Dean-Stark apparatus
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The Dean-Stark apparatus or Dean-Stark receiver or distilling trap is a piece of laboratory glassware used in synthetic chemistry to collect water (or occasionally other liquid) from a reactor. It is used in combination with a reflux condenser and a batch reactor for continuous removal of the water that is produced during a chemical reaction performed at reflux temperature.
Two types of Dean-Stark traps exist – one for use with solvents with a density less than water and one for use with solvents with a density greater than water.
The Dean-Stark apparatus in the laboratory typically consists of vertical cylindrical piece of glass (the trap), often with a volumetric graduation on its full length and a precision tap on the bottom very much like a burette. The top of the cylinder is a fit with the bottom of the reflux condenser. Protruding from the top the cylinder has a side-arm at less than right angles. At the end the side-arm makes a sharp turn so that the end of the side arm is vertical as well. This end connects with the reactor.
During the reaction, vapors containing the reaction solvent and the component to be removed travel out of reaction flask up into the condenser, and then drip into the distilling trap. Here, immiscible liquids separate into layers. When the top (less dense) layer reaches the level of the side-arm it can flow back to the reactor, while the bottom layer remains in the trap. The trap is at full capacity when the lower level reaches the level of the side-arm--beyond this point, the lower layer would start to flow back into the reactor as well. It is therefore important to syphon or drain the lower layer from the Dean-Stark apparatus as much as needed.
More rarely encountered is the model for solvents with a density greater than water. This type has a tube at the bottom of the side-arm to allow the organic solvent at the bottom to flow back into the reaction vessel. The water generated during the reaction floats on top of the organic phase.
This piece of equipment is usually used in azeotropic distillations. A common example is the removal of water generated during a reaction in boiling benzene. An azeotropic mixture of bezene and water distill out of the reaction, but only the benzene (density=0.8786 g/cm3) returns. Water (density=1.0 g/cm3) collects in the trap.
This equipment can be used in cases other than simple removal of water. One example is the esterification of butanol with acetic acid catalyzed by sulfuric acid. The vapor contains 63% ester, 24% water and 8% alcohol at reflux temperature and the organic layer in the trap contains 86% ester, 11% alcohol and 3% water which is reintroduced. The water layer is 97% pure.
Another example is the esterfication of benzoic acid and n-butanol where the ester product is trapped and the butanol, immiscible with the ester flows back into the reactor. Removing water in the course of these esterfications shifts the chemical equilibrium in favor of ester formation.
