Why not simply add a field to the document library called "Approver" and make it a lookup to the list of approvers you have.  User can select the approver and then start the workflow.  You can either make the field required, or put a check in the workflow to validate the field is populated. 

You could also create a SharePoint security group and add your approvers to this. Then configure the field to select only from this group. As ‌ states, make the field required and you should be fine.

Hi Brendan

Thanks for the reply.  Good idea!  One issue.  In my List I have 3 columns; Approver, Approver Type, and Approver Lookup.  The Approver Lookup column is a person or group column that is a Name (with presence).  When I try to select this as a lookup column from the document library this column is not listed. Only the Approver column is listed.  This Approver column is just a Single Line of Text column (eg. "Mike Smith").  That's probably not good enough to pass to the workflow as a valid approver?

Please advise

Andy

No, you're correct.  You would need to use a "Query List" and query you approver list for for when the Approver equals Mike Smith, then return the Approver Lookup field for that entry.  Then feed that into your approval task. 

Or go with ‌ solution and use a SharePoint group. 

Hi Andy,

Agree with this. Using a SharePoint group and adding a field of type person or group to your list that only allows selections from within that SharePoint group should be the best and easiest way to achieve your goal.

This way your users will have the same user experience when working with people picker fields rather than have a drop down here and a people picker there to select people.

Also you will have values of type "person" in your workflow which makes it easier for you to add them as assignee in further tasks or notifications you might use in your workflow.

Kind regards,

Enrico