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Another possible absorber problem to address is excessive carryover of the solvent with the stripped gas. The remedy is to replace the mist eliminators in the absorber towers and downstream knockout drums with higher-efficiency types. Figure 5 illustrates the operation of a horizontal mesh-type mist eliminator pad in an absorber tower. Simply retrofitting a conventional pad with a modern high-efficiency design can reduce solvent carryover by 90%.
Generally the optimum mist eliminator performance in terms of both efficiency and capacity is achieved by providing a vane mist eliminator downstream of a vertical mesh pad with horizontal flow, as shown in Figure 6 for a knockout drum. In that case, the mesh agglomerates fine mist droplets into larger ones that are then effectively captured by the vane unit.
In mesh-vane assemblies such as this, maximum mist elimination efficiency can be obtained by using Amistco Double-Pocket Vane units, shown in Figure 6, instead of conventional vanes. The special vanes in these units shield captured liquid from re-entrainment due to buffeting by the flowing gas. There are ways to accomplish such retrofits without welding new supports to the vessel walls, such as Amistco’s double expansion rings shown in Figure 7.
Besides improving hydrogen purity and throughput, retrofitting mist eliminators in the absorbers and related knockout drums in ways such as these can drastically cut losses of expensive solvent.
In the compressor train, increased hydrogen production may cause mist carryover from the knockout drums that are typically located before each compressor stage. The results are compressor slowdown and eventually shutdown due to pitting corrosion. The separation capacity of each of these knockout drums can be easily increased by 50% when the existing mist eliminator is replaced with a more advanced variety as explained before for the absorber towers and knockout drums. The result is increased hydrogen production coupled with ultra-high efficiency, providing maximum protection for the compressors against liquid carryover damage.
As for relieving constraints in various other separator columns and vessels in the hydrogen plant, the opportunities there are as described before for absorber towers and knockout drums: providing higher-capacity internals and improving the mist eliminators.
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