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| Sulphuric Acid on the WebTM | Technology Manual | DKL Engineering, Inc. |
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Knowledge for
the Sulphuric Acid Industry Introduction
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The most critical stage in
the manufacture of sulphuric acid is the oxidation of sulphur dioxide (SO2) to
to sulphur trioxide (SO3) using vanadium pentoxide catalyst. In order for the reaction to proceed, the catalyst
must be heated up to its ignition temperature, typically 400 to 420oC prior to
introducing the sulphur dioxide containing gas. The
heating of the catalyst beds as well as other items of equipment, prior to startup is
achieved using a preheat system. In sulphur burning plants
a supplementary fuel may be burnt in the sulphur furnace for refractory brick curing and
preheating the contact section of the acid plant. In metallurgical acid
plants a separate furnace and heat exchange system burning a supplementary fuel is used to
preheat the acid plant. During operation with
low SO2 strength gases,
insufficient heat is produced or recovered to maintain the catalyst bed temperatures. In this situation, the preheat system is operated
to provide the additional heat. In acid regeneration
plants a combination of both systems are used to preheat the plant. The regeneration furnace is preheated using a
supplementary fuel for refractory brick curing and preheating. A separate furnace and heat exchange system is
used to preheat the acid plant contact system as well as preheating combustion air for the
regeneration furnace during normal operation. The preheat system
consists of a furnace, heat exchanger, combustion air and tempering air fan and stack. Natural gas or fuel oil is typically burned in the
furnace and tempering air or recycle gas is added to cool the combustion gases to a
suitable temperature before entering the heat exchanger. Heat
is exchanged between the combustion gas and process gas before being discharged from a
local stack. Preheat systems are used
for startup and on-line applications. Startup
preheaters provide necessary heat to raise the catalyst bed to its ignition temperature
and heat other equipment prior to introduction of SO2 gas and as such do not
operate continuously. On-line preheat systems
are operated continuously during low SO2 strengths to maintain the catalyst
beds at ignition temperature. On-line systems
are commonly used in metallurgical and acid regeneration plants. Three systems,
illustrated in Figure 1.1 are presented here in order of increasing overall thermal
efficiency. The choice of system is dependent
on the overall thermal efficiency required by the client.
The type of fuel burned is
generally specified by the client. Generally
there is a primary fuel which is used most of the time and a secondary fuel which is used
when the primary fuel is not available. Materials
must be specified to accommodate the burning of either fuel. Occasionally a third fuel such as propane is
specified for the pilot. Gas and Light Fuel Oils versus Heavy Fuel
Oils The design of the preheat
furnace and exchanger will differ depending on the type of fuel used. Table 4.2 compares the difference in designs.
Assuming that the
combustion reaction occurs with no heat interchange between the furnace and its
surroundings, then the total heat content of the entering fuel and air plus the heat of
combustion is available to raise the temperature of the product of combustion. This relationship is represented by the following
heat balance: HProducts = HReactants + HCombustion Complete combustion of the fuel is assumed. This is the highest temperature which can be expected from the combustion process and aids in the selection of materials for the furnace. The acid dewpoint
temperature is important for the design of the preheat system to minimize corrosion and
extend the life of the furnace and preheat exchanger. Several methods are available for calculating the acid dew point temperature. The methods will generally give a dew point temperature within 20oC of each other. The most conservative temperature should be used for the design.
The combustion equipment for a preheater consists of the following components:
There are a number of burner manufacturers supply burners for a wide range of fuels and ratings. A standard 'off-the-shelf' burner meeting the required rating is generally specified since no special features are required for an acid plant preheater. The burner will generally be equipped with a spark ignited propane/LPG/natural gas pilot assembly. For liquid fuels a supply of high pressure air is generally required for atomizing the fuel. A wide range of burners can be obtained from suppliers such as Bloom Engineering and Hauck Manufacturing. The valve train provides for the isolation and control of the flow of fuel. The valve train generally consists of a skid mounted assembly of piping, fittings, shut-off valves, control valves and instrumentation. If a liquid fuel is specified, an oil pump set is generally required to deliver the fuel oil to the burner at the required pressure. The oil pump skid will consist of skid mounted oil pumps (generally gear type positive displacement), filters and valving. Fans are required to deliver combustion and dilution air to the burner and furnace. In some designs combustion and dilution air requirements are supplied by the same fan. A package of instrumentation and controls are supplied with the preheater to control the operation of the burner. Instrumentation consists of temperature measurement, pressure switches, flow switches, UV flame detector, control valves, etc. Controls are required to regulate fuel and air flow, provide shutdown logic, control startup and shutdown sequences, and to generally monitor and maintain safe combustion. Early control systems were hardwired with relays and timers providing the control logic. Modern control systems are now micro processor based. The preheat exchanger is
simply a gas-to-gas heat exchanger with combustion gases on one side and either air or
process gas on the other side. There are many different types of preheat exchangers
depending on the vendor. The normal shell and tube designs are most common with a
few plate type heat exchangers and units with finned tube bundles. If condensation of the combustion products could occur, it is recommended to place the hot furnace gases on the tubeside of a horizontal unit. This ensures that condensed liquids will be forced out into the outlet vestibule. As well, it is easier to clean the tube side of the exchanger. When condensation is not a problem, the hot furnace gases are placed on the shellside. Combustion gases that tend to foul heat exchanger surfaces should be directed on the tube side for ease of cleaning. The heat exchanger is
manufactured in either stainless steel or carbon steel depending on the design
temperatures. The heat exchanger may be either horizontal or vertical. Depending on the preheat system chosen, the stack
for the system may be attached to the heat exchanger or self-supporting. The photo shows the shell side of a horizontal preheat exchanger that will be mounted piggy-back on top of the preheat furnace. This design uses thermal insulation on the inside of the shell. Furnace Gas Temperature to Heat Exchanger The maximum inlet
temperature of the furnace gases to the heat exchanger is limited by the mechanical design
of the exchanger. The maximum temperature is
independent of the type of fuel burned. For all designs using
stainless steel preheat exchangers the design temperature is 750oC. The preheat exchanger should be designed for a
maximum temperature of 800oC at which point an interlock will shutdown the
preheat system. For designs using carbon
steel preheat exchangers the design temperature is 550oC. The overall thermal
efficiency is defined as follows: % Overall
Heat removed in preheat exchanger This value may be
specified by the client. The initial capital
cost of the preheat system usually increase with overall thermal efficiency. For preheat systems used
in startup applications, the main concern is to keep the initial capital cost low. By definition, the operating costs are low since
the preheat system is not operated continuously. The
standard type of preheat system, with an overall thermal efficiency of 50-60% is commonly
used for this application. For on-line application,
operating costs should be minimized since the preheat system operates frequently or
continuously. This can be achieved by
increasing the overall thermal efficiency of the standard preheat system by recycling the
furnace gases and preheating the combustion air. An
overall thermal efficiency greater than 72% can be achieved in this manner. When the preheat system is used to preheat process gas containing SO2, condensation may occur resulting in corrosion of the heat exchanger. To minimize condensation, the tubewall temperature should be maintained above 180 to 200oC. As the temperature
approach at either end of the heat exchanger is reduced, the log mean temperature
difference decreases requiring more area for the same duty and consequently increasing the
size of the heat exchanger. To avoid
excessively large exchangers the recommended minimum temperature approach at either end of
the exchanger is 15oC. The minimum amount of
combustion air recommended for burning natural gas is twenty percent above the
stoichiometric amount. Variable firing rates are
required to enable operation of the preheater over a wide range of plant operating
conditions from plant heat up to supplementary heating during low gas strengths. The turndown that can be achieved will be
dependent on the type of burner used and the type of fuel burned. For natural gas and light fuel oils a turndown of
8:1 should be specified. For heavy fuel oils
a turndown of 6:1 is typical. |
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Copyright© 2005,
2006, 2007, 2008 DKL
Engineering, Inc., All Rights Reserved |
Preheater furnace lined with high temperature ceramic fibre insulation in
stack bond construction which is held in place with stainless steel anchors. The
interior surfaces of the fibre insulation is sealed and protected with a thin layer of
insulating rigidizer such as Unifrax Top Coat "M" or equal. Thi stype of
lining is most suitable for gas and light fuel oils. For heavy fuel oils a
refractory bricklined is most suitable since the fuel oil will not be absorbed into the
lining material.
