TEG dehydration unit

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. 

Figure 7.1 Simplified process flow diagram for a TEG dehydration unit.

Fire tube heat density depends on the type of draft used for design. Most TEG dehydration units are designed for natural draft, and for such conditions, the maximum fire tube heat density used is 24 kj/(mm2 h) [1]. The general equation of heat density is... 

Table 5.8 General Specification of a TEG Dehydration Unit (Continued) S.

The program to design TEG dehydration unit, dehydration.exe, has been developed for sizing major equipment and estimate the heat exchanger loads. Designing individual exchangers has not been considered in this program. 

The biggest disadvantage of the standard mole sieve dehydration unit is the cost. As a rule of thumb, a mole sieve unit costs 1.5 times that of a similar TEG unit (Kohl and Nielsen, 1997). 

The glycol circulation rate for Case 1 is equivalent to 9.7 gal of TEG per lb of water removed and in Case 2 it is 5.9 USgal/lb. These are well in excess of the typical 2 to 4 USgal/lb typically recommended for a natural gas dehydration unit. Zabcik and Frazier (1984) do not offer an explanation as to why the glycol circulation rate is so high. 

Ho et al. (1996) describe briefly the dehydration unit at the Wayne-Rosedale injection project. The operating conditions for this dehydration unit are given in table 7.5. Note the values given are the operating conditions and not the design conditions. Unfortunately, no details about the TEG side of the process are reported. 

The treated gas leaving the M D E A unit will require downstream dehydration since the pipeline gas specifications require a minimum water dew point. This is best achieved with a conventional TEG glycol dehydration unit, as the gas is very lean without condensable hydrocarbons. 

The design and operation of a trailer-mounted vacuum reclaimer which was used to process the TEG of a number of dehydration units by the La Vaca Gathering Company, is described by Armstrong (1979). The unit was designed to process 0.5 gpm of glycol, but actually handled up to 2.0 gpm. The reclaimer operates in a semi-batch mode, i.e., feed is continuous at 0.5-2.0 gpm until sufficient non-volalile material accumulates in the heater vessel to raise the boiling point to 400 F at 28 in. of mercury vacuum. The reclaimer is then shut down and cleaned out. 

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... 

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. 

The degree of dehydration that can be attained with a glycol solution is primarily dependent on the extent to which water is removed from the solution in the reconcentrator. The operation of atmospheric pressure distillation units for water removal is limited by the maximum temperature that can be tolerated without excessive decomposition of the glycol (about 400°F for TEG). Concentration of TEG to 98.5 to 99.0% is attainable in a simple atmospheric pressure still. When significantly higher concentrations are needed to meet stringent gas dehydration requirements, the use of an enhanced stripping technique is necessary. 

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