Nov-2024

Retrofitting vane pack separator for improved column performance

Case history of a syngas production unit, which experienced high methanol content in the tail gas of the scrubber when operated at 116% of original design loads.

Han Yongchun, Wu Zhijiang and Ang Chew Peng
Sulzer Limited

Article Summary

In the production of syngas, methanol is often used for the absorption of acid gases, such as CO₂ and H₂S, as it is critical to minimise methanol content in the tail gas due to environmental regulations. The process typically involves a scrubber where wash water is used to reduce methanol content. Upgrading the existing vane pack separator in the reabsorber, upstream of the scrubber, brings about a significant reduction in methanol content in the tail gas, meeting the client requirement of less than 35 ppm.

Process flow and column internals
In the syngas production unit of a coal chemical plant in East Asia, the tail gas exiting the reabsorber consists mainly of CO₂ with some nitrogen and a trace amount of methanol. This tail gas goes through a heat exchanger before entering the bottom of the scrubber, where the gas is contacted with wash water to absorb the residual methanol, as per the simplified process flow in Figure 1. High methanol content in the tail gas exiting the top of reabsorber C1 will affect the methanol content in the tail gas exiting the top of scrubber C2. The wash water feed rate in the scrubber is a process variable to adjust the methanol concentration in the tail gas.

The only measurement of methanol concentration in the tail gas exiting reabsorber C1 is after heat exchanger E1. From operational experience, it is observed that the methanol concentration in the tail gas after heat exchanger E1, before entry to scrubber C2, must be less than 350 ppm to achieve a methanol concentration of less than 35 ppm in the tail gas from the top of the scrubber.

With the columns operating at 116% of original design loads, the methanol concentration in the tail gas after heat exchanger E1 outlet was high (around 900-1,400 ppm). Even with more wash water in scrubber C2, the methanol concentration in the tail gas from the scrubber could not be reduced to less than 35 ppm. Attempts to optimise the operating temperature and pressure of the reabsorber, in addition to the adjustment of the wash water flow rate in scrubber C2, did not bring about significant process improvements.

Reabsorber C1 was equipped with 73 trays in the column and a gas/liquid separator at the gas outlet. A hydraulic evaluation of the existing column internals was performed, and the results revealed that the existing trays are adequate to handle the higher loads. However, the existing gas/liquid separator, which was a vane pack, was operating beyond the overdesign margin.

A conventional vane pack was specified in the original design. In general, vane packs can capture liquid droplets in the range of 10-20 um and are widely used in petrochemical and gas treatment industries. Compared to knitted mesh mist eliminators, vane packs have the following unique advantages:
· Higher gas handling capacity, typically 1.5 to 3 times of mesh pads
· Flexible arrangement: it can be installed vertically or horizontally
· Big vane spacing against plugging
· Robust design.

As the existing vane pack was inadequate for the new loads, the proposed solution was to retrofit the gas/liquid separator to reduce the methanol concentration in the tail gas from reabsorber C1 to meet the methanol emission criteria of less than 35 ppm in the tail gas from scrubber C2.

Detailed evaluation of existing vane pack
The plant owner requested an evaluation of the existing vane pack’s adequacy. To analyse the actual column performance, Sulzer collected operating data, including the tail gas analysis with methanol concentration. Table 1 summarises the hydraulic evaluation of Mellachevron H51Z in the design case and actual operation case. Note that Mellachevron ‘MCV’ is the trademark for Sulzer’s vane pack.

The actual operating capacity was 116% of the design case, while the existing vane pack was designed with a maximum turn-up of 110%. As the operating loads were beyond the maximum design range, there was a chance of potential liquid droplet entrainment at the vane pack considering:
· Gas velocity was exceeding maximum design velocity.
· The complex vapour trajectory into the vane pack located at the column side wall due to lack of space.
· The presence of the inlet pipe and the flash box, which were affecting the vapour flow trajectory.

All these issues caused an increase in liquid droplet entrainment in the vane pack outlet, resulting in an increase in methanol content in the tail gas exiting reabsorber C1.

In the design of vane packs, both vapour flow trajectory and vapour velocity are important to performance. Poor vapour distribution into the vane packs leads to vapour velocity being higher or lower than design guidelines. If the vapour velocity is too high, re-entrainment will happen. If the vapour velocity is too low, the inertial force will be insufficient, resulting in an increase in droplet cut-off size.

The MCV H51Z in reabsorber C1, with layout as seen in Figure 2, was located at the side wall of the column due to lack of space. The housing stretches 900mm into the column of diameter 3,800mm, affecting the vapour flow below the housing. The layout of the housing and column internals (tray deck, draw-off, and flash box) is not symmetrical, which may cause poor vapour distribution into the vane packs.

A computational fluid dynamic (CFD) simulation was performed to simulate the fluid flow parameters, including gas distribution quality, gas flow trajectory, gas velocity, and pressure drop at the vane pack entry. Figure 3 shows the complex vapour flow trajectory, while Figure 4 shows the gas load factor at the vane pack entry. The gas load factor 'λ’, also called the K-factor, is used to indicate vapour velocity. K-factor is influenced by many parameters, including separator internal type, operation pressure, vapour, and liquid physical properties. As observed in the CFD, the vapour flow distribution at the vane pack entry is non-ideal and would lead to excessive entrainment, as follows:
· The inlet pipe and the flash box not only obstruct the vapour flow but also re-entrain extra liquid droplets.
· The vapour rising from the tray below, at the opposite end of the vane pack, will reach the top of the column and then be routed back to the vane pack entry, generating vapour stream circulation during the process, as shown in Figure 3.
· Complex vapour flow patterns with multidirectional velocity vectors, influencing the MCV capture of liquid droplets and liquid drain.
· Obvious red zones of high velocity at the bottom of the vane pack, potentially leading to excessive entrainment.

Revamp solution
The main motivation of the revamp is to reduce the methanol concentration in the tail gas from scrubber C2 to less than 35 ppm, achieved through lower methanol content from the overhead of reabsorber C1. To achieve this stringent requirement, it was observed that the methanol concentration after heat exchanger E1 must be less than 350 ppm. The revamp targets are as follows:
· Capacity at 116% of original design.
· Methanol concentration after heat exchanger E1 less than 350 ppm.
· No hot works on column shell.
· Turnaround within four days.