Method for selective extraction of natural gas liquids from “rich” natural gas

1. A method for selective extraction of natural gas liquids from “rich” natural gas, comprising the steps of:

positioning a storage vessel for liquid natural gas (LNG) at a straddle plant facility that has a series of separators operating at different selected hydrocarbon dew points in order to condense different natural gas liquids (NGLs);

mixing a rich natural gas stream entering a first of the separators with a stream of LNG from the storage vessel by direct mixing to lower a temperature of the rich natural gas stream to a selected hydrocarbon dew point for a selected natural gas liquid in order to condense the selected natural gas liquid in the rich natural gas stream, the entire rich natural gas stream after LNG addition entering the first separator, a remainder of the rich natural gas stream exiting the first separator, after removal of the selected natural gas liquid, passing to a subsequent one of the series of separators; and

mixing the remainder of the rich natural gas stream sequentially entering each of the separators after removal of the selected natural gas liquid with a stream of LNG from the storage vessel by direct mixing to lower a temperature of the remainder of the rich natural gas stream to the selected hydrocarbon dew point in order to condense a next selected natural gas liquid in the remainder of the rich natural gas stream, the entire remainder of the rich natural gas stream after LNG addition entering each separator, the temperature of the rich natural gas stream being controlled by controlling addition of LNG to the rich natural gas stream, separation taking place sequentially with natural gas liquids having progressively lower hydrocarbon dew points.

2. A straddle plant, comprising:

a storage vessel for liquid natural gas (LNG);

a series of separators adapted to operate at different selected hydrocarbon dew points in order to condense different natural gas liquids (NGLs);

a first mixing station for a first of the separators adapted to mix LNG from the storage vessel with a rich natural gas stream entering the first of the separators by direct mixing to lower a temperature of the rich natural gas stream to a selected hydrocarbon dew point for a selected natural gas liquid in order to condense the selected natural gas liquid in the rich natural gas stream, the entire rich natural gas stream after LNG addition entering the first separator, a remainder of the rich natural gas stream exiting the first separator, after removal of the selected natural gas liquid, passing to a subsequent one of the series of separators; and

a mixing station for each subsequent separator adapted to mix LNG from the storage vessel with the remainder of the rich natural gas stream sequentially entering each subsequent separator, the entire remainder of the rich natural gas stream after LNG addition entering each separator, whereby a temperature of the rich natural gas stream is lowered to the selected hydrocarbon dew point in order to condense a next selected natural gas liquid carried in the remainder of the rich natural gas stream, the temperature of the remainder of the rich natural gas stream being controllable by addition of LNG to the remainder of the rich natural gas stream to just achieve the selected hydrocarbon dew point, separation taking place sequentially with natural gas liquids having progressively lower hydrocarbon dew points.

3. The method of claim 1 , involving a further step of passing natural gas through a heat exchanger to effect a heat exchange with relatively colder natural gas prior to entering each separator, thereby reducing the amount of LNG addition required to achieve each hydrocarbon dew point.

4. A method for selective extraction of natural gas liquids from “rich” natural gas, comprising the steps of:

positioning a storage vessel for liquid natural gas (LNG) at a straddle plant facility that has a series of separators, including a heavier hydrocarbon fractions separator, a propane separator, a carbon dioxide separator, and an ethane separator operating at different selected hydrocarbon dew points in order to condense heavier hydrocarbon fractions, propane, carbon dioxide, and ethane, respectively;

mixing a rich natural gas stream entering the heavier hydrocarbon fractions separator with a stream of LNG from the storage vessel by direct mixing to lower a temperature of the rich natural gas stream to a selected hydrocarbon dew point for heavier hydrocarbon fractions in order to condense the heavier hydrocarbon fractions in the rich natural gas stream, the entire rich natural gas stream after LNG addition entering the heavier hydrocarbon fractions separator and a remainder of the rich natural gas stream exiting the heavier hydrocarbon fractions separator, after removal of heavier hydrocarbon fractions, passing to the propane separator;

mixing the remainder of the rich natural gas stream entering the propane separator with a stream of LNG from the storage vessel by direct mixing to lower a temperature of the remainder of the rich natural gas stream to a selected hydrocarbon dew point for propane in order to condense the propane in the remainder of the rich natural gas stream, the entire remainder of the rich natural gas stream after LNG addition entering the propane separator and the remainder of the rich natural gas stream exiting the propane separator, after removal of propane, passing to the carbon dioxide separator;

mixing the remainder of the rich natural gas stream entering the carbon dioxide separator with a stream of LNG from the storage vessel by direct mixing to lower a temperature of the remainder of the rich natural gas stream to a selected hydrocarbon dew point for carbon dioxide in order to condense the carbon dioxide in the remainder of the rich natural gas stream, the entire remainder of the rich natural gas stream after LNG addition entering the carbon dioxide separator and the remainder of the rich natural gas stream exiting the carbon dioxide separator, after removal of carbon dioxide, passing to the ethane separator; and

mixing the remainder of the rich natural gas stream entering the ethane separator with a stream of LNG from the storage vessel by direct mixing to lower a temperature of the remainder of the rich natural gas stream to a selected hydrocarbon dew point for ethane in order to condense the ethane in the remainder of the rich natural gas stream, the entire remainder of the rich natural gas stream after LNG addition entering the ethane separator;

the temperature of the rich natural gas stream entering each separator being controlled by controlling addition of LNG.

5. The method of claim 4 , involving a further step of passing natural gas through a heat exchanger to effect a heat exchange with relatively colder natural gas prior to entering each separator, thereby reducing the amount of LNG addition required to achieve each hydrocarbon dew point.