14 replies to this topic

[teikhongg]

Hi all,

 

I am currently on working on a project to design a boiler/steam generator which produces superheated steam at 700 - 1100 degree Celcius for hydrogen production using steam methane reforming method. Can anyone give me a good and reliable source where I can read about the design, maintenance, start-up and shut down protocols, safety measures and etc.? Any help and reply will be very much appreciated

 

Teik Hong

[thorium90]

Generate superheated steam at 700-1100C? Current SMR designs and its associated equipment don't generate such high steam temperatures.

 

There are many past projects, reports and books on SMR technology, should be easy to dig out from google. Perhaps you have something more specific to ask?

 

Attached pdf taken from Haldor Topsoe website.

Attached Files

•  Topsoe_high_flux_steam_reform.pdf   1.36MB   108 downloads

Edited by thorium90, 01 October 2013 - 12:24 AM.

[teikhongg]

700-1100C is what I got from google.. I was just given a PFD to work on. In the design, Waste Heat Boiler is used to generate 370C of steam to be fed into the reformer unit together with desulphurised natural gas. The mixture of natural gas and steam is then preheated to 500C before the reaction begins and then to 850C where the reforming reaction occurs. The task is to pick one of the equipments involved in the PFD to design and improve on it and I've chose to do design the boiler.

 

Is it safe to assume that Waste Heat Boiler is one of the most efficient type of boiler since it uses recovered heat rather than other sources of heat such as coal? And if possible, can you tell me why must the steam and natural gas enter the reformer at 370C to be preheated to 500C and then 850C to reform? Can I just heat both to 850C before feeding them into the reformer to react? I'm sorry to ask this because the lecturer gave us the design basis done by seniors and we have no idea how to design came about. We are assuming that the seniors just designed the process using patents that they find but that fact wouldn't help us to fully understand the process because we do not have the patent.

[thorium90]

700-1100C I believe you refer to the mix of steam and feed gas to the reformer, not just superheated steam. The reactions require an excess of steam and as the reactions are endothermic, require high temperatures.

 

A waste heat boiler is as the term implies, to recover waste heat. Burning coal to produce the heat to boil water is not known as waste heat, it is the main purpose and is the useful heat. The reformer outlet gases are hot and its heat content can be recovered and is thus appropriately named waste heat.

 

Natural gas is not pure methane and entering the prereformer turns the heavier hydrocarbons to methane. Performing some of the reforming in the prereformer reduces the risk of hydrocracking in the main reformer and allows one to run at higher temperatures which is better for endothermic reactions. The prereformer also take the brunt of the damage if the sulphur absorber before that experiences problems. It is easier to change stuff in the simple prereformer than in the larger reformer. Note that the prereformer is simply a packed bed adiabatic reactor. Steam generation will also be reduced.

Attached Files

•  SMR Process Schematic - Air Products.jpg   160.54KB   7 downloads

Edited by thorium90, 01 October 2013 - 10:40 AM.

[PingPong]

The reformer reactor is a set of catalyst filled tubes in the radiant section of a fired heater. The reforming reaction is endothermic.

In the convection section of that heater the fluegas preheats the natural gas feed, the steam and the combustion air (for the burners).

 

Natural gas and steam are preheated separately until about 370 - 390 ^oC, and after the natural gas is desulphurized the natural gas and steam are mixed and further preheated to about 530 - 650 ^oC (exact temperature depends on heater design by licensor), which is simply the crossover temperature from convection section to radiant section (reformer reactor inlet). In the radiant section the mixed feed is further heated to about 870 - 880 ^oC at the heatet outlet. 

There is absolutely no advantage in separately heating the natural gas and the steam until 530 - 650 ^oC, or even 870 - 880 ^oC before mixing them.

 

Is it safe to assume that Waste Heat Boiler is one of the most efficient type of boiler since it uses recovered heat rather than other sources of heat such as coal?

Do you think it is efficient to use a hot gas stream of ~ 880 ^oC to generate saturated steam of only ~ 255 ^oC ?

This is done because of metallurgical reasons, such as metal dusting (google that).

[teikhongg]

So i suppose it is cheaper to use Waste Heat Boiler rather than reinforcing the other types of boiler to overcome such metallurgical reasons? Because my project is suppose to upgrade the boiler, so will it be advisable if I were to pick other types of boiler and reinforce the wall to overcome metal dusting and integrate the heat energy released from the reformer to other equipments?

[thorium90]

A boiler is just a type of heat exchanger. I dont see how you could upgrade by changing the type of exchanger. There are reasons why certain types of exchangers are used for certain applications. Wall material is a problem of materials science. Using different alloys or coatings. As for integrating heat energies, that could be a possible topic. Have you heard of pinch analysis?

[teikhongg]

Yes I've studied pinch analysis last year. It's just that since we are only required to focus on one equipment, there won't be a need to do pinch analysis which involves the whole plant. However, by doing so I can determine the amount of heat energy required to produce 350C of steam more accurately because having 800C of waste heat is will definitely be more than enough of heat energy right? So I'm sure there will be leftover which I can integrate it somewhere else. Well of course that is just an assumption since I haven't start any calculations yet. Anyway I've just looked up on google and it seems that it is quite a common practice to install an economizer to a boiler to increase it's overall efficiency. So i guess I will go with that as a type of 'upgrade'. Thank you very much for your replies Anyway, do you guys know what books should I look into for thermal and mechanical designs of waste heat boiler and economizer? Are the calculations similar to shell and tube heat exchanger? Also where should I start to look for the start-up and shutdown protocols, maintenance method, and etc?

[thorium90]

There are many heat transfer books out there as well as books on heat exchanger designs including, so I guess it shouldnt be too hard to find. Others might have good recommended books for you. Still, adding heat exchangers anywhere will affect other heat exchangers too as the heat duties for others will change. One doesnt randomly place unit operations anywhere on a PFD unless you can prove its impact on other unit operations too.
Startup and shutdown procedures as well as maintenance of equipment are very specific to the plant and equipment. There is no generic way of starting a type of plant. Unless you are prepared to come up with specific detailed designs on all valves, controllers, control systems, units, auxiliary units, utilities, electrical, mechanical and process details in the entire plant by what would normally take many people in an EPC team to do, I wouldnt suggest you touch anything on it.
 
Just to clarify, you were given a project topic by your lecturer to upgrade an equipment in an SMR plant that your seniors have designed and you chose the boiler to upgrade?

Edited by thorium90, 04 October 2013 - 07:31 AM.

[teikhongg]

Yes I did. On the part where I chose the boiler I mean. You sound like I shouldn't have chose the boiler because there is nothing much that I can do about a boiler hahaha well I chose to do the boiler because in my uni, we are taught to design heat exchangers, distillation columns, reactors, separation columns and etc so as a year 4 student, they want us to do more than what we had done last year. So they are expecting things that are not so easily found in books like azeotropic distillation, extractive distillation, crystallizer, and of course boiler: which is why I chose to do boiler because those mentioned before are not in the SMR plant. The lecturer had mentioned that designing a boiler can be more interesting that it seem because there are lots of factors to consider due to the high temperature involved and that there is a reason why someone needs a license in order to run a boiler. And of course we had to give our topics soon so we actually didn't have much time to really look into all equipments before deciding on which to design and upgrade. Anyway, now that I've chose to do the boiler, I need to do more than just designing a simple heat exchanger. We are not expected to design a perfect boiler that will work but hopefully as specific as it can be with only internet and books as my source of information.

 

But as for now, I'm planning to do a detailed study on water feed treatment, blowdown, hazop, corrosion, complications and how to overcome, detailed design on all important parts of a boiler, and of course need to draw the boiler for the spec sheet as well. You had mentioned that the startup and shutdown procedures as well as the maintenance is very specific to the plant and equipment but isn't there a generic way to do so where numbers and calculations are not involved? Or am I too naive to think it that way?

[thorium90]

So your task is to design a new boiler and not just "upgrade" an existing boiler. Ok. I think the scope you have outlined is pretty good.

Perhaps a generic method of startup can be seen as follows. Pressurize the feed streams, introduce the feed streams into the first equipment, stabilize the parameters, introduce into the next equipment, stabilize the parameters and so on so forth until the entire unit is started. Shutting down can be seen as the opposite in reverse; depressurize the streams into the last equipment, stop feed into that equipment, depressurize the streams into the second last equipment, stop feed into that equipment and so on so forth until the entire unit is stopped.

Edited by thorium90, 04 October 2013 - 11:19 PM.

[teikhongg]

Alright I will take note of this. Thank you very much! Will start working on my design soon and will post here again when I hit into a wall again

[teikhongg]

Hi all, just to clarify some things:

 

Now that I've discovered that I need a superheater in my system, I will need to design an economiser, boiler and superheater. Also I've just discovered that Kern's Process Heat Transfer book is no longer available in my library (Probably somebody stole it or something) so I will be using Coulson and Richardson's 'Chemical Engineering Designs' Volume 6. Is it safe to perform design calculations for these 3 equipments using the same design calculations for shell and tube heat exchangers in general?

 

If I can use the same design calculations, are there be any specific calculation factors that I must consider since there will be a phase change in the waste heat boiler? If I were to consider the phase change which mean a drastic increase in volume, must I then calculate the volume of the shell like how I calculate a kettle reboiler?

 

Thanks in advance

Edited by teikhongg, 09 October 2013 - 09:54 AM.

[teikhongg]

Since the design basis is not that close to industrial designs, I do not have to design a perfect WHRU that works in industries but I do need to design one that follows all the working principles so these are my understandings so far but please tell me if I am correct or not:

 

Economiser: Heats feed water to saturated liquid

Boiler: Heats saturated liquid to saturated steam

Superheater: Heats saturated steam to superheated steam

 

Economiser and superheater can be designed as normal shell and tube heat exchangers but boiler must be designed as kettle reboiler.

 

All units are very close to each other so pressure drop can be ignored.

 

Am I right so far? Feedbacks will be very much appreciated

Edited by teikhongg, 15 October 2013 - 07:46 AM.

[thorium90]

They are all located at different places. The superheater is in the convection section and heats steam and NG feed. The boiler is just after the reformer. The economiser is after the shift reactor.
There a couple of threads that talk about boiler design, would be advantageous to search the forums.

Edited by thorium90, 15 October 2013 - 08:16 AM.