Permeation: A New Competitive Process for Offshore Gas Dehydration

ABSTRACT Usual process for gas dehydration implement absorption (glycol chemical process) or adsorption (molecular sieves). The new dehydration process as described in this paper will be a strong competitor with these usual processes, especially for offshore applications, due to its simplicity and space and weight saving. This new membrane based process is discussed along major constraints of the application, suitable characteristics of the adapted permeation membranes and gain versus conventional gylcol in offshore application. DEHYDRATION OF NATURAL GAS The dehydration of natural gas or associated gas in a traditional operation on production fields, as it allows:to transform these gases so they may be transported (protection against corrosion and hydrates formation),to make these gases meet certain specifications (for gas-lift, liquefaction). The usual methods implement absorption (glycol chemical process) or adsorption (molecular sieves, silica gels); the latter being mainly used to obtain extreme purities (1 ppm for liquefaction for examp1e). Although they have been well experienced, the different processes present many disadvantages and particularly within offshore operations. Thus, the process most currently used, with triethylene glycol (TEG), requires a furnace (safety problem), is expensive, heavy, cumbersome (high platform induced costs) and sometimes difficult to operate (problems of foaming, operators permanently on the site). This usual dehydration processes have been the subject of many studies developed by different oil companies, industrial manufacturers and skilled engineers. They seem to have reached ceiling. For further improvement, the bas i s of the process, itself, must be reviewed and a mutation process must be implemented. Today gas permeation seems very attractive and promising. GAS PERMEATION - PRESENT SYSTEMS Gas separation by selective permeation has progressed for more than five years with the first MONSANTO units utilized to separate hydrogen from gas, such as CO, N2 and hydrocarbons, in refineries and ammonia plants. The application of permeation techniques for the treatment of gas on a production field begins today with different systems proposed by U.S. companies. These systems are aimed to be used within the scope of the many U. S. enhanced oil recovery projects, using miscible CO2 (TEXAS, ROCKY MOUNTAINS, GULF COAST). They will allow to separate and enrich the C02 produced with associated gases, so it can finally be injected in the reservoir. Here are the main characteristics of these permeation processes:physical and passive method: partial pressure is the driving force,reduced weight and dimensions,the process may be automated and telecontrolled,modular and consequently, evolutive process. DEHYDRATION OF HYDROCARBON GASES BY GAS PERMEATION Taking into account the advantages observed with the existing permeation systems, and also the fact that the intrinsic characteristics of many polymer membranes present a very high se1ectivity between water and vapor and hydrocarbons, the development of a permeation based system appears highly attractive, and this especially in offshore operations.