Solving Mass Balance One Tank at a Time
Mass balance of a refinery is a necessary tool for controlling the efficiency of its loss control. Many of our clients focus on the volume measurement since it directly impacts the bottom line, rightfully so. However, they fail to take accurate density measurement especially for their inventory. Taking measured density values for inventory can improve the accuracy of the plant mass balance, as a result improving how efficiently loss control is managed. Many sites treat their inventory as non-important since inventory is a balance sheet item and avoid measuring inventory density. Instead they use pre-set density values which translates into errors in the mass balance. What is astonishing is that the API gravity is usually measured and available to use, instead monthly mass balances are closed using the same static density month after month. If we categorize tanks into 3 groups: crude, intermediates, and finished products. It’s fair to assume that crude tanks are tested for density before entering units. As a result, the lab has measured density values on most, if not all crude tanks. This is especially important if those tanks are co-mingled with different grade of API gravity.
An important consideration is whether the content of the tank changed before the end of the month. If it has, one cost effective method is to calculate a weighted average density based on the tank composition volume, this approach will yield a more accurate measurement instead of static densities. Finished products don’t typically change tanks and a CoA from the lab is a precise measure of density for month end close. It’s important to remember if you are using fixed density to adjust for the summer and winter gasoline blends as the API gravity difference can have a significant seasonal swing in your mass balance. For intermediates or other tanks these may need to be sampled and tested for. The monetary impact on density correction will depend on size of refinery and amount of inventory held end of month. In a recent case the three crude tanks alone impacted 3MM lbs. of mass resulting in 0.1 percentage point in mass balance. Although the solution above may seem trivial, it is usually the fault of stale procedures that have been passed down coupled with following the status quo.
This blog was written by Poria Kanozi, Engagement Manager at Trindent Consulting. He has extensive experience implementing Hydrocarbon Loss Control programs for international refineries and improving their processes and operations.