Technology - HEROS
March 5, 2003

Introduction

Lurgi’s heat recovery system HEROS, recovers waste heat from the acid circulating system and produces low pressure steam for process use or power generation.  The key element is a venturi absorber located upstream of a conventional interpass absorption tower.  HEROS can be easily shutdown allowing the interpass absorber to operate as usual.  This design feature allows HEROS to be easy installed into an existing sulphuric acid plant.

Another key part of the system is the material of construction for the venturi absorber and the sulphuric acid cooler/steam generator.  Numerous materials of construction were tested in the laboratory and in an operating acid plant.  The material was required to handled sulphuric acid in the concentration range of 95 to 100% H2SO4 at temperatures as high as 190°C (374°F).   The material selected from all the test work is 1.4575 also known as Cronifer 2803 Mo produced by VDM.

The material (1.4575) was first used as the material of construction for a venturi absorber installed in 1988.  Two heat exchangers were also fabricated in 1.4575.  The aim was to test the material in high temperature (120-160°C) concentrated acid.  The results proved satisfactory with corrosion rates practically immeasurable.

The first HEROS was installed in 1988 as part of a retrofit of an existing interpass absorber acid system.  The existing venturi absorber system was adapted for the higher operating temperatures and steam generator system was added.  The system operates with acid entering the venturi absorber at 172°C (342°F) and exits the venturi at 195°C (383°F).  The hot acid is circulated through the steam generator by a vertical pump.  Steam at 5 bar (73 psi) is produced for use on site.

The steam generator is a kettle type boiler with hot acid flowing through the tubes and low pressure steam generated on the shell side.  Material 1.4575 is used for the tubes and tubesheet which is flange mounted to the shell.

Instrumentation and control for HEROS consists of the following:

• Acid concentration

• Steam pressure

• Acid pump tank level

• Corrosion rate monitoring (continuous)

• Leak monitoring

• Boiler water conductivity and pH

The advantages of HEROS are:

• Production of low pressure steam (up to 10 bar)

• A material of construction that can tolerate a range of acid concentrations (95 to 100%) at high temperature with low corrosion rates

• HEROS can be shutdown and the plant would continue to operate with the conventional interpass absorber tower

• Acid concentration control is conventional

• HEROS can be easily installed into an existing plant

As of 1998, four HEROS have been installed ranging in size from 4 to 116 t/h of low pressure steam production.