8 replies to this topic

[skylee8888]

Hi all

I am a student of 3rd year of Chemical Engineering. I have been asked to design a waste heat boiler to relieve the hot temp of the process fluid before entering the catalytic reactor. The process fluid is mainly sulphur dioxide, nitrogen and oxygen in a sulphuric acid plant. Well the waste heat boiler that I need to design is located just after the sulphur burner/ furnace reactor, the process gases leave the reactor at a temperature of 1020oC and pressure of 1.6 barg and then enter the waste heat boiler. The gases leave the w.h.b at 410oC

The gases leaving the reactor are mainly sulphur dioxide, oxygen and nitrogen. The flow rate of the gases is 92607 kg/h (where N2 = 64269kg/h, SO2 = 17651kg/h and O2 = 10685kg/h)

The steam required from the waste heat boiler is at the follow conditions:

Pressure: 40 bara
Temperature: about 400-450oC (superheated steam)

The boiler feed water is taken from the utilities manager at a temperature of 105oC and a pressure of 41bara.

I was wondering if you can give me ideas how can i tackle the design of the w.h.b. Will a vertical thermosyphon reboiler with steam drum manage to do solve this issue?

What are the basic stages in desgin and what books or journals do I need to read.

Well all suggestions are welcome as this is my first design and I will appreciate any help from people with experience in this field

Thanks

[Padmakar Katre]

Dear,

Based on the above data you can calculate the steam generation rate. Addition to this I am giving some links which may help you,
http://www.hrsgdesign.com/design0.htm
http://www.firecad.n...defaulthrb.aspx
If you can get the copy of one of the books from the list below, then you will have better idea of the system,

http://www.google.co...nG=Search Books

[skylee8888]

Hi Padmaker,

I appreciate the help n hint given by you, I've look into some books regarding on furnace design and found some interesting answer. However, it is still confusing if a vertical or horizontal thermosyphon reboiler to be used as some would recommend water tube and some fire tube based on the same application. The arguement given that water tube reboiler is more suitable because it is not cost effective to implement the boiling fluid at high pressure at the shell side, hence water tube. However, fire tube reboiler is said to be more suitable due to the corrosiveness of the fluid and implying that high pressure steam at the tube side is also not very economical. So how do i judge from here as the few books i read about operates at the same operating condition and application....

your reply is highly appreciated

[Art Montemayor]

Skylee:

I responding upon your request. But before proceeding into the details of what you should do to resolve this problem, allow me to point out that you have basically put together a very BAD POST because you are causing confusion and mistakes in understanding what you try to state. This has nothing to do with the fact that English is not your native language. It has more to do with you not using your natural brain to think before you write and also in communicating in a practical and accurate manner. For example, look at what you write:

“I have been asked to design a waste heat boiler to relieve the hot temp of the process fluid before entering the catalytic reactor.”

And,

“Well the waste heat boiler that I need to design is located just after the sulphur burner/ furnace reactor, the process gases leave the reactor at a temperature of 1,020 oC and pressure of 1.6 barg and then enter the waste heat boiler.”

How can you first say that the Waste Heat Boiler (WHB) is BEFORE the catalyst reactor, and then state that the WHB is AFTER the reactor???? This simply doesn’t make any sense and shows that you are not paying attention to what you write and certainly not proofing what you write. I will allow you to explain what you really mean to state by generating a detailed Process Flow Diagram (PFD) together with the identification of all streams and their conditions. Please show all basic data and any calculations that you have already done. You should have generated this detailed PFD as soon as you were assigned the problem, so it should not take you very long to submit it. Please submit all your information in an Excel Workbook that we can all comment in and show our recommendations.

This is not a hard problem, but you must come up with the Basic Data and the PFD. That is the MINIMUM WORK EFFORT that you should put into this project up to now. You will be expected to do some serious research on the production of Sulfur Dioxide for the production of Sulfuric Acid. That information is well documented for many years and you should be spending your time searching your local technical engineering libraries and the Internet. My copy of “The Chemical Process Industries” by R. Norris Shreve; McGraw-Hill; 2nd Edition; (1956) gives a detailed description of the production of Sulfur Dioxide as well as the production of Sulfuric Acid. As a 3rd year ChE student, you should have had this textbook in one of your courses – or something similar to it.

Be SPECIFIC in defining what you are expected to do. For example, WHAT TYPE of design are you referring to? Is it a process design? Or is it a Mechanical Design? We can’t do a mechanical design on a process that is captive technology and patented. Equipment like sulfur burners and systems are all corportate technology and any professor in any university knows that you can’t be expected to know the details of something that is closely guarded in secrecy agreements. Therefore, be specific in what you are asking for. Chemical Engineering Students are expected to do Process Designs, not Mechanical Designs.

I am attaching a sketch of what I would apply as a Waste Heat Boiler Unit Operation on the hot Sulfur Dioxide gas stream.

Await your reply.

Attached Files

•  SO2 Waste Heat Boiler.xls   900.5KB   203 downloads

[skylee8888]

Hi Montemayor,

I appreciate for your quick response and I apologise for the confusion. Perhaps I should stick to the word sulphur burner instead of furnace reactor as there are some books refer sulphur burner to that. Probably I should re-describe the whole process again and attached is the simplified flow diagram around the waste heat boiler. I'll try and explain as detail as possible.

Here, I am asked to design a waste heat boiler which is connected directly to a sulphur burner where the combustion of sulphur is highly exothermic and leaves the burner at 1000degree C. Due to the high temperature, a waste heat boiler is said to reduce the temperature of sulphur dioxide and other gases (O2 and N2) to 420degree C before entering catalytic reactor. This is to prevent damages that could be done onto the catalyst. The sulphur dioxide and oxygen will then react with the presence of catalyst in the catalytic reactor to form sulphur trioxide.

The gases leaving the sulphur burner are mainly sulphur dioxide, oxygen and nitrogen. The flow rate of the gases is 92607 kg/h (where N2 = 64269kg/h, SO2 = 17651kg/h and O2 = 10685kg/h)

The steam required from the waste heat boiler is at follow conditions:
Pressure: 40barg
Temperature: 205degree C (saturated steam) (I gave the wrong info previously here)

After seeing your attachment, I'm impressed with the configuration that you recommend as I'd never see such design before and it looks interesting which i may consider to study on it. However, I've paid quite some attention to thermosyphon reboiler attaching to a steam drum before this. Now that you highlighted this new idea, would you mind to give me more advice (probably the pros and cons) between these two system as I'd never dealt with any of them.

Thanks.

Attached Files

•  Simplified version of PFD around WHB.xlsx   11.94KB   116 downloads

Edited by skylee8888, 21 February 2011 - 02:25 PM.

[Art Montemayor]

Skylee:

You have not responded with answers to my request.

Is your design a mechanical or process design?

Where is the DETAIL on the PFD? All you have done is copied what is found in the text book I quoted - except that it is even more simplified and NO DETAILS as to flows, temperatures, and pressures. You are not being honest with us by admiting to the truth: You should have already generated the detailed PFD and at least started the process heat and mass balance. If you have already done this work, why don't you share with those who can most help you?

How are you proposing to have the Sulfur furnace built and fabricated: design operating pressure, configuration, materials of construction, and configuration? This information is important because it will be almost the same description for the Waste Heat Boiler.

Please respond to my requests - you can either dedicate the time and effort or reject my requests. That is your decision. I am returning your workbook as Rev1.

Await your reply.

Attached Files

•  Simplified PFD around WHB Rev1.xlsx   14.56KB   102 downloads

[skylee8888]

Montemayor:

My problem here is to pick the type of W.H.B as well as designing the process design of the WHB. Sorry that I had not met your request because my PFD was mostly done in microsoft visio whereas my mass & heat balance is a little bit not organise in the excel spreadsheet. So i extracted and put it into the description at the post but forgotten to put it into the excel sheet. Anyway I had input the conditions into the PFD as revision 2. Sorry for that. If there is still lack of information, please let me know.

For Sulphur furnace, I was thinking if cylindrical furnace (horizontal) could work. Brick-lined and refractory-lined carbon steel is the only economical material of construction that is suggested by most books. Also, Quoted from the handbook of sulphuric acid manufacturing: "Brick and refractory linings insulate the carbon steel equipment and ducting, and reduce actual metal skin temperatures to below about 900°F. Below 900°F carbon steel has sufficient mechanical strength and oxidation scaling resistance". However, I have no idea how to determine the design optimum pressure.

Cheers

Attached Files

•  Simplified%20PFD%20around%20WHB%20Rev2.xlsx   15.18KB   106 downloads

[Art Montemayor]

Skylee:

First and foremost, this is a student work assignment, right? What you are expected to know about how to design and fabricate a real-life waste heat boiler is the same that I am expected to know about how to land on the moon – absolutely nothing. Therefore, what you are expected to produce is a best-effort design, based on what little you know. But it must be practical and explained as to why.

The answer always is related to using common sense. For example, the most reasonable and common sense solution is to use the same vessel that is proposed to serve as the sulfur furnace to also house the waste heat generation equipment (steam drum, bent tubes, condensate & mud drum). This is so because:

• The pressure and temperature to be handled are nearly both the same;
• This type of assembly will save on capital costs by eliminating the use of two vessels and inter-connecting piping;
• This type of assembly will save on costly pressure drops between vessels;
• Both services require the same kind of construction and materials – carbon steel pressure vessel (for design pressure approx. 2 bargs) with internal acid brick + refractory lining and sealing.

Who cares whether the furnace+waste heat vessel is horizontal or vertical? The configuration that results in the best process arrangement will determine the outcome. That’s how an engineer justifies his configuration design. There is nothing “optimum” about the design pressure. It is what it has to be: whatever is required for the downstream reactor + all the pressure drops to get there.

Personally, I think you are laboring because of your disorganization – in your work output and in your logical thinking of how to resolve this problem. Keep it simple and direct. You seem to be getting mixed up with details that have no importance at all. That is why I have stressed that you first generate the basic work you require: a detailed heat and mass balance backed up by a detailed PFD. If you had already done this requirement, you could have arrived at the same common sense results by now.

[kkala]

"what you are expected to produce is a best-effort design, based on what little you know. But it must be practical and explained as to why". Art Montemayor
Indeed, for a proprietary result specialized groups have worked for years; an exercise has to be delivered in a short time, so a result so optimized as in industry cannot be obtained; however physical laws and engineering judgment are applied, developed and checked through an exercise. Searching for existing industrial cases is of course useful, yet it had better not consume the majority of available time.
To this effect waste heat boilers for sulfuric acid plants should be specifically searched for, e.g.
http://www.sulphuric...eam/boilers.htm, http://vganapathy.tr...com/sulfur.html, http://zzboilers.en....cineration.html, http://www.iffco.nic...bd?OpenDocument. Following can be seen from them.
1. Waste heat boilers in sulpfuric acid production can be either fire tube or water tube (1st url)
2. Water tube boilers are preferred in sulfuric acid plants burning pyrites -dirt of tubes has to be external (3rd url).
3. However water tube boilers are also in use for plants burning sulfur (4th url).
I have seen a pyrite burning plant having a water tube boiler like what "SO2 waste heat-boiler.xls" indicates. Total of drums & tubes could be moved away by crane.
Same factory had two sulfuric acid plants, burning sulfur and producing steam (~40 kgf/cm2 g, 400 oC). I could not look into their boilers, apparently not maintained so often as the pyrite plant.
I would accept water tube boiler for a sulfuric acid plant burning sulfur (it can be met, so it works). The water tube boiler (with two drums) to produce 40 bara steam seems cheaper to me in capital cost, compared to a fire tube boiler of same capacity.

Edited by kkala, 23 February 2011 - 10:06 AM.