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Oilfield & Energies



Optimizing crude distillation unit operations

By Sandeep Ram Mohan, Senior Product Marketing Specialist at Aspen Technology - 18/01/2018

Sandeep Ram Mohan, Senior Product Marketing Specialist at Aspen Technology, explains how refineries’ operation efficiencies may be improved with ‘Advanced Process Simulation Technology’.
 
The crude distillation unit (CDU) segments crude oil within the refinery into different products that are processed to create automotive fuels or petrochemicals. Energy makes up approximately two-thirds of a refinery’s operating costs (not considering crude oil costs).1
 
A typical CDU needs crude oil to be heated to temperatures of 360-380°C (around 700°F), which consumes approximately 2% of the crude oil that the CDU processes.2 The CDU operation impacts the rest of the refining process units, making it vital to a refinery’s bottom line. Clearly, driving optimal CDU performance is a key goal.
 

Challenges

The main challenges faced by refineries segmenting crude oil include: changes in crude oil composition that affect the quality and quantity of a refinery’s products; fouling of heat exchangers; visibility of column operations; and general complexity of the CDU.
 
Changes in composition: Refineries must accurately analyse the economic and processing feasibility of the crudes available, with the aim of choosing crude blends that are compatible with their existing processes, facilities and markets. However, even for a refinery processing crude oil from just one source, the composition of the crude oil can vary with the depth of the well and year of production.
 
The nature of crude oil introduces further analysis challenges. Crude oil is a complex combination of hydrocarbons – approximately 600 different hydrocarbons have been identified in crude oil.3 Analysing these complex crudes or blends requires technological prowess.
 
Fouling of heat exchangers: Up to 70% of the energy required to heat crude oil is recovered from hot streams tapped out of the crude distillation column using a network of heat exchangers called crude preheat trains.4 These exchangers are frequently fouled by the crude oil they heat, diminishing their heat transfer capacity. This means the fired heaters have to work harder, burning additional fuel and adding to operating costs.
 
Throughput reduction, by way of increased hydraulic resistance, is considered the most significant cost of fouling for most oil refineries.5 Nevertheless, cleaning heat exchangers can take 3-14 days, depending on the severity of fouling, and can cost up to $50,000 per heat exchanger — not to mention the lost revenue from the downtime incurred.6 Most refineries predict fouling based on historic trends. In such cases, crude compositions or process conditions, which both impact fouling levels, are accounted for in a very limited way.
 
Additionally, the various heat exchangers employed in the CDU are difficult to analyse without the refinery operators’ expert knowledge.
 
Visibility of column operations: To understand the internal operation of a distillation column, hydraulic and thermal analyses of its operation are needed. The complexity discourages operators. However, the sensitivity of the column operations for different operational factors, and the implication of those operations on profitability, make it crucial for the refinery’s business health.
 
Flooding is the most common capacity limitation in distillation columns. When a column floods, tray efficiency diminishes, separation deteriorates and products are produced off-spec. To avoid the onset of flooding, operators cut throughput, which causes the plant to lose capacity. This highlights the importance of operators having insight into the operation of their crude distillation columns.
 
General complexity of the CDU: Typically, the CDU entails multiple recycle streams running between the crude distillation columns and the heat exchanger units in the crude preheat train. The flow rate and temperature of each stream depends on the functioning of the distillation column, which affects the crude preheat train’s heat transfer and thereby the fuel consumption of the fired heaters. Accurate analysis of the operation of such integrated systems requires multiple iterations.
 

Finding solutions

The following capabilities are essential to overcome the challenges mentioned above.

Assay management
Complex combinations of hydrocarbons in crude oil pose a challenge when characterizing a crude oil assay or blend of assays. To accurately predict flow rates and properties of refinery process streams, one must characterize every stream in the refining process in terms of a uniform set of components. Assay management capabilities in the energy industry’s standard process simulation software, Aspen HYSYS (Aspen Technology, Bedford, MA, USA), can break down the constituents of crude oil or any blend thereof into distinct components.
 
These are used to represent every stream across the refinery model, enabling the process simulator to accurately predict the yields and properties of streams at any point.
 
Integrated heat exchangers
Accurate simulation of heat exchanger operations in the context of a broader process simulation is important. Process simulation technology advancements allow engineers to put rigorous models of heat exchangers into the process flowsheet from within the integrated software, without having to switch between different software.
 
The mechanical information in rigorous heat exchanger models helps simulate or predict potential operational issues. Rigorous fired heater models can also accurately simulate fuel consumption.
 
A quick and powerful solver
The nature of the distillation process warrants a rigorous thermo-hydraulic analysis of the distillation column operation, made possible by the latest advancements in process simulation technology. More importantly, it presents the results of the analysis in an easy-to-read graphical format. This is critical for engineers who are looking to quickly decipher the results of the thermo-hydraulic analysis and decide on the corrective actions they should take to address operational challenges.
 
Process optimization and model calibration are significant challenges that arise when dealing with simulation models of process units with multiple recycle streams and integrated heat exchanger networks, such as in the CDU. This is due to the complexity in integrated systems that requires multiple iterations before the process simulator ‘solver’ can converge the integrated model. The conventional ‘sequential modular’ solver used by process simulators can be slow to converge such models. However, with the ‘equation-oriented’ solver technology in Aspen HYSYS, integrated CDU models can be solved faster by solving them concurrently.
 

Positive Prospects

Process optimization technology can potentially realise savings for refineries at every level, from crude preheat train monitoring through to integrated CDU monitoring. Modern refineries have the technology to run assets at maximum capacity while reducing risk, although not all have leveraged its value. Nevertheless, improvements in CDU operations can significantly boost refineries and many have already started profiting from these solutions.
 
 
References
1. Emerson. Round Rock, Texas: Emerson Process Management, 2012.
2. Gadalla M. Cairo, Egypt: Chemical Engineering Dept at The British University in Egypt, 2012
3. Giles HN. In: The Significance of Tests of Petroleum Products: A Report. Bridgeport, NJ: ASTM International, 2003.
4. Yeap BL. PhD Thesis. Cambridge University, Department of Chemical Engineering, Cambridge,UK, 2003.
5. Yeap BL et al. In: Trans IChemE, Part A, Chemical Engineering Research and Design. Cambridge, UK: Institution of Chemical Engineers, 2003: pp 53-71.
6. Joshi HM, Brons G. Heat Exchanger Fouling and Cleaning: Fundamentals and Applications. Santa Fe, New Mexico: ECI Digital Archives, 2003.
 
 
Author:
Sandeep Ram Mohan, Senior Product Marketing Specialist at Aspen Technology, Bedford, Massachusetts, United States
T: +1 855 882 7736