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 
2

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 7,998bto 

8,000

14.4 14.4 14.4 + 2.6 Heel + 14.4 Heel

Vent to control 

device

7,998bto 

8,000

0.7c14.4 14.4 + 2.6 Heel + 14.4 Heel

Vapor balance 8,000 0 14.4 Heel + 14.4

Methylene 

chloride

Uncontrolled 7,988dto 

8,000

133 133 133 + 110 Heel +133 Heel

Vent to control 

device

7,988dto 

8,000

6.6e133 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 
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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 
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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):

 

 psi  lb lbmole

ft psia lbmole R R

gal ft gal Edep 2.4 32 /

14.7 2.4

17.1 2.4

ln

(10.73 3 / ) 530

8,000 3/ 7.48  

















  





= 2.57 lb methanol emitted

 

 psi  lb lbmole

ft psia lbmole R R

gal ft gal Edep 8.3 85 /

14.7 8.3

23 8.3

ln

(10.73 3 / ) 530

8,000 3/ 7.48  

















  





= 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.
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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

A. 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.

B. 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.
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C. 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.