Vapor return carries the vapor pushed out of a truck during loading through a closed line back to the storage tank the liquid came from; it does not treat the vapor, it only balances two vapor spaces. Vapor recovery treats the collected vapor in a unit and recovers the solvent in it as liquid product. A vapor combustion unit also treats the collected vapor, but nothing is recovered. Which of these is needed depends on the emission limits the site must meet, the volatility and value of the solvent, whether one product or several are loaded, and whether there is a closed tank for the vapor to return to.
The truck-to-rack vapor connection is covered on the bottom loading page and pressurized LPG on the LPG vapor return page; this page deals with solvents in atmospheric or low-pressure tanks.
Where loading vapor comes from
As a truck compartment fills, the gas inside is pushed out. That gas has three sources: vapor from residue of the previous load, vapor the truck brought back from its last delivery point, and vapor generated by the new product during loading. When a low-volatility product follows a volatile one, the previous load's vapor is still expelled even though the new product itself emits little; the compartment's history therefore affects the vapor load.
The loading method also matters. In splash loading, where the fill pipe is lowered only part way into the compartment, turbulence and liquid-vapor contact are high, vapor generation increases and droplets can be entrained in the expelled gas. With a submerged fill pipe reaching almost to the bottom, or with bottom loading, the outlet stays below the liquid surface for most of the fill and turbulence is greatly reduced.
How vapor return works, and its limits
In vapor return, the compartment's vapor connection is piped to the vapor space of the storage tank being drawn from. The volume freed as liquid leaves the tank is filled by vapor displaced from the truck, so the vapor stays in a closed loop instead of going to atmosphere. The same principle runs in reverse during unloading: as the truck empties, vapor from the storage tank moves into it and the truck carries that vapor to its next loading point.
The limits are clear at solvent sites. The loop only works between identical products or products whose vapors can safely mix; if vapors of different solvents meet in a shared return line, product quality in the tank they return to can suffer. Atmospheric tanks tolerate only a narrow pressure and vacuum range, so pressure drop in the return line, tank pressure-vacuum valve settings and the back pressure allowed at the truck vapor adapter have to be considered together. In nitrogen-blanketed tanks, the returning gas is largely nitrogen plus solvent vapor, and the blanketing system has to be designed to handle it.
How a vapor recovery unit (VRU) works
A vapor recovery unit separates organic components from the collected vapor and returns them to storage as liquid. The main principles are refrigeration and condensation, capture in a liquid (absorption), capture on a solid surface (adsorption) and compression; commercial units often combine them.
Method | Principle | Points to watch with solvents |
Refrigeration and condensation | Vapor is cooled until its components condense and the liquid is separated | Solvent freezing point and moisture in the vapor; icing on cold surfaces |
Absorption | Vapor is contacted with a liquid that dissolves and holds it | Compatibility of the absorbent with the solvent, and recovered product mixing with the absorbent |
Adsorption | Vapor is held on beds, usually activated carbon, which are regenerated by vacuum or another method | Bed heating and carbon interaction with some solvent vapors; suitability confirmed with vendor and process data |
A key question at solvent sites is where the recovered liquid goes. Solvent recovered from a bay that loads a single product can go back to the product tank. When several solvents feed the same unit, the recovered liquid is a mixture that may need to be collected in a separate tank, reprocessed or handled as waste.
Vapor combustion unit (VCU)
A vapor combustion unit burns the collected vapor at high temperature to destroy the organic components; no product is recovered. It is considered where the vapor mixture is highly variable, the recoverable product is of low value, or the vapor is unsuitable for recovery. Because the unit is an ignition source, devices that stop flame propagation in the vapor line, the unit's location relative to hazardous areas and safe isolation of the vapor line are all part of the design.
Efficiency depends on more than the unit
The US EPA emissions estimating guidance states that the control efficiency of recovery units ranges from 90 to over 99 percent, depending on the nature of the vapors and the type of equipment. The same guidance stresses that not all displaced vapor reaches the unit: leaks in the truck and the collection system reduce collection efficiency. US regulation likewise defines the vapor collection system separately from the vapor processing system that recovers or oxidizes the vapor. The result therefore depends as much on truck vapor tightness, verifying the connection on every load and the integrity of the collection line as on the unit itself.
Which approach fits which situation?
Situation | Suitable approach | Reason |
Single product, closed storage tank, vapor mixing not an issue | Vapor return | Vapor goes back to its source; no processing unit needed |
Emission limit cannot be met by return alone, valuable product | Vapor recovery | Solvent is recovered as liquid and returned to storage |
Many different solvents, variable vapor composition | Combustion, or separate handling of the recovered mixture | A recovered mixture cannot go back to product |
Return is possible but residual vapor remains | Return and recovery combined | Return cuts the load and the unit handles what is left |
Which emission limit applies depends on national regulation, the site permit and the product class. The EU gasoline vapor legislation, for example, requires displaced vapor to go through a vapor-tight connection to a recovery unit with an hourly outlet concentration limit; it does not apply directly to solvents but is a useful design reference.
Safety and monitoring on the vapor line
The vapor line carries a potentially ignitable mixture between the rack and the storage tank or processing unit. Design covers devices that stop flame propagation, collection and draining of liquid that condenses at low points, adding the vapor hose connection to the loading permissive chain, and monitoring line pressure and vacuum. Whether loading may continue when the recovery or combustion unit is not running is also defined as part of the permissive logic.
How TLY Enerji contributes
Depending on project scope, TLY Enerji provides engineering support at solvent loading racks: defining pressure and flow measurement on the vapor return line, vapor connection permissives and unit status signals, supplying the related instruments and valves, integrating these signals into the PLC or SCADA system, and site installation, testing and commissioning. Selection of the recovery or combustion unit itself, and the scope of supply boundary, are clarified project by project.
Data needed for a vapor management review
· List of solvents loaded, with vapor pressure or flash point data
· Product arrangement per bay and whether the same truck is loaded with different products
· Storage tank type, pressure-vacuum settings and blanketing status
· Applicable emission limits and site permit conditions
· Loading method and number of arms loading at the same time
Related pages
· LPG vapor return systems: Vapor balancing and line sizing for pressurized liquefied gas.
· Bottom loading systems for fuel terminals: Vapor collection in gasoline loading and why regulation favors bottom loading.
· Solvent truck loading systems: Controlling the truck vapor space during loading.
· Solvent truck unloading systems: Vapor balancing and suction-side issues during unloading.
Questions about vapor return and recovery
Does vapor return eliminate emissions entirely?
No. Return keeps vapor in a closed loop, but the vapor can reappear elsewhere, for example through the tank's pressure valve or at the truck's next loading. Leaks in the truck and lines also lower collection efficiency. At sites with strict emission limits, return is often the first step that cuts the load, and the remaining vapor is handled in a processing unit.
Can different solvents share one vapor return line?
Generally not recommended. Vapors of different solvents mix in a shared line and can condense in the tank they return to, contaminating the product. A shared line may be considered for product groups where vapor mixing causes no quality issue, based on a product compatibility review. For high-purity products, a separate return line or a processing unit instead of return is preferred.
Can recovered solvent be sold again?
That depends on the purity of the recovered liquid. At a bay loading a single product, recovered solvent can go back to the product tank and its quality is confirmed by sample analysis. Where vapors from several products reach the same unit, the liquid is a mixture that may not meet product specification and needs separate collection or reprocessing.
When is a vapor combustion unit the preferred option?
A combustion unit is considered when vapor composition is highly variable, when the quantity or value of recoverable product is low, or when the vapor is unsuitable for recovery methods. No product is recovered, and because the unit is an ignition source, flame propagation protection in the vapor line and proper siting are essential design topics.