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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.


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.


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.