Joule-Kelvin effect, Throttliing heat transfer.pptx

Joule- kelvin effect, throttling

1. Throttling & The Joule- Thomson Experiment  Throttling process involves the passage of a high pressure fluid through a narrow constriction.  The effect is the reduction in pressure and increase in volume This process is adiabatic as no heat flows from and to the system, but it is not reversible. It is not an isentropic process The entropy of the fluid actually increases  Such a process occurs in a flow through a porous plug, a partially closed valve and a very narrow orifice.

2. In this experiment gas is forced through a porous plug and is called a throttling process In an actual experiment, there are no pistons and there is a continuous flow of gas A pump is used to maintain the pressure difference between the two sides of the porous plug . In a refrigerator compressor is enough, additional pump not required

3.  In the experiment for different sets of 𝑃 𝑖 , 𝑇 𝑖 P f and 𝑇 𝑓 is measured and plotted  Consider a series of experiments in which 𝑃 𝑖 and 𝑇 𝑖 are constant ( 𝐻 𝑖 constant) and the pumping speed is changed to change 𝑃 𝑓 and hence 𝑇 𝑓  Since the final enthalpy does not change, we get points of constant enthalpy

4. Mechanical Work Done is zero, but there if ‘Flow Work’ dW = 𝑃 𝑓 𝑉 𝑓 − 𝑃 𝑖 𝑉 𝑖 The overall change in internal energy of the gas is ߜ Q = dU + dW For Adiabatic Expansion ߜ Q is 0 = ( 𝑈 𝑓 − 𝑈 𝑖 ) + 𝑃 𝑓 𝑉 𝑓 − 𝑃 𝑖 𝑉 𝑖 𝑈 𝑓 + 𝑃 𝑓 𝑉 𝑓 = U i + 𝑃 𝑖 𝑉 𝑖 But Enthalpy is H = U + PV 𝐻 𝑓 = 𝐻 𝐼 Hence, in a Throttling process, enthalpy is conserved.

5. 5 A smooth curve is placed through the points yielding an isenthalpic curve . {Note that this is not a graph of the throttling process as it passes through irreversible states.} • • • • • • • • P f , T f isenthalpic curve P i, , T i P f T f

6.  This is therefore an ISENTHALPIC expansion and the experiment measures directly the change in temperature of a gas with pressure at constant enthalpy which is called the Joule- Thomson coefficient (μ). μ = 𝜕𝑇 𝜕𝑃 𝐻  For expansion, 𝜕 P is negative , if 𝜕𝑇 is also is negative and therefore a positive value for μ corresponds to cooling on expansion and vice versa. dU = dQ - dW dQ = dU + dW = 0, for adiabatic dU = - dW = U f – U i = W i - W f If there is no expansion work, only flow work happens, then, W = PV U f – U i = P i V i - P f V f U f + P f V f = U i + P i V i H i = H f

7.  The gas which is initially at a state represented by the point P as shown in fig., is undergoing Joule-Thompson Expansion.  It will experience the rise in temperature till the point Q is reached, and thereafter the temperature decreases with further decreases in pressure.  The slope of the isenthalpy curve is equal to the Joule-Thompson coefficient as per the relation.  It is positive only in the region where pressure is less than that of Q and is Zero at point Q, where the isenthalpy exhibits a maximum. A smooth curve is placed through the points yielding an isenthalpic curve

8. μ μ < temperature increases μ = temperature remains constant μ > temperature decreases

9. 9 If point a on the diagram (  < 0) is a starting point and point b is the final point, then the T of the gas will rise, i.e. we have heating. If, on the other hand, we start at point c ( > 0) and go to point d , then the T of the gas will drop, i.e. we have cooling. As higher initial starting temperatures are used, the isenthalpic curves become flatter and more closely horizontal. These curves are horizontal lines for an ideal gas. As noted on the plot there will be a maximum inversion T, the value of which depends on the gas. For cooling to occur, the initial T must be less than the maximum inversion T. For such a T the optimum initial P is on the inversion curve.

10. Theory behind temperature drop during throttling process When there is a decrease in internal energy of the gas during expansion It produces temperature decrease and cooling effect When there is an increase in internal energy of the gas during expansion It produces temperature increase and heating effect This can be associated with molecular level kinetic energy of molecules. After expansion when molecules have lots of volume and space, molecular interactions and collisions reduce, which produces a reduction in K.E. Of molecules, directly affecting temperature drop, as per the relation: K.E. = 2/3 ​k B ​.T ..........(1) k B ​= ​R/N A ​ It should be noted that the gas at high pressure just after leaving the valve/obstacle/restriction Has small increase in temperatures due to the compression effect, until expansion Becomes dominant More importantly, throttling process is a Sharp/sudden decrease of pressure and drastic increase of volume, which leads to changes in temperature

11.  This also tells us that we cannot just use any gas at any set of pressures to make a refrigerator, for example - At a given pressure, some gases may be cooling but others may be heating  The proper choice of refrigerant will depend on both the physical properties, esp. the Joule-Thompson coefficient as well as the mechanical capacity of the equipment being used.  Thus, we cannot just exchange our ozone-depleting freon in our car's air conditioner with any other coolant unless the two gases behave similarly in the pressure - temperature ranges of the mechanical device, i.e., they must have the same sign of ' m u’ at the pressures the equipment is capable of producing.  Generally, to use a more environmentally friendly coolant, we need to replace the old equipment with new equipment that will operate in the temperature range needed to make " m u” positive  The sign of the Joule–Thomson coefficient, μ, depends on the conditions  The temperature corresponding to the boundary at a given pressure is the ‘inversion temperature’ of the gas at that pressure

12. Application Of throttling process  The throttling process is commonly used for the following purposes : For determining the condition of steam (dryness fraction) For controlling the speed of the turbine Used in refrigeration plants Liquefaction of gases. In the Linde technique as a standard process in the petrochemical industry In many cryogenic applications. At room temperature, all gases except hydrogen, helium, and neon cool upon expansion by the Joule–Thomson process when being throttled through an orifice; these three gases experience the same effect but only at lower temperatures.

13. Simple refrigeration cycle

14. Show Starting from 2 nd working equation,

15. As we know, Throttling is isenthalpic process, dH = 0, implies,

16. Natural Gas Liquefaction process

17. IMPLIES,

18. R ef e re n c e s “A Textbook of Chemical Engineering Thermodynamics.” K.V. Narayanan PHI Learning, 6 th ed., pp 127,150, 214. 2. J.H. Noggle, "Physical Chemistry", 3rd ed., Harper Collins, 1996 pp 104ff. 3. R.G. Mortimer "Physical Chemistry", Benjamin/Cummings, Redwood City, Calif., 1993, pp 70-73.