Abstract: A compact sulfur recovery system is disclosed which includes an upflow orientation for the gases through a primary structure including a catalytic partial oxidation reaction zone, a first temperature-control zone, a first Claus catalytic reaction zone, a second temperature-control zone, a first liquid sulfur outlet, and a first effluent gas outlet. The upward flow of the gases puts the hottest gases in contact with the tubes and tube sheet in the waste heat boiler where there is greater confidence in having liquid water in most continuous therewith. In some embodiments, a secondary structure follows the primary structure and comprises a second Claus catalytic reaction zone, a third temperature-control zone, a second liquid sulfur outlet, and a second effluent gas outlet. One or more components of the system employ heat transfer enhancement material in the temperature-control zones, and one or more components deter accumulation of liquid sulfur in the Claus catalytic reaction zones.