Glycol dehydration unit regenerator

An alert on hydrocarbon emissions from glycol dehydration units issued by 77te American Petroleum Institute, in connection with their Specification for Glycol-Type Gas Dehydration Units (1990), contains a mass balance for a 10 MMscfd TEG unit operating at 800 psia and 130°F with a feed gas containing 100 ppm benzene. The study indicates that 10% (3 tons per year) of the benzene is absorbed and discharged in the regenerator vapor stream. In this... 

A typical triethylene glycol (TEG) dehydration unit is made up of two main components. The absorber, often known in the industry as the glycol "contactor" and the regenerator, is normally based on direct fired reboiling. The feed gas enters the bottom of the contactor and travels upward. The glycol enters the top of the tower and travels down. Thus dry gas leaves the top of the contactor and rich glycol (containing more water) leaves the bottom. 

The pressure of the rich glycol stream is dropped to near atmospheric and is sent to the regenerator system. Heat is added to the bottom of the column, and the water is driven out of solution to produce the lean glycol stream, which is returned to the contactor. The water exits as the offgas from the top of the regenerator still. In the case of an acid gas dehydration unit, this offgas steam contains H2S and C02 for reasons that will be discussed later, and cannot be vented to the atmosphere. In an acid gas dehydration... 

In gas dehydration service, triethylene glycol (TEG) will absorb limited quantities of BTEX from the gas. Based on literature data, predicted absorption levels for BTEX components vary from 5-10% for benzene to 20-30% for ethylbenzene and xylene [2]. Absorption is fa vored at lower temperatures, increasing TEG concentration and circulation rate. The bulk of absorbed BTEX is separated from the glycol in the regeneration unit and leaves the system in the regenerator overhead stream. 

Molecular. sieves are the most expen.sive, but provide the lowest dew point. They are normally used to dehydrate natural gas feed streams for cryogenic hydrocarbon recovery units. Type 4A is the type most commonly employed, but 3A is gaining favor because it is less active catalytically and can be regenerated at a lower temperature than 4A. A disadvantage of molecular sieves for gas dehydration is that they can be fouled by impurities in the gas, such as amine, caustic, chlorides, glycol, and liquid hydrocarbons. However, fouling problems can be minimized by the installation of a buffer layer of one foot or more of activated alumina (for amine, caustic, and chlorides) or activated carbon (for glycol or hydrocarbon liquids) on top of the molecular sieve bed (Veldman, 1991). 

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