Mar-2018

Maximising CDU performance

Scalable simulation of crude unit operations enables refineries to build increasingly rigorous models to enhance performance

SANDEEP RAM MOHAN
Aspen Technology

Article Summary

Crude oil refineries typically run on tight profit margins, usually less than 7%.¹ Therefore, any opportunity to mitigate costs is of significant interest. Given this, it is ironic how many refineries continue to miss out on significant cost-saving opportunities from their crude distillation unit (CDU) operations.

Could this be due to a lack of awareness around new opportunities created by the latest advancements in process simulation technology or because of misconceptions about the amount of resources and time required to implement meaningful solutions? Whatever the reality, key opportunities are missed and profit is left on the table.

Exploring the CDU
Energy makes up two-thirds of a refinery’s operating costs (not considering crude oil costs).² The CDU, which exists in every refinery, is the primary unit that separates crude oil into different products that are processed to create automotive fuels or petrochemicals.

A typical CDU requires unprocessed crude oil to be heated to temperatures between 360°C and 380°C (around 700°F),³ which consumes an equivalent of approximately 2% of the crude oil the CDU processes.⁴ The CDU operation has a significant impact on the rest of the refining process units, making the processing unit critical to a refinery’s bottom line.

Variations in crude oil
Changes in crude oil composition can have a significant impact on the quality and quantity of a refinery’s products. This limits the refinery’s options in the crude oil it can process, as it is constrained by its existing processes and facility, as well as the products demanded by its market.

Refineries must accurately analyse the economic and processing feasibility of the various crudes available to choose the most optimal crude or blend of crudes. Even for a refinery processing crude oil from the same source, the composition of the crude oil extracted from the same well can vary with the depth of the well and the year of production.⁵

In addition, the nature of crude oil introduces further challenges to this analysis. Crude oil is a highly complex combination of hydrocarbons. Approximately 600 different hydrocarbons have been identified in crude oil.6 Analysing these complex crudes, or their blends, by breaking down their compositions and representing them in terms of numerous sub-components, requires considerable technological prowess.

Fouling of heat exchangers
Some 60-70% 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.7 These heat exchangers are frequently fouled by the crude oil they heat, diminishing their heat transfer capacity and leaving the burden of heating crude oil to the required temperature on the fired heaters. The additional fuel burned makes up for the heat transfer loss due to fouling and adds to the refinery’s operating cost. Fouling also increases hydraulic resistance in the heat exchangers.

Throughput reduction, by way of increased hydraulic resistance, is considered the most significant cost of fouling for most oil refineries.8 On the other hand, cleaning heat exchangers can take anywhere between three and 14 days depending on the severity of fouling and can cost up to $40000-50000 per heat exchanger – not to mention the lost revenue from the downtime incurred.9 The economic benefit from cleaning is not the same for all heat exchanger units, so the challenge for refineries is determining the right cleaning schedule. Fouling is not something refineries can readily measure during operation, which is why most predict fouling based on historic trends. In such cases, crude compositions or process conditions, which are both critical factors affecting fouling levels, are accounted for in a very limited way.10 This clearly does not equal an optimal maintenance strategy.

Additionally, the various types of heat exchangers employed in the CDU such as shell and tube, fired heaters, air coolers and even plate exchangers, requires the refinery operators’ knowledge to accurately analyse the operation of each of these equipment types to efficiently manage CDU operation.

Visibility into column operations
To understand the internal operation of a distillation column, both hydraulic and thermal analyses of its operation need to be completed. The complexity involved in the analysis discourages many refinery operators from carrying it out. The sensitivity of the column operations for different operational factors and the implication those operations have on the profitability of the refinery makes it crucial for the refinery’s business health.

Distillation columns are susceptible to many operational issues such as flooding, weeping, foaming and entrainment, among others. 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. In addition to destabilising the operation of the column, flooding can also cause issues such as cavitation of the bottom pump. To avoid the onset of flooding, operators cut throughput, which causes the plant to lose capacity.11 This highlights how important it is for refinery operators to have clear insights into the operation of their crude distillation columns.

Complexity of the CDU
The CDU entails multiple recycle streams running between the crude distillation columns and the various heat exchanger units in the crude preheat train. The flow rate and temperature of each of these streams depends on the functioning of the distillation column, affects the crude preheat train’s heat transfer and thereby affects the fuel consumption of the fired heaters. Accurate analysis of the operation of such integrated systems requires multiple iterations. Moreover, the different types of equipment in the unit, such as distillation columns, shell and tube heat exchangers, fired heaters, air coolers and plate exchangers, raises the expertise required to perform an effective analysis.

Usability of computer based solutions
Unfortunately, there are shortcomings with many of the computer based solutions available in the market for analysing and monitoring CDU operations. Many require considerable expertise, not only for set-up but also for maintenance. For instance, these solutions might not support streamlined workflows for validating the model against plant data to ensure that they represent the refinery’s present operating conditions.

Many refineries do not have the expertise required to manoeuvre such cumbersome work processes and rely upon external consultants to perform these core functions. The steep learning curve required to gather enough proficiency to manage these systems discourages refineries from taking the time to implement them. The difficult nature of some of these solutions hampers a refinery’s ability to grow its internal expertise. Many refineries do not have access to technological partnerships and therefore are forced to stay dependent on expensive external consulting companies. These factors have contributed to many refineries lacking basic computer based simulation models of their CDU.