portion is expanded to provide the refrigeration for liquid production as discussed hereinafter the first exhaust stream 18 becomes a feed stream 19 having oxygen from the exhaust stream 18 the feed stream 19 for the autoclave 20 is mixed with a recycled waste gas stream 28 to achieve a combined feed stream oxygen purity of around 21 the mixing of streams 19 28 preferably occurs external to the autoclave 20 at a pre mixer apparatus 23 as shown in fig2 so that a single feed stream 25 enters the autoclave 20 alternatively the streams 19 28 can be introduced separately as in fig1 for mixing within the autoclave 20 this mixing as would result from either of the embodiments of fig1 and 2 maintains the same operating conditions in the autoclave 20 as occurs with the original air fed operation the recycled stream 28 has an oxygen content of about 10 while the feed stream 19 has an oxygen purity of around 99 5 with argon being its main impurity high purity oxygen is used to minimize the buildup of inerts in the system 10 a lower oxygen purity can be used such as 95 but this would result in a higher loss at a purge stream 30 or 30 a of the system 10 the combined streams recycle waste 28 plus feed 19 are provided to the autoclave 20 as in either of fig1 or 2 where a significant portion of the contained oxygen is consumed in the autoclave 20 a waste gas stream 32 u201c waste stream u201d exits the autoclave 20 at about.