Boilers deaerator venting

Knox has provided the following graphs for estimating the required vent steam from boiler feedwater deaerators. Vent steam rate depends upon the type of deaerator (spray or tray type) and the percentage of makeup water (in contrast to returning condensate). Low makeup water rates require relatively lower steam vent rates, but there is a minimum rate required to remove CO2 from the returning condensate. 

NOTE Some recirculation of ammonia and amine takes place within the overall boiler plant system, although at higher pH much of the ammonia is lost at the deaerator vent. In practice, this recirculation coupled with low er-than-theoretical C02 liberation (as a result of the incomplete breakdown of sodium bicarbonate when present in the boiler) typically results in a reduced amine-demand for any particular boiler pressure. This reduced demand, compared with the apparent demand, results in real cost savings. 

As methanol emissions continue to be monitored more closely, there are some methods of reducing the methanol in the deaerator vent. The vent stream could be condensed and sent to the reformer or the steam system. Catalytic combustion could be used to reduce the methanol. A scrubber system could be added to remove the methanol. In some instances a condensate stripper is added instead of the deaerator to remove the ammonia, methanol, and other contaminants from the condensate. This system recycles the vent stream to the reformer as process steam and the bottoms are mixed with the incoming boiler feedwater makeup. However, this system adds considerable capital cost to a project. 

Thus, in this example, assumption of the deaeration steam allows the steam balance to be closed. However, this is based on an assumed deaerator flow. The actual flow to the deaerator can be calculated from a heat balance around the deaerator. Figure 23.23 shows the flows into and out of the deaerator. If the boiler feedwater flow and condensate flows are known, together with an assumed value of the vent steam, then the flowrate of deaeration steam can be calculated from an energy balance. 

In another process plant, the steam system operates with a large amount of LP vent. To prevent loss of valuable condensate due to LP vent, a condenser was installed to cool down the LP steam and return LP condensate back to the deaerator. Although this solution saves condensate, it did not resolve the LP long issue. It was later identified that a driver switch could help to reduce the LP vent In the boiler house, there are three forced draft (FD) fans currently run by MP-LP extraction turbines but fans have spare motor drivers. The operation policy acceptable to the plant was to use steam turbines for reliability reasons. The engineer wanted to establish the value of the driver switch to minimize the LP dump. 

One approach involves fihn-forming materials, such as sodium silicate, oils, or polyphosphates. Sodium sihcate reduces corrosion but cannot prevent it entirely. A very successful approach is the use of long-chain nitrogenous compounds as film formers for condensate and return lines. They do not normally accumulate in the boiler because they are either eliminated at the vent of the deaerating heater or steam distil from the boiler water. 

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