Fractionsation Unit: Purpose Of The Fractionation Process

Fractionation Unit

Purpose of the Unit

The Fractionation Unit supplies ethane and propane, at the required purity, as make up to the refrigeration cycles. In addition, methane is sent to the HP fuel gas system and pentane to the LP fuel gas system. NGLs are re-injected into the LNG product and hydrocarbon condensate is produced to the required vapour pressure and directed to storage.

Process Introduction

The Fractionation Unit consists of four fractionation columns in series:

• a De-methaniser

• a De-ethaniser

• a De-propaniser

• a De-butaniser

The unit is closely integrated with the scrub section in order to achieve optimum cold economy and a minimum processing/recycle of 'excess' components. Any excess light components removed

The two-phase feed at -20C and 30 bara, which contains considerable levels of methane and nitrogen, is routed to the top of the column. The falling liquid is then stripped of methane and nitrogen by rising vapour produced, by exchange with Heat transfer fluid (HTF), in a De-methaniser Reboiler. The methane and nitrogen have to be removed to allow proper recovery of ethane in the downstream De-ethaniser column.

The De-methaniser column overheads are mixed with Natural Gas Liquids (NGLs) from the other three fractionation columns in an NGL collection header, cooled against low pressure propane in a heat exchanger from 15C to -29C, and separated in a Natural Gas Liquid Re-injection

Vapour from the top of the De-ethaniser column is condensed against medium pressure propane in an overheads condenser before passing into a reflux drum. From the reflux drum ethane refrigerant make up, in vapour form, is supplied to the Liquefaction Unit and ethane liquid is pumped back to the column as reflux or to the Refrigerant Storage Unit for refrigerant make up purposes as required. The bottoms from the De-ethaniser Column, at 128C and 27 bara, are fed to the De-propaniser Column through a level control valve. Flashing across the valve reduces the pressure to 16 bara at