absorb.txt
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Absorption Units May 1995 Technical Disclaimer References to abatement equipment technologies are not intended to represent minimum or maximum levels of Best Available Control Technology (BACT). Determinations of BACT are made on a case-by-case basis as part of the New Source Review (NSR) of permit applications. BACT determinations are always subject to adjustment in consideration of specific process requirements and recent developments in abatement technology. Additionally, specific health effect concerns may indicate stricter abatement than required by the BACT determination. The represented calculation methods are intended as an aid in the completion of acceptable permit submittals; alternative calculation methods may be equally acceptable if they are based on, and adequately demonstrate, sound engineering assumptions or data. These guidelines are applicable as of the date of this document, but are subject to revision during the permit application preparation and review period. It is the responsibility of applicants to remain abreast of any guideline or regulation developments which may affect their industries. BACT Guidelines All BACT determinations shall be made given consideration for the technical practicability and economic reasonableness of controlling emissions on a case by case basis. BACT guidelines have been established, however, individual determinations may require more or less than the levels given below. Removal efficiency: Generally, absorption units are expected to achieve a minimum of 99% removal efficiency. To verify a removal efficiency, a permit may require several means of determining compliance with specified emission allowables (see Methods of Compliance). The type of solvent used in an absorber is directly related to the removal efficiency. A permit engineer will check to see if certain streams are soluble in the solvent or if the appropriate chemical reaction between liquid and gas can take place. This is especially important if the absorber is designed to handle multi-component streams. The temperature of the incoming solvent greatly affects the absorbing properties as well. As the temperature of the solvent increases, its ability to absorb gases normally decreases. For this reason, a maximum temperature of solvent is often specifically put into a permit to ensure adequate removal efficiency. The removal efficiency is also highly dependent on the circulation rate of the solvent. As the circulation rate of the solvent decreases, the removal efficiency decreases. Monitoring of the circulation rate is also often required to ensure compliance with removal efficiencies. For non-continuous operations the applicant must demonstrate that the proposed absorption system will be able to handle the instantaneous load directed to the absorber. For instance, adequate flow rates will be required for an absorber designed to handle upset and maintenance procedures. Also, vapors directed to an absorber from barge, tank and/or truck cleaning operations will need to be checked for chemical reactivity and/or solubility for each chemical compound proposed. Design Justification for certain design features of absorbers/scrubbers (such as tower cross sectional area and adequate packed bed heights) should be supplied by the applicant. An applicant may submit computer generated emission estimates (such as Sim Sci program estimates) based on absorber design features to support a claim. In addition, a permit engineer may require vendor data supporting a company's claims. Permit conditions will be written to address specific concerns regarding design specifications. The following are basic design specifications commonly looked at by permit engineers. Solubility and Chemical reactivity Regardless of what type of absorber is used, an important factor in reviewing these units is the solubility of the gas in the absorbing solution or the chemical reactivity between the gas and the contacting liquid. Many absorbers that directly absorb the gas contaminant use water as the solvent. Some gases that are commonly absorbed by pure water include ammonia and acetic acid. It is important to verify that the solute will indeed be absorbed by the solvent. Often, a permit engineer will reference an Engineers' handbook to determine the absorption tendencies between solute and solvent or may request vendor data describing the absorption tendencies between solvent and solute. It is also important to monitor the solute concentration in the water if the water is being re-circulated. This is done to verify that there is sufficient clean water present to remove the contaminant gas. Absorbers that are designed to remove acid gases such as HCL and SO often use a caustic solution that will 2 chemically react to produce a salt. This salt may later be removed from the liquid effluent. The caustic is carefully chosen for its reactivity with the contaminant gas. The permit engineer will check to see if an adequate caustic circulation rate (batch operation) is being represented in the permit application and how often the inventory of caustic is being purged. Also, the required pH for a caustic solution will be reviewed. For example, since the pH of a solution is a logarithmic function, a small decrease in pH can result in a large drop in caustic strength. This causes concern as far as the timing of the batch changeouts. Permitting the incoming caustic solution at a high pH allows time for corrective action to be taken before the absorber effectiveness is compromised. A more accurate measure of caustic strength is measured by a titration test. Both pH and titration testing have been approved and limits for one or the other are often put into permits to ensure adequate caustic strength. Sample calculations are included in the following section. Flooding A situation may arise in countercurrent flow towers where an inlet gas flow rate is so high that it interferes with the downward flow of the solvent liquid. This may cause an upward flow of the liquid through the tower. This situation is known as flooding. Most absorbers are designed to operate at no more than 70% of the maximum gas velocity that can cause flooding. The reviewing engineer may require vendor data supporting the fact that flooding will not occur inside of the absorber. Items that may lead to flooding include high inlet gas flow rates, low liquid circulation rates and small diameter towers to name a few. Pressure Drop Pressure drop across a packed bed tower is a common way of determining if flooding is occurring. It is also an effective way of determining if something else has gone wrong inside of the absorber. A large pressure drop across a packed bed could also indicate plugging in the bed and/or deterioration of the packing itself. A high pressure drop can also indicate a variety of other abnormalities with the absorber that can ultimately result in a low gas removal efficiency. On-stream time and reliability For both continuous and non-continuous operations, absorbers are expected to operate effectively over the entire operating range and be on-line 99% of the time. Applicants may be required to have spare equipment such as ready to operate liquid circulation pumps and spray nozzles in case of mechanical failures. The applicant should also address how emissions will be minimized during equipment downtime. Methods of Compliance To verify a removal efficiency, a permit may require stack sampling or continuous emission monitoring of the exhaust stack of the absorber. Also, monitoring of solvent temperatures, flow rates, batch strength, pressure drop and pH may be required as a surrogate for a direct measurement of emissions to ensure absorber performance. This is commonly done as a way of demonstrating continuous compliance with the Maximum Allowable Emission Rate Table.