MEMORANDUM

TO: Docket EPA-HQ-OAR-2012-0522

FROM: Gerri G. Garwood, P. E., EPA/OAR/OAQPS/SPPD

DATE: March 16, 2015

SUBJECT: Use of Pressure Drop as an Operating Parameter

A. INTRODUCTION During the proposal of the Phosphoric Acid Manufacturing and Phosphate Fertilizer Production Risk and Technology Review and St andards of Performance for Phosphate Processing, the EPA received comments about th e removal of pressure drop as an operating parameter on certain scrubbers. This memo explai ns the rationale behind the agency’s decision.

B. DISCUSSION Pressure drop is a critical scrubber de sign element and an important indicator of performance for most wet scrubbers. Because pressure drop remains fairly constant during steady state operations, a significan t change in pressure drop (either an increase or a decrease) indicates a problem with the scrubbing system. This is particularly true for high energy scrubbers. However, the EPA has previously note d that pressure drop across the scrubber is not a parameter of interest in spray towers and othe r low energy scrubbers beca use it is not directly related to the effectiveness of particle capture 1 . Additionally, due to the small magnitude of pressure drop in a low energy scrubber, it is more difficult to measure a significant change in pressure drop. A low energy scrubber is generally defined as a scrubber that utilizes a pressure drop through the system of less than 5 inches of water column 2 . Low energy scrubbers, such as spray

1 Wet Scrubber Inspection and Evaluation Manual . Prepared for U.S. Environmen tal Protection Agency, Office of Air Quality Planning and Standards. September 1983. EPA-340/1-83-022.

2 The EPA notes that 5 inches of water column is the general cut-off for low energy scrubbers in several documents, including the instructor’s guide to APTI course 413 – Control of Particulate Emissions, March 1980 and the student manual for APTI Course 445 - Inspection of Particle Contro l Devices, Third Edition, February 2003. This is also noted in the Monitoring Knowledge Base (see: http://cfpub.epa.gov/oarweb/mkb/contechnique.cfm?ControlID=27 ).towers, are effective in applica tions requiring control of gaseous pollutants or large particles. High energy scrubbers, such as venturi scrubbe rs, are useful in app lications requiring high removal efficiencies of small particulate. It is important to note that we are not saying that a low energy scrubber is not an effective control device, nor should a low energy scrubber be forced to operate at higher pressure drops. We are simply stating that monitoring pressure drop on a low energy scrubber is of limited value. There are different operating parameters that ar e more informative of a low energy scrubber’s performance. Although not inclusive, the followi ng are examples of operating parameters that can be useful in determining the performance of a low energy scrubber. Generally more than one operating parameter is necessary to gain a complete picture of the scrubber’s performance. (1) The ratio of the liquid flow rate and gas fl ow rate (or L/G) in the scrubber is an indicator of whether enough water is present to provide adequate scrubbing for the amount of gas flowing through the system. (2) In acid gas scrubbers, pH measurements of the scrubber liquor is an indicator of whether enough caustic is available in the system to achieve neutralization of the

acid. (3) Changes in scrubber liquid specific gravit y can be an indication of changes in pollutant concentration in the scrubber liqui d. An increase in pollutant concentration in the scrubber liquid can decrease the scrubber’s efficiency. (4) Spray nozzle pressure can provide an i ndication of proper atomization of the scrubbing liquid. Water droplet formation increases the surface area for liquid and gas contact. Loss of water droplet forma tion can result in loss of absorption.