Process to make both nitric and sulfuric acid

Abstract

Stirred acid resistant shallow cylindrical reactors are used to produce both nitric and sulfuric acid from a feed gas stream arranged to contain both sulfur dioxide and nitrogen oxides passed over or through the mixed acids. The homogeneous catalytic mixture of sulfuric and nitric acids uses the highly oxidizing nitrosyl ion to further oxidize the gaseous oxide stream to sulfuric and nitric acids. Oxygen or air then oxidizes the nitrosyl ion reduction products back to nitrosyl ion for further reaction. The acids are separated by distillation, and concentrated using heat from the burner and the reaction heat. The modified sulfur burner used operates at temperatures to oxidize some of the nitrogen in the air. The temperature required may be obtained by increasing the oxygen of the air by pure oxygen. More nitrogen oxides may be produced by a glow discharge into the burner air or burning of ammonia. Any heavy metals such as mercury will be first oxidized then precipitated as sulfates.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a concept drawing of the mode of entry and mixing of the feed stream gases not showing thermal recycling or the burners necessary, but showing the lined stirred reactor and the simplest method of obtaining a massive, reactive, gas-solution interface that will normally exceed bubble cap towers in either of the entrance modes, free gas stream or perforated coil.

FIG. 1B shows the preferred method of contacting the feed gas with the oxidizing mixed acids in the stirred reactor using a perforated ring bubbler and rapid stirring.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT

In the drawing 1A, 1 is the inlet of the burner gasses after cooling to 80 degrees Celcius during which major fly-ash and other particles resulting from the burning of solids will have dropped out. Extra oxygen or air has a separate inlet 2, and 3 is an optional inlet for NOx made by burning ammonia or by a glow discharge which also adds oxygen not used to make NOx as the gasses proceed to the flow meters 4 to provide the data to control the flow of the various gas streams to the mixing chamber 5 just prior to entering the stirred reactor 8 containing the mixed acids 6 by the inlet pipe 7. A gas outlet 15 when air rather than pure oxygen is used leads to a sampler, not shown but necessary in batch mode, with recycle to the inlet 7, also not shown. Additions to the burner gas stream 1, may be necessary to obtain a mixture that produces economical and usable quantities of the two acids which is especially so for nitric which is at least twice as valuable as sulfuric.

FIG. 1A is the simplest, effective form of the invention and the cheapest used in the batch mode. It is expected that it will dominate small batch applications of the invention in such uses as for small mining operations, shut in sour gas wells, small town garbage incinerators or any small incineration operation where intermittent operation is the norm and where ejected small amounts of the un-reacted feed gas are tolerable making collection and sale of nitrogen impractical.

The gas inlet 7 provides the cooled gas feed stream, balanced as needed from data from flow meters 4 relayed to a small computer which will operate control valves, not shown, on the various parts of the feed stream. The pressure to get a sufficiently rapid flow of gas assymtotic to the wall of the lined shallow cylindrical reaction vessel will be of the order of 5 to 10 inches of water above ambient, provided by a centrifugal Teflon gas pump, not shown, following cooling and mixing of the gas stream. This pump will usually not be necessary as presentation of the original gases [or a pulverized solid] to the burners will have required some pressure. The pressure drop on cooling of the burner gases could easily be acquired initially by calculation and refined by experience to allow a back up pump if needed to supply the required pressure.

The gas discharged between the liquid surface and the cover (shown only in 1B, 14) would be expected to be turbulent, or above the Reynolds Number as it passes through a space between the sealed cover and the rapidly stirred mixed acids in the reactor 8, in counter-current flow. The reacting gas will decrease in volume, especially if pure oxygen is used in the burners, and as the SO2 and NOx react. The residual gas stream will slow down because of friction such as interaction with the catalytic solution surface and the cover as it moves towards the greater gap between the gas and the liquid 6 at the centre. The “stripped” gas is expected to be mainly nitrogen especially if air is used as the oxidizer of the nitrosyl ion reduction product back to nitrosyl ion. There are two alternative gas outlets 13 and 15, but only 15 is shown in FIG. 1A. The gas outlet 15 in FIG. 1A, presumes a rapid stripping of the feed stream gas when placed opposite of the inlet 7, though it could be placed just behind the inlet gas elbow 7 or in the centre of the cover.

Two effects other than absorption and reaction, cause the gas flow to reduce the radius of its initial direction assymtotic to the reactor's wall 8. The rapid stirring of the catalytic acid mixture will cause the meniscus to climb up the reactor's wall and decrease the distance between the cover 14 and the liquid. The centre of rotation will then be lower, affording more space for gas.

The second factor is that the interfacial friction at the gas-liquid interface and the interaction with the curved wall and cover 14 of the reactor 8 will reduce the velocity of the gas, reducing its momentum to maintain straight line translation. Again this suggests that the stripped gas outlet 13 in 1B is the more appropriate. However, the arrangement in FIG. 1A is that used effectively in a one litre pilot plant apparatus in my laboratory.

In FIG. 1B, an alternative method of introducing the gas stream 7 to the reactor 8 is shown. This arrangement is reminiscent of, again, the laboratory pilot plant of one litre capacity, where in one mode the gas stream entered at the bottom of the liquid reaction mixture through a glass frit. The calculated reaction rates for the two modes of entry were sufficiently similar in this small volume to be ignored. But compared to the contemplated 0.178 cubic meters or 178 L in the commercial sized reactor, it can only be assumed that the extra complication of FIG. 1B and the gas injection perforated Teflon tube 9 will be preferred for continuous long term production of the two acids.

For best performance, the Teflon ring has graduated perforations, the smallest near the gas entrance and the largest just before the end of the ring. It is important that the gas entrance perforated ring 9 not be flooded. Flooding will occur if the perforations are too large and if the diameter of the ring is too high and of course if the gas pressure is not appropriate to the mixed acid depth and the dimensions of perforations and the ring 8. From this it can be argued that various lengths, sizes of perforations and gas pressure should be allowed for as the solid or gas feed source is varied, or a multiple push on Teflon fitting with suitable top cocks can best serve the process. Having the possibility of rapid change of tube would serve two uses; if the tube became largely blocked by solids not removed and if the feed to the burners were suddenly changed, changing the pressure in the reactor, to give minimum interruption to production.

Further, in the configuration of the invention shown in FIG. 1B the perforated Teflon coil 9 is supported above the bottom of the stirrer in the reactor 16 with the perforations on the bottom, so that any particulate matter would more likely be blown out by the gas. An accumulation of unwanted solids can be swept to the centre of the reactor to a finished acid exit port, not shown, allowing them to be removed in the normal way with the finished acid then separated by decantation.

In practice, the “finished acid” must be removed, or if not finished, removed to another stirred reactor. FIG. 1B does not show this. The laboratory tests showed that a second stirred reactor was un-necessary in a batch mode operation. It is easily visualized that should it be found expedient to have several stirred reactors in series when operating the invention in the continuous mode, that centre extraction of the mixed and enhanced acid and its addition to the top of a second stirred reactor would be effective.

Again in a batch mode use of the invention, as the feed gas made available neared its end, the rate of stirring could be reduced so that the heavier, more concentrated acid mixture would accumulate near the bottom, as noted in the charging of lead acid batteries (the so-called “layering effect”) and be drawn off leaving an effective starting mixture in the reactor in anticipation of another, batch mode run. A specially shaped reactor with a dished downward form with centre withdrawal port would be appropriate and in no way breach the invention principles.

In both modes, since the oxidation reactions occurring in the stirred reactor(s) are all exothermic, at least a stand-by heat extraction device such as an immersed Teflon or glass coated coil with a pumped liquid capable of extracting anticipated reaction heat must be provided.

Claims

1-11. (canceled)

12. A method for using a mixture of nitric and sulfuric acid to form a homogenous catalytic solution of nitrosyl ion, a gas containing sulfur dioxide, nitrogen oxides and oxygen in ambient air where this gas mmixture is converted to both sulfuric and nitric acids when passed into a stirred reactor via reaction with the nitrosyl ion, the reduction products of the nitrosyl ion re-oxidized by the residual oxygen in the air or added oxygen, the gas being obtained from a) sulfur burned in excess air or with extra oxygen at temperatures above 950 degrees Celcius to convert some of the nitrogen in the air to nitrogen oxides andb) alternatively the feed to the burner is flue gas containing excess oxygen from the burning of an effluent gas from stored animal manure, landfill burnables or incineration of dried, powdered solid such as animal manure, sewage sludge or other waste material containing both sulfur and nitrogen compounds

13. The method according to claim 12, the burner may be a sulfur burner or a sulfur burner modified to burn dried, powdered solids at temperatures of 1100 degrees Celcius and higher containing sulfur and nitrogen

14. The method according to claim 12, where a high voltage produces a glow discharge in air gives more nitrogen oxides which are subsequently fed into the feed gas.

15. The method according to claim 12, an ammonia burner adds nitrogen oxides to the gas stream

16. The method according to claim 12, the cooled gas stream enters the stirred reactor through an inlet, and at about 80 degrees Celcius, and the reaction heat is removed by a heat exchanger to maintain the tempperatures at less than 80 C., as the reaction mixture is stirred by a magnetic stirring bar, at a rate sufficient raise the meniscus at least two centimeters above the centre

17. The method according to claim 12, the stirred reactor is acid resistant glass or is acid resistant, plastic-lined steel

18. The method according to claim 12, the starting acid concentrations may be as low as 0.1 M in sulfuric acid and less than one molar in nitric acid; the best starting concentrations are five and three molar respectively

19. The method according to claim 12, heat of reaction and from the burners is used to separate the two acids by distillation and to process them to the desired concentration, and to heat or dry the incoming waste material or air, or to co-generate electric power

20. The method according to claim 12 where more than one reactor is used, the reactors to be in series when more than one reactor is required to efficiently remove all of the SO2, NOx and heavy metals.

21. The method according to claim 12 when air is used as the oxidant, the remaining out-gas stream is almost pure nitrogen, which is compressed for sale using some if any co-generated electric power.

22. Heavy metals present in the fuel are first oxidized then precipitated as the sulfates in the strong sulfuric acid produced in both in the reaction vessel and in the still producing sulfuric acid for sale.