3 replies to this topic

[Pasqualino]

Hi there,

 

I am being asked a question for an assignment involving binary distillation. The question reads "the feed, which is in the liquid state and at its boiling point..."

 

The wording of this confuses me. The feed itself contains hexane and octane.

 

Of the available states of the feed, termed q (sub-cooled, saturated liquid, liquid & vapour mixture, saturated vapour, or super-heated vapour), I'm inclined to say it is saturated liquid.

 

Can anyone assist with this?

[Art Montemayor]

This is such a basic-basic query that it needs some clarification in order to be of positive help to you and to other students.  In order to help you it is necessary to have some background as to your level of learning:

• Have you already successfully passed such requisite courses as general chemistry and physical chemistry?
• Are you presently taking your first chemical engineering course?  If so, what semester or period are you presently in?
• What Chemical Engineering course has presented this question to you?  Is it your first Unit Operations course, involving distillation?  If so, have you already gone through heat and mass balances, the McCabe Method, and the Ponchon-Savarit Method of resolving the number of stages in a distillation column?

Please respond to the above in order to give us a clear picture as to why and how you (or any other student) could be “confused” about how or why a feed can (or cannot) be fed to a distillation column at its boiling point (at process pressure - not atmospheric)?  The first steps in a basic and elementary McCabe Method of solution clearly define the “q” line and the role it plays in determining the final number of distillation stages.  It’s slope defines the “quality” or state of feed to the column.  Refer to the attached illustration.

 

Perhaps your problem is not one of engineering learnings but one of language skills.  You may not know what the words “boiling point” and “saturated liquid” mean in English.  A saturated liquid is a specific and proper definition of a liquid that is at its boiling temperature.  A liquid at its boiling point is a tricky definition generally used by chemists to identify a liquid boiling at atmospheric pressure (usually defined as sea level).  This type of definition is for purposes of identifying liquids relative to their atmospheric boiling points.  However, we engineers (being more practical and dealing with the real world, outside of the laboratory) require to know the boiling point of liquids at process conditions - which involves pressures above atmospheric.  That is why we generally always prefer to deal with the term “saturated liquid”.  This is a liquid that is boiling under process pressure - whatever that may be.  This condition is illustrated in the liquid water that is boiling inside of a boiler, creating steam.  It is also illustrated on the saturated liquid portion of the curve in a a Mollier Diagram - which you will have to understand and use when you take your first Thermodynamics course.

 

If it were me that received this problem statement, I would insist that the professor or instructor that issued it define what they mean by "in the liquid state at its boiling point".  Do they mean boiling at atmospheric or at process pressure?

 

I hope this lengthy discussion helps to give you a firm handle on how to visualize and understand the problem at hand and handle it accordingly.

 

 Q-line_slopes.png   6.06KB   1 downloads

[Pasqualino]

Having passed the basic requisite courses, this is the second specific chemical engineering course after having learned about Heat Transfer. This subject is regarding Processes of Separation, and is my first subject involving unit operations, distillation and the McCabe-Thiele method.

 

The diagram you have provided is exactly what I was referring to regarding the specific states of the feed, and you are correct that my confusion is around the language used here. If my language skills are correct, when you stated:

 

A saturated liquid is a specific and proper definition of a liquid that is at its boiling temperature.

 

This immediately cleared my confusion.

 

Perhaps this wording was used intentionally to probe our understanding of feed states. I will be asking the instructor to explain this wording further.

[Art Montemayor]

Paz:

 

Thank you for responding timely and confirming our concerns.

 

This, in my opinion, is a classical problem that is often confronted by many chemical engineering students.  Some professors and instructors, unfortunately, are either lackadaisical or simply LAZY when teaching the fundamentals of Unit Operations.  Students cannot, by normal standards, be aware that there are certain routines, procedures, understandings, or habits cultivated by academia that are foreign or not understood by the general public.  This creates a lot of misunderstandings or - worse - ignorance of what knowledge is being taught.

 

I personally have been the victim of such examples of bad teachings - some by graduate students used as lab instructors or lay teachers who really have little interest in the well-being and knowledge being passed on to undergraduate students.  This type of "instructor" really has only interests in his/her own future and aspirations.  Even some practicing engineers often acquire this type of lazy attitude when communicating with peers or associates.  They, for example will fail to use the proper and specific units of process values - such as saying psi, instead of either psig or psia.  Some people are simply naive and tend to believe that you and I can understand what they are thinking without them describing specifically what they mean.  This, I believe, is the culprit in your specific case.

 

I would take the indicated instructor to task and insist that he/she specifically state what are the specific conditions of the feed in question - at the process conditions of pressure and temperature.  This is a phase equilibria application (one that you will have to conquer in the near future, especially in your thermodynamics courses) and an important one to master when dealing with fluids at their phase boundaries.

 

I hope you capture and retain the importance of this aspect of specific and accurate communications in engineering.