Noncondensible vapor venting

Process Description. Reactors used in the vapor-phase synthesis of thiophene and aLkylthiophenes are all multitubular, fixed-bed catalytic reactors operating at atmospheric pressure, or up to 10 kPa and with hot-air circulation on the shell, or salt bath heating, maintaining reaction temperatures in the range of 400—500°C. The feedstocks, in the appropriate molar ratio, are vaporized and passed through the catalyst bed. Condensation gives the cmde product mixture noncondensable vapors are vented to the incinerator. 

Provide a place from which noncondensable vapors may be vented Partial condensation... 

For a surface condenser to work properly, noncondensable vapors must be sucked out of llie shell side. This is done with a two-stage jet system, as shown in Fig. 18.3. When I was first commissioned the jets, they were unable to pull a good vacuum. Moreover, water periodically blew out of the atmospheric vent. I found, after considerable investigation, that the condensate drain line from the final condenser was plugged. 

Approximately 1 of air enters the extractor with each 1 of material. To maintain the extractor under a slight vacuum, the air must be continuously vented from the extractor. At the extractor temperature of 60°C, an equilibrium condition will occur where the vent gas exiting the extractor will contain approximately 10 parts of solvent vapor for every one part of air (see Figure 12). The solvent vapor and air exit the extractor and are typically condensed in an extractor condenser. The extractor condenser is a shell and tube vessel with the vapors typically on the shell side and the coohng water on the tube side. The noncondensable vapors exiting the extractor condenser pass on to the vent condenser. 

Figure 11-14 Forced-circulation evaporator with internal heating element A, heating element B, vapor head C. liquor-return pipe D, circulating pump E, deflector F, cylindrical baffle G, noncondensed-gat vent H, condensate outlet. (Swenson, Courtesy of American institute of Chemical Engineers.)...

Venting Rates Noncondensibles enter the system with the feed or by system leaks. These noncondensibles must be vented at a rate that is sufficient to maintain pressure control and heat transfer. If venting rates are too low, the loss of heat transfer must be recovered by additional steam at a higher pressure to increase the temperature differences for heat transfer. However, excessive venting of the noncondensibles does not improve operation high venting rates will require additional heating steam to offtet the losses from vapors vented to the atmosphere or condenser. 

Only noncondensable vapors should be discharged. (Pressure relief valves capable of venting liquids should discharge through a separator or flare drum so that the liquids are recovered, and also to prevent the liquids falling to the ground.)... 

One of my friends, Steve, made such an error in designing a debutanizer. He failed to account for the methane and ethane in the existing debutanizer feed stream. The sample from the 25-psig feed vessel was taken in a bottle. The lighter components weathered off prior to lab analysis. The results were, for Steve, rather catastrophic. The debutanizer overhead product could not be fully condensed. The noncondensable vapor pressured up the overhead reflux drum. The noncondensables were vented, along with 30 percent of the butane, to... 

Fig. 1. Natural ckculation evaporators where C = condensate, E = entrainment return, F = feed, N = noncondensibles vent, P = product or concentrate, S = steam, V = vapor, and M = knitmesh separator (a) horizontal-tube, (b) short-tube vertical, (c) propeUer calandria, and (d) long-tube reckculating.

In any event, noncondensable gases should be vented well before their concentration reaches 10 percent. Since gas concentrations are difficult to measure, the usual practice is to ovei vent. This means that an appreciable amount of vapor can be lost. 

To nelp consei ve steam economy, venting is usually done from the steam chest of one effecl to the steam chest of the next. In this way, excess vapor in one vent does useful evaporation at a steam economy only about one less than the overall steam economy. Only when there are large amounts of noncondensable gases present, as in beet-sugar evaporation, is it desirable to pass the vents directly to the condenser to avoid serious losses in heat-transfer rates. In such cases, it can be worthwhile to recover heat from the vents in separate heat exchangers, which preheat the entering feed. 

For partial condenser systems, the pressure can be controlled by manipulating vapor product or a noncondensible vent stream. This gives excellent pressure control. To have a constant top vapor product composition, the condenser outlet temperature also needs to be controlled. For a total condenser system, a butterfly valve in the column overhead vapor line to the condenser has been used. Varying the condenser cooling by various means such as manipulation of coolant flow is also common. 

Accumulation of Noncondensihles - Noncondensibles do not accumulate under normal conditions since they are released with the process vapor streams. However, with certain piping configurations, it is possible for noncondensibles to accumulate to the point that a condenser is "blocked". Such a condition could occur if an automatic vent control valve failed closed for a period of time. This effect is equal to a total loss of coolant, and thus need not be considered separately. 

The vapor outlet should preferably be connected to the flare system. However, when the safety valve releases and other streams tied into the drum contain only a small quantity of noncondensible hydrocarbons or inerts, and where no pollution problems are anticipated, then an atmospheric vent is acceptable, subject to the following conditions ... 

Finally, the receiver will accumulate any noncondensable (or hard-to-condense) components that have accidentally entered the system. Air left in the vessels on start-up is one such example. Traces of methane and ethane in a propane refrigerant system is another. These light vapors may be vented from the top of the receiver during normal operations. 

In some cases, as with pulp-mill liquors, the evaporator vapors contain constituents more volatile than water, such as methanol and sulfur compounds. Special precautions may be necessary to minimize the effects of these compounds on heat transfer, corrosion, and condensate quality. They can include removing most of the condensate countercurrent to the vapor entering an evaporator-heating element, channeling vapor and condensate flow to concentrate most of the foul constituents into the last fraction of vapor condensed (and keeping this condensate separate from the rest of the condensate), and flashing the warm evaporator feed to a lower pressure to remove much of the foul constituents in only a small amount of flash vapor. In all such cases, special care is needed to properly channel vapor flow past the heating surfaces so there is a positive flow from steam inlet to vent outlet with no pockets, where foul constituents or noncondensibles can accumulate. 

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