CA1108598A - Indirect gas to gas heat exchanger for use with a gas stream containing condensible corrosive constituents - Google Patents
Indirect gas to gas heat exchanger for use with a gas stream containing condensible corrosive constituentsInfo
- Publication number
- CA1108598A CA1108598A CA333,500A CA333500A CA1108598A CA 1108598 A CA1108598 A CA 1108598A CA 333500 A CA333500 A CA 333500A CA 1108598 A CA1108598 A CA 1108598A
- Authority
- CA
- Canada
- Prior art keywords
- gas
- heat exchanger
- gas stream
- enclosure
- zone
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
- 239000000470 constituent Substances 0.000 title claims abstract description 31
- QAOWNCQODCNURD-UHFFFAOYSA-N Sulfuric acid Chemical compound OS(O)(=O)=O QAOWNCQODCNURD-UHFFFAOYSA-N 0.000 claims abstract description 104
- 238000009833 condensation Methods 0.000 claims abstract description 45
- 230000005494 condensation Effects 0.000 claims abstract description 45
- 239000006096 absorbing agent Substances 0.000 claims abstract description 22
- 238000010521 absorption reaction Methods 0.000 claims abstract description 22
- 238000000034 method Methods 0.000 claims abstract description 18
- RAHZWNYVWXNFOC-UHFFFAOYSA-N Sulphur dioxide Chemical compound O=S=O RAHZWNYVWXNFOC-UHFFFAOYSA-N 0.000 claims abstract description 16
- 238000004519 manufacturing process Methods 0.000 claims abstract description 5
- 239000007789 gas Substances 0.000 claims description 311
- AKEJUJNQAAGONA-UHFFFAOYSA-N sulfur trioxide Chemical compound O=S(=O)=O AKEJUJNQAAGONA-UHFFFAOYSA-N 0.000 claims description 20
- 238000006243 chemical reaction Methods 0.000 claims description 18
- 239000002253 acid Substances 0.000 abstract description 25
- 239000003595 mist Substances 0.000 abstract description 24
- 230000007797 corrosion Effects 0.000 abstract description 23
- 238000005260 corrosion Methods 0.000 abstract description 23
- 230000003197 catalytic effect Effects 0.000 abstract description 15
- 235000011149 sulphuric acid Nutrition 0.000 abstract description 2
- 239000001117 sulphuric acid Substances 0.000 abstract 1
- 230000000977 initiatory effect Effects 0.000 description 5
- 238000004581 coalescence Methods 0.000 description 4
- 238000001816 cooling Methods 0.000 description 4
- 238000010438 heat treatment Methods 0.000 description 4
- 230000002745 absorbent Effects 0.000 description 3
- 239000002250 absorbent Substances 0.000 description 3
- 239000012530 fluid Substances 0.000 description 3
- 150000002500 ions Chemical class 0.000 description 3
- 239000007788 liquid Substances 0.000 description 3
- QSHDDOUJBYECFT-UHFFFAOYSA-N mercury Chemical compound [Hg] QSHDDOUJBYECFT-UHFFFAOYSA-N 0.000 description 3
- 229910052753 mercury Inorganic materials 0.000 description 3
- NINIDFKCEFEMDL-UHFFFAOYSA-N Sulfur Chemical compound [S] NINIDFKCEFEMDL-UHFFFAOYSA-N 0.000 description 2
- 239000000443 aerosol Substances 0.000 description 2
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 description 2
- 239000007791 liquid phase Substances 0.000 description 2
- 239000000463 material Substances 0.000 description 2
- 229910052760 oxygen Inorganic materials 0.000 description 2
- 239000001301 oxygen Substances 0.000 description 2
- 239000002245 particle Substances 0.000 description 2
- 239000012071 phase Substances 0.000 description 2
- 229910052717 sulfur Inorganic materials 0.000 description 2
- 239000011593 sulfur Substances 0.000 description 2
- 238000009834 vaporization Methods 0.000 description 2
- MHCVCKDNQYMGEX-UHFFFAOYSA-N 1,1'-biphenyl;phenoxybenzene Chemical compound C1=CC=CC=C1C1=CC=CC=C1.C=1C=CC=CC=1OC1=CC=CC=C1 MHCVCKDNQYMGEX-UHFFFAOYSA-N 0.000 description 1
- 229910000975 Carbon steel Inorganic materials 0.000 description 1
- 101100238304 Mus musculus Morc1 gene Proteins 0.000 description 1
- LEHOTFFKMJEONL-UHFFFAOYSA-N Uric Acid Chemical compound N1C(=O)NC(=O)C2=C1NC(=O)N2 LEHOTFFKMJEONL-UHFFFAOYSA-N 0.000 description 1
- TVWHNULVHGKJHS-UHFFFAOYSA-N Uric acid Natural products N1C(=O)NC(=O)C2NC(=O)NC21 TVWHNULVHGKJHS-UHFFFAOYSA-N 0.000 description 1
- DQSGVVGOPRWTKI-QVFAWCHISA-N atazanavir sulfate Chemical compound [H+].[H+].[O-]S([O-])(=O)=O.C([C@H](NC(=O)[C@@H](NC(=O)OC)C(C)(C)C)[C@@H](O)CN(CC=1C=CC(=CC=1)C=1N=CC=CC=1)NC(=O)[C@@H](NC(=O)OC)C(C)(C)C)C1=CC=CC=C1 DQSGVVGOPRWTKI-QVFAWCHISA-N 0.000 description 1
- 238000009835 boiling Methods 0.000 description 1
- 239000010962 carbon steel Substances 0.000 description 1
- 239000003054 catalyst Substances 0.000 description 1
- 238000001311 chemical methods and process Methods 0.000 description 1
- 150000001875 compounds Chemical class 0.000 description 1
- 238000010276 construction Methods 0.000 description 1
- 238000001035 drying Methods 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- 230000008030 elimination Effects 0.000 description 1
- 238000003379 elimination reaction Methods 0.000 description 1
- 239000010419 fine particle Substances 0.000 description 1
- 238000009413 insulation Methods 0.000 description 1
- 238000012423 maintenance Methods 0.000 description 1
- 230000007257 malfunction Effects 0.000 description 1
- 230000003647 oxidation Effects 0.000 description 1
- 238000007254 oxidation reaction Methods 0.000 description 1
- 230000005855 radiation Effects 0.000 description 1
- 238000003303 reheating Methods 0.000 description 1
- 230000003014 reinforcing effect Effects 0.000 description 1
- 229940116269 uric acid Drugs 0.000 description 1
- 239000012808 vapor phase Substances 0.000 description 1
- 230000008016 vaporization Effects 0.000 description 1
- 238000013022 venting Methods 0.000 description 1
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 1
Landscapes
- Heat-Exchange Devices With Radiators And Conduit Assemblies (AREA)
Abstract
Abstract of the Disclosure The addition of external heat to a gas stream in duct work passing from the intermediate absorber to the heat exchanger employed to raise the temperature of the gas stream containing residual sulfur dioxide to its catalytic iniation temperature in a multiple contact/multiple absorption process for the manufacture of sulphuric acid in an amount sufficient to maintain the gas stream above the dew point of sulfuric acid is employed to eliminate corrosion in both the ductwork and the heat exchanger(s). There is also provided by the invention an indirect gas to gas heat exchanger especially adapted to take account of the fact that the cooler gas stream contains condensible corrosive constituents. This exchanger immediately follows intermediate absorption and includes a zone maintained at a temperature above the condensation temperature of sulfuric acid to prevent condensation of acid mist on the heat exchanger surfaces.
Description
INDIRECT GAS TO GAS HEAT EXCHANGER FOR USE WITH
A GAS STREAM CONTAINING CONDENSIBLE CORROSIVE
CONSTITUENTS _ _ _ Field of the Invention This invention relates to gas to gas heat exchangers, and particularly to such exchangers for use with a cooler gas stream containing condensible corrosive constituents.
Cross-reference to related application Reference may be made to our copending application Serial No. 196,729, filed 3rd April 1974, from which the present ; application is divided, which discloses and claims a multiple contact/multiple absorption process for the manufacture of sulfuric acid and oleum, which process includes improvements, to avoid the severe corrosion problems encountered in sulfuric acid plants based on the multiple contact/multiple absorption principle.
Background of the Invention Many sulfuric acid plants based on the multiple contact-multiple absorption principal have been designed and built over the past ten years. In this chemical process, a gas stream containing sulfur dioxide and oxygen is passed through a plurality of catalytic conversion stages with intermediate cooling of the gas stream to remove the exothermic heat of reaction to convert a substantial quantity of the sulfur dioxide contained in the gas stream to sulfur trioxide.
The gas is then passed to an intermediate absorption stage where the formed sulfur trioxide is absorbed from the gas stream.
There is commonly employed as the absorbent, sulfuric acid in a concentration between about 98 and 99 per cent. Some processes utilize the sulfuric acid absorbent at elevated temperatures to provide part of the heat required for reheating the residual gas stream going to secondary conversion stages to its catalytic .. ii-,, ~8S98 niatioll temp~l~ture. When the resl~ual gas stream is ~OUyhL to its cataly~ic initiation temperature, it is introduced to a next catalytic stage or stayes where the residual sulfur dioxide is converted to sulfur trioxide for absorption prior to venting the gas stream to the atmosphere.
Depending upon the number of conversion and absorption stages employed, extremely high overall conversions are achieva~le.
It is, for instance, feasible to achieve a 99.9 per cent or more conversion of sulfur dioxide to sulfur trioxide.
A problem has existed in the maintenance of the equipment and the ductwork used for returning the gas stream from the intermediate absorber to what may be termed a cold heat exchanger used alone, or in conjunction with other heat exchangers to preheat the gas stream to its introduction temper-ature for the next catalytic conversion stage or stages.
There have been extreme corrosion problems in the ductwork due to the presence of acid mist and to the condensation of previously vaporized sulfuric acid in the gas stream, as well as corrosion at the inlet end of the heat exchanger due to sulfuric acid condensation and as condensation of acid mist where acid mist elimators are inefficiently operated.
Because the gas stream leaving the intermediate absorber is in equilibrium with the acid used in the intermediate absorber, it contains free sulfuric acid which has been found to condense on cold metallic surfaces which are below the dew point of sulfuric acid leading to extreme corrosion problems.
In addition, sulfuric acid mist can form as a result of insufficient drying of process air, or gas, which permits water vapour to react with sulfur trioxide formed later in the process. The free particles of acid mist which exist as an i8598 aerosol can coalesce due to the turbulence of flow in the duct-work into the cole heat exchanger and then adhere to the inner surfaces of the heat exchanger leading to additional corrosion problems.
It is known that many sulfuric acid plants based on the multiple contact/multiple absorption principle have exper-ienced severe corrosion problems, for example in the ductwork and the cold heat exchangers, some of which have been known to have completely deteriorated within six to nine months of use.
This represents a considerable loss in both equipment and down-time required to replace equipment.
Definition of the Invention It is therefore an object of this invention to provide a new indirect gas to gas heat exchanger of the kind specified and with which theproblem of corrosion of the surfaces thereof is at least appreciably reduced.
In accordance with the present invention there is provided an indirect, gas to gas heat exchanger adapted to heat a relatively cool gas stream containing at least one condensible corrosive constituent to an elevated temperature by indirect heat exchange with a gas at a higher temperature than the condensation temperature of the condensible constituent, the heat exchanger comprising a gas feed heater having a gas inlet;
a gas outlet header having a gas outlet; a plurality of spaced gas flow conduits communicating with the gas feed header and the gas outlet header to provide a plurality of passageways for flow of one of the gases to participate in indirect heat exchange between the headers; an enclosure surrounding the conduits to provide a passageway for the flow of the other of the gases about the conduits and having a gas inlet and a gas i8598 outlet; and means which restrict the gas flow in a zone of the enclosure so as to establish inside the enclosure a sufficiently stagnant or quiescent gas zone adjacent to the point of entry of the relatively cool gas stream into the heat exchanger, the said restrict means being such that -the said zone in use is kept at a temperature above the condensation temperature of the said corrosive constituent by the flow of the high temperature gas through the heat exchanger, whereby any condensate from the relatively cool gas stream entering the said zone will be vaporized, the restrict means permitting the vaproized condensate to pass readily back from the said zone into the gas stream, so as to prevent condensation of heat exchanger surfaces of the said constituent from the incoming cool gas stream to be heated.
Also in accordance with the invention there is provided an indirect, gas to gas heat exchanger for preventing condensation of at least one condensible corrosive constituent contained in a relatively cool gas stream which is to be heated to an elevated temperature by a gas at a temperature higher than the condensation temperature of the said constituent, the heat exchanger comprising:
a) a gas feed header having a gas inlet for the high temperature gas:
b) a gas outlet header having a gas outlet for the high temperature gas after being cooled by indirect heat exchange with the relatively cool gas stream;
c) a plurality of spaced gas flow conduits communicating with the gas inlet header and the gas outlet header to permit flow of the high temperature gas from the gas inlet header to the gas outlet header;
d) an enclosure surrounding the spaced gas flow ~138598 conduits and having an enclosure gas inlet for the relatively cool gas stream, which inlet is spaced from the gas outlet header, and an enclosure gas outlet down-stream from the enclosure gas inlet to permit flow of the said gas stream from the heat exchanger after indirect heat exchange wi.th the high temperature gas stream flowing through the conduits; there being e) an open baffle plate adjacent the enclosure gas inlet, positioned between the enclosure gas inlet and the gas outlet header in spaced relationship from the gas outlet header and the said gas flow conduits, the said open baffle restricting the gas flow in a zone of the enclosure so as to maintain inside the enclosure a sufficiently stagnant or quiescent gas zone adjacent the enclosure gas inlet, the said zone being between the baffle plate and the gas outlet header, and the said zone in use being kept because of the open baffle plate at a temperature above the condensation temperature of the corrosive condensible constituent in the relatively cool gas stream by the flow of the high temperature gas stream through the conduits, the open baffle plate being sufficiently open that any condensate from the relatively cool gas stream entering the said zone will be vaporized and can pass readily back from the said zone into the gas stream, so as to prevent condensation on the heat exchang-er surfaces of the said constituent from the incoming gas stream to be heated.
Further in accordance with the invention there is provided an indirect, gas to gas heat exchanger for preventing condensation of at least one condensible corrosive constituent in a relatively cool gas stream which is to be heated to an elevated temperature by a gas at a temperature higher than the condensation temperature of the said constituent, the heat 359~3 exchanger comprising:
a) a gas feed header having a first gas inlet for the relatively cool gas stream;
b) a gas outlet header having a first gas outlet to permit flow from the heat exchanger of the said gas stream after being heated by indirect heat exchange with the high temperature gas;
c) a plurality of spaced gas flow conduits communicating with the gas inlet header and the gas outlet header to permit flow of the relatively cool gas stream from the gas inlet header to the gas outlet header;
d) an enclosure surrounding the conduits and having an enclosure gas inlet and an enclosure gas outlet for flow of the high temperature gas into and through the heat exchanger in indirect heat exchange with the relatively cool gas stream the enclosure further including an additional inlet for the high temperature gas; there being e) an open baffle plate between the said additional gas inlet and the gas inlet header in spaced relationship from the said gas inlet header and the conduits, the said open baffle restricting the gas flow in a zone of the enclosure so as to maintain inside the enclosure a sufficiently stagnant or quiescent gas zone between the baffle plate and the gas inlet header, the said zone in use being kept at a temperature above the condensation temperature of the corrosive condensible constituent in the relatively cool gas stream entering the gas inlet header and conduits by high temperature gas entering the said zone via the additional gas inlet, whereby any condensate from the relatively cool gas stream entering the said zone will be vaporised and passed back from the said zone ~8598 - into the gas stream,so as to prevent condensation on heat exchanger surfaces of the said constituent from the incoming gas stream to be heated.
Description of the Drawings Indirect gas to gas heat exchange apparatus which are particular preferred embodiments of the invention will be described, by way of example, with reference to the accompanying diagrammatic drawings, wherein:-FIGURE 1 is a cross-sectional illustration of the ductwork leading from an intermediate absorber to the next, or cold indirect gas to gas heat exchanger showing one means of providing a supply of external heat to the gas stream passing to the heat exchanger and also illustrating means providing a quiescent zone in the heat exchanger to provide a hot zone for vapourization of acid mist, the feed from the intermediate absorber being to the shell side of the exchanger, and FIGURE 2 is an alternate embodiment of Figure 1 and for the situation where the gas stream from the intermediate absorber is passed through the tubes of the heat exchanger.
,-' Description The present invention is applied to apparatus employed in processes for the manufacture of sulfuric acid by what is known as multiple contact/multiple absorption system.
The first aspect of the invention involves elimination of corrosion in ductwork employed to transport a gas stream following intermediate absorption of formed sulfur trioxide to a next or cold gas to gas heat exchanger in which the gas stream is to be heated, in whole or in part, to its catalytic initiation temperature for introduction to a next catalytic oxidation stage.
There is also provided means to eliminate settled or settling of sulfuric acid and acid mist within the heat exchanger itself by providing a stagnant or semi-stagnant high temperature zone which will prevent condensation of sulfuric acid as well as agglomerated acid mist on select heat exchanger surfaces to prevent their corrosion.
The problems to be solved may be understood by a consideration of the typical operation of a multiple contact/
multiple absorption sulfuric acid plant. After passing a gas stream containing sulfur dioxide and oxygen through a plurality of catalytic conversion stages with intermediate cooling between stages, the gas stream is passed to an intermediate absorption tower containing, as the absorbent, 98-99 per cent sulfuric acid.
Following intermediate absorption to remove formed sulfur trioxide, the gas stream is passed to a gas to gas heat exchanger to preheat the gas stream, in whole or in part, to a temperature required to initiate conversion of .
....
~ 598 ¦ residu~l sulur dioxide to sulfur trioxlde in one or ¦ morc catalytic conversion stages. This occurs prior to absorp~ion of sulfur trioxide formed from the residual sulfur dioxide in the gas stream in a final absorption sta~e or st~es.
Consider a plant operating at a capacity of about 1200 metric tons per day sulfuric acid and having 8 duct between the intermediate absorber and the next or cold gas to gas heat exchanger employed to reheat, whole or in part, the gas stream following intermediate absorption back to its catalytic initiation temperature for introduction to a next catalytic stage or stages.
For a plant of this capacity, the ductwork, generally having a diameter of about 6.5 feet and a length of about 65 feet, provides an external heat transfer area sub~ect to heat loss by convection and radiation to the surroundings of about 1300 square feet.
Under normal operating conditions process gas flow from an intermediate absorber through the duct is about
A GAS STREAM CONTAINING CONDENSIBLE CORROSIVE
CONSTITUENTS _ _ _ Field of the Invention This invention relates to gas to gas heat exchangers, and particularly to such exchangers for use with a cooler gas stream containing condensible corrosive constituents.
Cross-reference to related application Reference may be made to our copending application Serial No. 196,729, filed 3rd April 1974, from which the present ; application is divided, which discloses and claims a multiple contact/multiple absorption process for the manufacture of sulfuric acid and oleum, which process includes improvements, to avoid the severe corrosion problems encountered in sulfuric acid plants based on the multiple contact/multiple absorption principle.
Background of the Invention Many sulfuric acid plants based on the multiple contact-multiple absorption principal have been designed and built over the past ten years. In this chemical process, a gas stream containing sulfur dioxide and oxygen is passed through a plurality of catalytic conversion stages with intermediate cooling of the gas stream to remove the exothermic heat of reaction to convert a substantial quantity of the sulfur dioxide contained in the gas stream to sulfur trioxide.
The gas is then passed to an intermediate absorption stage where the formed sulfur trioxide is absorbed from the gas stream.
There is commonly employed as the absorbent, sulfuric acid in a concentration between about 98 and 99 per cent. Some processes utilize the sulfuric acid absorbent at elevated temperatures to provide part of the heat required for reheating the residual gas stream going to secondary conversion stages to its catalytic .. ii-,, ~8S98 niatioll temp~l~ture. When the resl~ual gas stream is ~OUyhL to its cataly~ic initiation temperature, it is introduced to a next catalytic stage or stayes where the residual sulfur dioxide is converted to sulfur trioxide for absorption prior to venting the gas stream to the atmosphere.
Depending upon the number of conversion and absorption stages employed, extremely high overall conversions are achieva~le.
It is, for instance, feasible to achieve a 99.9 per cent or more conversion of sulfur dioxide to sulfur trioxide.
A problem has existed in the maintenance of the equipment and the ductwork used for returning the gas stream from the intermediate absorber to what may be termed a cold heat exchanger used alone, or in conjunction with other heat exchangers to preheat the gas stream to its introduction temper-ature for the next catalytic conversion stage or stages.
There have been extreme corrosion problems in the ductwork due to the presence of acid mist and to the condensation of previously vaporized sulfuric acid in the gas stream, as well as corrosion at the inlet end of the heat exchanger due to sulfuric acid condensation and as condensation of acid mist where acid mist elimators are inefficiently operated.
Because the gas stream leaving the intermediate absorber is in equilibrium with the acid used in the intermediate absorber, it contains free sulfuric acid which has been found to condense on cold metallic surfaces which are below the dew point of sulfuric acid leading to extreme corrosion problems.
In addition, sulfuric acid mist can form as a result of insufficient drying of process air, or gas, which permits water vapour to react with sulfur trioxide formed later in the process. The free particles of acid mist which exist as an i8598 aerosol can coalesce due to the turbulence of flow in the duct-work into the cole heat exchanger and then adhere to the inner surfaces of the heat exchanger leading to additional corrosion problems.
It is known that many sulfuric acid plants based on the multiple contact/multiple absorption principle have exper-ienced severe corrosion problems, for example in the ductwork and the cold heat exchangers, some of which have been known to have completely deteriorated within six to nine months of use.
This represents a considerable loss in both equipment and down-time required to replace equipment.
Definition of the Invention It is therefore an object of this invention to provide a new indirect gas to gas heat exchanger of the kind specified and with which theproblem of corrosion of the surfaces thereof is at least appreciably reduced.
In accordance with the present invention there is provided an indirect, gas to gas heat exchanger adapted to heat a relatively cool gas stream containing at least one condensible corrosive constituent to an elevated temperature by indirect heat exchange with a gas at a higher temperature than the condensation temperature of the condensible constituent, the heat exchanger comprising a gas feed heater having a gas inlet;
a gas outlet header having a gas outlet; a plurality of spaced gas flow conduits communicating with the gas feed header and the gas outlet header to provide a plurality of passageways for flow of one of the gases to participate in indirect heat exchange between the headers; an enclosure surrounding the conduits to provide a passageway for the flow of the other of the gases about the conduits and having a gas inlet and a gas i8598 outlet; and means which restrict the gas flow in a zone of the enclosure so as to establish inside the enclosure a sufficiently stagnant or quiescent gas zone adjacent to the point of entry of the relatively cool gas stream into the heat exchanger, the said restrict means being such that -the said zone in use is kept at a temperature above the condensation temperature of the said corrosive constituent by the flow of the high temperature gas through the heat exchanger, whereby any condensate from the relatively cool gas stream entering the said zone will be vaporized, the restrict means permitting the vaproized condensate to pass readily back from the said zone into the gas stream, so as to prevent condensation of heat exchanger surfaces of the said constituent from the incoming cool gas stream to be heated.
Also in accordance with the invention there is provided an indirect, gas to gas heat exchanger for preventing condensation of at least one condensible corrosive constituent contained in a relatively cool gas stream which is to be heated to an elevated temperature by a gas at a temperature higher than the condensation temperature of the said constituent, the heat exchanger comprising:
a) a gas feed header having a gas inlet for the high temperature gas:
b) a gas outlet header having a gas outlet for the high temperature gas after being cooled by indirect heat exchange with the relatively cool gas stream;
c) a plurality of spaced gas flow conduits communicating with the gas inlet header and the gas outlet header to permit flow of the high temperature gas from the gas inlet header to the gas outlet header;
d) an enclosure surrounding the spaced gas flow ~138598 conduits and having an enclosure gas inlet for the relatively cool gas stream, which inlet is spaced from the gas outlet header, and an enclosure gas outlet down-stream from the enclosure gas inlet to permit flow of the said gas stream from the heat exchanger after indirect heat exchange wi.th the high temperature gas stream flowing through the conduits; there being e) an open baffle plate adjacent the enclosure gas inlet, positioned between the enclosure gas inlet and the gas outlet header in spaced relationship from the gas outlet header and the said gas flow conduits, the said open baffle restricting the gas flow in a zone of the enclosure so as to maintain inside the enclosure a sufficiently stagnant or quiescent gas zone adjacent the enclosure gas inlet, the said zone being between the baffle plate and the gas outlet header, and the said zone in use being kept because of the open baffle plate at a temperature above the condensation temperature of the corrosive condensible constituent in the relatively cool gas stream by the flow of the high temperature gas stream through the conduits, the open baffle plate being sufficiently open that any condensate from the relatively cool gas stream entering the said zone will be vaporized and can pass readily back from the said zone into the gas stream, so as to prevent condensation on the heat exchang-er surfaces of the said constituent from the incoming gas stream to be heated.
Further in accordance with the invention there is provided an indirect, gas to gas heat exchanger for preventing condensation of at least one condensible corrosive constituent in a relatively cool gas stream which is to be heated to an elevated temperature by a gas at a temperature higher than the condensation temperature of the said constituent, the heat 359~3 exchanger comprising:
a) a gas feed header having a first gas inlet for the relatively cool gas stream;
b) a gas outlet header having a first gas outlet to permit flow from the heat exchanger of the said gas stream after being heated by indirect heat exchange with the high temperature gas;
c) a plurality of spaced gas flow conduits communicating with the gas inlet header and the gas outlet header to permit flow of the relatively cool gas stream from the gas inlet header to the gas outlet header;
d) an enclosure surrounding the conduits and having an enclosure gas inlet and an enclosure gas outlet for flow of the high temperature gas into and through the heat exchanger in indirect heat exchange with the relatively cool gas stream the enclosure further including an additional inlet for the high temperature gas; there being e) an open baffle plate between the said additional gas inlet and the gas inlet header in spaced relationship from the said gas inlet header and the conduits, the said open baffle restricting the gas flow in a zone of the enclosure so as to maintain inside the enclosure a sufficiently stagnant or quiescent gas zone between the baffle plate and the gas inlet header, the said zone in use being kept at a temperature above the condensation temperature of the corrosive condensible constituent in the relatively cool gas stream entering the gas inlet header and conduits by high temperature gas entering the said zone via the additional gas inlet, whereby any condensate from the relatively cool gas stream entering the said zone will be vaporised and passed back from the said zone ~8598 - into the gas stream,so as to prevent condensation on heat exchanger surfaces of the said constituent from the incoming gas stream to be heated.
Description of the Drawings Indirect gas to gas heat exchange apparatus which are particular preferred embodiments of the invention will be described, by way of example, with reference to the accompanying diagrammatic drawings, wherein:-FIGURE 1 is a cross-sectional illustration of the ductwork leading from an intermediate absorber to the next, or cold indirect gas to gas heat exchanger showing one means of providing a supply of external heat to the gas stream passing to the heat exchanger and also illustrating means providing a quiescent zone in the heat exchanger to provide a hot zone for vapourization of acid mist, the feed from the intermediate absorber being to the shell side of the exchanger, and FIGURE 2 is an alternate embodiment of Figure 1 and for the situation where the gas stream from the intermediate absorber is passed through the tubes of the heat exchanger.
,-' Description The present invention is applied to apparatus employed in processes for the manufacture of sulfuric acid by what is known as multiple contact/multiple absorption system.
The first aspect of the invention involves elimination of corrosion in ductwork employed to transport a gas stream following intermediate absorption of formed sulfur trioxide to a next or cold gas to gas heat exchanger in which the gas stream is to be heated, in whole or in part, to its catalytic initiation temperature for introduction to a next catalytic oxidation stage.
There is also provided means to eliminate settled or settling of sulfuric acid and acid mist within the heat exchanger itself by providing a stagnant or semi-stagnant high temperature zone which will prevent condensation of sulfuric acid as well as agglomerated acid mist on select heat exchanger surfaces to prevent their corrosion.
The problems to be solved may be understood by a consideration of the typical operation of a multiple contact/
multiple absorption sulfuric acid plant. After passing a gas stream containing sulfur dioxide and oxygen through a plurality of catalytic conversion stages with intermediate cooling between stages, the gas stream is passed to an intermediate absorption tower containing, as the absorbent, 98-99 per cent sulfuric acid.
Following intermediate absorption to remove formed sulfur trioxide, the gas stream is passed to a gas to gas heat exchanger to preheat the gas stream, in whole or in part, to a temperature required to initiate conversion of .
....
~ 598 ¦ residu~l sulur dioxide to sulfur trioxlde in one or ¦ morc catalytic conversion stages. This occurs prior to absorp~ion of sulfur trioxide formed from the residual sulfur dioxide in the gas stream in a final absorption sta~e or st~es.
Consider a plant operating at a capacity of about 1200 metric tons per day sulfuric acid and having 8 duct between the intermediate absorber and the next or cold gas to gas heat exchanger employed to reheat, whole or in part, the gas stream following intermediate absorption back to its catalytic initiation temperature for introduction to a next catalytic stage or stages.
For a plant of this capacity, the ductwork, generally having a diameter of about 6.5 feet and a length of about 65 feet, provides an external heat transfer area sub~ect to heat loss by convection and radiation to the surroundings of about 1300 square feet.
Under normal operating conditions process gas flow from an intermediate absorber through the duct is about
2 68,000 standard cubic feet per minute, or about 333,000 lbs. per hour. Considering a normal exit temperature of the gas stream from the intermediate absorber at about 180F, an ambient temperature of 32F and a wind~vel~city of 5 miles per hour normal to the duct, and a specific 2 heat for the gas stream of about 0.24 BTU/lb., the process gas stream under these conditions will cool at least about 4F or more, depending on other climatic conditions such as cloud cover, daytime, nighttime conditions and the like.
In this situation, the approximatc heat loss from
In this situation, the approximatc heat loss from
3 ~he process duct, itself, is about 244 BTU/hr. Isq. f~., :'~'~,;~ ~' ~
~ ~ 8 59 8 .
1 or ~bout 318,000 ~TU's per hour for the entire length of the 65 fo~t duct.
The gas stre~m exiting the intermediate absorption tower is, of course, in equilibrium with the sul~uric acid absorben~. From established vapor phase data for a vapor in equilibrium with the sulfuric acid it was determined that for an absorption tower containing 98.6 per cent H2SO4, that in cooling the gas in the duct to about 4.0F, the vapor pressure of the sulfùric acid would reduce from 0.04 millimeters of mercury to about 0.035 millimeters of mercury.
This corresponds to a partial pressure reduction of about 0.005 millimeters of mercury for the sulfuric acid vapor.
l This heat loss occurs by two means, one is by the cooling of the process gas stream by reduction in the amount of sensible heat of the gas and also heat liberated by latent heat of condensation o sulfuric acid when it is condensed from the gas phase into the liquid phase within 2 the gas stream.
For these conditions, approximately 6.75 lbs. per hour of sulfuric acid can be condensed from the vapor to the liquid phase. The acid condenses out as small droplets and coalesce as sulfuric acid liquid which accumulate on 2 the metallic surfaces of the cool duct as well as in the heat exchanger.
At this rate of condensation, approximately 27 short tons per year of acid will condense and accumulate in the duct or gas to gas heat exchanger. It must be raised to 3 its boiling point or evaporated back into the gas phase ~ 5~8 1 ¦ or removcd from dr~in connections.
¦ If it is ever permitted to form ln the fir~t place, there will be liquid sulfur~c acid of about 98.6% concentratlon ~n contact with metallic surfsces. At this concentration, from known corrosion curves for carbon steel which is a normal material for construction of ductwork and heat exchanger tubes in sulfuric acid plants, as well, the corrosion rate for 98.6~/. sulfuric acid above 300F, is in excess of 200 mils per year which accounts for the ductwork and heat exchanger tubes las~ing only a limited period of time, namely, from about 6 to 8 months under normal plant operating conditions where for tubes and ductwork having a normal wall thickness of 134-120 mils.
With reference now to the Drawings, to overcome the 1 problem of corrosion occurring as a consequence o condensation of sulfuric acid, there is provided to duct~ork 10 leading from the intermediate absorber ~not shown) to the gas to gas heat exchanger 12, a surrounding source of heat capable of transferring into the gas stream an amount of 2 heat sufficient to maintain the gas stream at a temperature above the dew point of sulfuric acid.
As shown in the Drawings, this may consist of a spiral array of tubes 14 in spaced relationship shown or preferably spaced longitudinal tubes positioned axially along the length of and around the perip~ery of duct 10 q~ ~Z~ 10 J
through which steam, Dowtherm(or a similar heat transfer media is passed. Between tubes 14 is an air space 16 heated by the transport of fluid through the tubes 14.
There is provided a reinforcing layer 18 to maintain 3 the gas space 16 typically an air space between tu~es 1 14 and an outside ~nsulation 20 to reduce the heat rc~uirements for the systcm nnd assure that the gas space bctween tubes 14 will always be at an excess tcmperature so that a driving force will exist to maintaln a flow of heat into the gas stream passing through ductwork 10.
In general, the amount of heat provided through tubes 14 should be sùfficient to maintain air space 16 between the tubes 14 and the insulation 20 at least about 100 to about 200F above the average temperature in the gas stream in order to insure sufficient heat will be transferred from the external heating source to the internal surface of the duct carrying the process gas stream in order to prevent condensation of sulfuric acid.
In the alternative, there may be employed an electric resistance heater in place of tubes 14, or simply to surround the duct with a second jacket through which a fluid heating medium is passed. The latter ~acketed method is the least preferred since any fluid heating media may tend to enter and contaminate the sulfuric acid plant gas should leak into the inner duct occur.
By the expedient of maintaining a positive flow of heat into the gas stream to maintain it at a temperature above the dew point of sulfuric acid, between the 2 intermediate absorption tower and heat exchanger 12, corrosion due to condensation of sulfuric acid in both duct 10 and heat exchanger 12 will be eliminated. Accordin~ly, the lifetime of both costly and essential equipment associated with the operation of multiple contact/multiple absorption processes 3 for sulfuric acid manufacture will be preserved.
.' .. ,~
1~ ~1 3598 1 ~hilc the heating systcm surrounding duct 10 will prcvent condcnsation during stcady state operation, cond~ns~tion of sul~ur~ic acid or other condensiblc sulfurou8 compounds may occur during upset conditlons. These may bc dur~ng start up operations or a malfunction in the duct hcating system itself. As a precau~ion, there may be provided dam 19 which surrounds the periphery of duct 10 just beore heat exchanger 12 to collect the condensates passing along duct 10 to prevent ~heir entry lnto heat exchanger 12. Associated with dam 19 is drain 21 having valve 23 the opening of which permits collected acid and the like to be discharged from duct 10.
Drain 21 also serves as a means to monitor the gas flowing through duct 10. If during operation gas issues when valve 23 is opened then the operator is assured that . the inner surface of the ductwork is dry and free of . condensate. If liquid issues, then condensation is indicated which may require supplying additional heat to the duct to prevent condensation or simply that an upset condition ; 20 exists.
Another material problem associa~ed with corrosion in ductwork and heat exchanger is the condensation of sulfuric acid and coalescence and condensation of acid mist on heat exchanger surfaces. Acid mist exists as an aerosol of fine particles of sulfuric acid which may escape a mist eliminator or similar device associated with the effluent of the intermediate absorber.
There is also provided as part o~ this invention a modifie~ cold gas to gas heat cxchanger whose cons~ruction 3 avoids coalescence and settlemcn~ and a~tendant corrosion _ ~ _ ~ 3i598 1 of thc heat exchanEer sur~aces due to drop out o~ coalescence of sulfuric acid and sulfuric acid mist particles.
A typical gas to gas heat exchanger consists of a gas inlet header having a gas inlet and a gas outlet header 5 having a gas outlet. These headers are interconnected by a multiplicity of conduits such as tubes or spaced plates.
An enclosure surrounds the conduits and has a gas inlet and n gas outlet to permit a gas stream passing through the conduits to be heated or cooled by a gas stream flowing along the conduits in the enclosure.
Typical of these gas to gas heat exchangers are shell and tube heat exchangers and plate to plate heat exchangers.
. In many processes such as the multiple contact!
multiple absorption process, the gas stream to be heated contains constituents which are condensibleon exchanger surfaces particularly at the point where the gas stream enters the heat exchanger. As mentioned above,an example is the gas stream returning from the intermedîate absorber.
Fig. l represents the situation ~ihere the gas from the intermediate absorber is passed on the enclosure or shell side on the heat exchanger 1~ in heat exchange with the gas from a catalytic conversion stage passing through tubes 30 of the heat exchanger.
In the situation illustrated in Fig. 2 the gas 2 from the intermediate absorber is passed through the tubes 30 of heat exchanger 12 in heat exchange with the gas from a ____ ____ ____ 3 ____ ;~
~1~85g8 1 converslon sta~e passin~ through the shcll side of hcat exchan~er 12.
~ ith respect to both situations heat cxchanger 12 consists of an ou~er shell 22 and an lnlct 24 or the gas flo~7ing from onc or more of the conversion stages by the prior to or following intermediate absorp~ion, an inlet header 26 for either the gas from a conversion stage or the gas from an intermediate absorber, and an upper transverse tube sheet 28 over ~hich tubes 30 are normally flared. There also exists a lower transverse tube sheet 32 to ~hich the tubes are connected by flares to provide an outlet header 34 ~here the gases exit to additional heat exchanger, or catalyst conversion stage via line 40 whichever may be the case.
In the situation depicted in Fig. 1 the gas returning from the intermediate absorber at a temperature at about 180F is heated on the shell side of heat exchanger 12 by passage through passageways 36 to exit by line 38 at a temperature of about 820F to the next conversion stage, assuming that only one heat exchanger is employed to heat the gas back to its catalytic initiation temperature.
If more than one heat exchanger is employed the gas stream will exit at a lower temperature due to a reduction in heat exchange surface.
A gas stream from a catalytic conversion stage at a temperature from about 950 to about 1150F is passed through the tubes 30 of heat exchanger 12 to transfer heat ¦ to the gas from the intermediate absorber and normally to ¦ its desired catalytic initiation temperature of about 820F.
30 ¦ Because ~he gas stream entering exchanger 12 from s9~
the intermediate absorber may drop in temperature to a point below the condensation temperature of sulfuric acid in the system and any entrained sulfuric acid mist will tend, due to turbulence and the like, to collect, condense or deposit on tubes 30 and transverse tube sheet 32 and initiate their corrosion.
To eliminate the possibility of this occurring there is provided in accordance with the present invention, an open intermediate baffle plate 42. Open baffle plate 42 provides a restriction of the gas flow to and from the corresponding zone between the baffle plate and transverse tube sheet 32 so as to maintain this as a sufficiently stagnant gas zone.
The high temperature of the gas stream flowing through tubes 30 maintains the stagnant gas zone above the condensation temperature of sulfuric acid mist, namely above about 650F.
Any sulfuric acid mists would condense from the gas stream and settle through the openings in open baffle plate 42 into zone 44 will, because of the high temperature in the zone, be vaporized back into the gas stream to prevent a settlement and entrainment of sulfuric acid mist and the associated problems of corrosion in the heat exchanger, the openings in the baffle plate being sufficiently large to permit the flow necessary to produce this action.
Also the quiescent zone 44 in conjunction with the duct 10 provides for a positive transfer of heat into the gas stream to always maintain the sulfuric acid in the ~as stream in the vapor state to prevent thereby its condensation on tubes 30 and transverse tube sheet 32 to eliminate corrosion problems.
In the situation illustrated in Figure 2, the gas from the intermediate absorber enters header 26 for passage through tubes 30 in heat exchange with the gas stream at a temperature from about 780F to about 850F entering 1 from lin~ 2~ lnto tlle sl-ell sidc of tl~c heat exchanger, and ex~ts from hcader 34 by linc 46 to additional heat absorber8 or heat exchan~crs at the approximate temperaturcs shown.
In thls instance, because tubes 30 are normally secured to the transverse tube sheet by a flaring operation there is provided depressions on tube sheet 28 upon which ~ulfuric acid and/or acid mist can collect and condense.
They may also collect and condense on the walls of inlet header 26. To avoid this possibility, a portion of the gas stream from the conversion stage at a temperature of about 780F to about 850F is bypassed by an external insulated line 48 or an equivalent internal bypass ~not shown) to zone 44 in which there is maintained a semi-stagnent zone because of the existence of baffle plate 42. The amount lS of gas bypass to zone 44 is about 3 to about 20% of the total gas flow.
In the alternative a portion 43 of baffle plate 42 can be removed to allow for a flow of high temperature converter gas across tubes 30 to exit 46. The effect however, is the same, namely to maintain the tubes 30, tube sheet 28 and the walls of header 26 to a temperature sufficient to prevent condensation of sulfuric acid and acid mist.
This effectively maintains the upper portions of tubes 30 and more importantly tube plate 28 and the ad3acent walls of header 26 above the condensation temperature of sulfuric acid and acid mist so that any sulfuric acid and acid mist which might tend to settle onto these surfaces will be vaporized back into the gas stream coming 3 from the intermediate absorber. This avoids corrosion of . "-~
1085~i8 1 tubc platc 28, tubes 30 as well as the ad~acent walls of header 26.
As shown in Fig. 2 thc entire heat exchanger is preferably insulatcd to further aid in avoiding the S creation of a problem o~ corrosion.
In the instance of the situation shown in Fig. 2 vaporization of sulfuric acid or acid mist back into the gas stream is done indirectly whereas in the situation shown in Fig. 1 reboiling o the acid mist back into the gas stream is direct. Both, however, are accomplished by providing a high temperature zone to raise those portions - of the heat exchanger subject to normal corrosion due to the condensation of sulfuric acid or coalescent of acid mist free of corrosive condensates.
For a plate- to plate heat exchanger baffles corresponding to baffle 42 would be positioned between plates on the plate sides opposed the plate sides communicating with the headers.
In summary, the principal improvement to the gas to gas heat exchangers resides in providing a zone within the heat exchanger that is maintained by the flow of hot process gas such as converter exit gas above the condensation temperature corrosive constituents such as sulfuric acid and acid mist to prevent corrosion due to condensation 2 or coalescence of these corrosive constituents on hea~
exchanger surfaces.
~ ~ 8 59 8 .
1 or ~bout 318,000 ~TU's per hour for the entire length of the 65 fo~t duct.
The gas stre~m exiting the intermediate absorption tower is, of course, in equilibrium with the sul~uric acid absorben~. From established vapor phase data for a vapor in equilibrium with the sulfuric acid it was determined that for an absorption tower containing 98.6 per cent H2SO4, that in cooling the gas in the duct to about 4.0F, the vapor pressure of the sulfùric acid would reduce from 0.04 millimeters of mercury to about 0.035 millimeters of mercury.
This corresponds to a partial pressure reduction of about 0.005 millimeters of mercury for the sulfuric acid vapor.
l This heat loss occurs by two means, one is by the cooling of the process gas stream by reduction in the amount of sensible heat of the gas and also heat liberated by latent heat of condensation o sulfuric acid when it is condensed from the gas phase into the liquid phase within 2 the gas stream.
For these conditions, approximately 6.75 lbs. per hour of sulfuric acid can be condensed from the vapor to the liquid phase. The acid condenses out as small droplets and coalesce as sulfuric acid liquid which accumulate on 2 the metallic surfaces of the cool duct as well as in the heat exchanger.
At this rate of condensation, approximately 27 short tons per year of acid will condense and accumulate in the duct or gas to gas heat exchanger. It must be raised to 3 its boiling point or evaporated back into the gas phase ~ 5~8 1 ¦ or removcd from dr~in connections.
¦ If it is ever permitted to form ln the fir~t place, there will be liquid sulfur~c acid of about 98.6% concentratlon ~n contact with metallic surfsces. At this concentration, from known corrosion curves for carbon steel which is a normal material for construction of ductwork and heat exchanger tubes in sulfuric acid plants, as well, the corrosion rate for 98.6~/. sulfuric acid above 300F, is in excess of 200 mils per year which accounts for the ductwork and heat exchanger tubes las~ing only a limited period of time, namely, from about 6 to 8 months under normal plant operating conditions where for tubes and ductwork having a normal wall thickness of 134-120 mils.
With reference now to the Drawings, to overcome the 1 problem of corrosion occurring as a consequence o condensation of sulfuric acid, there is provided to duct~ork 10 leading from the intermediate absorber ~not shown) to the gas to gas heat exchanger 12, a surrounding source of heat capable of transferring into the gas stream an amount of 2 heat sufficient to maintain the gas stream at a temperature above the dew point of sulfuric acid.
As shown in the Drawings, this may consist of a spiral array of tubes 14 in spaced relationship shown or preferably spaced longitudinal tubes positioned axially along the length of and around the perip~ery of duct 10 q~ ~Z~ 10 J
through which steam, Dowtherm(or a similar heat transfer media is passed. Between tubes 14 is an air space 16 heated by the transport of fluid through the tubes 14.
There is provided a reinforcing layer 18 to maintain 3 the gas space 16 typically an air space between tu~es 1 14 and an outside ~nsulation 20 to reduce the heat rc~uirements for the systcm nnd assure that the gas space bctween tubes 14 will always be at an excess tcmperature so that a driving force will exist to maintaln a flow of heat into the gas stream passing through ductwork 10.
In general, the amount of heat provided through tubes 14 should be sùfficient to maintain air space 16 between the tubes 14 and the insulation 20 at least about 100 to about 200F above the average temperature in the gas stream in order to insure sufficient heat will be transferred from the external heating source to the internal surface of the duct carrying the process gas stream in order to prevent condensation of sulfuric acid.
In the alternative, there may be employed an electric resistance heater in place of tubes 14, or simply to surround the duct with a second jacket through which a fluid heating medium is passed. The latter ~acketed method is the least preferred since any fluid heating media may tend to enter and contaminate the sulfuric acid plant gas should leak into the inner duct occur.
By the expedient of maintaining a positive flow of heat into the gas stream to maintain it at a temperature above the dew point of sulfuric acid, between the 2 intermediate absorption tower and heat exchanger 12, corrosion due to condensation of sulfuric acid in both duct 10 and heat exchanger 12 will be eliminated. Accordin~ly, the lifetime of both costly and essential equipment associated with the operation of multiple contact/multiple absorption processes 3 for sulfuric acid manufacture will be preserved.
.' .. ,~
1~ ~1 3598 1 ~hilc the heating systcm surrounding duct 10 will prcvent condcnsation during stcady state operation, cond~ns~tion of sul~ur~ic acid or other condensiblc sulfurou8 compounds may occur during upset conditlons. These may bc dur~ng start up operations or a malfunction in the duct hcating system itself. As a precau~ion, there may be provided dam 19 which surrounds the periphery of duct 10 just beore heat exchanger 12 to collect the condensates passing along duct 10 to prevent ~heir entry lnto heat exchanger 12. Associated with dam 19 is drain 21 having valve 23 the opening of which permits collected acid and the like to be discharged from duct 10.
Drain 21 also serves as a means to monitor the gas flowing through duct 10. If during operation gas issues when valve 23 is opened then the operator is assured that . the inner surface of the ductwork is dry and free of . condensate. If liquid issues, then condensation is indicated which may require supplying additional heat to the duct to prevent condensation or simply that an upset condition ; 20 exists.
Another material problem associa~ed with corrosion in ductwork and heat exchanger is the condensation of sulfuric acid and coalescence and condensation of acid mist on heat exchanger surfaces. Acid mist exists as an aerosol of fine particles of sulfuric acid which may escape a mist eliminator or similar device associated with the effluent of the intermediate absorber.
There is also provided as part o~ this invention a modifie~ cold gas to gas heat cxchanger whose cons~ruction 3 avoids coalescence and settlemcn~ and a~tendant corrosion _ ~ _ ~ 3i598 1 of thc heat exchanEer sur~aces due to drop out o~ coalescence of sulfuric acid and sulfuric acid mist particles.
A typical gas to gas heat exchanger consists of a gas inlet header having a gas inlet and a gas outlet header 5 having a gas outlet. These headers are interconnected by a multiplicity of conduits such as tubes or spaced plates.
An enclosure surrounds the conduits and has a gas inlet and n gas outlet to permit a gas stream passing through the conduits to be heated or cooled by a gas stream flowing along the conduits in the enclosure.
Typical of these gas to gas heat exchangers are shell and tube heat exchangers and plate to plate heat exchangers.
. In many processes such as the multiple contact!
multiple absorption process, the gas stream to be heated contains constituents which are condensibleon exchanger surfaces particularly at the point where the gas stream enters the heat exchanger. As mentioned above,an example is the gas stream returning from the intermedîate absorber.
Fig. l represents the situation ~ihere the gas from the intermediate absorber is passed on the enclosure or shell side on the heat exchanger 1~ in heat exchange with the gas from a catalytic conversion stage passing through tubes 30 of the heat exchanger.
In the situation illustrated in Fig. 2 the gas 2 from the intermediate absorber is passed through the tubes 30 of heat exchanger 12 in heat exchange with the gas from a ____ ____ ____ 3 ____ ;~
~1~85g8 1 converslon sta~e passin~ through the shcll side of hcat exchan~er 12.
~ ith respect to both situations heat cxchanger 12 consists of an ou~er shell 22 and an lnlct 24 or the gas flo~7ing from onc or more of the conversion stages by the prior to or following intermediate absorp~ion, an inlet header 26 for either the gas from a conversion stage or the gas from an intermediate absorber, and an upper transverse tube sheet 28 over ~hich tubes 30 are normally flared. There also exists a lower transverse tube sheet 32 to ~hich the tubes are connected by flares to provide an outlet header 34 ~here the gases exit to additional heat exchanger, or catalyst conversion stage via line 40 whichever may be the case.
In the situation depicted in Fig. 1 the gas returning from the intermediate absorber at a temperature at about 180F is heated on the shell side of heat exchanger 12 by passage through passageways 36 to exit by line 38 at a temperature of about 820F to the next conversion stage, assuming that only one heat exchanger is employed to heat the gas back to its catalytic initiation temperature.
If more than one heat exchanger is employed the gas stream will exit at a lower temperature due to a reduction in heat exchange surface.
A gas stream from a catalytic conversion stage at a temperature from about 950 to about 1150F is passed through the tubes 30 of heat exchanger 12 to transfer heat ¦ to the gas from the intermediate absorber and normally to ¦ its desired catalytic initiation temperature of about 820F.
30 ¦ Because ~he gas stream entering exchanger 12 from s9~
the intermediate absorber may drop in temperature to a point below the condensation temperature of sulfuric acid in the system and any entrained sulfuric acid mist will tend, due to turbulence and the like, to collect, condense or deposit on tubes 30 and transverse tube sheet 32 and initiate their corrosion.
To eliminate the possibility of this occurring there is provided in accordance with the present invention, an open intermediate baffle plate 42. Open baffle plate 42 provides a restriction of the gas flow to and from the corresponding zone between the baffle plate and transverse tube sheet 32 so as to maintain this as a sufficiently stagnant gas zone.
The high temperature of the gas stream flowing through tubes 30 maintains the stagnant gas zone above the condensation temperature of sulfuric acid mist, namely above about 650F.
Any sulfuric acid mists would condense from the gas stream and settle through the openings in open baffle plate 42 into zone 44 will, because of the high temperature in the zone, be vaporized back into the gas stream to prevent a settlement and entrainment of sulfuric acid mist and the associated problems of corrosion in the heat exchanger, the openings in the baffle plate being sufficiently large to permit the flow necessary to produce this action.
Also the quiescent zone 44 in conjunction with the duct 10 provides for a positive transfer of heat into the gas stream to always maintain the sulfuric acid in the ~as stream in the vapor state to prevent thereby its condensation on tubes 30 and transverse tube sheet 32 to eliminate corrosion problems.
In the situation illustrated in Figure 2, the gas from the intermediate absorber enters header 26 for passage through tubes 30 in heat exchange with the gas stream at a temperature from about 780F to about 850F entering 1 from lin~ 2~ lnto tlle sl-ell sidc of tl~c heat exchanger, and ex~ts from hcader 34 by linc 46 to additional heat absorber8 or heat exchan~crs at the approximate temperaturcs shown.
In thls instance, because tubes 30 are normally secured to the transverse tube sheet by a flaring operation there is provided depressions on tube sheet 28 upon which ~ulfuric acid and/or acid mist can collect and condense.
They may also collect and condense on the walls of inlet header 26. To avoid this possibility, a portion of the gas stream from the conversion stage at a temperature of about 780F to about 850F is bypassed by an external insulated line 48 or an equivalent internal bypass ~not shown) to zone 44 in which there is maintained a semi-stagnent zone because of the existence of baffle plate 42. The amount lS of gas bypass to zone 44 is about 3 to about 20% of the total gas flow.
In the alternative a portion 43 of baffle plate 42 can be removed to allow for a flow of high temperature converter gas across tubes 30 to exit 46. The effect however, is the same, namely to maintain the tubes 30, tube sheet 28 and the walls of header 26 to a temperature sufficient to prevent condensation of sulfuric acid and acid mist.
This effectively maintains the upper portions of tubes 30 and more importantly tube plate 28 and the ad3acent walls of header 26 above the condensation temperature of sulfuric acid and acid mist so that any sulfuric acid and acid mist which might tend to settle onto these surfaces will be vaporized back into the gas stream coming 3 from the intermediate absorber. This avoids corrosion of . "-~
1085~i8 1 tubc platc 28, tubes 30 as well as the ad~acent walls of header 26.
As shown in Fig. 2 thc entire heat exchanger is preferably insulatcd to further aid in avoiding the S creation of a problem o~ corrosion.
In the instance of the situation shown in Fig. 2 vaporization of sulfuric acid or acid mist back into the gas stream is done indirectly whereas in the situation shown in Fig. 1 reboiling o the acid mist back into the gas stream is direct. Both, however, are accomplished by providing a high temperature zone to raise those portions - of the heat exchanger subject to normal corrosion due to the condensation of sulfuric acid or coalescent of acid mist free of corrosive condensates.
For a plate- to plate heat exchanger baffles corresponding to baffle 42 would be positioned between plates on the plate sides opposed the plate sides communicating with the headers.
In summary, the principal improvement to the gas to gas heat exchangers resides in providing a zone within the heat exchanger that is maintained by the flow of hot process gas such as converter exit gas above the condensation temperature corrosive constituents such as sulfuric acid and acid mist to prevent corrosion due to condensation 2 or coalescence of these corrosive constituents on hea~
exchanger surfaces.
Claims (6)
1. An indirect, gas to gas heat exchanger adapted to heat a relatively cool gas stream containing at least one condensible corrosive constituent to an elevated temperature by indirect heat exchange with a gas at a higher temperature than the condensation temperature of the condensible constituent, the heat exchanger comprising a gas feed header having a gas inlet; a gas outlet header having a gas outlet; a plurality of spaced gas flow conduits communicating with the gas feed header and the gas outlet header to provide a plurality of passageways for flow of one of the gases to participate in indirect heat exchange between the headers; an enclosure surrounding the conduits to provide a passageway for the flow of the other of the gases about the conduits and having a gas inlet and a gas outlet; and means which restrict the gas flow in a zone of the enclosure so as to estab-lish inside the enclosure a sufficiently stagnant or quiescent gas zone adjacent to the point of entry of the relatively cool gas stream into the heat exchanger, the said restrict means being such that the said zone in use is kept at a temperature above the condensation temperature of the said corrosive constituent by the flow of the high temperature gas through the heat exchanger, whereby any condensate from the relatively cool gas stream entering the said zone will be vaporized, the restrict means permitting the vaporized condensate to pass readily back from the said zone into the gas stream, so as to prevent condensation on heat exchanger surfaces of the said constituent from the incoming cool gas stream to be heated.
2. An indirect, gas to gas heat exchanger for preventing condensation of at least one condensible corrosive constituent contained in a relatively cool gas stream which is to be heated to an elevated temperature by a gas at a temperature higher than the condensation temperature of the said constituent, the heat exchanger comprising:
a) a gas feed header having a gas inlet for the high temperature gas:
b) a gas outlet header having a gas outlet for the high temperature gas after being cooled by indirect heat exchange with the relatively cool gas stream:
c) a plurality of spaced gas flow conduits communicating with the gas inlet header and the gas outlet header to permit flow of the high temperature gas from the gas inlet header to the gas outlet header:
d) an enclosure surrounding the spaced gas flow conduits and having an enclosure gas inlet for the relatively cool gas stream, which inlet is spaced from the gas outlet header, and an enclosure gas outlet down-stream from the enclosure gas inlet to permit flow of the said gas stream from the heat exchanger after indirect heat exchange with the high temperature gas stream flowing through the conduits; there being e) an open baffle plate adjacent the enclosure gas inlet, positioned between the enclosure gas inlet and the gas outlet header in spaced relationship from the gas outlet header and the said gas flow conduits, the said open baffle restricting the gas flow in a zone of the enclosure so as to maintain inside the enclosure a sufficiently stagnant or quiescent gas zone adjacent the enclosure gas inlet, the said zone being between the baffle plate and the gas outlet header, and the said zone in use being kept because of the open baffle plate at a temperature above the condensation temperature of the corrosive condensible constituent in the relatively cool gas stream by the flow of the high temperature gas stream through the conduits, the open baffle plate being sufficiently open that any condensate from the relatively cool gas stream entering the said zone will be vaporized and can pass readily back from the said zone into the gas stream, so as to prevent condensation on the heat exchanger surfaces of the said constituent from the incoming gas stream to be heated.
a) a gas feed header having a gas inlet for the high temperature gas:
b) a gas outlet header having a gas outlet for the high temperature gas after being cooled by indirect heat exchange with the relatively cool gas stream:
c) a plurality of spaced gas flow conduits communicating with the gas inlet header and the gas outlet header to permit flow of the high temperature gas from the gas inlet header to the gas outlet header:
d) an enclosure surrounding the spaced gas flow conduits and having an enclosure gas inlet for the relatively cool gas stream, which inlet is spaced from the gas outlet header, and an enclosure gas outlet down-stream from the enclosure gas inlet to permit flow of the said gas stream from the heat exchanger after indirect heat exchange with the high temperature gas stream flowing through the conduits; there being e) an open baffle plate adjacent the enclosure gas inlet, positioned between the enclosure gas inlet and the gas outlet header in spaced relationship from the gas outlet header and the said gas flow conduits, the said open baffle restricting the gas flow in a zone of the enclosure so as to maintain inside the enclosure a sufficiently stagnant or quiescent gas zone adjacent the enclosure gas inlet, the said zone being between the baffle plate and the gas outlet header, and the said zone in use being kept because of the open baffle plate at a temperature above the condensation temperature of the corrosive condensible constituent in the relatively cool gas stream by the flow of the high temperature gas stream through the conduits, the open baffle plate being sufficiently open that any condensate from the relatively cool gas stream entering the said zone will be vaporized and can pass readily back from the said zone into the gas stream, so as to prevent condensation on the heat exchanger surfaces of the said constituent from the incoming gas stream to be heated.
3. An indirect, gas to gas heat exchanger for preventing condensation of at least one condensible corrosive constituent in a relatively cool gas stream which is to be heated to an elevated temperature by a gas at a temperature higher than the condensation temperature of the said constituent, the heat exchanger comprising:
a) a gas feed header having a first gas inlet for the relatively cool gas stream;
b) a gas outlet header having a first gas outlet to permit flow from the heat exchanger of the said gas stream after being heated by indirect heat exchange with the high temperature gas:
c) a plurality of spaced gas flow conduits communicating with the gas inlet header and the gas outlet header to permit flow of the relatively cool gas stream from the gas inlet header to the gas outlet header:
d) an enclosure surrounding the conduits and having an enclosure gas inlet and an enclosure gas outlet for flow of the high temperature gas into and through the heat exchanger in indirect heat exchange with the relatively cool gas stream the enclosure further including an additional inlet for the high temperature gas; there being e) an open baffle plate between the said additional gas inlet and the gas inlet header in spaced relationship from the said gas inlet header and the conduits the said open baffle restricting the gas flow in a zone of the enclosure so as to maintain inside the enclosure a sufficiently stagnant or quiescent gas zone between the baffle plate and the gas inlet header, the said zone in use being kept at a temperature above the conden-sation temperature of the corrosive condensible constituent in the relatively cool gas stream entering the gas inlet header and con-duits by high temperature gas entering the said zone via the additional gas inlet, whereby any condensate from the relatively cool gas stream entering the said zone will be vaporized and passed back from the said zone into the gas stream, so as to prevent condensation on heat exchanger surfaces of the said con-stituent from the incoming gas stream to be heated.
a) a gas feed header having a first gas inlet for the relatively cool gas stream;
b) a gas outlet header having a first gas outlet to permit flow from the heat exchanger of the said gas stream after being heated by indirect heat exchange with the high temperature gas:
c) a plurality of spaced gas flow conduits communicating with the gas inlet header and the gas outlet header to permit flow of the relatively cool gas stream from the gas inlet header to the gas outlet header:
d) an enclosure surrounding the conduits and having an enclosure gas inlet and an enclosure gas outlet for flow of the high temperature gas into and through the heat exchanger in indirect heat exchange with the relatively cool gas stream the enclosure further including an additional inlet for the high temperature gas; there being e) an open baffle plate between the said additional gas inlet and the gas inlet header in spaced relationship from the said gas inlet header and the conduits the said open baffle restricting the gas flow in a zone of the enclosure so as to maintain inside the enclosure a sufficiently stagnant or quiescent gas zone between the baffle plate and the gas inlet header, the said zone in use being kept at a temperature above the conden-sation temperature of the corrosive condensible constituent in the relatively cool gas stream entering the gas inlet header and con-duits by high temperature gas entering the said zone via the additional gas inlet, whereby any condensate from the relatively cool gas stream entering the said zone will be vaporized and passed back from the said zone into the gas stream, so as to prevent condensation on heat exchanger surfaces of the said con-stituent from the incoming gas stream to be heated.
4. A heat exchanger as claimed in claim 3, wherein a by-pass conduit connects the additional gas inlet to the enclosure gas inlet.
5. A heat exchanger as claimed in claim 4, wherein the by-pass conduit is located internally of the enclosure.
6. A heat exchanger as claimed in any one of claims 1 to 3, in which the relatively cool gas stream is the effluent of a sulfur trioxide absorber in a multiple contact/multiple absorption process for sulfuric acid manufacture which contains at least condensible sulfuric acid and the high temperature gas stream is a gas stream exiting a stage for the conversion of sulfur dioxide to sulfur trioxide.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA333,500A CA1108598A (en) | 1973-04-23 | 1979-08-07 | Indirect gas to gas heat exchanger for use with a gas stream containing condensible corrosive constituents |
Applications Claiming Priority (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US35336473A | 1973-04-23 | 1973-04-23 | |
| CA196,729A CA1079481A (en) | 1973-04-23 | 1974-04-03 | Process and apparatus for prevention of corrosion in a multiple contact-multiple absorption sulfuric acid manufacturing operation |
| CA333,500A CA1108598A (en) | 1973-04-23 | 1979-08-07 | Indirect gas to gas heat exchanger for use with a gas stream containing condensible corrosive constituents |
| US353,364 | 1989-05-17 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1108598A true CA1108598A (en) | 1981-09-08 |
Family
ID=27163407
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA333,500A Expired CA1108598A (en) | 1973-04-23 | 1979-08-07 | Indirect gas to gas heat exchanger for use with a gas stream containing condensible corrosive constituents |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1108598A (en) |
-
1979
- 1979-08-07 CA CA333,500A patent/CA1108598A/en not_active Expired
Similar Documents
| Publication | Publication Date | Title |
|---|---|---|
| CA1168593A (en) | Exhaust gas treatment method and apparatus | |
| EP0130967B1 (en) | Heat recovery from concentrated sulfuric acid | |
| US4031862A (en) | Economizer | |
| AU2012239081B2 (en) | Process for production of sulphuric acid | |
| JPH0446887B2 (en) | ||
| CN101626977A (en) | Process for the production of sulfuric acid | |
| NL8220242A (en) | HEAT EXCHANGER WITH HEATED PIPES. | |
| CA1079481A (en) | Process and apparatus for prevention of corrosion in a multiple contact-multiple absorption sulfuric acid manufacturing operation | |
| US5094826A (en) | Method and arrangement for the denitrification and desulfurization of hot waste gases, particularly from furnaces | |
| EP3551578B1 (en) | A process for increasing the concentration of sulfuric acid and equipment for use in the process | |
| US4470449A (en) | Economizer arrangement | |
| CA2845912A1 (en) | Heat recovery system | |
| CA1108598A (en) | Indirect gas to gas heat exchanger for use with a gas stream containing condensible corrosive constituents | |
| CN106082136A (en) | A kind of sulfur recovery tail gas condensation separation equipment | |
| CN206793088U (en) | A kind of sulfuric acid vapor condenser of wet method Sulphuric acid | |
| WO2011137506A1 (en) | Shell and tube heat exchangers | |
| CN208990543U (en) | White integral process system is taken off for steel mill's sintering flue gas denitration | |
| CN109562939B (en) | Process for producing sulfur trioxide | |
| CA1205614A (en) | Process and an apparatus for the preparation of sulfuric acid | |
| US4391617A (en) | Process for the recovery of vaporized sublimates from gas streams | |
| US4478809A (en) | Economizer arrangement | |
| US4400369A (en) | Apparatus and process for recovering heat emanating from the shell of a thermal reactor in a sulfur recovery plant | |
| US20110272124A1 (en) | Shell And Tube Heat Exchangers | |
| US2142855A (en) | Sulphuric acid contact process | |
| RU2373989C2 (en) | Method of simultaneous cleaning and utilisation of flue gases, and multiblock plant for implementation thereof |
Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| MKEX | Expiry |