The main raw materials are hydrogen and toluene to produce benzene and methane known as hydrodealkylation process (HDA). The feed specifications of the process were defined at 25C and 3500kPa before they are heated in a furnace to 725C. The thermodynamic package was used is Peng Robinson in Aspen HYSYS simulator.
The furnace was modelled as a heater for simplification purpose of the simulation with a pressure drop of 20 kPa. Fired heater packages usually involve modelling the combustion process and heat transfer of process transfer (required heater duty). Modelling the combustion process is not provided in this report. However, this can be done usually using the fuel gas composition, the combustion air composition, the ratio of air to fuel and the fuel flow rate.
The reactants are then fed into the reactor specified as plug flow reactor (PFR) with pressure drop of 20 kPa, volume 300 m3- 1 tube, diameter 4m -, void fraction 0.9). The primary reaction of the HDA process is: Toluene + H2 → Benzene + CH4
Irreversible
(equation…) (equation …)
Another undesired reaction involves the decomposition of benzene into Biphenyl and hydrogen as follows: 2 Benzene
Biphenyl + H 2
↔
Reversible
(equation…)
PFR – PFR – used used for high production rates and for short residence time reactions.
The reactor must not exceed 800C to prevent significant cracking.
High exothermic heat of reaction – increase the amount of hydrogen added to the reactor (excess reactant).
In practical instances, the ratio of hydrogen to toluene should be high to prevent coking.
Initially, for the non-recycle scenario, we can classify our reaction set as consecutive. This is because reaction rea ction 2 cannot occur until reaction reacti on 1 has taken t aken place. When dealing with a set of consecutive reactions, we must look at the activation energy of the reactions to determine optimal reactor operating conditions. The activation energy of reaction 1 is greater than
reaction 2, therefore we should use a decreasing temperature profile through our PFR in to favour reaction 1 as much as possible.
In the scenario with the recycle stream, reactions 1 & 2 will occur simultaneously as reaction 2 reactants will be present in the feed stream. The activation energy for the desired reaction 1 is greater than the activation energy for reaction 2 – therefore, heuristically, we should operate the reactor at a high temperature to favour reaction 1. This can be confirmed by analysing the reaction kinetics using an Arrhenius expression.
Figure ? (need to be reproduced)
From the above plot it can be seen that, to achieve the greatest comparative favourability for the desired reaction to the unwanted secondary reactions, we must operate the reactor at a higher temperature.
General Heuristic: A batch or plug flow reactor should be used for multiple reactions in series.
The outlet of the reactor is fed into flash separator after being cooled to condensate the liquid for further separation process. The flash is modelled at the minimum operating pressure (1000 kPa) that most of the product (benzene) could be recovered.
Heuristics: for the initial simulation adhere to the following suggestions for convergence – set the actual number of trays to twice the minimum value, set the reflux ration to 1.2 times the minimum value. These values can be taken from shortcut methods.
Column 1 should ideally operate with a partial condenser in order to allow the highly volatile components (hydrogen and methane) to leave the system easily. Partial condensers should be used when components in the system have BPs below -40C. The recycled biphenyl stream could be passed through a reactive distillation column – this would allow the benzene that is produced to be separated and recovered before it can take part in the reverse reaction as the removal of the product would drive the production of more. Tray spacing should equal: S=0.5D^0.3 A safety factor of 10% on the minimum number of trays should be used. A safety factor of 25% on the reflux ratio to account for reflux pump efficiency The L/D diameter of a column should ideally be between 20 and 30. Bubble Cap trays should be used when low liquid and vapour flowrates are present in the column
STANDARD HEURISTICS FROM LECTURES Easiest separation first, sequences that remove the lightest components alone one by one should be favoured, favour splits that produce as close to equimolar flows in t he top and bottom products as possible - remove hazardous, heat sensitive and corrosive products first – remove very highly volatile components as soon as possible.
The mass and energy balance were obtained using Aspen HYSYS and summarized in figure (1) and (2).
The design basis of hydrogen is 160 kmol/hr and 58 kmol/hr for toluene. The production of
benzene designed to be 3168.24 kg/hr with a purity of 96.6% (molar fraction). The energy balance (figure 2) presents all heat flow required and released from the system. However, its important to mention that the enthalpy of component is not considered in here, however this can be found in the simulation model.
•Relevant Decision for the Recycle Structure – do we want to use an excess of one reactant at the reactor inlet? Is a gas compressor required? Do we want to shift the equilibrium conversion? •There is no rule of thumb for selecting the purge composition or the ratio of hydrogen to toluene required in the new feed for the process containing the rec ycle structure. •We want to ensure there is sufficient hydrogen at the reactor inlet to prevent coking within the reactor – therefore it is desirable to recycle the hydrogen. •Our PFR reactor is originally operating at a very high conversion so a recycle stream may not be needed •For the secondary reaction, we want to shift the equilibrium to the right to produce mor e benzene. To do this, we must maximise the amount of hydrogen at the inlet. This is another reason to recycle hydrogen back to the inlet of the reactor.
•
Heuristic: Whenever there is a light reactant (BP < -48C) and a light feed impurity (Methane
on the recycle) use a gas recycle and a purge stream for the first design. •
The purge stream should generally have the lower flowrate
•
Purge is required in order to provide a sink for the methane. The methane produced does not
take part in any of the reactions and will therefore build up in the system if a n infinite recycle was used with no purge. However, some of the methane must be recycled through the system due to the difficult and highly economically inefficient separation of hydrogen and methane – we would look at introducing a membrane separation unit for separation. •
Methane and Hydrogen are non-toxic species, so a purge system is feasible (however both are
highly flammable, so it is highly likely that a purge system would simply be connected to a flare tower). •
SUGGESTIONS: Recycle the Biphenyl to extinction by passing it back through the reactor
with excess hydrogen to favour the reverse reaction and generate methane – the reaction conditions would have to favour the reverse reaction – by increasing the concentration of biphenyl I the react or – the equilibrium would shift to the left and more benzene would be produced provided hydrogen is in excess.
•
RECYCLE OPTIMISATION – To determine the optimum purge/recycle ratio, an optimisation
of the recycle process needs to be carried out. This is due to the costs associated with recycle – recycling large amounts of material will result in higher operating costs (i.e compressor/pump duties) but will reduce raw material costs. Whereas a low recycle and large purge will have the opposite economic effect. The CAPEX and OPEX should therefore be calculated for a variety of purge ratios in the simulation to see which value would minimise the over related costs.
