6 replies to this topic

[speakas]

Hi,

 

I am currently completing my final year design project on a TEG regeneration still column for an offshore installation.

 

I have a few questions regarding some aspects of the design and the validity of some of my procedure. 

 

1. I have read that most setups have the still column directly bolted onto the reboiler through a flange. To my understanding, a flange is determined by its class (150#, 300# etc), and in the case of the still column operating at more or less atmospheric pressure, I would guess the flanged connection between the column and the reboiler would be a 150# class. 

 

From my understanding, a column out in the open has skirts and is bolted to the ground. The specifics of the skirt and bolts would dependent on factors such as weight load (bending moment, internals of column, externals, wind load etc). How would I apply this to the flanged connection of my still column to the reboiler? Is it even necessary? 

 

2. For the reflux coil located at the top of the reflux column, I cannot find much specific guidelines on its sizing, and would like to validate by methdology. I took a U value of 170kW/m²K as recommended by PTS (or DEP). This gave me a required surface area of 6.2m². 

 

I assumed that I would require 25 coils (assumed to be tightly packed, and completely horizontal), and hence found that I would require a surface area of 0.25m² per loop. Using the nominal pipe sizes, I would then calculate specifics such as internal an external diameter of coil, length and height of coil from top to bottom.

 

This gave me the following results

NPS = 2 inches

inner diameter of coil = 0.36m

outer diameter of coil = 0.48m

distance between column wall and coil = 0.086m

 

I don't know if these values are considered acceptable, or if my assumptions are reasonable. I have also yet to work out the distance required between the liquid collector and distributor between the top of the bed and the reflux coil, I am just guessing it would be typical ranges dependent on the type of distributor , correct me if I am wrong about this. 

 

3. I am not too sure if I havent been looking hard enough, since my column is too small for manholes, I would require flanges around for installation and maintenance of internals. I am under the impression I would require flanges at the top of the column above the reflux coil, one at the feed chamber, and one obviously at the reboiler connection. Would there be a specific rule on the minimum required distance between the flange connections and the Torispherical and its "flange/skirt". I am going to guess the middle and bottom flange can be anywhere reasonable, and its distance to internals such as distributors and packing support should not effect anything. 

 

In summary, I am having issues finding the overall height of my column due to spacing of internals, unsure of reflux coil sizing, and also connection between reboiler and column.

 

For reference, my columns process requirements is as followed:

 

column internal diameter = 0.65m / 25.6 inches

random bed packing = 1.05m below feed, 1.5m above feed (PTS/DEP procedure).

 

Sorry for the lengthy post. I am in desperate need of some help/clarification, and any I would be thankful if I could get some advice. Apart from the process design requirements, the mechanical design have proven to be very vague and difficult. 

 

Thanks and Regards,

speakas

Edited by speakas, 24 October 2016 - 02:31 PM.

[Art Montemayor]

Speakas:

 

I think you are having your problems with your student problem because you haven't studied the application of TEG dehydration sufficiently.  You need to know the basis and details of the unit operations and why they are applied.  You also have to apply good old common sense in your mechanical design.  Think like a chemical engineer and then apply some horse sense.

 

Refer to the attached documents and note how the sketch shows you how the TEG regeneration system is put together.  The stripper generally sits on top of the reboiler simply because they are relatively small vessels - as process vessel go.  Study the amount of TEG flow rate you need for your dehydration and you will see that it is very small and, as such, requires a relatively small stripper-reboiler.  Knowing that, the only reason to put a flange on the stripper bottom is for assembly in the field or for future maintenance - both of which may not even be needed.  Common chemical sense tells you that the stripper operates a little above atmospheric pressure.  Why would you use a 150# flange on a vessel that is not considered a "pressure vessel"??  Refer to ASME code definitions.  If a flange is really needed, I would simply cut a corresponding pair of flanges from a 1" steel plate, machine them on the facings and drill for bolt holes as needed.

 

The so-called reflux coil is not used to furnish any reflux.  Study your Unit Operation here where the total vapors going up and out the stripper are essentially water and stripping gas.  How much TEG do you think is going up the stripper when it is at 400 ^oF and boils at 545 ^oF?  Nil.  So therefore, there is no TEG being refluxed back.  Strippers don't use or need reflux.  Just look at the McCabe-Thiele or Ponchon Savarit diagrams and you will see no reflux is employed in the operation.  All of these coils are sized empirically and used to simply cool down the overheads vapors and maybe trap out any entrained TEG.

 

Your stripper can't be over 10 feet tall and at 2 ft diameter I'd put 4" flanges at the top and bottom for filling the loose packing and dropping the same.

 

What do you mean by " liquid collector and distributor between the top of the bed and the reflux coil"?  All you need is to feed the hot rich TEG at the top of the packed bed and allow any liquids formed in the top coil to drop by gravity.  You can feed the rich TEG into the top packing and let the packing distribute the downward flow.

 

What are you describing with a torispherical head?  Why don't you simply prepare and submit a standard, required Data Sheet on your Stripper and Reboiler?  Is your reboiler fired or are you using a hot therm oil in a tube bundle?  You fail to describe what you are trying to design.  This system is one of the simplest in chemical engineering and all you should be doing first and foremost is describing it with a flow diagram and Data Sheets for the equipment.  That way, our members could really help you out.  All this writing, trying to describe a unit operation with words is very confusing and wastes time, effort, while causing confusion.  Use graphics and Data Sheets.

 

Why is your bed packing a METER below the rich TEG feed nozzle?  What are you trying to do??  You state you're using a packed bed.  Fine.  So why do you have issues with "internals"???  All you need is a firm, stout bed support for the packing.  At the very small vapor flows in this type of operation you may not need a top hold down plate for your packed bed, so you really have no reason to expect to ever have access to the top of the stripper.  I have never had to open up or go into a TEG stripper.  These units work for well over 10-15 years without any need to go inside or even drop the packed beds.  Just keep your TEG clean with your strainers and filters and nothing will clog or plug your system.

 Dehydration with Glycol - PetroWiki.docx   4.15MB   95 downloads

 TEG Dehydration of Natural Gas.docx   1.1MB   107 downloads

[speakas]

Art,

 

Thanks for the guidance.

 

 

I have attached a quick sketch I did of what I now have currently in mind for the still column. 

 

The amount of gas we are dehydrating is 800MMSCFD. 

 

I mostly followed the guidelines outlined in DEP. 

 

As quoted from the documentation

 

A total of 1.8 m of packing is recommended, 0.6 m below the feed point and 1.2 m above. Although this is effectively two stages, the cost of the additional packing is small and provides a design margin......The feed should be introduced in a 0.5 m high entrance chamber. This allows rich glycol surges to settle out and subsequently drain through the bottom section of packing. On columns greater than 300 mm diameter, the packing depths should be increased by 0.3 m each. Since larger packing is used, it will have higher HETPs.

 

 

Applying this to my design, I took a slightly higher HETP of 0.75m rather than 0.6m below the feed point, and added the 0.3m, which gave me the 1.05m height of bed packing. Maybe this is too conservative, and calculations show that only 1 theoretical stage (plus reboiler and condenser) is required for it to work. I am really unsure about this, I liked the idea of having a design margin and just went along with having a bed below and above the feed. It is 38mm random packed pall rings.

 

In terms of reflux, based on my simulation results, it is as what you described. Very very little amounts of TEG actually go to the top of the column, and I did not realise much change when the reflux was changed.  I think whatever I have right now is good enough, but I am just unsure if any constraints exist that I am not conforming to. I may remove the top distributor if it is not required, and have the column setup as you described, with only 1 set of packed bed below the feed nozzle. 

 

The reason why I treated the column as if a pressure vessel is because I took my design pressure to be 3.5barg. My reasoning to this was in the event of overpressure, I the column would need to withstand 3.5barg to route the fuel/stripping gas to the LP Flare KO drum from the PSV. Leading me to treating the whole mechanical design of the column as a pressure vessel. Hence the toripsherical head, 150# flange etc. 

 

Leading onto the flanged connection to the reboiler. I was thinking that the weight, external and internal loads and bending moments on the column should at the very least effect how to size the flanged connection. I read on another forum that, in this case I should take the weight stresses and bending moments acting on the column, and then add it to the hydrostatic test pressure of the column, and then fit it to a suitable class. I am not anticipating this to be more than class 150 regardless, since the column isnt particularly any large. But I need to show a basis and methodology to give a strong argument that what I have designed will hold and work.

 

To address my 3^rd point in my original post. I think I was a little confused in the beginning about how a flange is connected. It’s not a problem now.

 

I understand its a simple operation. What you described to me is a very practical and efficient approach to it. However, I need to show a basis, procedure and result in my design report, otherwise my design may not have a strong basis (which leads to me losing marks).

 

Thanks for your time in responding to my queries. I really appreciate the help. 

Attached Files

•  still column.png   25.13KB   23 downloads

Edited by speakas, 25 October 2016 - 06:00 AM.

[speakas]

I thought I might just add a bit of scope of work here if it helps. 

 

I am to design the still column of the TEG regeneration. My TEG re-concentrator consists of the still column, direct-fired reboiler, stripping column and surge tank. I will not be designing the stripping column, surge tank or reboiler (apart from some basics like duty, number of fire tubes and general volume). I dont need to worry about the other units in the gas dehydration system. The TEG system is the typical one.

 

The lean glycol circulation is 90gpm, dehydrating 800MMSCFD of gas. 

 

In my report I would need to detail process design, mechanical design and operational design. Mechanical design includes wall thickness, head thickness, support/foundations, flanges, reinforcements to openings, and manholes (if applicable). 

 

Thanks again

Edited by speakas, 25 October 2016 - 01:08 PM.

[Art Montemayor]

Speakas:

 

I am glad to read that you recognize that our comments are meant only for your benefit and key learnings.  If you feed our members with positive and detailed information, we will be enabled to help you in many ways that should improve your project's results - and possibly your final grade.  But we will not do your work or thinking for you.  Our help is intended to assist you in learning and understanding what chemical engineering is all about and how to use those learnings.  You have to do the work, the hard thinking, and the ultimate decisions.

 

Do not use any acronyms without identifying them.  We don't have time to guess or to look them up.  What is meant by "DEP"?

 

Your submitted sketch is first class.  I wish every student would spend time and effort on theirs because this helps a lot in understanding what you are submitting.  However, in order to help out we have to know what heat source you are using in your reboiler.  You have not answered my query on this.

 

Now that we know the size of your feed gas flow rate, we have a more detailed understanding of what size factors may be involved in your design - both from a process and a mechanical point of view.  However, you fail to tell us the design water content in your product dried gas.  Without the gas dew point identification, we can't determine if you are using stripping gas or not.  If you are using stripping gas, you must state that.  That would identify the need for a stripping section between the reboiler and the usual TEG surge drum and would identify the tower on top of the reboiler as a "still" and not a stripper.  That is the usual criteria used in industry to identify both types of operation.

 

If you already know the required flow rate of your lean TEG, why don't you let us know that?  You should already have an idea of the estimated TEG contactor, Still, Reboiler, and Stripper sizes and should share that information with our members.

 

Are you venting your water vapor+stripping gas to flare, to a burner, or flare?  What is the back pressure exerted on the reboiler?  That will set your reboiler pressure and temperature.  Are you confronting any BTX pollution issues on this process?

 

Once again, have you done all the research, study, and sample calculations involved in natural gas TEG dehydration?  Texts like Gas Purification (Kohl & Neilsen), NGPA Databook, Gas Conditioning & Processing (Campbell), etc., etc. should already have been studied and noted by you.  You can also download a document from our Website on the subject, Glycol Dehydrator Design Manual 1976, for free that would - in my opinion - be of great help and information for you.  Go to: http://www.cheresour...y-hall-of-fame/and select it from there.

[Art Montemayor]

Speakas:

 

I posted my last comments just as you finished posting yours.

 

There is NO "typical" TEG system in the gas industry.  In order for you to know that, you must read and study all the information I've mentioned in my last post.

 

I'm afraid you've behaved as most prior students seeking help: you don't share all the vital and necessary basic data or scope of work that you're assigned with to solve a problem, but you expect us on the Forum to guess or divine that.  We can't.  You should have told us your scope of work and the recent data you've now furnished in your last post at the very initial post you made.  We could have saved a lot of work and effort.

 

In addition to the information I've already given you, I can add that the reflux coil, or condenser, is usually installed in the top of the glycol still column to maintain the still column overhead temperature between 99 °C to 102 °C using the rich glycol stream coming from the TEG contactor (or TEG flash drum).  The reflux coil is intended for condensing the generated glycol vapors from the reboiler and mainly allow only the water vapor to escape out of the top of the still.

 

The temperature in the overhead line is usually  controlled using a manually operated globe valve installed in the piping to bypass rich glycol flow around the reflux coil.  The temperature in the overhead is set manually at start-up to yield an overhead temperature of approximately 210 - 215 oF and is monitored using a temperature indicator/transmitter.  In some remote onshore or other locations, a temperature indicator may be all that is installed on the still overhead line and located where it can be read by an operator while manually adjusting rich glycol flow to the condenser.

 

The coil can be constructed of 304 or 316 SS material and can be removable through the top of the glycol still column by employing a full flange on the top of the still.  The reflux coil should be designed in accordance with ASME B31.3 and be rated for the same design pressure and temperature as the TEG contactor.  Here, you have to use safety sense and take into consideration that the flow control valve at the TEG contactor can fail open and allow all the downstream piping to reach contactor pressure - if you are not using a flash drum.  Here, you can appreciate that you have to be told about any potential safety hazards that can affect your mechanical design downstream (or maybe upstream as well).

 

Issues like number and type of nozzles, flanges, etc. depends a lot on the expected maintenance and inspection expected in the future.  The size and type of surge drum is dictated by the scope of work (maintenance and storage requirements) and other factors decided upon by Project management and not by the process designer.

[speakas]

Art,

 

My mistake about not posting all the relevant information and scope as required. I should have identified that I am mainly focusing on the mechanical design at this stage, and have already determined and simulated the process. 

 

I hope the scope I have given you so far would allow you to judge the problems I am currently facing in the mechanical design. 

 

I will try and address as much as I can in the post. 

 

 

Are you venting your water vapor+stripping gas to flare, to a burner, or flare?  What is the back pressure exerted on the reboiler?  That will set your reboiler pressure and temperature.  Are you confronting any BTX pollution issues on this process?

 

In the event of overpressure, the PSV will send the water vapor + stripping gas to the LP Flare K.O. drum, since I assumed that would be the safer option. I am not too sure what a burner is. I haven't really looked heavily into the reboiler. Its not explicitly part of my scope, and I was only planning to get the basic sizing as outlined in the method found in Gas Dehydration Field Manual. 

 

 

 

 

All the pipe sizing calculations I performed were as per ASME B31.3. I found all the piping in the TEG circuit to be a 3 inch Schedule 40 pipe (design pressure at 13MPag), while keeping the liquid velocity between 1-3m/s. In terms of loads from offshore vibration, I am not sure if that size will be able to hold. If not, a 4 inch Schedule 40 also fits the criteria and I can change to that.  Of course schedule 40 was not required at lower pressures, but from what I know it is more economical in terms of capital cost. 

 

As for the reflux coil, I sized it to 20% of the total rich glycol flow rate. Currently, I am using a 2 inch Schedule 80 SS316L. BP group standards  actually specified a minimum of Schedule 80 for the coil, Shell Design and Engineering Practice (DEP) did not address it. I could not keep the velocity criteria in this case. I am unsure of my assumptions regarding the geometry was reasonable or not, but I felt the results were reasonable. 

 

Going back to the still column mechanical design. You mentioned earlier that the column is not a pressure vessel, however since I took the design pressure as 3.5barg, am I correct in assuming that this would effectively allow me to treat it as a pressure vessel, and size the wall and head thickness accordingly?

 

Based on the diameter of 0.65m and the design pressure, the wall thickness I acquired with a corrosion allowance of 4mm, gave a minimum thickness of 5mm. It would definitely be different if I were to size based on hydrostatic test pressure of an atmospheric vessel so I would appreciate your clarifications on this matter. I will check if thicker walls is required towards the bottom of the column at a later stage. 

 

As for the connecting flange at the bottom of the column, am I correct to assume of fitting the design pressure, and all stress and bending moments acting on it and fit it to a class of flange? I have not found a clear explanation about this. I feel like it makes sense but would also appreciate external thoughts on it. 

EDIT: I just reailsed my body flange is non-standard. I will try following the flange sizing as found in Coulsons and Richardsons Chemical Engineering Design Vol 6.

 

Funny you mentioned the surge tank. My academic supervisor assigned the stripping tower and surge tank to my group member for his design. Some slightly contradicting opinions regarding that matter.

 

I think that was the only clarifications/issues I had.

 

If you require any more information, or want more clarification from me please dont hesitate to ask. 

 

Thanks again for your help Art. It is much appreciated. 

Edited by speakas, 25 October 2016 - 05:24 PM.