Great finally you come by; To same extent I am familiar with Phosgen .I agree with the so called standard method. But it occur to me start some other way ! this is a forum and we can kick in other ideas.
So. Ok, alkaline have an electrolytic alkaline cell with NaCl .. At the Anode side I will buble in CO2. Because os alkalinity CO2 will be converted into CO3-- + 2Na + at the same time we are going to produce at the Anode Cl2, NaOCOONa -- + 2 Cl2 ---- 2NaClO + CO , CO + Cl2 ---- ClCOCl
Chlorine comes from electrolysis of NaCl.
Now old timer , fire up your criticism
Nothing in mind but exercise .