A purge ratio is the ratio of the volume of hydrogen in the purged gas to the volume of hydrogen in the makeup gas. Purging is required to prevent the buildup of inert gases and light hydrocarbons in the recycle gas. The quantity of purge directly influences the purity of the hydrogen in the recycle gas. For low-sulfur feeds such as naphtha, the purge ratio required is small. For heavy, high-sulfur feeds the purge ratio required to maintain the purity of recycled hydrogen is quite high. Typical purge values used are shown in Table 2-1.
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