Method for producing concentrated oleum or sulfur trioxide

Abstract

Methods and associated production plants are disclosed for producing concentrated oleum or sulfur trioxide therefrom with high purity, and particularly with minimal sulfur dioxide. The methods involve use of a single contact, single absorption sulfuric acid plant operating under an unconventional set of parameters including: reacting sulfur and high purity oxygen in a combustion reactor to form sulfur dioxide at a concentration between 11.5 and 20% by volume and with an atypical oxygen to sulfur dioxide molar ratio ≥ 1; atypically catalytically converting > 96% by volume of the sulfur dioxide to sulfur trioxide in a contact apparatus; and atypically recycling > 95% by volume of the total gases from an absorption apparatus to the combustion reactor. Concentrated oleum with < 300 ppm dissolved sulfur dioxide can desirably be produced with sulfur utilization ≥ 80% using a smaller, cost-effective sulfuric acid plant. The co-production of unnecessary sulfuric acid can also be reduced.

Description

[0001] METHOD FOR PRODUCING CONCENTRATED OLEUM OR SULFUR TRIOXIDE

[0002] Technical Field

[0003] The present invention pertains to methods and associated plants for producing high purity, concentrated liquid oleum and / or concentrated SOs gas. In particular, it pertains to producing oleum and / or SO3 with very low levels of dissolved SO2 and with high sulfur utilization while reducing co-production of unnecessary sulfuric acid.

[0004] Background

[0005] At this time, the dry contact process is the primary process used to produce sulfuric acid commercially (developed in 1831 by P. Phillips). Other associated products can also potentially be produced including pure SO2 gas and / or liquefied SO2 for sale or as a gas mixture for use in downstream processes as well as oleum in various concentrations and / or concentrated SO3 gas for making ultrapure sulfuric acid or for speciality applications such as sulfonation reactions.

[0006] Typically, the contact process involves obtaining a supply of sulfur dioxide by combusting a supply of sulfur with ambient air and then oxidizing the sulfur dioxide with oxygen in the presence of a catalyst (typically vanadium oxide) to accelerate the reaction in order to produce sulfur trioxide. The reaction of sulfur dioxide to sulfur trioxide is reversible and exothermic and it is important to appropriately control the temperature of the gases over the catalyst in order to achieve the desired conversion without damaging the catalyst and / or the contact apparatus which incorporates the catalyst.

[0007] The produced sulfur trioxide is then converted to sulfuric acid by absorption into a concentrated sulfuric acid solution with subsequent water addition to form a now more concentrated acid to maintain the acid concentration. This indirect reaction of the sulfur trioxide with water avoids the consequences of directly dissolving sulfur trioxide into water which is a highly exothermic reaction. The absorbing of the sulfur trioxide is usually done in one or more absorption towers.

[0008] In some instances, a portion of the produced sulfur trioxide is first used for oleum production by absorption into sulfuric acid or dilute oleum. Sulfur trioxide dissolves readily into sulfuric acid and high oleum concentrations can be produced. The absorbing of the sulfur trioxide to make oleum is usually done in one or more oleum absorption towers.

[0009] Common configurations of sulfuric acid plants based on the contact process include a single contact, single absorption or “SCSA” sulfuric acid plant and a double contact, double absorption or “DCDA sulfuric acid plant. As their names imply, the former involves a single stage of catalytic conversion of SO2 to SO3 in a contact apparatus and a single absorption stage of SO3 into sulfuric acid in an absorption apparatus while the latter involves two stages of both catalytic conversion and absorption. The former type of plant can have advantages in simplicity and cost but the latter can have advantages in efficiency and reduced emissions.

[0010] Numerous variations of SCSA and DCDA and other sulfuric acid plants have been disclosed in the art. For instance, ambient air is commonly used as the source of the oxygen required in the process. The use of ambient air is inexpensive and the conventional process operating at approximately 11-12 vol% SO2 into the contact apparatus perfectly balances the O2: SO2 molar ratio required for high conversion and the maximum allowable temperature in the first catalyst bed. The disadvantage of using air is that each required molecule of oxygen also comes with approximately four molecules of inert gas (mainly nitrogen and argon) which must also flow through the plant, therefore requiring very large equipment to handle the entire gas flow. As an alternative, the use of oxygen enriched air or pure oxygen as the oxygen source has also been suggested in the art. Such use can allow for a reduction in size for much of the equipment in the plant in principle but the combustion process results in exceedingly high temperatures which need to be addressed. For instance, in US3803298, a method for production of high purity concentrated SO2 using concentrated oxygen is disclosed which requires multiple stages of combustion to limit reactor temperatures. In another approach, reactor temperatures can be kept desirably low by recycling a portion of the gases following conversion in the contact apparatus (as disclosed in US4046866). Further still, combinations of such approaches may be considered.

[0011] In other variations of sulfuric acid plants, the contact apparatus employed for catalytically converting the sulfur dioxide to sulfur trioxide may differ. For instance, an adiabatic converter or a (pseudo)isothermal converter may be employed. As an example, W02008 / 052649 discloses a process for the continuous catalytic complete or partial oxidation of a starting gas containing from 0.1 to 66% by volume of sulfur dioxide plus oxygen, in which the catalyst is kept active by means of pseudo-isothermal process conditions with introduction or removal of energy. This process and apparatus disclosed are commercially available under the trade-marks CORE™ and CORE-S™.

[0012] Details regarding the many conventional options available and preferences for sulfuric acid production and the contact process are well known and can be found for instance in “Handbook of Sulfuric Acid Manufacturing”, Douglas Louie, ISBN 0-9738992-0-4, 2005, published by DKL Engineering, Inc., Ontario, Canada. Further, in WO2023 / 097389, details regarding a variety of prior art sulfuric acid production plants plus those of a novel, improved system comprising two recycle circuits can be found.

[0013] While the main objective in much of the art has been the production of sulfuric acid, attention has also been directed towards the production of oleum and / or sulfur trioxide. For example, this appears to be the objective in patent applications such as EP0002737, GB637585, and GB1413539. In all the aforementioned prior art however, the underlying approach is directed towards, and / or involves, maximizing the production of SO2 whether air or pure oxygen is used in the combustion. Indeed, generally the reason for using high purity oxygen is precisely to create a very high concentration of SO2 in the product gas stream, thereby reducing the overall gas volume that needs to be handled. As a result though, the oleum obtained following absorption contains substantial concentrations of dissolved SO2, e.g. of order of 10,000 ppm or greater which, without further purification, is orders of magnitude too high to be suitable for applications requiring pure oleum and / or SO3 gas. In applications in which the primary intended products are pure oleum and / or SO3 gas and where it is otherwise desirable to minimize the production of sulfuric acid, both the high level of dissolved SO2 in the oleum and any overproduction of sulfuric acid are unwanted and complicating.

[0014] There remains a desire then for methods and sulfuric acid plants to be developed that can more efficiently produce pure oleum and / or SO3 gas while reducing any unwanted co-production of sulfuric acid. The present invention addresses this desire and provides other benefits as disclosed below.

[0015] Summary

[0016] Methods for producing concentrated oleum or sulfur trioxide with high purity, and specifically with minimal sulfur dioxide, have been developed. The methods involve use of a single contact, single absorption (SCSA) sulfuric acid plant that operates according to a novel set of parameters. Concentrated oleum with very low amounts of dissolved sulfur dioxide (e.g. < 300 ppm) can be produced with sulfur utilization > 80% using a smaller, simpler and less costly sulfuric acid plant. High purity sulfur trioxide can be produced therefrom. And the amount of potentially unwanted sulfuric acid that is necessarily coproduced can be reduced.

[0017] The SCSA sulfuric acid plant relevant to the invention has much in common with certain plants in the prior art. Specifically, the relevant sulfuric acid plant used in producing concentrated oleum in accordance with the invention comprises: a supply of sulfur, a supply of oxygen at > 85% purity by volume, a reactor for the combustion of sulfur and oxygen, a reactor gases heat exchanger, a contact apparatus, an absorption apparatus comprising an oleum absorber, a recycle circuit comprising a pump and fluidly connecting an outlet from the absorption apparatus to an inlet of the reactor, and a discharge stack. Further, the steps involved in the operation of such a plant also have much in common with those in the prior art. That is, the method of the invention comprises the steps of: reacting sulfur from the sulfur supply and oxygen from the oxygen supply to form sulfur dioxide in the reactor; cooling hot outlet gases comprising sulfur dioxide obtained from the reactor in the reactor gases heat exchanger; catalytically converting the sulfur dioxide in the cooled gases obtained from the reactor gases heat exchanger to sulfur trioxide in the contact apparatus; absorbing sulfur trioxide from gases obtained from the contact apparatus into dilute oleum in the absorption apparatus to produce concentrated oleum; recycling a portion of gases from the outlet of the absorption apparatus to the inlet of the reactor; and discharging the remaining gases from the outlet of the absorption apparatus to atmosphere via the discharge stack.

[0018] The method of the invention however is characterized in that it is operated according to the following unconventional combination of parameters: the concentration of sulfur dioxide in the hot outlet gases obtained from the reactor is between 11.5 and 20% by volume; the oxygen to sulfur dioxide molar ratio in the cooled gases obtained from the reactor gases heat exchanger is > 1 ;

[0019] > 96% by volume of the sulfur dioxide in the cooled gases obtained from the reactor gases heat exchanger is catalytically converted to sulfur trioxide in the contact apparatus; and the portion of gases recycled from the absorption apparatus comprises > 95% by volume of the total gases from the outlet of the absorption apparatus. Inherently then, this means that the purge gas flow from the plant which is obtained from these total gases is < 5% by volume.

[0020] In certain exemplary embodiments, the oxygen to sulfur dioxide molar ratio in the cooled gases obtained from the reactor gases heat exchanger can be in the range from 1 to 1.75. Further, the contact apparatus in the plant can comprise an adiabatic converter or a pseudo-isothermal converter (e.g. a tubular pseudoisothermal converter). Further still, the supply of pure oxygen can be obtained via a pressure swing adsorption plant or a vacuum pressure swing absorption plant. Other alternatives can be considered too (e.g. a cryogenic air separation plant). And the absorption apparatus may comprise more than one oleum absorber, e.g. two oleum absorbers installed in series.

[0021] In an exemplary and preferred embodiment, the absorption apparatus in the SCSA sulfuric acid plant also comprises a sulfuric acid absorber downstream of the oleum absorber which absorbs sulfur trioxide not absorbed by the oleum absorber. The sulfuric acid absorber has an inlet fluidly connected to the outlet for outlet gases from the oleum absorber, an inlet for concentrated sulfuric acid, an outlet for more concentrated sulfuric acid and an outlet for outlet gases.

[0022] In preferred embodiments, the SCSA sulfuric acid plant comprises a purge gas scrubber and the method includes steps for scrubbing the remaining gases from the absorption apparatus prior to the step of discharging them to atmosphere via the discharge stack. These steps include: directing the remaining gases from the outlet of the absorption apparatus to the purge gas scrubber and scrubbing the remaining gases in the purge gas scrubber to remove sulfur dioxide and sulfur trioxide. In such a case, the purge gas scrubber can be a hydrogen peroxide-based purge gas scrubber but any other type of sulfur dioxide scrubber known in the art may be considered.

[0023] Advantageously, the method can produce concentrated oleum in which the amount of dissolved sulfur dioxide is < 300 ppm. In addition, the sulfur utilization associated with the method can desirably be >80%. Further still, the method allows for smaller plant sizes to be used to produce a given amount of pure oleum. In addition, a drying tower is not required (is absent) in such a SCSA sulfuric acid plant.

[0024] Related methods of the invention can also be used to produce concentrated sulfur trioxide gas in mostly similar SCSA sulfuric acid plants. Such plants however may additionally include an oleum circulation loop fluidly connecting the inlet for dilute oleum to the outlet for concentrated oleum in the oleum absorber, and a sulfur trioxide evaporator in the oleum circulation loop. In such a case, the methods comprise the steps of: producing concentrated oleum according to the aforementioned method, then directing the concentrated oleum produced to the sulfur trioxide evaporator via the oleum circulation loop, and evaporating concentrated sulfur trioxide gas from the concentrated oleum. The reduced concentration oleum is then returned to the oleum absorber via the circulation loop.

[0025] In an exemplary embodiment for producing concentrated sulfur trioxide gas, the absorption apparatus in the SCSA sulfuric acid plant also comprises a sulfuric acid absorber downstream of the oleum absorber. The sulfuric acid absorber has an inlet fluidly connected to the outlet for outlet gases from the oleum absorber, an inlet for concentrated sulfuric acid, an outlet for more concentrated sulfuric acid and an outlet for outlet gases. The absorption apparatus can optionally comprise a second oleum absorber in the oleum circulation loop. The second oleum absorber typically would operate with a lower oleum concentration than the first oleum absorber and would be installed in series therewith.

[0026] The invention can also include a SCSA acid plant if configured in such a way that it can only produce concentrated oleum and / or sulfur trioxide according to one of the aforementioned inventive methods, e.g. if the plant equipment is designed and arranged to only allow operation according to the inventive method or if the control hardware was programmed using PROM, or programmable read only memory devices, and thus necessarily operated in accordance with the inventive methods.

[0027] Brief Description of the Drawings

[0028] Figure 1 is a schematic of a SCSA (single contact, single absorption) sulfuric acid plant for producing pure oleum in accordance with the invention. Pure oxygen is used in the sulfur combustion and a substantial amount of flue gas after the absorption step is recycled. Figure 2 shows a flowchart of the basic inventive method steps involved in producing pure oleum using the SCSA sulfuric acid plant of Figure 1.

[0029] Figure 3a is a schematic of the absorption apparatus and oleum circulation loop in a SCSA sulfuric acid plant for producing pure sulfur trioxide gas in accordance with the invention.

[0030] Figure 3b shows a schematic of alternative absorption apparatus and oleum circulation loop in a SCSA sulfuric acid plant used to produce pure sulfur trioxide gas in accordance with the invention.

[0031] Figures 4a and 4b show schematics of prior art sulfuric acid plants used to produce oleum in the Comparative Examples.

[0032] Detailed Description

[0033] Unless the context requires otherwise, throughout this specification and claims, the words "comprise", “comprising” and the like are to be construed in an open, inclusive sense. The words “a”, “an”, and the like are to be considered as meaning at least one and are not limited to just one.

[0034] The words "high purity oxygen”, “enhanced oxygen” or “pure oxygen” are to be considered as meaning oxygen in concentrations equal to or exceeding 85% by volume.

[0035] “Oleum” herein refers to liquid solutions of sulfur trioxide in sulfuric acid.

[0036] “Pure oleum” refers to solutions consisting essentially of oleum, i.e. with essentially no dissolved SO2 nor other components.

[0037] The term “sulfur utilization” refers to the amount of sulfur atoms that end up being absorbed into the intended product oleum or product SO3 gas divided by the amount of sulfur atoms contained in the sulfur supplied to the reactor, expressed as a percentage. For clarity; only sulfur atoms absorbed from the reactor gases in a sulfuric acid plant are included in this definition and any sulfur atoms already contained in sulfuric acid and / or dilute oleum entering the plant from elsewhere are excluded. Any sulfur atoms contained in sulfuric acid exported from the plant (by-product acid) are also excluded.

[0038] The trade-mark CORE-S™ refers to the molten salt cooled tubular reactor of the technology disclosed in the aforementioned W02008 / 052649. The present invention relates to methods for producing concentrated oleum or sulfur trioxide of high purity and particularly with minimal sulfur dioxide present. A single contact, single absorption (SCSA) sulfuric acid plant is employed but which unconventionally includes a recycle circuit and is operated under an unconventional set of parameters. The invention is particularly useful in applications that only require pure oleum and / or sulfur trioxide and in which the co-production of sulfuric acid is preferably minimized. Compared with conventional such plants, the invention provides for a significant reduction in the required plant size and hence cost (e.g. >20% reduction expected) with less requirement for equipment and maintenance along with a reduction in emissions and a greater ease in making such plants modular.

[0039] Oleum consists of sulfuric acid with dissolved SOs. It is typically expressed as the amount of SO3 dissolved in the total amount of liquid, e.g. 20% oleum contains 20wt% SO3 and 80wt% H2SO4. Oleum can be produced by dissolving SO3 in a circulating stream of more dilute oleum in a similar fashion as SO3 can be dissolved in circulating sulfuric acid in the acid absorption towers of a sulfuric acid plant. In such a plant, because oleum has a high SO3 vapor pressure, not all of the SO3 in the incoming gas is dissolved in the oleum and typically therefore the oleum absorber (e.g. an oleum absorption tower) is installed directly upstream of the primary acid absorption tower. Any remaining SO3 in the gas leaving the oleum absorption tower is absorbed in the primary acid absorption tower and converted into sulfuric acid. When the primary product is oleum, sulfuric acid from the primary acid absorption system is typically fed to the oleum tower. If the aim is to produce SO3 gas then the concentrated oleum from the oleum absorption tower can be directed to a SO3 evaporator where gaseous SO3 is created and the now more dilute oleum is returned to the oleum absorption tower in a closed loop. In such a system no sulfuric acid feed is required for the oleum system. More than one oleum absorption tower in series may be used to improve sulfur utilization as the subsequent oleum absorption tower operates at lower oleum concentration and therefore can absorb more SO3 from the gas.

[0040] The method of the invention involves operating a SCSA sulfuric acid plant under an unconventional combination of parameters which include: reacting sulfur and oxygen in a combustion reactor to form sulfur dioxide at a concentration between 11.5 and 20% by volume with an atypical oxygen to sulfur dioxide molar ratio > 1, atypically catalytically converting > 96% by volume of the sulfur dioxide to sulfur trioxide in a contact apparatus, and atypically recycling > 95% by volume of the total gases from an absorption apparatus to the combustion reactor. Inherently then, the purge amount under such operating conditions is thus < 5% by volume. Under such conditions, concentrated oleum can be produced with low dissolved sulfur dioxide concentration, preferably below 300 ppm by weight, with a sulfur utilization > 80% in a smaller, cost-effective SCSA sulfuric acid plant. Further, the co-production of unnecessary sulfuric acid is also reduced. Figure 1 shows a schematic of a SCSA sulfuric acid plant which is configured to (e.g. by way of programming) produce pure oleum in accordance with the invention. Here, SCSA sulfuric acid plant 1 comprises a supply of enhanced or pure oxygen 2 (i.e. > 85% purity by volume), a supply of sulfur 3, reactor 5 for the combustion of sulfur and oxygen, a reactor gases heat exchanger 6, contact apparatus 7, and absorption apparatus 8. In the embodiment shown, absorption apparatus 8 comprises oleum absorber 12 (which can be a dual oleum absorber) and sulfuric acid absorber 13. Sulfuric acid absorber 13 has an inlet fluidly connected to the outlet for outlet gases from the oleum absorber, an inlet for concentrated sulfuric acid, an outlet for more concentrated sulfuric acid and an outlet for outlet gases (note: none of these inlets or outlets are called out in Fig. 1). Downstream of absorption apparatus 8 is optional additional equipment 10 for additional desired process steps (e.g. purge gas scrubbing) as is well known to those in the art, followed by discharge stack 11. Sulfuric acid plant 1 further comprises recycle circuit 20 comprising pump 21 that fluidly connects an outlet from absorption apparatus 8 to an inlet of reactor 5. Sulfuric acid plant 1 may be provided with a pre-programmed control system (not shown) such that plant 1 produces oleum in accordance with the invention. (Those skilled in the art will also readily appreciate that plants like that shown in Figure 1 typically involve numerous more physical elements, e.g. superheater, internal or external gas / gas heat exchangers, boiler, or the like, inputs & outputs, and more interconnections than are shown in the simplified schematics here. These have been omitted to avoid clutter but are well known to those in the art.)

[0041] As illustrated in the flowchart of Figure 2, the basic inventive method steps involved in producing pure oleum using the SCSA sulfuric acid plant of Figure 1 are similar to those used in a conventional plant. That is the method comprises a series of steps including reacting 31, cooling 32, catalytically converting 33, absorbing 34, recycling 35, and discharging 36. In reacting step 31, oxygen from the enhance or pure oxygen supply 2 and sulfur from the sulfur supply 3 are reacted to form sulfur dioxide in reactor 5. In cooling step 32, hot outlet gases comprising sulfur dioxide obtained from reactor 5 are cooled in reactor gases heat exchanger 6. In catalytically converting step 33, the sulfur dioxide in the cooled gases obtained from reactor gases heat exchanger 6 are catalytically converted to sulfur trioxide in contact apparatus 7. In absorbing step 34, sulfur trioxide from gases obtained from contact apparatus 7 is absorbed into dilute oleum provided to oleum absorber 12 in absorption apparatus 8 at inlet 8a to produce concentrated strong oleum obtained at outlet 8b. As shown, the embodiment in Fig. 1 also includes optional sulfuric acid absorber 13 and thus absorbing step 34 also involves absorbing sulfur trioxide into concentrated sulfuric acid in sulfuric acid absorber 13 to form more concentrated sulfuric acid. In recycling step 35, a substantial portion of gases from the outlet of absorption apparatus 8 are recycled to the inlet of reactor 5. In discharging step 36, the remaining gases from the gases outlet of absorption apparatus 8 are discharged to atmosphere via discharge stack 11. However, the inventive method is characterized in that in an unconventional manner the concentration of sulfur dioxide in the hot outlet gases obtained from reactor 5 is between 11.5 and 20% by volume, and the oxygen to sulfur dioxide molar ratio in the cooled gases obtained from reactor gases heat exchanger 6 is > 1. (For comparison’s sake, in conventional embodiments using ambient air as the supply of oxygen, a representative [SO2] is 11.5% and the associated O2: SO2 molar ratio is 0.822.) Further, in the inventive method, > 96% by volume of the sulfur dioxide in the cooled gases obtained from reactor gases heat exchanger 6 is catalytically converted to sulfur trioxide in contact apparatus 7. (For comparison’s sake, even though it is known that greater conversion values can be achieved by employing more catalyst in the contact apparatus, in conventional embodiments it is generally considered to be an inefficient use of additional catalyst and thus uneconomical and undesirable overall to do so. Instead, typically only from about 95 to 95.5% of the sulfur dioxide is converted in conventional plants upstream of the first acid absorption tower.) Further still, in the inventive method, atypically almost all the gases from absorption apparatus 8 are recycled. That is, the portion of gases recycled from absorption apparatus 8 comprises > 95% by volume of the total gases from the outlet of absorption apparatus 8. As a consequence, this results in an atypically small amount of purge flow coming from such plants, namely < 5% by volume.

[0042] In exemplary operation of SCSA sulfuric acid plant of Figure 1, the oxygen to sulfur dioxide molar ratio in the cooled gases obtained from the reactor gases heat exchanger can be in the range from 1 to 1.75. Operation in this range has a further advantage that it does not create an enriched oxygen atmosphere (O2 concentration > 21%) inside the equipment, which otherwise may require additional safety precautions. In one embodiment shown in Figure 1, contact apparatus 7 has been considered to be an adiabatic converter (as discussed in inventive example 1 below) or in a further embodiment has been considered to be a pseudoisothermal converter (as illustrated in inventive example 2 below).

[0043] In a desirable embodiment, supply of pure oxygen 2 can comprise a pressure swing adsorption plant or a vacuum pressure swing absorption plant. However other supplies of pure oxygen may also be considered, e.g. cryogenic air separation or by-product oxygen from another process e.g. water electrolysis. While use of a supply of enhanced or pure oxygen in the inventive method requires such additional apparatus when compared to plants that use ambient atmospheric air for their oxygen supply, an advantage of the present invention is that a drying tower is not required to remove water from the ambient air to prevent plant corrosion as is commonly otherwise required. A further advantage is that unlike for operation with ambient air, it also allows the sulfuric acid plant to be operated with any desired oxygen concentration in the gas. As is known to those skilled in the art, the conversion of SO2 to SO3 is increased when oxygen concentration is increased and operating with a higher oxygen concentration is therefore advantageous. A further advantage is that unlike for operation with ambient air it is possible to recycle gases from the absorption apparatus to the reactor without significantly increasing the overall gas flow through the plant and thus overall SO2 conversion can be maximized.

[0044] Oleum absorber 12 in absorption apparatus 8 may be simple or a more complex arrangement, e.g. a system comprising two or more oleum absorbers. Further still, it is generally considered desirable for optional additional equipment 10 to include a purge gas scrubber (not called out in Figure 1) for purposes of scrubbing the remaining sulfurous gases from absorption apparatus 8. An exemplary preferred purge gas scrubber is a hydrogen peroxide -based purge gas scrubber. In such a case, prior to the step of discharging the remaining gases to atmosphere via discharge stack 11, operation of plant 1 includes the steps of directing the remaining gases from the outlet of absorption apparatus 8 to the purge gas scrubber and then scrubbing the remaining gases in the purge gas scrubber to remove sulfur dioxide and sulfur trioxide. The sulfur dioxide and sulfur trioxide removed from the gas react with the hydrogen peroxide solution to form dilute sulfuric acid which can be directed to the acid absorption system and thus advantageously no effluent from the purge gas scrubber need to be disposed.

[0045] The inventive method employs high purity oxygen (> 85% and preferably >90% oxygen) instead of ambient air. Drying of ambient air in the sulfuric acid plant is thus not required and only high purity oxygen without moisture enters the plant. Consequently, there is no need for a drying system (e.g. drying tower and associated equipment) in the plant. Further, the use of high purity oxygen allows the O2:SO2 molar ratio to be freely selected. Higher oxygen concentrations allow for improved conversion and thus a higher SO2 concentrations can be used at the inlet of the contact apparatus while achieving the same SO2 concentration at the inlet of the absorption apparatus. This concentration is now only limited by the maximum allowable temperature at the exit of the first catalyst bed in the contact apparatus. Typically, if the contact apparatus is a conventional 3-bed adiabatic converter, this limits the SO2 concentration to approximately 13% by volume. However, if the contact apparatus is a pseudo-isothermal converter (e.g. the CORE-S™ molten salt cooled tubular pseudo-isothermal reactor) SO2 concentrations up to 20% by volume may be considered. A higher SO2 concentration at the inlet of the contact apparatus results in a higher SO3 concentration after the contact apparatus 7, and thus provides for improved sulfur utilization as more SO3 can be absorbed in the oleum absorber.

[0046] The method aims to obtain concentrated oleum of high purity and particularly with low concentrations of dissolved SO2 (i.e. < 300 ppm). The SO2 remaining after catalytic conversion determines the dissolved SO2 concentration in the oleum. As SO2 to SO3 conversion decreases with increasing SO2 inlet concentration (as dictated by the chemical equilibrium), the SO2 concentration at the oleum absorber inlet cannot be increased to very high levels without exceeding the limits on dissolved SO2. This limits the allowable inlet SO2 concentation to the contact apparatus unless conversion is improved. The low concentration of dissolved SO2 can be achieved by limiting the SO2 concentration in the gas leaving the contact apparatus (e.g. to <0.7vol%) or to a lesser extent by adjusting the operation of the oleum absorbers employed.

[0047] Limiting the SO2 concentration in the gas leaving the contact apparatus can be achieved by 1) controlling the SO2 concentration entering the contact apparatus to relatively low levels (e.g. <11.5%) SO2 since higher [SO2] would shift the equilibrium away from SO3); 2) controlling the O2 concentration entering the contact apparatus to relatively high levels (e.g. >11.5%, since higher [O2] shifts the equilibrium towards SO3); and 3) loading more than the conventional amount of catalyst in the contact apparatus (to push the catalytic conversion process to be very close to equilibrium). The invention allows the SO2 and O2 concentrations to be controlled independently, thus enabling operation at high O2 to SO2 molar ratio (e.g. > 1.0) at higher than conventional SO2 concentration (>11.5 vol%) which improves overall conversion and thus reduces the [SO2] at the inlet of the oleum absorber.

[0048] Operating conditions, in particular oleum concentration and temperature, can be used to influence the amount of SO2 dissolved in the concentrated oleum. In general, lower oleum concentration and higher oleum temperature result in a reduction in dissolved SO2. Generally, the concentrated oleum concentration is dictated by the downstream process or application requirements and there can be limited opportunity for adjustment. The operating temperature can generally be adjusted, but a higher operating temperature results in an increase for the oleum vapor pressure and therefore a reduction in the amount of SO3 that can be absorbed and thus a lower sulfur utilization.

[0049] The invention provides for a much simpler, smaller and less expensive plant to produce desirably pure oleum product. While the requirement for a supply of enhanced or pure oxygen (e.g. via VPSA or other) adds cost, the elimination of a drying tower and smaller plant size can still result in significant net capital savings.

[0050] A further advantage of the invention is less unwanted by-product sulfuric acid may be produced. The byproduct sulfuric acid here is the concentrated sulfuric acid produced in sulfuric acid absorbers or towers in the plant (e.g. sulfuric acid absorber 13). In applications where pure gaseous SO3 is the desired product, such by-product acid is a waste product that must be handled, stored and transported off-site at considerable effort and investment cost and with reduced sales value. By-product acid production is a function of water content of the oxygen source and sulfur utilization for the plant.

[0051] In prior art plants using ambient air, water in the ambient air is generally the most significant source of water entering the plant. This water is removed in a drying tower by absorption into concentrated sulfuric acid. To maintain a constant sulfuric acid concentration, a portion of the SO3 obtained from the reactor must therefore be dedicated to react with the water that is absorbed. At a representative 3.6 vol% ambient water level, this limits the sulfur utilization in the plant to about 67.5%. In the invention, dry enhanced or pure oxygen is used which contains essentially no water, and thus no by-product sulfuric acid need be produced and hence greater sulfur utilization can be achieved.

[0052] In inventive embodiments employing two (dual) oleum absorption stages, the sulfur utilization can be further increased and the amount of dissolved SO2 further reduced by adjusting the operating temperature of the oleum absorbers employed. Operating the first oleum absorber at a relatively higher temperature reduces the amount of SO2 that is dissolved in the oleum, but results in higher SO3 concentration in the gas leaving the first oleum absorber. But this SO3 can be absorbed in the second oleum absorber which can be operated at a relatively lower temperature to increase SO3 absorption and improve sulfur utilization. As a result, the sulfur utilization may for instance be increased to >80% for a plant comprising a single oleum absorber but to >90% for a plant comprising dual oleum absorbers

[0053] In the present invention, the recycling of unconverted SO2 to the reactor also achieves lower sulfur losses to the discharge stack or any of the tail gas scrubber present when compared to prior art processes using ambient air. Recycling reduces the amount of gas discharged to atmosphere and a further reduction in plant size can be achieved when compared to conventional designs due to much lower by-product acid production.

[0054] In the recycling step, most of the unconverted SO2 together with the small amounts of N2 / Ar gases present are recycled to the reactor whereas only a small remaining portion is purged from the plant. Specifically, > 95% by volume of the total gases from the outlet of the absorption apparatus is recycled and thus < 5% is purged. (The amount that is recycled primarily depends on the concentration of the oxygen used and the concentration of inerts in the gas going to the contact apparatus.) The purge stream consists of the inert gas (mainly N2 / Ar) contained in the supplied pure oxygen feed and is removed to prevent accumulation in the system. Because most of the gas is recycled, the heat exchangers used to reheat gas, the secondary catalyst bed(s), the heat exchangers used to cool the gas from the contact apparatus and the secondary absorption tower in typical prior art DCDA plants are not required. Of course, this further allows for a reduction in the cost of the plant.

[0055] Optionally, the purge gas can be discharged directly via the discharge stack without further treatment. Alternatively, additional SO2 can be removed therefrom using a purge gas scrubber (as part of optional additional equipment 10 in Figure 1). Because the gas volume involved here is very small compared to that in a conventional DCDA (e.g. reduced by >97.5%), the purge gas can be easily treated at low cost and plant emissions can easily be kept desirably low.

[0056] The invention can also desirably be used to produce pure sulfur trioxide from the pure oleum produced via the aforementioned methods. For such purposes, the SCSA sulfuric acid plant used would be similar to that shown in Figure 1 yet would also comprise an additional oleum circulation loop and a SO3 evaporator. Figure 3a shows a schematic of that portion of such a plant which comprises the absorption apparatus and the additional oleum circulation loop features. As shown, oleum circulation loop 40 is fluidly connected between the absorption apparatus outlet 8b and inlet 8a with SO3 evaporator 41 provided in circulation loop 40. As before, SO3 obtained from contact apparatus 7 would be absorbed in absorption apparatus 8 to produce pure concentrated oleum. This concentrated oleum would then be directed from outlet 8b to SO3 evaporator 41 where a portion of the SO3 dissolved in the oleum would be evaporated as an essentially pure SO3 stream. Dilute (lower concentration) oleum then would leave SO3 evaporator 41 as a liquid and be returned to inlet 8a of absorption apparatus 8 for reuse. Note that any SO2 present and dissolved in the circulating oleum would also be evaporated from the stream in the evaporator thereby degrading the purity of the produced SO3. Because only a portion of the SO3 contained in the oleum is evaporated along with the SO2, the concentration of SO2 in the produced SO3 gas can be 4 to 6 times higher than in the concentrated oleum. This is why having a minimal amount of SO2 dissolved in the produced oleum can be so important. In such an arrangement then, the oleum is merely a carrier of SO3 and the sulfur utilization associated with the process determines the size of the required plant. A lower utilization results in more gas being required at a certain SO2 concentration in order to produce the oleum desired, and as a result also increases the amount of unwanted by-product sulfuric acid that is produced from any SO3 not absorbed in oleum absorber 8 and which is removed before gas is recycled to the reactor.

[0057] Ideally, the concentration of the dilute oleum returned to oleum absorber 8 should be as low as possible, but due to the corrosive nature of the liquid, the return concentration is typically kept above about 15 wt% oleum. Also ideally, the concentration of the concentrated oleum obtained from oleum absorber 8 should be as high as possible, but as the concentration increases, the partial pressure of SO3 increases and less SO3 is absorbed from the gas in oleum absorber 8.

[0058] In a preferred embodiment, the absorption apparatus comprises a second oleum absorber in the oleum circulation loop, e.g. a dual oleum tower system as shown in Figure 3b. Use of such a system improves the overall absorption of SO3 from the gases as the first oleum tower operates at the concentrated oleum concentration (with high vapor pressure) and the second oleum tower operates at an intermediate concentration (set by mass balance) with a lower concentration. Thus, such a dual oleum tower system can remove more SO3 even though the dilute / concentrated oleum concentrations involved remain the same.

[0059] In the embodiments shown in Figure 3a and 3b, all the dilute oleum provided to oleum absorber 8 is expected to come from oleum circulation loop 40. However, in other embodiments this is not required. It is also possible that the dilute oleum comes from another source. In either case, there is no need to produce sulfuric acid from SO3 contained in the concentrated oleum stream and SO3 is only added to increase the oleum concentration. In other embodiments, it is possible that the dilute oleum source provided to oleum absorber 8 comes from a stream of concentrated sulfuric acid from a sulfuric acid absorber in the plant. The produced concentrated oleum now would not be cycled back to oleum absorber 8 in an oleum absorption loop.

[0060] While the above description discloses the general arrangement and operation of certain embodiments of the invention, those of ordinary skill will appreciate that certain specifics may need to be modified somewhat in accordance with differing situations and plant apparatus. It is expected however that those of ordinary skill will readily be able to make such modifications based on the disclosed teachings and the following Examples for guidance. The following Examples have been included to illustrate certain aspects of the invention but should not be construed as limiting in any way.

[0061] Examples

[0062] Re oleum production

[0063] For comparison purposes, several different comparative and inventive sulfuric acid plant designs used to produce oleum were considered and modelling results were obtained for each. Figures 1, 4a and 4b show schematics of those sulfuric acid plants which were considered. Except where indicated, the components appearing in Figures 4a and 4b are similar to those in Figure 1 and were operated in a like manner.

[0064] Comparative Example 1 involved a comparative double absorption, double contact or DCDA sulfuric acid plant supplied with sulfur and ambient air as shown in Figure 4a. Being of double absorption, double contact design, the plant comprises second contact apparatus 7i and second sulfuric acid absorber 13i. The oleum absorber in this embodiment comprises a single oleum tower and no portion of gases from the absorption apparatus are recycled to the reactor. The DCDA sulfuric acid plant shown in Figure 4a is considered representative of typical, conventional commercial DCDA sulfuric acid plants. In this example the sulfur utilization was about 52.1% and the amount of dissolved sulfur dioxide in the produced concentrated oleum was < 300 ppm. The remainder is generally converted to by-product acid or leaves via the stack as SO2.

[0065] Comparative Example 2 on the other hand shows a comparative DCDA sulfuric acid plant like that shown in Figure 4a except that it has been optimized to produce desirable oleum product. In this regard, the operation of contact apparatus 7 has been changed to increase the conversion. Further, a dual oleum adsorption tower arrangement has been employed as the oleum absorber and a lower concentration of dilute oleum has been supplied to the oleum absorber. In this embodiment, oleum production is limited by the ambient humidity in the ambient air. As shown, with a water content @ 3.6 vol%, a minimum of 32.5% of the supplied sulfur must be used to produce sulfuric acid product and thus the sulfur utilization was limited to about 67.4% with any remainder leaving as dissolved SO2. Like the embodiment of Comparative Example 1, the amount of dissolved sulfur dioxide in the produced concentrated oleum was again < 300 ppm.

[0066] A yet further comparative embodiment that may be considered from the prior art is that of Comparative Example 3 involving the SCSA sulfuric acid plant shown in Figure 4b. This embodiment is like that shown in Figure 4a in many ways except that a second contact apparatus and second sulfuric acid absorber are not employed. Further, in this embodiment, pure oxygen is supplied to the reactor instead of ambient air and a second reactor 5i and a second reactor gases heat exchanger 6i are provided for purposes of combusting the sulfur in two stages to prevent overheating. This embodiment also comprises recycling circuit 20 and pump 21 for recycling gases from the absorption apparatus 8 to the first reactor 5. The use of pure oxygen instead of air in this example resulted in 40 vol% SO2 in the stream entering the contact apparatus, hence allowing for a reduction in the size of contact apparatus 7 and other downstream equipment in the plant. The flowrate in recycling circuit 20 and the resulting SO2 concentration going to the contact apparatus is determined by the maximum temperature in reactor 5 and the number of combustion stages (2 stages in this embodiment but prior art shows that up to 60% concentration can be achieved using more stages). Due to the high SO2 concentration, the conversion achieved in the contact apparatus was reduced to 85%. In this embodiment, 95.6% of the supplied sulfur was converted to oleum and just 3.5% to sulfuric acid. 1.8% by weight of the SO2 produced in the reactor ended up dissolved in the oleum produced or out the stack. A high % of sulfur was thus converted to oleum but the high residual [SO2] in the reactor gases results in unacceptably high dissolved SO2 in the oleum (i.e. >3500 ppm).

[0067] Inventive Example 1 involved a SCSA sulfuric acid plant like that shown in Figure 1 and as discussed in detail in the preceding Description. In this inventive example, contact apparatus 7 has been considered to be an adiabatic converter. Further, the C^SCE molar ratio has been increased to 1.538 and sulfur utilization is 86.3%. While the sulfur utilization was slightly lower than in Comparative Example 3, the residual [SO2] in the reactor gases was now desirably < 0.5 vol% and the concentration of dissolved SO2 was less than 300 ppm.

[0068] Inventive Example 2 was similar to that of Inventive Example 1 except that contact apparatus 7 was a tubular pseudo-isothermal converter (e.g. a CORE™ converter) rather than an adiabatic converter. This allowed for a higher SO2 concentration at the contact apparatus and in this embodiment the SO2 concentration was 19.5% andthe O2:SO2 molar ratio was 1.026. Forthis embodiment, the sulfur utilization was 92.7% and the dissolved SO2 concentration was less than 300 ppm.

[0069] The following table summarizes the various operating parameters assumed for each of these examples along with the various results obtained from modelling.

[0070] “ Maximum oleum production is limited by water content in gas. The amounts shown are the maxima achievable for the stated moisture in the ambient air. Re sulfur trioxide gas production

[0071] Figures 3a and 3b show two different inventive designs that can be used to produce pure sulfur trioxide gas using concentrated oleum obtained from the SCSA sulfuric acid plant depicted in Figure 1. These figures both show schematics of the relevant absorption apparatuses and oleum circulation loops. The concentrations of SO3 at various locations in the plants are also shown in these figures. Again, unless otherwise specified, the various components in each of these embodiments shared a common design and were operated in a like manner to those in Figure 1.

[0072] Figure 3a shows a schematic of the absorption apparatus and oleum circulation loop in an inventive SCSA sulfuric acid plant for producing pure sulfur trioxide gas. The embodiment shown here comprises oleum circulation loop 40 and SO3 evaporator 41 in this loop. As shown, oleum circulation loop 40 fluidly connects dilute oleum inlet 8a to concentrated oleum outlet 8b in absorption apparatus 8. In this embodiment, absorption apparatus 8 merely comprises oleum absorber 12 and the dilute oleum provided to oleum absorber 12 is supplied solely from oleum circulation loop 40. Gaseous pure SO3 product is obtained from outlet 41a in SO3 evaporator 41 which can then be used to produce other products e.g. ultra pure sulfuric acid as shown. For this embodiment, the sulfur utilization was 61.5% and the dissolved SO2 concentration was less than 300 ppm.

[0073] Figure 3b shows a schematic of an alternative embodiment to that of the absorption apparatus and oleum circulation loop shown in Figure 3a. Absorption apparatus 8 in the embodiment shown here comprises second oleum absorber 12i which also appears in oleum circulation loop 40. As shown, a portion 40a of oleum circulation loop 40 fluidly connects dilute oleum inlet 12a to concentrated oleum outlet 12ib in absorption apparatus 8. This arrangement resulted in a greater total amount of SO3 absorbed in the oleum. For this embodiment, the sulfur utilization was 87.7% and the dissolved SO2 concentration was less than 300 ppm.

[0074] All of the above U.S. patents, U.S. patent applications, foreign patents, foreign patent applications and nonpatent publications referred to in this specification, are incorporated herein by reference, in their entirety, including U.S. Provisional Patent Application No. 63 / 550,179 filed February 6, 2024, unless otherwise stated. Aspects of the embodiments can be modified, if necessary to employ concepts of the various patents, applications, and publications to provide yet further embodiments.

[0075] While particular elements, embodiments and applications of the present invention have been shown and described, it will be understood, of course, that the invention is not limited thereto since modifications may be made by those skilled in the art without departing from the spirit and scope of the present disclosure, particularly in light of the foregoing teachings. Such modifications are to be considered within the purview and scope of the claims appended hereto.

Claims

Claims1. A method for producing concentrated oleum with little dissolved sulfur dioxide in a single contact, single absorption sulfuric acid plant, the plant comprising: a supply of sulfur; a supply of oxygen at > 85% purity by volume; a reactor for the combustion of sulfur and oxygen; a reactor gases heat exchanger; a contact apparatus; an absorption apparatus comprising an oleum absorber having an inlet for gases from the reactor gases heat exchanger, an inlet for dilute oleum, an outlet for concentrated oleum and an outlet for outlet gases; a recycle circuit comprising a pump and fluidly connecting an outlet from the absorption apparatus to an inlet of the reactor; and a discharge stack; the method comprising reacting sulfur from the sulfur supply and oxygen from the oxygen supply to form sulfur dioxide in the reactor; cooling hot outlet gases comprising sulfur dioxide obtained from the reactor in the reactor gases heat exchanger; catalytically converting the sulfur dioxide in the cooled gases obtained from the reactor gases heat exchanger to sulfur trioxide in the contact apparatus; absorbing sulfur trioxide from gases obtained from the contact apparatus into dilute oleum acid in the oleum absorber in the absorption apparatus to produce concentrated oleum; recycling a portion of gases from the outlet of the absorption apparatus to the inlet of the reactor; and discharging the remaining gases from the outlet of the absorption apparatus to atmosphere via the discharge stack; characterized in that: the concentration of sulfur dioxide in the hot outlet gases obtained from the reactor is between 11.5 and 20% by volume; the oxygen to sulfur dioxide molar ratio in the cooled gases obtained from the reactor gases heat exchanger is > 1 ;> 96% by volume of the sulfur dioxide in the cooled gases obtained from the reactor gases heat exchanger is catalytically converted to sulfur trioxide in the contact apparatus; and the portion of gases recycled from the absorption apparatus comprises > 95% by volume of the total gases from the outlet of the absorption apparatus.

2. The method of claim 1 wherein the oxygen to sulfur dioxide molar ratio in the cooled gases obtained from the reactor gases heat exchanger is in the range from 1 to 1.75.

3. The method of claim 2 wherein the contact apparatus comprises an adiabatic converter or a pseudo-isothermal converter.

4. The method of claim 1 wherein the amount of dissolved sulfur dioxide in the produced concentrated oleum is < 300 ppm.

5. The method of claim 1 wherein the sulfur utilization is >80%.

6. The method of claim 1 wherein the supply of oxygen comprises a pressure swing adsorption plant or a vacuum pressure swing absorption plant.

7. The method of claim 1 wherein the absorption apparatus comprises a sulfuric acid absorber having an inlet fluidly connected to the outlet for outlet gases from the oleum absorber, an inlet for concentrated sulfuric acid, an outlet for more concentrated sulfuric acid and an outlet for outlet gases.

8. The method of claim 1 wherein the single contact, single absorption sulfuric acid plant comprises a purge gas scrubber and the method comprises: prior to the step of discharging the remaining gases to atmosphere via the discharge stack, directing the remaining gases from the outlet of the absorption apparatus to the purge gas scrubber and scrubbing the remaining gases in the purge gas scrubber to remove sulfur dioxide and sulfur trioxide.

9. The method of claim 8 wherein the purge gas scrubber is a hydrogen peroxide-based purge gas scrubber.

10. The method of claim 1 wherein the single contact, single absorption sulfuric acid plant is absent a drying tower.

11. A method for producing concentrated sulfur trioxide gas in a single contact, single absorption sulfuric acid plant, the plant comprising: a supply of sulfur; a supply of oxygen at > 85% purity by volume; a reactor for the combustion of sulfur and oxygen; a reactor gases heat exchanger; a contact apparatus;an absorption apparatus comprising an oleum absorber having an inlet for gases from the reactor gases heat exchanger, an inlet for dilute oleum, an outlet for concentrated oleum and an outlet for outlet gases; an oleum circulation loop fluidly connecting the inlet for dilute oleum to the outlet for concentrated oleum in the oleum absorber; a sulfur trioxide evaporator in the oleum circulation loop; a recycle circuit comprising a pump and fluidly connecting an outlet from the absorption apparatus to an inlet of the reactor; and a discharge stack; the method comprising: producing concentrated oleum according to the method of claim 1 ; directing the concentrated oleum to the sulfur trioxide evaporator via the oleum circulation loop; and evaporating concentrated sulfur trioxide gas from the concentrated oleum.

12. The method of claim 11 wherein the absorption apparatus comprises a sulfuric acid absorber having an inlet fluidly connected to the outlet for outlet gases from the oleum absorber, an inlet for concentrated sulfuric acid, an outlet for more concentrated sulfuric acid and an outlet for outlet gases.

13. The method of claim 11 wherein the absorption apparatus comprises a second oleum absorber in the oleum circulation loop.

14. A single contact, single absorption sulfuric acid plant configured to produce concentrated oleum according to the method of claim 1.

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