11 replies to this topic

[Chase]

Hi there, 

For my project, I have to design a stripper column in an ammonia plant. The information I have is the flow rate, temperature and pressure of MDEA and CO₂ entering the stripper. Generally not sure where to start? Would it be easier to design a tray column or packed?  I've been doing a lot of reading and I've come across many different methods and equations for calculating diameter, HETP, etc. I have attached an example below. Are these equations correct? If not could you point me towards the right direction and let me know where I should be looking.

 

Thank you for your time.

 

Edited by Chase, 28 February 2013 - 01:46 PM.

[Art Montemayor]

What do you mean by: "Generally not sure where to start?"

If you don't know where to start, your cause is already lost.  Get down to serious engineering business and ask yourself:

 

What is my process supposed to do?

What capacity is my process supposed to meet?

 

When you answer those simple, common sense questions you will know where to start.  First of all, you had better know just exactly what it is that a "Stripper" does and what it is supposed to do in your case.  To give you a BIG hint and at the risk of doing your work for you, I can tell you that you are supposed to remove CO2 from a Rich MDEA solution stream.  How much CO2 are you supposed to remove?

 

If you know the MDEA process and design basis for acid gas loadings, you should be able to establish the CO2 loadings in both the Rich and the Lean MDEA solutions.  The difference between those figures is the amount of CO2 you are to remove in the stripper.  Now you can calculate the reboiler duty to carry out the stripping by doing a material balance around the stripper / reboiler.  That's how you start.

 

Now you are ready to generate some of your own work and if you care to, you can submit it to this Forum for review and recommendations, suggestions, critiques, and checking.  Submit your work in an orderly Excel workbook showing all calculations and design basis.

[kkala]

Besides <http://www.cheresour...0-mea-solution/> can be of some introductory help. Better to explain meaning of symbols in presented equations (post no 1) and clarify units, if equations are not dimensionless.

[Chase]

Thank you Art for your feedback. I was referring to the design calculations by that. That is what I believed it was doing but thank you for clarifying. I have done a basic mass balance and attached it.

Thank you kkala for the link. Here are the meaning of symbols:
G = Mass flowrate of gas
G' = Mass flux of gas

A = Size of packing
σ = Surface tension of liquid
µ = Overall viscosity of feed stream

β = L/HG
L = Molar liquid flow rate
G = Molar gas flow rate
H = Henrys law constant
x2 = Solute contents in liquid inlet stream mol frac
x1 = Solute contents in liquid exit stream mol frac
y1 = Solute contents in gas at bottom mol frac


Thank you once again to both of you for your time.

Attached Files

•  MDEA-CO2.xlsx   30.38KB   232 downloads

Edited by Chase, 01 March 2013 - 10:22 PM.

[kkala]

Thanks sincerely for having the labor to explain symbols, facilitating use of formulas. I assume that σ is in dyn/cm, μ in cP, G'=mass flux of gas. Can you advise, if not so?

[Chase]

σ =  mN/m

 μ = Pa s

G' = mass flux gas

[Art Montemayor]

Chase:

 

You are a student in chemical engineering.  It is essential and very important that you know from the outset that you never - and I repeat: NEVER - start a design project on a distillation or absorber column without knowing up front what exact and specific type of internals you propose to use in that column.

 

If you don't know (or haven't decided) what type of internals you propose to use in your column, you are wasting a lot of valuable time and effort on your part and everyone else's.  What I am telling you here is what your Unit Operations instructor or professor should have already told you or is telling you.  If he/she haven't done so already, then you have a very inept and poor Unit Operations instructor.  The type of internals (whether packed bed or trays) should be decided on a variety of other things than just preferences or guesses.  That decision has an impact on such important chemical engineering factors as: turn-down ratios, column diameter, effective vapor-liquid distribution, capital costs, operating and maintenance costs, safety issues, operating flexibility, cleanliness of fluids, etc., etc.

 

My strong and experienced advice is to first make your basic decision on what type of internals you intend to employ before attempting to calculate the column diameter or size.  You must walk before you can run.

[Chase]

Thank you once again Art for your valuable input. I should have made clear that I have selected the internals at the start. I have chosen packed column - random packing - intalox saddle - from the research I've done. I would just like to find out if the equations I've presented are correct?

 

Thank you.

Edited by Chase, 04 March 2013 - 08:20 PM.

[Pilesar]

Amines are used in solution with water for CO2 removal. MDEA is typically not more than 50% of the solution. The composition in your spreadsheet is missing the water. For a brief summary of design parameters, see http://www.amines.com/mdea_comp.htm

[kkala]

I made a brief search, mainly in books, without success in tracing mentioned equations. Probably they are expressed as diagrams existing in books. Algebraic expressions have got necessary in the computer age. Or probably some of them are specific for the intalox saddles (size?) adopted. My experience is quite limited and have not looked into "manuals" of packing suppliers (e.g. Glitsch, Koch).

Concerning HETP equation, liquid surface tension in milli N/m is same as in dyn/cm;  fraction (σ/20) seems right. Such fractions are not very big or small numbers as a rule (actually being correction factors from one fluid to another).

But the viscosity fraction (μ/0.2), with μ expressed in Pas, could be a reason for further check. Feed overall viscosity μ concerns gas. Viscosity of gases are in the order of (say) 0.02 cP = 20E-6 Pas = 20 μPas, <http://en.wikipedia.org/wiki/Viscosity>. Thus (μ/0.2) ~ 1E-4, quite small number. This may be correct, but confirmation would be good.

Can Chase give more information on where the equations have been found, by whom each of them was "published" (if name is known), etc? In this way members with experience will have a better background to help.

[Pilesar]

The operating conditions of your tower throw up some caution flags. Confirm your temperature profile. 76C at top and 84C at bottom is too cool. Remember that your solution is half water so will have a similar boiling point. It should boil at the bottom and condense at the top. Pressure of 15 bar seems too high as strippers usually operate a bit above atmospheric pressure. The material balance must be right before you can properly design the equipment. I think you may have jumped ahead to the detail design phase of your project too soon. 

 

Trays are usually used for strippers, but you can use packing. I would choose ceramic over plastic just to avoid the problems plastic can cause at higher temperatures. When you calculate your diameter, be sure to add a bit because foaming can be a problem in these towers. 

[junaid8942]

Chase your calculation are pretty good. the preferable dia for tray is taking minimum 2 ft below this we replaced tray from packing.

 

The packed tower design concepts are listed below which help you to design the absorber column:

 

1) Packed towers almost always have lower pressure drop compared to tray towers.

 

2) Packing is often retrofitted into existing tray towers to increase capacity or separation.

 

3) For gas flow rates of 500 ft3/min (14.2 m3/min), use 1 in (2.5 cm) packing, for gas flows of 2000 ft3/min (56.6 m3/min) or more, use 2 in (5 cm) packing.

 

4) Ratio of tower diameter to packing diameter should usually be at least 15.

 

5) Due to the possibility of deformation, plastic packing should be limited to an unsupported depth of 10-15 ft (3-4 m) while metal packing can withstand 20-25 ft (6-7.6 m).

 

6) Liquid distributor should be placed every 5-10 tower diameters (along the length) for pall rings and every 20 ft (6.5 m) for other types of random packing.

 

7) For redistribution, there should be 8-12 streams per sq. foot of tower area for towers larger than three feet in diameter. They should be even more numerous in smaller towers.

 

8) Packed columns should operate near 70% flooding.

 

9) Height Equivalent to Theoretical Stage (HETS) for vapor-liquid contacting is 1.3-1.8 ft (0.4-0.56 m) for 1 in pall rings and 2.5-3.0 ft (0.76-0.90 m) for 2 in pall rings.

 

11) Packing support is used to carry the weight of the wet packing while allowing free passage of the gas and liquid. Gas inlets are provided above the level where the liquid flows from the bed.

 

12) Liquid distributor is used to maintaining a uniform flow of liquid throughout the column. For small diameter columns, a central open feed pipe or one fitted with a spray nozzle may well be adequate.