Revamps 2024 Issue

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

I n 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 signif- icant 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 enter- ing 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 meth- anol 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 metha- nol 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 col- umn 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 petrochemi- cal 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

Tail gas Methanol < 35ppm

Heat exchanger

Original design case and actual operation

Wash water

Vane pack before revamp

Original

Actual

design case

operation (2022)

Tray deck 73# 72# 71# 70#

Mass flows to reabsorber C1

Vapour flow Liquid flow

100% 100% 100% 100%

116% 116% 116% 134%

Hydraulic evaluation

Actual gas velocity

Pressure drop

across the vane pack

C1 Reabsorber

C2 Scrubber

Figure 1 Simplified process flow of tail gas

Table 1

Revamps 2024