Acid dew point

Example 6.4 The process in Fig. 6.2 is to have its hot utility supplied by a furnace. The theoretical flame temperature for combustion is 1800°C, and the acid dew point for the flue gas is 160°C. Ambient temperature is 10°C. Assume = 10°C for process-to-process heat transfer but = 30°C for flue-gas-to-process heat transfer. A high value for for flue-gas-to-process heat... 

Figure 6.30 shows the grand composite curve plotted from the problem table cascade in Fig. 6.186. The starting point for the flue gas is an actual temperature of 1800 C, which corresponds to a shifl ed temperature of (1800 — 25) = mS C on the grand composite curve. The flue gas profile is not restricted above the pinch and can be cooled to pinch temperature corresponding to a shifted temperature of 145 C before venting to the atmosphere. The actual stack temperature is thus 145 + 25= 170°C. This is just above the acid dew point of 160 C. Now calculate the fuel consumption ...

Small amounts of sulfuric acid mist or aerosol are always formed in sulfuric acid plants whenever gas streams are cooled, or SO and H2O react, below the sulfuric acid dew point. The dew point varies with gas composition and pressure but typically is 80—170°C. Higher and lower dew point temperatures are possible depending on the SO concentration and moisture content of the gas. Such mists are objectionable because of both corrosion in the process and stack emissions. 

Under normal operating conditions, the concentration of the trioxide is unlikely to exceed 10 ppmv, but this is sufficient to elevate the acid dew point to around 422 K (300°F). This places a limit on the lowest acceptable back-end temperature if acid condensation and resulting corrosion problems are to be avoided. 

When cooling combustion flue gas for heat recovery and efficiency gain, the temperature must not be allowed to drop below the sulfur trioxide dew point. Below the SO3 dew point, very corrosive sulfuric acid forms. The graph in Figure 1 allows determination of the acid dew point us shown in Example 1. 

Acid dew point The temperature at which a vapor containing an acid appears as condensate on a cool surface, causing corrosion. 

The production of sulfuric acid from condensing water vapor is dependent upon the acid dew point temperature, which varies from... 

The split between the radiant and convection section heat varies according to the design. Casing losses are usually between 1 and 3% of the heat release from combustion. The heat loss from the stack is constrained by the desire to avoid any condensation of water vapor in the convection section. If there is any sulfur present in the fuel, then the condensate will be corrosive. The temperature at which the flue gas starts to condense is the acid dew point. For sulfurbearing fuels, the temperature of the flue gas is normally... 

Obviously, the lower the stack temperature, the higher the furnace efficiency. As already noted, it is desirable to avoid condensation in the convection section and the stack. If there is any sulfur in the fuel, the condensate will be corrosive. There is thus a practical minimum to which a flue gas can be cooled without condensation causing corrosion in the stack, known as the acid dew point. If there is any sulfur in the fuel, the stack temperature is normally kept above 150 to 160°C. Natural gas can normally be cooled to... 

In Figures 16.27 and 16.28, the flue gas is capable of being cooled to pinch temperature before being released to atmosphere. This is not always the case. Figure 16.29a shows a situation in which the flue gas is released to atmosphere above pinch temperature for practical reasons. There is a practical minimum, the acid dew point, to which a flue gas can be cooled without condensation causing corrosion in the stack (see Chapter 15). The minimum stack temperature in Figure 16.29a is fixed by acid dew point. Another case is shown in Figure 16.29b where the process away from the pinch limits the slope of the flue gas line and hence the stack loss. 

FIG. 24-56 Calculated sulfuric acid dew points, as a function of SO3 content, for various flue gas water vapor concentrations. Courtesy W. M. M. Huijbregte,... 

Acid Dew Point For fossil fuels, the acid dew point temperature is that temperature at which the actual mixed acid vapor pressure equals the mixed acid vapor saturation pressure. The mixed acid dew point can be approximated by the sulfuric acid dew point (Fig. 24-56). It can be described as a function of the SO3 and water content of the flue gas (Huijbregts). These concentrations result from the sulfur, hydrogen, and free water content of the fuel the relative humidity of the air and the amount of excess air used. Using the equation of Ver-hoff, where T is degrees K and P is mm Hg (see OUces, A.G.) ... 

Conventional Economizers Conventional economizers can be constructed from relatively inejq)ensive materials, such as low-alloy carbon steels, if they will be operated dry on the gas side, with flue gas side metal temperatures above the acid dew point. This practice is done to protect the economizer from corrosion, caused by the acidic flue gas condensate. Conventional economizers can also be con-... 

Obtain data on the quantity and composition of other emissions from the combustion of the synthetic fuels such as particulate loading, particulate morphology, hydrocarbons, chlorides and flue gas acid dew point temperature. 

Figure 1.3 shows a single drum overhead system. Double drum systems are also used. The difference between the two systems is the reflux temperature at the top of the tower. In the single drum system, total liquid condensation occurs in the overhead condensers. The reflux will be cool and will keep the tower top cool. It is advisable to check the hydrochloric acid dew point vs partial pressure to determine the anticipated location of corrosion. For example, tower top temperatures above 250° F (120°C) can transfer corrosion to the cold reflux. Where dew point conditions exist in the tower, It may be desirable to add ammonia to the reflux to neutralize the acid. 

Both neutralizers are injected in the fractionator overhead line in order to be present when the dew point of hydrochloric acid in solution is reached. It is important to use a quill to inject neutralizers or inhibitors because drip injection can cause dissolution of the protective scale on the inside of the pipe, which can result in corrosion and erosion in that area. Often, however, neutralization is not accomplished, and severe corrosion from hydrochloric acid still occurs at the dew point. The pH is controlled at the overhead receiver water draw because dew point pH measurement is not feasible. One method of controlling the dew point pH is to recycle water from the drum to the overhead line. This water buffers the condensate at the hydrochloric acid dew point and also provides water in which the ammonia can dissolve. 

With sulfur-containing fuels, attention has to be paid to the acid dew point of the flue gas. Acid dew point in flue gas is shown as a function of sulfur content in the fuel oil in Figure 7. To avoid corrosion in the upper bundles, it has to be ensured that the tube skin temperature is, in all cases, higher than the acid dew point. 

Figure 7. Acid dew point as a function of sulfur content In fuel oil...

In the past, specifically prior to the required installation of SO2 scrubbers, most chimneys were subjected to hot, dry, seldom acidic conditions. Typical flue gas temperatures exceeded 400°F and were therefore above the acid dew point during normal plant operation. Furthermore, the plant itself experienced fewer shutdowns due to the intermittent operational difficulties inherent within the scrubbed gas systems themselves. Thus, chimney linings were not exposed to severe acidic conditions other than at infrequent start-ups and shutdowns, For this reason, wet acid corrosion was not a major design factor, and chimneys were relatively simple to design, construct and maintain. There were a few relatively common designs, which will be briefly described. 

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