Abstract

The paper deals with the introduction of membrane based separation processes to treat natural gas according to pipeline specifications. Polymeric membranes have been tested in the frame work of a project sponsored by the European Union (Brite EuRam III, Contract No. BRPR-CT 98–0804). The focus of the work was the dehydration of natural gas streams by means of a glassy membrane and hydrocarbon dewpointing by means of an elastomeric membrane. Engineering aspects and experimental results will be discussed as well as the potential for technical applications.

Introduction

Polymer membranes are used commercially to separate air, to remove hydrogen from mixtures with nitrogen, carbon monoxide or hydrocarbons, to separate carbon dioxide from natural gas, hydrocarbon vapors from air or process gas streams and to control the water vapor dewpoint of compressed air. Gas membrane separations compete with cryogenics and a variety of adsorption and absorption processes (activated carbon adsorption, pressure swing adsorption, amine treatment etc.).

Membrane permeation is a pressure driven process. The partial pressure difference between the feed side and the permeate side has by far the greatest impact on the performance of a membrane separator. This pressure difference directly influences the membrane area required to achieve the desired separation at given feed conditions. Another important process criteria is the ratio of feed pressure over permeate pressure, which has to be established in accordance to the membrane selectivity in order to achieve an efficient separation. In Figure 1 the economic trade offs in membrane process optimization are depicted. It is a direct dependence of product purity: product losses, investment costs and operating costs.

  • The retentate (product) purity can be increased by the installation of more membrane area or the increase of the pressure ratio (feed pressure over permeate pressure).

  • More membrane area or higher pressure increases the stage cut (permeate flow volume over feed flow volume).

  • The feed flow rate impacts the retentate purity. Higher feed flow rates reduce the retentate purity but increases the permeate concentration of the faster permeating compound.

  • The capacity of vacuum pumps or feed compressors are influenced by the recycled permeate or applied permeate pressure.

The membrane selectivity determines the achievable product purity. The economic efficiency will decrease if the required product purity will approach 100%. Often hybrid systems are more efficient to produce clean product streams. Membrane separation can be combined for recovery and second purification with condensation, absorption and adsorption.

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