Disclosed is an apparatus for heating a viscuous liquid slurry by direct steam injection in small scale installations. A process tube, through which the slurry flows, is surrounded by a pressure vessel. The pressure vessel acts as a high pressure steam boiler tank, heating the process tube. The process tube, having a length to accommodate mixing of the slurry within, act as a reaction chamber, into which steam is injected via a direct steam injection assembly. The integration of the injector, reactor, and boiler avoids the need for an external boiler and associated plumbing.