Lars Erik Øi, Associate Professor, Telemark University College, Porsgrunn, Norway Elisabeth Tyvand Selstø, Process Engineer, Kvaerner Stavanger, Norway
for presentation at GPA Europe's meeting in Bergen 13th - 14th May 2002
Natural gas dehydration is normally performed by absorption in triethylene glycol. Regeneration by traditionaldistillation achieves about 1 weight % water in regenerated glycol. Use of stripping gas with or without an extra column, Coldfinger, Drizo and the Read Cycle are processes that achieve less than 1 % water in regenerated glycol. These processes are difficult to model because of special details, and convergence is a traditional problem in regeneration calculations. Models for all the glycolregeneration processes have been developed in the process simulation program HYSYS with the Peng Robinson equation of state. The models can also be implemented in other process simulation programs and with other vapour/liquid equilibrium models. Parameters in the models have been adjusted to simulate actual regeneration plant performance. The models have also been used to evaluate alternative regenerationprocesses.
Natural gas is dehydrated to avoid corrosion or pipeline plugging due to ice or hydrate formation. The traditional way to dehydrate natural gas is by absorption in triethylene glycol (TEG). Normally, the water is removed from the (rich) glycol by distillation, and then the regenerated glycol is recycled back to the absorption column. Figure 1 shows a typical process.After absorption, soluble gases like CO2 are first released from the glycol in a flash tank, and then the glycol is heated before it flows to a distillation (regeneration) column. The regeneration column has a reboiler and a condenser. Regeneration of the glycol by atmospheric distillation at about 200 °C achieves about 1 weight % water in regenerated glycol. The regenerated glycol is pumped back tothe absorption column and heat exchanged against the rich glycol and cooling water.
Condenser Regeneration column
Absorption column Wet gas
Reboiler Glycol pump
Figure 1: Traditional TEG absorption and stripping process
To achieve less than about 1 weight % water in TEG, there are several special regeneration processes available. Examples areuse of stripping gas with or without an extra column, Coldfinger , Drizo  and the Read Cycle . There is not much available information in literature about simulation of such processes. GPSA  and Kohl  are general references for dehydration processes. The aim of this paper is to show how different glycol regeneration processes can be modelled in a simulation program. Then it isshowed how these models can be used to evaluate alternative regeneration processes. This paper is based on a M.Sc. thesis from 2001  by Elisabeth Tyvand Selstø at Telemark University College with Lars Erik Øi as responsible supervisor.
SIMULATION OF A TRADITIONAL TEG DEHYDRATION PROCESS
Models for traditional TEG dehydration processes are given from the vendors of the processsimulation programs HYSYS, PROSIM, PRO/Vision and ASPEN PLUS. Except for this, there is not much information about simulation of dehydration processes in the literature. A paper about dehydration simulation in PROSIM was presented at a GPA conference in 1992 . At Hyprotech 2000, two papers about simulation of glycol dehydration were presented, with emphasis on steady state simulation  anddynamic simulation . An example of a HYSYS simulation of a traditional TEG dehydration process is given in figure 2.
Figure 2: HYSYS simulation of a traditional process
Compared to the general process in figure 1, the flowsheet includes a pre-cooler and a pre-scrubber to remove liquid water in the simulation. In the simulation, the pre-scrubber ensures that the gas to the absorption...