The high pressure liquid flows into the low pressure separator. Typically the low pressure vessel operates at 30 to 60 psig. Below 20 psig, there will not be enough pressure to push the accumulated liquids into adjacent tanks. Above 60 psig, natural gasoline condensate will generate excessive vapors when it is introduced to a storage tank.
The low pressure separator’s purpose is to separate three phases:
• Brine
• Natural Gasoline Condensate
• Evolved Vapors.
When the high pressure liquid flashes in the low pressure separator, substantial volumes of hydrocarbon vapor are generated. For example, when one barrel of a typical natural gasoline condensate is depressured from 1000 psig to 65 psig, roughly 1.3 moles of 28 molecular weight is vented through the ow pressure separator’s back pressure regulation. A typical composition of this flash gas is:
Carbon Dioxide………. 3%
Methane……………….. 53%
Ethane………………….. 22%
Propane………………… 13%
Butanes…………………. 6%
Pentanes Plus…………. 3%
The condensate is drawn off to control the separators liquid level, while the brine is withdrawn to hold the condensation-brine interface level. It is quite important that the gas supply used to operate the liquid level dump valves not be withdrawn from the low pressure separator itself. The moisture content of gas withdrawn from a 40 psig vessel will be 18 times higher than gas flowing from a 1000 psig high pressure separator. Also, any surge in the liquid level in the low pressure separator will cause a liquid carryover into the gas supply to the liquid dumps. For these reasons, the first step in correcting level control problems in low pressure separators is to connect a source of high pressure gas (dried if possible) to the liquid level dump valves.
If the dump valves are operating properly, but surges of liquid from the high pressure separator cause natural gasoline to blow out of the low pressure separator’s vent, raise the pressure setting on the back pressure controller. This will force liquid out of the low pressure separator at a greater rate.
