In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: Of the 82 patients studied after 1 year , 23 had primary and 59 recurrent disease .
### Output Text:
Of the 82 patients studied after 1 year , 23 had primary and 59 recurrent disease . 