WO2023025372A1

WO2023025372A1 - Sulfuric acid concentration plant

Info

Publication number: WO2023025372A1
Application number: PCT/EP2021/073393
Authority: WO
Inventors: Max HOSTETTLER
Original assignee: Bertrams Chemieanlagen Ag
Priority date: 2021-08-24
Filing date: 2021-08-24
Publication date: 2023-03-02

Abstract

A plant for the concentration of sulfuric acid comprising a feeding line, a rectifying column, at least one concentration unit, an acid cooling and discharge unit a condensing unit, a product line, and a vacuum system, with at least one electrical heating device in each concentration unit, each heating device comprising a multitude of heating elements, each heating element being surrounded by a ceramic sheathing, the heating elements being contained in a glass tube for insulation against surrounding hot sulphuric acid, the quartz tube being closed at its bottom end.

Description

Sulfuric acid concentration plant

The present invention relates to a plant for the concentration of sulfuric acid, in particular of sulfuric acid which originates from the drying of humid gases, such as chlorine produced by the electrolysis of an aqueous NaCI solution.

Sulfuric acid is used in many processes as dehydrating and drying agent to remove water from other liquids or gases. The spent acid needs to be concentrated before re-using it in the process. Examples for drying processes with highly concentrated sulfuric acid are chlorine drying and chemical dehydration reactions, to name only two. Highly sophisticated recovery plants allow a recovery rate close to 100%.

The possibility of releasing waste acid into the environment has become increasingly restrictive and costly. Recovering spent sulfuric acid avoids waste and high costs for waste treatment. The recovered sulfuric acid can be fully recycled into the process, where it originates from.

Concentrated sulfuric acid is a highly corrosive liquid. Recovery plants have to take account of this and need to be made from corrosion resistant materials. This holds in particular for devices that come into contact with hot and concentrated sulfuric acid, such as - among others - pumps, heating elements, mixing devices and storage tanks.

It is therefore important to improve both the recovery process/conditions and the plant equipment in order to achieve a prolonged lifespan as well as a cost reduction.

The present invention relates to a plant for the concentration of sulfuric acid comprising a feeding line, a rectifying column, at least one concentration unit, an acid cooling and discharge unit a condensing unit, a product line with a product acid cooling system, and a vacuum system, wherein there is at least one electrical heating device in each concentration unit, each heating device comprising a multitude of heating elements, each heating element comprising heating wires surrounded by a ceramic sheathing, the heating elements being contained in a tube made of suitable corrosion resistant glass, preferably quartz glass, for insulation against surrounding hot sulfuric acid, the glass tube being closed at its bottom end.

The present concentration plant comprises conventional units, as used in many existing plants. There are improvements in some of the units, as are detailed below. The improvements may be used in the plant each by itself, all together or in any combination.

Most important and of particular interest is the design of the electrical heating device or devices used in the concentration units. Electrical heating devices are regularly used in plants, where high- pressure steam is not available and the necessary temperatures of more than 200°C can only be reached by electrical heating. As the heating devices are surrounded by hot and highly concentrated sulfuric acid, a specific design of the heating device is necessary.

According to the invention there is at least one heating device in each concentration unit, each heating device consisting of a closed glass body, preferably made of quartz glass and containing one or multiple heating wires. The heating wires are positioned and fixed in the glass body with ceramic sheathing or insulation in such a way that electrical insulation is ensured, while heat radiation is provided by the heating wires and ceramic insulation to ensure sufficient heat transfer to the sulphuric acid liquid. The insulation is made from ceramic materials.

The heating elements are contained in a glass tube that immerses into the sulfuric acid to be heated. Quartz is particularly suited for the transfer of thermal radiation, thus allowing an effective heat transfer into the surrounding sulfuric acid. Of course, the tube has a closed bottom end, the electrical connections entering the tube at its top end.

It is highly preferred to make use of a quartz tube. Preferably, the tube has a surrounding flange for support and better handling, the flange being located above the sulfuric acid level. Also, the part of the tube not immersed into the sulfuric acid comprises insulation material, such as glass, stone or ceramic wool.

Another improvement relates to the rectifying column, where conventionally used ceramic packing is replaced by a packing made from a corrosion resistant plastic material, preferably PFA (perfluoralkoxypolymer). It has turned out that conventional ceramic packings tend to break, the pieces effecting the performance of both the column and the pre-concentration unit. Frequent replacement is required. Though being more expensive, a PFA packing has a much longer lifespan and thus compensates for higher costs. Frequent maintenance of the unit is avoided.

Moreover, the rectifying column has a newly developed support grid for the packing, the support grid being made from ferrosilicium. This material is much more robust and stable than the conventionally used glass, ceramic or PTFE grids. Ferrosilicium is highly acid and temperature resistant.

In its top part, the rectifying column comprises a feeding device for distributing pre-heated feed acid onto the packing. According to one aspect of the invention, the feeding device is in the form of an acid resistant tube comprising a great number of holes or openings for spraying the feed acid onto the packing of the rectifying column. Suitable materials are quartz, ceramics and high alloyed steel, preferably glass fibre reinforced PTFE. For an even distribution of the feed, the holes of the feeding device are arranged at well-defined angles, e.g. at 0°, ± 14° and ± 26° with respect to the vertical.

According to another aspect of the invention, the plant comprises a mixing device, where hot product acid is cooled down by injection into cold product acid before being passed on to a heat exchanger for heat transfer to feed acid. The mixing device preferably is an injection tube where hot product acid is mixed with and injected into a stream of cold product acid.

Whereas commonly a dwell tank is used to mix and cool down concentrated product acid at very high temperatures of about 200°C, the present invention makes use of the mixing tube to avoid such dwell tank. Dwell tanks are expensive, as they are made of highly corrosion resistant material, mostly of enamelled steel. The mixing tube mixes cold product acid with the hot acid coming from the concentration unit in such a way that the hot acid is given into the centre of the flow of cold acid. This avoids you thermal stress. A permanent recirculation allows to cool down the hot product acid from about 200°C to less than typically 70°C. As a consequence, all parts downstream from this mixing tube, in particular a pump made of corrosion resistant material, e.g. Hastelloy for passing product acid to a subsequent heat exchanger, do not come into contact with hot acid, and a dwell tank is avoided.

Another improvement relates to the neutralisation of corrosive compounds in the off gas. Sulfuric acid originating from a chlorine production plant always contains a certain amount of chlorine. Same as sulfuric acid, chlorine is highly corrosive. Neutralization of the chlorine released from the sulfuric acid in the concentration process is therefore desirable.

As acid is used for chlorine drying, it contains dissolved chlorine. During the acid concentration process chlorine is released. As a consequence, the vapours coming from the rectifying column contain chlorine. Due to the high content of chlorine the vacuum pump downstream from the rectifying column should be made of a chlorine resistant material, for example titanium, which is expensive. In order to optimise the costs and life time of the vacuum pump, a neutralisation unit in the vapour line is preferred.

In order to neutralise the chlorine released during the acid recovery process, the vapour exhaust line comprises a neutralization unit, which is equipped with an injection device for injecting a dilute sodium hydroxide solution. The sodium hydroxide solution is injected in a controlled way, depending from the acid and/or chlorine content determined by analyzing of the condensate of the vacuum pump downstream from the rectifying column. The neutralization unit is designed as a scrubber.

In the neutralization unit a diluted sodium hydroxide solution and cold water are injected into the vapours, this resulting in a washing effect. The solution unit, is mixed with the cold water, which will further optimise the pump performance. The sodium hydroxide will react with the chlorine to form NaCI and NaOCI. Minor traces of chlorine and sodium hydroxide will remain, but in so low quantities that a standard vacuum pump made of steel, preferably stainless steel can be used.

Details of the invention are given in the attached drawings and the description pertaining thereto.

In the drawings, there is shown in Fig. 1 a general presentation of a sulphuric acid concentration plant,

Fig. 2 a quartz tube to be used in a concentration unit,

Fig. 3 a rectifying column to be used a pre--concentration unit,

Fig. 4 a mixing device for cooling product acid,

Fig. 5 a general presentation of the off-gas unit.

Figure 1 is a general presentation of a sulphuric acid concentration plant such as used according to the invention. Such concentration plants are used for example in connection with a chlorine drying facility in a caustic soda production plant.

A feed acid line 1 passes heat exchanger 2, where heat is exchanged with product acid from product line 3, and supplies feed acid to rectification column 4. The feed acid is sprayed onto a filling of packing rings 5 for pre-concentration. Heat is supplied by means of steam. For preconcentration the feed acid, which normally has a concentration of 70 - 80 % is all collected in horizontal evaporator 6. The off-gas, which is rich in chlorine, leaves rectification column 4 via line 7b. The overflow of pre-concentrated sulphuric acid from evaporator 6 is passed via line 8 to concentration units 9 and 10. Concentrated product acid leaves the plant via product line 3 to heat exchanger 2 and cooling unit 11. Off-gas leaves the concentration units 9 and 10 via line 7a and is fed into the bottom of rectifying column 4.

Rectification column 4, horizontal evaporator s, concentration units 9 and 10, and off-gas lines 7a and 7b are run under a vacuum, such as below 100 mbar abs., to assure removal of gases like water and chlorine.

Concentration units 9 and 10 comprise heating elements 12 for additional heat transfer. This is necessary, if there is no supply of hot steam with temperatures beyond 200°C available. The heating elements 12 consist preferably of quartz glass tubes which contain heating rods. The quartz tubes are immersed into the sulphuric acid provided from horizontal evaporator 6. Details are given below.

Figure 2 shows a heating device to be used in a concentration unit according to the present invention. The quartz tube 12 comprises a multitude of electric heating elements 13, four being shown, fed by a thyristor unit via insulated feeding cables 14a. The electrical heating rods have a ceramic sheathing 14b consisting of a multitude of ceramic elements. The part of the quartz tube 12 not being immersed into hot sulphuric acid is filled with insulation material 14c to minimise heat loss. Distance elements 15 ensure a correct distance between the individual heating rods, also to prevent shortcuts. Heat transfer to the sulphuric acid is by radiation.

Figure 3 shows the rectifying column 4 with the packing 5 of packing rings rings made from a plastic material resistant to sulphuric acid, preferably PFA, the inlet 16 for off-gas from horizontal evaporator 6 at the bottom, inlet 17 for the off-gas from concentration units 9 and 10 via line 7a, and the exit 19 for off-gas into line 7b. Feed acid is fed in via line 1 and perforated tube 20 and sprayed onto the packing ring filling 5, flowing down countercurrent of fed in off-gas from line 7a and produced in horizontal evaporator 6. Perforated tube 20 consists of a corrosion resistant material, such as quartz glass, ceramics and high alloyed steel, preferably glass fibre reinforced PTFE. The packing ring filling 5 preferably consists of a plastic material, such as PFA, and is supported by a grid 21 , which consists of a metallic material, preferably ferrosilicium

Figure 4 shows a mixing device 22, which is arranged within product line 3 to cool down product acid coming from concentration units 9 and 10 in order to protect downstream equipment from contact with hot concentrated sulphuric acid. This allows the use of standard equipment, such as a pump made of corrosion resistant material such as Hastelloy, and avoids a dwell tank for cooling product acid.

Mixing device 22 provides a stream of cool concentrated sulphuric acid (arrows) from cooling device 11 , into which the hot product acid from line 3 is injected. This provides an initial stream of hot product acid surrounded by cool product acid. Upon mixing, temperature is lowered to less than 70°C.

Mixing device 22 comprises the tube of hot acid line 3, incoming tube 24 for cold concentrated acid, tube 24 leading into a wide tube 25 surrounding hot acid line 3. A half round wall 28 at the mouth of incoming tube 24 separates the flow of cold acid into an upwards stream surrounding tube 26 and a downwards stream further diluting the product acid stream with cold acid. The longitudinal baffle 27 on tube 26 supports the mixing and cooling process.

Figure 4a is a section of Fig. 4 along the line A - A’ showing the half round wall 28 at the month of incoming tube 26 and the arrangement of the longitudinal baffle 27. Figure 5 shows the neutralisation unit 28 with the off-gas line 7b from the rectifying column 4, which passes condenser units 30a and 30b on its way. Condenser units 30a and 30b form together with tank 35, where waste water and condensate are collected, the condensate system. Neutralisation unit 28 functions as a scrubber, dilute sodium hydroxide solution via line 32 and cold water via line 33 being injected for neutralisation of corrosive constituents of the off-gas. Chlorine is reacted to form NaCI and NaOCI. Vacuum pump 31 arranged downstream from the neutralisation unit, which also together with pump 34 provides the vacuum for the vacuumized units 4, 6, 9 and 10 of the plant, is thus protected from contact with corrosive gases, which allows the use of conventional material for the pump instead of for example titanium. The amount of dilute NaOH injected into neutralization unit 29 via line 32 is calculated from the chloride contents measured in the condensate by established techniques. A dosing pump (not shown) provides the correct volume stream.

Claims

8 Claims

1 . A plant for the concentration of sulfuric acid comprising a feeding line, a rectifying column, at least one concentration unit, an acid cooling and discharge unit a condensing unit, a product line, and a vacuum system, characterised by at least one electrical heating device in each concentration unit, each heating device comprising a multitude of heating elements, each heating element being surrounded by a ceramic sheathing, the heating elements being contained in a glass tube for insulation against surrounding hot sulphuric acid, the quartz tube being closed at its bottom end.

2. The plant according to claim 1 , wherein the heating elements are wires embedded in an multitude of ceramic elements.

3. The plant according to claim 1 or 2, wherein the quartz tube comprises insulation material in its part projecting the sulfuric acid level in the concentration unit.

4. The plant according to claim 1 , wherein the rectifying column contains a packing made from PFA.

5. The plant according to claim 4, wherein the rectifying column comprises a packing support grid made from ferrosilicium.

6. The plant according to claim 4 or 5, wherein the rectifying column comprises in its top part a feeding tube having holes for an even distribution of feed acid. 9

7. The plant according to claim 6, wherein the feeding tube is made from glass fibre reinforced PTFE.

8. The plant according to claim 1 or 4, wherein in a mixing device hot product acid is cooled down by cold product acid in a pre-cooling step before entering a heat exchanger for heat transfer to feed acid.

9. The plant according to claim 8, wherein hot product acid is passed through an injecting device for injection into cool product acid.

10. The plant according to claim 9, wherein the mixing device comprises a mixing tube inserted into the discharge tube for hot product acid and a feeding tube for cold product acid, the feeding line injecting cold product acid into the mixing tube and into the acid stream exiting from the mixing tube.

11. The plant according to claim 1 , 4 or 8 comprising a vapour exhaust line, wherein the exhaust line is equipped with a neutralization unit comprising an injection device for injecting low concentrated sodium hydroxide solution and cold water into the exhaust vapours for neutralisation.

12. The plant according to claim 11 , wherein the exhaust line comprises a vacuum pump.

13. The plant according to claim 11 or 12, wherein in the exhaust vapours the chlorine contents is neutralised in a controlled way.

14. The plant according to claim 13, wherein an exhaust vapour condensate system is provided with a measuring device for determining the acid and/or chlorine content, the measurements serving as a basis for the controlled injection of sodium hydroxide solution.