Noncondensables, venting

A greater amount of steam would be generated if the noncondensible vent was treated using catalytic incineration rather than absorption. The... 

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.

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. 

Figure 17.5 Pressure control by condenser flooding, (a) Control valve in condenser outlet ib) flooded reflux drum (c) flooded reflux drum with automatic noncondensables venting [d) hot vapor bypass (c) a poorly pip hot vapor bypass if) control valve in condenser inlet. (Part c from "Unusual Operating Histories of Gas Processing and Olefins Plant Columns, H. Z. Kister and T. C. Hower, Jr., Plant/Operations Progi a, vol. 6. no. 3, p. 153 (July 1987). Reproduced by permis-...

Poor steam quality can reduce the performance of the steam jets used to vent the noncondensibfes from the evaporator system. Also, steam quality can cause erroneous readings of steam rates during the evaluation of the system and can indicate poorer efficiencies than are actually experienced. Air leaks will overload the noncondensible venting system this reduces the vacuum capability of both mechanical pumps and steam jets and increases the system pressures. In every case an Increase In pressure will reduce the AT. The AT loss is overcome by Increasing the steam pressure and rate to maintain capacity. 

The applicability of the SulFerox process to geothermal gas clean up depends on the partition of H2S between the condensate and noncondensable gases, which depends on the level of ammonia in the steam and the condenser design. As the ammonia level rises, the amount of H2S dissolved in the steam condensate increases. This makes the SulFerox process less attractive because separate condensate treatment can be required. Since the noncondensable gases are contacted with much less water with indirect cooling than with direct contact condensation, less H2S is dissolved and a major fraction remains in the gas phase. Therefore, operating plants that have been retrofitted with surface condensers are more likely to yield improved economics when the SulFerox process is used to treat the noncondensable vent gas. 

One problem with flooded condenser pressure control is related to the need to occasionally vent noncondensables. This vent valve must not leak when closed. Since the drum is normally full of liquid, a leaking vent valve will pass liquid. Many pounds of product can be lost in this way. A butterfly control valve with a soft, rubberized seat is a good choice for a remotely controlled, noncondensable vent valve. 

Calcium C rbon te. Calcium carbonate, like R2O2, affects sulfuric and oleum consumption in the HF process. Sulfuric acid loss is approximately 0.98% H2SO4 for each percentage of CaCO. The carbon dioxide evolved by the reaction increases the noncondensable gas flow, and because it carries HF, contributes to yield losses in the vent stream. 

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. 

The HCl gas is absorbed in water to produce 30—40% HCl solution. If the HCl must meet a very low organic content specification, a charcoal bed is used ahead of the HCl absorber, or the aqueous HCl solution product is treated with charcoal. Alternatively, the reactor gas can be compressed and passed to a distillation column with anhydrous 100% Hquid HCl as the distillate the organic materials are the bottoms and are recirculated to the process. Any noncondensible gas present in the HCl feed stream is vented from the distillation system and scmbbed with water. 

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. 

The noncondensable gases eventually reach the condenser (unless vented from an effect above atmospheric pressure to the atmosphere or to auxiliary vent condensers). These gases will be supplemented by air dissolved in the condenser water and by carbon dioxide given off on decomposition of bicarbonates in the water if a barometric condenser is used. These gases may be removed by the use of a water-jet-type condenser but are usually removed by a separate vacuum pump. 

The top pressure controller varies the level of liquid in the condenser, so it, like the reboiler, must have extra surface for the derating required for control. Many other control methods also require some control surface. If noncondensibles are present, a vent should be provided. Otherwise, they collect at the liquid seal. With large amounts of noncondensibles, another type of system should be considered. 

For steam side noncondensibles, a proper vent is required. A small amount of noncondensibles can greatly lower the steam side heat transfer coefficient. The improper removal of condensate is another way to reduce... 

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 ... 

A similar reservoir is desirable where multiple loads discharge through individual steam traps to a common pump. Provision should then be made for venting noncondensables and flash steam which reach the reservoir. The exhaust line from the pump can then be connected to the same vent line. 

The oxygen thus released concentrates in the upper zone of the deaerator with other noncondensable gases, passes through a small vent condenser, and then exits the DA through an atmospheric vent together with a limited amount of steam. 

The mist mixture strikes a baffle plate that separates water, steam, and other gases. Hot, gas-free water then falls to a lower storage compartment while the steam and noncondensable gases rise and are vented. 

Device used to vent noncondensable gases from a boiler during startup and shutdown procedures. 

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