Overall design of low temperature processes

Abstract

A methodology for the overall design and analysis of low temperature processes is proposed. A low temperature process such as the cold end of ethylene consists of three main components; the process (usually distillation), the heat exchange system (cold box and other exchangers), and the refrigeration system. The task of overall design is complex since all three components are interlinked and interactive. The proposed approach allows the designer to resolve these interactions and combine all three components in one design task. The approach is based on a combination of Pinch Analysis and Exergy Analysis. The overall design of low temperature processes is dominated by the shaftwork or power consumption of the refrigeration system. The concept of “Shaftwork Targeting” from Pinch Analysis is extended to encompass the design of the entire process. This allows the designer to identify the refrigeration shaftwork benefit related to individual process units thus obtaining “process shaftwork targets” ahead of design. The overall procedure commences with the identification of appropriate column modifications using the concept of “Column Targeting”. The concept of “process shaftwork targeting” is then used to establish the net shaftwork benefit which results from any column modifications. This benefit is traded off against the capital cost of the column modifications thereby enabling the designer to preoptimise distillation column ahead of design. This pre-optimlsation by-passes the usual repeated column and refrigeration system simulations. Finally, the column modifications are “fine tuned” incorporating the actual choice of refrigeration levels and heat exchanger network (HEN) configuration. The refrigeration system and the HEN are also optimised at this final stage. The overall procedure systematically modifies each of the design components ensuring that promising design options are easily identified at an early stage and all major design options are considered. The methodology has been tested on several ethylene and LNG applications yielding an average of 15% shaftwork savings in addition to the savings obtained through established Pinch Analysis procedures.