(50b) Retrofit and Off-Line Operational Optimisation of Heat-Integrated Crude Oil Distillation Systems

Crude oil distillation is an energy intensive and complex process. To reduce the large energy demand of crude oil distillation, heat integration is implemented. This distillation system (i.e. distillation units and heat exchanger network, HEN) needs to perform a cost-effective separation over a broad range of scenarios, such as changes in product yields or throughput. Operational optimisation and retrofit projects are frequently implemented to adapt existing distillation systems to such diverse scenarios.

To support this goal, this paper presents an approach to perform operational optimisation and retrofit of existing distillation systems. In this approach, the distillation units (CDUs) and HEN are optimised together to obtain cost-effective and acceptable designs.  The optimisation approach explores changes in the CDUs and HEN simultaneously, exploiting the synergy within the system. This simultaneous approach leads to far better results, compared with changes to the CDUs or HEN alone.

In the proposed approach, industrially-relevant variables are manipulated during optimisation; for example, flow rates of distillation products, pump-arounds and stripping steam and split fractions of stream splitters in the HEN. For retrofit, HEN structural modifications are easily constrained to ensure that practicable solutions can be achieved.

In this methodology, variables related to product quality (e.g. ASTM D86 temperatures, flash points, density, etc.) can be readily modelled and monitored. Thus, only solutions that meet product quality specifications are considered during optimisation. Other practical constraints related to the CDUs can be considered, such as column flooding or pump capacity.

Additionally, it is possible to monitor and constrain the heat transfer area (existing and additional area) of individual heat exchangers during optimisation. Thus, practical constraints related to heat transfer area (e.g. lower and upper bounds on additional heat transfer area, plant layout restrictions, etc.) can be easily captured and implemented in the optimisation framework.

The methodology is showing itself to be powerful for addressing cases of industrial-relevance, including a constrained and relatively complex system consisting of a prefractionator, a distillation unit, a vacuum distillation unit and the associated HEN. Results show that industrially-applicable designs with reduced operating costs and relatively low capital investment can be obtained using this methodology.