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Gulf Guardian Award Winners 2004
Business Category - 1st Place
| Project #: | GG-04-23 | |
| Company: | ExxonMobil Baton Rouge Refinery, LA | |
| Project Name: | Voluntary Nitrate Emission Reducation | |
| Category: | Business | |
| Project Type: | Nutrient Enrichment |
Summary: At the ExxonMobil Baton Rouge Refinery, nitrate emissions from the refinery to the Mississippi River were voluntarily reduced by two thirds from 4.51 million pounds/year in 1999 to 1.47 million pounds/year in 2003. Further reduction to 1.15 million pounds/year is projected for 2004. ExxonMobil’s management team established this voluntary reduction objective and supported it through a five-year effort that included an extensive engineering analysis. Process operations were modified to run two ammonia strippers in parallel. Additionally, an innovative method of operating the refinery waste water treatment facility under anoxic conditions, previously believed to be infeasible, was implemented. This reduction effort was entirely voluntary and was achieved without capital expenditure.
Description: The ExxonMobil Baton Rouge Refinery processes nearly 500,000 barrels of crude oil daily on a 1,000-acre site in the northern part of East Baton Rouge Parish. A biological water treatment plant with a capacity of 14 million gallons/day discharges effluent to the Mississippi River under an NPDES Permit.
After nitrates were added to the national Toxic Release Inventory in 1995, the nitrate content of this effluent (4.51 million pounds per year) was identified as the largest component of the Baton Rouge Refinery's TRI emissions. Effluent nitrates are created when elemental nitrogen from the refinery process finds its way to water effluent, usually as ammonia. When these streams are oxygenated in the water treatment plant, nitrates form and are discharged. Management established a voluntary objective to reduce these nitrate emissions.
ExxonMobil Research & Engineering, the corporation's central engineering branch, conducted an extensive engineering study of plant operations. The study addressed sources of nitrogen entering the refinery, operations that could recover nitrogen from water and redirect it back into the process, and water treatment methods that could convert nitrates back into elemental nitrogen.
Nitrogen enters the refinery from several sources, largely contained in crude oil. In processing the crude oil, nitrogen is converted to ammonia that can concentrate in water streams. Units called ammonia strippers capture ammonia from the water and return it to the crude oil process. Beginning in 2000 larger quantities of steam were injected into the stripping process to recover greater amounts of ammonia. This resulted in a reduction to 3.55 million pounds/year in 2000, a reduction of 21percent from 1999.
Study of the refinery ammonia strippers identified a way to modify the operation so that two strippers can run simultaneously instead of two strippers operating in alternate periods. The parallel operating method was tested and evaluated, with results showing that the change would almost double the ability of the strippers to remove ammonia from waste water and carry it back into the refining process. The modified stripper operation was established as routine in May 2002.
Process engineers studied the operation of the biological treatment plant, particularly the aerators that inject oxygen into the wastewater to ensure high dissolved oxygen levels. Previously it was believed that shutting down aerators to reduce oxygen levels (and thereby reduce nitrates) could lead to incomplete removal of organic wastes and increased production of ammonia. However an extensive program of laboratory and in-plant testing showed that turning off aerators in specific patterns would cause denitrification. This desirable reaction causes nitrogen (which would otherwise form ammonia or nitrate) to be converted to nitrogen gas. Nitrogen gas is an inert, harmless element that makes up about 80% of the air that we breathe. An innovative method was devised to operate different sets of aerators in combination while controlling dissolved oxygen at lower levels than previously thought feasible. Operators were trained in the new methods, and the new dissolved oxygen levels were set as routine targets for evaluation during 2002.
As a result of the parallel stripper operation and modified aerator operation,
nitrate emissions were reduced to 1.51 million pounds in 2002. This performance
was maintained in 2003, with emissions of 1.47 million pounds — a reduction
of 3.04 million pounds/year or 67 percent less than in 1999. Emissions
in 2004 have been 1.15 million pounds/year, a reduction of 75 percent
from 1999. This establishes that the reductions are both substantial —
a projected reduction of 3.4 million pounds/year — and sustainable over
long periods. This entire reduction translates directly to reduced nutrient
loading to the Mississippi River and to the Gulf of Mexico regions offshore
of Louisiana and Mississippi.
