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Gas Ejectors for Vapour Recovery from Crude Oil Tanks

Oil Production sites and Refineries use large tanks for storing Crude Oil. Crude Oil is a mixture of various hydrocarbons. When stored in a tank, the lighter fractions in the crude mostly Methane and Ethane flash off from the crude oil. These Methane & Ethane in vapour form occupy the space between the floating roof and the Tank liquid level.
The vapours are flashed continuously and keep on the collecting over the liquid level. This leads to increase in pressure inside the tank. This pressure increase is hazardous as increases gas leakage through the floating roof and also raises fire concerns.
To avoid any mishaps the tanks need to be periodically vented to keep the pressures within limits. During the venting process, some amount of vapour are released from the tank into the flare gas header. The vapour may then be flared or recovered from the flare system.
The storage tanks are very large in size and contain huge crude oil quantities. Naturally the amount of vapour vented throughout the day is considerable. These vapours are actually a fuel rich gas and has significant value if recovered.

But recovery of vapours is not an easy task and fraught with below challenges
• Availability at low Pressure
Generated vapours are typically at atmospheric pressures and need to be compressed in order to reuse them in the plant’s fuel gas network
• Sour Service
Crude Oil tends to have high concentrations of H2S. H2S possess a risk to the plant infrastructure and hence needs to be separated or neutralized during recovery

Gas Ejectors operate on the basis of jet Venturi principle and utilize energy in compressed fuel gas for their operation. During operation the gas Ejector displays a special ability to create vacuum at its inlet and at the same time creating a compressive effect between its inlet and outlet.
This enables the gas Ejector to utilize the vacuum to suck vapours from the storage tanks and then compress it to the required pressure. The compressed vapour can be reinjected into the fuel gas grid after appropriate treatment.

The system is similar to an Ejector based Flare Recovery but has following advantages:
• Each tank or set of tanks can be provided with its independent recovery system, thus reducing the load on the flare recovery system.
• The vapour generated in crude storage tanks are very rich in Methane & Ethane and if recovered independently, mixing with other gases in the flare headers can be avoided.

Gas Ejectors are suited for Vapour Recovery from Crude Oil tanks due to their inherent characteristics
• The vapour generation in each Crude Storage facility varies drastically. Gas Ejectors can be easily customized and hence are designed as per each facility.
• Can be manufactured in materials compatible with the sour service requirements.
• Gas ejectors do not contain any moving parts and have large internal passage areas, hence are not vulnerable to liquid entry or clogging by solid particles.
• As they have no moving parts, maintenance requirements are very less. Also it reduces largely the risk of abrupt breakdowns, helps in maintaining the regular production schedule
• Fuel gas is available at high pressures in all Oil & Gas Facility and hence no additional infrastructure is typically required.