The amount of hydrocarbon emissions from tank vehicle loading and unloading activities is a relatively small portion of the total for the upstream petroleum industry. Nonetheless, where these activities occur they can be a major local source of emissions. There are two basic control strategies that may be taken: install separate systems to control loading losses from the tank vehicles and storage losses from the tanks, or implement a system to balance or exchange vapours between the tanks and tank vehicles and add a common vapour control device if needed. The first approach has the advantage that it may avoid any air contamination of vapours in the storage tanks and thereby reduce the costs of controlling storage losses. However, there also may be some economy in going to a common control system. If storage losses are substantially greater than loading losses it may be best to employ separate control systems. Otherwise, the economics may support the use of a vapour exchange system and common control device. If the product being handled is sweet and has a relatively low vapour pressure (say less than 34.5 kPa), then a vapour exchange system alone may provide adequate emissions control. Each case should be evaluated on its own to determine the best solution.
In the absence of any specific controls, emissions from loading and unloading activities may be attributed to three major effects:
· physical displacement of residual vapours by the incoming liquid,
· evaporation effects promoted by agitation of the liquid during the transfer process, and
· leakage/spillage during the connection/disconnection of transfer lines and during the transfer process.
The first two types of emissions will emanate from the receiving tank (i.e., the tank vehicle or the storage tank). With respect to the last type, a certain amount of the product that is spilled or leaked onto the ground may be recovered during cleanup efforts. The rest will tend to evaporate and volatilize over time due to natural processes including biodegradation caused by micro-organisms in the soil.
The amount of emissions from the receiving tank will depend on the initial concentration of hydrocarbon vapours in the vapour space, the amount of liquid received, the rate and method of liquid transfer and the volatility of the product being moved. The initial concentration of hydrocarbon vapours in an empty transport compartment may be attributed to the evaporation of residual liquid left over from the previous cargo, and will depend on the physical and chemical characteristics of this material. The concentration of hydrocarbon vapours in a storage tank will depend on the physical and chemical characteristics of the product being stored in the tank, the physical characteristics of the tank and the current liquid level in the tank. In either case, the gaseous effluent will generally comprise both air and hydrocarbon vapours.
The following are some specific features of crude hydrocarbon liquids that may have a significant impact on the emission control options for loading and unloading operations:
· he liquid is often very volatile (i.e., it is not fully weathered). Consequently, there is a potential for the volume of emissions to be much greater than the actual volume of liquid that is transferred. In addition, the concentration of hydrocarbon vapours in the emissions will be higher than might otherwise be expected.
· The product may often be sour which can result in nuisance or hazardous concentrations of H2S occurring in the emissions.
Vapour Exchange Systems
A portion of the vapours displaced during transfer operations may be recovered by interconnecting the vapour spaces of the storage tanks to the vapour space of the storage compartment on the transport vehicle. Then, when liquids are transferred between a truck/rail car and one of the tanks, the displaced vapours are simply exchanged for liquid product. This control method also helps to inhibit evaporation losses since the vapours are usually saturated or nearly saturated with hydrocarbons.
A typical vapour exchange system at an oil battery is usually quite simple. The storage tanks are equipped with a vapour balancing system and a single vapour exchange line which runs from the truck terminal to the closest of the tanks. The interconnecting piping is sloped to provide natural drainage of any condensation that may occur, and is constructed of fibreglass to minimize the weight to be supported. There are two connections to the vapour exchange line at the truck terminal: one for filling operations and one for emptying or unloading operations. A block valve and male Kamloc fitting are provided at each of these connection points plus there is a flexible hose to connect to the top of the truck vapour space. The fill connection is equipped with a check valve to prevent any backwards flow of vapours into the truck and a flame arrestor to help protect the tanks in the event of a fire during transfer operations.
There are two truck loading methods available: top filling with vapour return, and bottom filling with vapour return. The latter approach is most common in the upstream petroleum industry, although both types may be encountered depending the type of product being handled and the type of facility. Bottom loading tends to be less turbulent and therefore results in less vapour growth during a transfer operation.
The use of vapour exchange systems at gasoline retail service stations is reportedly able to reduce emissions from product deliveries to underground storage tanks by 93 to 100 percent (U.S. EPA, 1985). However, these systems will tend to be less effective in the upstream petroleum industry as the transported hydrocarbon liquids (crude oils, condensates and natural gas liquids) are usually not stabilized or fully weathered and thus may be very volatile. A more reasonable control range for upstream petroleum applications is probably 50 to 90 percent.
The following are some technical requirements that should be taken into account in the design of the vapour balancing/recovery lines (Williams etal., 1986):
· The transported vapour/ air mixture can be in the flammable range when transported therefore flame arrestors and detonation relief devices should be installed at the loading facility.
· To promote the movement of the vapour the pressure head of the tanks and transport vehicles could be raised. Care must be exercised to ensure the total relieving capacity of the relief valves is sufficient for the service and the piping is of satisfactory size for efficient flow.
· There could be liquids accumulating in the system. These must be detected and pumped out to a collection vessel to prevent build up.
· The transport vehicles must be compatible with the vapour exchange system. This may require some modifications to the transport vehicle itself.
· Proximity of the loading equipment to the storage tanks can be an important consideration in determining whether a common vapour exchange and balancing is feasible (API, 1993). The piping required to return the vapours to the storage tanks can be very costly.
· Vapour balancing is not possible when product is being transferred to or from a source that does not have a vapour space (for example, pipelines and tanks equipped with floating roofs).
Oxidation
The emissions generated from product transfer operations involving tank vehicles may often contain explosive levels of air. Detonation arrestors coupled with vapour enriching, inerting or diluting systems can be used to prevent such conditions and allow safe treatment or recovery of the vapours. However, special incinerators are available to safely dispose of explosive mixtures without the problem of flashback from the combustion unit into the vent piping and back into the tank or loading area.
Flashbacks are prevented by using a liquid (water or glycol) seal and a special burner head with full inter-lock safety controls. The burner head is designed to act as a flame arrestor or quenching device and provides the first level of protection. The safety features of the burner are achieved by using very small gas orifices or slots for the vapour openings. These openings must be smaller than the required quenching distance; usually a 50 percent safety factor is necessary to account for heating of the burner head from radiation and other operating conditions (Straitz, 1987). The opening must also be of at least the minimum quenching thickness - usually a safety factor of 100 percent is used to prevent the flame from entering when the vapour stream is within the flammable range.
Between transfer operations the incinerator is normally shut down. As a truck moves into loading position and the start-up signal is given (either manually or automatically), the unit undergoes an automatic safety check to ensure proper liquid-seal level for flashback protection. The pilot is then ignited and the loading pumps go into operation. This sequence takes between 10 and 20 seconds. If the vapours contain mostly air, gas-fired burners are activated to ensure complete combustion. Ambient air is added to the combustion chamber as needed to minimize the generation of excessive temperatures and assure sufficient oxygen for the complete combustion of the vapours. If significant temperature excursions occur and the system cannot be brought back into adjustment, the process will be shut down automatically. Automatic shutdown also will be triggered by flame failure or if the fluid level in the liquid seal drops too low.
Although a vapour disposal unit is generally used alone, it can be used in conjunction with a vapour recovery unit. This type of unit is designed to handle peak loads or to increase the overall efficiency of a vapour recovery system.
Small systems are available. These compact systems are fully automatic, with or without optional heat-recovery sub-systems.
Properly designed and maintained incinerators will provide destruction efficiencies of more that 99.8 percent. The resulting emissions will include CO2, CO, and NOx. If the vapours are sour, there will also be some SO2 emissions.
If the vapours contain H2S, the incinerator will have to be designed to achieve proper dispersion of the resulting SO2.
The incinerator must be located a safe distance from the storage tanks. This may preclude its use at small sites.