Catalytic Cracking Process

Demonstration Experiment on Video

Catalytic Cracking Process

Objective: Catalytic Splitting of Hydrocarbons

Peter Keusch

German version

Chemicals:
paraffin in pastille form 57-60 (Merck)
bead catalyst
10 ^-2 M aqueous bromine (1.28 g bromine / 800 mL H₂O)
glass wool

Apparatus and glass wares:
2 round bottom flasks 29/32, 100 mL
distilling tube 29/32, 14/20
thermometer 14/20
vacuum take-off adapter (straight) 29/32
three-way T-shape stopcock
gas syringe 100 mL
crystallizing dish
cylinder (gas washing bottle) with stopper 250 mL
powder funnel

Hazards and safety precautions:

Bromine is highly toxic if inhaled, ingested or comes in contact with the skin.

Bromine water is harmful if ingested or inhaled. Prolonged skin contact can cause burns. Eye irritant - lengthy contact will lead to eye damage.

Safety goggles and protective gloves must be worn. The bromine solution should be prepared in a fume cupboard!

Experimental procedure:

The experimental set-up (see figure below) consists of a simple distillation apparatus. A gas collection syringe is attached via a three-way stop cock to the hose connection of the receiver take-off adapter. The plunger of the gas syringe is positioned at the 0 mL mark. The receiver flask is immersed in a water bath (crystallizing dish).

Using a Bunsen burner the distillation flask (1) filled with 20 g of the bead catalyst and 30 g of paraffin is heated. The three-way stop cock (5) is turned in a manner allowing the gas liberated in the reaction mixture to pass directly into the gas syringe (3). The plunger moves outward. When the plunger of the gas syringe has reached the 100 mL mark the burner is removed. The plunger (4) is slowly pushed back into the syringe to allow the trapped gas to escape through a glass tubing (6), provided that the three-way stopcock has been turned to the adequate position. The gas flowing out is ignited. The glass tubing contains (6) a plug of quartz wool which serves as a flame trap.

Experiment set-up

Before igniting the gas evolved care must be taken to ensure that all the air has been displaced from the apparatus! The first two volumes of the gas collected in the syringe (100 mL) are removed. The third gas sample is ignited.

Safety goggles must be worn!

The liquid collected in the receiver flask (2) is mixed with 100 mL of bromine water in a cylinder fitted with stopper.

Results:

When heating paraffin with a bead catalyst liquid and gaseous products, burning with a bright flame, are produced. The catalyst turns dark and becomes black finally. Both, the liquids and the gases decolorize bromine water.

crack

Video clip (Download RealPlayer .rm file)

Discussion:

Paraffins are composed of hydrocarbons containing up to 40 carbon atoms connected in straight chains (normal paraffin) or ramified (iso-paraffin) or chains with a cycle at the end (cyclo-paraffin). Such chains are called alkanes. To crack them, paraffin is heated with a bead catalyst. Short chained hydrocarbons are formed - saturated and unsaturated hydrocarbons. Also the colorless gas released contains alkenes, which are shorter carbon chains than alkanes and have a double carbon bond. Alkenes turns bromine solution from red-brown to colorless.

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