Removing non existant users from NTFS folder permissions

I have NTFS folders showing members by their SID. Looks like these members are no longer employed and have been deleted, but the SID still shows. I want to script the removal of these. What I have currently works only if I use the username and domain, but I need to remove based on SID. Can this script be changed to accomplish this? Have any ideas how I should go about doing this?

Code obtained from Microsoft Scripting guys:

$user = 'domain\user'
$folders = "C:\test" 
$acls = Get-Acl -path $folders 
$outputObject = @() 

Foreach($acl in $acls) 
{ 
 $folder = (convert-path $acl.pspath) 
 Write-Progress -act "Getting Security" -status "checking $folder" -percent ($i/ $folders.count*100) 
  Foreach($access in $acl.access) 
  { 
    Foreach($value in $access.identityReference.Value) 
     { 
       if ($value -eq $user) 
          { 
           $acl.RemoveAccessRule($access) | Out-Null 
          } 
     } #end foreach value 
  } # end foreach access 
 Set-Acl -path $folder -aclObject $acl 
$i++ 
} #end Foreach acl

Please mark my post as helpful or the answer or better yet.... both! :) Thanks!