Knowledge for the Sulphuric Acid Industry
Sulphuric Acid on the Web
Acid Plant Database
Boiler Feed Water
Materials of Construction
DKL Engineering, Inc.
Contact Section - Preheat Systems
September 6, 2004
Recycle and Preheat Air System
Types of Fuel
Gas and Light Fuel Oils versus Heavy Fuel Oils
Adiabatic Flame Temperature
Acid Dew Point Temperature
Furnace Gas Temperature to Heat Exchanger
Overall Thermal Efficiency
Minimum Temperature Approach
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 fuel is burned with combustion air and the combustion gases are cooled using tempering air. The combustion and tempering air enter the furnace essentially at ambient temperature. The hot combustion gases are cooled in the preheat exchanger before being discharged to atmosphere. The overall thermal efficiency is typically 50 to 60%.
The fuel is burned with combustion air and the combustion gases are cooled using a portion of the cooled gases leaving the heat exchanger instead of tempering air. This reduces the total amount of air required. The total flow to the stack is reduced due to elimination of tempering air. The overall thermal efficiency is higher than the standard system since heat is not wasted by heating the tempering air which goes up the stack. The overall thermal efficiency is typically 72 to 80%.
Having previously reduced the combustion air to its minimum value and using the maximum amount of recycle possible, only by recovering the heat in the stack gases can the overall thermal efficiency be increased. A portion of the stack gas heat can be recovered by heating up the combustion air. As the overall thermal efficiency increases the size of the air preheater increases, thus good judgement must be used to put an upper limit on the efficiency. For any specific efficiency, the amount of heat recovered in the air preheated can be varied so that the best size combination for the air preheat exchanger and process gas heat exchanger can be achieved. The overall thermal efficiency is typically greater than 80%.
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.
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.
|Gas and Light Fuel Oils||Heavy Fuel Oils|
|Combustion Chamber||Combustion takes place in refractory lined chamber||Combustion takes place in refractory lined chamber|
|Furnace Mixing Chamber||Ceramic fibre blanket lined. Since the fuel is relatively clean burning there is no problem with the lining absorbing the fuel.||Refractory lined. Heavy fuel oils are difficult to burn so refractory is used to ensure fuel cannot be absorbed into the lining.|
|Preheat Exchanger||Combustion gases enter on shell side since fouling is not a problem. Gas is vertically up the shell.||Combustion gases enter on tube side since the fouling problem requires the ability to clean the tubes. Gas flow is downwards or horizontal to allow draining of the tubes.|
|Inlet Vestibule||Carbon steel/ceramic fibre lined shell. The base is generally refractory lined to allow access without damaging the liner.||Carbon steel/refractory lined.|
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.
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.
|One type of furnace available is designed such that tempering air or recycle gas supplied to the furnace plenum at the burner end of the furnace, is circulated through the annular space between the suspended refractory wall and the steel shell. Thus the refractory and the shell are continually cooled. This type of design ensures long refractory life, minimizes heat losses and results in a cool running furnace. Tempering air or recycle gas is introduced into the combustion chamber through specially designed air-inlet slots in the refractory wall. This results in uniform mixing of the air with combustion gases within the furnace and eliminates the possibility of hot spots.|
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.
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:
Heat removed in preheat exchanger
Thermal = ------------------------------------------------------
Efficiency Heat input to the furnace by the fuel
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.