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High-purity ethane from wet NGL (TIA)

Producing high-purity ethane (95+ LV% ethane) in an NGL fractionation plant requires a refrigerated condenser for the deethaniser.

Chyuan Chen
Wood Group Mustang
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Article Summary
If the NGL feed is wet or saturated with water, hydrates can form in the top section of the deethaniser and in the deethaniser condenser where the operating temperature may be below the hydrate formation temperature. Formation of hydrates can plug the equipment and piping to hinder the operation. The conventional method of preventing hydrate formation is to use dry desiccant beds or a glycol contactor to remove water content in the NGL feed. However, both methods utilise a complicated dehydration unit that not only requires additional capex and opex but also additional space. To overcome these shortcomings, Wood Group Mustang has developed a novel high-pressure deethaniser that produces 95+ LV% ethane from wet NGL without the need for a dehydration unit. This commercial-scale plant is currently in operation and has been running successfully.

Typically, molecular sieve or silica gel beds are used to dehydrate light liquid hydrocarbons such as LPG or NGL. As shown in Figure 1, a dry desiccant system consists of adsorption and desorption (regeneration) beds with a complicated switching valve arrangement. The regeneration system consists of a regen gas heater, regen gas cooler, regen gas compressor, and other miscellaneous items. A dry desiccant dehydration system is a batch-operated system. The periodic switching among adsorption, heating and cooling cycles is complicated although the switching operations can be manipulated by an automatic timer control.

The operation of a glycol dehydration unit is simpler than that of a dry desiccant unit. However, a glycol unit is commonly used for gas dehydration (instead of liquid dehydration) due to the small amount of glycol used in the contactor. In order to use a glycol unit, a vapour stream needs to be drawn from and returned to the deethaniser after it is dehydrated. This means the deethaniser column requires extra length for the internal vessel heads that are necessary for side vapour/liquid draws and returns (see Figure 2). In addition, a dehydrated gas compressor or a side draw liquid return pump may be needed in order to compensate for the pressure drops in the draw piping, return piping and the glycol contactor. A glycol dehydration unit consists of a glycol contactor and a glycol regeneration system. The glycol regeneration system usually includes a minimum of a flash drum, a glycol reboiler/still column, a glycol/glycol exchanger, a high-head lean glycol pump, and a glycol cooler. Furthermore, injecting stripping gas to help regeneration is necessary in some cases.

As the pressure increases, the temperature of the deethaniser condenser increases rapidly, while the hydrate formation temperature increases slowly. Based on this phenomenon, high-purity ethane can be obtained by increasing the deethaniser pressure without dehydration (see Figure 3). For example, the temperature of the deethaniser overhead condenser is about 46°F at 390 psig and 63°F at 490 psig to produce ethane at 95+ LV%, while the hydrate point of this high-purity ethane is about 51°F at 390 psig and 61°F at 490 psig. Comparing the operating temperature and the hydrate point, hydrates form in the condenser at 390 psig but do not form at 490 psig if the NGL is not dehydrated. Although the operating pressure of a deethaniser is typically 200-400 psig, which is well below the critical pressure of the hydrocarbon mixture, Wood Group Mustang has designed a deethaniser that operates successfully near 500 psig at the bottom of the deethaniser to produce 95+ LV% ethane from wet NGL without dehydration.

The cost of a high-pressure deethaniser column is higher than that of a conventional deethaniser column, but the savings from not needing a dehydration system and from the reduction in horsepower of the refrigerant compressor due to a higher condenser temperature can compensate for this increased capex cost. The increased requirement of the deethaniser reboiler duty caused by a higher pressure in the deethaniser column can be offset by the reduction in depropaniser reboiler duty due to a higher outlet temperature of the deethaniser bottoms. Therefore, the capex and opex of the high-pressure deethaniser design without dehydration are not significantly different from those of a conventional deethaniser design with dehydration.

In conclusion, a novel deethaniser process without dehydration has been developed and proven a successful alternative in commercial operation. This process eliminates the need of installing a complicated molecular sieve or glycol hydration unit, providing simplicity for both installation and operation.

This short case study originally appeared in PTQ's Technology In Action feature - Q3 2015 issue.
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