VAPOUR RECOVERY DURING FUEL LOADING Ben Adamson Principal Engineer Refrigeration Engineering Pty Ltd, NSW Australia ben_adamson@refeng.com.au ABSTRACT Volatile fuels such as gasoline and naphtha, and to a lesser extent jet fuel and diesel, have significant vapour pressure and the amount of vapour lost during fuel handling and transfer can be significant. For example, in transferring gasoline into a road tanker, the loss often exceeds 0.1% of the fuel transferred. When fuels are transferred several times in the chain between the refinery production unit and reaching the end users tank, the losses accumulate. These vapour losses are both an economic loss to the owner of the fuel, and an undesirable environmental emission. Capture of the vapours is technically simple, and once captured, the vapour can either be vented, incinerated or recovered. Recovery is the preferred option, and in many cases the value of the product recovered can pay for the cost of the vapour recovery system in a short time often less than two years, and sometimes less than one year. This paper describes vapour recovery units (VRUs) using refrigeration to condense the hydrocarbon vapours, and gives examples of detailed economic payback analysis from actual projects. FUELS AND THEIR VAPOUR The vapour pressure of fuels is commonly expressed as the Reid Vapour Pressure (RVP), which is measured at 100 F (37.8 C) and other specific conditions, and is an indication of fuel volatility. For common fuels typical values of RVP are shown in table 1 below. Fuel RVP, kpa RVP, bar RVP, psi Gasoline 40-60 0.4-0.6 6-9 Jet fuel 10-20 0.1-0.2 1-3 Diesel 15-20 0.15-0.2 2-3 Kerosene 1-4 0.01-0.04 0.1-0.5 Table 1 RVP of common fuels Some variation in RVP is a natural outcome of variations in refinery feedstock and processes, but RVP is also controlled by the refiner, such as higher RVP for winter blends of gasoline, and lower RVP for summer blends. Table 1 shows that gasoline is by far the most volatile of common fuels. This high volatility, coupled with its high volume of production and use, means that gasoline emissions are by far the largest source of fuel-related VOC emissions. This paper is devoted to gasoline unless otherwise noted, although the same technology can recover many different hydrocarbon and inorganic vapours. Actual vapour pressure varies from RVP, mainly depending on temperature, as shown in Fig. 1. For example, based on ideal gas laws, at atmospheric pressure (101 kpa), air saturated with vapour from the gasoline shown in Fig.1, at 40 C, will contain 55/101 = 55% by volume hydrocarbons.
VAPOUR RECOVERY DURING FUEL LOADING Page 2 of 8 Vapour pressure, kpa 90 80 70 60 50 40 30 20 10 0 0 10 20 30 40 50 60 Temperature C Fig.1 Variation of vapour pressure with temperature for typical gasoline (RVP 50 kpa) VAPOUR CONCENTRATION IN TANK VENTS Vapour immediately above the liquid surface in a tank will be saturated, but at higher levels in the tank, the air/vapour mixture will be somewhat less than saturation. Hence the concentration of vapour vented from the top of the tank will vary as the tank is filled, from a low concentration when the tank is empty, to near saturated when the tank is near full. The variation in concentration between the start of filling and end of filling of a tank will vary with tank size and shape, whether the tank is clean or has previously been used, relative density of the hydrocarbon vapour compared to air (or other vapour used for tank blanketing), loading method (bottom loading, submerged top loading, splash top loading etc), but a typical curve is shown in Fig. 2 below. Saturation, % 100 90 80 70 60 50 40 30 20 10 0 0 20 40 60 80 100 Tank level, % Fig. 2 Variation of tank outlet vapour concentration with tank level Upper curve used tank Lower curve clean tank For a tank which has previously been used in the same service and contains residual vapour from the previous filling, as shown in the upper curve in Fig.2, the average outlet concentration would be in the range 50-60% of saturation, or in the case of gasoline, around 25-35% by volume. OVERALL VAPOUR LOSSES There are three main opportunities for vapour loss between the refinery and the vehicle tank; At the loading terminal, when road tankers are filled from terminal bulk tanks (Fig. 3) At the retail station, when the road tanker unloads into station underground tanks (Fig. 4) At the retail station, when the vehicle tank is filled from the station tank (Fig.5).
VAPOUR RECOVERY DURING FUEL LOADING Page 3 of 8 Fig. 3 Vapour loss at loading terminal Fig. 4 Vapour return during unloading at retail station Fig. 5 Vapour loss during vehicle loading
VAPOUR RECOVERY DURING FUEL LOADING Page 4 of 8 In each case, an air/gasoline vapour mixture is displaced from a tank as the tank is filled. The total of these three losses will vary with temperature, but an average figure for climates such as Iran is approximately 0.5% of fuel transferred, although this will vary significantly between cooler northern Iran and the warmer south. This total loss is divided approximately as Tanker filling at terminal 0.15% Tanker unloading at retail station 0.15% Vehicle filling at retail station 0.20% Total 0.50% VAPOUR EMISSION CONTROL There are three ways to handle the vapour displaced at each stage of the process above, Vent vapour to atmosphere or flare Return vapour to the source tank Recover vapour as liquid Venting directly to atmosphere is obviously undesirable environmentally, and wastes a valuable product. There are also health and safety issues around venting a flammable mixture containing many different hydrocarbons, some of which are damaging to health. Venting to a flare reduces environmental, health and safety effects, but is still a loss of product. Vapour return is commonly used during tanker unloading at retail stations, by connecting the tank vent to the tanker vapour space, as shown in Fig. 4. This is simple, very effective, and low cost when tankers and stations are equipped with suitable vapour connections. It is used widely in Europe, North America, Australia and many other countries. Vapour return is also used to a lesser extent in Stage II emission controls, returning vapour from vehicle tanks to station underground tanks. Stage II controls are used in parts of USA and Europe, and are being introduced in China starting in 2008, but are still not widely used due to technical difficulties of effective sealing at the wide variety of vehicle filling point arrangements. Vapour recovery, where the vapour mixture is taken to a vapour recovery unit, is used widely at loading terminals in Europe, North America, Australia and many other countries. When combined with vapour return during unloading at retail stations, more than 50% of total emissions can be readily recovered. VAPOUR RECOVERY TECHNIQUES There are four main types of vapour recovery unit in commercial use today; Carbon adsorption Lean oil absorption Membrane separation Condensation by refrigeration The most fundamental difference between the various vapour recovery units is the different processes by which they separate the VOC from the air, but they also differ in running costs, maintenance, waste products and other aspects. The first three techniques are well covered in other literature, and will not be described here. Condensation by refrigeration is an old technique that is making a resurgence using new technologies which eliminate the disadvantages of the earlier refrigerated systems.
VAPOUR RECOVERY DURING FUEL LOADING Page 5 of 8 VAPOUR RECOVERY PROCESS A brief schematic of the refrigerated vapour recovery process is shown in Fig. 6. Fig. 6 Process schematic of refrigerated vapour condensation process The mixture a hydrocarbons and air is cooled in several steps. First, it is cooled from ambient temperature to about +3 C. In this first stage, most of the heavy hydrocarbons and the water vapour carried with the air condense to liquid. This stage recovers 20-60% of total hydrocarbons, depending on inlet conditions. In the second stage, the vapour is further cooled to -30 C, and after the second stage, typically about 94-98% of inlet hydrocarbons have been condensed. The third stage cools the vapour to around -75 C to -80 C, and after the third stage the overall recovery is typically over 99%. The final vapour temperature after the third stage determines the final hydrocarbon concentration, and is set to suit the outlet emission requirements applicable to the particular project. After the third stage, the cold cleaned air is used to improve refrigeration system efficiency and in this process it is warmed back to +15 C before discharge to atmosphere. Recovered liquids drain from the unit to a separator, where the small amount of water condensed from the air is separated by gravity, and the hydrocarbon liquid is then pumped away. As the vapour and recovered liquid has only contacted stainless steel and aluminium heat exchanger surfaces, the liquid is clean and uncontaminated, and is usually sent directly back into the loading line or to the bulk storage supply tank from where it can immediately be sold. It is not necessary to reprocess the liquid before resale. Most of the water vapour which is carried with the air condenses out in the first cooling stage, but a small amount remains after the first stage, and condenses out forming ice in the second and third stages. It is necessary to remove this ice, and once every day the vapour condenser is taken out of service and the cold sections warmed to above 0 C to melt off the accumulated ice. The defrost process takes only about 3 hours, so the unit can be on line for up to 21 hours per day, and defrosting for 3 hours. If it is essential to operate 24 hours per day, two vapour condenser sections are provided, served by a single common refrigeration system. One vapour condenser is on line for recovery, while
VAPOUR RECOVERY DURING FUEL LOADING Page 6 of 8 the other out of service for defrost. The two vapour condensers change over automatically without interrupting vapour flow. A typical dual-coil VRU for 24-hour service is shown in Fig. 7. Fig. 7 Dual-coil refrigerated vapour recovery unit The amount of water condensed from the air is very small - a 400 m 3 /hour gasoline vapour recovery unit operating in summer conditions will condense about 500 L/hour of hydrocarbon liquid and 3 to 6 L/hour of water. The water will contain less than 0.1% gasoline, plus traces of MTBE if applicable, and should be sent to an oily water drain system if available. In refrigerated VRUs, the hydrocarbons are recovered directly as condensed liquid, which can be easily measured to prove recovery amount. Other VRU technologies (carbon, membrane and lean oil) all recover the hydrocarbons as a vapour which is then absorbed into another liquid in an absorber tower. Measurement of recovered amount in this case then requires measurement of difference between the liquid flow into and out of the absorber as these flows may differ by only 1% or less, accurate measurement of the difference can be difficult. These other technologies also require access to a large tank of absorbent liquid to supply the absorber tower, which may not always be convenient or practical. REFRIGERATION PROCESS The three refrigeration cycles are very simple. In each stage, refrigerant vapour from the hydrocarbon vapour condenser is compressed, and then condensed to liquid in a condenser. High-pressure liquid from the condenser is passed through a control valve into the refrigerant evaporator, where the refrigerant vaporises, cooling and condensing the hydrocarbon vapour mixture. The refrigerant vapour then returns to the compressor. Refrigerants used are normally hydrocarbons, propylene for the first and second stages, and ethylene for the third stage. Hydrofluorocarbon ( Freon type) refrigerants can be used, but hydrocarbons have advantages of lower cost, lower power and much lower environmental effects in the event of loss. For example, the global warming potential (GWP) for the most common new HFC refrigerant, R-134a, is 1300 compared to GWP = 3 for both ethylene and propylene. Hydrocarbons and HFC refrigerants both have zero ozone depletion effect.
VAPOUR RECOVERY DURING FUEL LOADING Page 7 of 8 NEW TECHNOLOGY The new technology included in modern refrigerated VRUs has resulted in higher reliability, lower power, lower cost, almost zero maintenance and simpler operation compared to earlier designs. Compressors are generally hermetically sealed. These are a larger, industrial version of the compressors used in domestic and commercial refrigeration, with conversions to permit use in hazardous areas. As a result, these compressors never require oil changes and have no shaft seals to wear or replace. The refrigeration circuit is totally sealed, eliminating possibility of system contamination and resulting in little or no maintenance and a compressor life usually over 20 years. Typical hermetic compressors are shown in Fig. 8. Fig. 8 Hermetic compressors with Ex d terminal enclosures Refrigerant condensers are usually air-cooled, though water-cooled are available if required. Maintenance on these is only occasional cleaning of exchanger fins. The hydrocarbon vapour condensers are in a clean, non-fouling service, as the condensed hydrocarbon liquids are excellent solvents which constantly wash the exchanger surfaces to maintain them in a clean condition. Earlier refrigerated VRUs were not able to operate below about 30% of design vapour flow. This can be a problem in loading terminals where vapour flow can vary rapidly during the day, depending on the number of loading arms in operation at any time, and there may be periods of no flow at times. With new technology in digitally-controlled electronic control valves, modern refrigerated VRUs can continue to operate down to zero vapour flow, with reduced power at the no-flow condition. These electronic control valves are standard refrigeration-industry valves with modifications for hazardous area duty. Energy recovery from the cold cleaned air has been used to reduce power requirements, and energy use is approximately 0.15 kwh per cubic metre of vapour processed. This is an annual average it will be a little higher in summer and a little lower in winter. ECONOMICS In small systems with low recovery (for example, less than 150 m 3 /hr loading rate, operating less than 8 hr/day) economic analysis will show a low return on investment, with payback usually exceeding 4
VAPOUR RECOVERY DURING FUEL LOADING Page 8 of 8 years. However, in larger systems with long operating hours (for example, over 500 m 3 /hr loading rate, operating over 18 hr/day) return on investment is high and payback period may be extremely attractive. A recent study at a Middle East road tanker loading terminal showed the following figures: Maximum loading rate Daily loading Daily liquid recovery Annual value of recovered liquid Annual operating cost (power, maintenance) US$70 000 Installed cost US$1 600 000 Payback period 12.1 months 800 m 3 /hr, operating 24 hrs/day 9 000 000 litres 10 700 L or 7860 kg (at 99% recovery) US$1 650 000 (at US$600/tonne, 350 days/year) CONCLUSION Emission of large quantities of gasoline vapour is hazardous to health of terminal operators, as well as being a safety hazard due to its flammability. It is also a significant environmental pollutant, and has substantial value. Refrigerated vapour recovery is an established technology which can provide attractive economic returns as well as solving a health, safety and environmental problem. REFERENCES Compilation of Air Emission Factors for Petroleum Distribution and Retail Marketing Facilities (API publication 1673). Compilation of Air Pollutant Emission Factors, Volume 1: Stationary Point and Area Sources, Chapter 5, Petroleum Industry (US EPA publication AP-42)