Absorption and Stripping

Introduction

In the chemical industry, it is necessary to selectively remove a constituent (e.g. SO3) from a gas mixture by dissolving it in a liquid (e.g. H2SO4), or to free a liquid (e.g. weak H2SO4) of dissolved (e.g. SO2) gases by contact with an inert gas (e.g. Air).  These unit operations of chemical engineering are known as absorption and stripping, respectively.

Driving forces and processes are in opposite direction in the two cases, but exactly the same forces are present.

Equilibrium Factors

In a particular system, for a given concentration of a constituent in the liquid, there is an equilibrium concentration in the vapor above it.

If the actual vapor concentration is higher than the equilibrium concentration, absorption occurs. If the actual vapor concentration is lower than the equilibrium concentration, stripping occurs. At equilibrium, there is no mass transfer.

The driving force is the difference between the actual concentration and the equilibrium concentration. The rate of mass transfer is directly proportional to the distance from the equilibrium.

The solubility of gases in liquid increases at higher pressure and lower temperature, hence the driving forces for absorption increase under these conditions and the driving forces for stripping decrease.

Absorption

Absorption is a mass transfer process that applies to the removal of solute gases in the presence of inerts (i.e. oxygen and nitrogen). In a sulphuric acid plant, the primary absorption process is the absorption of sulphur trioxide (SO3) into concentrated sulphuric acid (H2SO4).

The absorption process can be represented by the two-film theory which states that the overall mass transfer coefficient is related to the mass transfer coefficients in the gas and liquid phase.

In the absorption of SO3 into H2SO4, the process is said to be gas-phase controlled. This means that the rate of absorption of SO3 is more a function of the gas properties than the properties of the liquid. Once the SO3 reaches the liquid, the rate at which SO3 passes through the liquid and reacts to form H2SO4 is so fast that the process is limit by what is happening in the gas.

Stripping

Stripping is the opposite of absorption and involves the removal of dissolved gases in the liquid by an inert gas. In a sulphuric acid plant, the primary stripping process is the removal of dissolved SO2 from weak and concentrated sulphuric acid streams.

In these stripping operations, the process is liquid phase controlled rather than gas-phase controlled as is the case with the absorption of SO3 into H2SO4.

Drying Systems

Introduction

The Drying Acid System absorbs the water vapor remaining in the gas after it leaves the Gas Cleaning Section of the plant.  The primary reason for drying the gas is to avoid corrosion caused by wet SO2 gas before the converter and by wet SO3 after conversion and to avoid acid condensation during shut-downs and thus protect the catalyst from degradation.   Drying the gas will also help to maintain a clear stack by avoiding the formation of excessive amounts of sub-micron mist particles that may over load the mist eliminators in the Final Tower.

A typical Drying Acid System consists of the following items of equipment:

a)        Drying Tower

A typical Drying Tower is a vertical cyclindrical vessel designed to contact process gas and strong sulphuric acid (93 to 98.5% H2SO4) for the purpose of drying the gas.  The tower may be constructed of specialty alloys such as the high silicon stainless steels, nickel-chromium alloys or the more traditional brick lined carbon steel.  The tower will be equipped with a packing support, packing, acid distributor, and mist eliminator.

b)        Acid Pump Tank

A pump tank is required to hold the acid that is circulated in the drying acid system.   The pump tank may be constructed of specialty alloys or the more traditional brick lined carbon steel.  The pump tank holds sufficient acid to enable the circulating pumps to operate and provides sufficient room to hold the acid that drains down out of the system when the circulation is shutdown.  In some designs the pump tank is a standalone vessel or can be partially or fully integrated with the Drying Tower as is the case in a pump boot arrangement.

c)       Acid Pump

An acid pump is required to circulate the acid from the pump tank up to the distributor inside the Drying Tower.  The acid flows by gravity over the packing and drain out the bottom or side of the Drying Tower back to the pump tank.   The pump can be a vertical submerged centrifugal pump in which case it would be mounted in the pump tank.  External vertical centrifugal pumps are also available as well as horizontal centrifugal pump types, although the latter is uncommon.

d)        Acid Cooler

The absorption of water into sulphuric acid is an exothermic reaction and will cause the temperature of the acid to rise unless the heat of absorption is removed.  The heat is typically removed in either a plate heat exchanger or anodically protected shell and tube heat exchanger.

e)       Piping

Acid piping is required to carry the acid from one piece of equipment to the next.  Materials vary considerably depending on the acid concentration, temperature and cost.

f)        Instrumentation and Controls

Instrumentation and controls are required to monitor the operation of the system and control its operating parameters.   Acid concentration, flow and temperature are the most important factors in ensuring the system is performing its function of gas drying as efficiently as possible

Pump Tanks

Introduction

Pump tanks in a strong acid circulating system provide the following functions:

reservoir to hold sufficient acid from which to fill the system on initial start up reservoir to contain the run back of acid when the system is shutdown reservoir from which the acid circulation pumps draw a convenient location to add dilution water and cross flows from other circulating systems

To adequately perform the above functions the pump tank must be properly sized and designed.  The most common types of pump tanks are vertical cylindrical and horizontal cylindrical tanks.  Materials of construction are the same as for the towers; brick lined or stainless steel.Pump boots are closed coupled to the tower such that the bottom of the tower and the pump boot provide the same functions as a standalone pump tank.SizingProperly sized pump tanks will ensure trouble free operation of the acid circulating system.  The following information is required to size a pump tank:

Type and size of pump Run back volume of system

When vertical submerged pumps are use, the length of the pump will generally set the height for vertical cylindrical tanks or the diameter of horizontal cylindrical tanks.  Pump manufacturers generally offer standard pump lengths but custom lengths are also available.  The diameter or height of the tank must take into consideration that the pumps will require a minimum undersuction which is the distance from the pump suction to the floor of the tank.  If the minimum undersuction

is no provided the pump may not operate properly.  As well, the height of the pump mounting nozzle must also be considered when determining the height or diameter of the tank.  Once the height or diameter of the tank is set the other major dimension must be determined.  For vertical tanks the diameter is calculated and for horizontal tanks it is the length that must be determined.  The diameter or length is calculated by setting the operating levels in the tank, knowing the required runback volume and setting the freeboard or ullage at the top of the tank.  Vertical submerged pumps require a minimum liquid level above the pump suction to prevent vortexing and drawing air into the pump suction.  This requirement sets the minimum operating level of the tank.  The normal operating level is typically set 150 mm (6") above the minimum level.  The high level is typically set 150 mm (6") above the normal operating level.  These levels provide sufficient flexibility in operation such that the level in the tank can vary but alarms and nterlocks will not be activated all the time.There must be sufficient volume in the tank between the high operating level and the freeboard to accomodate the runback volume of the system.   Setting the operating levels in the tank fixes the height available for runback.   In a vertical tank, the diameter is set so the height of the cylinder equals the volume of the runback.  In a horizontal tank, the length is set so that the runback can be accomodated in the required height.

Level Measurement

There are many options available for level measurement in an acid pump tank. The choice will depend on several factors such as cost, reliability, accuracy, etc.

Bubbler

Bubbler type instruments are used in many existing plants and are still specified for new plant applications. The instrument consists of a bubbler tube which is inserted into the tank and extends nearly to the bottom of the tank. Instrument air enters the top of the tube and bubbles out the bottom of the tube. A certain air pressure will be required to overcome the liquid head in the tube to allow air to bubble out the bottom of the tube. The air pressure required is measured by a pressure transmitter and the pressure reading can be converted to a level measurement. For the instrument to work properly, there must be a constant flow of air or bubbles out the bottom of the tube, hence the name bubbler.

The ‘high side’ of the pressure transmitter measures the pressure in the bubbler tube. The transmitters ‘low side’ is piped to the vapour space of the pump tank. This compensates for the fact that the pump tank may be operating under a negative or positive pressure depending on the design of the plant.

Bubblers are generally quite reliable but problems may be encountered with the operation of the instrument.

The instrument will stop working on loss of instrument air supply The bubbler tube must be corrosion resistant. Any holes in the bubbler tube will cause a false level reading. Fluctuations in the pump tank vapour space pressure may cause false level readings. Air flow to the bubbler must be constantly checked to ensure sufficient air is being delivered to the bubbler tube.

Capacitance

The capacitance level transmitter measures the dielectric constant values which are present in all materials to determine exact changes in liquid level. The probe is made up of two electrode plates and a guard wire which are completely encapsulated in PTFE. When liquid rises or falls against the probe, the dielectric material in the medium bridges the signal across the active and ground plates to complete the circuit. The change in capacitance value is converted by the processing electronics into a proportional 4 to 20 mA signal output. Capacitance level measurement works best with conductive liquids greater than 100 µS and/or non-conductive liquid greater than 20 dielectric constant units.

Radar

Use of radar type instruments in acid pump tanks for level measurement is relatively new. The instrument is mounted in a nozzle in the top of the tank. A radar signal is emitted from the instrument, travels to the surface of the liquid and is reflected back to the instrument. The time it takes for the signal to travel the distance from the instrument to the liquid surface and back again is converted to a level reading.

One major advantage of this type of instrument is that no part of the instrument contacts the liquid. The instrument should operate trouble free as long as the part of the instrument that is exposed to the vapour space is corrosion resistant.

Actual experience with this type of instrument shows that it is operates reliably most of the time. However, there are times when the instrument reading would fluctuate wildly for no apparent reason. The result would be that the level control valve would sometimes open fully causing the pump tank level to decrease resulting in a low level and loss of pump flow and flow over the tower. These wild fluctuations would sometimes occur when the plant was shutdown and only acid flow over the towers was being maintained. 

It was surmised that with the plant shutdown, the pump tanks were no longer being vented back to the inlet of the drying tower and the accumulation of acid mist/SO3 in the pump tank vapour space was causing interference with the instrument.

Acid Distributors

Introduction

The primary function of an acid distributor is to uniformly distribute acid across the packing.  Failure to do this will affect the performance of the tower.  The characteristics of a good acid distributor are:

Uniform acid distribution (> 21.5 points/m² or > 2 points/ft²) Low tendency to plug or foul Minimal splashing or droplet formation Does not constrict the flow of gas at the top of the tower Ease of installation and removal Corrosion resistant

There are many different designs of distributors that have been used over the years and by different plant designers.  All distributors generally fall into two categories: Trough Type Distributors Pipe Type Distributors

Pan type distributors have been used in the past for smaller towers but are never used in new installations.Mist Eliminators

Introduction

Within an acid plant there is the necessity to remove acid mist and droplets from the gas stream exiting the drying and absorption towers.  The primary reasons for trying to capture, collect and remove the mist and droplets are:

To prevent damage to downstream equipment To avoid undesirable atmospheric emissions To recover valuable acid from the gas stream

Acid mist and droplets are formed in one of three ways; Mechanical, Condensation or Chemical Reaction.Mechanically formed droplets usually range in size from 10 to 100 microns.  They generally form when acid is re-entrained due to localized high velocity gas tearing droplets away from a liquid film or from the splashing or spray generated from a liquid distribution device.Mist or fume are much smaller in size (3 microns or less) and are generated from the sudden or shock cooling of hot gas containing sulphur trioxide.   Chemical reaction between sulphur trioxide and water will also produce a mist or fume.  This generally occurs at the inlet of the absorber towers.Fibre filters have proven to be an effective device for capture, collection and removal of mist and droplets from gases.  In sulphuric acid applications, fibre bed mist eliminators are used to remove acid mist and entrained droplets from the process gas exiting the drying and absorbing towers.  Proper and effective removal of the acid mist and droplets will extend the life of the downstream equipment and prevent undesirable atmospheric emissions.Acid MistThe size and quantity of mist generated will depend on the type of plant, gas source, operating parameters, acid strength, type of distributor, etc.  In a sulphur burning plant the quality of sulphur affect the amount of mist generated.  'Dark' sulphur will produce considerably more mist than 'bright' sulphur due to the amount of hydrocarbon/water present in the sulphur.  The following table summarizes the typical mist loads and particle size for various types of plant and duties.

Mist Load Particle SizeComments

Drying Tower

Sulphur Burning500 mg/m³ + 3 m, relatively large

Spent Acid, Metallurgical175 to 3,530 mg/m³ 0.6 to 10 m, 1.0 m mean

Intermediate Absorber

No Oleum, Bright Sulphur500 to 1,766 mg/m³ 1 to 2 m, fine to moderate

No Oleum, Dark Sulphur3,000 mg/m³ fine to moderate

Metallurgical500 mg/m³ fine to moderate

Oleum2,000 mg/m³ fine to moderate

stronger oleum produces smaller particles

Final Absorber

No Oleum 500 mg/m³ moderate

Oleum 2,000 mg/m³ 1 mstronger oleum produces smaller particles

Spent Acid 3,000 mg/m³ 2 m average

Absorber (Single Absorption)

No Oleum500 to 700 mg/m³ 1 to 2 m, moderate

Oleum2,000 mg/m³ 0.6 to 1 m

stronger oleum produces smaller particles

Crossflow Stripper500 mg/m³ high granulometry

Product Stripper 500 mg/m³ high granulometry

Acid Concentrator10,000 mg/m³

Wet Process35,300 to 100,000

mg/m³2 m average

Collection MechanismsMist or droplets are collected in four different ways: 

1. Inertial Impaction 2. Direct Interception 3. Brownian Movement/Diffusion 4. Induced Electrostatic Forces

The first three collection mechanisms will occur to varying degrees in all fibre bed mist eliminators. 

Inertial Impaction

Large droplets (3 microns or larger) are collected when their momentum prevents them from following the gas streamlines around a fibre.  The momentum of the droplet causes it to leave the streamline and strike the fibre and become collected.  Since momentum is the product of mass and velocity, it follows that large droplets will be collected more efficiently than small droplets travelling at the same velocity.

Interception

Interception of a droplet occurs when the size of the particle allows it to follow the gas streamline around an object in its path.  As the particle follows the gas streamline around the object it may come sufficiently close to the object such that it will touch the object and become collected.  Interception as a collection mechanism is less important than inertial impaction.

Brownian Diffusion

Extremely small acid particles or mist are so small that they do not follow the gas streamlines but exhibit a random path as they collide with gas molecules.  These submicron particles will be collected when they collide or touch an object. 

Types of Mist Eliminators

There are generally two types of mist eliminators: Impaction and Brownian Diffusion Types so-called because of the primary collection mechanism employed in their design.  Impaction type mist eliminators employ impaction and to a lesser extend interception methods to capture, collect and remove acid mist.  As such, impaction devices are effective for the the larger particles and are less efficient for the smaller submicron particles.  To collect the submicron particles, a Brownian diffusion device must be used.

Mesh Pads

Mesh pads operate primarily on inertia impaction and are efficient in removing particles 5 microns or larger.  Mesh pads are size for relatively high gas velocities which relies on the fact that the size of the particle that can follow the gas streamline decreases as gas velocity increases.  However, at higher gas velocities, the possibility of droplet re-entrainment occurs.  At lower gas velocities, the momentum of the particle decreases which decreases the collection efficiency.  The effective operating of a typical mesh pad is approximately 30 to 110% of design gas flow.

Mesh pads are usually made of woven metal wire that is crimped and formed into a flat pad and fitted into the tower.  The mesh is held together by a grid place above and below the pad. 

To enhance the collection efficiency of a mesh pad, glass or PTFE fibres can be co-knitted with the wire to form a composite pad.  The smaller diameter glass or PTFE fibres increase the number or targets in the pad without the need to make the pad denser which increases pressure drop.

Impaction Candles

Impaction candles utilize inertial impaction as the primary means of particle collection but they offer improved collection of particles in the 1 to 3 micron range that mesh pads are only capable of removing to a small degree.

Impaction candles are made of glass fibres hand packed in between two metal cages or machine wound onto the inner cage with an outer cage added on top.  The candles are installed in either the hanging or standing position.

Normal bed velocities are in the range of 1.27 to 1.63 m/s (250 to 320 ft/min).  Since inertial impaction is the primary collection mechanism, the turndown capability is limited to 75% of the design value.

Brownian Diffusuion Candles

All three collection mechanisms are employed in Brownian diffusion candles but Brownian diffusion is the mechanism which allows these elements to achieve the high collection efficiencies. 

Impaction candles are made of glass fibres hand packed in between two metal cages or machine wound onto the inner cage with an outer cage added on top.  The candles are installed in either the hanging or standing position.

Design bed velocities are very low and range from 0.025 to 0.2 m/s (5 to 40 ft/min) depending on the pressure drop and collection efficiency desired.   In contrast to the other type of mist eliminators, the collection efficiency of Brownian diffusion candles increases as the gas velocity decreases.  At lower velocities the residence time of the mist particle increases and the fibre bed so the chance that it will be captured increases.

 Acid Pumps

Introduction

Pumps are used to circulate and transfer acid of all concentrations from one piece of equipment to another.  In a sulphuric acid plant, two types of pumps are used predominately; Vertical Submerged and Horizontal Centrifugal Pumps.

Submerged Vertical Centrifugal Pumps

Submerged vertical centrifugal pumps for strong acid service have become the standard in the industry for several reasons.  The pumps are installed in a closed vessel so any leaks are contained in the vessel and do not pose a hazard to personnel.  An additional feature may be that the shaft seal is not in contact with the acid but simply acts as a gas barrier. Tthis feature eliminates the problem of leaks at the shaft seal that is a problem with horizontal centrifugal pumps.

Chas. S. Lewis & Co., Inc.  

Lewis offers pumps specifically developed for 93 to 99% sulphuric acid and oleum service up to 2000 m³/h (8000 USGPM) capacity and 10 to 50 m (30-165 ft) liquid head. Pumps are available in sizes 2½ through 14 in standard material specifications and lengths to 12 ft. Custom lengths are available on request.

Features of Lewis Pumps are:

LEWMET® nickel-chrome allow used for critical parts which are subject to high velocity, corrosion and abrasion such as impellers, wear rings, journals, and bearings.

Casing and other major parts supplied in Lewis Process Iron with a large corrosion allowance for extended life.

Matching joints incorporate acid proof O-rings and sealed cap-nut bolting to prevent corrosion of vital mechanical components.

One piece Carpenter 20 Cb-3 shafts of premium yield strength, straightened and fitted with PTFE cover for high temperature duty.

 All Lewis Pumps feature a shaft seal that is not exposed to the liquid being pumped. A simple stuffing box is sufficient to seal the rotating shaft in tanks that operate at or near atmospheric pressure. For applications where the pump tank is under pressure or vacuum a deeper stuffing box is available with provision for a lantern gland for use with an external sealing medium such as instrument air.

FRIATEC-Rheinhütte GmbH & Co    Rheinhütte offer vertical submerged centrifugal pumps in capacities up to 1200 m³/h.The features of the Rheinhütte pump are:

Compact double volute casing to balance out radial loads Closed impeller Low flow velocity giving minimal corrosion damage Shaft column separate from discharge pipe Modular construction 22 pump sizes with 5 different shaft diameters and thrust bearing lanterns Materials (1.4136 HRS, Sigu) are available to handle concentrated sulphuric acid (99-99.5%) up to a a

temperature of 260°C (500°F).  All wetted shaft sleeves sealed with O-rings, therefore no crevice corrosion

Ensival-Moret produce a vertical submerged centrifugal pump for strong acid service with capacities up to 1600 m³/h.  The pump differs in design from the other suppliers in that the central column that houses the pump shaft is also used to bring the liquid up and out of the pump.  The features of the Ensival-Moret pump are:

Fully relieved gland packing Multi-stage construction if required to achieved required head Fully symmetrical pump casing which enbles the submerged bearings to operate under zero load Stuffing box protected the bearing in the gaseous pahse against corrosion by acid vapours

Two different arrangements are available which differ in the way the acid exits the central column.  Arrangement 2 is shown in the picture.

  ARRANGEMENT 1 PUMPS

ARRANGEMENT 2 PUMPS

Horizontal Centrifugal Pumps

The selection of pumps for sulphuric acid service should always take into consideration the acid concentration, temperature and service. Horizontal centrifugal pumps are rarely used for circulation duty in a sulphuric acid plants. This duty is reserved for the vertical submerged centrifugal pump.

However, horizontal centrifugal pumps are often used for transfer, loading and unloading of concentrated sulphuric acid. The material of construction for the casing and impeller is usually alloy 20. The shaft seal can be either packing or mechanical seal. If packing is used the operator must be aware that some leakage across the seal is required and that there is the potential for more severe leaks and acid spray if the packing should fail.

Weak acid pumps are generally horizontal centrifugal pumps.  Plastic materials of construction are generally more suited for this service than metallic pumps.  Plastics are resistant to a wide range of concentration and are inexpensive compared to some of the exotic alloys required to handle some acid concentrations.  Rubber lined pumps may also be used in weak acid service depending on the fluid being pumped.

To reduce the possibility of a leak at the pump seals, magnetic-drive pumps of suitable materials of construction can be used.

There are many manufacturers and suppliers of horizontal centrifugal pumps suitable for sulphuric acid service.  The key is the right selection of materials and specification of the right type of seal.

Acid Cooling 

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.

Plate Heat Exchangers

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:

316L SS Weak acid applications

904L SS Weak acid applications

254 SMO Weak acid applications

Hastelloy® G-30 Weak acid applications

Alloy C-276 Weak/strong acid applications

Hastelloy® D-205™ Alloy Strong Acid applications

Alloy 33 Strong acid applications

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.

Piping

Introduction

Piping is used to convey fluid from one location or vessel to another location.  Without piping there would be no process.  The importance of piping is equal to that of any piece of equipment.  Liquid must be conveyed safely, efficiently and economically from one point to another.

Materials used for acid piping have evolved with the design of acid plants over the years and with the introduction of new materials. 

The two most common materials used for the main acid circulating lines are cast/ductile iron and alloy materials.  The main differences, advantages and disadvantages are listed in the following table.

Grey Cast Iron/Ductile Iron Alloy System

High potential for leaks due to large number of flanges Piping is welded with minimum number of flanges thus minimizing the chances of leaks

High corrosion rates, up to 2.0 mm/year (80 mpy) or high in turbulent areas

Low corrosion rates, typically < 0.1 mm/year (4 mpy)

High installation cost due to heavy weight and large number of items

Low weight. Piping can be shop spooled to minimize field installation costs.

High maintenance cost Minimal maintenance required.

Require large inventory of fittings Inventory of spare parts is minimal

Low velocities (i.e. large pipe diameters to avoid excessive corrosion and erosion rates

Higher design velocities are permitted thus reducing line sizes.

Low ductility. Risk of brittle failure especially with cast iron High ductility which minimizes risk of brittle failure

 

Design Velocities

Acceptable velocities in acid piping will depend on a number of different factors such as:

Pipe material Acid concentration Acid velocity Impurities in the acid Acid temperature Intermittent or continuous flow Solids Flow conditions and turbulence

The following are general guidelines for maximum design velocities in various pipe materials.

Carbon Steel

Carbon steel is by far the most economical material of construction for conveying strong sulphuric acid but it has its limitations in terms of operating conditions which limits its use to but a few areas of the plant.  Carbon steel in the presence of strong sulphuric acid will corrode to form a thin film of iron sulphate on the surface of the metal.  It is this film of corrosion product that once formed prevents further corrosion of the underlying material.  However, this protective film is very unstable an is easily disturbed and removed.  It is this reason that the use of carbon steel is limited to handling acid at ambient temperatures and low velocities such as in product acid transfer and loading lines.

The acid velocity in the line should not exceed 0.5 m/s (1½ ft/s).

Mondi™

The permissible velocity in Mondi™ piping varies with the line size and temperature.  The velocities given in the following table are maximums.

Line SizeTemperature

<90°C (<194°F) >90°C (>194°F)

6" 1.5 m/s (5.0 ft/s) 1.5 m/s (5.0 ft/s)

8" 1.8 m/s (6.0 ft/s) 1.8 m/s (6.0 ft/s)

10" 2.1 m/s (6.9 ft/s) 2.1 m/s (6.9 ft/s)

12" 2.3 m/s (7.7 ft/s) 2.3 m/s (7.7 ft/s)

14" 2.6 m/s (8.5 ft/s) 2.6 m/s (8.5 ft/s)

16" 2.8 m/s (9.3 ft/s) 2.6 m/s (8.5 ft/s)

18" 3.0 m/s (10.0 ft/s) 2.6 m/s (8.5 ft/s)

>20" 3.0 m/s (10.0 ft/s) 2.6 m/s (8.5 ft/s)

Sandvik SX®

For all practical purposes there is no upper limit to the acid velocity in SX piping but generally velocities in the range of 2 to 3 m/s are used for pressure drop reasons.

General

The following figure shows maximum recommended acid velocities in various materials and at various temperatures.  The change in velocity at a particular temperature is not exact and should only be used as a guideline.

Flange GuardsThe weak point in any piping system are the connections between spool pieces, in-line instruments, valves, and equipment.  When the fluid being carried is not dangerous, leaks at these connections is not a critical safety issue, however, when the fluid is hot concentrated sulphuric acid, safety becomes a major concern.In acid piping systems all connections should be flanged connections.  Screwed connections are not recommended under any circumstance.   Leak may occur at the flange connections due to corrosion, wearing out of the gasket, poor or inproper installation.  If the system operated under high pressure, leaks may result in acid spraying out into the operating area.This hazard can be mitigated by the use of flange guards.   Flange guards is a safety shield that complete encloses the flange preventing the sprayout of acid.  The spray is contained and is converted into a drip which is less hazardous.Flushing and HydrotestingThere are two opinions as to whether or not sulphuric acid lines should be flushed with water and/or subjected to hydrotesting during construction.   The concern is that if water remains in the lines when sulphuric acid is introduced, there may be a violent reaction due to the sudden mixing of water and acid.   As well, the formation of weak acid in the system may lead to increased corrosion and eventual leaks.  Alloy piping system are more easily damaged by weak acid than ductile iron systems.The case for flushing and hydrotesting acid lines is that leaks can be detected before acid is introduced thereby reducing hazards during the initial filling and circulation of acid in the lines.  This is of particular advantage for ductile iron piping systems which have flanges at each fitting and spool piece.  For alloy piping systems the number of flanges is greatly reduced so the need to hydrotest the system is less.Flushing and hydrotesting can be done safely provided certain precautions are taken.  The first step is done during the design phase when the piping is laid out.  Low points and pockets should be avoided and if they are present, a drain

should be provided.  After flushing and hydrotesting the lines, all water must be drained from the system and the line purged with air until all moisture has been removed. Gaskets (Strong Acid)

Introduction

The following are recommended gaskets for use in strong sulphuric acid service.

Acid Concentration : 90% to 100% H2SO4  Temperature: 248°F (120°C)

The ability of a gasket to effect a seal depends not only on the quality of the materials, but also on the thickness of the material, the design of the flanges, the amount of pressure exerted on the gasket by the bolts, and how the gasket is assembled into the flanges and tightened.  Consult the manufacturer for specifics regarding your application.

Samples

Gylon 3540

Gylon 3454

DURLON 9000

Valves

Introduction

Manufacturer Garlock Flexitallic DurablaThermoseal

Inc.

DesignationGYLON

3500 GYLON 3540

GYLON 3545

Sigma 511

Sigma 600 DURLON 9000

KLINGERtop-chem-2000

Colour

Fawn with

Black Brand

White with Black Brand

White with Black Brand

Fawn White - -

Compositon PTFE with

Silica

Microcellular PTFE

Microcellular PTFE

PTFE with Silica

Highly Compressible

PTFE

PTFE with

Silica

PTFE

Temperature - Maximum                      - Minimum(Continuous)

500°F (260°C)

-350°F (-

212°C)

500°F (260°C) -350°F (-212°C)

500°F (260°C) -350°F (-212°C)

500°F (260°C) -350°F (-212°C)

500°F (260°C) -350°F (-212°C)

520°F (271°C) -350°F (-212°C)

-

Pressure - Maximum (Continuous)

1200 psig (83

bar)

1200 psig (83 bar)

1200 psig (83 bar)

1235 psig(1/16"

thickness)

1235 psig(1/16"

thickness) 1500 psig (103 bar)

-

Compressibility Range (ASTM F36) 7-12%

70-85% (1/16"

thickness)

60-70% (1/16" thickness)

7%(1/16"

thickness)

68%(1/16"

thickness) 8-16% 2%

(ASTM F36J)

Recovery (ASTM F36) 40%

8% (1/16"

thickness)

15% (1/16" thickness)

44%(1/16"

thickness)

6%(1/16"

thickness) 40% 55%

(ASTM F36J)

M Values : Thickness     - 1/16" 5.0 3.0 2.6

1.4 1.4-

-

     - 1/8"5.0 3.0 2.0

1.4 1.4-

-

     - 3/16"- 2.0 2.0

- --

-

     - 1/4"- 2.0 7.0

- --

-

Y Values : Thickness     - 1/16"

2750 psi

1700 psi 1500 psi 2300 psi 1600 psi

- -

     - 1/8" 3500 psi

2200 psi 2200 psi 2300 psi 1600 psi

- -

      - 3/16"- 2200 psi 2200 psi

- --

-

      - 1/4"- 2500 psi 3700 psi

- --

-

               

A variety of valve types and materials are used throughout the strong acid system.  The corrosive nature of sulphuric acid requires that special attention be paid to materials of construction to ensure long service life and reliable operation.

Plug Valves

Manufacturer XOMOX XOMOX DURCO DURCO

Model Tufline 067 Tufline 061 G4 T-41

Type Sleeved Fully Lined Sleeved Fully Lined

Size Range ½” to 18” ½” to 8” ½” to 18” ½” to 8”

Body Material

Ductile Iron316 SS / A351 CF8M

Alloy 20 / A351 CN7M

CD4MCu

Carbon Steel or Ductile Iron

Ductile Iron

316 SS / A351 CF8M

Durimet  20 / A351 CN7M

CD4MCu/Durcomet 100

Durcomet 5 (High Si SS)

Ductile Iron

Plug Material

Ductile Iron

316 SS / A351 CF8M

Alloy 20 / A351 CN7M

CD4MCu

Stainless Steel or Ductile Iron

Ductile Iron

316 SS / A351 CF8M

Durimet  20 / A351 CN7M

CD4MCu/Durcomet 100

Durcomet 5 (High Si SS)

Ductile Iron

Sleeve PTFE N/A PTFE N/A

Lining N/A PFA or FEP N/A PTFE

Connection 150 lb Flanged 150 lb Flanged 150 lb Flanged 150 lb Flanged

Wrench/Lever Operator

½” to 4” ½” to 4” ½” to 4” ½” to 4”

Gear Operator 4” to 18” 4” to 8” 4” to 18” 1" to 8"

 

Plug valves are used for isolation purposes and on/off service only. They are available in sizes from ½” through to 18”. Sizes larger than 8” diameter are not normally used partly due to cost and the increased flange-to-flange dimensions. For isolation purposes, gate valves are more suitable in the larger sizes.

The selection of materials for the body, plug and other wetted parts will depend on the service conditions.

Butterfly Valves

Butterfly valves are used primarily for flow control and regulation.  The seat design will determine if the valve will provide positive shutoff.  A swing-thru seat design should not be used for isolation as it will not provide positive shutoff.

Manufacturer DURCO LEWIS XOMOX  

Model BTV 2000 - Tufline  

Type Fully Lined Metallic, Swing-Thru Metallic  

Size Range 2" to 24" 4" to 24" 3" to 48"  

Body MaterialDuctile Iron (2" to 12")Cast Steel (14" to 24")

Lewmet Alloy 20 (1)  

Disc MaterialDIPA-PFA

encapsulated nickel plated ductile iron

Lewmet Alloy 20 (1)  

Lining PTFE N/A N/A  

Connection Lug or Wafer Wafer Lug or Wafer  

Wrench/Lever 2" to 8" Consult Vendor Consult Vendor  

Operator

Gear Operator 2" to 24" Consult Vendor Consult Vendor  

   

(1) Other materials available, consult vendor.The selection of materials for the body, disc and other wetted parts will depend

on the service conditions.

Gate Valves

Gate valves are used isolation purposes.  They are not designed for throttling service.

Manufacturer LEWIS      

Model        

Type Double Disc      

Size Range 2" to 24"      

Body MaterialProcess Iron (A278

class 35)Lewmet

     

Disc Material Lewmet      

Stem Alloy 20      

Seat Lewmet      

Handwheel Non-rising      

Stem Rising      

The selection of materials for the body, disc and other wetted parts will depend on the service conditions.

Globe Valves

Globe valves are used flow control. 

Manufacturer LEWIS      

Model        

Port Type Parabolic      

Size Range 1/2" to 6"      

Body Material Lewmet      

Alloy 20

Plug Material Lewmet      

StemLewmetAlloy 20

     

Seat Lewmet      

       

The selection of materials for the body, plug and other wetted parts will depend on the service conditions.New ProductsAVT (Advanced Valve Technologies) is pleased to announce the launch of its new Sulphuric Acid Valve, able to handle an

unusually wide range of sulphuric acid duties. The new ANSI150 and PN16 valves, based on a proprietary Novolac material technology, will accept duties in concentrations from 1% to 99%, working at temperatures up to 190°F (90°C). This ability to handle a wide range of concentrations in Sulphuric service is an exciting development for an industry where extremely expensive exotic alloys struggle to cover a wide range of concentrations whilst PTFE lined valves suffer problems due to liner failure and exterior attack. Furthermore, the new AVT valves typically offer a 30% to 80% cost saving over these alternative valves, and a 6 week manufacturing lead-time for project volumes - compared to 15 weeks for some alloy materials. The new Sulphuric Acid Valve joins the growing range of AVT products, already recognised for "class busting" performance in Hydrochloric Acid and other corrosive media. Advanced Valve Technologies (AVT) is the world leader in applying reinforced composite technologies to valves and related products. Their range of 20 bar (300psi) rated full port ball and double offset performance butterfly valves offer outstanding corrosion resistance, 60% weight savings over metallic valves, outstanding pressure-temperature characteristics and a compelling cost of ownership for applications in the Chemical, Offshore and Marine markets.