三甘醇再生

TRIETHYLENE GLYCOL REGENERATION IN NATURAL GAS

DEHYDRATION PLANTS: A STUDY ON THE COLDFINGER

PROCESS

F. Gironi, M. Maschietti, V. Piemonte, Università degli Studi di Roma “La

Sapienza”, D. Diba, S. Gallegati, S. Schiavo, Comart SpA, Ravenna, Italy

This paper was presented at the Offshore Mediterranean Conference and Exhibition in Ravenna, Italy, March 28-30, 2007. It was selected for presentation by the OMC 2007 Programme Committee following review of information contained in the abstract submitted by the authors. The Paper as presented at OMC 2007 has not been reviewed by the Programme Committee. ABSTRACT

Natural gas pipeline transportation requires very low water content in the gas stream in order to avoid condensation or hydrate formation. To reach this goal, when triethylene glycol (TEG) is used to dehydrate natural gas, after the absorption step TEG must be regenerated to levels substantially above 98.5-99.0 % by weight available from atmospheric distillation of glycol-water mixtures. In order to regenerate TEG to higher purity levels some of the methods used require a stripping gas, a solvent or to perform the distillation under vacuum. A simpler method to perform a further dehydration of TEG is the use of a water exhauster, known as Coldfinger, where the vapour in equilibrium with the liquid to be dehydrated is continuously condensed and removed. In this work, the Coldfinger apparatus was modelled and a study on the most relevant operating parameters was carried out. A process simulation of a natural gas dehydration plant, provided with a Coldfinger water exhauster for TEG regeneration, was performed on a case study. It was shown that the dehydration process with Coldfinger unit is capable of reaching current water content specifications in a simple and economic way.

INTRODUCTION

Natural gas at the producing well contains significant quantities of water vapour. Typically, the gas is water-saturated at the condition of pressure and temperature of the well and a dehydration process is required. In fact, water content must be reduced in order to prevent liquid water condensation and hydrate formation in the pipeline transportation system. Nowadays, typical values of allowable water content in the gas transmission lines range from 70 to 120 mg/Nm3 [1].

Among methods available for natural gas dehydration, absorption by means of triethylene glycol (TEG) is one of the most common. Water removal from the gas stream takes place by means of countercurrent contact between the gas, fed to the bottom of a contactor tower, and TEG, which is a liquid with a great affinity for water, fed at the top of it.

The crucial part of the process is represented by TEG regeneration. If the water-rich TEG is distilled in a simple atmospheric column, TEG can not be regenerated to levels above 98.8-98.9 % by weight. This is caused by the reboiler operating temperature, which can not be fixed at temperature above 204 °C. In fact, this tempe rature must be regarded as an upper limit for TEG processing, because of thermal degradation at higher values [2,3].

In the past these regeneration levels were sufficient because values of allowable water content in the lean gas were higher and regeneration was commonly performed in a simple atmospheric still column. On the other hand, in order to reach current water content specifications, it is necessary to regenerate TEG up to levels substantially above 99.0 % by weight.