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Bulk separation of gas-liquid mixtures
PTQ Outlook, Q2 2011
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; and
- 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 Schoepentoeter allows for a considerably smaller feed entry section of the vessel, thus a reduction of 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 the efficiency of the Schoepentoeter.
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.
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 15000 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;
- The tilt of the Schoepentoeter not exceeding 1% of the column diameter or 15 mm, whichever is smaller;
- Thermal expansion during normal operation and transient conditions – for instance start-up and shutdown - should be also considered; and
- For mega-sized Schoepentoeter devices, those with 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 shall 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.
Figure 1: Tests were performed at different column load factors and flow parameters. At higher column load factors, the entrainment of the Schoepentoeter Plus is even less than one-third of the conventional one
In most of the cases, the conventional Schoepentoeter has been proven to provide very good performance, and even to 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 a 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 may 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 column load factor is above 0.09 m/s.
Research and development
Figure 2: The improvement is achieved without any significant increase in pressure drop
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 Centre in Amsterdam.
The aim of the study was to optimise the separation efficiency without compromising the hydraulic capacity, in particular, 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. The goal was achieved by modifying the back end of the vane from a straight and flat vertical plate to a sloped and curling plate — the so-called catching rim.
The catching rim collects the separated liquid and conveys it into a rivulet heavy enough to win the upward momentum of the ascending vapour and reach the bottom section of the tower, thus minimizing the entrainment. The tests were performed at different capacity factors and flow parameters.
At low column load factors, no major difference was measured: both the conventional and new Plus devices performed sufficiently. At higher capacity factors, typically encountered in several industrial columns, the separation efficiency of the Plus version was consistently higher than the conventional one; the entrainment was even less than one-third for values typically encountered in several industrial columns. The improvement was achieved without any significant increase in the pressure drop (see Figure 2).
A new correlation for the prediction of the entrainment was developed by analytical regression of the experimental data, which considers the effect of the new vanes. A new tool has been engineered to manufacture the catching rim.
Computational fluid dynamics study
Within the last decade, computational fluid dynamics (CFD) has reached such maturity that it is now considered an indispensable analysis and design tool in a wide range of industrial applications, including for feed entry sections of distillation towers or gas-liquid separators. Therefore, a CFD study was performed to check the efficiency of the feed inlet device in terms of vapour distribution. For this scope, the flash zone of a refinery vacuum tower was modelled and analysed with both the devices. The following operating conditions were set:
- a feed inlet nozzle momentum of 6370 Pa;
- a column load factor of 0.097m/s;
- a collector tray with a 30% open area above the Schoepentoeter; and
- a combined bed of Mellapak™ 125X and Mellagrid™ 64X structured packings above the collector tray.
The vertical vapour velocities over the horizontal plane were checked at different tower elevations, in particular, underneath the combined bed of Mellagrid and Mellapak in the wash section. There is no significant difference between the two devices: the vapour distribution efficiency is good for both the distributors.
Fields of application
In general, the Schoepentoeter Plus could be used in all applications suitable for a conventional device, such as separation in oil and gas upstream units or distillation in the oil and gas downstream plants. This article mainly focuses on the second application. There are cases where there is no need for the Schoepentoeter Plus, such as when the inlet device is used for a single-phase stream and no significant benefit in distribution efficiency would be achieved. The higher cost of the Plus version makes the conventional one more attractive.
The best candidates for installation of the Plus device are vacuum towers, crude distillation main fractionators and hydrocracking main fractionators in oil refineries, where the separation efficiency of vapour from liquid plays a significant role in the performance of the units.
Case study: vacuum tower revamp
The column is located at a major European refinery. The main duty of the tower is the recovery of light and heavy vacuum gas oil (LVGO and HVGO) from the long residue coming from the primary distillation of the crude oil. The feed, preheated up to 400–420°C and partially vapourised, accesses the flash zone of the column through the feed inlet device, which performs a bulk separation of the liquid from the vapour as well as a vapour distribution in the gas compartment of the column.
The liquid drops down to the stripping section for the ultimate recovery of the light hydrocarbons and is finally drawn off as short residue from the bottom of the tower. The vapour phase is fractionated into LVGO and HVGO in the upper sections. The LVGO is drawn off at the top section of the tower. A pumparound provides the column with the duty necessary to condense the right amount of vapours coming from the wash section.
The HVGO is generally the first useful side cut above the flash zone. A pumparound provides the column with the duty necessary to condense the right amount of vapours coming from the wash section. A portion of the condensate — the wash oil — is pumped back to the bed below to control the quality of the drawn-off product. Among other factors, such as feed composition, wash section configuration, and operating parameters, the quality of the HVGO may also be affected by the separation efficiency of the feed inlet device.
Concerns at the existing tower
Figure 3: Coke in the wash bed leads to a higher pressure drop and lower recovery of distillates, resulting in shorter plant run length and unexpected shutdown
The flash zone of this column was originally equipped with a conventional Schoepentoeter. Since the separation efficiency was lower than expected, the liquid carryover to the wash section (entrainment), which was made of the heaviest hydrocarbons and should, indeed, have followed the short residue at the bottom of the tower, was higher than expected. As consequence, the slop wax flow rate (generally an unwanted product) was consistently higher than foreseen.
In an attempt to maximise the yield of the HVGO while minimising the production of slop wax, the wash oil was substantially reduced, even below the minimum, causing a deterioration of the wash bed performance:
- Poor quality HVGO: a high Conradson carbon residue (CCR) and metals content with negative impact on the downstream fluid catalytic cracking (FCC) unit. These results led to lower liquid yields and a higher catalyst make-up rate than expected;
- Coking up of the wash bed: higher pressure drop and less recovery of distillates; this effect led to shorter plant run length, unexpected shutdown, and thus reduced plant utilisation factor and increased maintenance costs (See Figure 3).
After an in-depth investigation and detailed analysis of the tower performance, Sulzer decided to replace the existing conventional Schoepentoeter with the Plus version. The wash bed was replaced due to coke formation, and the existing combination of Mellagrid and Mellapak was kept. In addition, the two pumparound beds were replaced with the same type of packing within the scheduled maintenance program of the unit during the overall turnaround of the refinery. All the other tower internals were retained. The tower has recently been started up.
Increased separation efficiency
The Schoepentoeter Plus is a tool with which to improve the bulk separation efficiency of gas-liquid mixtures. The main fields of application are the refinery towers in vacuum distillation units, crude distillation units and hydrocracking plants. The best fit is the revamp of vessels equipped with radial feed inlet devices In new columns, the higher cost of the Plus version may make the conventional device more attractive, provided that the performance requirements are not excessively high.
Schoepentoeter is a mark of Shell. Mellagrid and Mellapak are marks of Sulzer Chemtech.
Giuseppe Mosca is the Global Refinery Technology Manager of Sulzer Chemtech in Milan, Italy. He leads the design of mass transfer components for distillation towers, absorbers and strippers, and is involved in process simulation, revamping proposals, troubleshooting, commissioning of tower internals and start-up assistance of fractionation equipment. He holds BS and MS degrees in chemical engineering from the University La Sapienza in Rome.
Pierre Schaeffer is an R&D Engineer in the Laboratory for Mass Transfer Technology at Sulzer Chemtech in Winterthur, Switzerland. As a specialist in experimental methodology, he contributes to the development of seperators and fractionation trays.
Bart Griepsma is a Senior Mechanical Specialist with Sulzer Chemtech in Winterthur. He is currently responsible for global support in mechanical engineering, global technical sales support for mechanical revamping proposals, and for mechanical design optimisation and product improvement.
Harry Kooijman is a Senior Separations Equipment Consultant for Shell Global Solutions International and is a Subject Material Expert for transport properties (diffusion). He has an MS degree in chemical engineering from Delft University of Technology, Netherlands, and a PhD in distillation from Clarkson University, New York.