the role of conductive packing in direct contact humidification-dehumidification regenerators - part...

International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),

ISSN 0976 – 6359(Online), Volume 5, Issue 12, December (2014), pp. 126-138 © IAEME

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THE ROLE OF CONDUCTIVE PACKING IN DIRECT

CONTACT HUMIDIFICATION-DEHUMIDIFICATION

REGENERATORS - PART I: THEORETICAL ANALYSIS

Abdelhakim Hassaboua*, Markus Spinnlerb, Wolfgang Polifkeb

aQatar Environment and Energy Research Institute,

Qatar Foundation, P.O.Box 5825, Doha, Qatar

bLehrstuhl für Thermodynamik, Technische Universität München,

D-85747 Garching, München, Germany

ABSTRACT

In solar desalination by humidification-dehumidification (HDH) technique, heating the air

prior to entering a humidifier enhances the production of fresh water. This phenomenon can be used

to increase the system’s productivity in a multiple effect heating humidification (MEHH) process. In

this study, spherical conductive packing elements to enhance the evaporation and condensation rates

have been applied to achieve the MEHH as well as multiple effects of cooling/dehumidification

(MECD) while air passes through the successive packing layers in the evaporator and condenser

respectively.

This paper discusses the theoretical background and potential role of conductive packing, as a

heat and mass exchanger, on the effectiveness of heat and mass transfer processes both on micro and

macro scale levels in HDH cycles under steady state conditions. Preliminary experimental analysis

of the new HDH system equipped with direct contact packed bed regenerators in both evaporator and

condenser will be presented for proof of concept. Comprehensive experimental and numerical

investigations covering a wide range of operation conditions will be presented in following

publications.

Keywords: Solar Desalination; Humidification-Dehumidification; Direct Contact Heat Exchangers;

Evaporation; Condensation; Phase Change Materials

INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING AND

TECHNOLOGY (IJMET)

ISSN 0976 – 6340 (Print)

ISSN 0976 – 6359 (Online)

Volume 5, Issue 12, December (2014), pp. 126-138

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1. INTRODUCTION

Solar distillation can be classified as small or large scale, based on the plant size and

production capacity. The basin-type solar stills and humidification-dehumidification systems are well

known examples of the small scale systems. Solar driven Multi Stage Flash (MSF) and Multi-Effect

Distillation (MED) are typical examples of medium and large scale solar distillers. Basin-type solar

distillers require large land areas and have a relatively low productivity compared to other

desalination technologies. As a consequence, humidification-dehumidification (HDH) technique

using packed beds was introduced as a promising technique early in 1950's. As reported by Hodges

[1], a solar operated HDH process can produce 5 times higher output than a basin-type solar distiller

of the same solar collecting area. The HDH technique is especially suited for seawater desalination

when the demand for water is decentralized. Advantages of this technique include flexibility in

capacity, moderate installation and operating costs, simplicity, and possibility of using low

temperature energy [2].

However, desalination with humidification-dehumidification of air has been under

development over the last three decades, leaving some problems unsolved. Comprehensive literature

coverage and fair knowledge base on HDH systems are available in [2, 3]. Al-Halaj and Selman [9]

illustrated the needs for more work to be performed to increase the productivity and decrease the cost

of product water of these units.

To improve the performance of HDH systems, previous studies have investigated various

configurations of the HDH cycle. These efforts focused on improving heat and mass exchanger

designs or using multi effect humidification of air. A multitude of alternative water-heated HDH

systems has been presented. However, few interesting approaches focused on direct air heating

compared to direct water heating systems.

In most of the previously investigated systems, forced convection has been used for actuating

the air flow between evaporator and condenser. Obviously multi-staging has attracted considerable

attention in the latest developments, especially in Germany, i.e. Müller-Holst [4], Brendel [5] and

Chafik [7].

Müller-Holst [4] used free convection between vertical fleece mats and vertically hanging

heat exchanger plates as evaporator and condenser. Here, a multi-stage behaviour has been induced

by means of different self-establishing convection vortices between evaporator and condenser.

Another approach has been presented by Brendel [5], who used a multistage evaporator/condenser

set-up that is run at different brine mass flows under forced convection. However, this design seems

to be far too complex for application in isolated arid regions, where simplicity of operation and

maintenance is an essential design criterion.

It is well understood that fresh water production efficiency can be significantly enhanced in

HDH systems by heating air in addition to heating water. This fact has been experimentally proved

by Klausner et al. [6], as he observed that the fresh water production rate has increased when air was

heated prior to entering the diffusion tower. A multiple effect heating-humidification (MEHH) solar

desalination unit has been developed by Chafik [7], as shown in figure (1.a). In this desalination

process, the heating/humidifying procedure is conducted in multiple stages to load the air with a high

amount of water vapor and to reduce the volume of circulating air. Air is heated in each stage, where

seawater is sprayed at the end of the stage to humidify the air until it is saturated, as represented by

the zigzag lines in figure (1.b). With the aim to reduce the costs and land footprint of the MEHH

system, the solar air heater and the humidifier were combined into a single multi-stage unit. The air

is cycled between heating and humidification several times in the successive stages to attain higher

humidity content. As the air was recycled in a closed multiple-stages arrangement, vapor humidity

values up to 20% (by weight) and beyond can be attained compared to < 6% in a single stage

process.



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(a)

(b)

Figure 1: Multi effect of heating and humidification of the HDH desalination unit by Chafik [7];

(a) integrated collector-humidifier, (b) representation on the psychometric chart

Such a high concentration can be achieved in the MEHH process at operating temperatures

less than 90°C, while it can be attainable in a single stage when air is heated up to very high

temperatures. As depicted in figure (1.b), the highest red arrow represents the theoretically high

temperature of air that has to be reached (~250°C) to load it with the same humidity in a single stage

process. However, Chafik [7] reported that this unit is quite costly, while the integrated air collector-

humidifier represents 40% of the total cost. It is worth mentioning that high operating temperatures

have substantial negative influence on the solar collector efficiency. Moreover, it is anticipated that

the heat and mass transfer coefficients between air and water are smaller than using a humidifier

separated from the solar collector. Furthermore, solar water heaters have better thermal efficiency

than solar air heaters under same weather conditions. This reveals that the multi-staging arrangement

of Chafik [7] would have no significant impact on the system‘s energetic efficiency, as higher energy

input is required for the higher yield compared to single stage units.



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On the condenser side, direct contact condensers have several advantages that tend to

alleviate the inherent problems widely experienced in conventional condensers, where heat and mass

transfer take place through heat exchange surfaces with high thermal resistances. Scale formation

and degradation of heat and mas transfer efficiency due to existence of non-condensable gases

represent the most important problems encountered in conventional condensers.

Direct contact condensers exhibit higher intensity of heat and mass transfer with minimal

resistances, on contrary to noncontact heat exchangers with high resistances through tube walls in

heat exchangers, and provide large specific interfacial areas with minimum pressure loss. It has been

reported in previous studies [15–16] that direct contact condensation is a promising technique for

small-scale HDH desalination units. Simplicity, low cost, elimination of fouling tendency, relatively

high heat transfer rates and low pressure drop per unit volume represent the most important features

of direct contact condensers, as argued by Dawoud et al. [15]. In this study, direct-contact

condensation has been adopted for improving the performance of the HDH cycle.

A new approach that has been introduced by the authors [13-14] in this context is the use of

encapsulated phase change material (PCM) as conductive packing medium both in the evaporator

and condenser while consequently applying free or forced convection. A preliminary experimental

analysis of the new HDH system equipped with fixed packed bed PCM regenerators in both

evaporator and condenser has been introduced and analyzed in [14]. The PCM materials were

proposed to be integrated in the HDH thermal desalination cycle for improving the thermal

performance in the evaporator and condenser. The main objective of incorporating PCM elements in

the evaporator and condenser was for heat storage, as a backup for part-time night operation and

during transient solar irradiation behaviour in cloudy hours. However, it was discovered during

analysis of steady state conditions that multiple-effects of heating/humidification (MEHH) and

cooling/ dehumidification (MECD) of air passing through the successive PCM layers in the

evaporator and condenser respectively seem to play an important role for the system efficiency.

Theoretical analysis indicated that the multiple-effects phenomena are attributed to the existence of

thermally conductive packing media, which act as heat and mass exchangers in the two columns.

Thus, this study focuses on the effect of thermal conductivity rather than thermal storage

capacity or solid-liquid phase transitions of the packing. The MEHH of air, which was investigated

by Chafik [7], is approaching the main interest of the present work. The novel approach for locally

establishing MEHH in direct contact heat exchangers of the HDH cycle using conductive media will

be elaborated and deeper analysis of experimental results will be presented in this study. The thermal

behaviour of the new evaporator and condenser technology in the closed air loop HDH unit is

discussed and analyzed. Comparative performance of the HDH system will be thoroughly

investigated and clearly documented using conductive and non-conductive packing media (empty

plastic spheres having the same size as conductive packing, i.e. PCM spheres) in the evaporator and

condenser. The effect of various parameters on the plant performance and productivity will be

investigated.

2. PROPOSED SYSTEM AND OPERATION CYCLE

The operation cycle and flow diagram of the proposed HDH system equipped with PCM

packing and its processes is illustrated schematically in figure (2). The plant comprises three closed

loops; the air loop, the hot water loop, and the cooling water loop. Hot water is sprayed into the top

of the evaporation column, where it forms a thin liquid film over the spherical packing surface while

in contact with the low humidity countercurrent air stream. Due to partial evaporation and convective

heat transfer, the brine cools down and leaves the evaporation unit concentrated at a lower

temperature. To keep the salinity concentration within a certain limit, part of the outlet warm brine is

blown down while the other part is re-circulated and mixed with the feed seawater makeup, and



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hence the cycle is repeated. The brine and seawater makeup is heated through recovery of latent heat

of condensation in a heat exchanger downstream of the condenser. The brine is then heated up

further through a solar collector during day time or bypassed to the external thermal buffer to avoid

heat losses through the collector during night time. The solar collector should be oversized to store

the excessive energy during day time in the external thermal buffer to be utilized for night operation.

Figure 2: Schematic layout of the PCM-Supported HDH desalination unit [13]

The humidified air is guided by forced convection into the air cooler (dehumidifier) where

the sub-cooled water is gently sprayed in tiny droplets at the top of the condenser and trickles

downward by gravity concurrently with the warm humidified air. The liquid droplets eventually

form a liquid film on the surface of the spherical packing where it gets in direct contact with the

concurrent flow of warm humid air at liquid-gas interface. The air continuously circulates in closed

loop cycles, as a carrier medium, to transfer the generated water vapor from the evaporator to the

condenser where it is condensed back into highly purified fresh water.

3. THEORETICAL ANALYSIS

In this section, potential role and influences of conductive packing on the effectiveness of

heat and mass transfer processes in the evaporator and condenser will be introduced and discussed to

a certain depth both on micro and macro scale levels to help developing better understanding of the

underlying physics. This will also help in interpretation of the numerical and the experimental



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results, which will be discussed in following publications. The subsequent discussions will provide a

qualitative expectation of the temperature profiles of different phases over the packed bed height.

These qualitative profiles would make it possible to predict the system’s behaviour and influences on

distillation rates.

3.1 Potential Role of Conductive Packing in the Evaporator (MEHH) The main physical difference between the PCM evaporator and condenser investigated in the present

study and a conventional packed bed is that the later is equipped with nearly non-conductive packing

elements. Unexpected effects appeared when comparing PCM with Non-PCM packing during steady state

operation conditions led to a new focus of the study. The effect of thermal conductivity of PCM packing on

the heat and mass transfer effectiveness of the HDH cycle represents the main focus of this analysis, hence

PCM packing will be refered to as “the conductive packing” throughout this analysis.

The energy balance and energy flow in the conventional packed beds with non-conductive

packing are only governed by the interaction between both fluid phases (i.e. hot and cold streams).

On the other side, for the conductive packing, the solid phase contributes to the energy balance and

the interstitial energy flow between fluid phases. Since there are wetted areas and dry patches, there

is simultaneous heat and mass transfer entirely between all phases in both evaporator and condenser

in existence of conductive packing. Therefore, the physical problem under investigation involves

multiscales; on the pore scale or the micro scale level as well as on the macro scale level or the

overall balance, which both mutually affect the sensible and latent heat transfer components at

different interfaces on the micro-scale level.

3.1.1 Micro scale level

Existence of conductive packing in the evaporator and condenser creates a parallel path for

heat and mass transfer between gas and liquid through the packing elements due to partial wetting of

the packing surfaces as shown in figure 3. Let the subscript “s” identify the solid particles and

subscripts “l” and “g” designate the liquid water and humid air (air-water vapor gas mixture)

respectively. Consider a rectangular finite slab of thermal conductivity k, which is covered on one of

its sides by hot liquid water and subjected to ambient air on the other side as shown in figure (3.b).

• Due to the higher liquid temperature, there will be two sensible heat transfer components

between water and gas (Qlg) and water and solid (Qls).

• The interaction between gas and solid phases (Qsa) arises due to the temperature difference

between both of them.

• At the liquid gas interface, the latent heat of vaporization becomes part of the energy balance,

and the mass diffusion (mv) caused by the vapor pressure gradient affects both heat and mass

balances.

On the other hand, when evaporation depends on the interfacial vapor pressure, the sensible

energy flow components impact the film temperature at liquid gas interface and consequently affect

the latent heat component. At steady state conditions, the heat flow from liquid to solid phase

equilibrates with the heat flow from solid to air and the local temperatures of all phases remain

constant. This analysis indicates that thermal conductivity represents one of the key parameters that

control local heat and mass transfer not only at solid-liquid and solid-gas interfaces but also at the

liquid gas interface. In fact, there is a trade off between direct heat and mass transfer at the liquid-gas

interface and the heat flow from liquid to gas across the solid medium which creates the multiple

effects of heating and humidification (MEHH) in the evaporator and similarly multi-effects of

cooling and dehumidification in the condenser (MECC).



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3.1.2 Macro scale level

The temperature and enthalpy of conductive packing is both space and time dependent as a

result of the transient conduction inside the packing elements, and thermal stratification in water and

gas phases along the packing height. However, due to the stratification of the heat source

temperature (i.e. the hot water), stratification exists along the packing height in different layers. This

in turn creates a multiple effect heating/humidification or cooling/dehumidification in the evaporator

and condenser respectively. The MEHH in the evaporator is illustrated in figure (4).

The conductive packing media proposed theoretically serve as temporary heat exchangers

that virtually increase the sensible and latent heat transfer interfacial area between water and air and

create a multi-effect heating and humidification and multi-effect cooling and dehumidification along

humid air passages in the evaporator and condenser, respectively. Such combined effects of creating

additional parallel and serial interfaces for heat and mass transfer along the packing height would

increase the air carrying capacity for water vapor and maximize evaporation and condensation rates

in the closed air cycle.

Figure 3: Illustration of local heat and mass transfer flow between different components in the evaporator; (a) one packing element, (b) finite element

b a



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When the unsaturated airflow gets in contact with hot saline water, a certain quantity of water

vapor diffuses from water into the air, which results in a temperature reduction of water as well as a

higher salt concentration. The maximum amount of water vapor is limited by the saturation

conditions of air, which is mainly dependent on its temperature under constant pressure, see figure 5.

Once the air is saturated, it can not carry more water vapor unless its temperature is further

increased. As a well known fact, the air carrying capacity for water vapor has an exponential

dependence on its temperature. In order to achieve a certain humidity content, the air temperature

could be increased at once to the corresponding temperature (as represented by the dashed arrow on

figure (4) or in a multiple stages of heating and humidification as depicted by the successive red and

blue solid arrows on the same figure). The red arrows correspond to sensible heating of air along the

dry patches in the packing, blue arrows correspond to adiabatic humidification along the liquid-gas

interface. In the first case, the heat source is required to be available at a high temperature, which in

turn will be reflected on increasing the collector area and hence the principle and operating costs. In

the later case, the heat source can have a lower quality, such as a waste heat source or simple flat

plate solar collector.

In order to achieve the maximum amount of water vapor in the evaporation chamber, the

incoming water temperature should be high enough so that the maximum amount of water can be

evaporated to the surrounding air. The maximum temperature of the incoming water cannot exceed

85°C due to the high material costs associated with such high temperatures and to avoid scale

formation at high temperatures. Furthermore, the efficiency of solar collectors would be reduced at

such high temperatures.

This particular problem inspired the concept of utilizing conductive packing media in the

evaporator to create the MEHH locally (as illustrated by figure (4), without the need either to heat

water to higher temperatures beyond 85°C or to extract air at intermediate points in the evaporator,

heat it in external air heaters and return it back to the same point, which would neither be technically

nor economically feasible.

Given axial temperature stratification along the packed height, the solid temperature at the

top layers of the bed is higher than that of the gas phase, while the reverse may hold true at the

bottom layers when the heat capacity flow of hot water is low. Thus, the expected behaviour of the

system in this case is that there is heat flow from gas to solid in the lower part of the column (i.e.

cooling effect) and from solid to gas in the upper part. This implies that there should be a certain

height of the bed at which the heat flow between gas and solid is zero (breakeven point), as depicted

in figure (6). Thus, if the system design is to be optimized so that at every specific location along the

packing height there would be always heat flow from solid beads to the gas mixture in the evaporator



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(and to the cooling water in the condenser), this will result in a more profound MEHH in the

evaporator or MECD in the condenser. These idealized temperature profiles are illustrated

qualitatively in figure (7) for the evaporator and condenser respectively. Under such circumstances,

the conductive packing would have much higher impact on the distillate yield.

The energy flow between all and each pair of phases plays the major role in plotting the

system behaviour under specific boundary and geometrical conditions. These particular and

geometrical conditions. These particular concept proposed in the present study for improving the

performance of the HDH cycle. The main advantage of integrating a cycle. The main advantage of

integrating a induce self establishment of MEHH and MECD locally.

Figure 7: Qualitative illustration of ideal temperature profiles, left: evaporator, right: condenser

3.2 Potential Role of Conductive Packing in the Condenser (MECD)

In the suggested PCM condenser, the physical phenomenon of direct contact condensation is

characterized by the transport of heat and mass through the gas (i.e. humid air)-liquid interface as

well as across the dry patches at the gas-solid interface along the packing height. Due to continuous

cooling of the conductive packing elements by the subcooled liquid layer, supplementary cooling

and condensation take place at the gas-solid interface simultaneously with that at the gas-liquid

interface; as a result, heat and mass transfer rates are enhanced.

Since axial stratification occurs entirely in all involved phases, the phase temperature is a

function of distance and hence the solid phase temperature profile plays an important role in the

condenser performance. However, unlike the evaporator, the gas phase remains saturated at local

temperatures throughout the MECC process since the condensation process takes place at both gas-

liquid and gas-solid interfaces.



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Thermal stratification in the condenser is not as strong as in the evaporator because of the

lower heat source (gas) temperature in comparison with the heat source (hot water) temperature in

the evaporator and due to the concurrent flow in the condenser. In the concurrent flow, it is

anticipated that most of the condensation takes place in the first top layers of the bed, where the

cooling water temperature is at its lowest level while the gas temperature is at its maximum level.

Hence, the water temperature increases downwards due to condensation and cooling of humid air.

4. EXPERIMENTAL RESULTS AND PROOF OF CONCEPT

An experimental setup for the HDH system has been designed and built as a prototype at

Technische Universität München (TUM). The setup consists of an evaporator and condenser made of

two cylindrical packed beds. PCM encapsulated in spherical plastic shells were used as a conductive

packing and examined in comparison with low conductive empty spherical shells with the same size.

A detailed description of the experimental facility, procedures and comprehensive experimental

analysis will be presented in a following publication.

The experimental results showed a productivity enhancement of the evaporator and

condenser in existence of conductive packing. The plots on figure (8) illustrate that the differences

between inlet and outlet gas and inlet and outlet liquid temperatures in the evaporator and condenser

of the conductive PCM packing are higher than those of low conductive empty spheres packing (ES).

Although the boundary conditions were typically the same on the evaporator side, the outlet water

temperature for the conductive PCM packing was about 3 ºC lower than that of the empty spheres.

On the condenser side, due to technical reasons, the inlet cooling water temperature of the PCM

system was higher than that of the empty spheres; nevertheless the PCM productivity was 14.6%

higher than that of the empty spheres.

The plots on figure (9) illustrate the improvement of productivity and Gained Output Ratio

(GOR) for the conductive PCM packing. The GOR is a measure of the overall efficiency of thermal

desalination systems, illustrating how much desalinated water can be produced per unit of energy

expended. Moreover, the performance analysis indicates that the effectiveness of PCM evaporator

and condenser has increased by 3 and 6% respectively compared with the empty spheres packing. It

could be interpreted that this particular behaviour of the PCM based system represents the potentially

important role of establishing multiple effects of heating and humidification due to the existence of

thermally conductive packing in the system.

Figure 8: Average inlet and outlet liquid and gas temperatures, left: evaporator, right: condenser



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It is worth to mention that this behaviour can also be realized using other kinds of sensible

heat storage or conductive media since it is not associated with the solid-liquid phase change

phenomena inside the PCM packing, but rather related to its thermal conductivity.

Figure 9: Evolution of productivity and GOR for conductive (PCM) and nonconductive (ES)

packing elements

5. CONCLUSIONS

This paper discusses the theoretical background and potential role of thermally conductive

packing as a heat and mass exchanger on the effectiveness of heat and mass transfer processes both

on micro and macro scale levels in HDH cycles under steady state conditions. Spherical conductive

packing elements to enhance the evaporation and condensation rates have been applied in the

evaporator and condenser. The potential role of conductive packing is to achieve multiple effect

heating humidification (MEHH) and multiple effects of cooling/dehumidification (MECD) while air

passes through the successive packing layers in the evaporator and condenser respectively.

The presented experimental analysis has shown a proof of concept. From the analysis, it is

clear that there is indeed an effect of the conductive packing on the effectiveness of heat and mass

transfer processes in the HDH system. The solid phase thermal conductivity represents the key

parameter that controls local heat and mass transfer not only at solid-liquid and solid-gas interfaces,

but also at the liquid gas interface.

The presented experimental analysis has shown a proof of concept. From the analysis, it is

clear that there is indeed an effect of the conductive packing on the effectiveness of heat and mass

transfer processes in the HDH system. The solid phase thermal conductivity represents the key

parameter that controls local heat and mass transfer not only at solid-liquid and solid-gas interfaces,

but also at the liquid gas interface.

ACKNOWLEDGEMENTS

This research project is supported by the Middle East Desalination Research Center

(MEDRC) through project number 05-AS-003 and the Technische Universität München (TUM),

Germany. The financial support is gratefully acknowledged.



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Nomenclature

c Concentration, [mol.m-3]

ES Empty spheres

h Heat transfer coefficient [W.m-2.K-1]

k Thermal conductivity, [W.m-1.K-1]

m Mass flow rate, [kg.s-1]

PCM Phase change material

& q Heat flux, [W.m-2]

T Temperature, [K]

t Time, [h]

∆T Temperature difference, [K]

V Volume, [m3]

Z Coordinate of axial direction or packing height, [m]

Subscripts

a Air

g Gas (air-vapour mixture)

hw Hot water

l Liquid

s Solid

sat Saturation conditions

w Water

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