1. Packed Column Humidifier
Mallare, Tristan Howell C.
5ChEC

2. Humidification
Evaporative humidification has effect when a liquid is in direct contact with a gas. The liquid evaporates into the gas because of the vapor concentration difference that exists between the liquid and the gas at the interface. Thus, the humidity of the gas increases along the tower.

3. Packed Column
Packed column is a hollow tube, pipe, or other vessel that is filled with a packing material. The packing can be randomly filled with small objects like Raschig rings or else it can be a specifically designed structured packing.
The purpose of a packed bed is typically to improve contact between two phases in a chemical or physical process.

4. Types of Packings

5. Structured Packing

6. Random packing

7. Choice of packing
Structured packings give a high surface area with a high void fraction. The advantage of structured packings over random dumped packings is their low pressure-drop, which is very significant in the gas turbine economy, and effective mass transfer characteristics.
The cost of structured packing per cubic meter is significantly higher than the random packing; however, the much higher efficiency of the structured packing compensates the higher cost.

8. Rationale

9. Evaporative Gas Turbine Cycle (EvGT)
also known as the humid air turbine (HAT). The EvGT concept involves addition of water vapor to the compressed air by evaporation of water in a humidifier. Humidification of the compressed air utilizes the low value exhaust gas heat since humidification water is used as the heat sink in cooling of the exhaust gas.

10. Characteristics of EvGT
The evaporative cycle is basically characterized by:
• High efficiency; • Low specific investment costs; • Low NOx emissions; • Good part-load features; • Quick start-up time; and • Compact size.

11. Humidification in EvGT
Humidification in the EvGT system is a process of adding water vapor to the compressed air stream by evaporation. The increase in the working medium (air-water vapor mixture) mass flow rate without additional compressor work, together with the enhanced heat recovery potential concomitant with the introduction of water vapor, boosts the electric power output and the overall efficiency of this cycle.

12. Feedstock Dry air inlet (Mass rate, Humidity, Temperature) G=0.9 kg/s
H= kg H2O / kg dry air
T= C
 Water inlet (Mass Rate, Temperature)
L=3.5 kg/s
T= C
Working pressure: x 10^5 N/m2
Required: Outlet air humidity of 0.1 kg H2O/kg air

13. Assumptions Outlet air is saturated
Mass and heat transfer occurs only within the system
The mass is transferred only in one direction, from the water to the air stream

14. Materials of construction
Stainless steel
does not readily corrode, rust or stain with water as ordinary steel does. There are different grades and surface finishes of stainless steel to suit the environment the alloy must endure. Stainless steel is used where both the properties of steel and corrosion resistance are required.
Due to these properties, stainless steel will be used for the packing and internals where large contact area with the water and air is expected. Also stainless steel is also used for high temperature applications.

15. Materials of construction
Carbon steel
Carbon steel will be used for the shell and outer structures of the packed column because of its durability and cost efficiency.

16. Diagram

17. Heat and mass balance
The mass transfer can be expressed as a function of latent heat transfer because of the phase change. The heat is conserved in the water vapor and conveyed to the air-water vapor mixture, resulting in a rapid enthalpy increase.

18. The exit water flow can be determined if the exit compressed air temperature is specified or vice versa. Since the exit air is assumed to be saturated its wet-bulb and dry-bulb temperatures are equal and its humidity can easily be determined. The exit water flow is the inlet flow less the water evaporated into the compressed air. The exit water flow is:

19. The heat transferred out of water is defined by:

20. Packed bed diameter
The pressure drop, produced as a result of the design air and water flow rates, specifies the tower cross-sectional area, which in turn determines the capacity. In other words, if the pressure drop is specified, the required tower capacity can be determined. Usually, the packed-bed is designed to operate at the highest economical pressure drop to ensure efficient mass transfer.
The pressure drop and flooding velocity can be determined using the generalized pressure drop correlation of Eckert as modified by Strigle (shown in the next slide)

22. Nog
Mass transfer dominates the humidification process here. Thereby, the height of the packed bed is determined by the mass transfer duty delegated to it and the mass transfer rate. The driving force for this operation can be expressed by implementing the number of gas-phase transfer units (NOG) methodology. Required NOG can be obtained by integrating the area between the operating line and equilibrium curve with respect to the average driving force

23. Numerical Calculation of Nog
The solutions to the heat and mass transfer equations and the integral for number of transfer units require exact knowledge of the mass flow rates and the condition of the fluids at any point along the humidifier. If the humidifier height is theoretically divided into very small discrete segments, the thermodynamic conditions of the fluids change marginally over each segment and may be assumed constant; then, the equations can be solved numerically.

24. Theoretically dividing the column into 5 segments

25. Hog
HOG is defined as that depth of packing required producing a change in the composition (which also can be shown by the change of enthalpy of humid air) equal to the mass transfer driving force causing that change. Basically, the height of a transfer unit represents the rate of mass transfer for a specific packing.
Totally satisfactory methods for predicting the height of a transfer unit do not yet exist; however, a great number of correlations have been published. These correlations must take into account the physical properties of the system; the gas and liquid flow rates; and the bed diameter and height.

26. The following general model can be used to calculate HOG from the mass transfer coefficient:
Where
G’ = Humid air flux
hD = Mass transfer coefficient

27. The mass transfer process can be expressed by the dimensionless numbers Reynolds, Schmidt and Sherwood. c, n and m are constants
After using the definition of the dimensionless numbers for packed beds, leads to an expression for mass transfer coefficient:

28. Substitution gives the expression for the height of a gas transfer unit:

29. If the diffusivity of one gas in another (DG) at a reference temperature (T°) and pressure (p°) is known, DG for any given temperature and pressure can be obtained by the following relation:

30. Accordingly, if the height of a gas transfer unit for any reference condition (H°OG) is known, a relation for prediction of HOG, after simplification, can be completed as follows:

31. If the humidity of the compressed air is relatively low, the ideal gas law may be applied to simplify the HOG relation. This means that terms containing only temperature and pressure can replace density and mass rate in the equation. Density's dependence can be expressed by ρ∝p/T
Similarly the gas velocity dependence is u∝(T/p)1/2 Further, the viscosity term is almost equal to unity and may be neglected. Applying these assumptions gives the following equation:

32. For mass transfer in packed beds, the constants n and m can be assigned to and 0.33 respectively, leading to the following relation (Petrovic and Thodos correlation):

33. The impact of liquid flow rate is absent in this equation
The impact of liquid flow rate is absent in this equation. As mentioned previously, in gas-film controlled systems the mass transfer coefficient is a function of both the liquid and the gas flow rates. In the humidification system a higher gas rate increases the HOG value. A higher liquid rate decreases the HOG value. This effect should be added to the equation. It is estimated that a 10 percent increase in the liquid rate will improve, i.e. decrease, the HOG value by approximately 4 percent, which results in the correction term (Lo/L)0.39:

34. Packing Height
The packing height (Z) is the height required to achieve a certain humidification operation can be determined by multiplying NOG with the height of a gas phase transfer unit HOG:

35. Wetting rate
Water distribution has a significant impact on the performance of the humidifier. An ideal distributor should be uniform and continuous water distribution, has minimal gas pressure drop and resistant to plugging and fouling. Orifice distributors are often considered to be the best alternative in the chemical industry, since they provide discrete, continuous streams with minimal flow variation from one point to another. Wetting rate is defined as the volume flow rate of water per wetted perimeter.

36. The effective mass transfer process suffers with decreasing interphase contact. For any packing, there is a minimum wetting rate for effective use of the packing surface area. In selecting a packing, it is necessary to check that the water rate is above the minimum wetting rate. If the water flux in the packing is low, a packing with a low specific area should be employed.
Minimum wetting rates of /m s

37. Heuristics
Packings of random and structured character are suited especially to towers under 3 ft dia and where low pressure drop is desirable. (Walas)
Packed towers should operate near 70% of the flooding rate (evaluated from Sherwood and Lobo correlation). (Ludwig)
Temperature of exit water must be at least 4 C above of wet bulb temperature of inlet air (Agren)

38. Design calculations

53. Equipment Spec Sheet

56. 3D model

58. Appendix

59. From Dalili, F. , Humidification in Evaporative Power Cycle.

61. References
Morris, G.A. and Jackson, J., (1953) Absorption Towers, Butterworths.
Leva M (1954) Chemical Engineering Progress 50(10): 51
Lobo WE et al. (1945) Transaction of the American Institute of Chemical Engineers 41: 693.
Strigle RF Jr (1994) Packed Tower Design and Applications. Houston: Gulf Publishing.
Robbins LA (1991) Chemical Engineering Progress, May, p. 87.
Dalili, F. (2003) Humidification in Evaporative Power Cycle. Stockholm, Sweden
Ågren, N., (2000) Advanced Gas Turbine Cycles with Water-Air Mixtures as Working Fluid, Doctoral Thesis, Dept. of Chemical Eng. Tech./Energy Processes, Royal Inst. of Tech., Stockholm, ISSN