Gas sweetening is a purification process employed to remove acidic contaminants from sour natural gas streams prior to sale. It includes removal of hydrogen sulphide (H2S) and carbon dioxide (CO2) when these components are above the levels permitted by industry regulations, before it can be passed to the pipeline.
Historically, oil and gas operators conducted gas sweetening (or acid gas removal) using amine systems. However, membrane systems have gained notable traction due to their ease of operation, flexibility, smaller footprint and lower capital requirements – especially in high CO2 or trimming applications where amine technology is not applicable. CO2 selective cellulose acetate (CA) membranes have been the technology of choice since the mid-1980s due to their high performance characteristics in applications where there are large flows, high H2S and CO2 concentrations, or where facilities are situated in remote locations.
Moreover, membrane systems are typically skid-mounted, meaning the cost, scope and time required for site preparation is minimal. The only major operating cost is replacement of the membrane modules. Membranes also avoid the need for managing solvent storage and handling, not to mention disposal of spent amine solution, and therefore offer installation and operating costs that are some 30 percent lower than amine systems. Amine gas treatment and the regeneration of its solvents also demand high energy consumption, which adds to the operational cost and environmental footprint.
The issue with conventional membrane technology however, is that CA and other material cannot maintain their performance over time due to natural aging of the membrane material. CA is also irreversibly damaged when it is exposed to water, water vapor or liquid hydrocarbons (HC), with exposure often occurring as a result of the insufficient pre-treatment processes, or unexpected changes in the composition of the gas or the reservoir.
For operators, the degradation in performance of CA over time presents considerable risk to their operational costs (OPEX). In addition, CA modules must typically be replaced at a rate of 20-30 percent annually. Nevertheless, new membrane materials developed at laboratory scale and commercially tested offer significant benefits over currently available commercial membranes and promise major improvements in industrial separation process applications.
Cellulose acetate technology limitations
Despite being a relatively simple process, gas permeation through thin film membrane technology has grown rapidly in popularity since it was first introduced in 1979. The range of applications covers the supply of pure or enriched gases including: Helium (He), Nitrogen and CO2 from air; the separation of acidic gases such as CO2 and H2S; and the separation of Hydrogen in the petrochemical and chemical industries.
Membrane separation is based on selective gas permeation where the separation of a mixture of gases is performed with the objective of obtaining one component in pure form. Membranes are typically made from spiral wound CA, or polymers and copolymers in the form of flat film or hollow fiber tubes. The membrane characteristics together with the operating conditions (e.g. gas composition, temperature, pressures, flow characteristics) determine the specific rate of permeation for each gas component.
Under standardised conditions for comparison, CA membranes have a carbon dioxide-methane selectivity of 6-12. This is significantly below the selectivity calculated from pure gas measurements, and highlights the effect of membrane plasticisation. In CA, acetyl (CH3CO) groups are attached to long molecular chains of cellulose, and exposure to moisture, heat, or acids causes the groups’ bonds to break, releasing acetic acid. This structural breakdown causes the acetate to become brittle with decay and potentially shrink.
Fujifilm recognised and understood the intrinsic limitation of the CA material for gas treatment applications. An innovative proprietary technology was therefore developed and commercialised in partnership with ProSep offering a highly-functional and cost-effective gas sweetening membrane element.
Basics of the membrane process.
The Fujifilm Apura Membrane (Apura) is a multilayer composite with high CO2/HC selectivity that makes modules extremely robust towards and resistant to aromatics and water. It is produced as a flat sheet and formed by packaging several of these flat sheets into a spiral-wound module that is then inserted into steel pressure-vessel tubes.
A spiral-wound module consists of membrane envelopes that are constructed of one sheet of membrane folded over and glued back to back along the edges with the membrane dense film surface facing outward. Spacers are used to keep the membrane sheets separated ensuring the availability of channels for both the feed and permeate streams to flow. The open end of each membrane envelope is attached to a stainless steel tube that has been drilled with holes to allow permeate gas to exit.
Under the influence of differential pressure across the membrane, Apura uses spiral wound membrane modules to remove acidic gases such as CO2 and H2S from the natural gas feed stream. This results in an enhanced caloric product stream (known as residue) that is low in acidic gases and a waste stream that is lean in hydrocarbons and rich in CO2 and H2S. The result is a lower capital expenditure (CAPEX) and OPEX for end users over alternative process solutions.
The Apura technology platform has been in development for almost a decade and has seen tens of millions of dollars invested in research and development (R&D). Fujifilm, which itself has transformed its R&D and manufacturing operations, partnered with ProSep for commercialisation of Apura due to ProSep’s proven track record as a provider of integrated process separation solutions, to global operators both offshore and onshore.
ProSep personnel have designed, engineered, fabricated, installed, and commissioned more than 200 membrane separation units for natural gas applications in 15 countries worldwide. ProSep’s installed membrane separation units are treating over 1 billion standard cubic feet per day (SCFD) globally. The company has extensive experience with gas separation membrane performance characterisation and the potential contamination effects that may occur. Moreover, it is the only provider with experience in using both hollow fibre and spiral wound membranes.
Written by John Sabey, ProSep, and Onno Gerrits, Fujifilm. Edited from report by Joseph Green
Read the article online at: https://www.hydrocarbonengineering.com/special-reports/30042015/membrane-technology-hydrocarbon-opex-081/