6 replies to this topic

[CENEERAJ]

why radial flow packed bed reactors not very common as axial flow packed bed reactors, when the former is superior to later in giving less pressure drop and better conversion per pass?

[PingPong]

... better conversion per pass

Where did you read that wisdom?

[CENEERAJ]

 

... better conversion per pass

Where did you read that wisdom?

 

the point is, at low pressure drop achievable.....catalyt particle size- lower particle diameter can be used to increase catalyst efficiency factor and therefore more feed conversion....thats the wisdom........now please answer the query

[PingPong]

Every process design is an economic optimum.

 

A smaller diameter catalyst particle will have shorter pores than a somewhat larger particle, but I think that most of the action is at the pore area in the outer layer of the particle anyway, even in the smallest catalyst particles.

 

A radial flow reactor requires a bigger, and therefor more costly, reactor vessel for the same amount of catalyst than a normal axial reactor. Simply stated: a radial reactor contains more empty volume then an axial one.

 

A radial reactor is only suitable for single phase flow, not for mixed phase.

So what do you want (if pressure drop or mixed phase flow are not determining): more useless space in a radial reactor, or a little more useless catalyst material in the center of somewhat bigger particles in an axial reactor.

 

A lot of catalyst particles are extrudates. These need to have a certain strength to prevent them from crushing before even reaching the reactor, or when dumped into the reactor. To have the benefits of small diameter related effectiveness, as well as the benefits of larger diameter related increased strength and lower pressure drop, vendors invented the quadralobe and trilobe catayst extrudates:

Edited by PingPong, 10 January 2015 - 12:03 PM.

[Light]

Neeraj

 

As PingPong rightly mentioned, it is not always that pressure drop is the governing factor in all the cases. Hence you cannot generalize that "An axial-radial flow reactor gives better conversion per pass than an axial flow reactor."

Edited by Light, 11 January 2015 - 03:36 AM.

[CENEERAJ]

Neeraj

As PingPong rightly mentioned, it is not always that pressure drop is the governing factor in all the cases. Hence you cannot generalize that "An axial-radial flow reactor gives better conversion per pass than an axial flow reactor."

I will appreciate it if you please explain a case where pressure drop isnt a governing factor and nor is the rxn multiphase( that is a strong point by PingPong)and axial flow reactor is preferred based on some other governing factor .Thank you for the reply........

Edited by CENEERAJ, 12 January 2015 - 08:54 AM.

[PingPong]

Radial reactors are used where the designer wants the absolute pressure over the whole bed to be as low as possible. Pressure drop is minimised not because it is a problem but because the higher pressure at the bed inlet has a negative impact on the conversion. That is usually the case in processes where the number of moles of gas increases due to the reaction, and the conversion is limited by equilibrium. That is often the case with dehydrogenation reaction such as catalytic reforming of heavy naphtha to produce aromatics plus hydrogen.

 

But instead of a real radial reactor sometimes a horizontal reactor with a shallow bed and vertical downward flow is used to minimise difference in pressure over the catalyst bed, such as in CATOFIN.

Edited by PingPong, 12 January 2015 - 11:49 AM.