In November, 1990, the sulfuric acid unit at du Pont's plant in La Porte voluntarily installed new facilities to reduce sulfur dioxide (SO2) from its process vent stack at a cost of over $6,000,000. Because the sulfuric acid unit was in operation prior to 1971, it operates a "grandfather" status which allows operation under less rigorous standards than those of sulfuric acid plants which began operations after New Source Performance Standards (NSPS) were initiated. Despite this, stagnant weather conditions, SO2 ground level concentrations may only exceed normal limits. The only feasible way to reduce SO2 emissions was to increase the conversion. This has been accomplished by converting the existing absorption sulfuric acid plant to a dual absorption facility. This meets NSPS for sulfuric acid plants and will reduce SO2 emissions current 45 lb./ton of acid to less than 4 lb./ton of acid. efficiency will increase from 97% to 99.7%. The project has provided facilities to convert the La Porte's existing absorption sulfuric acid process to a dual absorption process using Enviro-Chem (engineering contractor) dual absorption technology. Absorption process improves current SO2 to SO3 conversion from 99.7% and thereby reduce SO2 emissions to the maximum allowable for burning plants. In simple terms, the SO2 previously existing the stack be converted to usable SO3, reducing stack emissions by over 90%.
The La Porte sulfuric acid unit formerly manufactured sulfuric "single absorption" technology by burning elemental sulfur and sulfur spent acids. Sulfur is burned in the sulfur furnace with pre-dried air to form sulfur. Spent acid is burned in the spent furnace with natural gas and containing gases from the spent furnace flow through scrubbing an equipment, and then combined with sulfur derived SO2. The "single absorption" process is limited to a conversion of about 98% for burning elemental sulfur. But when spent acids are with auxiliary fuels, the conversion is lowered to about 97% due to the reduced oxygen level. At this lower conversion, an emission of about 45 lb. SO2/ton H2SO4 can be expected. The dual absorption process dramatically reduces exit SO2 emissions by increasing SO2 to SO3 conversion efficiency. This is done by installation of a new final catalytic converter after the primary absorption step (oleum tower). The removal of SO3 in the oleum tower moves the SO2S03 equilibrium condition in the final converter well to the right. This reduces the final SO2 concentration to a level about 1/10th of that possible ahead of the primary SO3 absorption site. A second (final) absorption step is required to absorb SO3 formed in the new converter before the gas is discharged to the stack. This "dual absorption contact process" has been the basis to all new construction since the implementation of the NSPS. The dual absorption process for control of SO2 emissions has been thoroughly demonstrated as both an acceptable and efficient process that meets NSPS criteria without generation of additional waste by-products or otherwise affecting the environment. It is concluded that the standard dual absorption process with an emission level of 4 lb. SO2/ton H2SO4 represents best available control technology for an existing sulfuric acid plant.
SO2 emissions from the stack has been reduced by 90%. The SO2 emission rate of 3500 ppm (while burning elemental sulfur and spent acids) was reduced to 1/10th, i.e., to less than 350 ppm. SO2 emissions of 2000 ppm (with sulfur only) was reduced to less than 200 ppm.
Details of Reductions
Additional Information :
This is primarily an environmental project installed to reduce SO2 emissions. Even though some economic benefits are achieved due to improved conversion, it is not enough to meet the standard industry accepted return on investment (ROI).