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Publication numberUS3285711 A
Publication typeGrant
Publication dateNov 15, 1966
Filing dateApr 24, 1963
Priority dateApr 24, 1963
Publication numberUS 3285711 A, US 3285711A, US-A-3285711, US3285711 A, US3285711A
InventorsAlfred E Stanford
Original AssigneeExxon Research Engineering Co
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Inert flue gas system
US 3285711 A
Abstract  available in
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Claims  available in
Description  (OCR text may contain errors)

2 Sheets-Sheet 1 Filed April 24, 1963 ALFRED E. STANFORD- INVENTOR PATENT ATTORNEY Nov. 15, 1966 A.- E. STANFORD INERT FLUE GAS SYSTEM 2 Sheets-Sheet 2 Filed April 24, 196:5

53? com ALFRED E. STANFORD INVENTOR WWW PATENT ATTORNEY United States Patent O Delaware Filed Apr. 24, 1963, Ser. No. 275,249

2 Claims. (Cl. 23--281) The present invention concerns an improved flue gas inerting system. In particular, it relates to a system which utilizes flue gas derived from boilers as an inerting atmosphere for filling cargo holds in tankers in order to prevent an explosive atmosphere from forming. It further relates to an improved system 'for producing truly inert flue gases in that such gases will not be corrosive to the tank walls and pipes in the tankers holds.

The cargo tanks in oil tankers normally contain hydrocarbon vapor-s in the space above a loaded cargo. Also, during a ballastvoyage, enough residual cargo usualfly remains (unless special cleaning has been provided) to saturate the vapor space with hydrocarbons. A combustible vapor, gas or mist will not burn or explode, however, if the oxygen content of the atmosphere in the hold is reduced below a certain definite value, which varies with the combustible material under consideration. It is known that the most critical point from the danger viewpoint, occurs when the tanker is being unloaded. As the cargo is pumped out, it is necessary to replace the lost liquid volume with gas in order to maintain pressure balance. If an air vent is used, the atmosphere in the hold will contain a greater than critical concentration of oxygen and any flame or spark could set off the entire cargo. It is therefore imperative that this venting gas be one that will yield an atmosphere having a lower than critical level of oxygen. In most cases, if the oxygen concentration of the atmosphere in the hold is kept below approximately 12%, ilames will not propagate nor will explosions take place, regardless of the concentration of the combustible that may be present. As examples, the minimum oxygen content for gasoline to burn is 14.4%, while for benzene it is 13.9%.

One possible solution would be to use the classic inert gas atmospheres to vent the cargo holds during unloading. Gases such as helium, nitrogen, carbon dioxide, argon, etc., are known for their inability to support combustion. However, such inert gas systems would be far too expensive to be used in commercial tankers "having gargantuan ho'ld volumes. Therefore, pseudo-inert gases have been utilized in place of the more expensive true inert gases. Since the main objective of these psuedo-inert gases is to provide an oxygen poor atmosphere, an excellent source of these gases is the [flue gases produced by the combustion of fuel oil in the boilers.

An example of the composition of flue gas produced by the combustion of No. 6 oil burned in a Cleaver- Brooks burner, Model CB-SO is given below:

PRODUCTS OF COMBUSTION OF NO. 6 FUEL OIL 1 1 The oil contained 2.53% sulfur and 385 ppm. vanadium with 0.09% ash 2 The 002 in the fine gas was 13.9%; no was found at a stack temperature of 320 F.; the Oz concentration is approximately 3%.

3 1,490 ppm.

"ice

It is obvious from an examination of the above products that such a flue gas is far from inert in fact. Both sulfur gases are extremely acid and are very corrosive to metal when brought in contact with it over a period of time. Furthermore, the combustion of fuel oil yields substantial amounts of carbonaceous particulate matter which of necessity must be removed before the flue gas is allowed to enter the cargo holds. If traditional methods of scrubbing gases are used to remove the above contaminants, large quantities of water will be carried in the resulting gas stream. This water, then, will become the main corrosion causing agent and will therefore greatly reduce the effective use-'life of the cargo .ho'lds.

It is therefore an object of this invention to prevent flame and explosions .in the cargo holds of tankers by providing therein a non-combustion supporting atmosphere. It is a :further object of this invention to provide a truly inert atmosphere which will be non-corrosive to the cargo holds.

The system of the present invention produces an inert fi'ue gas by the fol-lowing steps. Flue gas, produced in the boiler of the tanker or other cargo vessel, is led from the boiler uptakes to a scrubber which preferably uses salt water. The scrubber serves to remove virtually all of the soot and other particulate matter. Additionally, the scrubber cools the flue gas from its original temperature of about 350 F. to about F. Of course, it would be possible to utilize a fresh water scrubber here also, but salt Water is far more convenient and is preferable for that reason.

The acid sulfur gases are then removed from the flue gas stream by passing the gas through an alkali scrubber. Suitable alkali solutions for this purpose can be made from caustic salts such as NaOH, Na CO Na PO KOI-I,

other similar agents known to the art. It is preferred that the scrubber contain Na CO for the purposes of the present invention.

It is within the scope of this invention to utilize a combined caustic-salt water scrubbing step to simultaneously effect the particulate and acid gas removal and temperature reduction. In such an embodiment the caustic agent is added directly to the salt water stream prior to its circulation through the scrubber.

Gas, freed from particulate matter and acid gas, is then chilled by passing it through a dehumidifying coil. The flue gas is cooled to about 45 F. and at this temperature most of the water in the gas stream is condensed out. The cooling element may be a type employing Freon or similar halogenated parafiinic gases, chilled water, or any other refrigerant known to the art, as coolant, and can be used in various ways. It can be used to cool the gases directly by heat exchange through a cooling coil. Another possibility is to use the cooling coil to chill water shunted off from the main scrubbers intake and then use this water to wash the flue gases arising from the main scrubber. Other variations will suggest themselves to one experienced in the refrigerant art.

The cooled, dehumidified gas is then passed through a heater in which the gas temperature is raised to about -F. in order to prevent condensation in the pipingas the gas is passed to the cargo holds. A preferable form of heater is a steam air heater since the steam used for its operation is readily available on marine vessels.

The flue gas inerting'system of the present invention will be more readily understood by reference to the appended drawings and the description thereof.

FIGURE 1 is a diagrammatic representation of a typical flue gas inerting system in a marine cargo vessel.

FIGURE 2 is a diagrammatic view of a specific embodiment of an inert gas system of the present invention.

Turning now to FIGURE 1, a general view of a flue gas inerting system is shown in relation to other facilities in a marine cargo vessel Flue gas, produced by the combustion of fuel oil in the boilers (not shown) is withdrawn from flue by outlet line 11. The flow rate of the flue gas is controlled by butterfly valve 12 as it passes to line 13. In vessels that have both a port and a starboard boiler, this flue gas tap system is reproduced identically by flue 1 4, outlet line 15, butter-fly valve 16 and line 17. The .gas flows are combined in the latter case into a single line 18 which serves as an inlet for the scrubber unit 19. Salt water for the scrubber is taken aboard via line 20 by the suction action of pump 21. The salt water is then fed into scrubber 19 by line 22. The salt water rate of flow is controlled by valve 23. Salt water discharge from scrubber 19 is returned to the sea by means of outlet line 24. A control valve 25 regulates the out-let flow.

The dehumidified inert flue gas emerging from the scrubber unit passes through gas outlet line 26. A safety cutout valve 27 is located in this line to prevent excessive pressure differentials from building up. Blower 28, powered by steam turbine 29, forces the gas into gas supply header 30. Control of gas flow in the header is maintained .by means of swing check valve 31 and butterfly isolation valve 32.- Inert flue gas is supplied to individual cargo holds 34 by bleed off branch lines such as line 33 which taps off gas from supply header 30. Each branch line is equipped with a butterfly valve such a valve 35 to allow control over the course and amount of gas flow effective targets for fine dust. It is probable that for small particles, most of the scrubbing action of the impingement baffle plate may be attributed to the spray droplets formed by the transfer of momentum from gas jets to the liquid flowing over the perforated plate, rather than to the impingement of the particles on the baflles.

Another desirable type of scrubber for use in element 105 is one which utilizes a plurality of trays of bubble caps to contact flue gas countercurrently in once-through operation. The bubble caps are the familiar round bell caps with slots spaced equally around their lower periphcry. The total slot area for each cap is about 12 square inches and there are about 49 caps per tray. Flue gas contacts the sea water in passing through the slots in the bubble caps. At the maximum flue gas flow rate for this type unit, about 4,300 s.c.f.m., the slot velocity is near 18 feet per second. Proportionately lower slot velocities would occur for lower flue gas flow rates. In general the range of flue gas flow rates would be about 2,000 to 4,300 s.c.f.m.

The cooling efficiency of both the above described countercurrent scrubber elements was tested in use aboard tankers. A measure of cooling efficiency for the scrubbing operation is the so-called approach temperature. This temperature is the difference between the flue gas temperature leaving the scrubber and the temperature of 1 Sea water scrubber element only. 1 Sea water scrubber element and a separate Na CO absorber.

to the holds. The gas is bled directly into each hold through a tank head 36.

Turning now to FIGURE 2, a detailed diagrammatic view of the inert gas system is shown in a specific embodiment. Flue gas from the boilers 100a and 10017 is bled from flue 1000 by line 101. The flow of flue gas through this line i controlled by buterfly valve 102. The gas then passes into the scrubber unit 103 by way of gas inlet line 104.

The first element in the scrubber unit is a sea water scrubber .105. The flue gas vapors entering the scrubber are intimately contacted with a countercurrent stream of sea water. A suitable scrubber for thi purpose utilizes a plurality of impingement bafile stages. The perforated plate in each impingent lbaflle stage consists of a sheet having about 600 holes per square foot, giving an open area of about 22 percent. Each set of impingent bafiles is arranged so as to have a bafile located directly above each perforation. Flue gas passes through the holes in the perforated plate and impinges on the baffles. The relatively high velocity, in the order of 75-100 feet per second, required by the aerosol in the perforations is effective in atomizing the sea water at the edges of the perforations. The resultant mist particles carry into the vapor s ace above the baffle stage and continue to act as The impingement baflle scrubber, using 250 g.p.m. of sea water, has an approach temperature which appears to hold fairly constant over the range of flue gas flow encompassing 2000-3000 s.c.f.m. (38 F., with an average of about 5 F.). The bubble cap scrubber, using 200 g.p.m. sea water shows an approach temperature of about 13 F. over the range 2700-4300 s.c.f.m. The temperature differential increases to about l7-18" F. when a low flow of about 700 s.c.f.m. In general, the amount of sea water used in either type of scrubber will be in the range of about 5 to 280 gallons/ 1000 s.c.f. flue gas. A preferable range is about 65-125 gallons/1000 s.c.f. flue gas.

Sea water for scrubber unit 103 is obtained through line 106 and is regulated by control valve 107. Since the inerting system will be used most extensively for unloading operations on the vessel, it would be expected that this will occur mainly while the vessel is in harbor. Since most harbors have a great deal of suspended matter in the water and since one of the objects of the scrubber system is to remove particulate matter, it is therefore highly desirable to run the inlet sea water through a strainer 108.

In a preferred embodiment, the sea water is then lead into mixing chamber 109 where Na CO is added to form a saturated solution in the sea water. The Na CO enriched sea water then passes into scrubber unit 103 by 5 way of line 110. By using Na CO enriched sea water, scrubber element 105 is actually performing a dual role. One role being that of particulate scrubber and the second being that of S absorber.

The particulate-free, SO -free flue gas then rises through scrubber unit 103 to the dehumidification zone 111. At this point the gas is at a temperature of about 80 F. and contains a substantial amount of water vapor. The rising gas is contacted with chilled sea water having a temperature of about 36 F. This sea water is obtained by tapping the main sea water supply line 106 at 113 and running the diverted sea water portion through refrigeration unit 114. Coolant for this refrigeration unit may either be a Freon type gas which is cycled through appropriate compression equipment (not shown) or else it may take the form of cold water supplied from the vessels air conditioning system if an appropriate steam-jet system is already used for that purpose. As previously indicated, the inert gas system would be used most extensively when unloading the vessel. Since this, in the case of most crude products, will take place in harbors in the temperate zone, the loss of some or all air conditioning efliciency during the unloading period will not impose a very great burden on the crews comfort.

The chilled sea water will descend countercurrently to the rising flue gas vapors in unit 103. As the water descends, it will be warmed to about the temperature found in scrubber element 105 and will then join the sea water inlet flow coming from line 110. The combined sea water streams are discharged through line 115 to the sea. It is possible, however, to recycle at least some of the exhaust stream by means of line 115a to the Na CO enriched stream thereby cutting down on the amount of NA CO needed to maintain an efficient absorbing concentration.

After passing through the chilled sea water, the flue gas is at a temperature -of about 45 F. and is thereby effectively dehumidified since very little moisture vapor will be retained by the gas at that temperature. The inert flue gas then passes through heater 116, which preferably utilizes steam coils, wherein the gas temperature is raised to about 110 F. The warmed gas exits unit 103 by means of line 117. An automatic cut-out valve 118 controls the flow rate of the gas to the cargo holds. Distribution of the gas is effected by passing it through steam turbine blower 119. The distribution flow rate is controlled by check valve 120, and butterfly valves 121 and 122. Cargo hold 123 receives the flue gas flow through inlet head 124.

In order to guarantee the best particular and S0 removal efficiency of the scrubber, a recirculation line 133 is incorporated from the discharge of the blower 119 to the suction piping of the scrubber 104. A modulated butterfly valve 134 is installed in line 133 to control the amount of recirculation. The purpose of this connection is to guarantee that the design gas flow rate and velocities are maintained constant through the scrubber trays no matter what the cargo discharge rate is from the tanks.

In addition, the system is provided with an alternate piping arrangement to permit isolation of the scrubber components and the taking of suction for an atmospheric intake for purposes of gas freeing. Gas freeing air is drawn in through air intake 125. Blind 126 is opened thereby allowing air to enter scrubber unit 103. The air passes up into line 117 and is then diverted into the cargo piping system 129 by opening blind 127 (which is normally closed during the inerting process) and closing butterfly valve 121. Cargo tank 123 can then be blown free of the inert gas atmosphere by the opening of cargo tank suction valve 128. Exhaust gas is released to the atmosphere by closing valve 122 and opening the tank hatch 132.

The system as described above will yield a truly inert flue gas. The concentration of S0 in this gas is in the range of 0.5-5 ppm. as opposed to a concentration of about 3000 ppm. in the untreated flue gas. Furtherrosive than even scrubbed flue gas.

6 more, water vapor content is about 0.80% as contrasted to the 9.68% originally found in the flue gas.

In order to more dramatically prove the effectiveness of the present system, a comparative corrosion rate test was run utilizing a sea water scrubber of the impingement bafille type identified as Unit A against a scrubber system of the present invention also using an impingement baffle type element identified as Unit B. The tests were conducted on corrosion test rack-s exposed to the scrubbed flue gas from each of the flue gas systems for six months. The results of the tests are summarized in Table 2.

Table 2 CORROSION RATES FROM TEST RACKS Corrosion Rate Material Unit A, m.p.y. Unit B, m.p.y.

Carbon Steel:

11 3. 1 12 2. 8 11 3. 0 12 2.8 Average 11.5 2. 9

It is therefore apparent that flue gas produced by the system of the present invention is very much less cor- Therefore, the use of such a system will substantiall increase the useful life of all components in the vessel contacted by the flue gas.

While several specific embodiments of the instant invention have been recited with some particularity, it should be emphasized that these embodiments should not be considered as being limiting of the scope of the invention. It is well within the province of an engineer skilled in the art 'to modify the system in many ways without departing from the spirit of the invention.

What is claimed is:

1. A system for the preparation of inert flue gases comprising in combination, means for producing flue gases and means for treating said flue gases and removing oxides of sulfur therefrom and for inerting a cargo tank at ambient temperature, said flue gas treating means including a scrubber first stage having an inlet adapted to be connected to a source of flue gas, a conduit arranged to be connected to a supply of water, means connected to said conduit for introducing an aqueous solution of an alkaline inorganic compound into said scrubber first stage, means for intimately contacting said flue gases with said solution; a dehumidifier second stage connected and arranged to receive the scrubbed flue gases from said first stage for removing water vapor from said flue gases, said dehumidifier second stage including separate means connected to said conduit and having cooling means therein for placing cooled water from said conduit into direct contact with the scrubbed flue gases, and means for directing the spent dehumidifying cooled water from said dehumidifier second stage into said scrubber first stage, means separate from said dehumidifier second stage for raising the temperature of the dehumidified flue gases after they leave said dehumidifier second stage thereby lowering the relative humidity of said gas, and means for directing said flue gases from the discharge of said treating means to said cargo tank.

2. A system in accordance with claim 1 including conduit means for recirculating a portion of the flue gases discharged from said treating means to the inlet of said scrubber first stage.

(References on following page) References Cited by the Examiner UNITED STATES PATENTS Howard et a1. 23178 Healy et a1. 22088 Beensen et a1. 23281 Colvin et a1. 22088 8 7 2,756,215 7/1956 Burgess et a1. 2523'72 2,787,530 4/1957 Staiger 23281 2,944,987 7/1960 Potter et a1. 252-372 MORRIS O. WOLK, Primary Examiner.

JULIUS GREENWALD, M. WEINBLATT, J. I. MULLEN, J. H. TAYMAN, IR., Assistant Examiners.

Patent Citations
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Referenced by
Citing PatentFiling datePublication dateApplicantTitle
US3776164 *Oct 1, 1971Dec 4, 1973Kongsberg Vapenfab AsApparatus for filling and, if desired, emptying inert gas poor in oxygen into or from holds in ships, respectively
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US3852408 *Sep 21, 1972Dec 3, 1974Lone Star Steel CoProcess for the removal of sulfur dioxide from carrier gases
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Classifications
U.S. Classification422/168, 252/372, 220/88.3, 440/88.00C, 440/88.00M, 114/77.00R, 440/89.00R
International ClassificationB65D90/44, A62C3/10
Cooperative ClassificationB65D90/44, A62C3/10, A62C99/0018
European ClassificationA62C99/00B2, A62C3/10, B65D90/44