Maybe Aspen Plus is giving the best results for new facilities, while Aspen Hysys is giving average results for operating units.

how you have model a reactor and find reactor volume?

I have inserted initial feed stream conditions, kinetic data of the reaction, and put value of length and diameter of the reactor to determine the volume of reactor suitable for 98% conversion of reactant. 

My guess is that you have accidentally input different kinetic information - possibly a units error.  You should be able to do a hand calculation to see  which one is correct.

If you have kinetics, then try obtaining the volume of the reactor using the performance equations of the reactor and compare the results. 

I'm surprised to see such difference in the results between Aspen Plus and Aspen HYSYS. These two products should be identical and developed by the same company Aspen Tech.

Please make sure that you use the same thermodynamic model and same type of reactor. For example, in Aspen HYSYS, you can model four different types of chemical reactors.

In my capstone course (CENG-416) in the chemical engineering curricula  which I teach at Yale University, I use Aspen HYSYS and I know that Aspen Plus should be identical.

Khalil, Y.F. (2014). Chemical Engineering Process Design (CENG-416). Yale University, New Haven, CT 06520

Professor Yehia Khalil, Yale University, USA

Fellow of the University of Oxford, United Kingdom

I have used same thermodynamic model and use same reactor for both cases. I tried it different way. But still there is a difference. For volume of 15 m3, aspen plus provided a conversion of 99.5% and aspen hysys provided 93.5%. Approx. 6% efficiency difference. Further, aspen hysys simulation is linked only with reactor volume, no matter what is dia, length etc. While in aspen plus, the change in conversion is observed with change in dia, length or no of tubes.

Muhammad,  You could use a Gibbs reactor to check (and hopefully confirm) that equilibrium predictions are the same.  There are a few cases where simulators have used incorrect thermodynamic information.   For example a few simulators have (and may still) incorrectly use liquid phase equilibrium values  as if it was  vapor phase equilibrium for C6 paraffin reactor equilibrium because of an error in one of the widely used research documents on the subject.  

It is interesting that changing L/D affects the kinetics for Aspen Plus but not  Aspen Hysys.  This probably means "catalyst contacting" correlations turned on in the Aspen Plus the simulations but not in the Aspen Hysys simulation.  You can look for a setting to turn this off or you can specify high mass velocity reactor dimension  (High Length/Diameter) to assure good catalyst contacting.  It is intersting but this does not explain why Hysis (with no catalyst contacting effects modeled) has a lower conversion prediction than Aspen Plus.

It may also help if you let us know what reactions you re trying to model and what conditions you are operating at..

Dear Rick, I have used gibbs reactor for both softwares, inserted same values and obtained same outlet composition and flow rate. 

I have varied L/D ratio between 20-50 (volume constant) but there is no effect in aspen hysys simulation. However, the conversion obtained is different with different L/D ratios. 

I am using plug flow reactor for methylcyclohexane dehydrogenation to toluene and hydrogen. Operating conditions include 360C, 2 bar and 100 kgmole/hr with feed comprising of 99.8% MCH, 0.05% H2 and 0.15% toluene. 

k is 1.65 and Ea is 55.45 kJ/mol. Order of the reaction is 1. 

At 360 C and 2 barg I expect methylcyclohexane and toluene to be vapor phase - there should not be any significant maldistribution/catalyst contacting issues in a vapor phase reactor you may need to make an adjustment to the aspen plus model to remove the impact of mass velocity on catalyst contacting.. Also  360 C is also a low temperature to form aromatics.  Commercial naphtha reformers operate around 500-540 C.

Aromatics formation is endothermic you will need to  heat  the feed to a higher temperature and/or attach an energy stream to get the reaction to occur.  You may also have to adjust values for rate constants to get a more reasonable results.

I think u should to check your units and conversion fectors  ...

It's impossible there is a huge deference in result..

ASPEN and HYSYS is like black box. You do not know what the numerical procedures were used, and therefore know nothing. In addition, you should have specified the type of reactor. Volume refers tank reactors - batch reactor, a semi-batch reactor or CSTR, while the pyrolysis (topics) is usually carried out the tubular reactors. In the case of CSTR and tubular type reactors  may occur multiple steady states. Which state was computed, you do not know. And it may be the answer to your question.