Heat exchange – With adjustor for heat – or exchange material – flow – Branched flow
Reexamination Certificate
1995-08-08
2001-04-03
Atkinson, Christopher (Department: 3743)
Heat exchange
With adjustor for heat, or exchange material, flow
Branched flow
C165S159000, C165S100000, C126S101000
Reexamination Certificate
active
06209624
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to gas-to-gas heat exchangers for use in sulphuric acid manufacturing plants involving heat exchange between air, sulphur dioxide and sulphur trioxide and also said heat exchangers for use with combustion gases in a preheater system.
BACKGROUND OF THE INVENTION
Plants for the manufacture of sulphuric acid involving either the burning of elemental sulphur or oxidation of metal sulphides to produce sulphur dioxide for subsequent oxidation to sulphur trioxide followed by absorption into sulphuric acid are very large generators of process heat. This process heat comes from the exothermic burning or absorption processes and is generally used for many purposes, such as the heating of gases or raising steam.
The SO
2
oxidation is carried out in a series of uncooled catalyst beds of a catalytic converter with heat being removed between beds and before the SO
3
-containing gases are passed to absorber(s) for SO
3
removal. In sulphur burning sulphuric acid plants the bulk of the heat removed from the SO
3
-containing gas is transferred into steam systems with only limited pre-heating of process or other gases. In plants using an SO
2
gas source, the heat is almost completely used to pre-heat incoming cold, dry SO
2
feed gas, or, in addition, in the case of the so-called “double absorption process”, the cold SO
2
-containing gas returning from the gases in the first or intermediate absorber. Where surplus heat must be rejected from such plants, often the heat is rejected to either atmospheric air or to plant tail gas in special exchangers designed for this purpose.
Inter-bed exchangers used in such processes are, typically, known as Hot, Intermediate, or Hot IP Exchangers. The Hot Exchanger is normally associated with cooling of the hot gas leaving the first catalyst bed and the other exchangers with cooling of the gases between beds
2
and
3
. In all of these exchangers the heat is transferred to colder SO
2
-containing gases which then pass either directly or through other exchangers to catalyst beds. The gases leaving catalyst beds en route to absorption steps are normally cooled by heat transfer with cold SO
2
-containing gases from either an acid plant main blower or an Intermediate absorber. It may also be cooled by heat transfer with air in what is known as “SO
3
Coolers” or “Air Heaters” or by plant tail gas, in which case the apparatus becomes known as a Tail Gas Heater.
Classic heat transfer between SO
2
- and SO
3
-containing gases uses counter-current shell-and-tube heat exchangers in which one gas flows through the tubes of the exchanger and the other gas flows through the shell space as directed by baffles within the shell space. These exchangers are, typically, quite large and for colder duties are made of carbon steel. More recent plants use stainless steel as a construction material for hotter duties such as involving the cooling of the hot SO
3
gas leaving beds
1
or
2
where the SO
3
gas is hottest. In most large plants, the exchangers are fabricated on site as they are too large and heavy to allow of reasonable transportation. Tubes of exchangers are arranged in a number of different ways with baffles to match the tubing layout. In some cases the tubes are distributed throughout the shell space and either single or double-segmental baffles have been used. In other designs, the tubes are arranged in the form of annular bundles wherein gas flows radially through the bundle from an open core to a tube-free outer annulus and returns as required. The number of passes across the bundle and the tubing layout will depend on the size of gas flows, thermal efficiency needed and the pressure differences available to cause flow through the shell space.
In plants where heat is rejected from the process to atmosphere, early plants used simple bare gas ducting to cool gases between beds, such as, for example, between a third and fourth catalyst bed in a small single absorption plant. Such apparatus was simple but not effective in rejecting large quantities of process heat. Induced draft heat exchangers were subsequently used to reject significantly larger quantities of heat. The pressure difference available using stack draft was, however, small and, accordingly, fans or blowers were introduced to provide adequate pressure differences and allow the size of such equipment to have reasonable physical dimensions. Where an exchanger handled air that was used elsewhere in process, SO
2
-containing gas, or plant tail gas, the main acid plant blower provided the driving force for gas flow and separate blowers were unnecessary. Where air was heated and rejected directly to atmosphere a separate air blower was used.
Each of the exchangers described hereinbefore was based on counter-current heat transfer with the two gases entering at opposite ends of the exchanger. Problems exist if the metal of the exchanger becomes too cold or too hot. Gas streams found in sulphuric acid plants normally contain condensible compounds, such as small quantities of sulphuric acid vapour, either from entrained acid from drying operations, from reaction of SO
3
formed in the reaction with moisture from inadequate drying, or from hydrocarbons present in the elemental sulphur if sulphur is used. As a result, there is the possibility of sulphuric acid condensation from such gases when the temperature of the metal exchanger falls below the condensation temperature. This condensation produces significant corrosion. Although the condensation temperature is normally not a factor in the hotter exchangers, it is a problem in colder exchangers such as Cold or Cold IP Exchangers, SO
3
Coolers or Tail Gas Heaters.
Where the condensation risk is serious, special measures are often taken to keep metal temperatures above the minimum at which condensation takes place. One such technique is to recycle hot air from the exit of a SO
3
cooler back to its inlet. This corrective action is widely used, but requires a much larger fan and heat exchanger and, hence, larger capital and operating costs. Where tail gas is being heated, there is little prospect of a recycle stream without the need for a separate fan and the operator is, thus, normally forced to accept any condensation that results. Such equipment is therefore very dependent on the quality of the drying and mist elimination equipment upstream.
Conventional exchanger designs result in large exchangers having high flow resistance due to the large gas flows involved. The large exchangers also often have significantly different thermal expansions between adjacent parts of the exchangers. Cracked tube sheets, broken tube-to-tube sheet joints and leaks can result from excessive differential thermal stresses in such units.
The shell and tube exchanger having a shell full of tubes has fallen into disfavor in the last two decades as the shell and adjacent tubes have significantly different thermal expansions and generate excessive stresses on tube-to-tube sheet joints or on tube sheet-to-shell joints. Heat transfer varied significantly from tube-to-tube in the shell space and the unit used many more tubes than necessary. Baffle arrangements included single and double segmental baffles with the problem being common to both baffle arrangements.
In an alternative design, the tubes of the exchanger are confined between chords with open dome spaces on each side of the tube bundle for gas flow between cross-flow passes. With single segmental baffles, this arrangement provides for gas transfer from one shell pass to the next in the dome space where no tubes are located. Better heat transfer is provided as all of the tubes are located in a zone where good gas flow is assured but pressure drop in the shell space is high. This design has also been used with double segmental baffles. In the double segmental baffle variation, gas flows either around and parallel to tubes in the central portion of the bundle or in the two dome spaces which are free of tubes. The gas flows from the edge of the bundle to the centre of the bundle and then b
Atkinson Christopher
Pillsbury Madison & Sutro LLP
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