Vacuum Tower Demister and Mist Eliminator Selection

Vacuum service is pressure-drop sensitive. A demister that looks acceptable in a normal-pressure vessel can reduce capacity if the tower has little pressure margin.

Read pressure drop as part of the tower balance

In vacuum distillation and evaporation, even a modest additional pressure loss can change upstream absolute pressure, boiling temperature, vapor volume and available capacity. The allowable demister pressure drop should therefore come from the tower pressure balance, not from a generic catalogue limit.

State whether the limit applies to a clean pad, a wetted operating pad or an end-of-run fouled condition. Comparing a supplier’s dry clean value with a plant’s wet fouled measurement is not a valid check.

Design concerns

Low vapor density affects the Souders-Brown density term and greatly changes actual volumetric flow. It does not, by itself, define the applicable K factor or prove that a particular derate is required. The designer should use actual flow and densities at vacuum conditions together with a project K value supported for the service, not normal flow or a generic preset.

Fouling from heavy hydrocarbons, salts or coke fines can make dense mesh risky. Vane packs or lower-density mesh may be considered when plugging tolerance matters.

Convert each vapor case to actual volume at the local pressure, temperature and composition. Check the cross-sectional area that is truly open after support beams, wall gaps and other internals. Then review liquid load, entrainment source and required droplet removal. Velocity screening alone cannot choose between a fine mesh and an open vane.

Select around the contaminant

• Relatively clean vapor and fine entrainment: knitted mesh may provide compact separation where clean and operating pressure drop are acceptable.
• Sticky heavy liquid, coke fines or salts: open vane profiles or other fouling-resistant internals may provide more stable area and easier irrigation.
• Variable service: a combined arrangement can be evaluated, but added height, pressure drop and drainage interaction must be included.
• Predominantly vapor condensation after the pad: changing the mist eliminator may not solve a downstream temperature or pressure problem.

Identify whether deposits arrive as entrained solids, precipitate locally or form from condensation and reaction. The mitigation differs: upstream control, temperature management, irrigation and media geometry are not interchangeable.

Installation details

Vacuum towers are often large, so pad segmentation and manway access are critical. Section width, hold-down design, bypass sealing and support ring condition should be confirmed before ordering.

A pad that leaves a wall gap or is compressed poorly can create bypass. In vacuum towers, bypass can look like a process design problem even when the root cause is mechanical fit-up.

Survey the vessel diameter at the support elevation, ring width and flatness, beams, nearby packing or distributors, hold-down arrangement and full manway route. Large-diameter section maps should prevent continuous joints that create a preferred bypass path. The drawing should show flow direction and installation sequence.

Use operating trends before a shutdown

Compare tower pressure profile, throughput, product contamination and wash or irrigation history. A gradual rise in section pressure drop suggests shrinking open area; a sudden change can reflect liquid accumulation, a process upset or displaced internals. No single trend proves the root cause.

During inspection, photograph the upstream and downstream faces before cleaning. Map deposits, erosion, wall gaps, crushed sections and clean channels. Retain representative deposit samples where the site procedure allows.

RFQ data

Send operating pressure, temperature, vapor flow, liquid properties, vessel ID, available height, manway size, fouling history, material preference and outage schedule. If drawings are missing, send photos and hand measurements.

Add minimum, normal, maximum and upset vapor cases, local gas density, allowable clean and operating pressure drop, liquid loading, contaminant description, irrigation details, inlet and outlet geometry, support survey and required removal criterion. Distinguish measured values from design data and estimates.

The PI should freeze media family, offered style or geometry, material set, finished dimensions, section layout, support and wall sealing, inspection evidence and packing. If performance is guaranteed, separately freeze the operating and droplet basis plus acceptance method. A build-to-drawing replacement does not imply tower-capacity performance.

Use the fouling guide to interpret deposits and the installation guide for large-section access.

Engineering source and transfer limits

Koch-Glitsch’s Severe Services identifies high-vacuum, refinery and evaporator duties as pressure-drop- and fouling-sensitive, and explains why open vane paths and irrigation can be considered where mesh plugging is a risk. This supports the selection logic; proprietary pressure-drop and capacity figures apply only to the named geometry and stated basis.