HUMIDIFICATION: AN APPROACH TO THE PROCESS

A. CASTILLO

Department of Chemical Engineering, Antioquia University, Medellin, Colombia

April 6th, 2014

ABSTRACT

There is a wide variety of unit operations used in a chemical process and that are studied separately, since the physical and chemical principles that govern each are similar, facilitating their understanding to allow better use of them on a given process. In this paper some aspects of the humidifying operation, process description, advantages and disadvantages of it and some important applications of this unit operation will be discussed.

Keywords: unit operations, chemical process, humidification.

1. Introduction

The humidification and dehumidification operations involving mass transfer between a pure liquid phase and a permanent gas is almost insoluble in the liquid. These operations are somewhat simpler than the absorption and desorption, since the liquid contains only one component and no concentration gradients or resistance to mass transfer in the liquid phase. [1].

2. Process Description

Humidification is an operation that is to increase the amount of steam present in a gas stream; may increase vapor by passing the gas through a liquid which evaporates into the gas. This transfer into the gas stream takes place by diffusion at the

interface and there is simultaneously heat transfer and mass.

The processes taking place in the humidifying operation are:

1. A stream of hot water is contacted with dry air (or low moisture content).

2. Part of the water evaporates and cools the interface.

3. The body of liquid then transfers heat to the interface, and thus cools.

4. In turn, the evaporated water is transferred to the air interface, so that humidified [2]

3. Applications

There is a wide number of applications for this process. It will remember three important uses of it:

a. Cooling tower:

When a hot liquid is contacted with an unsaturated gas, part of the liquid evaporates and the temperature thereof drops. The most important application of this principle is found in the use of cooling towers to reduce the temperature of recycled water used by condensers and heat exchangers in chemical plants, power plants and air conditioning units. Cooling towers are a large diameter columns with types of special packaging designed to provide good liquid-gas contact with a low pressure drop. The hot water is distributed to the packing by means of spray nozzles or a grid notched pipes or troughs. The air passes through the packing by fans or induced draft forced, or in certain designs is driven by natural convection. In the huge concrete towers natural draft, used primarily in conjunction with power plants, packaging occupies only the section of the base; the rest of the tower acts as a chimney to create air flow. [1]

b. Adiabatic saturator

It is a device with two inlets and one outlet, with a negligible heat transfer with the surroundings. In this device enters an air mixture with a temperature T and a pressure P (both are known), but with unknown specific humidity ω and water at a temperature Tas which is adding on the same amount as it evaporates so that the amount of water is constant.

When the air mixture passes through the device, has contact with the surface of water so that this mixture, if not saturated, increase its water vapor content. The energy needed to evaporate water in the saturator comes from the air mixture, resulting in a drop in temperature of the air mixture entering. With a sufficiently long length saturator, the mixture will leave the device in its saturation point at a saturation temperature Tas, known pressure P and a specific humidity ω'. [1] c. Psychrometric chart

A psychrometric chart presents physical and thermal properties of moist air in a graphical form. It can be very helpful in troubleshooting greenhouse or livestock building environmental problems and in determining solutions. An important use for this chart is the analysis of cooling and dehumidification of air for human comfort (commonly called air conditioning). This is not entirely correct, for the term “Air Conditioning” refers to any and all phases of cooling, heating, ventilating, filtering, distribution, etc. of air to meet the requirements of the conditioned space. The cooling and dehumidification of air is the phase of air conditioning which concerns the refrigeration service engineer, for it normally requires the use of mechanical refrigeration equipment. In order to produce the required cooling and dehumidification for the conditioned space, the refrigeration equipment must be working properly and have the correct capacity for the application. [4]

4. Advantages and Disadvantages

The main advantage of a humidification process is the cost. If it is compared to another process, is cheaper, because no require a great amount of energy (low energy consumption). Other advantage is the cost of maintenance, because is a process that is non-reactive. For this reason, the corrosion (an important problem in process devices) is less compared to a reactive process.

5. Reference

[1] Mc.Cabe. Operaciones Unitarias en Ingeniería Química. Séptima Edición, pp 647-664

[2] http://ocw.unican.es/ensenanzas-tecnicas/operaciones-y procesos/materiales/BLOQUE2-OyP.pdf

[3] M. Moran, H. Shapiro, Fundamentals of engineering thermodynamics. Segunda edición, pp. 573-574

[4]http://www.rses.org/assets/serviceapplicationmanual/630-16.pdf