4 replies to this topic

[mubarakahmad]

why enthalpy remains constant in throtling process while the temperature decreases after the throtling. enthalpy should decrease when temperature decreases.

[thorium90]

well, its not 100% isenthalpic in reality. But the pressure drops across the valve and so temperature drops too.

Think this picture sums up the math behind it.

http://www.learnther...sson-C/pg09.php

Edited by thorium90, 17 February 2013 - 07:29 AM.

[ankur2061]

mubarakahmad,

 

Refer to the link below to understand how a throttling process is isenthalpic:

 

http://www.engineers...ing_process.htm

 

http://en.wikipedia....–Thomson_effect

 

In the Wikipedia link refer the portion related to free expansion which is also called as throttling process and is isenthalpic. Also refer the section where the mathematical proof is provided as to how the specific enthalpy remains constant for an ideal gas.

 

Hope this helps.

 

Regards,

Ankur

[Art Montemayor]

Mubarakahmad:

 

In my opinion you are failing to understand the reasoning and logic behind an isenthalpic process because you are not applying basic thermodynamic logic to the operation.

 

It all starts with an energy balance around the system and this quickly resolves itself through logic and arithmetic (there is no mathematics involved) into the simple relationship, DH = 0.  This applies to an irreversible process.  It is fully explained in Smith & Van Ness’ classic text book, “Introduction to Chemical Engineering Thermodynamics”.  Since DH = 0, the value of enthalpy is constant throughout the process.  In the majority (but not all) of cases, this results in a temperature decrease of the produced fluid.

In thermodynamics, you must first understand the process taking place and its effect on the surroundings.  The application of thermodynamics is not based on mathematics; it is based on the application of thermodynamic logic as it applies to its basic laws.  Once you establish the basis, you use math to define the operation.  You must first understand fully the Thermodynamic Laws before attempting to define them using mathematics.  For example, the concept of reversibility is important as it segregates the isenthalpic process from the isentropic one: the isenthalpic process yields a cold fluid product without generating useful shaft work; the isentropic process yields a colder fluid product while also generating useful shaft work.  Both processes are assumed as adiabatic (no - or - little heat transferred with thesystem).  It is obvious that the isentropic process is the most desirable in the industrial world – but it has its tradeoffs.

 

You state: “enthalpy should decrease when temperature decreases”.  This is not true in an adiabatic process carried out irreversibly.  The simplest and most direct way to understand the principle is to use a Mollier Diagram or a T-S Diagram.  Follow the process through the Diagram and you will become aware of what is truly taking place.
 

I hope this helps to explain the processes to you.

[kkala]

1. <http://www.cheresour...ure-calcualtion> deals with "isenthalpic" and "isentropc" processes, but it may help understanding previously mentioned principles  by examples, a fruitful way in thermodynamics.

2.  Enthalpy H and internal energy U are related through H=U+pV (p=pressure, V=volume). For ideal gases pV=contant for given temperature and U depends only on temperature; in throttling change of P does not affect U or pV, temperature and H remains same upstream and downstream. For real gases U, pV are generally dependent of pressure; in throttling H remains same, but temperature can increase or decrease (more usual), depending on  fluid and conditions. Joule-Thomson coefficient (in books of Physics or Physical Chemistry) can give more insight.

For instance, saturated steam 25 bara (224 oC) throttled to 10 bara will get a temperature of about 190 oC, as one can calculate using steam tables.

A case of increasing U with decreasing temperature for steam can be seen in <http://www.cheresour...saturated-steam> (not possible, if gas were ideal).  

Edited by kkala, 18 February 2013 - 05:27 AM.