Strong Acid System - Acid Cooling 
June 22, 2003

Introduction

The absorption of water and sulphur trioxide into sulphuric acid is an exothemic reaction resulting in an increase in the acid temperature as it exits the tower.  This heat must be removed before the acid is returned to the tower.   The function of the acid cooler is remove this heat from the acid stream and to reject it to the environment or recover the heat as useful energy.

In early acid plant designs this energy was considered low grade heat and the energy was simply rejected to the environment usually via a cooling water system.  Early acid cooling systems consisted of cast iron 'serpentine' coolers, so-called because of their shape.  They were generally cooled with cooling water being sprayed on the outside with the acid flowing on the inside. 

These coolers had several disadvantages, such as:  

• Installations on large plants required large plot areas
• The water spray often caused a low level plume or fog in the area
• Corrosion of the coolers results in leaks
• The concrete foundations deteriorated
• Water side fouling reduced heat transfer
• High potential for leaks due to the many gaskets
• Acid leaks were sometimes catastrophic exposing personnel to hazards

Some cast iron cooler installations still exist in a few acid plants.

In the mid 1960's CIL (Canada) began the search for a better alternative.  After many years of development work and testing, the anodically protected shell and tube stainless steel acid cooler was introduced to the industry.   The shell and tube units were more compact than the cast iron coolers and the all welded construction virtually eliminated the potential for leaks.  The key to the success of the shell and tube acid cooler is the use of anodic protection.

Anodically protected stainless steel air-cooled heat exchangers were also designed and built but were of limited success.  Anodic protection allowed higher operating temperatures of 120°C (250°F) instead of the lower 85°C (185°F) maximum temperature of conventional air coolers.  The higher operating temperatures resulted in smaller surface areas, reduction in the number of fans, lower operating horsepower and higher operating efficiencies.  The problem with the design is the difficulty in anodically protecting the tube side of a heat exchanger.  For the system to protect the material the current must be able to reach down the entire length of the tubes and this is very difficult to achieve on the tube side of an exchanger.

Plate heat exchangers are a popular alternative to shell and tube acid coolers because of their compact size and lower costs.  They offer very high heat transfer coefficients compares to shell and tube acid coolers.  Materials of construction such as alloy C-276 allow the units to operate without anodic protection.  The price advantage is still maintained even though more corrosion resistant and expensive materials are used compared to 316L SS because the plates are very thin.  Anodic protection of plate heat exchanger using 316L SS plates was attempted but ran into the same problems as the attempt to commercialize anodically protected air coolers.  Alloy C-276 plate heat exchangers were limited to a maximum inlet temperatues of 90°C (194°F) for corrosion reasons.  The introduction of Hastelly D-205 and Cronifer 2803 Mo as a plate material allows acid temperatures to go beyond the 90°C (194°F) restriction.

Spiral heat exchangers are another option for acid cooling.   They offer many of the same advantages as plate heat exchangers.

A seldom used cooling option are PTFE tank coils.  There unique design do not lend itself to the typical acid plant and are used in special cases where other exchangers are not suitable.

A fairly recent advancement are shell and tube coolers constructed entirely of high silicon stainless steels such as Sandvik SX.  Using this material eliminates the need for anodic protection.  Acid is no longer restricted to the shell side of the exchanger as is the requirement for anodically protected acid coolers.