7 replies to this topic

[drdave]

In one of our packed column treating ammonium carbonate solution to separate out 90~92% vapour ammonia from top, We are feeling that there is higher pressure drop of 1500 mmWC in total.
There are two sections each of 8Meters height having 35mmPP pallrings in 1.5 Meter tower dia.We understand that pressure drop can be reduced by better & bigger & efficient packings and accordingly hydraulics is estimated & found satisfactory.
Now, we are not able to say effects on mass transfer efficiency it should not be adversely affected.Can we say that Reduction in pressure drop will help reducing power consumption by centrifugal compressor for vapour ammonia???

Attached Files

•  packed_column.doc   19.5KB   115 downloads

[Art Montemayor]

drdave:

You say you have a "feeling" that you are experiencing a pressure drop of 1.5 meters of water column (>2 psi). Are you not able to directly measure this dP? This is a exceptionally high pressure drop across a packed column. Of course, this is just my general observation, based on conventional packed column designs. It could be that your application is an exceptional one. Without any further or additional basic data, it is difficult to make a judgement on your operation - other than just generalities.

You fail to state the original design specifications of the stripper (?) and the actual conditions it presently finds itself operating under, so we can't really tell much.

You give us physical dimensions and from my experience, 1.5-inch Pall Rings seem to be too big a dimension for a packed tower that is 59 inches in diameter. I would suspect liquid channeling and poor liquid-vapor contact. Normally, for a column this "slender" (I consider a column with a height/diam ratio of approx. 13 to be slender), I would expect 1-inch size Pall Rings. I also would expect that 8 meters is a large height to employ for a packed bed without having liquid re-distributors. But again, I have to generalize since I don't know the liquid loadings, the vapor rates, the operating pressures, etc.

Can you give ALL the process specifications, details and history on this operation? Without detail, one can't do much but generalize and speculate. Of course you can "say that Reduction in pressure drop will help reducing power consumption by centrifugal compressor ". An reduction in column pressure drop will make for less energy consumption and a more economical operation. Employing larger Pall Rings will directly affect your mass transfer and efficiency. As I said, your column already seems to be using over-sized Pall Rings (as compared to the column diameter).

Additionally, your attached Word document is nothing more than a duplication of your post. It contains no other basic data or details.

[Art Montemayor]

DrDave:

I have taken the liberty of copying your personal, edited, email to me and posting it here on our Forum. This is the normal, expected methodology of responding to queries. We initiate a thread with an original posting, and fellow Forum Members subsequently add their valued comments and ideas on the same thread. This allows for organized and shared information on the common subject theme. Please respond to the individual threads directly on the Forum, and not through personal email. Personal email does not allow all members to share in the same information and potential solutions and ideas are lost or never exploited.


“Sir,

First thanks for your care to our query.

My clarifications are mentioned below, point wise:

1) We have measured both bed pressure drop with standard pressure gauge and it is 1,500 mm WC in toto in both beds wherein bottom bed pressure drop of 850 mm WC. Estimated bedwise pressure drops are 640 mm WC (Top) & 660 mm WC (Bottom) based on Treybal Mass transfer formulae.

2) 8 meters for each bed is really too large a bed height, but it is that way since its inception in 1982. Whole separation unit gives normal off gas treatment & acceptable in composition, except higher pressure drop.

3) Original gas & liquid composition with respect to pressure & temperature are almost matching with present actual gas & liquid composition. I would like to send a self explanatory block diagram to simplify a process look to my problem but not possible.

4) By provision of efficient packing (e.g., structured one), If there is a reduction of pressure drop of 800 mm WC, how can we get power saving on gas handling centrifugal compressor? In fact, adiabatic compression based power formula gives power with respect to compression ratio (P2/P1) and not based on Hydraulic (= Non-compressible) power formula where delta P is put in to get fluid horse power. Here, I want to say that centrifugal compressor is constant pressure ratio machine. Hence, tower packing pressure reduction does not help to save power.”

Attached Files

•  Ammonium_Carbonate_Stripper.doc   38KB   110 downloads

[siretb]

35 mm plastic rings is not a too large size for a 1500mm diameter column. the ratio of diameters is 1500/35=43; the most conservative approach is to use 40, and sometimes I allow for as low as 20.
As you write, a bed of 8 meters is probably too much.

It is likely, that if you change your random packing for structured packing, for the same efficiency (same HTU) you will have less pressure drop. It would be useful to segment each bed, into two beds, with a liquid collector in between. The benefits of a good distribution profile, asssociated with a good structured packing will more than offset the lost space for the collector.
Note that using metal rings instead of plastic rings would help, the void fraction is higher with metal rings, for at least same efficiency.

Can you give us additional process information (flows, operating conditions,...) so that we could help you more? Certainly, a pressure drop of 1500mm WC is huge, it is also likely that the operating point is not satisfactory.

[Art Montemayor]

DrDave:

I hope that you are able to open the simplified Flow Diagram I prepared, citing some of the basic data requirements for analysis. There are other data that would also be useful: packing factor, irrigation rate, liquid loading rate, etc.

Structured packing is definitely a step in the right direction, in my opinion, in order to minimize pressure drop across a column. I always attempt to restrict my packed column beds to approximately 3 meters of height. This is applied together with re-distribution trays for maximum efficiency in mass transfer. I have found that greater bed heights are prone to channeling and bad liquid-vapor contact.

The Brake Horsepower (bHP) requirement of a centrifugal compressor is given by the GPSA engineering Data Book as:

bHP =[ZRT1/(n-1/n)] [(P2/P1)^((n-1)/n) -1]

where,
Z = average compressibility factor
R = universal gas constant
T1 = suction gas temperature
n = polytropic exponent
P1 = suction pressure
P2 = discharge pressure

As you can see, at the same discharge pressure and temperature conditions, the bHP (or work input) is less when the compressor's inlet pressure is increased (in this case, by a lowering of the packed beds' pressure drop). I do not agree with your description that a centrifugal compressor is a constant pressure ratio machine. Any centrifugal compressor can vary its suction or discharge conditions independently, depending on its design and operating characteristics. If you compress with a lower pressure difference across the compressor it is common sense that you are doing less work - and therefore require less horsepower demand and consumption, lessening your energy bill.

You state: "Original gas & liquid composition with respect to pressure & temperature are almost matching with present actual gas & liquid composition". However, composition has little to do with the pressure drop across the packed beds. It is the liquid and gas rates that determine, in large part, the pressure drop - the ultimate pressure upset being when the column undergoes flooding. Again, structured packing is a good recommendation and consultation with a recognized and capable supplier can be of help to you.

[drdave]

Sir,
Let me clarify first that I am doctor by initial (DR Dave) and not actually.
Of course, I am a practising chemical engineer.
This time, I am enclosing the sketch with data to make my problem easy to check from any process aspects.
first of all, I say that I have got a good tips from you to explore my process problem.
Still, Following points come to my mind:
(1) Power saving obtained is not much convincing to justify cost of structured packings.
(2) By stating centrifugal compressors are constant pressure ratio one, I mean to say ,Change of suction pressure will adjust discharge pressure accordingly to possible extent sothat P2/P1 will remain almost constant.
(3) Tower sketch indicates pure vapor ammonia(~91%W) to the suction of centrifugal compressor whereas CO2 absorbed liquor will follow subsequent recovery processing.Here, process exchange should not alter given composition at all .Here,I understand that pressure reduction may induce acute stripping action.Hence, I am seeking solution for certain quantification method to say whether composition will deviate much or not by certain value.
Thanks & Kind regards.

Attached Files

•  Slide1.JPG   43.91KB   54 downloads

[drdave]

Art & Sireth,

Thanks for an effective process threads to weave a solutions on my end.
I have submitted additional process informations as much as possible to get your guidance.

Suppose,I am trying 30mm Hypak packings (SS 316 is MUST for process)based on calculations showing pressure drop reduction by ~500 mmWC.It will not meet Tower diameter to packing diameter ratio concept as explained earlier.(ok I am to check packing factor data of 145 as given in literature data too) What should be inferred from it?
Mass transfer efficiency in terms of NH3 in vapour & CO2 in liquid still remains qualitative & doubtful.Will you please throw a light on it?
Thanks in advance, sir.
Dattatreya