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Mar-2016

The impact of tower internals on column performance

Full scale testing and analysis of operation demonstrate the importance of tower internals on the performance of mass transfer packings

MICHAEL SCHULTES
Raschig GmbH
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Article Summary
Fractionation Research Inc. (FRI) is the most modern independent test institute in the world for performing tray, random packing, and structured packing tests under rectification conditions on an industrial scale test column (see Figure 1). In 1998, Raschig Super-Ring No. 2 was tested at FRI for the first time, and FRI high performance tower internals were selected for this test. However, in 2012, Raschig elected to retest Super-Ring No. 2. In this retest, the company used state of the art tower internals. This article shows the impact of tower internals on packing performance, comparing both FRI test results.

In addition to the FRI test results, the article provides information about Raschig’s large scale liquid distributor test facility. The test facility was built to improve large and small scale distributor designs as a development project and to demonstrate their performance after fabrication for industrial columns. Finally, the article describes an industrial design example for which modern tower internals were applied.

Tests with two sets of tower internals
The development over the past few decades of the most modern packing generations and tower internals, such as distributors, hold-down plates and support plates, have seen pronounced and noticeable design improvements.

High performance tower internals used 10 to 20 years ago in mass transfer columns are currently often considered to be low or standard performance devices, especially when they are used with high performance packings. When Raschig Super-Ring No. 2 was tested at FRI in 1998 for the first time, the FRI high performance tubed drip pan (TDP) distributor and the FRI support plate and hold-down plate were selected for the test (see Figure 2).

However, in 2012, Raschig elected to retest Super-Ring No. 2. In this retest, the company used a more modern, narrow trough type distributor that is very open to the gas phase compared to FRI’s TDP distributor. In addition to the high performance DT-S distributor which Raschig used in 2012, FRI employed a modern Raschig SP-1 support system and HP-1 hold- down device.

The impact of the TDP distributor on the performance of Super-Ring No. 2 from 1998 has been discussed in detail in previously published papers.1,2,3 In short, the narrow gas space of the TDP distributor and the large volume of sub-cooled reflux filling up in the pan type construction initiated heavy condensation of the vapour in the gas risers and caused pre-flooding of the distributor and finally of the whole test column.

Figure 3 shows the pressure drop and efficiency comparison of the Super-Ring No. 2 when tested in 1998 using FRI’s ‘high capacity’ TDP pan type distributor and when tested in 2012 utilising Raschig’s high performance DT-S distributor. The packing performance comparison presented is for the iso-butane/n-butane distillation system at 165 psia (11.4 bar) and the cyclohexane/ n-heptane system at 24 psia (1.62 bar).

For the test systems shown, the pressure drop decreased by 22-37% and the capacity improved by 13-18%, especially with the DT-S distributor. Figure 3 also demonstrates an efficiency improvement of 6-14%.

The DT-S distributor offers a much greater open space to the gas phase that facilitates the gas phase passing by, which consequently impacts the pressure drop and capacity measurement. It is important to note that open distributor designs are important for processes with high gas rates, as is the case under FRI test conditions. One has to consider that under conditions with limited gas rates and high liquid rates (absorbers and desorbers for example), deck type distributors with reduced open gas area are state of the art designs, allowing an easy passage of liquid over the entire distributor deck.

Raschig’s large scale liquid distributor test centre
Raschig has developed expertise in designing high performance liquid distributors/tower internals. To test these liquid distributors, the company built one of world’s largest liquid distributor test centres in Germany. Thus, it can test liquid distributors up to 12 m in diameter at full scale. For larger column diameters, Raschig tests these distributors in sections. Further smaller scale test facilities are available in Germany.

Figure 4 shows a performance test Raschig implemented for a large scale DT-S trough type distributor. The liquid was fed into the distributor via a liquid feed pipe header and a multiple parting box system. This test facility enables the measurement of the ‘drip point related coefficient of flow variation’ as well as the ‘drip area related coefficient of flow variations’. These measurements are supported by an automated collecting system.

The company is also equipped to test combined tower internal systems. For example, Figure 5 shows a dummy shell erected to simulate a column section for a two phase flashing feed. The tested column section consists of two 24in tangential feed lines directing the feed into a flashing gallery. Below the flashing gallery, a trough type distributor is positioned to be tested for its ‘drip point related coefficient of flow variation’.

High capacity CO2 absorption column with modern tower internals
In 2014, a new CO2 absorber was delivered to Australia, where a liquefied natural gas plant in Northern Territory went into operation in 2006. The CO2 absorber was a replacement unit due to capacity limitations of the existing column. The facility is known to employ state of the art engineering and environmental technology and, wherever possible, local and regional resources.

The plant uses the ConocoPhillips Optimised Cascade Process, an LNG liquefaction technology that employs two trains in one design to increase reliability. Through a dual lift operation, the 38 m long and 530 t piece was successfully loaded onto a heavy duty trailer. It was pulled by four primary movers. Together, the absorber and the prime movers stretched 85 m and weighed more than 750 t, making it the largest road haul in the state’s history.

The CO2 absorber was designed with state of the art tower internals. Such a typical absorber design with Raschig’s high performance tower internals is shown in Figure 6. To minimise the overall cost of a mass transfer tower, the column shell, the packing, and the internal components of the tower must be considered. Due to the use of the high performance Raschig Super-Ring, the diameter of the high pressure column shell could be reduced, which resulted in a reduced overall capital cost.

At the top of the amine absorber, a high performance droplet separator and three wash trays were used to avoid loss of amine and piperazine contained within the off-gas.

Raschig provided a high capacity feed pipe design that directs the high liquid flow rate into a special parting box to de-aerate the liquid and to reduce/eliminate any foam build-up. From the parting box, the liquid is directed to a high quality deck type RP 2 liquid distributor that allows the liquid to be very uniformly distributed across the deck. An equal liquid head above the distribution holes ensures a homogeneous liquid flow to the bed below.

Wide open hold-down plates type HP 1 and support plates type SP 1 were applied to keep the packing in place even if unusual column set-up or flooding were to occur. Between the packed beds, a high quality liquid redistributor was designed to ensure proper remixing of the liquid and gas phases. By design, the redistributor ensures both a homogeneous liquid flow to the bed below and a homogeneous gas distribution to the bed above.
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