Oil Production Tanks Methodology

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September 1, 2000 Page: 1

EMISSION CALCULATION PROTOCOL FOROIL PRODUCTION TANKS

1. PURPOSE OF METHODOLOGY AND APPLICABILITY

The purpose of this assessment is to develop an accurate methodology to estimate

emissions associated with heavy crude oil storage tanks. Emissions from heavy crude

oil storage tanks are of particular interest since most of the oil production in the San

Joaquin Valley is heavy crude oil.

Headspace gases in crude oil storage tanks contain reactive organic gases (ROG),

which are released into the atmosphere. Venting of headspace gases during routine

tank operation can be due to flashing losses, working losses, breathing (standing)

losses, or a combination of these processes.

The San Joaquin Valley Unified Air Pollution Control District (SJVUAPCD) includes

emissions associated with heavy crude oil storage tanks in its emissions inventory.

These emissions are divided into working losses and breathing losses. The following

emissions inventory codes are affected:

310-995-1600 Crude Oil Production Tanks

310-314-1600 Crude Oil Production Oil Water Separators

2. BACKGROUND INFORMATION

To accurately assess emissions from an oil production tank, both the volume of

headspace gas released per day and the concentration of ROG in the headspace gas

need to be known.

To date, the Reid vapor pressure (RVP) method (ASTM Method D-323)1

has been used

to estimate the ROG vapor pressure in the tank headspace. While this method may be

appropriate for light crude and refined petroleum products, it does not work well with

heavy crude oils. The RVP method was developed for use with hydrocarbons with high

vapor pressures, and tends to overestimate emissions for heavy crude oil.

Measurements made under the Heavy Oil Storage Tank (HOST) program have shown

1 American Society for Testing and Materials




that the RVP gives excessively large vapor pressure values due to non-regulated gases

such as methane and carbon dioxide released during the measurement process.2

In

addition, the ASTM procedure calls for the RVP test to be performed at 100°F, whereas

the crude oils in many of the oil storage tanks are at significantly higher temperatures.

A nomograph can be used to perform a temperature correction on the measured RVP,but many tanks operate at temperatures beyond the range of the nomograph. Also, the

possibility of errors introduced by using the nomograph has not been established.

Under the HOST program, an improved procedure was developed to determine ROG

vapor pressures for crude oil storage tank headspaces. The method was developed at

LBNL and is entitled "Test Method for Vapor Pressure of Reactive Organic Compounds

in Heavy Crude Oil Using Gas Chromatography." The method uses gas

chromatography analysis to determine the ROG vapor pressure from the sample. The

HOST study demonstrates that the ROG concentration from the liquid sample

correlates well with the ROG concentration in the tank headspace. This procedure can

be used in place of the RVP method to determine the ROG vapor pressure of heavy

crude oils for use in the AP-42 equations3

for tank working and breathing losses.

2.1 ROG Losses From Tanks

As mentioned above, there are three processes that generate ROG losses from tank

headspaces: working, breathing (standing), and flashing. Flashing losses areaddressed in a separate section and will not be discussed here. Chapter 12 of the

AP-42 document provides procedures and equations to estimate emissions from fixed

roof tanks. In addition to a number of tank parameters, one of the inputs to the AP-42

calculations is the vapor density, which is derived from the RVP. In the case of heavy

crude oil tanks, the vapor density can be derived from the ROG vapor pressure

determined from the HOST study. The existing equation is presented below:

AP-42 Equation

2In 1995, the Heavy Oil Storage Tank (HOST) Working Group was formed to review heavy crude

emissions information and develop appropriate emissions estimation methodologies. The committeeenlisted the assistance of staff from the Lawrence Berkeley National Laboratory (LBNL) to evaluateexisting methods, perform laboratory and field measurements, and make recommendations on how toestimate heavy crude emissions.

3 U.S. Environmental Protection Agency, Compilation of Air Pollution Emission Factors, Volume I, Section

12, Storage of Organic Liquids. U.S. Environmental Protection Agency, 4th Edition, 1993.




Wv = [ Mv Pva ] / [ R Tla ]

where:

Wv = vapor density, (lb / ft3)

Mv = vapor molecular weight, (lb/lb-mole) [the HOST group recommends 100]Pva = ROG vapor pressure at average liquid temperature (psia)

R = ideal gas constant, (10.731 psia ft3 / lb-mole temp[°R])Tla = average liquid temperature at surface (°R)

Once the vapor density is calculated, working and breathing losses can be estimated.

3. METHODOLOGIES USED BY OTHER DISTRICTS

Most air pollution control districts currently use the methodology developed by the

California Air Resources Board (CARB) to estimate crude oil storage tank emissions.4

The methodology is based on the AP-42 estimation equation for working and breathing

losses. It assumes that open top tanks and tanks with open holes or roof openings do

not have emissions greater than those for fixed roof tanks.

More recently, the CARB developed Method 150 to assess emissions from heavy oil

storage tanks over a 24-hour period. Method 150 was used in some of the HOST

testing. It was found that a number of tanks were not well-suited for the method, the

procedure is time-consuming, and it does not lend itself to routine tank testing. A more

detailed discussion is provided in the Emissions Calculation Protocol for Oil Production

Flashing Losses.

4. DESCRIPTION OF POSSIBLE METHODOLOGIES

There are three known options to estimate oil production tank emissions. The first

option is to retain the existing approach. The method uses RVP in the AP-42 formula

and ignores flashing losses. This option is considered unsatisfactory by both regulators

and industry. Both the ASTM Method that describes the RVP apparatus and procedure

and AP-42 state that RVP is unsuitable for viscous petroleum products. Measurements

4California Air Resources Board, Emissions Inventory Procedural Manual Volume III Methods for

Assessing Area Source Emissions, October 1997.




conducted under the HOST program show that RVP does not provide an accurate

measure of the ROG vapor pressure. It is generally recognized that the District's

previous inventory can be improved by making appropriate modifications.

The second option is to retain the AP-42 formula replacing RVP in the equation with theLBNL method for determining ROG vapor pressure. This option maintains the basic

framework provided in AP-42, with modifications to provide an improved assessment of

heavy crude ROG vapor pressures. Emissions obtained from the methodology to

estimate flashing losses would be included in the total emission estimates.

The third option is to conduct on-site measurements of emissions from all permitted

tanks. This option would yield the most precise results; however direct measurements

of all tanks would be costly and time-consuming.

5. RECOMMENDED APPROACH

The recommended approach is to modify the emissions inventory by using the LBNL

method for determining ROG vapor pressure in place of the RVP and to include

flashing losses estimated in accordance with the methodology explained in the

Emission Calculation Protocol for Oil Production Flashing Losses.

Based on production rates or throughput of the tanks, the ROG emissions can beextrapolated to the population of tanks in the San Joaquin Valley. The District's

database of oil storage tanks will be updated to reflect tanks that are no longer in

operation. The number of tanks in operation will be determined by using data compiled

for the District's "Control Profile Development Project" based on the District's master

permit database.

6. STRENGTHS AND WEAKNESSES OF PROPOSED METHODOLOGY

This approach provides incremental improvements over the existing method. The basic

concepts described in AP-42 generally are considered satisfactory and only corrections

such as improved ROG vapor pressures are needed to obtain accurate emission

estimates. By incorporating an improved technique for determining the ROG vapor

pressures from heavy crude oils, the methodology is expected to generate an

emissions inventory that more closely reflects actual emissions. There may be




consensus regarding use of this approach since it is a product of the HOST Study

Group, which consists of representatives from a number of regulatory agencies,

petroleum industries, and trade organizations.

7. SAMPLE CALCULATION

The AP-42 Calculation Equation presented in Section 2 above indicates that the vapor

density is directly proportional to the value of the vapor pressure used. From AP-42,

both working losses and breathing losses are proportional to the vapor density, and

consequently proportional to the ROG vapor pressure.

Heavy crude oils analyzed using the RVP method indicates that vapor pressures

typically are in the range of 0.1 to 0.5 psia. Measurements conducted using the

proposed method as part of the HOST program have yielded an average ROG vapor

pressure of 0.04 psia. If 0.3 psia is used as an average RVP for heavy crude oils, it can

be seen that the RVP method significantly over-estimates emissions when used with

AP-42 (0.3 psia (Reid) / 0.04 psia (HOST) = 7.5 times actual vapor pressure).

7.1 Working Loss Sample Calculation

Assume that a tank is 20 feet high by 30 feet diameter

The liquid level initially is at the 5 foot level and is filled to the 15 foot level. The changein the oil level is 10 feet.

The gas displaced by filling will be:

10 ft change in level x (3.14 x 15 ft x 15 ft) = 7065 cu ft

(The area of the tank is pi r squared, pi = 3.14159 and r = 15 ft)

The ROG released will be:

7065 cu ft x [ ROG (psia) / 14.7 psia ]

If the ROG concentration is 0.05 psia, the ROG released will be 24 cu ft. This can be

converted into pounds of ROG released, using a molecular weight of 100:




100 lb-mole x 24 cu ft / (379 cu ft/ lb-mole) =

6.3 lb ROG released

7.2 Breathing Loss Sample Calculation:

Assume a tank with 1000 cu ft headspace starts the day with a headspace temperature

of 60°F. It reaches a maximum temperature of 110°F in the afternoon. The volume of

gas vented due to expansion (using the ideal gas law) is:

1000 cu ft x [(110 + 460)/(60 + 460)] = 1096 cu ft expanded volume

1096 cu ft - 1000 cu ft = 96 cu ft volume vented due to thermal expansion

The ROG released will be:

96 cu ft x [ ROG (psia) / 14.7 psia ]

Again, if a ROG concentration of 0.05 psia is used, the volume of ROG released will be

0.3 cu ft.

The same conversion process as above will be used to calculate the ROG emissions:

100 lb-mole x 0.3 cu ft / (379 cu ft/ lb-mole) =

0.08 lb ROG released




7.3 Flashing Loss Sample Calculation:

(Based on the Emission Calculation Protocol for Oil Production Flashing Losses)

Assume that the gas-oil ration (GOR) is 1.5 cu ft/barrel (bbl) (either from directmeasurement or the Vasquez-Beggs Equation) and the daily throughput of a tank is

100 barrels of oil.

The volume of gas flashed off will be:

1.5 cu ft/bbl x 100 bbl = 150 cu ft

Assume a ROG headspace concentration of 0.05 psia, (the same as above)

150 cu ft x [0.05 psia /14.7 psia] = 0.5 cu ft

Convert to pounds of ROG released, using a molecular weight of 100

100 lb-mole x 0.5 cu ft / (379 cu ft/ lb-mole) =

0.13 lb ROG released

The total emissions per tank can be applied to all tanks based on the throughput orproduction rate of each tank.

Oil Production Tanks Methodology

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