**FROM:** David Randall, RTI International

**TO:** Nick Parsons, EPA/OAQPS **FOR:** EPA Docket No. EPA

- HQ

- OAR

- 201 0

- 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 sy stem to a control device. The total emissions include emissions from both the facility that owns the storage vessel and the facility

whe re 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 S ynthetic 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 6 3 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 s ys tem to a control device or b y 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 s ys tem to a control device, we conducted an anal ys is 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 model2

liquids and transfer condition s 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. Emission s 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 pre ssure (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 Unco ntrolled 7,998 b to 8,000 14.4 14.4 14.4 + 2.6 Heel + 14.4 Heel Vent to control device 7,998 b to

8,000 0.7 c 14.4 14.4 + 2.6 Heel + 14.4 Heel Vapor balance 8,000 0 14.4 Heel + 14.4 Methylene chloride Uncontrolled 7,988 d to 8,000 133 133 133 + 110 Heel + 133 Heel Vent to control device 7,988 d to

8,000 6.6 e 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 chlorid e: Up to 12 gal volatilize to saturate the vapor space (the amount of methylene chloride that evaporates depends on the volatilization rate). e Meth yl ene 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 (K N) and working3

loss product factor (KP ) are both 1. Note that for K N to be 1 we assumed the storage vessel would have less than 36 turnovers per ye ar. 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 meth yl ene 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 immediatel y upon entering the truck. Thus, if onl y 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. I f the tank truck is designed to handle the pressure, then the final pressure in the tank truck w ould 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 tr uck, 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 anal ys is 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 from4

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):



##       lbmole lb psi R R lbmole psia ft gal

ft gal E dep / 32 4

. 2 4

. 2 7

. 14 4

. 2 1

. 17 ln 530 ) / 3 73

. 10 ( 48

. 7 / 3 000 , 8        

        = 2.57 lb methanol emitted



##       lbmole lb psi R R lbmole psia ft gal

ft gal E dep / 85 3

. 8 3

. 8 7

. 14 3

. 8 23 ln 530 ) / 3 73

. 10 ( 48

. 7 / 3 000 , 8        

        = 110 lb methylene chloride emitted Displacement emiss ions 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 meth yl ene 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 anal ys is 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 dev ice per vehicle transfer would be 0.7 2 lb of methanol and 6.6 lb of methylene chloride (4.4 lb x 0.05 = 0.7 2 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 scenari o 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.5

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 methyl ene chloride) are emitted as the tru ck 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 Unc ontrolled 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 0.7 2 (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 1 40 to 2 50 6.6 (plus heel) to 1 40 (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 b y routing through a closed vent s y stem 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 un der both the vapor balancing and vent to control device scenarios are essentially the same.6

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/rel oading 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 sy stem 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 Sy nthetic Organic Chemi cal Manufacturing Industry

and Other Processes Subject to the Negotiated Regulation for Equipment L eaks. October 4, 2006. U.S. EPA. (1995, January). Compilation of Air Pollutant Emission Factors , Volume 1: Stationary Point and Area Sources, AP

- 42, Fifth E dition, U.S. Environmental Protection Agency , Office of Air Quality Planning and Standards.