Liquid Viscosity – an overview
Viscosity is the property that influences the flow of the gas, and it depends on the temperature of the gas. During energy balance and design calculations of equipment for a particular process, the material which will be handled influences all controlling parameters of the methods required for designing.
In most cases viscosities of gases are to be calculated at temperatures that are higher than normal and due to the temperature effect on the viscosity, flow behavior will vary significantly and the right value should be used in the calculation of transport equations,
All these data can be obtained from the research papers and data books which will be charged for some amount of money, for a rough calculation and design of the skeleton of the equipment on the drawing board a formula can be used for this purpose which
μ/μo= [T/To] n
The above equation is well used for every design calculation where
μ = viscosity at absolute temperature T (the temperature where viscosity is required)
μo = viscosity at temperature 0oC (273 K)
T= absolute temperature in K
To= 273 K
n is the valve which is determined by experiments
Selection of,’n’ value:
carbon dioxide and light hydrocarbons =0.9
steam and sulfur dioxide
The ‘n’ value can be applied to the substance which is similar to the above materials as it is the physical property, same group compounds will behave in the same way, for example, alcohol group, all alcohol compounds will have more or less the same physical behavior and the units will be same the units of absolute viscosity take at 273 K. So, all you need is the absolute viscosity of the gas.
Viscosity Index Experiment
Scope: The tables and equations included in the method provide a means for calculations of the viscosity index of the petroleum product from its viscosity at 40oC and 100oC, This method provides tables of oils with viscosities at 100oC between the values of 2 and 70 Centistokes(mm2/ sec) equations are provided for calculating basic values for oils having viscosities about 70 Centistokes at 100oC.
Definition: A viscosity index is an empirical number indicating the effect of a change in temperature and viscosity of an oil. A low viscosity index signifies a relatively large change in viscosity with temperature and vice versa.
Apparatus: Viscometer consisting essentially of a cup with an orifice in the center of the base, which may be closed by a ball and socket value, water bath, thermometer, recover time recorder, etc.
Procedure: Insert a cork in the bottom of the viscometer cup and fill the cup to the prepared sample to such a height that the leading plug on the value is just immersed when the taller is vertical. Insert a second cork provided with a central hole and groove at one side through which the steam of the valve may be passed into the upper end of the cup and pass the thermometer so that its bulb is approximately at the geometrical center of the same, then suspend the cup to its rim in the water bath maintained with 0.01o of the test temperature throughout the test maintain the bath temperature at this value by stirring frequently. Lift the valve and suspend the valve support to start the time-recording device when the liquid in the receiver when the liquid reaches 75ml mark.
Equation and Calculation:
(L-U)/(L-H) X 100 = (L-U)/D X 100
U: kinematic viscosity at 40oC of oil whose viscosity index, is to be evaluated.
L: kinematic viscosity at 40oC of an oil of zero viscosity index having the same viscosity at 100oC as the oil whose viscosity index is to be calculated.
H: kinematic viscosity at 40oC of an oil of 100 viscosity index having the same viscosity at 100oC as the oil whose viscosity index is to be calculated.
s.no Temperature Volume collected Time taken
1 example 100 deg C 200 ml 924 sec
kinematic viscosity of redwood viscometer is = 0.0026 tr – 1.561/tr
where tr is the time of efflux collecting.
Physical Properties like mole weight, melting point, viscosity, the standard free energy of formation, etc are available at this free data bank
Determination of the viscosity of the liquid can be done by various instruments:
1. Capillary viscometer
2. Orifice viscometer
3. High-temperature high shear rate viscometer
4. Rotational viscometer
5. Falling ball viscometer
6. Vibrational viscometer
7. Ultrasonic viscometer