A Success Story: Extending Molecular Sieve Service Life by Double
Authors: Al-Khamis Mohammed Jassim 
Published: February 2016

Uthmaniyah Gas Plant (UGP) has achieved 100% additional service life of the gas molecular sieve desiccants. This new record in gas desiccants service life was a result of careful data monitoring and online sampling which was used to gain confidence to continue using the desiccant despite the vendor's advice of changing it. The paper will discuss the process description of the area where the desiccants are used (NGL Recovery) and the role of this material in the area which is basically dehydration. After that, the paper will discuss the dehydration process in general and at UGP specifically.

Then, the performance profile of the desiccants through thier seven years service life will be demonstrated and explained. The performance of the desiccants depends on two main factors: the water adsorption capacity and the pressure drop. The paper will then explain the factors that contributed to the extension of the service life of the desiccants. These factors are mainly the loading plan, two-step regeneration, regernation at the same adsorption pressure, using the activated aliumna at the top of the dessicants as a guard layer, and the close monitoring.

Introduction:

Uthmaniyah Gas Plant uses Molecular Sieve (Desiccants) to remove water from the sweet gas feeding Natural Gas Liquid (NGL) Recovery Area. NGL is recovered by cooling the sweet gas to a cryogenic temperature of -140 oF. At this low temperature, no water shall be in the sweet gas. Otherwise, hydration will form and blockage will take place in the downstream equipment. The water content allowed going to the NGL recovery area is 10 PPM maximum. The molecular sieve material is composed of zeolite and binder. The zeolite is the active material and where the water is adsorbed. The binder is a small amount of clay that enables the fabrication of the molecular sieve. Usually the service life of the desiccants is 750 cycles which is equal to almost 3 years at normal gas loading. Uthmaniyah has successfully achieved 1380 cycles (7 years).

Process Description of NGL Recovery Area:

Uthmaniyah Gas Plant has four identical NGL Recovery Modules. The main objective of each module is to recover the NGL from the sweetened gas through a cascaded refrigeration process using ethane and propane as refrigerants. Each module consists of three chill trains, which are basically a series of chillers and heat exchangers. The sweet wet gas is fed to the module at 140 °F, and 450 psig (Fig.1). The gas is cooled at the 1st Chill train to a temperature of 65 °F. After that, the feed gas is taken to a three phase separator to remove liquid hydrocarbon and water from the feed gas.

The separated gas is saturated with water and thus is fed to the gas dehydrators to remove the moisture from it and avoid hydrate formation in the downstream equipment. At the same time the liquid hydrocarbon is sent to the liquid dehydrator to remove the moisture from it. The dry liquid hydrocarbon is sent to the De-methanizer tray 4 while the dry gas leaving the dehydrators continues to the second chill train where it is cooled to a temperature of -35 °F. And at the end of the train the liquids are separated from the gas in a two phase separator. The liquid hydrocarbon is sent to the De-methanizer tray 10 and the gas continues to the 3rd chill train for further cooling.

The gas is cooled at the 3rd chill Train to a temperature of -98 °F and the liquid is separated from the gas in a two phase separator. The liquid is fed to the de-methanizer tray 19 and the gas is used to cool the feed in the upstream heat exchangers. The De-Methanizer is used to stabilize the NGL and remove the light ends from it. Then the NGL product is drawn form the bottom of the column and sent to the shipping pumps.