Vapor Recovery

VOC and HAP emissions to the atmosphere cause pollution of the air we breathe. Governmental agencies have mandated the control of these emissions. These emissions can be controlled by destruction (incineration) or by recovery.
 
PETROGAS offers systems for the recovery of emissions using absorption, refrigeration or adsorption.
 
How To Choose The Correct VAPOR RECOVERY SYSTEM?
 

APPLICATIONS

Which type Vapor Control System is most effective?

No single technology offers the best solution for ALL vapor control applications.
 
PETROGAS designs and manufacturers Vapor Recovery Systems using ALL technologies and is, therefore, impartial to one type of technology. Petrogas looks at ALL of the inputs to fit the correct technology to the application. Other companies have one type of systems to apply for all applications.
 
Petrogas was the pioneer in vapor recovery. We are the oldest company in the field and the only one to offer all types of technologies.
 
Vapor Recovery technologies used is usually one of the following:

  • - Condensation
  • - Adsorption with Carbon Beds or Molecular Sieve
  • - Absorption

In order to choose the correct type of Vapor Recovery System you must look at the composition of the vapors, the concentration of the vapors, the flow rate of the vapors and the ambient conditions of where the unit will be located.
 

1. Condensation

Condensation of vapors is accomplished by lowering the temperature of the vapors.
 
The temperature may be lowered using one or a combination of the following:
 
A. Direct Expansion of a gas
 
Advantages
a. Low initial cost
b. Low operating cost if cost of nitrogen is not included
c. Simple operation
 
Disadvantages
a. High cost of nitrogen if not used for another purpose.
b. High maintenance.
 
Applications
 
If there is a use for the vaporized nitrogen after being used for condensation of the VOCs, this may offer an economic alternative.
 
B. Mechanical Refrigeration
 
Mechanical refrigeration, using a cascade refrigeration system is an expensive but operationally feasible alternative. Each stage is dependent upon the proper operation of the prior stage. It is very difficult to maintain a system operational at the specified temperature.
 
Disadvantages
a. High maintenance costs
b. Not effective with low VOC concentrations
c. The system is adversely affected by high humidity. Dehumidification of
the vapor stream is required or the system will require a defrost cycle. d. Secondary wastewater treatment required
e. Requires cryogenic temperatures -120 F to -200 F
f. Difficult to operate
g. Efficiencies are affected by flow rate and VOC concentration
h. High ambient temperatures cause serious problems.
i. The system requires a startup period of about 45 minutes and does not lend itself to intermittent operation.
 
Application
 
Best suited for the recovery of chemical vapors and organic vapors having condensing temperatures higher than -60 F.
 

2. Adsorption using Carbon Beds or Molecular Sieve

Adsorption relies on the capillary action of the many micro-pore spaces, or little tunnels, to capture the VOC molecules. Once these pores are full the VOCs are no longer captured and the carbon or molecular sieve must be regenerated. The higher the concentrations of VOCs the faster the pores become full. Gasoline type vapors emitted during loading are very high concentrations filling the pore spaces very quickly, requiring very large carbon beds or molecular sieve and rapid regeneration. Each time the beds are regenerated, some of the VOCs remain in the pore spaces and the capacity of the carbon or molecular sieve to capture the VOCs is reduced. The carbon beds must therefore be replaced very often at a very high cost. A small system $50,000, a large system $800,000.
 
Advantages
a. High recovery rate
b. Dilute mixtures of VOC's
c. Low concentrations of VOC's
 
Disadvantages
a. Dehumidification of the vapor stream is required, efficiency decreases as relative humidity increases.
b. Potential bed fires, spontaneous combustion, can occur at bed temperatures above 350 F due to catalytic oxidation generating exothermic heat.
c. Efficiency decreases with increased temperature
d. High corrosion potential, in presence of heat, moisture and recovered product HC1, chloroalcohols and other corrosives form.
e. Ethane, C2, fouls carbon bed very quickly.
f. Secondary wastewater treatment may be required.
g. Costly replacement of carbon is required, and it loses its adsorptive capacity each time it is regenerated.
h. Many components may "KILL" (destroy, foul, contaminate) the carbon bed.
i. "KILLED" carbon is a hazardous waste requiring special disposal.
j. Recovery efficiency drops when used with the lighter gasoline components.
k. Most costly system.
 
Application
 
Adsorption is most commonly applied to dilute mixtures of VOCs and air (i.e. paint booths, solvent cleaning) having a maximum practical inlet concentration of 10,000 ppmv (<1%).
 

3. Absorption

VOC recovery in a Refrigerated Lean Oil Absorption system consists of the vapor/air mixture entering the bottom of a packed tower, counter-flowing upward, impinging on absorbent wetted packing. A chilled absorbent fluid enters the top of the tower and begins a downward flow wetting the packing. The air exits the top of the tower stripped of the hydrocarbon vapors. The vapors, captured by the absorbent fluid, exit the bottom of the tower.
 
ADVANTAGES
a. Low pressure drop through system
b. Insensitive to varying flow rates
c. Insensitive to dirty vapor streams
d. Insensitive to varying vapor concentrations
e. Not affected by high humidity
f. Not significantly affected by high/low temperature
g. Simple operation
h. Low initial capital cost
i. Lowest operating costs
j. Revenues from the recovered VOCs yield an operating profit, not just an expense.
k. Absorbent medium does not need to be replaced nor is it consumed
j. No compression or blowers needed.
 
DISADVANTAGES
Less cost effective with low vapor concentrations
(I.e. less than 300 PPM)
Can require high amount of electricity for regeneration if fuel reboiler is not used
 
APPLICATION
Recovery of VOCs from low pressure, variable throughput and variable concentration, high humidity, high temperature streams.
 
Best choice for the recovery of hydrocarbons.
 
The BEST AVAILABLE CONTROL TECHNOLOGY for the recovery of hydrocarbon vapors at storage and loading terminals is the Refrigerated Lean Oil Absorption system.
 
Reasons:
 
1. High recovery efficiency - 99%
2. Low levels of emission - 0.003 # / 1000 gal. displaced.
3. Reliability - two years with a mean time between repairs of three months and ZERO truck rack down time due to vapor recovery system malfunction.
4. Vapors recovered as liquids have yielded 0.1- 1.0% of the volume of gasoline loaded, depending on many variables (i.e. temperature, vapor pressure, etc.). The recovered liquids, a high-octane gasoline, are returned to premium gasoline storage, currently valued at $2.50/gal. The system consumes only $1.45/day in electricity per 1,000 cu. ft. of throughput capacity and has no other operating costs.
5. Produces the most recovered liquids than any system
6. No hydrocarbons are trapped in the system.
7. The PETROGAS system is fully automatic and operates safely unattended.
8. The system has no waste products.