Internal Demisters

Demisters are sometimes installed in the intertray space to eliminate entrainment. Typical applications include some clean, low-liquid-load chemical services at close tray spacing ( 20 in), and refinery vacuum 

Internal demisters are expensive, have a high plugging tendency, and their sections may disintegrate. The author is familieir with one incident where chunks of wire mesh finm an internal demister tore loose and found their way to a draw pan above and to the bottom pump. For these reasons, they should only be installed in clean services experiencing entrainment problems that cannot be eliminated by alternate means. It is a good practice to install an internal demister only in the problem regions rather than above every tray in a column. In one case (278), it was foimd that demisters installed in the nonproblem regions did little to improve performance. 

Typically, an internal demister in the intertray space is about 2 in thick and contains wire mesh. Attention must be paid to correctly specifying the mesh type, size, and materials. In a refinery vacuum column, it is also important to keep the mesh wet (eg, by spraying wash oil above the demister) in order to inhibit coking. AVhen the service has a high plugging tendency, chevrons rather than wire-mesh may be the best demister choice (270). 

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Cleaning of sludge in absorption towers and tower packing Raschig rings/lntalox saddles and internals, demister pads, candle demisters ... 

Impingement demister systems are designed to intercept liquid particles before the gas outlet. They are usually constructed from wire mesh or metal plates and liquid droplets impinge on the internal surfaces of the mist mats or plate labyrinth as the gas weaves through the system. The intercepted droplets coalesce and move downward under gravity into the liquid phase. The plate type devices or vane packs are used where the inlet stream is dirty as they are much less vulnerable to clogging than the mist mat. 

Knockout vessels are the most common form of basic separator. The vessel contains no internals and demisting efficiency is poor. However, they perform well in dirty service conditions (i.e. where sand, water and corrosive products are carried in the well stream). 

The generated water vapor rises through a screen (demister) placed to remove entrained saline water droplets. Rising further, it then condenses on the condenser tube bank, and internal heat recovery is achieved by transferring its heat of condensation to the seawater feed that is thus being preheated. This internal heat recovery is another of the primary advantages of the MSF process. The energy performance of distillation plants is often evaluated by the performance ratio, PR, typically defined as... 

Effectiveness of Internals in Controlling foam. Originally, the separators contained limited internals—an inlet disengaging device, surge baffles, an anti-foam baffle, and a demister at the outlet. After severe foaming occurred, however, a bank of parallel plates (Dixon plates) was filled in each separator on one of the Ninian and one of the Brent trains The plates provide a large surface area and are sloped so that the separated oil flows dow n to the surface of the liquid. The installation is illustrated m Ftg S. The parallel plates arc 4 in. apart and extend from near the top of the vessel in the vapor space down to the normal liquid level. 

Other variants of the gas clean up system have been tried including shell and tube condensers, rotary particle separation in a gas centrifuge and other various filters and demisters. These various unit operations were recommended and tested by Marick International however, success was extremely limited and is summarised in Table 1. 

The net mass velocity of chlorine through a packed suction chiller is typically 200-250 tpd m . Pall rings are frequently used as packing, in beds 2.5-3 m high. This makes the packing requirement 0.01-0.015 m per tpd. Carbon steel is the main material of construction, but some of the internals are stainless steel or other material with better corrosion resistance. These internals include the demister pad and sometimes support rings and packing. 

Demister pads or mist eliminators, are important internals in process vessels. Anytime there is a continuous two phase flow, vapor and liquid, there is the possibility for liquid entrainment. If it is desirable to separate the liquid and vapor, or prevent liquid carryover in the vapor stream, the velocity must be kept sufficiently low to allow flie liquid droplets to fall out of the vapor stream. Demister pads are effective entrainment separators which allow operation at vapor velocities that would otherwise be excessive. 

The residue from an atmospheric distillation tower can be sent to a vacuum distillation tower, which recovers additional liquid at 0.7 to 1.5 psia (4.8 to 10.3 kPa). The vacuum, which is created by a vacuum pump or steam ejector, is pulled from the top of the tower. Relative to atmospheric columns, vacuum columns have larger diameters and their internals are simpler. Often, instead of trays, random packing and demister pads are used. 

Provide for intake of unpolluted air into internal combustion engines, gas turbines, and boilers. Demisters, filters, chemical filters, preheaters or air dryers may be required to remove adverse solid, liquid, or gaseous pollutants. 

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