Superstructure Optimization of Petroleum Refinery Design: Processing Alternatives for Naphtha Produced from the Atmospheric Distillation Unit

This research project concerns superstructure optimization for the design of petroleum
refineries focusing on the subsystem that considers the alternatives for naphtha produced
from the atmospheric distillation unit (ADU). The intricate complexities associated with this
process synthesis problem in general and the refinery design problem in specific
necessitates the development and implementation of a systematic and automated approach
that efficiently and rigorously integrate the elaborate interactions involving the design
decision variables. The primary objective of this research is to establish a systematic
procedure to determine the optimal topology of the refinery subsystem of naphtha produced
from the ADUusing the optimization or mathematical programming approach. Through the
identification of equipment, raw materials, products, and process alternatives in terms of the
different feasible choices of states (material streams) and tasks (process units) for the
mentioned subsystem, the first step is to represent the problem as the interconnections
between these elements in a network representation of a superstructure. Subsequently, an
optimization model is formulated with binary and continuous variables in order to arrive at
the optimum flowsheet design. The scope of this work is focused on the formulation of a
mixed-integer linear programming (MILP) and a generalized disjunctive programming
(GDP) optimization models. The independent design decision variables are flows of
materials and the continuous variables of stream flowrates with the discrete variables
denoting the existence of streams. Logical constraints are extensively incorporated in the
models to represent qualitative design knowledge through design specifications and
structural specifications on the interconnectivity relationships involving the states and the
tasks. Computational studies to demonstrate the implementation of the proposed modeling
approaches are carried out on the GAMS modeling language platform using the established
GAMS/CPLEX solver and the new code of GAMS/LOGMIP solver for the MILP and GDP,
respectively. Two design scenarios are considered as distinguished by the API gravity
(specific gravity) of the crude charge to the ADU. The optimal refinery topology generated
from the MILP and GDP model agree with the typical existing refinery topology. The way
forward for this project is to account for varying sulphur content in the crude charge as well
as to introduce nonlinearity in the composition modeling to obtain a more practical
representation of a real-world refinery design problem.