Dear All,

I just wanted to share a small initiative of my new company towards saving of water. This can be presented by you to your management for implementation which will help us to keep our globe safe and provide sufficient water savings for many families to upkeep their daily life.

The water is very important for all living things and currently it is of utmost importance for any coroporate house, individual, and governments to conserve water & promote conservation fo water.

In our present company the management has implemented waterless urinals in the corporate building. This is saving around 2 Litres of water / day / person. We have around 400 people working in this office which is therefore, saving around 2 x 400 x 300 = 240000 Litre / Year.

Now see the importance of this number. Each family (4 persons) need around 100 litre of water / day for hygeine & drinking. Therefore, it can serve for 2400 families for 1 year OR 40 families for 60 years of average life.

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Compared to other physical properties the advantage with diffusion coefficients is that they are fairly uniform for a given state. For gases the value of 10-5 m2/s gives you nearly always the correct order of magnitude. If this rough estimate isn't enough and you don't find tabulated values in the literature and you also don't want to make measurements you can try one of the prediction methods.

Quite a number of different correlations and methods have been proposed over the years but the one semi-empirical equation from Chen and Othmer (J. Chem. Eng. Data 7 (1962), 37) is preferable because

• It is Simple

• Sufficiently Accurate in most of the cases

• Availability of inputs required

According to Chen and Othmer the diffusion coefficient D1,2 for the diffusion of gas 1 in gas 2 at moderate pressures can be calculated from the following equation:

The equation is very simple to use & is given below.

D(1,2) = 6.04 x 10 ^ -9 x (T^1.81 / p) x ( (M1+M2)/M1/M2)^0.5 x (Tc1 x Tc2)^0.1405

x (Vc1 ^0.4 + Vc2 ^0.4)^2

Where

M1, M2 = Mol Wt of both components

Tc1, Tc2 = Critical Temp in K

Vc1, Vc2 = Critical Volume in Cm3/mol

P = System Pressure in bar

T = Temp in K

D1,2 = Diffusion Coeff in M2/sec

I hope it is useful for many of you.

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The order of magnitude of diffusion coefficients in liquids is 10^-9 m2/s with the bulk of data in the range between 0.5 x 10^-9 m2/s and 5 x 10^-9 m2/s. Note that these data also hold for gases dissolved in liquids.

Equation of Chen and Othmer for gases is used for

• Its Simplicity

• Sufficiently Accurate

• Availability of Input Data

Correlation for liquids is that of Wilke and Chang (AIChE J. 1 (1955), 264):

The equation is very simple to use & is given below.

D(1,2) = 7.4 x 10 ^ -12 x (T / mu) x (C x M2)^0.5 x (V1)^-0.6

Where

D(1,2) = is the diffusion coeff of solute 1 in Solvent 2 (M2/s)

T = Temperature in K

mu = dynamic viscosity of solvent 2 (mPa s)

M2 = Mol Wt of solvent 2

V1 = Molar volume of solute 1 at NBP (Cm3/mol)

C = association factor of solvent 2 as below

C = 2.6 for water

C = 1.9 for methanol

C = 1.5 for Ethanol

C = 1.0 for non polar solvents.

I hope it is useful for many of you.

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In my previous article on the topic, I suggested a method called dimoplon rule for estimating the heat capacity for solution with dissolved solids.

The method was giving a detailed estimate with accurate prediction for such values of specific heat capacities.

However, I am also giving a very quick method for such evaluations with some approximation. The results from this method are accurate enough for preliminary estimates and are not too far from experimental values compared to previous method.

The Quick method is known as Vosseller method proposed by him in 1973.

Cpsolution = 1 - 0.7 x W1

Where W1 is Wt% of solids in solution.

So for our example of Sodium Carbonate solution of 20% in previous post will result in as below.

For 20% solids W1 = 0.2

So specific heat of solution shall be 1 - 0.7 x 0.2 = 0.86

Now compare this with literature data & previous detailed method.

Literature data = 0.85

Dimoplon method = 0.848

Vosseller method = 0.86

So error is still only 1.2% but the method is quite quick for initial understanding of the system & calculations.

Similarly Vosseller also proposed this kind of equation for organic solutions with water where the equation is

Cpsolution = 1 - 0.45 X W1

Here W1 is the Wt % of Organics in water. So for 20% Ethanol mixture the heat capacity can be assumed as = 1 - 0.45 x 0.2 = 0.91 which is quite close to experimental value.

Hope it will be useful for many of you in different industries.

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