Pressure change due to energy transfer in a reactor 1

[mills82]

I am trying to get my head around the relationship between pressure and temperature.

We have a preheated recirculated process gas stream that is used to provide energy to a bed within a rotary drum. It has been calculated from the energy balance that the gas will enter at 350oC and leave at 250oC. Due to the nature of the process it is necessary to maintain a negative gauge pressure within the drum to prevent leakage out.

I am currently working on modelling this process in order to implement the control system which will maintain the mass flowrate, temperature of gas entering the reactor and the negative pressure.

There is a pressure drop within the reactor as the gas passes through the bed, however will there also be a change in pressure due to the drop in temperature by 100oC? I am aware that the ideal gas law can by used to calculate the change in pressure due to change in temperature, however will the temperature affect the volumetric flowrate and the pressure remain the same? Or is it a combination of the two?

Scratching my head over this one. Thinking about it that much, I am sure logic has clouded over some time ago!

Hope to hear from you.

Andy

 

[Latexman]

For gases in a closed system, the ideal gas law is about all you need. You have an open system, so the dynamics of the gas flowing thru the system is what's important. You still need the IGL to calculate density at any point that is important though.

Good luck,
Latexman

 

[sshep]

Hey Mills,

The change in temperature will manifest as a local change in density rather than a change in pressure. Pressure will be everywhere constant except as a result of the pressure drop due to flow. I think this was Latexman's clarification

You want to control the reactor temperature and pressure, so your manipulated variables will be inlet temperature and gas flow. The simplest model will probably break-up the reactor into a number of sections. The number of profile sections needed is based on the total pressure drop using rules of thumb as one would do for a compressible flow in a pipe. Using a sufficient number of discrete sections is easier than trying to obtain an integrated form of the profile equations.

For each section you will calculate a pressure profile (from pressure drop across the preceeding bed section) and also a temperature profile from the heat transfer across the preceding bed section. You can then calculate a molar hold-up based on either the inlet or average density=f(T,P) in each section of your reactor. You will have a fixed total flow at either the inlet or the outlet, and your model will tell you the flow, temperature, and pressure at the other boundary condition.

best wishes,
sshep