6 replies to this topic

[Tonu]

I am basically trying to design a CSTR in which there is biomass, and now when I pass gas through it , a certaing amount of gases would be generated, which would again be determined by the residence time for the reactant gases, which would be determined by the driving force between the inlet and the outlet.

My question is that whether the outlet velocity with which the gases would flow out would be depended upon the amount of gases that has been generated,

can there be a system where I do not know the oultlet flow rate, but I want to determine the outlet gas velocity by the amount of agses that has been generated ?

I know the problem may sound vague, and my understanding low. Any insights would help me immensely ,
thanks!

[Steve Hall]

Clearly, gas velocity is determined by the quantity of gas, its pressure, and the flow area.

Your reaction in the biomass follows some kind of rate equation that may also be affected by pressure, concentration at the boundary layer, temperature, and other factors.

Are you really intending to ask about gas velocity, or is your question centered on the quantity of gas generated by the reaction? Perhaps framing the question with a flow diagram will increase your understanding (you said it is "low"); I often find that writing down everything I know, including making a flow diagram with heat and material balance, gives me enough insight to ask the questions that lead to full understanding of the system.

[Art Montemayor]

You are making assertions that may / may not be correct with respect to what you are trying to do. I have to make assumptions in order to proceed and please correct me if I am wrong in these assumptions.

You are proposing to use a Continually Stirred Tank Reactor in processing a biomass. During the processing of the biomass, gases are generated (probably methane, CO, N₂, etc. ) in a mixture and these are constantly expelled from the reactor as they are formed. The reactor is maintained at essentially atmospheric pressure – or a few psi above it.

You want to know the amount of gases generated by the biomass and subsequently calculate the velocity these gases are being expelled at.

To your specific questions, I can respond as follows:

• whether the outlet velocity with which the gases would flow out would be depended upon the amount of gases that has been generated. Of course, the amount of gas generated determines the velocity the same gas assumes as it exits the reactor. The more quantity of gas expelled through a fixed diameter of outlet nozzle, the higher the velocity (at constant flowing temperature and pressure).
• Can there be a system where I do not know the outlet flow rate, but I want to determine the outlet gas velocity by the amount of gases that has been generated? You cannot determine the outlet gas velocity without knowing its rate of mass flow. That is a logical and physical reality. You must first determine the RATE at which the gas is being generated – either by calculation or by empirical measurements (such as in a pilot plant operation).

I don't understand your title to this thread as it relates to your questions. If you need to determine the gas outlet nozzle size for the reactor, then so simply state it. You seem to be laboring in logically expressing your needs.

[Tonu]

Apologies for not being articulate, but your insights would be helpful and help me take a step ahead in this exciting new field.

@Steven Hall
Perhaps framing the question with a flow diagram will increase your understanding (you said it is "low")
Thanks for the suggestion , have implemented it.

@ Art Montemayor
Thank you fro the reply sir,

You are proposing to use a Continually Stirred Tank Reactor in processing a biomass. During the processing of the biomass, gases are generated (probably methane, CO, N2, etc. ) in a mixture and these are constantly expelled from the reactor as they are formed. The reactor is maintained at essentially atmospheric pressure – or a few psi above it.

You want to know the amount of gases generated by the biomass and subsequently calculate the velocity these gases are being expelled at.
The above description is actually the question I wanted to frame,

@ All
I am actually not able to comprehend this:
say a reaction.

C+ O₂ -----> CO₂

takes place in a CSTR. I know the inital concentration of O₂ inside the chamber (same as initial feed), the inlet flow rate for oxygen and the carbon, and also the pressure and temperature.

I have framed the mass balance and energy balance equations for the same:

Accumulation = In - Out + generation

here I know the inlet mass flow rate for the gases and the solids.

For, Outlet mass flow rate for the carbon:
I want to refill it with the same amount of carbon that has converted into gases (i.e. keep it at steady state).

I am confused in calculating the outlet flow for the gases, i.e. ( Conc.(out) * area(out)* velocity(out))
here I know the area(out), but I do not know velocity out for the gases and the Conc.(out) - i.e., 2 unknowns in one mass balance equation.

If I know the inlet flowrate for the gases, and the rate equation , should'nt I be able to know how much of the gases that would be generated?

Do I need to fix the outlet flowrate and hence the outlet velocity to know how much reaction would take place inside the CSTR?

Thanks,
Tonu

[Art Montemayor]

Tonu:

You continue to be confused by a simple stoichiometric problem because you are failing to state (and consequently study) the basic data and the scope of your problem.

I am not criticizing you or lecturing. Rather, like many students who visit our Forum, you are not stating and defining your problem correctly nor identifying your needs. Here, I am assuming (since you don’t give us any personal background) that you are a student. This is a basic handicap that, unfortunately, too many chemical engineering students seem to acquire and have a hard time getting rid of. By forcing and training yourself to communicate clearly and concisely, you will also benefit from the problem’s understanding and ultimately timely solution. You seem to be trying to juggle equations and unknowns to divine a magical equation that yields your outlet gas velocity. The solution is much, much simpler than all that.

Please study the attached Excel Workbook that I have used to illustrate what you must do to arrive at what you say you need – the amount of Carbon Dioxide being emitted by the biomass reaction. Take particular care to read the notes I have added at the end of the calculations. This is the manner that the problem should be stated and attacked, in my opinion. The answer is quickly arrived at because you have the basic known data: the mass of carbon. You quickly arrive at the amount of carbon dioxide produced (in moles) and this is simply converted to volumetric measurement. What you don’t know is the rate at which the reaction is taking place. If you don’t know stoichiometry, you will have a very rough time with reaction kinetics – which normally would be the academic manner of studying and defining the reaction rate. In a practical manner – if this is a practical problem – you run a test reaction in a pilot plant and measure the reaction rate at different conditions. From this data you can obtain your real gas production rates – and amount of excess oxygen required to run the reaction the way you need it.

I have prepared the spreadsheet so that you can clearly follow the calculations and substitute the values of inputs as you want to (for example, the amount of carbon fed). The spreadsheets are protected – but without a password and can easily be un-protected.

I hope this helps to address your needs.

Attached Files

•  Gas Production from BioMass.xlsx   134.53KB   28 downloads

[Steve Hall]

Tonu,
With respect, your problem sounds like a textbook CSTR calculation. The only wrinkle that I see is the nature of the biomass (what happens to the unreacted solids). I suggest that you study your reaction textbook and solve the problem from there. Note that you can't fix the outlet flowrate; at steady state the outlet flow will be determined by the conditions in the reactor, the inlet flows, residence time, and the rate equation (assuming you have perfect mixing). Also, with your carbon tied up in a solid substrate, diffusion and mass transfer at the liquid-solid interface is probably important.

Art's comments about gathering data from test runs are important; the mass transfer aspects will be incorporated into the data if the pilot plant is scalable.

Steve

Edited by Steve Hall, 25 July 2012 - 10:12 AM.

[Tonu]

Thanks Art and Steve,
Helped me understand the basics better.
Shall work on the same.