(99d) Holistic Approach to the Revamping and De-Bottlenecking of Pre-Heat Trains

The energy efficiency and throughput of pre-heat trains can be improved by using process integration and through fouling mitigation. In this paper we show how the approaches can be combined in a systematic way. If fouling behaviour is ignored the use of process integration can lead to revamps in which both energy efficiency and plant capacity are reduced (sometimes with massive cost penalties). If system behaviour is not taken into account the application of fouling mitigation technology can prove ineffective. The first step in revamping a pre-heat train should be the collection and analysis of exchanger monitoring data. For the exchangers positioned below the de-salter the fouling rates are important and will be used to identify how cleaning strategies can be used to maintain production and energy efficiency. For the exchangers at the hottest part of the pre-heat train, fouling models are fitted to the data. These models will be used to identify mitigation opportunities and determine how changes in process integration affect fouling behaviour. The data obtained from the exchangers situated immediately after the de-salter provide an indication of how well the de-salter is currently behaving and determine the needs for the de-salter strategy. The next step is the thermo-hydraulic simulation of the existing plant structure. Fouling models embedded into this simulation indicate opportunities for fouling mitigation. Fouling mitigation within the existing structure can be achieved through modification of exchanger geometry, installation of tube inserts or re-placement with new exchangers such as welded plate units (such as Compabloc). The results of this exercise will be a list of possible fouling mitigation projects. Since, no changes to plant structure are involved, the capital investment required for these changes will be relatively low and the pay-back short. The next step is to look at how plant performance can be improved through changes in structure. Changes in structure can be used for both fouling mitigation and increased heat recovery through improved integration. Two simple procedures are outlined. One for the identification of structures giving increased recovery through integration is outlined the other for identifying changes that lead to mitigation of fouling. The understandings developed in the paper lead directly to procedures for optimising the strategy for de-salter temperature control.

One aspect of retrofit that is not considered here is the interaction between the crude oil distillation column and the pre-heat train. This is the subject of further work.