4 replies to this topic

[Art Montemayor]

ALL:

Your questions are NOT stupid. Here are my comments (in red) to them:

A high temperature and pressure fluid enters the throttle and experiences a pressure reduction. If there is a component within the fluid which is more volatile than the others and if the pressure reduction from the throttle has reduced the pressure below that component's vapor pressure, then that component will enter the vapor phase. (The fluid can be high temperature or low temperature – but it must exist upstream of the throttle valve as higher in pressure than the interior of the flash drum. Otherwise, there is no fluid flow in the direction of the flash drum. Yes, when you lower the pressure below a fluid's vapor pressure at saturation, it will vaporize ("flash") downstream of the throttle valve – you will create a 2-phase mixture.)

The drum attached to the throttle then provides the space for the vapor and liquid to separate from each other, where the liquid travels downward and the vapor upward. (The drum is not attached to the throttle. The throttle valve is piped to a nozzle – which is welded to the flash drum. The flash drum is sized according to the Brown-Souders equation and is designed to facilitate the separation of the 2-phase mixture resulting from the adiabatic – isenthalpic expansion of the feed fluid through the throttle valve.)

The process can be isenthalpic, isothermal or isentropic, with isenthalpic being the most common. (In the real, practical world of industrial processing, the throttling of the pressurized fluid is considered 100% isenthalpic. There is no useful work done. The process is definitely NOT ISENTROPIC and it practically cannot ever be isothermal. There is no practical way that anyone could keep the temperature constant during the throttling. If any human being can devise a device or gimmick that can ensure isothermal operation during throttling, I'd like to be informed of it.)

Now, my confusion stems from the throttle. Is the throttle placed inside the flash tank (A), outside and away from the flash tank ( B), or outside and directly before the flash tank (C )? See attached for picture. (The throttling valve can be inside the flash tank or outside, directly before the flash tank. I've designed and installed both varieties for specific engineering reasons. You normally want to stay away from "flashing" into a pipe - outside and away from the flash tank.)

Also, what exactly happens in a throttle? I know that the fluid flows into a constriction and the pressure drop is produced from the smaller volume. However, after the throttle, why doesn't the pressure return to the same amount it was before (like Bernoulli's equation indicates it should)? Is it because there's been momentum/frictional losses, which Bernoulli's doesn't account for? If there have been 'losses' and the system is adiabatic, shouldn't those momentum/friction losses be transferred into heat energy, increasing the temperature of the fluid after the throttle? (Your asserted knowledge is wrong. The resultant pressure drop is NOT produced from the smaller volume. The pressure drop across the throttle valve exists simply because the resultant products – primarily the produced vapor – are being continuously removed from the flash drum. This is a STEADY STATE process. The prerequisite condition is that there must be an existing steady state (continuous flow). Otherwise, the process cannot work. There is nothing "Bernoulli" about this simple fact. You must have systems downstream that are continuously taking your product vapor and the continuously drained liquid. It is that simple.)

However, it is indicated that temperature typically decreases. So is the above analysis incorrect? I also tried to think about the process this way; If the process is isenthalpic, the reversible equation for enthalpy is H = U + PdV. When pressure decreases, U (and temperature) should increase to keep H constant over the throttle. So by this analysis, temperature increases too. Where am I going wrong? (The easiest way to understand how the process works and where your thinking has been "warped" is to obtain a liberal-sized version of the Mollier (or the Temperature-Entropy) Diagram for Steam – Water. Draw the expansion ("throttle") process out on the diagram and you will become aware that you create a 2-phase mixture under the saturation dome curve - the portion we used to call "the big tit" in Chemical Engineering Thermo class. One portion of the expanded product mixture goes to the right-hand, saturated vapor part of the curve and the other portion goes to the left-hand, saturated liquid part. See the attached Excel workbook that I'm giving you. This example is for a refrigeration cycle, which is one of the immediate applications of this throttling process that you are studying and should be an indication to you how hugely important this process is in the real world. You are correctly spending your time in trying to fully understand this process and you will profit greatly as an engineer from this study time you are now putting in. This is very important, basic engineering that you will apply the rest of your life.)

Also, I've seen the word throttle used as a verb as well as a unit operation. What exactly is a throttle, physically? In class we are taught that a throttle is simply a constriction – like a smaller place in the pipe. So is it, or can it be, a valve? If so, why do some valves function better as throttles than others? (A throttling process is a Unit Operation that is described as a thermodynamics process. It is adiabatic and isenthalpic in nature. It is NOT SIMPLY A CONSTRICTION. A venturi tube is also a constriction – and it is not the same effect. Look to Thermo to explain the effect in detail. The throttle can take place through a constriction orifice or a valve -which is a variable constriction orifice. Some valves function as better constriction orifices than others – i.e., butterfly valves versus globe or needle valves.)
 CarbonDioxideRev1.xls   152.5KB   260 downloads

[Art Montemayor]

AlL:

If you are reading Smith & Van Ness on Basic Thermodynamics, then we are on the same "page". Please read my inserted worksheet in the CarbonDioxideRev1 which I have revised for your benefit. It pretty well explains the effect that takes place and describes it mathematically.

To answer your specific question, the effect of pressure drop is simply that of a driving force. It is analogous to the Fourier's equation for heat transfer and the basic effect that drives electricity to go from one energy level to a lower one - voltage drop. That's the best and simplest way I can describe it.

Study all the Mollier Diagrams that Smith & Van Ness give you - especially the ones for CO2, Water, Ammonia, and the light alkanes (methane, ethane, propane, butane, & Pentane). Note the change of phases and how the enthalpy and entropy curves are related to the phase changes. This is very important and confirms all that Thermo is based on. The ability to visualize the shape and position of the curves will be of extreme value to you later on as you progress into professional engineering. Please trust me on that.

I hope this helps.

[katmar]

A.L.L., I must compliment you on a very well stated question. Stating the question well is a HUGE step towards getting the right answer. There is very little I can add to the thorough and complete answer Art gave you (it is good to see that you appreciated is wonderful answer).

However, I would like to correct a mistake you made because it is a mistake I have seen PhD graduates make and it could impact on your understanding of many other operations in chemical engineering. You stated:

"First (correct me if I’m wrong), here is my understanding. A high temperature and pressure fluid enters the throttle and experiences a pressure reduction. If there is a component within the fluid which is more volatile than the others and if the pressure reduction from the throttle has reduced the pressure below that component’s vapor pressure, then that component will enter the vapor phase."

Individual components DO NOT separate out in this way. ALL the components flash to some extent. You must think of the vapor pressure OF THE MIXTURE, and not of the individual components. It is of course true that the more volatile components will be enriched in the vapor phase. If you think of a single component application (like steam condensate entering a condensate collection drum from a steam trap) you will see that part of the water flashes to steam and part remains as liquid water according to the heat balance. Similarly if you had a hot pressurised steam of water and ethanol you would find that the vapor phase has a higher concentration of ethanol than was in the feed stream, but there will be water in the vapor too. If the vapor pressure of the MIXTURE is above the pressure in the drum then there will be flashing, and the quantity flashed will be determined by the heat balance. The composition of the two phases will be determined by the vapor-liquid equilibrium behavior.

Sorry to be pedantic about this, but I have seen this mistake made too often and I have become rather sensitive to it.