19 replies to this topic

[aliadnan]

Hi Morin

You have not provided the basic data in your query that from which stream your extracting heat(liauid or gas) and if your using this heat to convert water into steam, then what pressure steam your generating.
Waste Heat Boilers are nothing different than Shell and Tube Heat exchangers, in which the hot fluid exchanges heat from the cold fluid. Now what kind of Shell and Tube exchanger your using depends upon the process conditions.

There are basically two types of Waste Heat Boilers.(My discussion is based on Steam producing Boilers)

Water Tube Boilers:

In these type of boilers, the water is in the tube side of the exchanger. After exchanging heat from the the Hot Fluid, the water is converted into saturated steam. This saturated steam can be converted into superheated steam (if required) by passing it from superheaters. Water tube boilers are used when the steam generated is of high pressure. The advantage of producing high pressure steam in the tube is due to the Meahanical deign considerations (It is economical to generate high pressure steam in tubes than in shell because the thickness of the small diameter tube will be lesser. One the other hand if you generate high pressure steam on the shell side a much thicker shell will be required).

Fire Tube Boilers:

In these types of Boilers the steam is generated in the shell side and these types of boilers are used to generate low pressure steam. And the Hot Fluid passes through the tube side.

Now coming to design of the Exchanger. It is very simple, go to your library and search for "Process Heat Transfer" by Kern and "Applied Process Design for Chemical and Petrochemical Plants" by Ludwig. These books are really handy and have a lot of data in them.

Return with the basic data for your design and I am sure there are so many experienced engineers who are willing to help you provided your clear your query.

I hope this helps.

Regards
Ali

[aliadnan]

Hi

First talking about the High temperature Shift Converter

The Rate Equation is as follows:

r = s*k [(A-x)*(B-x) - (C+x)*(D+x)/K]

where
A = initial conc. of H2O
B = initial conc. of CO
C = initial conc. of H2
D = initial conc. of CO2
x = conversion
k = Reaction rate constant
log10 k = 6.947 - 3830/T (T, temperature is in R)
K = thermodynamic equilibrium constant
K = exp (8240/T - 4.33) (T is in R)
s = activity factor, its value depends up the operating pressure of the reactor and can be taken as 4 for pressures above 20

atm
r= ft3 of gas converted per hour per ft3 of catalyst

Since you know the Equation for the Plug Flow Reactor, which is given by

1/SV = intergral of dx/r

SV = space velocity

Now you can put the value of the "r" in the above equation and solve the inetgral either graphically or with a computer

program.

The above Rate Equation can be integrated at constant temperature and the result is given by the following equation.

k / SV = k * (-q)^1/2 * { ln [2wx - u - (-q)^1/2] / ln [2wx - u + (-q)^1/2] - ln [-u - (-q)^1/2] / ln [-u + (-q)^1/2] }

where

w = K-1
u = K * (A+ + (C+D)
(-q)^1/2 = [u^2 - 4*w*v]^1/2
v = K * (A*-(C*D)

It has been said in the journal from where I have taken all this stuff that you can use the integrated form at constant

temperature if the adiabatic temperature rise along the length of the reactor is upto 100 F and the value of K(equilibrium constant) should be

calculated at the exit temperature of the reactor. While the value of k(rate constant) can be taken as the arithmetical average of inlet and outlet temperatures.

(All the above information is taken from the Journal of Chemical Engineering Progress Vol 58, No.

3, March 1962, pages 33-36. Author is James M. Moe)

Also vist the following links in which there is some more discussion on Water Gas Shift reactor

http://www.cheresour...?showtopic=1555 <------- Click here

Now about the Waste heat boiler, please provide us with the flowrates of gas and water, also the composition of the gas and

at what pressure the gas is entering the reactor.

Hope the above data helps.
Regards
Ali

[aliadnan]

Hi Morin

Man your desiging so many things . Now first talking about your Methanator, check out the following links where a lot of discussion have been done on the subject of "Methanator".

http://www.cheresour...9&hl=methanator <------ Click here

http://www.cheresour...4&hl=methanator <------ Click here

http://www.cheresour...5&hl=methanator <------ Click here

http://www.cheresour...5&hl=methanator <------ Click here

You can also get alot of information if you go to your university library and search this in Encylopedias of Chemical Engineering and books on Chemical Process industries.

Now talking about the typr of Exchanger you should use, well I admit that I am not an expert in selecting the correct type of exchanger and I am also learning it now. I hope Mr. Mantemayor will reply to this query and throw light on the selection of correct type of exchanger since he has a vast experience in this.
Well I am sure you will be producing the saturated steam on the tube side and your gas will be on the shell side (well if somebody thinks I am wrong please do correct me and explain the reason too).

Hope this helps you.

Regards
Ali

[Art Montemayor]

This is very interesting and necessary information for every young ChE that is going to be involved in heat transfer in his future career. I highly recommend morin (and any other interested young engineer or student) to go to the following very profitable website:

http://vganapathy.tr...om/boilers.html

Mr. V. Ganapathy is a world-reknown expert in heat transfer and especially in waste heat boilers. He lives in Chennai, India and has written numerous articles on the subject and a very popular book. I further recommend you download all the free material and software that he offers you --- but with the proviso that you earnestly read and study it at length. You have a lot to gain and profit from, but only if you study the matter carefully and at length.

How you configure your Waste Heat Steam Generator depends on a lot of the unknowns in your process that you are in control of:

1. What is the size (flow rate) of the waste heat stream? This will determine the size of the heat recovery exchanger you employ and its configuration.

2. What is the pressure of the hot process stream and how much pressure drop can be tolerated?

Because I’ve operated and managed a few steam reforming plants for the production of Hydrogen, I have a good idea of what I would employ if it were my project. But I don’t know the constraints and needs of morin’s proposed project, so I can’t be 100% certain my scheme would be the best. I know it would work (as it has in the past) but there may be other options. Please refer to the attached workbook to see what I would recommend as the configuration. Bear in mind that since I don’t know the capacity (or size) of the unit, I can’t be sure of the TEMA type of shell & tube exchanger I would employ. But basically, it would be a TEMA shell & tube. Depending on the physical size, it could be a BEU or Kettle type. If it is really too big for a workable BEU, then I would try to employ an external floating head type (type BEP) but only if the steam pressure is relatively low. The hot process gas would be on the tube side and there are definite process reasons for this.

I hope this helps you out.
 Waste_Heat_Boiler.xls   19KB   566 downloads

[blanchy]

I am in my final year and the title of my design project is Production of hydrogen using steam reforming. I am designing a waste heat boiler and i need information on how to start the design. can someone help? thanks in advance.

Hi morin,

I'm working in steam reformer plant as a process production engineer. If you contact me I would help you.

Regards,

blanchy

[Sylvia]

Hi,

I came across this forum as i was doing my research for my waste heat boiler design, and i find the responses are very useful. I would also like to seek advices from the experts as i am also currently working on my final year plant design project.

The process produces acetone via dehydrogenation of isopropanol. The objective of my waste heat boiler is to cool down my process stream (consisting of acetone, isopropanol, hydrogen, propylene and mesity oxide) from vapor phase to two phase (vapor and liquid) because of the presence of non condensable gas, hydrogen. My intention is to use dimineralized water as the cooling utility, which will then exit the exchanger in vapor phase (i am using the latent heat of the demin water). I have yet to confirm on my temperature and pressure conditions as i do not know what is appropriate, however my process stream enters at 400degC and 2 atm and the exits at 106.7degC and 1.85 atm.

The reason i am using demin water is because i wish to produce LP steam which will then be used for heating up my reactor inlet. Forgot to mention earlier, but the waste heat boiler inlet is from the reactor effluent. It is unsafe to exchange heat from reactor inlet and outlet directly, thus i have resort to indirect heat recovery. However, i would like to know what other options do i have to produce LP steam besides demin water? Also, i would like to know what is the best type of heat exchanger for my case, whether is it a floating head (due to the large temp difference) or a U tube (as referred to RN's case with the toluene exchanger)

thanks in advance!

regards,
sylvia

[Sylvia]

oh and one more thing is...is my process stream considered heavy fouling? from what i know, i do not think my process stream is corrosive. I need to know this in order for me to justify whether to use triangular pitch or square pitch.

thanks.

[Sylvia]

Good day,

As posted yesterday about my process stream operating conditions for my waste heat boiler, there has been some changes. My inlet remains at 400degC and 2 atm (because this is the optimum operating conditions for my reaction) however my outlet is now at 68.65degC at 1.85 atm. I have learnt that boiler feed water is used generate steam. My biggest concern now is how to determine the inlet/outlet temperature of demin water? Since i am only considering the nucleate boiling zone, i will only consider the latent heat, thus inlet and outlet temperature remains constant but with a phase change from liquid to vapor. I also know how to determine the corresponding pressure IF i know my temperature.

According to an example, a simple calculation to determine the demin temp is by deducting the assumed overall driving force from the outlet temperature of the process stream. so in my case, with a minimum delta T of 10degC, does it mean that my demin temp will be at (68.65 - 10) = 58.65degC? If so, my saturated pressure is abt 15kPag which is actually below atmospheric pressure. I have no idea if i have understood the example correctly, and would like to know what is the better choice of demin temp in my case.


Seems a bit hard to explain in words, but i hope i did make things clear.

Hope to receive a reply soon. I could really use some help now. Thanks


Regards,
Sylvia

[joerd]

Most of the time you would want to recover as much heat as possible as a useful type of steam - let's say 125 psig. You are correct that the outlet temperature will then be about 375 F (25 F above the steam temperature). Therefore, to do the final cooling, you'll use a second exchanger, a trim cooler, with air or cooling water.

[Sylvia]

There has been some changes to my earlier waste heat boiler, as i found out that the steam produced (outlet at 55degC) will not be sufficient to heat up my reactor feed afterall (to be heated up to 100degC) As an attempt to stil recover the heat from the reactor effluent, i am heating up cooling water from 25degC to 55degC instead and then sent to utilities section for further heating to produce steam. this way i am indirectly slightly reducing the cost of producing steam. However, another question come up to my mind, whether is it more economical to just buy all my utilities, or produce own steam but with higher capital cost? My plant production is only 15, 000 tonnes per year.

Regards,
Sylvia

[joerd]

Sylvia,

the quick way would be to convert utility cost to capital cost using a payout time, say 3 years. If your utility costs 10 cents per tonne, and you use 100,000 tonnes per year, you'll spend 10,000 $ per year. It would be cheaper to buy the utility if your capital cost of generating it exceeds 30,000$ in this case.
A better way would be to figure out the total lifecycle cost of each option (buying vs. generating). If your plant is designed to operate for 20 years, you can do the math taking into account capital cost, maintenance and operating cost, cost of capital (borrowing the money). Then, you get two values, one for the buying option, one for generating your own utilities, for the total of 20 years.
That said, it is often cheap to buy utilities if available near the site, if your plant is relatively small. Think of it as "core business" and economy of scale: a utility company can often produce electricity and steam cheaper than a small consumer can.

[aliadnan]

Hi Morin

Yes the Rate Equation for the LTS (Low Temperature Shift Converter) is the same for HTS. The only difference is that in this cause you have to use the following equation to calculate the Rate Constant

k = exp (12.88 - 3340/T)

where T is in R (This is for Copper-Zinc Catalyst which is used in LTS, I am sure you will be using this in your LTS).

s = activity factor for Copper-Zinc Catalyst in the rate equation is given by equation

s = 0.86 + 0.14P (for P < 24.8 atm)
s = 4.33 (for P > 24.8 atm)

Now as you have calculated the volume of Catalyst I suggest that you consult a good Reactor Design Book like Rase, Fogler, Smith (All these are Authors name of different Books). From these books you will get the design steps for the Fixed Bed Catalytic Reactor. You will also get the equations and method to for pressure drop calculation from these books.

Now talking about Adiabatic Temperature Rise. Since this is a Exothermic Reaction so heat will be produced as the reaction proceeds, the heat produced is not that much high that to control that we use Cooling Coils. The temperature of the gas stream rises as the gas stream flows through the reactor and this is know as adiabatic temperature rise (i.e. no heat enters or leave the system. The reactor is usually well insulated to stop the exchange of heat with the surrounding). Steam in excess is used to control the temperature rise, since steam increases the heat capacity of the system.

I hope that you understand what I am trying to convey.

Regards,

Ali

[Neilsen]

hello, i am also a final year student and i am currently design a water tube boiler. i would like to know whether if the boiler is divided into threee parts, which consists of economizer, superheater and evaporator. so i have to design all these three components? or i have to design the boiler based on it is sterling, A, O or D type boiler?thanks in advance.