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VAPOUR RECOVERY DURING FUEL LOADING

Ben AdamsonPrincipal Engineer

Refrigeration Engineering Pty Ltd, NSW [email protected]

ABSTRACT

Volatile fuels such as gasoline and naphtha, and to a lesser extent jet fuel and diesel, have significantvapour 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 fueltransferred. When fuels are transferred several times in the chain between the refinery production unitand reaching the end users tank, the losses accumulate. These vapour losses are both an economicloss 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 productrecovered can pay for the cost of the vapour recovery system in a short time – often less than twoyears, and sometimes less than one year.

This paper describes vapour recovery units (VRUs) using refrigeration to condense the hydrocarbonvapours, 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 ismeasured at 100°F (37.8°C) and other specific conditions, and is an indication of fuel volatility. Forcommon fuels typical values of RVP are shown in table 1 below.

Fuel RVP, kPa RVP, bar RVP, psiGasoline 40-60 0.4-0.6 6-9Jet fuel 10-20 0.1-0.2 1-3Diesel 15-20 0.15-0.2 2-3Kerosene 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 RVPis also controlled by the refiner, such as higher RVP for winter blends of gasoline, and lower RVP forsummer blends. Table 1 shows that gasoline is by far the most volatile of common fuels. This highvolatility, coupled with its high volume of production and use, means that gasoline emissions are by farthe largest source of fuel-related VOC emissions. This paper is devoted to gasoline unless otherwisenoted, 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. Forexample, based on ideal gas laws, at atmospheric pressure (101 kPa), air saturated with vapour fromthe gasoline shown in Fig.1, at 40°C, will contain 55/101 = 55% by volume hydrocarbons.

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th International Chemical Engineering Congress

Kish Island, Iran2-5 January, 2008

VAPOUR RECOVERY DURING FUEL LOADING Page 2 of 8

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Temperature °C

V a p o u r p r e s s u r e ,

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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 vapourvented from the top of the tank will vary as the tank is filled, from a low concentration when the tank isempty, to near saturated when the tank is near full. The variation in concentration between the start offilling and end of filling of a tank will vary with tank size and shape, whether the tank is clean or haspreviously been used, relative density of the hydrocarbon vapour compared to air (or other vapourused for tank blanketing), loading method (bottom loading, submerged top loading, splash top loadingetc), but a typical curve is shown in Fig. 2 below.

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Fig. 2 Variation of tank outlet vapour concentration with tank levelUpper curve – used tank Lower curve – clean tank

For a tank which has previously been used in the same service and contains residual vapour from theprevious filling, as shown in the upper curve in Fig.2, the average outlet concentration would be in therange 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).

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Fig. 3 Vapour loss at loading terminal

Fig. 4 Vapour return during unloading at retail station

Fig. 5 Vapour loss during vehicle loading

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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 isapproximately 0.5% of fuel transferred, although this will vary significantly between cooler northernIran 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 valuableproduct. There are also health and safety issues around venting a flammable mixture containing manydifferent hydrocarbons, some of which are damaging to health. Venting to a flare reducesenvironmental, 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 ventto the tanker vapour space, as shown in Fig. 4. This is simple, very effective, and low cost whentankers 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 IIemission controls, returning vapour from vehicle tanks to station underground tanks. Stage II controlsare used in parts of USA and Europe, and are being introduced in China starting in 2008, but are stillnot widely used due to technical difficulties of effective sealing at the wide variety of vehicle filling pointarrangements.

Vapour recovery, where the vapour mixture is taken to a vapour recovery unit, is used widely atloading terminals in Europe, North America, Australia and many other countries. When combined withvapour return during unloading at retail stations, more than 50% of total emissions can be readilyrecovered.

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 processesby which they separate the VOC from the air, but they also differ in running costs, maintenance, wasteproducts and other aspects. The first three techniques are well covered in other literature, and will notbe 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.

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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 ambienttemperature to about +3°C. In this first stage, most of the heavy hydrocarbons and the water vapourcarried with the air condense to liquid. This stage recovers 20-60% of total hydrocarbons, dependingon inlet conditions.

In the second stage, the vapour is further cooled to -30°C, and after the second stage, typically about94-98% of inlet hydrocarbons have been condensed. The third stage cools the vapour to around -75°Cto -80°C, and after the third stage the overall recovery is typically over 99%. The final vapourtemperature after the third stage determines the final hydrocarbon concentration, and is set to suit theoutlet emission requirements applicable to the particular project. After the third stage, the cold cleanedair is used to improve refrigeration system efficiency and in this process it is warmed back to +15°Cbefore discharge to atmosphere.

Recovered liquids drain from the unit to a separator, where the small amount of water condensed fromthe air is separated by gravity, and the hydrocarbon liquid is then pumped away. As the vapour andrecovered liquid has only contacted stainless steel and aluminium heat exchanger surfaces, the liquidis clean and uncontaminated, and is usually sent directly back into the loading line or to the bulkstorage supply tank from where it can immediately be sold. It is not necessary to reprocess the liquidbefore resale.

Most of the water vapour which is carried with the air condenses out in the first cooling stage, but asmall amount remains after the first stage, and condenses out forming ice in the second and thirdstages. It is necessary to remove this ice, and once every day the vapour condenser is taken out ofservice and the cold sections warmed to above 0°C to melt off the accumulated ice. The defrostprocess 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

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the other out of service for defrost. The two vapour condensers change over automatically withoutinterrupting 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 m3/hour gasoline vapour recovery

unit operating in summer conditions will condense about 500 L/hour of hydrocarbon liquid and 3 to 6L/hour of water. The water will contain less than 0.1% gasoline, plus traces of MTBE if applicable, andshould be sent to an oily water drain system if available.

In refrigerated VRUs, the hydrocarbons are recovered directly as condensed liquid, which can beeasily 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 absorbertower. Measurement of recovered amount in this case then requires measurement of differencebetween 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 accessto a large tank of absorbent liquid to supply the absorber tower, which may not always be convenientor 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 liquidfrom the condenser is passed through a control valve into the refrigerant evaporator, where therefrigerant vaporises, cooling and condensing the hydrocarbon vapour mixture. The refrigerant vapourthen returns to the compressor.

Refrigerants used are normally hydrocarbons, propylene for the first and second stages, and ethylenefor the third stage. Hydrofluorocarbon (“Freon” type) refrigerants can be used, but hydrocarbons haveadvantages of lower cost, lower power and much lower environmental effects in the event of loss. Forexample, the global warming potential (GWP) for the most common new HFC refrigerant, R-134a, is1300 compared to GWP = 3 for both ethylene and propylene. Hydrocarbons and HFC refrigerantsboth have zero ozone depletion effect.

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NEW TECHNOLOGY

The new technology included in modern refrigerated VRUs has resulted in higher reliability, lowerpower, lower cost, almost zero maintenance and simpler operation compared to earlier designs.

Compressors are generally hermetically sealed. These are a larger, industrial version of thecompressors used in domestic and commercial refrigeration, with conversions to permit use inhazardous areas. As a result, these compressors never require oil changes and have no shaft seals towear or replace. The refrigeration circuit is totally sealed, eliminating possibility of systemcontamination 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 vapourcondensers are in a clean, non-fouling service, as the condensed hydrocarbon liquids are excellentsolvents 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 canbe 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. Withnew technology in digitally-controlled electronic control valves, modern refrigerated VRUs cancontinue to operate down to zero vapour flow, with reduced power at the no-flow condition. Theseelectronic control valves are standard refrigeration-industry valves with modifications for hazardousarea duty.

Energy recovery from the cold cleaned air has been used to reduce power requirements, and energyuse is approximately 0.15 kWh per cubic metre of vapour processed. This is an annual average – itwill be a little higher in summer and a little lower in winter.

ECONOMICS

In small systems with low recovery (for example, less than 150 m3/hr loading rate, operating less than8 hr/day) economic analysis will show a low return on investment, with payback usually exceeding 4

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years. However, in larger systems with long operating hours (for example, over 500 m3/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 800 m3/hr, operating 24 hrs/day

Daily loading 9 000 000 litresDaily liquid recovery 10 700 L or 7860 kg (at 99% recovery) Annual value of recovered liquid US$1 650 000 (at US$600/tonne, 350 days/year) Annual operating cost(power, maintenance) US$70 000Installed cost US$1 600 000Payback period 12.1 months

CONCLUSION

Emission of large quantities of gasoline vapour is hazardous to health of terminal operators, as well asbeing a safety hazard due to its flammability. It is also a significant environmental pollutant, and hassubstantial value. Refrigerated vapour recovery is an established technology which can provideattractive 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 (APIpublication 1673).

Compilation of Air Pollutant Emission Factors, Volume 1: Stationary Point and Area Sources, Chapter5, Petroleum Industry (US EPA publication AP-42)

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