Surfactant Concentration On Aqueous LiBr Solution Absorption Rate Biology Essay

It is good known that little sums of wetting agents, such as 1-octanol, in the aqueous solution can increase the soaking up rate significantly. In this paper, experimental informations were obtained for soaking up of H2O vapor into an aqueous LiBr solution with different concentrations of 1-octanol.The experimental consequences showed that the surfactant concentration has a important consequence on the soaking up rate ; this is called the Marangoni instability.It has been concluded that, in order to clear up the soaking up sweetening phenomenon, it is necessary to understand the physicochemical facets of the soaking up procedure and the consequence of wetting agents on the sweetening of such procedure.

Additionally, it has been concluded that new attacks are needed to explicate the ascertained behavior.

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It is good known that the vapour soaking up rate into the absorptive solution is the cardinal procedure to better the thermic coefficient of public presentation ( COP ) of vapor soaking up air conditioning systems. The sweetening of the soaking up procedure leads to cut downing the heat money changer size of the absorber.For the uninterrupted control of any soaking up system, it is of import to supervise the belongingss of the binary solution at different points in the rhythm and under different infrigidation tonss.Assorted techniques are available to mensurate the concentration of the binary water/lithium bromide solution.

The most normally used method is the titration method ( Herold et al.,1996 ) , which can give a high truth when the process is performed by an experient and careful research worker. The drawback of this method is that it tends to be clip devouring, as it requires pulling a sample from the system solution.The uninterrupted measuring of concentration utilizing radioactive tracer sensors or mass spectrometers is really expensive ( Horn, 1969 ) .This survey describes an accurate process for obtaining the concentration of the aqueous Li bromide solution based on the measuring of electrolyte conduction.The survey besides presents an experimental probe to analyze the consequence of the surfactant concentration on the soaking up rate.

Measurement Techniques

The working ranges for vapour soaking up systems based on water/lithium bromide are ( 40-65 % ) for the concentration of the Li bromide in the binary solution and ( 20-100oC ) for the temperature of the solution.

These are the scopes investigated in this survey. The concentration samples of the binary solutions are prepared in a glass beaker and immersed in a changeless temperature H2O bath. A commercially available conduction metre with a constitutional temperature investigation is so used to mensurate the denseness and conduction of the solution.

Sample Preparation

Lithium Bromide is a white three-dimensional crystal or crystalline pulverization. It is disposed to break up in the moist air, fade out into H2O, intoxicant or ethanediol, and able to fade out into ether or pentanol to go colorless aqueous solution or pale xanthous transparent liquid.As the Li bromide is highly hygroscopic ( Lide D. R. et al.

-1994 ) , it is of import to follow a on the job process to guarantee that no wet from the ambiance is absorbed accidentally when fixing the mensurating sample. The samples were prepared from a pure dry Li bromide pulverization and deionised H2O. A digital mass balance was used to mensurate the multitudes of the Li bromide and the deionised H2O before commixture.

Temperature Measurement and Control

The reaction of the Li bromide pulverization and H2O is exothermal ; this produces a important rise in the temperature of the solution ( 20oC to 60oC ) . The temperature of the solution was controlled by a thermostatically regulated H2O bath to the desired value.The temperature of the solution was measured utilizing the conduction metre with the constitutional temperature investigation.

Conductivity Measurement:

For a cell of a unvarying cross subdivision ( A ) , with electrodes at either terminal separated by distance ( cubic decimeter ) , the conduction ( K ) is related to the conductance ( G ) , by equation ( 1 ) and has the units ( i? — 1m-1 or Sm-1 ) .

( Hamann et al. 1998 )( 1 )


In chief, the measuring of the conduction could be carried out in a cell, with rectangular electrodes of known country A M2 positioned fifty thousand apart. However, in pattern a figure of complicated corrections would hold to be made for acquiring the exact value of the conduction.

Rather than making this for all measurings, usage is now made of unequivocal conduction measurings for certain standard solutions carried out under really carefully controlled conditions in specially designed cells.An extended and thorough study of the market led the writer to reason that Omega bench-top CDB-420 conductivity-meters were the most suited. Conductivity ranges from 0 to 19.99S with declaration 0.01S and accuracy i‚±0.

5 % , the temperature scope is -10 to 105oC with truth i‚±0.5oC. It is besides concluded that the epoxy conduction cell model CDE-430-10-EP with changeless ( K=10 ) and constitutional temperature investigation is the most suited investigation to associate with the conduction metre.


A sample of known electrolyte conduction ( e.g.

0.745 g/litre Potassium Chloride solution ) has been used to graduate the instrument. Dissolving 0.745 gms of dried Analar Grade Potassium Chloride ( KCl ) into 1 liter of de-ionised H2O.

The solution has been placed into a H2O bath set to 25oC. When the temperature of the standardization criterion solution has reached steady province, the electrode has been placed in the solution and left for 5-10 proceedingss. The scope of 0-2000 i?­Siemens range on the conduction metre has been selected, and so the reading has been adjusted to 1413 i?­Siemens.

Data Correlation and Empirical Modelling

The experimental informations presented in tabular array ( 1 ) shows that for a changeless concentration, the conduction decreases with temperature for concentrations up to 55 % LiBr.

For concentrations higher than 55 % , the conduction increases as the temperature increases.Table ( 1 ) Electrolyte Conductivity for different concentrations






























64In order to develop an empirical theoretical account of the concentration as a map of temperature and conduction of the aqueous Li bromide solution, numerical curve tantrums have been produced for the experimental points of conduction versus temperature at peculiar concentrations. The best-fit equations are logarithmic of the signifier given by equation ( 2 ) .( 2 )The values of the invariables ai and Bi are given in tabular array ( 2 ) for different concentration Xi % .Table ( 2 ) invariables ai and bi for equation ( 2 )XI %Army IntelligenceBismuthXI %Army Intelligencebismuth40 %-15.436207.

8755 %-5.7245128.5845 %-12.565191.

9260 %12.61738.20750 %-8.5805156.

0865 %15.47318.921An equation is so constructed utilizing the above equations to bring forth a relationship giving the concentration of the Li bromide solution { X % } as a map of temperature { T, oC } and electrolyte conduction { C, multiple sclerosis } . This is presented in equations ( 2 ) to ( 5 ) .( 2 )( 3 )( 4 )( 5 )Whered1= 409.66d2= -1677.

2d3= -297.03d4= 942.01d5= 37.673d6= 103.56Figure ( 1 ) Water/Lithium Bromide Temperature vs. Conductivity for Different Concentrations.Figure ( 1 ) presents the experimental information of electrolyte conduction of the aqueous Li bromide versus the temperature.

These experimental informations agree with the tendencies presented by Hamann et al. , 1998. However, at higher concentrations, the conduction rises less quickly than expected from the extrapolation of the consequences ; this is due to the addition of the inter-ionic interactions as the average distance between ions lessenings ( Hamann et al.

, 1998 ) .

Experimental Test Rig

The general agreement of the experimental setup used in the present survey is shown in figure ( 2 ) .Figure ( 2 ) Measurement and command diagram of the trial rig.The measurings of assorted parametric quantities were taken in different places of the trial rig. The variables measured and controlled during the operation of the system were the temperature, force per unit area and concentration. Concentration was measured utilizing Electrolyte Conductivity-Temperature technique.

Working fluids

Absorbent solution

An aqueous solution of Li bromide incorporating approximately 61 % Li bromide is used ab initio as an absorbent. The working solution was prepared from pure dry Li bromide powder 99+ % “ Sigma-Aldrich merchandise ” and pure deionised H2O.

A digital mass balance was used to mensurate the multitudes of the Li bromide and the deionised H2O before blending. As the reaction of the H2O and the pulverization is exothermal, a binary solution of 61 % ( LiBr-Water ) was prepared and left to chill to the ambient temperature.


1-Octanol is used in the experimental probe for this work. The merchandise was bought from “ Sigma Aldrich Co.

Ltd. ” . The Molecular expression for the 1-Octanol is: C8H18O. This merchandise is besides known as Capryl intoxicant, n-Octyl intoxicant, Heptyl carbinol or 1-Hydroxyoctane. It has the undermentioned designations and belongingss:Molecular weight: 130.23Appearance: Clear, colorless liquid.

Solubility: Slightly soluble, 540 mg/L soluble in H2OBoiling point: 195 oC.Melting point: -16 oCSpecific gravitation: 4.5 g/LBrassy point: 81 oC.


Pure de-ionised H2O was used as the refrigerant in the evaporator vas.

Purpose of the experiment

The procedures in a LiBr absorber are coupled heat and mass transportation processes. As the H2O vapor is absorbed into the liquid, the latent heat associated with the stage alteration is released along with the heat commixture.

This energy release occurs at the vapour-liquid interface and causes the interface to be the location of highest temperature in the system. To let the procedure to go on, the energy must spread into the majority of the liquid. These two diffusion procedures are coupled in the sense that halting either procedure causes system variables to set in such a manner as to halt the other procedure. For the overall transportation to take topographic point, both procedures have to continue in tandem.

In the instance of the absorber, the mass transportation procedure controls the conjugate transportation. Therefore, design attempts to better overall absorber public presentation are focused on the mass transportation procedure ( Herold, et al 1996 ) . Several additives have been tested in the literature. Cul et Al, ( 1991 ) tested the impact of several additives in the soaking up. 2-ethyl-hexanol, n-Octanol such as 1-octanol and 2-octanol, decanol, 1-heptanol, soluble intoxicant as 2-methyl-2propanol, fluorinated wetting agent such as FC-430, and Ion extraction such as 15-crown 5 have been tested. The consequences showed that the 1-octanol has the best mass of H2O vapor absorbed during a 15minutes run by Lithium bromide solution of initial concentration of 60 % . 1-Octanol has been found to better the absorber public presentation.Experiments were performed by altering the figure of droplets of 1-Octanol, accordingly the distance between the droplets.

This in bend leads to the alteration of the facet ratio given by the ration of absorptive solution deepness to the distance between the linear droplets.Figure ( 3 ) shows the different additives places of the linear droplets. The linear droplets were located utilizing specially designed phonograph record ( 180mm diameter ) with figure of holes to let the additives to be in the coveted places as shown in the figure. The 1-Octanol droplets were positioned on the surface of the aqueous Li bromide solution utilizing a really all right pipette.

EXPERIMENT Appellation:

Primary concerns of avoiding the taint from experiment to experiment were taken into histories while planing the trial rig and the testing technique, since the minute sum of additives residue can greatly act upon the soaking up rate.

( Cul, et al 1991 ) . These concerns resulted in transporting out a figure of alterations on the trial rig so that it can be easy assembled and cleaned after each experiment.Extreme attention is taken to guarantee that the same initial conditions apply for each experiment. Parameters involved are initial temperature, force per unit area and the concentration of the aqueous Li bromide solution.The experimental process focused on bring forthing the same sum of H2O vapor by maintaining the H2O temperature invariable during the tally of the experiments.Table ( 3 ) shows the experiments appellations and the experimental agreements of the survey.Table ( 3 ) Experiments appellations and sweetening techniqueTest ModeExperiment Number and appellationNo of 1-Octanol dropletsSimple manner( 1 ) W/O

Surfactant additives manners( 2 ) Add33( 3 ) Add44( 4 ) Add66( 5 ) Add1616

Figure ( 4 ) Concentration fluctuation vs. clip for assorted experimental appellations

Experimental Consequence:

Figure ( 4 ) shows the fluctuations in the simple mode appellation of operation, without the usage of surfactant additives.

The figure besides shows the fluctuations of the consequence of the figure of droplets of 1-Octanol on the deliberate values of concentration alteration rate in ( hr-1 ) . It is clear from the figure that the sweetening of the soaking up procedure additions with the addition of the figure of the droplets up to a certain figure of droplets, in this instance four droplets.The add-on of more droplets, beyond four consequences in a bead in the enhancement procedure.

Increasing the additives to 16 droplets consequences in the surcease of the soaking up procedure.

Discussion of the Consequences:

In the majority of a liquid, molecules are capable to intermolecular forces, which averaged over clip are symmetrical and have no net consequence. At an interface where two stages meet, an instability of intermolecular forces arises because of differences in the molecular construction of the several stages. For illustration, at the H2O vapour/LiBr solution interface the liquid -liquid attractive forces are far stronger than either the vapour-vapour or the vapour-liquid attractive forces. Therefore at the vapour-liquid interface the surface molecules are pulled into the majority of the liquid.

Consequently, the surface bed of the Li bromide aqueous solution acting like an elastic membrane under tensenesss ( surface tenseness ) . When a wetting agent added to the binary solution interface, it replaces the H2O molecules at the fluid surface, with its hydrophobic caput groups, take downing the instability in the intermolecular forces and therefore cut downing the surface tenseness but the consequence of the surfactant depends on the surfactant concentration.

Low Surfactant Concentration:

The presence of one solute may impact the solubility of another ( Atkins, 1990 ) ; this is called the salt-out consequence. The salt-out consequence is the decrease of the solubility of a substance into H2O when another salt is added. This is because the solubility of LiBr into H2O ( 62 % at 35oC =1630 g/L ) is much greater than the solubility of 1-Octanol ( 540*10-3 g/L ) in H2O. When three droplets of 1-Octanol are added to the aqueous Li bromide solution, no sweetening has been observed. This is due to the fact that the solution is non saturated.

The little sum of additives will fade out in the solution. No 1-octanl will be on the surface, doing no sweetening to the soaking up procedure.If some of the surfactant molecules are dissolved in the solution, while the other molecules are adsorbed on the surface, harmonizing to Kashwagi ‘s hypothesis ( Rie and Kashwagi, 1991a & A ; b ) the droplets were non in a mirror place to do the Marangoni consequence.

As it requires the droplets to be confronting each other to do the formation of turn overing cells of convection.

Medium Surfactant Concentration

When 1-Octanol is added to the aqueous Li bromide solution, the wetting agent will fade out in the solution until the solution becomes saturated, in other words reaches its solubility bounds. Adding more surfactant, consequences in H2O be givening to adhere with the Li bromide alternatively of the surfactant molecules because of the higher solubility of LiBr in H2O. Consequently, some surfactant molecules will non be dissolved in the solution and are segregated to the surface of the solution.

The hydrophilic groups of the unintegrated wetting agents are so adsorbed on the solution surface, therefore diminishing the surface tenseness of the solution, while the hydrophobic group tends to be in the vapor volume. Wetting agents will be given to garner together and aggregate into droplets on the surface of the solution. The collection of the hydrophobic ironss of surfactant molecules ( caput groups ) with each other in the aqueous solution is to avoid the contact with H2O after their surface assimilation in the surface.

High Surfactant Concentration:

Adding more surfactant will be given to diminish the soaking up rate due to the followers:The short distance between the surfactant molecules helps the collection procedure. Collection of two or three molecules of wetting agent is non plenty to cut off the contact of hydrophobic groups with H2O molecules. So Several 10s of molecules ( or even more ) gather together and organize the accretion. The solution will move as a solution covered with wetting agents.Most of the surfactant molecule will pack together doing obstruction to the surface of the aqueous Li bromide solution towards the H2O vapor.

Water vapor will be impeded to spread through the surfactant bed.The local alteration of temperature during the soaking up procedure at the surface can take to a negative surface tenseness gradient ( critical temperature difference is reached ) which stabilises the liquid gesture, and accordingly, impede the heat and mass transportation through the surface. ( Kren et al, 1999 ) .


The consequences of conducted experiments for the soaking up procedure and the sweetening of mass transportation were discussed. Comparing the achieved consequences for each appellation, the undermentioned decision can be drawn:Marangoni instability can be moderately explained by the physical chemical science theories such as the salting-out consequence and the spreading consequence.Surfactant concentration and the solubility of the aqueous LiBr solution control the sweetening of the soaking up procedure in the undermentioned order:The low wetting agent concentration has no consequence in the soaking up procedure sweetening.

The medium surfactant concentration enhances the soaking up rate by increasing the Marangoni convection.High wetting agent concentration reduces the soaking up rate. As the wetting agents aggregates on the surface and impedes the H2O vapor to make to the LiBr- solution surface. In add-on to the high temperature gradient which will halt the Marangoni instability.


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