Strong Acid System - Acid Cooling - Plate Heat Exchangers
February 4, 2003

Introduction Plates Gaskets Process Limitations Temperature Control Leakage Control Extending the Life of Plates and Gaskets

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Introduction

The original idea for a plate and frame heat exchanger was patented in the latter half of the 1800’s but the first commercially successful design was introduced in 1923.   In the 1930’s plates pressed in thin gauge stainless steel were introduced.

A plate and frame heat exchanger consists of a frame in which closely spaced metal plates are clamped between a head and follower plate.  Fluid enters and leaves the plate pack through ports located in the corner of the plates.  Gaskets are located around the ports and the plate edges which prevent the mixing of fluids and the escape of liquid out of the plate pack.

Plates

Heat exchanger plates can be pressed out of any material that is ductile enough to be formed into a pressing.  Plate materials are typically 0.6 to 0.7 mm thick. 

It is very important that the material selected for a particular application be highly corrosion resistant to the process fluid being handled.  A corrosion rate that is acceptable for a vessel which has relatively thick wall would not be acceptable for a plate heat exchanger simply because the plate thickness is so much smaller.

Typical materials used in sulphuric acid plant applications are:

Gaskets

The selection of the proper gasket material must take into account the fluids being handled, operating temperature, and the sealing properties of the material itself.  Typical materials used in sulphuric acid plant applications are:

        EPDM               Weak acid cooling applications and water side

        Viton                 Weak/strong acid cooling applications

The plate heat exchanger should never be opened unless absolutely necessary as damage to the gasket may occur.  

After a few years of operation, the gasket at the hottest points may slowly lose their elasticity and the glue will no longer retain its full bonding strength.  This can lead to a section of the gasket loosening from the plate when the unit is open.  In some cases the gasket can be simply be glued back in place but in more severe cases the entire gasket may need to be replaced.

The normal life of Viton gaskets is about 5 to 6 years in normal service for strong acid cooling.   As a general rule of thumb, the life of the gasket will be reduced by half for every 10°C above 90°C that the unit operates at for prolonged periods of time. 

Process Limitations

Plate heat exchangers constructed of alloy C-276 plates and Viton gaskets impose certain limitations on the process in order to achieve reliable and extended operation of the heat exchangers.  The maximum allowable temperature into a plate heat exchanger is a function of acid concentration.  Exceeding the maximum temperature will result in higher corrosion rates and reduced life of the plate pack.

Concentration                 Maximum Temperature                   Expected
                                                                                    Corrosion Rate
        98%                             90°C (200°F)                     < 0.08 mm/year
        92%                             70°C (160°F)                                -
        70%                             60°C (120°F)                                 -

In 1992, Alfa Laval introduced Hastelloy® D-205™ alloy as a plate heat exchanger material for handling concentrated sulphuric acid up to 130°C (266°F).   If offers a longer lifetime, reduced maintenance costs and increased operational security.  D-205 is a nickel-based alloy with a high silica, chromium and copper content.  The silica and chromium form an oxide layer which povides outstanding corrosion resistance.  The high nickel and copper content ensures excellent corrosion resistance even when the acid concentration temporarily drops.  This feature, which is unique to D-205, make it superior to stainless steels containing silica. 

Alloy D-205 is most suitable for fairly good quality water with a moderately low chloride content.  A maximum chloride level of 100 ppm is recommended.

A relatively new material capable of handling the higher acid temperatures is Alloy 33 which compete directly with D-205.  References where this material has been used in sulphuric acid applications are pending.

The acid concentration should never be allowed to exceed 100% H2SO4 as the free SO3 will attack the gasket material causing it to swell.

The maximum permitted acid velocity for 98% H2SO4 is 3 m/s (9.8 ft/s) due to the risk of increased erosion and corrosion.

The cooling water outlet temperature should not exceed 40°C (104°F).  Carbonate fouling increases significantly at temperatures above 40°C.

Temperature Control

Bypassing fluid on the process side of the heat exchanger should always be used to control the process temperature.  Water flow should never be throttled or reduce as increased fouling will occur on the cooling water side.

The inability to maintain the required process temperature will generally indicate that the cooling water side of the heat exchanger is fouled.  This condition occurs when the acid side bypass is fully closed.

The temperature difference between the cooling water inlet and outlet will indicate the thermal performance of the unit since the cooling water flow through the exchanger should be constant.  Thus, higher cooling water outlet temperature will indicate a higher heat load on the unit.

Leakage Control

Every effort should be made to ensure that the process side operating pressure is higher than the cooling water side of the exchanger.  If a leak occurs in the unit, acid will leak into the cooling water and can be readily detected whereas a leak of cooling water into the acid is more difficult to detect.

An instrument measuring pH or conductivity of the cooling water leaving the exchanger is the simplest and method of detecting a leak.  If a leak is suspected the heat exchanger should be taken out of service, inspected and the necessary repairs performed.

Gasket leaks will always appear underneath the heat exchanger.  Tightening the plate pack by a few millimetres can eliminate some leaks.  If this does not work the unit will need to be taken out of service and the gaskets inspected. 

A plate heat exchanger should always be equipped with a plate pack cover which will prevent external leaks from spraying out and direct the liquid down.  A drip tray is often provided underneath the exchanger to collect any liquid and direct it to a safe location.  This is particularly necessary if the heat exchangers are located above grade on a platform.

Extending the Life of Plates and Gaskets

The most severe conditions in a plate heat exchanger are at the acid inlet.  Corrosion rates cane expected to be highest in this region.  The temperature will decrease as the acid is cooled in the exchanger so the corrosion rate will be less at the acid outlet.  The symmetry of the heat exchanger plates allows for the plates to be rotated so that the acid inlet end becomes the acid outlet and vice versa.  The effect of doing this is to extend the life of the unit by exposing the less corroded end of the plate to the more aggressive conditions.

The exact method of reversing the plates will depend on the geometry of the plates and their arrangement in the frame.  Consult the manufacturer’s instructions for details.