Muhammed, Tasneem
(2021)
Technical and economic assessment of process integration in gasoline production.
PhD thesis, University of Nottingham.
Abstract
In chemical engineering, process design is a fundamental attribute that includes process definition, simulation, and optimization. Commonly, superstructure approaches are used for process design. These approaches represent the process by integrating simpler unit blocks described by physical and chemical properties to retrofit complex chemical processes. The superstructure approaches follow certain objectives (e.g. economic, environmental and social) that can be optimized. This makes the implementation of superstructure approaches in process design a challenging task.
In this thesis, a superstructure-optimization framework is proposed considering economic criteria. The proposed superstructure representation divides the process into sections and then links these sections using a switch. This generates several pathways that are economically evaluated and then optimized. The framework is modelled using the HYSYS®-MATLAB® hybrid software platform. This platform implements two or more software programs and applications, each of which has a different specialist feature, to obtain a wider overview oi the process constraints, variables and conditions.
The proposed platform is applied in a case study. After generating process pathways, a trade-off between capital and operating costs are determined and then optimized employing the genetic algorithm (GA) method, which mimics the natural process of selection. The algorithm selects the most practical and economical process configuration design, considering alternative operating conditions, process pathways, materials and energy efficient units.
Gasoline production through isomerisation process of light naphtha was chosen as the case study, because the isomerisation process fulfils strict environmental regulations that call for lower energy consumption and cleaner fuel (e.g. fewer carcinogens substances such as aromatics). The process mainly converts the straight chain of normal paraffin to iso-paraffin. The isomerisation process superstructure was made of two sections: frontend and backend. The frontend process flowsheet includes three catalysts: chlorinated alumina-based, sulphated metal oxide-based, and zeolite. The backend process flowsheet includes five independent separation units, consisting of distillation column (DIS), eight-bed simulated moving bed adsorption (SMB), and three energy efficient side-stream distillation processes: absorption (AHP), bottom flash (BF) and vapour recompression (VRC) heat pumps.
The front end and backend flowsheets were linked by using the switch method, which connects the selected element(s) of the superstructure process without interacting with the other options. This adds up to 15 pathways made of the combination of the frontend and backend processes to produce gasoline. This enables the proposed platform to economically evaluate and optimize each combination independently.
As a result of evolving the process for 100 populations and 20 generations, a maximum net present value (NPV) of $220 million and maximum gasoline research octane number (RON) of 95 were obtained for the combination of sulphated metal oxide-based catalyst and the SMB flowsheet. However, coupling AHB and zeolite base catalyst to produce gasoline only achieved $180 million NPV and 90 RON. Also, the overall energy consumed (i.e. heating and electricity) and production cost of the optimal flowsheet combination were 0.33 GJ and $1.67 million per barrel gasoline, respectively. Thus, the developed method reduces the energy consumption and production costs by approximately 20% and 30% respectively over the conventional refinery.
In conclusion, our approach increases the gasoline production process NPV by approximately 15% over the base case. Applying the method for other applications and processes is recommended as long as the energy consumption and product quality are considered.
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