Height of a Packed Tower

Therefore, the gas mass velocity is 1372 lb/hr ft for 60% flooding condition with a gas mass flow of 1.28 x 10 lb/hr. It is determined that 3 towers are required each one 17.6 ft in diameter. The height of a transfer unit is a 11.4 ft and the total height of a packed tower is 22.8 ft. Pressure drop in the tower is 0.41 lb/in2. A summary of the composition of various streams is given in Table IV and stripper characteristics are given in Table III. 

As described in Problem MT0.3, the height of a packed tower can be calculated by... 

The height of a packed tower required for a given service is solely determined by the mass transfer characteristics of the system and is usually the first item to be addressed in design procedures. The tower diameter, which can be equally important, often appears as an afterthought toward the end of the proceedings. What we wish to do here is provide some guidelines for its early determination. 

The HETP of a packed-tower section, valid for either distillation or dilute-gas absorption and stripping svstems in which constant molal overflow can be assumed and in which no chemical reactions occur, is related to the height of one overall gas-phase mass-transfer unit Hqc by the equation... 

From this and the gas mass velocity, it is determined that 3 towers are needed, each one with a diameter of 18.5 ft. The tower height is determined by the height of a transfer unit which is 20.7 ft. 05). Therefore, the height of the packed tower is 41.4 ft. Other detailed tower characteristics are given in Table III. 

The height of the packing tower needed for the removal of a pollutant to a certain effluent concentration can be estimated using the following equations ... 

The rate of sensible-heat transfer from the liquid to the interface in a differential height of the packed tower, dz, is... 

The most common applications of this technique in distillation and absorption columns is for liquid level and liquid level interface detection, especially when normal level-measuring techniques suffer from plugging. Neutron backscatter techniques have also been used for froth height measurements on trays and downcomers, and for measuring the top and bottom of packed beds. One case history has been described (71) where downcomer froth height measurements using the neutron backscatter technique led to a detection of downcomer deposits which caused premature flooding of the column. The author is familiar with one case where this technique successfully detected overflow of a packed tower distributor. 

The following is a brief discussion for the calculation of the height of a packed absorption tower, Z. Because of space limitation, presentation is given for the case of transfer unit based on gas film ... 

Analyzing a packed tower involves both mass transfer and fluid mechanics. The mass transfer, detailed in Section 10.3, determines the height of the packed tower. In other words, it determines how tall the tower needs to be. This mass transfer is described as molar fiows, partly because of the chemical reactions that often occur. The fluid mechanics, described in this section, determines the cross-sectional area of the packed... 

Computation of Tower Height The required height of a gas-absorption or stripping tower depends on (1) the phase equilibria involved, (2) the specified degree of removal of the solute from the gas, and (3) the mass-transfer efficiency of the apparatus. These same considerations apply both to plate towers and to packed towers. Items 1 and 2 dictate the required number of theoretic stages (plate tower) or transfer units (packed tower). Item 3 is derived from the tray efficiency and spacing (plate tower) or from the height of one transfer unit (packed tower). Solute-removal specifications normally are derived from economic considerations. 

The height of a theoretical unit (Hqq) can be obtained from the aetual process conditions whenever possible. Tower packing manufacturers have information on a wide variety of specific packing systems that enable the determination of Hqc-Alternatively, this information can be estimated from values of K a using he following relationship ... 

Ammonia is removed from a 10 per cent ammonia-air mixture by scrubbing with water in a packed tower, so that 99.9 per cent of the ammonia is removed. What is the required height of tower The gas enters at 1.2 kg/m2s, the water rate is 0.94 kg/m2s and KGa is 0.0008 kmol/sm3 (kN/m2). 

Ammonia is to be recovered from a 5 per cent by volume ammonia-air mixture by scrubbing with water in a packed tower. The gas rate is 1.25 m3/s m2 measured at NTP and the liquid rate is 1.95 kg/m2s. The temperature of the inlet gas is 298 K and of the inlet water 293 K. The mass transfer coefficient is Kaa = 0.113 kmol/m3 s (mole ratio difference) and the total pressure is 101.3 kN/m2. What is the height of the tower to remove 95 per cent of the ammonia. The equilibrium data and the heats of solutions are ... 

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