(112a) Revamping, Debottlenecking and Optimizing Ethylene Plants Using Enhanced Heat Transfer Solutions

In the recent years the ethylene business encountered a complete change of its market structure caused by the shale gas boom in North America and the large trend for integration of refining and petrochemical industries. Thus the main drivers for the optimization of existing plants have become capacity upgrade, plant flexibility as well as adaptation to changing feedstock and reduction of carbon foot print. For new plants the minimization of the investment costs as well as carbon foot print are the keys to a successful business. In both cases highly reliable solutions are required to match the lifetime of 20 and more years of plants often located in harsh environment.

Efficient and reliable heat transfer solutions are required from the hot to the cold part of an ethylene plant. Focusing on the cold part, shell and tube heat exchangers equipped with enhanced heat transfer tubes can play a crucial role in clean as well as fouling services allowing both for OPEX improvements or CAPEX reductions.

Depending on whether optimizing a new grassroot ethylene mega plant or revamping / debottlenecking an existing plant enhanced heat transfer tubes offer multiple benefits :

• Capacity improvements of existing shell and tube heat exchangers installed in-place by supporting increased plant through-put ;

• Minimizing temperature differences and cold approaches allowing for maximizing efficiencies of refrigeration systems ;

• Improved operating range for critical rotating equipment by minimization of pressure differential between refrigerant reboilers and condensers ;

• Equipment size reduction as well as minimization of the number of shells per unit resulting in both plot space and cost reductions ;

• Improved heat integration using waste heat e.g. from quench water ;

These enhanced heat transfer tubes are dedicated  to horizontal shell and tube heat exchangers like thermosiphon and kettle type reboilers as well as overhead condensers. They are typically applied in C2 and C3 fractionation, splitting and refrigeration units in cold-end sections of ethylene plants. A wide range of pioneering references in revamp and grassroot applications since 2000 confirms the full thermal, mechanical and operational long-term viability of these solutions.

In the presentation different case studies will illustrate the benefits of enhanced heat transfer tubes depending on the individual application situation:

• Reboiler for a de-ethanizer overhead condenser service ;

• C3 splitter reboiler service in an open heat pump ;

• C3 refrigerant condenser performance improvement to either minimize the number of shells per unit or to optimize the heat exchanger efficiency.

Further to this, a recent development with an enhanced C3 quench-water heated reboiler did show very attractive and stable operation. Data collected and results from a C3 splitter unit installed at a naphtha-based steam cracker plant of an European ethylene producer will be presented. Two case studies will demonstrate the suitability and robustness of this technology in quench-water fouling services :

• C3 splitter reboiler with quench-water heating for heat integration

• Enhanced air-cooler for quench-water cooling as a most compact design.

All presented case studies rely on tubes with optimized heat transfer thanks to structured surfaces on the tube out- and inside. The outside enhancements are taylor-made either for enhanced nucleate boiling, enhanced condensation and gas heating. On the tube inside helical fin geometries are adjusted to support optimized heat transfer for 1-phase liquid and  gas or 2-phase flow. The design correlations and performance were verified in laboratory and scale-up testing as well as field qualifications.

To achieve this level of performances and, more important, to maintain it over time, the enhancement structures are manufactured through a cold rolling process by plastically forming fins from the original material out of the plain tube wall. The tubes are available in various grades of carbon steel (standard, low temperature carbon steel, 3.5 % Ni alloy) allowing for a wide range of operating temperatures, including cryogenic conditions.