What is a spray tower and its importance in mass transfer? What kind of gas solu
ID: 473588 • Letter: W
Question
What is a spray tower and its importance in mass transfer? What kind of gas solubility is required in it? The mass transfer resistance will be more in gas or liquid phase for spray tower? What are the HTU and WTU and their significance? How do you increase the value of HTU in a tower? In a wetted wall tower where ammonia, NH3, is desorbed from an ammonia-water solution into an air stream, the overall gas coefficient. Kg is 3.12 x 10^-9 k mol/(Am^2.s.Pa).At plane in the tower, the bulk concentration of the falling aqueous stream is 4 k mol/m^3 of solution and the partial pressure of ammonia in the rising gas stream is 3.04 x 10^3 Pa. For dilute solutions of ammonia in water the equilibrium partial pressure may be evaluated by Henry's law: If the gas phase comprises 75% of the total resistance to mass transfer, calculate the individual gas -film coefficient, k_g; the individual liquid-film coefficient, k_l; the overall liquid-film coefficient, K_l; the interfacial concentrations, p_aj and c_aj.Explanation / Answer
a) A spray tower (or spray column or spray chamber) is gas-liquid contactor used to achieve mass and heat transfer between a continuous gas phase (that can contain dispersed solid particles) and a dispersed liquid phase.Spray towers consist of vacant cylindrical vessels containing nozzles that spray liquid into the vessels. The inlet stream typically enters the bottom of the tower and moves upward, while liquid is sprayed downward from one or more levels. Counter-current flow contacts the gas with the lowest pollutant concentration to the cleanest scrubbing liquid.
Spray towers have been used effectively to remove large particles and highly soluble gases. The pressure drop across the towers is very low – usually less than 2.5 cm (1.0 in) of water; thus, scrubber operating costs are relatively low. However, the liquid pumping costs can be very high. Spray towers are constructed in various sizes – small ones to handle small gas flows of 0.05 m³/s (106 ft³/min) or less, and large ones to handle large exhaust flows of 50 m³/s (106,000 m³/min) or greater. Because of the low gas velocity required, units handling large gas flow rates tend to be large in size.
Spray towers can be very effective in removing pollutants if the pollutants are highly soluble or if a chemical reagent is added to the liquid.
In spray towers, gas phase resistance generally controls the mass transfer rate.
b) The NTU method is more appropriate because the changes in compositions of the liquid and vapour phases occur differentially in a packed column rather than in stepwise fashion as in trayed column.
The packed height (z) is calculated using the following formula:
z = N x H
where
N = number of transfer units (NTU) - dimensionless
H = height of transfer units (HTU) - dimension of length
The number of transfer units (NTU) required is a measure of the difficulty of the separation. A single transfer unit gives the change of composition of one of the phases equal to the average driving force producing the change. The NTU is similar to the number of theoretical trays required for trayed column. Hence, a larger number of transfer units will be required for a very high purity product.
The height of a transfer unit (HTU) is a measure of the separation effectiveness of the particular packings for a particular separation process. As such, it incorporates the mass transfer coefficient that we have seen earlier. The more efficient the mass transfer (i.e. larger mass transfer coefficient), the smaller the value of HTU. The values of HTU can be estimated from empirical correlations or pilot plant tests, but the applications are rather restricted.
The value of HTU is dependant on Gas to liquid ratio and the temperature. Hence to increase the value of HTU, we need to increase the operating temperature and G/L ratio.
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