7 replies to this topic

[lxz]

hello -

I have been given an assignment to calculate the max capacity of a distill column.
column data:
3ft dia.
column height: 60 ft
design: 180psig @425F
trays: 5, valve, spacing-18in
packing: depth 5' 7-1/2" -- type koch sulzer

what is inlet:
hydrocarbons - c4- c10 (4 chain carbons through 10 chain carbons)
4991 #/hr

what is outlet:
top (vapor) c4-c7
3177 #/hr

bottom (liq) c8-c10
2146 #/hr

there is also a reflux (condense top outlet stream)
1844 #/hr

I have calculated the max inlet flow rate apporx. 10000#/hr from the pump curve of the pump coming from the storage tank this mass flow # is with the control valve open 80%. It was confirmed by my mentor that the estimate was ok. However,I am concerned about flooding the column.

Basically I am a little jammed up on what the next step in my process should be, I can get whatever data I need, I have the info on all pumps , heat exchangers, pump curves, etc. I know calculating the outlet streams is not just a simple matter of scale - up. If someone could give me a little guidance on what my next step(s) should be I would be grateful. I can handle the calculations etc. All dimensions of the column, pumps, control valves are set, my assignment is to find the max. capacity of system as is, I am not allowed to recommend changes to equipment. I have all P & Id diagrams for everything on the system.

If someone is willing to give a little guidance I will get the work done and report back all calculations and conclusions.

thank you for assistance

[Art Montemayor]

Lxz:

You are a student and you have been given an assignment typical of what happens out in an industrial application. You've been told that you have the simulation output for the distillation of a new process and the company wants to save money and use an existing, idle distillation unit that is available for that use. What is the maximum amount of capacity that you can get out of the existing column when distilling the given feed?

You are given a lot of detailed basic data – most of which doesn't apply to the solution. Here is where you have to use your common sense. Go back to your basic course in distillation and look up the information on two classical Chemical Engineering pioneers: Mott Souders and Dr. George Granger Brown. In 1934 they published their method for sizing the capacity in distillation columns. If you use the SEARCH feature on our Forums you will find numerous threads on their famous equation:

Max. Allowable superficial velocity = K [(dL - dg)/dg]^0.5

where,

K = an empirical constant;
dL = Liquid density;
dg = gas (vapor) density.

Knowing the superficial velocity in that section of the tower, you can calculate the allowable diameter since you also know the gas or vapor rate going up the tower.

This equation is one of the really basic and great ones, developed over 75 years ago by two of the really great chemical engineers that ever used a slide rule. Souders went on to lead engineering in Shell Oil; Brown was (and has been) known as an outstanding academic in chemical engineering at Michigan.

Note that all you need to know is the vapor mass rate up the column and the liquid & vapor densities. The value of K is published in literature and references, depending on the application.

Obviously, you want to find out the difference between the bottom of the column and the top section. Run the above equation for the densities at the town sump as well as for the tower top section. You will get two different diameters; compare their difference and if the difference isn't great or it is more economical, use the biggest diameter of the two for the tower.

By using the SEARCH function, you can find further information on tower sizing on our Forums and also K values and references to them. Do not be lazy. Do the leg work and you will get results. The answer is easier than what you think.

Good luck.

[lxz]

hello-

i have taken into account the considerations and hopefully made progress

Since I already have my column diameter fixed at 3ft. I thought I could try to calculate the superficial velocity by use of area and G (gas mass flow). Superficial vel. is the empty vessel velocity? If so, I used the vapor exit stream b/c there is no vapor in the inlet stream.

U = (3177 #/hr)*(1/3600 sec)*(1/ vapor density)* (1 / area = 7.07 ft^2)
U = .5312 ft/s

I have my doubts about this b/c if K is constant than U would not change with mass flow as it does here. I could not find a K value, however I found that K is approx. between .25 and .4 depending upon application. I believe I need to calculate superficial vel. by K [(dL - dg)/dg]^0.5 . Then cal. the area from there since I known my diameter is fixed at 3ft I could compare the areas to each other and see which one is bigger. I did however find the densities from the top and bottom so I can run both calculations.

data:
density of liquid : 40.8
density of vapor : .235

-- i will keep at this and continue to post results along the way, if anyone notices mistakes do not hesitate to let me know, it will not hurt my spirits

thank you for the assistance

[Art Montemayor]

Lxz:

First some comments to make your life and everyone else's easier:

Please be explicit when you write your explanations and basic data. I don't know what "b/c" means. I do know what "porque", "a causa de", "a causa di", "a cause de" all mean. If you write in English, then use the English language. I (as well as the veteran and experienced contributors to the Forums) do not resort to gibberish and teenage "chat" talk. If we are to resolve serious engineering problems, we need serious engineering language that we can all understand in order to respond accurately and efficiently.

I am glad that you now understand that what you have is a relatively simple problem. However, it is based on identifying the vapor flow through your column. You have not mentioned this, so I must assume that you have missed this important point. You have to know the maximum and the minimum vapor flows in your column – and where exactly they are located. This is something you should already have calculated in making your column design with the basic data you are given.

You are correct in identifying that the superficial velocity is the "empty vessel velocity". You also state "I used the vapor exit stream b/c there is no vapor in the inlet stream". There may be no vapor in the inlet stream, but there certainly vapor going through the feed tray! And what about the vapor generated in the sump of the tower (from the reboiler)? From the way you write your response, I don't think you have a proper grasp on how a distillation tower works and what it can and can't do. If you are to resolve this problem correctly, you must have a complete and accurate understanding of what is occurring inside a distillation tower and why. Otherwise, you don't know what you are doing.

Why do you write "I believe I need to calculate superficial vel. by K [(dL - dg)/dg]^0.5"? I have told you that already! It is not a question of "believing" it yourself; it is a question of reading and studying what is in your Unit Operations course. That equation should have been studied by you already. If you have not been aware of that equation up to now, then admit it and accept the fact that you have missed out on the only way to "size" a proper diameter for a distillation column. You should then follow what I recommended: research this equation and the work of the authors – Souders and Brown. You will never regret doing this research on this equation because it will come up again and again if you are to become a Chemical Engineer

Good Luck

[lxz]

As I stated in another post I have not had unit operations. b/c is an often used abbreviation for because. I am currently studying Transport and Separations Process Principles (Includes Unit Operations) by: Christie Geankoplis 4th ed.

In the previous post I stated that I needed to calculate the superficial velocity by the eqn. stated involving "k". However, by unit analysis in the above eqn. I stated using the area will also give superficial velocity. Since I know my diameter is 3ft. then I know my area and can cal. the superficial velocity that way. I stated I had my doubts about that particularly because (b/c) the gas velocity I was using exit stream gas velocity.

"However, it is based on identifying the vapor flow through your column. You have not mentioned this, so I must assume that you have missed this important point. You have to know the maximum and the minimum vapor flows in your column "

That is correct I did miss that point, which is why I doubted by answer I knew something was in error. once I figured out what it was it was corrected. Perhaps it was assumed I knew more than what I did, or perhaps I made it appear that way. Regardless, I have begun to study the various separation processes on my own time since this is the only way to be certain to master what is needed.

thank you for assistance

[djack77494]

In the previous post I stated that I needed to calculate the superficial velocity by the eqn. stated involving "k". However, by unit analysis in the above eqn. I stated using the area will also give superficial velocity. Since I know my diameter is 3ft. then I know my area and can cal. the superficial velocity that way. I stated I had my doubts about that particularly because (b/c) the gas velocity I was using exit stream gas velocity.

"However, it is based on identifying the vapor flow through your column. You have not mentioned this, so I must assume that you have missed this important point. You have to know the maximum and the minimum vapor flows in your column "

That is correct I did miss that point, which is why I doubted by answer I knew something was in error. once I figured out what it was it was corrected. Perhaps it was assumed I knew more than what I did, or perhaps I made it appear that way. Regardless, I have begun to study the various separation processes on my own time since this is the only way to be certain to master what is needed.

thank you for assistance

lxz,
Sounds like an excellent and surprisingly practical type of problem; unusual for an academic setting.

In your initial post, you seemed to satisfy the requirement that your feed pump was adequate. I did not see a similar confirmation for the reflux or bottom (resid) pumps or for the overhead (gas or liquid) system. All must be confirmed to be adequate in a full system analysis. You must also consider possible limitations of your condensing and reboiling capabilities, and maybe a feed preheater.

As you approach an analysis of your distillation column's capability, you should simulate your system and get tray by tray liquid and vapor flowrates and properties. I'm not certain you are aware of this crucial fact. The simulation will be valuable for many purposes including all your flowrates and duties, but here I'm focusing on the column itself. Since you appear to be dealing with an existing column with both trayed and packed section of known properties, I would recommend that you use correlation specific to the trays and packing you have installed. You have the additional advantage of having your column internals provided by a well known and reputable vendor who offers excellent software for designing distillation columns using his trays and packings. (At least I think this is true though you refer to Koch Sulzer as if they were one company, which they are not.) Check out the Koch or Sulzer websites, as appropriate, and download the software(s) that pertain to your column internals so that you may rate them. You must have fully characterized the column's internal flowrates and properties to do this. The end result should be a full characterization of the column and other parts of your system, along with an understanding of the "bottlenecks".
Doug

[lxz]

hello-

sorry for the delay in update.

The column will not flood, i have also calculate the max. capacity of each pump (inlet, outlet-top, outlet - bottom). The bottom pump will be the limiting factor of the system. The system using control values to regulate flow which was something new to me, as I have never interned in the field before. Therefore I had to calculate max flow rate for each valve at 80% open. I was told 80% is the max opening where the operator still maintains control over the flow rate. In doing so I calculate max inlet at 14Mlbs/hr. since the inlet control valve was limiting. I did calculate the increase duty for heat exchangers for the system, however I had plenty of heat if needed. I gave the calculations to my mentor and said we were all clear, based on the calculations I am only using about 50% of what the heat exchanger is capable of. Once the 14Mlb/hr feed rate was field tested and verified my mentor asked me to analysis on enlarging the control valve to feed, this is where i determined the bottom pump was limiting with an enlarged inlet control valve.

Currently I am running cost analysis to determine if the bottom pump should be replaced. I have incurred another problem. The tank level where the inlet feed comes from decreases and I need to determine what the tank level needs to be in order to maintain a 14Mlb/hr inlet mass flow rate. Since it has been determined the 14Mlb/hr rate is doable my mentor now wants to increase to 17.5Mlb/hr and for this rate to be sustainable, i.e the feed tank level has to remain constant and not decrease.

I realize this is a general update. However the entire experience is new to me. There seems to be a "spiderweb" effect in a chem. plant. That is to say, changing one thing (however small) branches out and effects numerous operations all over, some of which I was unware of.

Any hints, comments, critics do not hesitate to let me know. I will keep posting updates for other interns in the chem. manufacturing sector.

thank you for assistance

[djack77494]

That is to say, changing one thing (however small) branches out and effects numerous operations all over, some of which I was unware of.

This is probably a fair statement. Some of this stuff is complicated.

The system using control values to regulate flow which was something new to me, as I have never interned in the field before. Therefore I had to calculate max flow rate for each valve at 80% open. I was told 80% is the max opening where the operator still maintains control over the flow rate.

Control loops with control valves are the "standard' in industrial process control. 80% is reasonable, though conservative; I like 85% as a maximum remembering that I need to be able to move in both directions if I really have control.

The column will not flood,

That's good. I assume you know that you must check flooding for every tray and every section of packing, and not just "the tower" as if it could be considered as a unit.

I had plenty of heat if needed. ... I am only using about 50% of what the heat exchanger is capable of.

The first statement makes no sense to me. You will always have plenty of heat unless you run out of the utility or process heat source that supplies that heat. The second statement is what i was looking for - your mechanical equipment is adequate.

The tank level where the inlet feed comes from decreases and I need to determine what the tank level needs to be in order to maintain a 14Mlb/hr inlet mass flow rate.

I detect faulty reasoning here. To maintain a 14Mlb/hr inlet mass flowrate = flowrate out of the tank if my understanding is correct, you need an inflow to the tank of 14Mlb/hr. If you have more, the tank level will increase, less and it will decrease. If you want a sustainable 17.5Mlb/hr feed, then you must feed 17.5 Mlb/hr into the tank. There's no rocket science here.
Doug