Process Simulation for Improved Energy Efficiency, Maximised Asset Utilisation and Increase in Feed Flexibility in a Crude Oil Refinery
a report by Zoltán Varga,1 István Rabi2 and Klára Kubovics Stocz2 1. University of Pannonia, MOL Department of Hydrocarbon and Coal Processing; 2. MOL plc, DS Development Process Engineering
Nowadays, the hydrocarbon industries have to face several challenges. Increasingly strict legislation on climate change forces refineries to take action to reduce their carbon dioxide emissions. Actions that can be taken to satisfy these requirements are the following:
• improving energy efficiency of existing processes;1–3
• use of alternative energy sources as well as fuel blending components being produced on a renewable basis;4–6
and
• the application of different carbon dioxide (CO2) capture technologies to extract CO2 from the refinery flue gases.7,8
The most attractive and economical way to reduce greenhouse gas emissions is a decrease in energy consumption, because this also contributes to reduce operational costs.
Additionally, the negative effects of the world economy force companies to maximise the utilisation and efficiency of their existing assets and optimise their feed supply and product slate. Furthermore, the hectic changes in crude oil and product prices, as well as the unpredictable geopolitical situation, make it inevitable to make the refinery process units capable of processing different types of crude oil.
The objectives of the present article include: •
heavy- and medium-type crude oils are summarised (see Table 1). Afterwards, the applicability of equipment for processing feed in higher capacity and/or different types was also checked. The capacity of tower internals was studied with company-specific software according to their vendor. The furnace of the ADU was investigated with FRNC–5 software for new cases.
The model of the distillation unit was also applied to analyse the heat exchanger network (HEN). The object of the retrofit case for an existing HEN is to check and to narrow the gap between the ideal and current energy consumption. Retrofit of a HEN is more common in everyday practice than the ‘grassroots’ design of a HEN.9,10
The
to investigate the possibility of increasing the crude oil processing capacity of one of the company’s atmospheric distillation units (ADUs) by 20%, determine the bottlenecks and suggest solutions to resolve them;
• •
to improve the energy efficiency of the ADU; and
to investigate the ability of the ADU to process different types of crude oil.
Methodology
The first step of the study was a test run to collect all the data required for modelling the unit, for example, yield and quality of products, process parameters (e.g. values of temperature and pressure, flow rate of refluxes and stripping steam, etc.) and constraints of parameters (e.g. minimum/maximum desalter and column inlet temperatures, maximum allowable pressures, maximum pump capacities, etc.). Data sheets and equipment drawings of the ADU (e.g. pumps, heat exchangers and column internals) were also collected. Based on these data, a detailed model of the ADU was built up with process engineering tools. During the simulation, the real and calculated heat and material balances were compared continuously. If any discrepancy was noticed it was clarified in the process unit.
After validating the model based on heat and material balances, the investigation of processing different types of crude oil at base and in 20% higher capacity was carried out. Product yields of the selected
© TOUCH BRIEFINGS 2011
following generalised steps were covered during the study. The first step of this analysis is to establish how much energy can be recovered by applying the installed surface area. Then, the existing design is analysed in terms of which exchangers transfer heat across the pinch. The remaining exchangers are analysed regarding the use they make of driving forces. Those units that are transferring heat across the pinch and/or make inappropriate use of driving forces are selected to be repositioned. In most cases, the results of the study suggested adding new heat exchangers to compensate for the suboptimality of the existing structure. HEXTRAN™ software was applied to design new heat exchangers in this study. Tedious calculation work of HEN analysis was saved by using SuperTarget™ software. Based on the results, the original model was changed and the applicability of the suggested HEN structure was checked by rigorous modelling. After that, an economic analysis was carried out. Retrofit of a HEN is a trade-off between energy saving and capital investment. If the new HEN suggested by the design improvement step satisfied given economic criteria, then a detailed engineering step was performed. If the HEN did not satisfy the criteria, another new HEN design was evaluated using the design improvement step until a HEN satisfying the criteria was found. After implementing the new HEN structure, a test run was carried out to evaluate the ‘real’ saving in energy consumption.
Results and Discussion Study of Processing Heavy Crude Oil
A model of the ADU processing heavy crude oil (Crude ‘A’) at base capacity was prepared in the first step. After analysing the calculation results of the HEN of the feed preheating line, it was noticed that the total rate of heat exchange between feed and products (46.62 megawatt [MW]) was well below what was possible considering the entire heat exchanger area and accepted fouling of the units (51.67MW), see ‘Test Run’ and ‘Case 1’ in Table 2. This difference was caused by the bypassing of some heat exchanger units due to their high pressure drop. The excess heat should be transferred to the crude oil in the atmospheric furnace, resulting in higher fuel
consumption and emissions of CO2 and other flue gases. The firing rate of the atmospheric furnace can be decreased by 4.1MW and CO2
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Refining Assets
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