Non-condensables

Separation of mixtures of condensable and non-condensable components. If a fluid mixture contains both condensable and noncondensable components, then a partial condensation followed by a simple phase separator often can give a food separation. This is essentially a single-stage distillation operation. It is a special case that deserves attention in some detail later. 

Still overhead—light naphthas. Steam and non-condensable gas 60-70... 

Can dangerous materials build up in the process, e.g. traces of combustible and non-condensible materials ... 

Just inside the shell of the tube bundle is a cylindrical baffle F that extends nearly to the top of the heating element. The steam rises between this baffle and the wall of the healing element and then flows downward around the tubes. This displaces non-condensed gases to the bottom, where they are removed at G. Condensate is removed from the bottom of the heating element at H. This evaporator is especially suited for foamy liquids, for viscous liquids, and for those liquids which tend to deposit scale or crystals on the heating surfaces. Vessel J is a salt separator. 

Figure 11. Vertical tube evaporator (A) Tube sheets (B) Downtake (C) Condensate outlet (D) Non-condensed gas outlet (E) Liquor inlet (F) Thick liquor outlet.

Evaporators, Horizontal-Tube Type - The basic horizontal-tube evaporator is illustrated in Figure 12. The body of this evaporator is the liquor compartment and is in the form of a vertical cylinder. It is closed, top and bottom, with dished heads, although the bottom may be conical. The lower body ring is provided on opposite sides with steam compartments, closed on the outside by cover plates and on the inside by tube sheets. Between these tube sheets are fastened a number of horizontal tubes. The two steam chests with their connecting mbes form the steam compartment, and the tube wall heating surface. Steam is introduced into one steam chest and as it flows through the tubes it washes non-condensed gases and condensate ahead of it, so that these are withdrawn from the opposite steam chest. 

In ordinary operation only condensate and non-condensed gases are removed from the exit steam chest. The connection for feeding the liquid to be evaporated may be attached to the body at any convenient point (D), but the discharge for thick liquor is usually in the center of the bottom (E). Suitable brackets are cast on the bottom to rest on the supporting steel. Most evaporators are provided with sight glasses. 

Figure 13. Steam outside tube evaporator (A) Shell (B) Tube sheets (C CJ Distributing plates (D) Vapor head (E) Baffles (F) Steam inlet (G) Condensate outlet (H) Non-condensed gas vent (J) Thick liquor outlet (K) Vapor outlet (L) Liquor feed box.

Cracking imposes an additional penalty in a vacuum unit in that it forms gas which cannot be condensed at the low pressures employed. This gas must be vented by compressing it to atmospheric pressure. This is accomplished by means of steam jet ejectors. Ideally, it would be possible to operate a vacuum pipe still without ejectors, with the overhead vapors composed only of steam. In practice, however, leakage of air into the system and the minor cracking which occurs make it necessary to provide a means of removing non-condensibles from the system. In addition to the distillation of atmospheric residuum, the lube vacuum pipe still is also used for rerunning of off specification lube distillates. 

The VPS overhead consists of steam, inerts, condensable and non-condensable hydrocarbons. The condensables result from low boiling material present in the reduced crude feed and from entrainment of liquid from the VPS top tray. The noncondensables result from cracking at the high temperatures employed in the VPS. Inerts result from leakage of air into the evacuated system. Steam and condensable hydrocarbons are condensed using an overhead water-cooled condenser. The distillate drum serves to separate inerts and non-condensables from condensate, as well as liquid hydrocarbons from water. Vacuum is maintained in the VPS using steam jet ejectors. 

The header is normally a 80 mm diameter pipe (50 mm may be adequate for small units) and is routed via an overhead pipe rack (which is generally sloped) to a non-condensible blowdown drum. 

Select a condensible blowdown drum for condensible releases, rather than the non-condensible type. If a condensible blowdown drum is not suitable for handling the total blowdown service (e.g., if cold liquids are involved), then a combination of a condensible and a non-condensible drum may be used. 

Lx)cate the blowdown drum (when the non-condensible type is used) at a minimum permissible spacing from the flare, to minimize condensation in the flare header. 

They are able to disengage oil mist better than non-condensible types. ... 

A typical non-condensible blowdown drum and its associated equipment and headers are illustrated in Figure 1. A single blowdown drum may be used for more than one process unit, if economically attractive. However, when this is done, all units served by it must be shut down in order to take the drum out of service, unless cross connections are made to another system of adequate capacity. Normally all closed safety valve discharges are combined into one header entering the drum, although separate headers and inlet nozzles are acceptable if economically advantageous. The following releases are also normally routed into the safety valve header ... 

Figure 1. Typical non-condensible blowdown drum arrangement.

The first vessel in the blowdown system is therefore an acid-hydrocarbon separator. This drum is provided with a pump to transfer disengaged acid to the spent acid tank. Disengaged liquid hydrocarbon is preferably pumped back to the process, or to slop storage or a regular non-condensible lowdown drum. The vented vapor stream from the acid-hydrocarbon separator is bubbled through a layer of caustic soda solution in a neutralizing drum and is then routed to the flare header. To avoid corrosion in the special acid blowdown system, no releases which may contain water or alkaline solutions are routed into it. 

Liquid hydrocarbons accumulated in non-condensible blowdown drums, originating from safety valves, closed drain headers, knockout drum drainage, etc. Facilities are normally provided at the drum for weathering volatile liquids and cooling hot liquids before disposal. 

As an alternative to special pressure slop storage, the necessary holdup may be provided in a non-condensible blowdown drum. 

Placing the fluid through the tubes is a consideration when special alloy materials aie needed for corrosion control, because the materials would be needed only on the tubes. If the corrosive material is in the shell, both the tubes and the shell would need to be protected with special alloy. It the fluid is at high pressure, it should be put in tubes because tubes can contain high pressure much more cheaply as they are much smaller in diameter than the shell. The low-pressure fluid would be in the shell. If the fluid contains vapor and non-condensable gases, heat transfer will be greater if it is placed in the tubes. If the fluid is scale forming it should be in the tubes, which can be reamed out. 

Based on handling pure liquids, without entrained air or other non-condensable gases, which adversely affect the pump performance. 

Absolute pressure at the pump inlet must not be low enough to release non-condensables of (2). If such release can occur, then the NPSHr would need to be increased above that of the cold water requirements to avoid cavitation and poor pump performance. 

Figure 6-3 illustrates a single-stage non-condensing ejector. In this type of installation the steam outlet from the ejector is either exhausted to atmosphere or on top of water in a sump. 

Figures 6-5 and 6-6 illustrate two-stage ejector installations with barometric and surface t) pe inter-after condensers respectively. The discharge of the steam non-condensables from the second stage jet of Figure 6-5 is exhausted to the atmosphere, while in Figure 6-6 the steam is condensed in the aftercondenser and, essentially, only non-condensables leave the ent of the aftercondenser. Figure 6-7A indicates a diagram of a three-stage barometric type installation.

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