SAFETY IS OF OVERRIDING IMPORTANCE. IT IS IMPERATIVE THAT YOU:

1. Wear eye protection at all times.

2. Make sure all electrical components and metal parts are well grounded.

3. Observe all the normal safety precautions. In addition, be aware of the following:

The normal (straight-chain) butenes comprise three isomers, namely 1-butene, cis-butene, and trans-butene. A refinery waste stream containing a 50-50 mixture of cis-butene and trans-butene is available at low cost. The Research Department has discovered a process which uses 1-butene and produces a valuable intermediate.

 Your goal is to develop a catalytic process which produces an equilibrium mixture of the three butene isomers, from which 1-butene can be removed by reaction, while the cis and trans isomers are recycled to the catalytic reactor.

In particular, you will find the temperature above which, at a given feed rate, the reactor effluent is at equilibrium with respect to the isosmerization reactions, and from the equilibrium composition you will find the three equilibrium constants (as functions of temperature) for the isomerization reactions.

You will also derive, from data taken at lower temperatures, estimates of the rate constants and activation energies for the isomerization reactions. Finally, you will look for other products of the catalytic reaction, including carbon deposited on the catalyst and other hydrocarbons resulting from butene pyrolysis (e.g. iso-octane) and possibly polymerization (e.g. octenes).

You may also measure the degree to which butenes are adsorbed on the catalyst at room temperature, and look for evidence that not all isomers are equally strongly absorbed.

Apparatus

The isomerization reactor system is shown schematically in the attached drawing. A roughly 50%-50% mixture of cis-butene and trans-butene is contained in a cylinder connected to a pressure regulator, rotameter and needle valve. A nitrogen cylinder is similarly connected. The butene and nitrogen streams mix and enter an 80 cm long and 20 mm ID Pyrex tube which passes through a Thermolyne tube furnace. The gas feed passes through a 10 cm long heating section packed with 5 mm stainless Raschig rings, a Pyrex wool plug, 5 cm of chromia-alumina catalyst powder (1% chromium by weight), and a second Pyrex wool plug. The tube is mounted at 45 degrees, and a steel weight following the catalyst bed prevents pressure drop across the catalyst bed from moving the catalyst. The effluent gas leaves the Pyrex tube through a tee (equipped with a silicone septum through which samples can be taken) and then to the inlet of the chemical hood system. A copper-constantan (or iron-constantan) thermocouple passes through the steel weight, is embedded in the catalyst and connected to a temperature indicator. The temperature in the oven is controlled by a potentiometer on the tube furnace, which is also equipped with a thermocouple connected to a meter.

After the run allow the reactor to cool. Remove the tube and look for evidence of carbon deposition. Look also for GC peaks that might correspond to pyrolysis products or polymerization products.

Theory

Three dependent reversible reactions can occur, namely:

Here x_k is the extent of the kth reaction and K_k is the equilibrium constant for the kth reaction. A₁, A₂, and A₃ correspond to 1-, cis-, and trans-butene. From the GC analyses, let y₁, y₂, and y₃ be the mole fractions of the three isomers. Then we have (assuming that partial pressure is the product of mole fraction and total pressure)

and

It is clear that

so that the three equilibrium constants are not independent.

For each run calculate K₁, K₂, and K₃ from the measured mole fractions. Show in a table the equilibrium constants as functions of temperature.

In the kinetic regime equilibrium is not fully attained. We expect that a combination of low temperature, high butene feed rate and high nitrogen dilution will move the reaction into the kinetic regime. Since the reactions are not independent, only two extents can be found from the analysis of an effluent sample. From these we can calculate the forward rate constants for A₁ to A₃ and for A₂ to A₃, assuming the backward rate constants are related to the forward constants via the equilibrium constants, and neglecting the conversion of cis-butene (A₁) to trans-butene (A₂).

1. It can be assumed that the GC peak area is proportional to the moles of a butene isomer, with the same proportionality constant for each isomer.

2. If the carbon deposition rate is higher than first-order in butene partial pressure, operation at low butene pressure may significantly reduce carbon deposition. The same result will be obtained by diluting the feed with nitrogen, which dilution also reduces the butene residence time. At a given temperature and butene feed rate, the addition of nitrogen will eventually bring the process from the equilibrium regime into the kinetic regime.

3. Assuming a feed rate of 200 scc/min of butene, the power needed to heat this stream to 400 C is only a few Watts, while the furnace can supply 1000 Watts. Thus, with a reasonable heat transfer coefficient, a short preheating section should be sufficient. The heat of isomerization is expected to be very close to zero, based on the similarity of the isomers.

4. The TA will start up the HP-6890 GC before the run. The Petrocol 150 capillary column is used with procedure butene01. Use a Hamilton gas syringe (well-purged) to inject samples of 2 cc volume. The butene peaks appear in the 25 to 30 minute retention time range.