ABSTRACT
The crude distillation unit is the first processing unit of a refinery that processes the raw crude oil
into intermediate products. The poor recovery of the desirable products from this unit leads to increased throughput of conversion units thereby increasing production cost. Increasing the production of the desirable products from this unit has direct impact on many other economic and operational objectives like energy demand, waste water treatment cost and greenhouse gases emissions. There is need to investigate the trade-offs between maximizing products yield and other conflicting objectives. This study is aimed at obtaining a computer based model that can simulate and optimize the performance of a crude distillation unit. The objective function consists of three contrasting objectives which include increasing products yield, minimizing energy, waste water and obnoxious gaseous emissions.
The simulation model based on a blend of crudes was developed with the operational data of the different columns that make up the unit. The assays of three Nigerian crudes (Agbami, Brass river and Ebok) were characterized to obtain their thermophysical properties. The blending of the crudes was carried out by formulating a linear optimization model to obtain a blend of API 31.22. The network of heat exchangers was designed with the Aspen Energy Analyzer using the pinch technology for targets to be set for the heating and cooling utilities. The concept of global warming potential was employed to target greenhouse gases emissions from the heating and electricity sources. A carbon trading scheme was developed to allocate greenhouse gases emissions quota to refineries and a carbon fee of $5 was charged per tonne of above-limit emission. A cost function was also attached to the treatment of the waste water. A multi-objective optimization model which considered revenue maximization, minimization of the energy as well as waste water and greenhouse gases emissions was developed and solved using the sequential quadratic programming tool embedded in Aspen Hysys subject to some sets of constraints.
The characterization results showed that Agbami, Brass river and Ebok crudes are light, medium and heavy crudes respectively. The energy analysis revealed that the process pinch temperature was 215oC and the heating and cooling demands were 4.034x108kJ/hr and 1.687x108kJ/h respectively. Greenhouse gases emissions from heating sources was 5.818x104lb/h while electricity sources was 7.727x104lb/h giving a total yearly greenhouse gases emissions from the unit as 491.576ktonne. The optimization gave a tremendous increase in the revenue from the crude distillation unit because there was an appreciable increase in the production of the more valuable products; the heat energy demand was minimized as it decreased from 360.0mmBtu/h to 355.7mmBtu/h while the greenhouse gases emission from the heating sources was minimized from 58180lb/h to 57390lb/h, however, a slight increase from 77270lb/h to 80390lb/h was observed from the electricity sources. The gross profit from the unit was increased from $7,572,334,144/yr to $7,755,498,160/yr and the Net Present Value of the process was $34,270,260,630. The sensitivity analysis revealed that gross profit is quite sensitive to carbon fee, energy usage and steam utilization.
