(151b) Optimal Design of the Integrated CGC and Depropanization System for An Ethylene Plant

As the most important and valuable facilities, Multistage Compression Systems are generally employed jointly with the Front-End Depropanization section in ethylene plants.  In most cases, the Charge Gas Compressor is designed with four or five stages of compression and aftercoolers, which allows the condensa­tion and remov­al of the heaviest components of the quench tower over­heads at the earliest possible time, thus avoiding expending energy on them in later compres­sion. Meanwhile, the low operating temperature of the compressor reduces the tendency for polymerization and fouling in the machine. The Front-End Depropanization section consists of a two tower system: high pressure (HP) column and low pressure (LP) column, with each tower operating at a different pressure. This offers a significant savings in refrigeration and minimizes the fouling problems experienced in a one-tower system.

This study is focused on energy consumption optimization by improved configuration design of a typical integrated CGC-DeC3 system of an ethylene plant. In order to study the integrated system behavior, four stages of compressors with suction drums, Caustic Wash Tower, Liquid Condensate Dryer, HP and LP Depropanizers with reboilers and condensers, C2 Convertor and other related process units are included in the case study  model. One innovative design is conducted under the consideration of introducing part of  the heavier stream from early stages of compression section directly into the fractionation section. Effluent from the 2nd stage of compression is split into two streams with one sent to the 3rd stage compressor, and the other directed to the  low pressure depropanizer, with the intention that saving compressing work on heavy components  in following compres­sion, as well as reducing high pressure depropanizer operation load. Key process variables such as split ratio, compression ratio, new feed tray location, column pressure are examined and determined by the optimization while the separation specification and safety requirements are satisfied.