Storage Tank Selection for Min Emissions & Oxy Contamination

1. Purpose

This procedure provides criteria for selecting atmospheric storage tanks that eliminate oxygen contamination and minimize hydrocarbon emissions.

2. General

The following problems may occur if oxygen (air) contaminates the feed to process units:

• Fouling of feed exchangers

• Premature plugging of reactors

• Corrosion from chemicals subjected to oxidation

To prevent oxygen contamination and avoid these potential problems, many Inflection Point Engineering licensed process units specify gas blanketed feed storage. The benefits of eliminating oxygen contamination, e.g., reduced maintenance and longer runs, have been validated commercially.

3. Regulations

Deciding what type of storage tankage to use hinges upon two main problems eliminating harmful oxygen (air) contamination and minimizing atmospheric hydrocarbon discharges to satisfy local air pollution requirements. Other issues include minimizing ingress of water and minimizing cost.

Procedures and IPE-TM-310-01 must be used together for the selection of storage tanks.

IPE-TM-310-01 and HSE-03 are based on U.S. EPA regulations as found in 40 CFR 60. Other jurisdictions may require less restrictive, or more restrictive, vapor pressure limitations. The installed tank type should be compatible with the local regulations. Use of a cone roof tank for hydrocarbons with a true vapor pressure greater than 0.75 psia (see section 4.2) may be acceptable if this meets those local regulations.

4. Types of Storage Tanks

4.1 Cone Roof Tanks (Atmospheric Vent)

Use this type of tankage to store water, soda ash solutions (for neutralization tanks), and caustic. This scheme is unacceptable to prevent oxygen contamination because these tanks expose the entire liquid surface to the vapor space above the liquid. During liquid level changes, and diurnal temperature changes, there is a high potential for oxygen contamination and evaporation. The subsequent emissions for these tanks are substantial for liquids with relatively high vapor pressures.

Cone roof tanks with atmospheric vents are often used for storing asphalt or for storing other hydrocarbons at a temperature greater than the boiling temperature of water. Consult the tank specialist when specifying the tank type for these types of services.

Wastewater can usually be stored in a cone roof tank. However, in cases where there are significant amounts of hydrocarbon or H2S in the wastewater, contact the tankage specialist to determine the tank type required.

4.2 Cone Roof Tanks (Pressure/Vacuum Vent Valves)

The first effective improvement made to cone roof tanks was to fit the tank with pressure vacuum vent valves (PV valves). A sealed tank with a PV valve represents a significant improvement in the ability to reduce evaporative losses. Note that the evaporative losses increase with vapor pressure. This type of tank may be gas blanketed and is commonly used for hydrocarbon liquids with a true vapor pressure of less than 0.75 psia (0.053 kg/cm2g) at 100F (38C). Depending upon the tank’s applications and contents however, their use may be limited to even lower vapor pressures. (Refer to Procedure , Table 1).

4.3 External Floating Roof Tanks (EFRT)

The floating roof design decreases the evaporative losses by an order-of-magnitude over cone roof tanks. The reason is the roof covers over 95 percent of the surface area. In addition, floating roof tanks are fitted with rim seals that further reduce emissions from the remaining exposed surface area. The emissions are limited to the amount that can escape past the rim seals and roof fittings and that which clings to the walls of the tank as the liquid level descends. These tanks do not reduce emissions or oxygen contamination to zero. EFRT's may not be gas blanketed.

Either EFRT's or Internal Floating Roof Tanks (IFRT) are normally specified for hydrocarbon products with a true vapor pressure greater than 0.75 psia (0.053 kg/cm2g) at 100F (38C). Depending upon the tank’s applications and contents, however, their use may be required for vapor pressures even lower than 0.75 psia. (Refer to Procedure HSE-03, Table 1).

Throughout the oil industry, EFRT's would be the primary choice with IFRT's used in special situations. As it turns out, most of the tanks specified by Inflection Point Engineering are in special situations. Contact the tank specialist if an external floating roof tank is being considered.

Crude oil is almost always stored in EFRT's, regardless of vapor pressure.

4.4 Internal Floating Roof Tanks (IFRT)

The IFRT is a floating roof inside a cone roof tank. Emissions are lower than those for a comparable EFRT as the effect of wind is diminished. Also, where highly hazardous toxic substances are stored, a properly designed internal floating roof tank may be the best choice (for example, benzene storage). When it is desirable to keep rainwater out of the product or feed (for example, MTBE), IFRT's are used. Also, when gas blanketing is required, IFRT's are normally used. IFRT's are normally specified for all aromatic type products (i.e. benzene, toluene, xylenes, etc.). IFRT's are sometimes used in water service to minimize oxygen contamination.

At Inflection Point Engineering, most if not all floating roof tanks will be of the internal floating roof type. Nitrogen blanketing is often specified to prevent oxygen contamination and that requires the cone roof. In other cases, the cone roof is desired to keep rain water from contaminating the product.

There are two types of internal floating roof tanks, open and closed. Open IFRT’s have holes in the shell at the top of the tank to allow air to freely ventilate the tank vapor space so as to avoid having the vapor space in the explosive range. Closed tanks do not have holes so the vapor space has the potential to be in the explosive range.

When Inflection Point Engineering specifies nitrogen blanketing, the IFRT must of course be of the closed type. If Inflection Point Engineering specifies an IFRT without nitrogen blanketing, then it should be of the open type.

Closed type IFRT’s will have slightly reduced emissions compared to an open type. However, the benefit of emissions reduction would be offset by the requirement to have nitrogen blanketing. Generally, do not specify a closed type IFRT to reduce emissions, but if this is required by a client, then add nitrogen blanketing. Do not specify a closed type IFRT without nitrogen blanketing.

Consult tank specialist if client requests that nitrogen blanketing be provided for safety considerations.

4.5 Cone Roof Tanks with Vapor Recovery/Destruction or Internal Floating Roof Tanks with Vapor Recovery/Destruction (Sealed System)

In locations with extreme emission limitations, it is sometimes necessary to collect and destroy vapors from storage tanks. Normally, cone roof tanks and IFRT’s are vented to the atmosphere right at the tank. With a sealed tank system, the vapors from the tank are collected and routed to a vapor control facility. The vapor control facility can either recover the vapor as a product or destroy the vapor by transforming it into H2O and CO2.

Common options for emission disposal in sealed tank systems are:

• Incineration
• Catalytic processes
• Vapor compression
• Refrigeration
• Activated carbon adsorption

These systems are used when very low or no emissions of vapors from the storage tanks are permitted. In most cases, an IFRT is all that is required. See Table 1 for a comparison of these systems.

Venting to the plant’s conventional flare is generally impractical. The flare is typically a long distance from the tanks, and the vent pressure is only a few inches of water. Furthermore, the tank would need to be designed for the back pressure during any relief event in the plant. Such a design pressure would normally be much more than that for which the cone roof tanks are designed.

Table 1 - Comparison of Vapor Recovery Systems for Cone-Roof Tanks

Figure 1 summarizes the relative losses from above ground storage tank improvements.

5. Gas Blanketing Eliminates Oxygen Contamination

The purpose of blanketing is to maintain a “pad” of an inert gas on top of the product to keep atmospheric contamination out and prevent the tank from going into a vacuum condition. The benefits of eliminating oxygen contamination, i.e., reduced maintenance, longer runs, and better product quality, have been validated commercially. In addition, the exclusion of air also provides safety by reducing the combustibility of the product’s vapors. The use of blanketing has essentially no effect on tank emissions.

Nitrogen is the preferred tank blanketing medium; it is almost always acceptable from a process standpoint and is usually the lowest cost option (as the nitrogen vapors can usually be simply vented to the atmosphere). Hydrocarbon vapors should not be used for tank blanketing unless the vapors are collected and disposed of properly. Inert gases other than nitrogen may be acceptable but need to be reviewed because they may be detrimental to the process. Procedure provides guidelines for determining the maximum nitrogen requirements for storage tank blanketing. Also, Tools and ” provide calculations on nitrogen blanketing requirements.

The two predominant factors when sizing any tank blanketing system are:

• Tank Capacity (for thermal inbreathing)
• Pump Out (withdrawal) Rate.

API-2000, “Venting Atmospheric and Low-Pressure Storage Tanks” provides guidance on nitrogen inbreathing requirements.

5.1 Tank Capacity

Tank capacity is important because the larger the tank, the larger the volume of tank inbreathing.

As the ambient temperature increases during the day, the tank and its contents (including the blanket gas) warm and expand, causing an increase in the pressure within the tank. As nightfall or a cooling rain shower comes, the tank and its contents cool, causing the pressure within the tank to drop toward a vacuum condition. At this point additional blanket gas must be introduced to maintain the positive pressure necessary to keep out moisture, air, and contaminants and to keep the tank from pulling a vacuum and collapsing in on itself. Larger tanks require greater volumes of blanketing gas.

See Table 2A or 2B of API-2000, “Venting Atmospheric and Low-Pressure Storage Tanks” for tank inbreathing requirements. These tables are based on air, but they are equally applicable for nitrogen.

For tanks with a capacity up to and including 20,000 barrels (3170 m3), API-2000 calls for 1.0 std ft3/h of nitrogen for each barrel of tank capacity.

For tanks with a capacity greater than 20,000 barrels (3170 m3), consult API-2000 or use T-611-01 for exact nitrogen requirements. The blanketing requirement can be approximated by the following equation:

Nitrogen std ft3/h = 31.5 (tank capacity, barrels) 0.657

5.2 Pumpout Rate

The second factor in sizing a blanketing system is the actual pumpout rate at which the product will be removed from the tank. Multiply the actual pumpout rate (gpm) by 8.0 to get std ft3/h of nitrogen.

5.3 Total Nitrogen Flowrate

Add the thermal inbreathing requirement and the pumpout std ft3/h together to come up with the total required flow rate of the blanketing system in std ft3/h. This total is the maximum amount of nitrogen that can go into the storage tank. Use this value to size the nitrogen blanketing equipment for the tank.

The amount of nitrogen required for blanketing large tanks, or complexes with a number of smaller tanks, can be extremely large and can entail considerable cost. If the required nitrogen rate is a concern, consult the tank specialist for recommendations. An overall design can be established to reduce the amount of nitrogen consumed and/or greatly reduce the size of the nitrogen supply facilities.

The 106 project specification should reflect nitrogen usage by tanks. Normal rates should not be shown. The peak rate, as calculated per above methods, should be shown. Also, an average rate should be shown. See tank specialist if detailed calculations on average nitrogen consumption is required. Normally, set average nitrogen consumption rate as follows

Average Rate = Thermal Inbreathing Rate * (3/480) + Pumpout Rate * X

Where X = % of time tank is being pumped out.

6. Suggested Philosophy for Future Designs

Future designs of Inflection Point Engineering processes should incorporate the following ideas:

• Determine early in the design what minimum tankage facilities are required by the client or the environmental regulating agencies.

• If the minimum allowable facilities allow oxygen contamination, stress to the customer the importance of feed quality and suggest that the facilities be upgraded to eliminate oxygen contamination.

• When facilities are required to prevent oxygen contamination, Inflection Point Engineering recommends, in most cases, an internal floating roof tank which uses nitrogen as the blanketing gas for the design of storage tankage.

Figure 1