass transfer schemes ? Multi-stage refrigeration schemes ? Isentropic rather than isenthalpic cryogenic expansion ? Integrated heat pump systems for several fractionation services ? Front-end rather than back-end hydrogenation

While most new mega-sized units incorporate a large number of the above features, several of the existing older units still operate with only a few such schemes. European and Japanese plants have generally been revamped and upgraded to a much greater extent than have US units. As a consequence the specific energy consumptions of the average US unit is approx 25% higher than their overseas counterparts.

Efficiency upgrades, in conjunction with emission reductions and life extension precautions should be a current objective for many US plants. The objective of this energy overview is to provide our industry colleagues with a reference list of potential efficiency improvements.