(175b) Optimization-Based Design of Energy-Efficient Crude Oil Distillation Systems with Pre-Separation Units

Crude oil distillation is an essential process in petroleum refining. A crude oil distillation system typically comprises a preheat train, crude oil distillation units with pump-arounds and side strippers and sometimes pre-separation units, such as preflash units or prefractionation columns. These systems are complex and energy intensive, and have a large number of degrees of freedom for design (namely, operating contidions and column structure) [1]. This complexity, together with the high operating costs, motivates development of new design approaches that can lead to optimal performance with respect to energy consumption, operating costs or total annualized cost (accounting for annualized capital expenditure).

Implementing pre-separation units (a preflash or prefractionator) in a crude oil distillation system provides an opportunity to reduce energy consumption, specifically in the furnace, decreasing fired heating demand and to exploit the strong interactions with the heat recovery system. Preflash units are added to existing crude units mainly for three reasons: to increase capacity, to improve the quality of the separation between components, and to improve heat integration. However, few crude oil distillation units are built with preflash or prefractionator units initially [2]. Moreover, the open literature does not provide a systematic design methodology for optimization of crude oil distillation systems with pre-separation units, where the methodology accounts for product quality and heat integration.

This work introduces a new optimization-based design approach to design energy-efficient and cost-effective crude oil distillation systems with pre-separation units. The approach accounts for interactions between the separation units and the heat recovery network and for capital-energy trade-offs.

The proposed approach is based on a simulation–optimization technique for the design of distillation columns [3]. The crude oil distillation column, together with its preflash unit, requires a wide range of degrees of freedom to be taken into account: stripping steam flow rates, pump-around duties and temperature drops, the column feed inlet temperature and the preflash temperature. In addition, the approach includes as a structural degree of freedom, the location at which the flashed vapor is fed to the main column.

A fixed column design and a given set of products are initially considered [4, 5]; then, column design is also taken into account integrating a rigorous tray-by-tray crude oil distillation system model [6, 7]. Rigorous simulations of the columns are carried out within Aspen HYSYS v8.6. An optimization framework using a stochastic algorithm (simulated annealing) is developed to select optimal values for structural and operational variables for the design of the heat-integrated crude oil distillation system with a pre-separation unit. Optimization bounds are defined based on sensitivity studies. To facilitate the optimization, an interface between Aspen HYSYS v8.6 and MatLab R2016a is established. Product quality specifications (in terms of ASTM T5% and T95%) are defined as inequality constraints; a penalty is applied to the objective function if these constraints are violated. Pinch technology is applied to address heat integration: the grand composite curve is used to evaluate the minimum demand for fired heating.

Objective functions considered in this work include: minimum hot utility demand, minimum operating cost and minimum total annualized cost. The optimization-based design approach then determines optimal column configurations and their corresponding operating conditions.

An industrially-relevant case study demonstrates the capabilities of our approach, and illustrates that the minimum hot utility demand of a crude oil distillation system can be reduced by introducing a pre-separation unit.

 

References

[1] M. Errico, G. Tola, and M. Mascia, 2009, Energy Saving in a Crude Distillation Unit by a Preflash Implementation, Applied Thermal Engineering, 29, 8-9, 1642–1647.

[2] A. Sloley, 2001,Designing and Revamping Crude Petroleum Sequences, Proceedings of the AIChE Spring National Meeting, Houston. 23th – 27th April 2001. Distillation Group Inc.

[3] J.A. Caballero, D. Milan-Yanez, I.E. Grossmann, 2005, Rigorous Design of Distillation Columns: Integration of Disjunctive Programming and Process Simulators, Industrial and Engineering Chemistry Research, 44, 17, 6760–6775.

[4] R.N. Watkins, 1979, Petroleum Refinery Distillation, Gulf Pub. Co., Houston, USA.

[5] L. Chen, 2008, Heat-integrated Crude Oil Distillation System Design, PhD Thesis, The University of Manchester, Manchester, UK.

[6] D. Ibrahim, J. Li, G. Guillén-Gonsálbes, M. Jobson, 2016, Optimisation-based Design of Heat-integrated Crude Oil Distillation Systems Using Rigorous Simulations and Surrogate Models, Proceedings of the 16th AIChE Annual Meeting, 13^th-18^th November, San Francisco, CA., USA.

[7] D. Ibrahim, M. Jobson, J. Li, G. Guillén-Gosálbez, 2017, Surrogate Models combined with a Support Vector Machine for the Optimized Design of a Crude Oil Distillation Unit using Genetic Algorithms, Proceedings of the European Symposium on Computer-Aided Process Engineering (ESCAPE-27), 1^st - 5^th October, Barcelona, Spain (provisionally accepted).