FROM:	David Randall, RTI International

TO:		Nick Parsons, EPA/OAQPS

FOR:		EPA Docket No. EPA-HQ-OAR-2010-0871

DATE:	December 1, 2011

SUBJECT:	Vapor Balancing Emissions Estimates for Storage Vessels

1.	Purpose

This memorandum estimates and compares total emissions for two storage
vessel control options: (1) vapor balancing and (2) routing emissions
through a closed vent system to a control device. The total emissions
include emissions from both the facility that owns the storage vessel
and the facility where the transport vehicle that supplies the storage
vessel is cleaned and/or loaded.

2.	Background

Vapor balancing is included as a compliance option for storage vessel
emissions in several current rules such as the National Emission
Standards for Hazardous Air Pollutants (NESHAP) From the Synthetic
Organic Chemical Manufacturing Industry (also known as the “HON”, 40
CFR part 63 subpart G), the Miscellaneous Organic NESHAP (40 CFR part 63
subpart FFFF), and the Pharmaceutical Production NESHAP (40 CFR part 63
subpart GGG). In each of these rules, vapor balancing was provided as an
option because it is at least as effective as the technology that
represented the MACT floor on which the emissions standard was based.
One of the requirements in the vapor balancing options is that emissions
from cleaning and/or reloading a transport vehicle that has been used in
vapor balancing control of a subject storage vessel must also be
controlled. The transport vehicle emissions may be controlled either by
routing through a closed vent system to a control device or by vapor
balancing to the storage vessel that supplies the liquid that is
refilling the transport vehicle. The required efficiency of the control
device is either 90 percent or 95 percent, depending on the rule.

In a letter submitted after the HON was amended to include the vapor
balancing alternative, an industry representative recommended that EPA
not require control of the transport vehicle because the amount of
residual chemical in the transport vehicle is the same regardless of how
displaced vapors from the storage vessel are handled (Alexius).

3.	Discussion

To evaluate the overall performance of the vapor balancing option versus
the option to route emissions through a closed vent system to a control
device, we conducted an analysis assuming liquid is transferred to the
storage vessel from a model transport vehicle. This model vehicle
contains 8,000 gal of liquid for transfer (plus the heel). Table 1 lists
characteristics for model liquids and transfer conditions used in the
emissions calculations, and Table 2 summarizes the estimated emissions
at both the facility that owns the storage vessel that is subject to
control requirements and the facility where the transport vehicle is
cleaned and/or reloaded. Emissions were estimated for an uncontrolled
scenario as well as the two control options noted above. Procedures used
to estimate the emissions are described in the sections below.

Table 1. Characteristics of model transferred liquids

Transferred liquid	Vapor pressure (psia)	Density (lb/gal)	Transfer
temperature (°F)

Methanol	2.4	6.6	70

Methylene chloride	8.3	11.0	70



Table 2.  Summary of emissions from storage tank control scenarios.

Stored material	Storage tank control scenario	Amount transferred to
storage tank, gal	Emissions, lb/transfer



	Displaced from storage tank	Cleaning/reloading facility (uncontrolled)a





Dedicated service	Clean the tank truck





Boundary Condition 1	Boundary Condition 2	Boundary Condition 1	Boundary
Condition 2

Methanol	Uncontrolled	7,998b to 8,000	14.4	14.4	14.4 + 2.6	Heel + 14.4
Heel

	Vent to control device	7,998b to 8,000	0.7c	14.4	14.4 + 2.6	Heel + 14.4
Heel

	Vapor balance	8,000	0	14.4	Heel + 14.4

Methylene chloride	Uncontrolled	7,988d to 8,000	133	133	133 + 110	Heel
+133	Heel

	Vent to control device	7,988d to 8,000	6.6e	133	133 + 110	Heel +133
Heel

	Vapor balance	8,000	0	133	Heel + 133

a “Boundary Condition 1” means the vapor space is immediately
saturated as liquid is withdrawn from the transport vehicle. “Boundary
Condition 2” means the air or nitrogen drawn into the transport
vehicle remains free of organic compounds while liquid is being
withdrawn from the transport vehicle. 

b Methanol: Up to 2 gal volatilize to saturate the vapor space of the
transport vehicle (the amount of methanol that evaporates depends on the
volatilization rate).

c Methanol: Assumes the 14.4 lb of methanol in the displaced vapor from
the storage vessel are routed to a control device that reduces emissions
by 95 percent.

d Methylene chloride: Up to 12 gal volatilize to saturate the vapor
space (the amount of methylene chloride that evaporates depends on the
volatilization rate).

e Methylene chloride: Assumes the 133 lb of methylene chloride in the
displaced vapor from the storage vessel are routed to a control device
that reduces emissions by 95 percent.

Scenario 1:  Uncontrolled 

Displacement emissions from filling the storage tank

For methanol, uncontrolled emissions were calculated using the equation
in AP-42 Chapter 7.1 for working losses from a fixed roof (i.e.,
Equation 1-29), except that we used the actual temperature, ideal gas
law constant, and unit conversion factors instead of using the constant
0.001 (U.S. EPA, 1995). We also assumed the working loss saturation
factor (KN) and working loss product factor (KP) are both 1. Note that
for KN to be 1 we assumed the storage vessel would have less than 36
turnovers per year.

	Emissions from storage vessel = (8,000 gal)(ft3/7.48 gal)(2.4 psi)(32
lb/lbmole)/

		(10.73 ft3psi/lbmole-°R)/(530°R)

	= 14.4 lb/transfer

For methylene chloride, uncontrolled emissions are calculated using the
following equation:

	Emissions from storage vessel = (8,000 gal)(ft3/7.48 gal)(8.3 psi)(85
lb/lbmole)/

	(10.731 ft3-psi/lbmole-°R)/(530°R)

	= 132.7 lb/transfer

What happens in the tank truck?

As liquid is drained from the truck, fresh air (or nitrogen) is drawn
into it. 

Boundary Condition 1 assumes that the air becomes saturated immediately
upon entering the truck. Thus, if only 8,000 gal can be removed from the
truck when unloading (either by transfer to the storage tank or by
evaporation into the truck vapor space), then approximately 2 gal of
methanol evaporate into the vapor space of the transport vehicle (14.4
lb/6.6 lb/gal) and 7,998 gallons are transferred to the storage tank.

Boundary Condition 2 assumes that the fresh air remains free of methanol
vapor until the transfer is complete. In this case, all 8,000 gal can be
transferred to the storage tank, and then 14.4 lb of the heel evaporate
to saturate the vapor space. 

The final amount of methanol vapor is the same under both boundary
conditions, but the amount of air is greater under Boundary Condition 2.
If the tank truck is designed to handle the pressure, then the final
pressure in the tank truck would be 17.1 psia (14.7 for the air plus 2.4
for the methanol). If the tank truck is not designed to handle such
pressures, a pressure relief vent would open one or more times and
release some of the vapor. Regardless of the pressure rating for the
tank truck, the amount of methanol in the truck (both liquid and vapor)
when it reaches the cleaning/reloading facility would be at least 14.4
lb less under Boundary Condition 2.

The analysis is the same for methylene chloride, except about 12 gal
evaporate into the vapor space (133 lb/11 lb/gal = 12 gal), and the
final pressure in the truck would be a maximum of 23.0 psia (14.7 +
8.3).

What happens at an uncontrolled cleaning/reloading facility if the truck
is in dedicated service?

Under Boundary Condition 1, the organic vapor in the 8,000 gal of vapor
space of the transport vehicle (14.4 lb methanol; 133 lb methylene
chloride) is emitted as the truck is reloaded with another 8,000 gal of
liquid.

Under Boundary Condition 2, if the tank truck did not vent as the liquid
evaporated to saturate the vapor space, then emissions will occur first
from depressurization and then from displacement. (If the tank truck did
vent, then we assumed the total emissions would be the same as if the
tank released emissions only at the reloading facility.)
Depressurization emissions can be calculated using Equation 32 in 40 CFR
63.1257 (subpart GGG):

 

	= 2.57 lb methanol emitted

 

	= 110 lb methylene chloride emitted

Displacement emissions are then the same as under Boundary Condition 1
(i.e., 14.4 lb of methanol or 133 lb of methylene chloride). The truck
is then filled with 8,002 gal of methanol (or 8,012 gal of methylene
chloride). We ignored the additional displacement emissions that would
occur from the extra 2 gal of methanol (or 12 gal of methylene chloride)
added under Boundary Condition 2. We also ignored the emissions from the
additional portion of a load that would be needed under Boundary
Condition 1 to achieve the same total amount of liquid transfer to the
storage vessel.

What happens at an uncontrolled cleaning/reloading facility if the truck
is cleaned?

Under both boundary conditions, methanol emissions consist of the 14.4
lb in the vapor space plus all residual liquid in the transport vehicle.
However, the residual liquid contains 14.4 lb less under Boundary
Condition 2 (i.e., the sum of the residual liquid plus the 14.4 lb vapor
is equal to the original heel). The same analysis applies to methylene
chloride, except the amount in the vapor space is 133 lb.

Scenario 2:  Control Storage Vessel Emissions by Routing Through a
Closed Vent System to a Control Device

Assuming the efficiency of the control device for the storage vessel is
95 percent, emissions from the control device per vehicle transfer would
be 0.72 lb of methanol and 6.6 lb of methylene chloride (4.4 lb x 0.05 =
0.72 lb and 133 lb x 0.05 = 6.6 lb).

Emissions at the transport vehicle cleaning/reloading facility would be
the same as under the uncontrolled scenario for storage vessels as
discussed above.

Scenario 3:  Control Storage Vessel Emissions by Vapor Balancing

Displacement emissions from filling the storage tank

Vapor from the storage vessel is routed to the tank truck from which the
storage vessel is being filled. Thus, there are essentially no emissions
at the storage vessel.

What happens in the tank truck?

No fresh air (or nitrogen) is drawn in. Thus, the full 8,000 gal of
liquid are transferred to the storage vessel. The vapor space contains
14.4 lb of methanol (133 lb of methylene chloride), and none of the heel
evaporates.

What happens at an uncontrolled cleaning/reloading facility if the truck
is in dedicated service?

The 14.4 lb of methanol vapor (133 lb of methylene chloride) are emitted
as the truck is refilled with another 8,000 gal of liquid.

What happens at an uncontrolled cleaning/reloading facility if the truck
is cleaned?

All of the heel and the 14.4 lb of methanol vapor (133 lb of methylene
chloride vapor) are emitted.

Summary of Emissions

Table 3 summarizes the total emissions from both the facility that owns
the storage vessel and the transport vehicle cleaning/reloading facility
for all three control scenarios. Emissions are per model vehicle load,
and the cleaning/reloading facility is uncontrolled.

Table 3. Total emissions under each storage vessel control scenario

Storage vessel control scenario	Total emissions when offsite facility is
uncontrolled (lb/truck load)

	Tank truck in dedicated service	Tank truck cleaned

Methanol



   Uncontrolled	28.8 to 31.4	14.4 (plus heel) to 28.8 (plus heel)

   Vent storage vessel to control device	15.1 to 17.7	0.72 (plus heel)
to 15.1 (plus heel)

   Vapor balance	14.4	14.4 (plus heel)

Methylene chloride



   Uncontrolled	266 to 376	133 (plus heel) to 266 (plus heel)

   Vent storage vessel to control device	140 to 250	6.6 (plus heel) to
140 (plus heel)

   Vapor balance	133	133 (plus heel)



4.	Conclusions

If the transport vehicle is in dedicated service and reloading
operations are uncontrolled, then total emissions from both the affected
facility and the offsite facility under the vapor balancing control
scenario are less than total emissions from both facilities when the
storage vessel emissions are controlled by routing through a closed vent
system to a control device.

If the transport vessel is cleaned before reloading, and the vapor space
in the transport vessel is close to saturated when transfer of liquid to
the storage vessel is complete (i.e., Boundary Condition 1), then total
emissions under both the vapor balancing and vent to control device
scenarios are essentially the same.

If the cleaning/reloading facility is controlled, the total emissions
from both the affected storage vessel facility and the off-site
transport vehicle cleaning/reloading facility would be significantly
lower than the total emissions from both facilities if the affected
facility controls the storage vessel emissions by routing through a
closed vent system to a control device (and no off-site control is
required).

5.	References

Alexius, B. (2006, October). Letter and attachments from B. Alexius, The
Dow Chemical Company, to R. McDonald, EPA/OAQPS. Vapor Balancing
Provisions of the National Emission Standards for Hazardous Air
Pollutants for the Synthetic Organic Chemical Manufacturing Industry and
Other Processes Subject to the Negotiated Regulation for Equipment
Leaks. October 4, 2006.

U.S. EPA. (1995, January). Compilation of Air Pollutant Emission
Factors, Volume 1: Stationary Point and Area Sources, AP-42, Fifth
Edition, U.S. Environmental Protection Agency, Office of Air Quality
Planning and Standards.

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