CA1079481A - Process and apparatus for prevention of corrosion in a multiple contact-multiple absorption sulfuric acid manufacturing operation - Google Patents
Process and apparatus for prevention of corrosion in a multiple contact-multiple absorption sulfuric acid manufacturing operationInfo
- Publication number
- CA1079481A CA1079481A CA196,729A CA196729A CA1079481A CA 1079481 A CA1079481 A CA 1079481A CA 196729 A CA196729 A CA 196729A CA 1079481 A CA1079481 A CA 1079481A
- Authority
- CA
- Canada
- Prior art keywords
- gas
- heat exchanger
- gas stream
- sulfuric acid
- 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
- QAOWNCQODCNURD-UHFFFAOYSA-N Sulfuric acid Chemical compound OS(O)(=O)=O QAOWNCQODCNURD-UHFFFAOYSA-N 0.000 title claims abstract description 149
- 238000010521 absorption reaction Methods 0.000 title claims abstract description 34
- 238000000034 method Methods 0.000 title claims abstract description 29
- 230000007797 corrosion Effects 0.000 title claims abstract description 28
- 238000005260 corrosion Methods 0.000 title claims abstract description 28
- 238000004519 manufacturing process Methods 0.000 title claims abstract description 8
- 230000002265 prevention Effects 0.000 title description 2
- 238000009833 condensation Methods 0.000 claims abstract description 45
- 230000005494 condensation Effects 0.000 claims abstract description 45
- RAHZWNYVWXNFOC-UHFFFAOYSA-N Sulphur dioxide Chemical compound O=S=O RAHZWNYVWXNFOC-UHFFFAOYSA-N 0.000 claims abstract description 33
- 239000002253 acid Substances 0.000 claims abstract description 33
- 230000003197 catalytic effect Effects 0.000 claims abstract description 23
- 239000007789 gas Substances 0.000 claims description 322
- AKEJUJNQAAGONA-UHFFFAOYSA-N sulfur trioxide Chemical compound O=S(=O)=O AKEJUJNQAAGONA-UHFFFAOYSA-N 0.000 claims description 42
- 238000006243 chemical reaction Methods 0.000 claims description 30
- 238000010438 heat treatment Methods 0.000 claims description 13
- 239000012530 fluid Substances 0.000 claims description 10
- 238000001816 cooling Methods 0.000 claims description 7
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 claims description 5
- 229910052760 oxygen Inorganic materials 0.000 claims description 5
- 239000001301 oxygen Substances 0.000 claims description 5
- 238000009413 insulation Methods 0.000 claims description 4
- 230000003014 reinforcing effect Effects 0.000 claims description 4
- 238000013022 venting Methods 0.000 claims description 4
- 235000011149 sulphuric acid Nutrition 0.000 claims description 3
- 239000012808 vapor phase Substances 0.000 claims description 3
- 239000001117 sulphuric acid Substances 0.000 claims description 2
- 239000003595 mist Substances 0.000 abstract description 25
- 239000006096 absorbing agent Substances 0.000 abstract description 21
- 230000000977 initiatory effect Effects 0.000 abstract description 7
- 239000000470 constituent Substances 0.000 description 10
- 230000002745 absorbent Effects 0.000 description 4
- 239000002250 absorbent Substances 0.000 description 4
- 238000004581 coalescence Methods 0.000 description 4
- 238000012546 transfer Methods 0.000 description 4
- 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
- 239000007791 liquid phase 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
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Chemical compound O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 2
- WWYNJERNGUHSAO-XUDSTZEESA-N (+)-Norgestrel Chemical compound O=C1CC[C@@H]2[C@H]3CC[C@](CC)([C@](CC4)(O)C#C)[C@@H]4[C@@H]3CCC2=C1 WWYNJERNGUHSAO-XUDSTZEESA-N 0.000 description 1
- 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
- 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
- 238000012423 maintenance Methods 0.000 description 1
- 230000007257 malfunction Effects 0.000 description 1
- 239000000463 material Substances 0.000 description 1
- 238000012986 modification Methods 0.000 description 1
- 230000004048 modification 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
- 238000011144 upstream manufacturing Methods 0.000 description 1
- 230000008016 vaporization Effects 0.000 description 1
Classifications
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
- C01B17/00—Sulfur; Compounds thereof
- C01B17/69—Sulfur trioxide; Sulfuric acid
- C01B17/74—Preparation
- C01B17/76—Preparation by contact processes
- C01B17/80—Apparatus
- C01B17/806—Absorbers; Heat exchangers
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
- C01B17/00—Sulfur; Compounds thereof
- C01B17/69—Sulfur trioxide; Sulfuric acid
- C01B17/74—Preparation
- C01B17/76—Preparation by contact processes
- C01B17/765—Multi-stage SO3-conversion
- C01B17/7655—Multi-stage SO3-conversion with intermediate absorption
Landscapes
- Chemical & Material Sciences (AREA)
- Organic Chemistry (AREA)
- Inorganic Chemistry (AREA)
- Physical Or Chemical Processes And Apparatus (AREA)
- Heat-Exchange Devices With Radiators And Conduit Assemblies (AREA)
- Chimneys And Flues (AREA)
- Separation By Low-Temperature Treatments (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 initiation temperature in a multiple contact/
multiple absorption process for the manufacture of sulfuric 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 exchange(s).
There may also be provided in the heat exchanger immediately following intermediate absorption, a zone maintained at a temperature above the condensation temperature of sulfuric acid to prevent condensation of acid mist on heat exchanger surfaces.
multiple absorption process for the manufacture of sulfuric 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 exchange(s).
There may also be provided in the heat exchanger immediately following intermediate absorption, a zone maintained at a temperature above the condensation temperature of sulfuric acid to prevent condensation of acid mist on heat exchanger surfaces.
Description
10794~1 PROCESS AND APPARATUS FOR PREVENTION OF CORROSION IN
A MULTIPLE CONTACT-MULTIPLE ABSORPTION SULFURIC ACID
MANUFACTURING OPERATION
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 initiation temperature.
' When the residual gas stream is brought to its catalytic initiation temperature, it is introduced to a next catalytic stage or stages 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 achievable. It is, for instance, feasible to achieve a 99.9 per cent or more conversion of sulfur dioxide to sulfur - 1 - ~
.
iO79481 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 temperature 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 - .
condlensation of acid mist where acid mist eliminators 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 vapor to react with sulfur trioxide formed later in the process. The free particles of acid mist which exist as an aerosol can coalesce due to the turbulence of flow in the duct-work into the cold heat exchanger and then adhere to the inner surfaces of the heat exchanger leading to the 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.
As yet no practical solution to these problems has been presented.
Summary of the Invention , According to the present invention there is provided a multiple contact/multiple absorption process for the manufacture of sulfuric acid and oleum in which a gas stream containing sulfur dioxide and oxygen is passed through a plurality of catalytic contact stages wherein sulfur dioxide ~()794~31 ls converted to sulfur trioxide with cooling between contact stages to remove the exothermic heat of reaction, followed by passing the gas stream to a~ least one absorption tower where the formed sulfur trioxide is separated from the gas stream by contact-with at least sulfuric acid and then passed by a duct work to agas to gas heat exchanger where the gas stream is heated at least in part to a temperature required for introduction to at least one additional catalytic contact stage where the residual sulfur dioxidé is converted to sulfur trioxide following which the formed sulfur trioxide~ extracted from the gas stream prior to venting the gas stream to the atmosphere, the improvement which comprises maintaining a positive flow of external heat into the duct work leading from an absorption tower to the gas to gas heat exchanger in an amount sufficient to maintain the gas stream above the dew point of sulphuric acid carried by the gas stream to maintain the sulfuric acid in the vapor phase to prevent corrosion of the duct work due to condensation of sulfuric acid on the surface of the duct work, the positive flow of external heat into the duct work being provided by the flow of a fluid heating media through.an array of spaced tubes in contact with the duct work, said tubes beinq surrounded by a reinforcing layer and an outside insulation to .- form gas occupied spaces between.said tubes, the amount of fluid heating media flowing through ~aid tube~ being sufficient to maintain the gas occupied spaces between the tubes from about 100F to about 200F above the temperature of the gas stream : passing through said duct work.
Also in accordance with the invention there is pro-vided a gas to gas heat exchanger consis30 header having a gas inlet, a gas outlet header having.a gas , .
"
1079~81 outlet, a plurality of spaced gas flow conduits communicating with said gas feed header and said gaS outlet header to provide a plurality of passageways for a ~low of first gas strean bet-ween said headers and an enclosure surrounding said conduits to provide a passageway for the flow of gas about said conduits for indirect heat exchange and having a gas inlet and a gas outlet and wherein said heat exchanger is employed to heat a relatively cool gas stream containing condensible corrosive constituents to an elevated temperature by indirect heat exchange with a high temperature gas stream the improvement which comprises providing a high temperature gas zone adjacent to the point of entry of said relatively cool gas stream into said heat exchanger above the condensation temperature of said cor~osivé constituents by the flow of the high temperature gas stream through said heat exchanger.
Further in accordance with the invention there is .~ provided a gas to gas heat exchanger for preventing condensation of condensible corrosive constituents contained in a relatively cool gas stream from one source to be heated to an elevated temperature by a high temperatuxe gas stream from another source which comprises:
(a) a gas feed header having a gas inlet for the high temperature gas stream;
` (b) a gas outlet header having a gas outlet for the high temperature gas strëam after being cooled by indirect heat exchange with the relatively cool gas stream;
(c) a plurality of spaced gas flow conduits communic-ating with said gas inlet header and said gas outler header to permit the flow of the high temperature gas stream from said gas inlet header to said gas outlet header;
107948~
(d) an enclosure surrounding said plurality of spaced gas flow conduits and having a gas inlet for said relati~ely cool gas stream spaced from said gas outlet header and a gas outlet upstream from said gas inlet to permit the flow of the relatively cool gas stream in indirect heat exchange with high temperature gas stream flowing through said conduits;.
(e) a baffle plate positioned between the gas inlet for said relatively cool gas stream and said gas outlet header in spaced relationship from said gas outlet header and said gas flow conduits to maintain a gas zone between said baffle plate and said gas outlet header at a temperature above the condensation temperature of the corrosive condensible constituents in the relatively cool gas stream, the temperature of the gas zone being maintained above the condensation temperature of the corrosive constituents by the flow of said high temperature gas stream through said conduits.
Further in accordance with the invention there is provided a gas to gas heat exchanger for preventing condensation of condensible corrosive constituents in a relatively cool gas stream from one source to be heated to an elevated temperature by a high temperature gas stream from another source which comprises: .
(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 for the relatively cool gas stream after being heated by indirect heat.exchange with the high temperature gas stream;
. (c) a plurality of spaced gas flow conduits ::~ communicating with said gas inlet header and said gas outlet header to permit the flow of the relatively cool gas from said gas inlet header to said gas outlet header;
D
107948~
(d) ~n enclosure surroundin~ said plurality of spaced gas flow conduits and having a second gas inlet for said hot gas stream and a spaced second gas outlet to perm:it the flow of the hot gas stream in indirect heat exchange S with relatively cool gas stream flowing through said conduits;
(e) a baffle plate positioned between the second gas inlet for the high temperature gas stream and said gas inlet header in spaced relationship from said gas inlet header and said gas flow conduits to maintain a high temperature zone between said baffle plate and said gas inlet header at a temperature above the condensation temperature of the corrosive condensible constituents in the relatively cool gas stream entering said gas inlet header and conduits by the flow of a portion of said high temperature gas stream by a by-pass conduit to the said high temperature zone.
Further in accordance with the invention there i8 provided a multiple contact/multiple absorption process for the manufacture of sulfuric acid and oleum in which a gas stream containing sulfur dioxide and oxygen is passed through a plurality of catalytic contact stages wherein sulfur dioxide is converted to sulfur trioxide with cooling between contact stages to remove the exothermic heat of reaction, followed by the passing of the gas stream to at least an absorption tower where the formed sulfur trioxide is separated from the gas stream by contact with at least sulfuric acid and then passed to a gas heat exchanger where the gas stream is heated~at least the temperature required for introduction to at least an additional catalytic contact stage where the residual sulfur dioxide is converted to sulfur trioxide following which the formed sulfur trioxide ~ .
10794~31 is extractcd from the gas stream prior to venting the gas stream to the atmosphere, the improved method of preventing corrosion in the gas to gas heat exchanger which comprises mai~taining~said gas to gas heat exchanger a high temperature S zone positioned at approximately the point of introduction of the gas stream from the absorption tower to said gas to gas heat exchanger and maintaining said zone at a temperature above the condensation temperature of sulfuric acid by the flow of a high temperature gas stream from a conversion stage through said high temperature zone to prevent condensation of sulfuric acid on heat exchanger surfaces.
It has been found that corrosion in the ductwork leading from an intermediate absorber employed in the ~rocess, to the next heat exchanger employed for preheating the gas to its cataly-tic initiation temperature can be eliminated by providing exter-nal heat to the ductwork in an amount sufficient to maintain the temperature of the process gas stream above the dew point of sulfuric acid or the sulfur trioxide and water contained in the gas stream.
Where acid mist also presents a problem of corrosion in the heat exchanger following intermediate absorption, it is proposed to employ at the inlet end an open zone where there is maintained a body of relatively stagnant flowing gas at a temp-erature sufficient to maintain heat exchanger surfaces above the condensation temperature of sulfuric acid to prevent condensation of sulfuric acid and acid mist on heat exchanger surfaces.
The Drawings , Fig. 1 is a cross-se~tional illustration of the duct-work leading from an intermediate absorber to the shell side of the next, or cold heat exchanger showing one meanC of providing .
~07948~
a supply of external heat to the gas stream passing to the heat exchanger and illustrat.ing the quiescent zone in the heat exchanger to provide a hot zone for vapourization of acid ~list.
Fig. 2 is an alternate embodiment of Fig. 1 and for the situation where the gas stream is passed through the tubes of the heat exchanger.
'." ~ ' ' ''~'~'' . .: ' .
_ g _ ;', ~
` D
Description According to the present invention, there is provided a modification to the process for 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 stagnent or semi-stagnent 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 1 0 7~ ~ 81 residual sulfur dioxide to sulfur trloxide in one or morc catalytic convcrsion stages. This occurs pr~or to absorption of sulfur trioxide formcd from the residual sulfur dioxide in the ~as stream in a final absorption sta~e or st~ges.
Consider a plant operating at a capacity of about 1200 metric tons per day sulfuric acid and having a 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 intermedi~te absorption back ~to its catalytic initlation temperature for introduction to a next ca~alytic 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 subject 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 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 velocity of 5 miles per hour normal to the duct, and a specific 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, ni~httime conditions and the like.
In this situation, the approximate heat loss from the process duct, itself, is about 244 BTV/hr./sq. ft., . '.
~ 10 7 9 4 8 l or about 318,000 BTU's per hour for the entire length of the 65 foot duct.
The gas stream exlt~ng the intermedlate absorption tower is, of course, in equilibrium with the sulfuric acid absorbent. From established vapor phase data for a vapor in equilibrium with the sulfuric acid it was detenmined 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 sulfuric acid would reduce from 0.04 mill~meters of mercury to absut 0.035 millimeters of mercury.
This corresponds to a partial pressure reduction of about 0.005 millimeters of mercury for the sulfuric acid vapor.
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 of sulfuric acid when it is condensed from the gas phase into the liquid phase within 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 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 its boiling point or evaporated back into the gas phase ~, .
~ 07 9 4 8 1 1 or r~mov~d from draln connections.
If lt is ever permltted to fonm in t~e first place, there will be liquid sulfurlc acid of ~bout 98.6% concentration in contact with metallic surfaces. At this concentration, from known corrosion curves for carbon steel which is a . normal mater~al for construction of ductwork and heat exchanger tubes in sulfuric acid plants, as well, the corrosion rate for 98.6% sulfuric ac~d above 300F, is in excess of 200 mils per year which accounts for the ductwork - 10 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 problem of corrosion occurring as a consequence o condensation of sulfuric acid, there is provided to ductwork . 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 beat sufficient to maintain the gas stream at a . temperature above the dew point of sul~uric ac~d.
. 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 periphery of duct 10 through which steam, Dowtherm or a æimilar heat transfer media is passed. Between tubes 14 is an air space 16 heated by the transport of fluid through the tubes 14.
m ere is provided a reinforcing layer 18 to maintain the gas space 16 typically an air space between tubes D
~0 7 9 4~ 1 1 14 and an outside lnsulation 20 to reduce the hest rcqulrements for the system md assure that the gas space between tubes 14 will always be at an excess temperature so that 8 driving force will exist to maintain a flow of heat into the gas stream passing through ductw~rk 10.
In general, the amount of heat provided through tubes 14 should be sufficient 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 l l~
condensation of suluric acid.
In the alternative, there may be employed an electric resi~tance 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 jacketed 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 ~ntermediate absorption tower and~heat exchanger 12, corrosion due to condensation of sulfuric acid in both duct 10 and heat ; exchanger 12 will be elimina~ed. Accordingly, the lifetime of both costly and essential equipment associated with the operation of multiple contact/multiple absorption processes .3 for sulfuric acid manufacture will b~ preserved.
~ 07 9 ~ ~1 1l~ile the heatin~ systcm surrounding duct 10 will prevent condcnsation durin~ steady state operatlont condens~tion of sulfuric acid or other condensible sulfurous compounds may occur during upset conditions. These may be S durin~ start up operations or a malfunction in the duct heating system itself. As a precau~ion, there may be provided dam 19 which surrounds the periphery of duct 10 just before heat exchanger 12 to collect the condensates passing along duct 10 to prevent their entry into heat 10exchanger 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 l ~lowing 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 preven~ condensation or simply that an upset condition exists.
Another material pxoblem 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 associa~ed wi~h the effluent o~ the intermediate absorber.
There is also provided as part o this invention a modified cold gas to gas heat oxchflnger whose cons~ruction avoids coalescence and settlement and attenclant corrosion .
~ . .. . . ..
10 7 9 ~ ~ ~
1 of thc h~at exchanger surfaces due tQ drop out or coalescence of sulfuric acicl and sulfuric acid mist particles. ~ --A typ~cal gas to gas heat exchanger consists of a gas lnle~ hca~er having a gas inlet and a gas outlet header S 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 a gas outlet to permit a gas stream passing through the conduits t-o 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 ~urfaces particularly at the point where the gas stream enters the heat exchanger. As mentioned above,an example is the gas stream returning from the intermediate absorber.
Fig. l represents the situation where the gas from the intermediate absorber is passed on the enclosure or shell side on the heat exchanger l~ 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 ~5 from the intermediate absorber is passed through the tubes 30 of heat exchanger 12 in he~t exchange ~ith the gas from a ____ ____ ___ 3 ____ /C
1 0 79 4~1 1 conversion sta~e passing throu~h the shell side of heat exchanger 12.
With respect to both situations heat ~xchanger 12 consists of an ou~er shell 22 and an inlct 24 for the gas ~lo~ling from on~ or more of the conversion stages by the prior to or follo~ing intermediate absorption, an inlet header 26 for either the gas from a conversion stage or the gas from an intermediate absorber, and an upper l transverse tube sheet 28 over which tubes 30 axe normally flared. There also exists a lower transverse tube sheet 32 to which the tubes are connected by flares to provide an outlet header 34 where 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 abou~ 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 ~50 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.
Because the gas stream entcring exchanger 12 from - /;i' D
10 79 4~ 1 1 the intcrmcdiate absorber may drop in temperature to a point below the condensation temperature of sulfuric acid in the ,ystcm ~nd any en~rained sulfurlc acid mist wlll tend9 <3ue 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. Baffle plate 42 maintains a stagnent gas zone between the baffle plate and transverse tube sheet 32~ The high temperature of the gas stream flowing through tubes 30 maintains the stagnent gas zone above the condensation temperature of sulfuric acid mist, namely above about 650F. Any sulfuric acid mists lS would condense from the gas stream and settle through the openings in baffle plate 42 into zone 44 will, because of the high temperature in the zone, be vapori~ed 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.
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 gas 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 Fig. 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 107 9 4~ 1 1 ¦ from line 24 into tl~c shell side of thc heat exchanger, and ¦ lexlts from hcader 34 by line 46 to additional heat absorbers ¦ or heat exchan~ers at the ~pproxlmate temperatures shown.
l In this instance, because tubes 30 are nonmally 5 ¦ secured to the transverse tube sheet by a flaring operation ¦ there is provided depressions on tube sheet 28 upon whlch ¦ sulfuric acid and/or acid mist can collect and condense.
¦ They may also collec~ and condense on the walls of inlet l header 26. To avoid this possibility, a portion of the 10¦ 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 of gas bypass to zone 44 is about 3 to about 20% of the total gas flow.
ln 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 ~ufficient 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 ad~acent 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 from the intermediate absorber. This avoids corrosion of ~ 10 7 9 ~ ~1 1 tubc platc 28, tubes 30 as well as the ~d~acent walls of header 26.
As shown in F~g. 2 the entire heat exchanger is pre~erably insulated to further aid in avoiding the creation of a problem of 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 of 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 ~uch as converter exit gas above the condensation temperature corrosive constituents such as sulfuric acid and acid mist to prevent corrosion due to condensation or coalescence of these corrosive constituents on heat exchanger surfaces.
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A MULTIPLE CONTACT-MULTIPLE ABSORPTION SULFURIC ACID
MANUFACTURING OPERATION
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 initiation temperature.
' When the residual gas stream is brought to its catalytic initiation temperature, it is introduced to a next catalytic stage or stages 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 achievable. It is, for instance, feasible to achieve a 99.9 per cent or more conversion of sulfur dioxide to sulfur - 1 - ~
.
iO79481 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 temperature 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 - .
condlensation of acid mist where acid mist eliminators 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 vapor to react with sulfur trioxide formed later in the process. The free particles of acid mist which exist as an aerosol can coalesce due to the turbulence of flow in the duct-work into the cold heat exchanger and then adhere to the inner surfaces of the heat exchanger leading to the 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.
As yet no practical solution to these problems has been presented.
Summary of the Invention , According to the present invention there is provided a multiple contact/multiple absorption process for the manufacture of sulfuric acid and oleum in which a gas stream containing sulfur dioxide and oxygen is passed through a plurality of catalytic contact stages wherein sulfur dioxide ~()794~31 ls converted to sulfur trioxide with cooling between contact stages to remove the exothermic heat of reaction, followed by passing the gas stream to a~ least one absorption tower where the formed sulfur trioxide is separated from the gas stream by contact-with at least sulfuric acid and then passed by a duct work to agas to gas heat exchanger where the gas stream is heated at least in part to a temperature required for introduction to at least one additional catalytic contact stage where the residual sulfur dioxidé is converted to sulfur trioxide following which the formed sulfur trioxide~ extracted from the gas stream prior to venting the gas stream to the atmosphere, the improvement which comprises maintaining a positive flow of external heat into the duct work leading from an absorption tower to the gas to gas heat exchanger in an amount sufficient to maintain the gas stream above the dew point of sulphuric acid carried by the gas stream to maintain the sulfuric acid in the vapor phase to prevent corrosion of the duct work due to condensation of sulfuric acid on the surface of the duct work, the positive flow of external heat into the duct work being provided by the flow of a fluid heating media through.an array of spaced tubes in contact with the duct work, said tubes beinq surrounded by a reinforcing layer and an outside insulation to .- form gas occupied spaces between.said tubes, the amount of fluid heating media flowing through ~aid tube~ being sufficient to maintain the gas occupied spaces between the tubes from about 100F to about 200F above the temperature of the gas stream : passing through said duct work.
Also in accordance with the invention there is pro-vided a gas to gas heat exchanger consis30 header having a gas inlet, a gas outlet header having.a gas , .
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1079~81 outlet, a plurality of spaced gas flow conduits communicating with said gas feed header and said gaS outlet header to provide a plurality of passageways for a ~low of first gas strean bet-ween said headers and an enclosure surrounding said conduits to provide a passageway for the flow of gas about said conduits for indirect heat exchange and having a gas inlet and a gas outlet and wherein said heat exchanger is employed to heat a relatively cool gas stream containing condensible corrosive constituents to an elevated temperature by indirect heat exchange with a high temperature gas stream the improvement which comprises providing a high temperature gas zone adjacent to the point of entry of said relatively cool gas stream into said heat exchanger above the condensation temperature of said cor~osivé constituents by the flow of the high temperature gas stream through said heat exchanger.
Further in accordance with the invention there is .~ provided a gas to gas heat exchanger for preventing condensation of condensible corrosive constituents contained in a relatively cool gas stream from one source to be heated to an elevated temperature by a high temperatuxe gas stream from another source which comprises:
(a) a gas feed header having a gas inlet for the high temperature gas stream;
` (b) a gas outlet header having a gas outlet for the high temperature gas strëam after being cooled by indirect heat exchange with the relatively cool gas stream;
(c) a plurality of spaced gas flow conduits communic-ating with said gas inlet header and said gas outler header to permit the flow of the high temperature gas stream from said gas inlet header to said gas outlet header;
107948~
(d) an enclosure surrounding said plurality of spaced gas flow conduits and having a gas inlet for said relati~ely cool gas stream spaced from said gas outlet header and a gas outlet upstream from said gas inlet to permit the flow of the relatively cool gas stream in indirect heat exchange with high temperature gas stream flowing through said conduits;.
(e) a baffle plate positioned between the gas inlet for said relatively cool gas stream and said gas outlet header in spaced relationship from said gas outlet header and said gas flow conduits to maintain a gas zone between said baffle plate and said gas outlet header at a temperature above the condensation temperature of the corrosive condensible constituents in the relatively cool gas stream, the temperature of the gas zone being maintained above the condensation temperature of the corrosive constituents by the flow of said high temperature gas stream through said conduits.
Further in accordance with the invention there is provided a gas to gas heat exchanger for preventing condensation of condensible corrosive constituents in a relatively cool gas stream from one source to be heated to an elevated temperature by a high temperature gas stream from another source which comprises: .
(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 for the relatively cool gas stream after being heated by indirect heat.exchange with the high temperature gas stream;
. (c) a plurality of spaced gas flow conduits ::~ communicating with said gas inlet header and said gas outlet header to permit the flow of the relatively cool gas from said gas inlet header to said gas outlet header;
D
107948~
(d) ~n enclosure surroundin~ said plurality of spaced gas flow conduits and having a second gas inlet for said hot gas stream and a spaced second gas outlet to perm:it the flow of the hot gas stream in indirect heat exchange S with relatively cool gas stream flowing through said conduits;
(e) a baffle plate positioned between the second gas inlet for the high temperature gas stream and said gas inlet header in spaced relationship from said gas inlet header and said gas flow conduits to maintain a high temperature zone between said baffle plate and said gas inlet header at a temperature above the condensation temperature of the corrosive condensible constituents in the relatively cool gas stream entering said gas inlet header and conduits by the flow of a portion of said high temperature gas stream by a by-pass conduit to the said high temperature zone.
Further in accordance with the invention there i8 provided a multiple contact/multiple absorption process for the manufacture of sulfuric acid and oleum in which a gas stream containing sulfur dioxide and oxygen is passed through a plurality of catalytic contact stages wherein sulfur dioxide is converted to sulfur trioxide with cooling between contact stages to remove the exothermic heat of reaction, followed by the passing of the gas stream to at least an absorption tower where the formed sulfur trioxide is separated from the gas stream by contact with at least sulfuric acid and then passed to a gas heat exchanger where the gas stream is heated~at least the temperature required for introduction to at least an additional catalytic contact stage where the residual sulfur dioxide is converted to sulfur trioxide following which the formed sulfur trioxide ~ .
10794~31 is extractcd from the gas stream prior to venting the gas stream to the atmosphere, the improved method of preventing corrosion in the gas to gas heat exchanger which comprises mai~taining~said gas to gas heat exchanger a high temperature S zone positioned at approximately the point of introduction of the gas stream from the absorption tower to said gas to gas heat exchanger and maintaining said zone at a temperature above the condensation temperature of sulfuric acid by the flow of a high temperature gas stream from a conversion stage through said high temperature zone to prevent condensation of sulfuric acid on heat exchanger surfaces.
It has been found that corrosion in the ductwork leading from an intermediate absorber employed in the ~rocess, to the next heat exchanger employed for preheating the gas to its cataly-tic initiation temperature can be eliminated by providing exter-nal heat to the ductwork in an amount sufficient to maintain the temperature of the process gas stream above the dew point of sulfuric acid or the sulfur trioxide and water contained in the gas stream.
Where acid mist also presents a problem of corrosion in the heat exchanger following intermediate absorption, it is proposed to employ at the inlet end an open zone where there is maintained a body of relatively stagnant flowing gas at a temp-erature sufficient to maintain heat exchanger surfaces above the condensation temperature of sulfuric acid to prevent condensation of sulfuric acid and acid mist on heat exchanger surfaces.
The Drawings , Fig. 1 is a cross-se~tional illustration of the duct-work leading from an intermediate absorber to the shell side of the next, or cold heat exchanger showing one meanC of providing .
~07948~
a supply of external heat to the gas stream passing to the heat exchanger and illustrat.ing the quiescent zone in the heat exchanger to provide a hot zone for vapourization of acid ~list.
Fig. 2 is an alternate embodiment of Fig. 1 and for the situation where the gas stream is passed through the tubes of the heat exchanger.
'." ~ ' ' ''~'~'' . .: ' .
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Description According to the present invention, there is provided a modification to the process for 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 stagnent or semi-stagnent 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 1 0 7~ ~ 81 residual sulfur dioxide to sulfur trloxide in one or morc catalytic convcrsion stages. This occurs pr~or to absorption of sulfur trioxide formcd from the residual sulfur dioxide in the ~as stream in a final absorption sta~e or st~ges.
Consider a plant operating at a capacity of about 1200 metric tons per day sulfuric acid and having a 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 intermedi~te absorption back ~to its catalytic initlation temperature for introduction to a next ca~alytic 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 subject 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 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 velocity of 5 miles per hour normal to the duct, and a specific 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, ni~httime conditions and the like.
In this situation, the approximate heat loss from the process duct, itself, is about 244 BTV/hr./sq. ft., . '.
~ 10 7 9 4 8 l or about 318,000 BTU's per hour for the entire length of the 65 foot duct.
The gas stream exlt~ng the intermedlate absorption tower is, of course, in equilibrium with the sulfuric acid absorbent. From established vapor phase data for a vapor in equilibrium with the sulfuric acid it was detenmined 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 sulfuric acid would reduce from 0.04 mill~meters of mercury to absut 0.035 millimeters of mercury.
This corresponds to a partial pressure reduction of about 0.005 millimeters of mercury for the sulfuric acid vapor.
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 of sulfuric acid when it is condensed from the gas phase into the liquid phase within 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 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 its boiling point or evaporated back into the gas phase ~, .
~ 07 9 4 8 1 1 or r~mov~d from draln connections.
If lt is ever permltted to fonm in t~e first place, there will be liquid sulfurlc acid of ~bout 98.6% concentration in contact with metallic surfaces. At this concentration, from known corrosion curves for carbon steel which is a . normal mater~al for construction of ductwork and heat exchanger tubes in sulfuric acid plants, as well, the corrosion rate for 98.6% sulfuric ac~d above 300F, is in excess of 200 mils per year which accounts for the ductwork - 10 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 problem of corrosion occurring as a consequence o condensation of sulfuric acid, there is provided to ductwork . 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 beat sufficient to maintain the gas stream at a . temperature above the dew point of sul~uric ac~d.
. 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 periphery of duct 10 through which steam, Dowtherm or a æimilar heat transfer media is passed. Between tubes 14 is an air space 16 heated by the transport of fluid through the tubes 14.
m ere is provided a reinforcing layer 18 to maintain the gas space 16 typically an air space between tubes D
~0 7 9 4~ 1 1 14 and an outside lnsulation 20 to reduce the hest rcqulrements for the system md assure that the gas space between tubes 14 will always be at an excess temperature so that 8 driving force will exist to maintain a flow of heat into the gas stream passing through ductw~rk 10.
In general, the amount of heat provided through tubes 14 should be sufficient 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 l l~
condensation of suluric acid.
In the alternative, there may be employed an electric resi~tance 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 jacketed 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 ~ntermediate absorption tower and~heat exchanger 12, corrosion due to condensation of sulfuric acid in both duct 10 and heat ; exchanger 12 will be elimina~ed. Accordingly, the lifetime of both costly and essential equipment associated with the operation of multiple contact/multiple absorption processes .3 for sulfuric acid manufacture will b~ preserved.
~ 07 9 ~ ~1 1l~ile the heatin~ systcm surrounding duct 10 will prevent condcnsation durin~ steady state operatlont condens~tion of sulfuric acid or other condensible sulfurous compounds may occur during upset conditions. These may be S durin~ start up operations or a malfunction in the duct heating system itself. As a precau~ion, there may be provided dam 19 which surrounds the periphery of duct 10 just before heat exchanger 12 to collect the condensates passing along duct 10 to prevent their entry into heat 10exchanger 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 l ~lowing 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 preven~ condensation or simply that an upset condition exists.
Another material pxoblem 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 associa~ed wi~h the effluent o~ the intermediate absorber.
There is also provided as part o this invention a modified cold gas to gas heat oxchflnger whose cons~ruction avoids coalescence and settlement and attenclant corrosion .
~ . .. . . ..
10 7 9 ~ ~ ~
1 of thc h~at exchanger surfaces due tQ drop out or coalescence of sulfuric acicl and sulfuric acid mist particles. ~ --A typ~cal gas to gas heat exchanger consists of a gas lnle~ hca~er having a gas inlet and a gas outlet header S 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 a gas outlet to permit a gas stream passing through the conduits t-o 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 ~urfaces particularly at the point where the gas stream enters the heat exchanger. As mentioned above,an example is the gas stream returning from the intermediate absorber.
Fig. l represents the situation where the gas from the intermediate absorber is passed on the enclosure or shell side on the heat exchanger l~ 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 ~5 from the intermediate absorber is passed through the tubes 30 of heat exchanger 12 in he~t exchange ~ith the gas from a ____ ____ ___ 3 ____ /C
1 0 79 4~1 1 conversion sta~e passing throu~h the shell side of heat exchanger 12.
With respect to both situations heat ~xchanger 12 consists of an ou~er shell 22 and an inlct 24 for the gas ~lo~ling from on~ or more of the conversion stages by the prior to or follo~ing intermediate absorption, an inlet header 26 for either the gas from a conversion stage or the gas from an intermediate absorber, and an upper l transverse tube sheet 28 over which tubes 30 axe normally flared. There also exists a lower transverse tube sheet 32 to which the tubes are connected by flares to provide an outlet header 34 where 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 abou~ 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 ~50 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.
Because the gas stream entcring exchanger 12 from - /;i' D
10 79 4~ 1 1 the intcrmcdiate absorber may drop in temperature to a point below the condensation temperature of sulfuric acid in the ,ystcm ~nd any en~rained sulfurlc acid mist wlll tend9 <3ue 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. Baffle plate 42 maintains a stagnent gas zone between the baffle plate and transverse tube sheet 32~ The high temperature of the gas stream flowing through tubes 30 maintains the stagnent gas zone above the condensation temperature of sulfuric acid mist, namely above about 650F. Any sulfuric acid mists lS would condense from the gas stream and settle through the openings in baffle plate 42 into zone 44 will, because of the high temperature in the zone, be vapori~ed 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.
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 gas 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 Fig. 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 107 9 4~ 1 1 ¦ from line 24 into tl~c shell side of thc heat exchanger, and ¦ lexlts from hcader 34 by line 46 to additional heat absorbers ¦ or heat exchan~ers at the ~pproxlmate temperatures shown.
l In this instance, because tubes 30 are nonmally 5 ¦ secured to the transverse tube sheet by a flaring operation ¦ there is provided depressions on tube sheet 28 upon whlch ¦ sulfuric acid and/or acid mist can collect and condense.
¦ They may also collec~ and condense on the walls of inlet l header 26. To avoid this possibility, a portion of the 10¦ 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 of gas bypass to zone 44 is about 3 to about 20% of the total gas flow.
ln 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 ~ufficient 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 ad~acent 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 from the intermediate absorber. This avoids corrosion of ~ 10 7 9 ~ ~1 1 tubc platc 28, tubes 30 as well as the ~d~acent walls of header 26.
As shown in F~g. 2 the entire heat exchanger is pre~erably insulated to further aid in avoiding the creation of a problem of 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 of 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 ~uch as converter exit gas above the condensation temperature corrosive constituents such as sulfuric acid and acid mist to prevent corrosion due to condensation or coalescence of these corrosive constituents on heat exchanger surfaces.
~6 j .,
Claims (12)
1. A multiple contact/multiple absorption process for the manufacture of sulfuric acid or oleum in which a gas stream containing sulfur dioxide and oxygen is passed through a plurality of catalytic contact stages, comprising the steps of converting sulfur dioxide to sulfur trioxide with cooling between contact stages to remove the exothermic heat of reaction;
followed by passing the gas stream to at least one absorption tower where the formed sulfur trioxide is separated from the gas stream by contact with at least sulfuric acid; then passing the gas stream by ductwork to a gas to gas heat exchanger where the gas stream is heated at least in part to a temperature required for introduction to at least one additional catalytic contact stage where the residual sulfur dioxide is converted to sulfur trioxide; followed by extracting the formed sulfur trioxide from the gas stream prior to venting the gas stream to the atmosphere; and maintaining a positive flow of external heat into the ductwork leading from the absorption tower to the heat exchanger by passing a fluid heating medium through an array of spaced tubes in contact with said ductwork, said tubes being surrounded by a reinforcing layer and an out-side insulation to provide between said tubes a gas space which is heated by the flow of the fluid heating medium through said tubes, the flow of external heat into the ductwork being in an amount sufficient to maintain the gas stream above the dew point of sulfuric acid carried by the gas stream to maintain the sulfuric acid in the vapor phase and thereby minimize corrosion of the ductwork due to condensation of sulfuric acid on the surfaces of the ductwork.
followed by passing the gas stream to at least one absorption tower where the formed sulfur trioxide is separated from the gas stream by contact with at least sulfuric acid; then passing the gas stream by ductwork to a gas to gas heat exchanger where the gas stream is heated at least in part to a temperature required for introduction to at least one additional catalytic contact stage where the residual sulfur dioxide is converted to sulfur trioxide; followed by extracting the formed sulfur trioxide from the gas stream prior to venting the gas stream to the atmosphere; and maintaining a positive flow of external heat into the ductwork leading from the absorption tower to the heat exchanger by passing a fluid heating medium through an array of spaced tubes in contact with said ductwork, said tubes being surrounded by a reinforcing layer and an out-side insulation to provide between said tubes a gas space which is heated by the flow of the fluid heating medium through said tubes, the flow of external heat into the ductwork being in an amount sufficient to maintain the gas stream above the dew point of sulfuric acid carried by the gas stream to maintain the sulfuric acid in the vapor phase and thereby minimize corrosion of the ductwork due to condensation of sulfuric acid on the surfaces of the ductwork.
2. A process according to claim 1, in which the gas space is maintained at a temperature of from about 100 to about 200°F above the temperature of the gas stream passing through said ductwork by the flow of the fluid heating medium through said tubes.
3. A process according to claim 1, wherein the fluid heating medium is steam.
4. A process according to any one of claims 1 to 3, in which the gas to gas heat exchanger contains a high temperature zone positioned at approximately the point of introduction of the gas stream from the ductwork, said zone being maintained at a temperature above the condensation temperature of sulfuric acid by the flow of gas from a conversion stage through said heat exchanger to prevent condensation of sulfuric acid on heat exchanger surfaces.
5. A process according to any one of claims 1 to 3, in which the gas to gas heat exchanger contains a high temperature zone positioned at approximately the point of introduction of the gas stream from the ductwork, said zone being maintained at a temperature above the condensation temperature of sulfuric acid by the flow of gas from a conversion stage through said heat exchanger to prevent condensation of sulfuric acid on heat exchanger surfaces, and in which the high temperature zone is stagnant and is heated indirectly by the gas from a conversion stage, the said gas being prevented from entering the zone.
6. A process according to any one of claims 1 to 3, in which the gas to gas heat exchanger contains a high temperature zone positioned at approximately the point of introduction of the gas stream from the ductwork, said zone being maintained at a temperature above the condensation temperature of sulfuric acid by the flow of gas from a conversion stage through said heat exchanger to prevent condensation of sulfuric acid on heat exchanger surfaces, and in which the high temperature zone is heated directly by the gas from a conversion stage, which gas is admitted to the said zone.
7. A process according to any one of claims 1 to 3, in which the gas to gas heat exchanger contains a high temperature zone positioned at approximately the point of introduction of the gas stream from the ductwork, said zone being maintained at a temperature above the condensation temperature of sulfuric acid by the flow of gas from a conversion stage through said heat exchanger to prevent condensation of sulfuric acid on heat exchanger surfaces, and in which the high temperature zone is heated directly by the gas from a conversion stage, which gas is admitted to the said zone, and in which from 3 to 20%
of the total gas flow from said conversion stage is passed through said high temperature zone.
of the total gas flow from said conversion stage is passed through said high temperature zone.
8. Apparatus adapted for use in the process according to claim 1, including a gas to gas heat exchanger for heating, by indirect heat exchange with a high temperature gas, a relatively cool gas stream from a sulfur trioxide absorption tower to a higher temperature demand by a catalytic conversion stage to which the gas stream is fed, and ductwork arranged for conveying the said gas stream from the tower to the heat exchanger and for heating the gas stream conveyed thereby to maintain its temperature above the dew point of sulphuric acid and thereby prevent condensation of corrosive acid on internal surfaces of the ductwork, the ductwork having an array of spaced tubes in contact therewith for the passage of fluid heating medium, the tubes being surrounded by a reinforcing layer and an outside insulation which in use provide a heated gas space enveloping the ductwork, and the gas to gas heat exchanger comprising (i) a gas feed header having a gas inlet, a gas outlet header having a gas outlet, and a plurality of spaced gas flow conduits communicating with said gas feed header and said gas outlet header to provide a plurality of passageways for flow of one of the gases to participate in indirect heat exchange between said headers; and (ii) an enclosure surrounding said conduits to provide a passageway for the flow of the other of the gases about said conduits and having a gas inlet and a gas outlet; the heat exchanger including means which in use establish a substantially stagnant or quiescent gas zone adjacent to the point of entry of the relatively cool gas stream into said heat exchanger, the said zone being kept at a temperature above the dew point of said corrosive acid by the flow of the high temperature gas through the heat exchanger so as to prevent condensation of corrosive acid from the gas stream entering the heat exchanger.
9. Apparatus according to claim 8, wherein the ductwork is connected to the enclosure gas inlet so as to feed the relatively cool gas stream into the enclosure for indirect heat exchange during its passage to the enclosure gas outlet with the higher temperature gas which in use is passed through the conduits, there being a baffle plate positioned adjacent the enclosure gas inlet, between the enclosure gas inlet and the said gas outlet header, in spaced relationship from said gas outlet header and the gas flow conduits to maintain the said gas zone between the baffle plate and the gas outlet header, the temperature of the said gas zone in use being maintained above the dew point of the corrosive acid by flow of the high temperature gas through conduits to the gas outlet header.
10. Apparatus according to claim 8, wherein the ductwork is connected to the gas inlet header so as to feed the relatively cool gas stream into the said header for flow through the conduits to the gas outlet header and gas outlet, the high temperature gas in use being fed via the enclosure gas inlet into the enclosure for indirect heat exchange with the cooler gas stream flowing in the conduits during its passage to the enclosure gas outlet, there being an additional gas inlet adjacent the gas inlet header for admitting high temperature gas to the enclosure, and a baffle plate positioned adjacent the enclosure gas inlet, between the additional gas inlet and the gas outlet header, in spaced relationship from said gas inlet header and said gas flow conduits to maintain the said gas zone between the baffle plate gas inlet header, the said gas zone comprising a portion of said high temperature gas admitted to the enclosure which in use maintains portions of the gas inlet header and of the conduits at a temperature above the dew point of the corrosive acid and prevents condensation thereon of the said acid from the gas stream entering the heat exchanger.
11. Apparatus according to claim 10, wherein a by-pass conduit connects the additional gas inlet to the enclosure gas inlet.
12. Apparatus according to claim 11, wherein the by-pass conduit is located internally of the enclosure.
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 (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US35336473A | 1973-04-23 | 1973-04-23 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1079481A true CA1079481A (en) | 1980-06-17 |
Family
ID=23388791
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA196,729A Expired 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 |
Country Status (8)
| Country | Link |
|---|---|
| JP (1) | JPS5942236B2 (en) |
| BE (1) | BE814042A (en) |
| CA (1) | CA1079481A (en) |
| DE (1) | DE2419096A1 (en) |
| FR (1) | FR2226358B3 (en) |
| GB (2) | GB1470334A (en) |
| IT (1) | IT1011766B (en) |
| NL (1) | NL7405426A (en) |
Families Citing this family (8)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPS56151334U (en) * | 1980-04-14 | 1981-11-13 | ||
| JPS57199279A (en) * | 1981-06-01 | 1982-12-07 | Toshiba Corp | Semiconductor luminous element and manufacture thereof |
| DK155723C (en) * | 1982-03-25 | 1989-10-09 | Haldor Topsoe As | METHOD AND APPARATUS FOR Sulfuric Acid Preparation |
| DE102006016786A1 (en) * | 2006-04-10 | 2007-10-18 | Triesch, Frank, Dr.-Ing. | Heat exchanger for heating or cooling of fluid flow, comprising internal or external bypass for continuous partial stream and heat carrying fluid flow |
| JP2008069053A (en) * | 2006-09-15 | 2008-03-27 | Japan Atomic Energy Agency | Heat exchanger having sulfuric acid decomposing function and so3 decomposing function and utilizing mist evaporation |
| EP2694177B1 (en) | 2011-04-06 | 2016-10-05 | Haldor Topsøe A/S | Process for production of sulphuric acid |
| DE102015112220A1 (en) | 2015-07-27 | 2017-02-02 | Outotec (Finland) Oy | Method and system for cooling a SO 2 and / or SO 3 and traces of water containing gas |
| CN106767044B (en) * | 2017-03-02 | 2023-05-16 | 洛阳明远石化技术有限公司 | Flue gas air cooler for catalytic reforming device |
Family Cites Families (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE1933460C3 (en) * | 1969-07-02 | 1973-11-15 | Metallgesellschaft Ag, 6000 Frankfurt | Device for intermediate absorption and heat exchange during the catalytic conversion of gases containing SO deep 2 with the production of sulfuric acid |
-
1974
- 1974-04-03 CA CA196,729A patent/CA1079481A/en not_active Expired
- 1974-04-18 JP JP49042828A patent/JPS5942236B2/en not_active Expired
- 1974-04-19 GB GB1728074A patent/GB1470334A/en not_active Expired
- 1974-04-19 GB GB4037576A patent/GB1470335A/en not_active Expired
- 1974-04-20 DE DE19742419096 patent/DE2419096A1/en not_active Withdrawn
- 1974-04-22 IT IT6828374A patent/IT1011766B/en active
- 1974-04-22 BE BE143489A patent/BE814042A/en not_active IP Right Cessation
- 1974-04-22 NL NL7405426A patent/NL7405426A/xx not_active Application Discontinuation
- 1974-04-23 FR FR7414046A patent/FR2226358B3/fr not_active Expired
Also Published As
| Publication number | Publication date |
|---|---|
| FR2226358A1 (en) | 1974-11-15 |
| IT1011766B (en) | 1977-02-10 |
| BE814042A (en) | 1974-08-16 |
| GB1470335A (en) | 1977-04-14 |
| GB1470334A (en) | 1977-04-14 |
| NL7405426A (en) | 1974-10-25 |
| DE2419096A1 (en) | 1974-11-14 |
| JPS5942236B2 (en) | 1984-10-13 |
| FR2226358B3 (en) | 1977-03-04 |
| JPS509593A (en) | 1975-01-31 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| MKEX | Expiry |