## Chemical enginering questions

plante1999 - 15-9-2013 at 11:11

I'm currently working on three project which are more engineering then chemistry, however, the engineering is for chemistry.

My problem lie in theoretical calculation, I thought of buying perry's chemical engineer handbook, but I'm unsure about if it contain what I need, I take any recommendation tough.

The first problem was that in a part of the set-up gas enter, a reaction fallow, release 1800 KJ per hour. The gas flow is 320 L per hour. The gas enter at RT. with calculations, much helped by dornier (much tanks to him) it was found that the temperature would be 13000K, which is way too much fallowing my "feeling" As such I must take other parameter such as heat dissipation by material.

My first question, of probably a series is:

How can I calculate dissipated energy in the system, considering that the shell is ether made of aluminium, stainless steel or ABS plastic.

Thanks.

plante1999 - 15-9-2013 at 13:18

I may have been unclear:

I mean that the gas enter in a chamber, of about 150 ml volume are heated by the reaction and then exit the reactor. I want to know the temperature inside theoretically to know what material to use, the volume and if I must use asbestos insulation.

Thanks again.

gsd - 15-9-2013 at 18:23

1) Radiation : Heat lost by radiation is proportional to the 4th power of the body's absolute temperature.

2) Increase the volume of the chamber, which will increase its surface area and heat dissipation by convection & radiation.

3) Dilute the reactants by adding inerts (eg if you are using O2 then use Air instead).

BTW, the adiabatic flame temperature of 13000 K appears extremely high. At this temperature your reaction may not proceed as written on paper as the reactants themselves may have transformed due to temperature effects.

Refer to any Std CE text book on thermodynamics ( Smith & Van Ness, etc) for further insight.

Gsd

plante1999 - 16-9-2013 at 08:11

Thanks, but doesn't the material affect energy dissipation rate?
gsd - 16-9-2013 at 08:31

Material does affect energy dissipation rate but it is very minor contributor to overall heat effects.

On closer look, you said 1800 KJ/hr of energy release taking place.
That is hardly a problem for any decent heat transfer equipment. It is roughly equivalent to (latent) heat released by 800 gms of steam condensing to liquid water over a period of 1 hr.

Recheck your calculations (energy released, Adiabatic Flame Temp etc). You may have made an error of order of magnitude in (mis)applying some conversion factor. If calculations are OK then the problem is relatively trivial from CE point of view.

Removing 1800 KJ/hr is no big deal even in a lab apparatus.

gsd

[Edited on 16-9-2013 by gsd]

plante1999 - 16-9-2013 at 08:44

If anyone wonder why I'm not telling exactly what I'm working on, is that I have the idea of making a prepublication with it.

simba - 20-9-2013 at 06:54

 Quote: Originally posted by gsd Material does affect energy dissipation rate but it is very minor contributor to overall heat effects.

How not? What about the thermal resistance of the material?

[Edited on 20-9-2013 by simba]

gsd - 20-9-2013 at 07:25

 Quote: Originally posted by simba How not? What about the thermal resistance of the material? [Edited on 20-9-2013 by simba]

The thermal resistance of the material comes in play when primary mode of heat transfer is Conduction.

For the problem at hand the primary modes of heat transfer are
1) Radiation - which is governed by absolute body temperature and the emissivity - which is a function of material.
2) Convection - which is governed by Temperature difference and Overall Heat transfer coefficient which in-turn has a very minor dependence on material of the body.

gsd

simba - 20-9-2013 at 07:49

Quote: Originally posted by gsd
 Quote: Originally posted by simba How not? What about the thermal resistance of the material? [Edited on 20-9-2013 by simba]

The thermal resistance of the material comes in play when primary mode of heat transfer is Conduction.

For the problem at hand the primary modes of heat transfer are
1) Radiation - which is governed by absolute body temperature and the emissivity - which is a function of material.
2) Convection - which is governed by Temperature difference and Overall Heat transfer coefficient which in-turn has a very minor dependence on material of the body.

gsd

I understand what you meant, I just think it was a little misleading, since there will always be some heat which is lost by conduction from the inside to the outside walls of the reactor. Although this loss is mininum in this case indeed.

Chemosynthesis - 28-9-2013 at 21:15

If you would like, I could point you in the right direction with some heat exchanger calculations. The surface area, materials in contact, and flow rates all factor into this type of calculation, and the coefficients should be easily found in Perry's. GSD did a very accurate overview.

[Edited on 29-9-2013 by Chemosynthesis]