Amount of Steam Required in MEA stripper 1

[Timewrap]

Dear Experts,

I'm doing a pilot plant study where I have a stripper which uses live steam to strip off acid gas to recover rich MEA solution. The MEA flow-rate is about 0.5 gpm and it contains H2S and C02. The exact composition of the acid gas is unknown at this point.

I'm looking for a general guideline for calculating amount of steam required in the stripper.

Thanks in advance for your help.

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[maddocks]

You can start with about 1 lb of steam per gallon of amine and vary from there. Plan on hitting the regen with about 30 lb/hr - should be interesting to try and control rates that low...

 

[ajs1972]

You can see on the book "Unit Operations of Chemical Engineering" from Warren L. Mc Cabe, Julian C. Smith and Peter Harriott (Mc Graw Hill) the equations for calculate the steam/acid relationship in stripping processes (gas absorption chapter).

 

[dcasto]

ummm, how do you plan on reconcentrating the MEA if you just inject the steam?

From the quicky tables, you will need 1100 BTU/gallon. .5 gpm means 550 BTU/min 33000 BTU/hr. The steam will have a net of 1100 BTU/lb, 30 lb/hr as maddocks stated. However, this is condensing and refluxed overhead. I'd guess you'd end up losing a lot of MEA in the overhead by the time you got a good amout of the CO2 released.

 

[maddocks]

In theory you could condense the overheads like normal and return a portion of the reflux stream to the tower and then dump the rest. It certainly wouldn't be suitable as boiler feedwater ever again. You could almost make it work out as water makeup but you'd still have to purge the reflux and basically dump a good portion of it. What are your thoughts dcasto?

 

[Timewrap]

Thanks for all the replies.

My regenerator operating temperature is around 121 deg C and the vapor pressure of MEA at that temperature is very low. The question is, whether I'll still be losing MEA with steam.

If I try to recover the MEA by condensing and refluxing back to the column, then this might lead to dilution of the MEA solution. Since the reflux will contain all steam plus little amount of MEA.

I'm thinking of a overhead condenser and a reflux drum to reflux back to the column. I want to control the pressure in the reflux drum and vent the co2 via a pressure control valve.

any more suggestion on this set up.

Thanks in advance

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[Timewrap]

Maddocks & dcasto,

I will appreciate your comments. Need your help.

My regenerator operating temperature is around 121 deg C and the vapor pressure of MEA at that temperature is very low. The question is, whether I'll still be losing MEA with steam. If I try to recover the MEA by condensing and refluxing back to the column, then this might lead to dilution of the MEA solution. Since the reflux will contain all steam plus little amount of MEA.I'm thinking of a overhead condenser and a reflux drum to reflux back to the column. I want to control the pressure in the reflux drum and vent the co2 via a pressure control valve. any more suggestion on this set up.Thanks in advanceTimewrap

 

[dcasto]

Lets think about this. If you just put steam (superheated to at least 121 C, then the first few pounds will condense to water just heating the feed up from 15 C to 100. Nextthe energy will transfer to the MEA to raise it too 121 and desuperheat the the steam as water in solution boils. If the temperature stays at 121 on the bottom, then the mixture will be at equalibrium with some percentage of MEA/water/CO2.

For every pound of steam added, then about .008 lbs of CO2 will be vaporized as will .011 lbs of MEA. You will continue to lose MEA just as fast as CO2. If you condense the overhead, you will get the MEA and the water back at a vey weak MEA solution and the bottom will still have CO2 and have to original water and the water you condensed to heat up the solution.

Bottomline, I don't think you can do this. You need to heat the solution, vaporizing the water in the solution to carry the CO2 out the top, then condense the overhead to get the water and MEA back and put it back in, You don't have to put it in at the top either. So build a internal or external coil on your still and keep the steam out of the column.

 

[Timewrap]

Thanks dcasto,

I'm considering an internal heater at the bottom to maintain 121 deg C. Let's me be clear what you are suggesting. The column will operate at near atmospheric pressure with a bottom temperature of 121 deg C. I have a overhead condenser to condense the vaporized water & MEA, it then passes through a reflux drum which will separate co2 and liquid. The condensed water & MEA will be refluxed back. The bottom product will be withdrawn on a level controller.

I'm planing to use IMTP 15 in my 4" column.

Appriciate your help

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[dcasto]

ready for lesson two?

In the bottom of the still column, you will have a steady state mixture of MEA and water, correct? If you have a steady state mixture, can you control the temperature of it? Lets make it simple, if you have a pot full of 100% pure water, can you set a temperature controller to make that pot of water be 105C?

So you cannot set the temperature of the bottom of the still to be 121C, the temperature is fixed at whatever the boiling point is for the mixture of MEA and water. You asked for how much steam was required to run the still and we said from the simple tables, 30 lbs/hr. Thats how much energy you need to add, not temperature, energy. So add that much energy to the still.

Next, you don't have to take all the condensed vapors from the top and put them back at the top, they could just drain back into the coulumn at the bottom. It works, but it takes a little extra energy to get the MEA stripped of CO2. This system gets rid of the pesky reflux pumps.

 

[Timewrap]

dcasto,

I'm sizing the overhead coendenser and the reflux drum.

How can I calculate the boil up rate in the MEA stripper?
I know the bottom temperature 121 deg C and the vapor pressure of MEA at that temperature.

Thanks for your help

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[Timewrap]

Dear Experts,

I'm trying to calculate the boil-up rate for a MEA stripper.

Need your help as how to proceed with the calculations.

Thanks in advance,

Timewrap.

 

[maddocks]

I don't normally consider boil-up rate in the stripper alone. You need to consider the entire system including the absorber, lean/rich exchanger, stripper, reboiler and condenser. I typically start with 1 lb of steam per gallon amine and then fine tune from there depending on how lean the amine has to be. If you have low treating pressure or a tight spec, you might need to overboil to ensure that the residual lean amine has been stripped well since the outlet gas can only be as good as your lean amine quality.

Get into the GPSA data book or a copy of the Kohl and Neilson text and analyze your entire system. Doing the stripper alone is no guarantee that the entire system will work.

 

[dcasto]

maddocks is correct.