Published in Downstream

Does your assay represent the crude actually received?

Friday, 09 October 2015 16:45 Written by 

In Petroleum refining world, crude oil cost accounts to around 90% of the overall expenditure incurred. Therefore, knowing what crude is worth and how to run it in a refinery begins with a good “Assay”.

But, why does crude oil quality vary…? What impact does it have on the refining business….?

As a matter of fact, all crude oil streams marketed are a blend of production from a few to hundreds of wells. The change in crude oil quality has a significant impact on two core areas of the refining business:

  • Impact on economics
  • Impact on operations

Impact on Refinery Economics:

Linear Programming (LP) tools are widely used in the refining world for strategic and critical decision making.

One key piece of input to an LP tool is the “Crude Assay Information”. LP tools essentially retrieve the yields and quality of the straight run and other crude linked streams from the assay information while performing scenario-based optimization. The accuracy of information inputted into the LP is thus of vital importance to enable accurate, feasible and profitable decision making.

Impact on Refinery Operability:

Lack of updated assay information of the crudes being processed may also lead to operational issues. Crude blended with significant quantities of condensate may limit the overhead condenser, crudes with lower API can have limitations in the Vacuum section while a crude with increased metal content can have processing issues in the downstream cat-cracker. Crude properties essentially drive how a given refinery will perform and how the assets generate sustainable and reliable profitability.

Therefore, a thorough understanding of these variations in yields, properties & contaminant levels in crude samples received is critical to selecting a crude slate and preparing and executing an operating plan.

Need of the hour:

A “Crude Parcel-wise Quality Monitoring Program” (CPQMP) is essentially required in order to:

  • Monitor incoming crude quality
  • Compare properties and yields to existing assay
  • Define tolerance limits for each critical crude oil property being tracked and the deviation from which shall determine need for an assay update
  • Indicate appropriate tools and data to generate updates
  • Help establish metrics to quantify improvement

Useful handles to update assay:

Following are the useful handles that help update crude assay information and thereby evaluate the impact of the changes in crude quality:

Crude Assay Management tools:

Crude assay management systems are reliable tools to convert paper assay data into usable information for LP/Simulation tools thus facilitating effective decision making. These tools have useful modules to generate revised detailed assay information upon updating minimal whole crude information on the existing crude files. Separation inefficiencies can be calculated from stable plant operation data and can be further fine-tuned to match desired cut-wise specifications during assay re-cutting.

Process Simulation Packages:

Process simulation packages not only include detailed thermodynamic packages and the ability to simulate pumps, compressors, exchangers, separators, and fractionators but also include the capability to model kinetic reactors, track and predict stream properties from crude through to the finished product and simulate an entire facility on a rigorous, non-linear basis.

Linear Programming tools:

Assay management tools and Process Simulation packages can be used individually or in conjunction to generate updated crude assay information for LP input. Updated assays better represent the current quality of crude received leading to more accurate crude valuation and drives more informed purchase decisions and better plant optimization.


The level of complexity of a quality-monitoring program can range from very comprehensive to very simple, depending upon individual needs and requirements. Following are the distinct advantages of practising such a program in a refinery:

  • Alerts Shippers & Producers that quality of crude supplied is being tracked on a continuous basis
  • Provides information required to enable timely action upon and make competitive trading decisions
  • It provides adequate information for refiners to feasibly plan & optimize plant operation
Last modified on Friday, 12 October 2018 10:22
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1 comment

  • Comment Link Hanmant Malkapure Tuesday, 13 October 2015 19:27 posted by Hanmant Malkapure

    Nice article. Enjoyed reading it. Keep it up. :)

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