Concentrating sulfuric acid (chiller test)

2010-03-06 14:34 by Ian

In my area, sulfuric acid can be purchased at automotive supply stores as battery electrolyte. This acid is useful as it stands, but higher concentrations are frequently required. This is the operation that was used to test the thermal load capacity of the coolant chiller.

The chiller was damaged and was upgraded and rebuilt. Details are on that page. This is the first real-world test of the chiller while under a moderate thermal load.

Here is what you get from the store:

The setup is that of a simple distillation. Acid was loaded into the boiling flask and distillation was started. The chiller was run at 6V @ 7.8A (about 47W). This is about 11.7% of the TEC’s capacity.

The solution reached its BP and distillate (primarily water) began to collect in the recovery flask. Equilibrium was reached with these values (from my notebook):
Plate temperature: 295C
Vapor temperature: 96C
Coolant temperature: 22C

So the TEC was able to maintain a delta-T of 74C at 11.7% of full working-power. Assuming that the thermal load came primarily from the condenser jacket, we could say a few things about the rate of the distillation. I seem to remember in the datasheet for the TEC that the maximum power consumption was 400W, but that the useful work done by the element was about 275W. The efficiency curve for the TEC is nonlinear in V, but treating it as such suggests that about 30W of work was being done inside the jacket.

These are my notes from the operation:
Input acid is about 35% sulfuric acid by weight.

20:12: 160mL of water was removed from the collection flask.
Additional acid was added to the boiling flask.

21:52: 200mL of water was removed from the collection flask.
Additional acid was added to the boiling flask.

01:40: 485mL of water was removed from the collection flask.
Additional acid was added to the boiling flask. Total volume of acid distilled is now (0.94)*2L.

03:03: 140mL of water was removed from the collection flask.

04:02: 95mL of water was removed from the collection flask.
Additional acid was added to the boiling flask.
Vapor temperature has been holding a range of 96-98C.
Total starting volume: 1.88L
Total water removed to this point: 1085mL
Remaining volume: 795mL

5:55: 160mL of water was removed from the collection flask.
The last of the initial acid was added. The total input volume is now 2.82L.

08:42: 270mL of water was removed from the collection flask.

11:34: 145mL of water was removed from the collection flask.

14:43: 150mL of water was removed from the collection flask.
15:49: 50mL of water was removed from the collection flask.

The concentrated acid was emptied from the boiling flask and allowed to cool to room temperature. A 10mL sample was taken in a volumetric flask and the density of the product measured.

Mass of dry 10mL volumetric flask: 9.33g
25.752g for 10mL of acid in flask
16.422g for 10mL of acid
Density of the product is 1.642g per mL.

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