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Whether operating at high pressures, dealing with liquids of low density, viscosity and surface tension, or attempting to satisfy a requirement for a high turndown ratio, it is difficult to separate the smallest droplets from the gas stream, since the velocity required to separate these small droplets may be higher than the critical liquid velocity above which re-entrainment will occur. In this operational situation, it is common to install a pre-conditioner or coalescing device upstream of the demister.

Zeta-pdm Ltd offers two different types of pre-conditioning or coalescing device, referred to as Mesh Type and ZP-090 Vane Type systems.

Installed as a coalescing device, both the Mesh Type and ZP-090 Vane Type systems are designed to operate at approximately three times the normal design velocity, when compared to the situation if used as a demisting device. In this capacity, they operate in a flooded condition, which has the effect of enlarging the sizes of the liquid droplets within the device. Since the droplet sizes are larger after passing through the pre-conditioner, they become easier to separate in the downstream demisting device and the gas/liquid separation efficiency is improved substantially.

Further, since the pre-conditioner inevitably creates a pressure drop, any flow mal-distribution will be reduced in the flow at the inlet to the demisting device. This has a secondary and beneficial effect on the overall efficiency of the gas / liquid separation process.

Finally, the incorporation of an upstream pre-conditioner/coalescing device ensures that the separator has a higher turndown ratio than normal. At low flow rates, the pre-conditioner takes over the demisting rolwe recommend the use of our proprietare of the downstream device. Under these conditions, it no longer operates in the flooded condition but reverts back to operation as a true demisting device.

Advantages of installing a pre-conditioner

Pre-conditioners are installed in both horizontal and vertical separators to:

  • Provide coalescence of the liquid droplets prior to engagement on the demisting device, substantially increasing the efficiency of the demisting system
  • Provide a pressure drop upstream of the demisting device, so that flow velocity variations (i.e. mal-distribution) are reduced across the frontal area of the demisting device, thereby increasing the separation efficiency
  • Ensure that the separator has a high turndown ratio ≥ 10:1.

Case Studies

Gas Scrubbers Flow Assurance

This is a before and after case study of a simple gas scrubber experiencing carryover problems. The original internals were an inlet deflector plate and a mesh pad only. These were replaced with a vane type inlet device and 48, 4G-FD® Demisting Cyclones.

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