KINETICS, CATALYSIS, AND REACTION ENGINEERING

An Experimental Study of the Kinetics of the Selective Oxidation of Ethene over a Silver on a-Alumina CatalystHysys- Ethylene Oxide Production

Ethylene oxide is widely used: it's an intermediate chemical in the production of ethylene glycol (used in synthetic fibres), it can be used as an industrial disinfectant and its high flammability range means it's also used in explosives.

We will simulate a reactor designed to oxidize ethylene to ethylene oxide in the presence of oxygen across a silver catalyst. A paper has been provided on the LMS (Borman and Westerterp) that provides all the necessary kinetic data for the simulation.

Pure ethylene is supplied at 100 kmol/hr at 25 oC and 3 MPa pressure. Oxygen is purified from air in a cryogenic distillation column in an upstream process 1 and is available at a purity of 98 mol% oxygen (balance nitrogen). Oxygen is supplied in the correct stoichiometric ratio for the desired reaction.

The gas mixture is sent to a reactor packed with silver catalyst. The reactor has a void fraction of 1.5 and a total volume of 1m3. Select a reasonable reactor length and aspect ratio. The reactor operates isothermally, typically at temperatures between 150oC - 450oC and pressures between 3 MPa and 5 MPa. You may assume no pressure drop across the reactor, or try to simulate a pressure drop using a higher order approximation (using the Ergun equation for example).

In the reactor, 3 reactions take place:

C₂H₄ + ½ O₂ → C₂H₄O (1) Ethylene Epoxidation

C₂H₄ + 3 O₂ → 2 CO₂ + 2 H₂O (2) Ethylene Combustion

C₂H₄O + 3O₂ → 2 CO₂ + 2 H₂O (3) Ethylene Oxide Combustion

The exhaust gases from the reactor are sent to an aqueous absorber-stripper column pair. Ethylene oxide is highly soluble in water, while CO2 and the reactant gases are not. When the exhaust gas stream from the reactor is contacted with water in a packed column, the EO will absorb and most other gases will not. The EO-water mixture can be sent to a second column (stripper) where increasing the temperature vaporizes the EO from the water stream. The exit water from the stripper column (which contains traces of EO) is recycled back to the absorber column.

You may assume the absorber-stripper column pair can be modelled in Hysys as a single (somewhat magical) component splitter. You may assume all the ethylene, oxygen, nitrogen and carbon dioxide leave from the top of the component splitter. You may also assume that 90% of ethylene oxide and 95% of the water leave in the bottom of the component splitter, on a molar basis.

The bottom of the component splitter is the product stream from the process. It is sent for further processing to improve the purity of the ethylene oxide (which you don't need to simulate).

The reactant rich gases from the top of the component splitter are recycled to the reactor feed, after 2% of the stream has been purged to prevent inert gas accumulation in the recycle loop.

Assume a 10 kPa pressure drop across the component splitter and any heat exchangers you install in your system.

The reaction kinetics are provided in Borman and Westerterp (the PDF). We will use model III from table 5 in this paper (shown below). The relevant kinetic constants for reaction 1 and 2 are given at the end of this paper. It has been assumed the density of the catalyst is 4 kg/L to adjust the forward and back rate constants- this makes the data much easier for you to simulate. Initially we will assume only reactions 1 and 2 take place- we of the 3rd reaction (complete combustion of ethylene oxide).

Hysys

1. Choose an initial guess for reactor temperature and pressure within the range provided and simulate the process described.

Hint: Make sure you construct the Hysys simulation sequentially and systematically, doing your best to verify each process is working before moving on to the next unit. Do not try to simulate the process in one go- something will go wrong and you will not be able to find the error.

2. Determine a metric to quantify the reactor performance then use this parameter to optimize the reactor temperature and pressure, within the range provided. If you choose to use case studies to optimize the process, make sure you include these plots in your report. Screen shots of case studies in Hysys are usually unreadable- you may prefer to export the case study data and replot it in excel.

3. Please submit your Hysys file as an XML case file through the LMS (which is one of the save options).

Task 1 - The 3rd Reaction

Borman and Westerterp (1995) has been provided on the LMS. Use it to find the kinetics for the 3rd reaction (complete oxidation of ethylene oxide) and include this reaction in your simulation. You will find it changes your optimal reactor conditions quite a bit. Re-run your case studies to identify an optimal temperature, at a reactor pressure of 3MPa.

Task 2- Heat Integration

If your simulation looks anything like mine, there are heat inlet and outlet streams all over the place. Perform a heat integration to minimize the net energy required. This does not just mean connecting heat sinks to heat sources- you must make sure you do not violate the 2nd law of thermodynamics. You will need to replace any heaters/coolers with heat exchangers and a working fluid that can move energy around your plant. Hysys is just a calculator and will let you design non-physical solutions.