6 replies to this topic

[Wondor]

I want to know the relation between the operating pressure VS reactor size.

 

Let's say I'm designing a chemical reactor, simply a fixed bed type reformer, and I have to decide the operating pressure.

The reactor will be installed in very confined space, so i have to consider its size carefully.

I am considering two cases.

 

(Case 1)

P1 = 20 bar, V1= 10m3, T1=600C

 

(Case 2)

P2 = 18 bar, V2= ???,    T2=600C

 

My simple thinking,

 

P1 * V1 = P2 * V2 (Boyle's Law)

 

V2= (P1/P2) * V1 = (20/18)*V1 = 1.11 * V1

 

So V2 is 11% larger than V1.

 

Is this correct?

 

 

 

 

 

[Bobby Strain]

Thermodynamics and reaction kinetics dictate reactor operating pressure and temperature. You don't usually have a choice on reactor operating pressure.

 

Bobby

[Art Montemayor]

Bobby is absolutely right.  First resolve the basic reaction requirements using thermodynamics and kinetics.

 

Once you establish that the reaction is possible and under what conditions, you next attack the required (or desired) reactor's physical size.

 

The ideal gas law has absolutely nothing to do with the sizing of a reformer.  How did you come to the decision to employ the ideal gas law?  Your response would give us all an insight on what is presently being taught in chemical engineering courses.

[Wondor]

@ Bobby, @Art Montemayor

 

As far as I understand, what you are saying is that I have no choice on calculating the reactor operating pressure BECAUSE a lab scientist will find out the best operating condition for that reaction.  And Chemical engineers have to meet those conditions in any way by designing proper reactor.  Am I right?  If so, that is not what I was wondering.

 

I have heard that Equipment and Piping size can be affected by its operating pressure, because the actual vol flow rate will be smaller at high pressure. (Let's say gas flow only)  My thinking was based on the volume flow rate change according to operating pressure change.  Let's say the reformer can be operated in a small range of operating pressure, 18.39~20.50 bar.  Please take a look at the attached table.  I calculated this by the Gibbs reactor Aspen plus v.8.4 simulation program.

 

FEEDGAS3 --> Refomer1(20.5 bar) --> EFFLU3​

FEEDGAS4 --> Reformer2(18.39 bar) --> EFFLU4

 

As shown in the attached Aspen printout table, the actual Vol. Flow rate is 17% larger at 18.39 Bar than at 20.50 bar.  So I could imagine the cross-sectional area of the Reactor and Pipings have to be enlarged in order not to increase space velocity of the gas. 

Attached Files

•  Picture1.jpg   92.98KB   3 downloads

Edited by Art Montemayor, 12 February 2016 - 03:22 PM.
spelling, grammar, composition

[Art Montemayor]

Is your "reactor" a conventional methane steam reformer for the production of synthesis gas or hydrogen?

The kinetics and thermodynamics of this very established reaction system are well known and published.  

 

If so, please acknowledge and tell us why you are applying the ideal gas law to the gas volume.  Tell us exactly what it is that you are trying to establish or find out.  The pressure at which steam reforming is carried out is not necessarily left to the engineering designer to "calculate".  It many times is determined by optimizing in conjunction with the required downstream pressure required..  I have operated steam reformers at pressures as low as 50 psig and upwards of 175 psig.  It all depends on other factors.  Space velocities are not the only criteria.  Catalyst selection and reforming temperatures also play a role - as do steam to carbon ratio.  What are your factors?

[Francisco Angel]

Dear Wondor:

I think Bobby and Art already explain a lot. Although your reasoning about using a greater pressure make some sense, you must critically analyse why you think the application of Boyle's law will give you an optimum value and with respect to what optimization criteria.

For example, some can argue that, given some mass flow, the volumetric flow will change by the ratio of pressures, and so if you want to keep the same residence time inside the reactor, the volume of it must be varied by this ratio also, but keep in mind that a different pressure will affect the rate of reaction inside the reactor, so the output number of moles will change.

Long story short, there are several aspects that must be considered in specifying the operating conditions, so state your problem clearly first, and decide which strategy to use to solve it.

Best regards.

[Pilesar]

Selecting a pressure for a process that can operate at several pressures is not necessarily straightforward. Higher pressure equipment is usually smaller in size for vapor phase volume reasons, but there may also be a requirement for thicker walled vessels and beefier piping. Lower pressure processes are generally considered inherently more safe because of the reduced energy in the system. There is not much difference between the two pressures you suggested. Your temperature is very high. Will the temperature really be the same at both pressures? If the materials of construction are costly, it may be economical to minimize the cost of materials by choosing the reactor pressure that requires the least amount of metal. You should take into account the wall thickness when you look at the economics. If you are looking only at the reactor and not the whole plant design, you have an easier comparison. If the reactor pressure also affects compressor costs, then the rotating equipment may be a larger influence on plant economics. The rigorous method is to estimate both complete plant designs and compare the final costs. For a rough screening, you likely just need to select reasonable criteria and document your decision for the professor.