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General Chemical Engineering Questions

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#1 ChemEng16


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Posted 03 March 2016 - 01:16 AM

Goodmorning everyone, I would like to ask a few general knowledge questions, even the shortest hint of information to any of these would be of utter value to me and be taken with great consideration and appreciation. I apologize in advance if my questions are not "competent"  enough , but all I can say is that we all start somewhere and have to learn. Additionally, I am not cutting roads and just asking questions, I have considered them myself and sincerely have no-one to ask but myself. I've tried to be concise and have separated my questions in the hopes to increase responses.
1) Many books claim that for isothermal (heterogeneous) reactions of order higher than 1 , a "reasonable" approximation of the effectiveness and concentration profile can be obtained using the equations derived (from the material balance on the catalyst pellet) for the 1st order reaction. How true is this assumption? Does anybody know the actual equations involved?(specifically for spherical catalyst pellets)
2) If you have a multicomponent system, consisting of multiple liquids and two gases, how do you evaluate diffusivity within the catalyst? What I have done is evaluated the effective diffusivity for the 2 gaseous components (using Chapman-Eskog theory, knuden diffusion , pore diffusion), and compared that with the liquid diffusivity i've calculated , but im not confident on having used the right method. What I have done is used the Stokes-Einstein equation to evaluate the diffusivity of each component separately and then found an average for the system. This average is ofcourse way slower than that of the gaseous components , so i've neglected effective diffusivity (I've only used the gaseous bulk diffusivity to find the external mass transfer coefficient). Is this nonsense?
3) If a shell and tube reactor has been properly designed to account for release of reaction enthalpy and the tubes containing the catalyst are small enough in size for efficient heat transfer , can it be assumed that there will be no temperature gradient within the catalyst?(ofcourse this is not completely true, but as an assumption it does not sound wrong to me..?)
4) I don't understand how pressure in a packed bed is "established" or "kept constant". I understand that when a gas is in a vessel it exerts pressure which is easily measurable.
But if we say for example a reaction inside a packed bed has to happen at 30 bar, how can we make sure the pressure is that? From what I gather , the pressure only refers to the bulk phase above the packed bed ,(..the pressure exerted by the pellets on the tube wall shouldn't matter for reaction conditions I gather). Also isn't 30 bar an enormous pressure for tubings to withstand? Ofcourse thickness can be adjusted, but wouldn't this be unbelievably uneconomical? or is it considered to be practically the best choice so to avoid excessive shell/vessel pressures and in that respect cut on capital costs. 
5) I have read, and then have observed in my calculations that to maintain strong control on  the temperature of the tubing process fluid constant, the service fluid  has to be of similar temperature. For me, it makes sense to use room temperature process water to cool down an extremely hot reactor, as it generates energy for alternative purpuses. However, my calculations shown that this wasn't suitable, and when I used  steam to cool the reactor , outcomes were a lot more satisfactory. But isn't it very expensive to operate a reactor with steam all the time?
6) Besides axial dispersion and negligible pressure drop what else has to be taken into account to assume plug flow in a trickle bed reactor?
7) In my Shell and Tube reactor , I've evaluated pressure drop on the tube side using Ergun's equation and on the shell side using  velocity,dynamic heads. This seems right to me because the most significant source of pressure drop (on the tube side) will be due to the packing?
8) As far as it concerns alkanes as biofuels, from research I've learned that the branched isomers are more effective for use as fuels for two reasons:
a) They release more energy during complete combustion
b )They have high flash points, thus don't evaporate/flash/burn prematurely causing effects on "knocking effects" on engines.  However, I have seen in many patents, academic journals and books etc that catalysts using for oligomerization are specifically pre-treated so to deactivate  external active sites. This means that the selectivity for the corresponding branched alkene oligomers is decreased drastically due to steric hindrance effects. However, after sufficient research, I did not find appropriate justifications. The I came up with was a quote in a catalysis journal , saying that use of catalysts for such reactions without pretreatment " leads to formation of complex structures" , without giving any insight on what it was referring.  I am aware that during oligomerization reactions there are a number of competing reactions , such as isomerization , catalytic cracking etc  , but what could this "complex structures " be referring to that its so significant so to decrease the product quality? The catalyst prior to this treatment is designed to favour oligomerization , especially at certain operating conditions , so the reason isn't to favour the wanted reaction. Could these complex structures be aromatics? But can aromatics be made during oligomerization? I think that if this were/is possible then it could justify this common pretreatment of oligomerization catalysts.
To whomever, read any of these, thank you very much for your time.

Edited by ChemEng16, 03 March 2016 - 01:18 AM.

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