Economizers, bypassing

Catalyst cost constitutes 15-20% of the capital cost of an SCR unit therefore, it is essential to operate at temperatures as high as possible to maximize space velocity and thus minimize catalyst volume. At the same time, it is necessary to minimize the rate of oxidation of S02 to S03, which is more temperature sensitive than the SCR reaction. The optimum operating temperature for the SCR process using titanium and vanadium oxide catalysts is about 38CM180oC. Most installations use an economizer bypass to provide flue gas to the reactors at the desired temperature during periods when flue gas temperatures are low, such as low-load operation. 

Fig. 22.1. Blowup of Chapter 2 l s economizer. Bypassing of input gas around the economizer (dotted line) is shown. Bypassing results in a warmer recombined output gas stream.

This requirement is greater than the cooling capacity (8.46 MJ) of the Section 22.3.1 economizer. Bypassing of gas around the economizer won t help because bypassing only decreases heat transfer. 

A new method of utilization of waste gas heat is provided by precalcining systems with bypass, in which up to 100 per cent of the kiln exit gases can be economically bypassed arrd be utilized in a steam boiler, without reqttiring arty eooling. 

Ammonia feed is shut off by a low temperature switch when the operating flue gas temperature drops below the minimum recommended value. This prevents deactivation of the catalyst from ammonium bisulfate deposition. This control feature is also applied during system startup and shutdown. An economizer bypass is used to maintain the flue gas temperature above the minimum recommended SCR operating temperature and an SCR bypass is shown (though not often provided). The SCR bypass is used to protect the SCR catalyst during startup and shutdown when the flue gas temperature can be below its dew point. Economizer bypasses are used at the Chambers, Indiantown, and Keystone power plants (Franklin, 1993). 

Keywords production decline, economic decline, infill drilling, bypassed oil, attic/cellar oil, production potential, coiled tubing, formation damage, cross-flow, side-track, enhanced oil recovery (EOR), steam injection, in-situ combustion, water alternating gas (WAG), debottlenecking, produced water treatment, well intervention, intermittent production, satellite development, host facility, extended reach development, extended reach drilling. 

Often, complete mixing cannot be approached for economic reasons. Inactive or dead zones, bypassing, and limitations of energy input are common causes. Packed beds are usually predominantly used in plug flow reactors, but they may also have small mixing zones... 

The definition of the patented process offers the opportunity to an innovative chemist to develop a process which bypasses the original patent claims and offers a new legally clear route to an economically attractive product. 

With shell boilers, economizers will generally only be fitted to boilers using natural gas as the main fuel and then only on larger units. It would be unlikely that a reasonable economic case could be made for boilers of less than 4000kg/hF and A100°C evaporative capacity. The economizer will incorporate a flue gas bypass with isolating dampers to cover for periods when oil is used and for maintenance. The dampers require electric interlocks to the selected fuel. 

Due to interruptible gas tariffs, it is often necessary to adopt gas as the primary fuel and burn oil in periods of peak loads. This means that the economizer has to be arranged so that when oil firing, the flue gases are bypassed around the economizer. The bypass duct must also contain a damper to simulate the economizer gas resistance so that the burner back pressure remains the same for both fuels. Figure 25.6 shows a typical installation layout. 

BYAS [Bypass ammonia synthesis] An economical process for expanding existing ammonia synthesis plants by introducing the additional natural gas at an intermediate stage in the process. The additional nitrogen in the air, which has also to be introduced, is removed by PSA. Developed and offered by Humphreys and Glasgow, UK. 

Unavoidable loss of gas is compensated via a feed valve for supplying virgin nitrogen into the circulation pipe. The exhaust gas of the process has to be bypassed for purification. After the separation of dust by a filter, the gas is heated to 400 °C for the catalytic combustion of the side products. The gas is then cooled down, and the excess oxygen is catalytically converted to water by using hydrogen. For economic reasons, the gas flow will recover the heat via a heat exchanger and then be cooled down by a gas cooler. 

Research is being conducted on the direct synthesis of ethylene glycol from synthesis gas. In one process very high pressures of 5,000 psi with very expensive catalysts Rhx (CO) are being studied. An annual loss of rhodium catalyst of only 0.000001% must be realized before this process will compete economically. At least five other alternate syntheses of ethylene glycol that bypass toxic ethylene oxide are being researched. 

Normally, Roots pumps are not as economical as gas ballast pumps for continuous operation at pressures above 40 mbar. With very large pump sets, which work with very specialized gear ratios and are provided with bypass lines, however, the specific energy consumption is indeed more favorable. If Roots pumps are installed to pump vapors, as in the case of gas ballast pumps, a chart can be given that includes all possible cases (see Fig. 2.74). 

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