Apr-2011

Bulk separation of gas-liquid mixtures

Effective gas-liquid separation is increasingly important to produce high-quality products from feedstocks of decreasing quality

Giuseppe Mosca, Pierre Schaeffer and Bart Griepsma Sulzer Chemtech
Harry Kooijman Shell Global Solutions International

Article Summary

The operating conditions of mixed phases and the requirements for separation efficiency may vary widely. Therefore, special care should be taken in selecting the most appropriate device to match the specific duty. For application to bulk gas-liquid separation, where generally not more than 95% of the liquid must be removed from the gas stream, the Schoepentoeter is a proprietary feed inlet vane device used for introducing gas-liquid mixtures into distillation columns or gas-liquid separators. It has two main functions:
• To separate the liquid from the gas
• To distribute the vapour in the gas compartment of the column.

The device accomplishes these objectives by slicing up the mixed-phase feed into a series of flat jets by means of properly distributed and oriented vanes. The jets dissipate a large part of the kinetic energy due to the vanes so that the vapour enters the gas compartment of the column in a smooth and uniform manner. The vanes also provide the mixed-phase feed with centrifugal acceleration to promote and/or enhance the separation of the liquid from the vapour — otherwise possible only by gravitational force.

For any given duty, the device allows for a considerably smaller feed entry section in the vessel, thus reducing the total column’s height and costs.

Process design parameters
The main design parameters for a Schoepentoeter are the sizing of the feed inlet nozzle, the flow parameter and the column load factor. These factors are important in predicting its efficiency. The sizing of the feed inlet nozzle of a vessel equipped with a Schoepentoeter should be based on the maximum flow rate, including the design margin. The internal nozzle diameter can be taken to be equal to that of the upstream feed piping to the vessel, provided that the maximum momentum criterion is satisfied. In some applications where the gas density is very low — for instance, in refinery vacuum towers — the velocity of the gas at the feed inlet nozzle should be somewhat lower than the critical velocity of the gas (the speed of sound of the gas mixture) to prevent choking or damage due to vibrations. The flow parameter is used to characterise the type of gas-liquid mixture entering the vessel or the relative importance of the liquid load approaching the feed inlet device. It is proportional to the ratio of the liquid mass flow to gas mass flow.

Additionally, the performance of the device — in particular, the separation efficiency — is greatly affected by the column load factor, also known as the capacity factor. This factor is proportional to the volume flow of the gas to the cross- section of the tower.

Separation efficiency
The separation efficiency of a feed inlet device for gas-liquid mixtures is normally defined by the ratio of the liquid flow rate separated from the gas stream and the liquid flow rate originally contained in the mixed-phase stream.
For a Schoepentoeter, the separation efficiency can be expressed as a function of the nozzle’s and column’s diameters, the column load factor, the flow parameter and the ratio of the surface tension of the liquid compared with the surface tension of water.

Mechanical design parameters
The device should be designed to comply with and satisfy the following mechanical requirements and criteria:
• A maximum operating load over the feed inlet nozzle of 15 000 Pa
• Withstand its own weight plus the weight of the fluid at process conditions
• Downward or upward deflection under operating loads not exceeding 1% of the nozzle diameter or 15 mm, whichever is larger
• Tilt of the Schoepentoeter not exceeding 1% of the column diameter or 15mm, whichever is smaller
• Thermal expansion during normal operation and transient conditions — for instance start-up and shutdown — should be also considered
• For mega-sized Schoepentoeter devices, those with a nozzle diameter exceeding 3m and length exceeding 9m, additional detailed mechanical strength calculations and vibration calculations should be performed.

There are cases — refinery vacuum tower revamps or flare system knockout drums — in which the Schoepentoeter is subject to loads even heavier than those mentioned above. Therefore, some additional measures should be taken to avoid vane tips being bent or broken, by for instance using thicker material or employing stiffening strips at the back of long, unsupported vane tips.

Established performance
In most cases, the conventional Schoepentoeter has been proven to match and even exceed expected performance. There are only a few applications, such as refinery vacuum towers, where the separation efficiency was measured to be lower than expected. Those measurements may have occurred because the liquid, separated by the vane, is not conveyed. Rather, it leaves the vane in the shape of a thin curtain, which, on its way to the bottom section of the tower, is subject to the upward momentum of the ascending vapour. A portion of the separated liquid (entrainment) may be carried to the feed entry zone of the tower. Therefore, the resultant separation efficiency can be lower than expected, especially under severe operating conditions that are commonly encountered in refinery vacuum towers, such as when the inlet nozzle momentum is above 7000–8000 Pa or the column load factor is above 0.09 m/s.

Research and development
Extensive research and development work was completed in the form of experimental tests and computational fluid dynamics analysis at the Sulzer Chemtech pilot plant in Winterthur and at the Shell Technology Center in Amsterdam.

The aim of the study was to optimise the separation efficiency without compromising the hydraulic capacity, particularly the pressure drop through the feed nozzle and the Schoepentoeter itself. The idea was to design a feature that would collect the separated liquid in a way to counterbalance the upward momentum of the ascending vapour. Several types of advanced vanes were tested.