Why is nicotinoyl chloride available as its hydrochloride salt? How can we separate them?
Unfortunately, i don't have access to the original reference from 1928 Über die Einwirkung von Thionylchlorid auf die Pyridin-monocarbonsäuren by Meyer and Graf, but it's cited in The Preparation of Nicotinyl Chloride from 1953.

In this later note, where dried and grounded potassium nicotinate was reacted with oxalyl chloride in anhydrous benzene and the resulting nicotinyl chloride eventually destilled off under reduced pressure (75-90 °C, 10-12 mm), the authors state: 


  On exposure to the air, the oily acyl chloride soon solidified to nicotinic acid hydrochloride.


With other words, the combination of moisture and the basicity of the pyridine moiety results in the decomposition of the product. This is apparently prevented by deactivation of the pyridine upon conversion to its hydrochloride.