The first separation unit in a petroleum refinery is the crude oil distillation column, where crude oil is separated into several products for downstream processing. Before it enters the column, the crude oil is pre-heated to around 400°C using a heat exchanger network (HEN) that exchanges heat between products and process streams and by a fired-heating furnace . Together, the distillation column and the HEN are known as the crude oil distillation system.
Crude oil distillation is among the most energy intensive processes in the refining and chemical industries; fuel consumed in the furnace has the energy equivalence of 1–2% of the total crude oil processed . For this reason, retrofit projects to increase productivity and to reduce energy consumption of the system are commonplace.
Retrofit projects aim to maximise the use of existing equipment, including by installing new equipment or introducing new technology. In crude oil distillation systems, the strong interaction between the distillation column and its associated HEN makes retrofit a complex problem with many degrees of freedom (operational, structural and flowsheet modifications to the distillation column and the HEN) and constraints (distillation column hydraulic limits, product specifications, HEN installed area, ΔTmin constraints, etc.).
This work proposes an optimisation-based retrofit methodology in which installation of a preflash unit, together with operational optimisation, aims to increase the productivity of an existing crude oil distillation system. Previous studies have shown that installing a pre-flash unit helps with the separation of the lighter components (reducing the vapour traffic in the distillation column), improves the hydraulic performance of heat exchangers as a result of reduced flow rates, and reduces the heat demand of the furnace [2, 3]. It has also been shown previously that operational optimisation can increase the profitability of the system and can reduce its energy demand .
The option of introducing a preflash unit increases the complexity of the retrofit problem, as it introduces additional degrees of freedom that have to be taken into consideration, namely the preflash location in the preheat train and the feed location in the main column of the preflash vapour.
In the methodology proposed in this work, the distillation column is modelled using rigorous simulation and following previous work , its hydraulic performance is assessed using hydraulic performance correlations implemented as a Matlab code. The column simulation results are also used within an optimisation-based HEN retrofit methodology  to simulate and retrofit the HEN.
The novelty of this methodology is the used of nested optimisation to find the best set of conditions that maximise the profit of the system for increased throughput. The crude oil distillation column operational parameters, the preflash location and the vapour feed stage are varied using the Global Search algorithm from the Matlab Optimisation Toolbox. For each proposed change to the column design and its operating conditions, HEN retrofit solutions (adding or deleting an exchanger, repiping or resequencing heat exchangers) are optimised using a Simulated Annealing algorithm.
The benefits of the proposed retrofit methodology are showed using an industrially relevant case study.
1. Smith, R., Chemical Process Design and Integration. 2005, Chichester, UK: John Wiley & Sons, Ltd.
2. Errico, M., G. Tola, and M. Mascia, Energy saving in a crude distillation unit by a preflash implementation. Applied Thermal Engineering, 2009. 29(8-9): p. 1642-1647.
3. Benali, T., D. Tondeur, and J.N. Jaubert, An improved crude oil atmospheric distillation process for energy integration: Part I: Energy and exergy analyses of the process when a flash is installed in the preheating train. Applied Thermal Engineering, 2012. 32(0): p. 125-131
4. Ochoa-Estopier, L.M., M. Jobson, and R. Smith, Optimisation of heat-integrated crude oil distillation systems. Part III: Optimisation framework. Industrial & Engineering Chemistry Research, 2015. DOI: http://dx.doi.org/10.1021/ie503805s.
5. Enríquez-Gutiérrez, V.M. et al., Retrofit of heat-integrated crude oil distillation columns. Chemical Engineering Research and Design, 2015. DOI: http://dx.doi.org/10.1016/j.cherd.2015.02.008.
6. Ochoa-Estopier, L.M., et al., Optimisation of heat-integrated crude oil distillation systems. Part II: Heat exchanger network retrofit model. Industrial and Engineering Chemistry Research, 2015. DOI: http://dx.doi.org/10.1021/ie503804u.