Tail gas expander

The elevated pressure in Proeess 2, shown sehematieally in Figure 4-6, again ineludes a tail gas expander as part of the eombustion air eompressor train (Figure 4-7). A typieal eontrol and instrument diagram for Proeess 2 is given in Figure 4-8. The nomenelature deseribed in Proeess 1 also pertains here. 

Tail gas expanders are thus an integral part of modern nitrie aeid plants. However, these turboexpanders are also part of a eombined turbomaehinery train eomprised of a prime mover and two or more eompressor easings. 

Tail gas expander without or with adjustable inlet guide vanes (11) and guide vane setting motor (12), labyrinth shaft seals with air sealing... 

Although this hook deals almost exclusively with modern turho-expanders, the tail gas expander in nitric acid plants enjoys a somewhat special relationship with the compressors that are almost always associated with this turhotrain. 

Seleetion of a tail gas expander depends on the temperature and pressure on the inlet eonditions of the nitrous gas. The pressure equals that of the eompressor outlet minus the pressure losses in the eyele. Depending on proeess and plant size, these losses amount to 0.3-2.0 bar. The inlet temperature may vary widely from plant to plant. Figure 4-19 shows an expander in the 10,000 kW power output eategory. 

Figure 4-19. Tail gas expander for 7.4 bar, 700°C maximum inlet and power output at 10,000 kW.

Most of the large tail gas expanders are axial type. Multistage uncooled expanders, illustrated in Figures 4-21 through 4-23, are... 

Figure 4-21. Tail-gas expander for a 945-t/day two-pressure nitric acid plant P = 11,200 kW, n = 5,850 rpm. (Source GHH-Borsig.)...

The other environmental impact consideration is noise. The air compressor with its tail-gas expander and steam-turbine drive could present a problem. Therefore, this unit should be enclosed in a brick building to contain the noise, with those working inside the building required to wear approved ear protection. 

A detailed design for the compressors would be required to determine the inlet and outlet velocities for the steam in the steam turbine, and the tail gas in the tail-gas expander. In this process, tail-gas expansion typically provides about 80% of the required energy with an efficiency in the production of shaft energy of less than 25%. 

The tail-gas expander takes tail gas at 620°C and approximately 950 kPa and reduces it to atmospheric pressure at a temperature at 290°C. This process enables the recovery of around 80% of the power requirements for the compressor. See Section F.2.1 for more details. 

K-IB Tail gas expander T-19 Start-up tank K-20 Steam turbine E-21 Condenser E-22 Bleach air cooler V- 23 Bleacher... 

---