You can't concentrate muratic (hydrochloric) acid by boiling it down.
Gases (such as HCl, which dissolved in water is what makes muriatic acid) are less soluble in solution when heated (the opposite of solids), which means that the gas (HCl) will be driven off resulting in a weaker acid.
Battery acid (sulfuric acid) is made of H2SO4 (mixed with water), which is a liquid with a high boiling point when pure, can be concentrated in this manner because the water will boil away before (at a lower temperature) than the H2SO4.
What you were thinking of, notfman, was distilling weak HCl acid and collecting the distillate with cold water in the condensor, because the vapor that comes off when heated is going to be higher in HCl concentration than the original acid at the beginning of the distillation.
The reason why you cannot concentrate 20% HCL by boiling it down is b/c it forms a maximum boiling point azeotrope that is 20%HCl. This means that if you had 10% hcl, and boiled it, it would concentrate in the flask until 20% HCL is obtained.. So, WATER will distill first. SWIM knows it sounds funny that water will be removed from a weaker HCl solution when distilled before the gas.
If you reduce the pressure, the concentration of HCl in the azeotrope will increase-- but not much.. at 50mm, the azeotrope is 23%HcL.
Also, about the boiling of battery acid--- Are you sure that water can be removed this way??? SWIM was under the impression that the max-boiling azeotrope distills at 300-something leaving the hydronium/HSO4 ions behind in the flask...
xoxo
Julia