CA1152285A - Process and apparatus for regenerating sulphuric acid - Google Patents
Process and apparatus for regenerating sulphuric acidInfo
- Publication number
- CA1152285A CA1152285A CA000353787A CA353787A CA1152285A CA 1152285 A CA1152285 A CA 1152285A CA 000353787 A CA000353787 A CA 000353787A CA 353787 A CA353787 A CA 353787A CA 1152285 A CA1152285 A CA 1152285A
- Authority
- CA
- Canada
- Prior art keywords
- acid
- stage
- weight
- torr
- pressure
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
- C01B17/00—Sulfur; Compounds thereof
- C01B17/69—Sulfur trioxide; Sulfuric acid
- C01B17/88—Concentration of sulfuric acid
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D1/00—Evaporating
- B01D1/0094—Evaporating with forced circulation
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D1/00—Evaporating
- B01D1/02—Evaporators with heating coils
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D1/00—Evaporating
- B01D1/26—Multiple-effect evaporating
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D1/00—Evaporating
- B01D1/30—Accessories for evaporators ; Constructional details thereof
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
- C01B17/00—Sulfur; Compounds thereof
- C01B17/69—Sulfur trioxide; Sulfuric acid
- C01B17/90—Separation; Purification
- C01B17/905—Removal of organic impurities
Landscapes
- Chemical & Material Sciences (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Organic Chemistry (AREA)
- Inorganic Chemistry (AREA)
- Vaporization, Distillation, Condensation, Sublimation, And Cold Traps (AREA)
- Organic Low-Molecular-Weight Compounds And Preparation Thereof (AREA)
Abstract
Abstract of the Disclosure The invention provides a process for regenerating contaminated sulphuric acid using indirect heating in apparatus of, or at least coated with, enamel, characterized in that the acid is first concentrated to an output concentration of from 60 to 80% by weight of H2SO4 in an indirectly heated, single-stage or, optionally, even multistage preconcentration unit; the acid is subsequently introduced into a high concentration unit where it is concentrated to a level of from 90% by weight to 98.3% by weight of H2SO4 at temperatures of from about 160°C
to 250°C and under a pressure of from about 30 Torr to 100 Torr; the acid is then delivered to a purification stage in which a temperature of from 220°C to 350°C is maintained; and the acid is purified under atmospheric pressure or reduced pressure; and an apparatus for carrying out the process described above.
to 250°C and under a pressure of from about 30 Torr to 100 Torr; the acid is then delivered to a purification stage in which a temperature of from 220°C to 350°C is maintained; and the acid is purified under atmospheric pressure or reduced pressure; and an apparatus for carrying out the process described above.
Description
~15~Z285 This invention relates to a process and an apparatus for regenerating contaminated sulphuric acid in several stages using indirect heating in apparatus of, or at least coated with, enamel. In the context of the invention, the term "enamel" applies to glass-like coatings which are acid-resistant and are capableof withstanding fluctuating temperatures.
More particularly, the present invention provides a process for regen-erating contaminated sulphuric acid using indirect heating in apparatus of, or at least coated with, enamel which is characterised in that the acid is first con-centrated in an indirectly heated, single-stage or multiple-stage preconcentra-tion unit to an output concentration of from 60 to âO % by weight of sulphuric acid, subsequently introduced into a high concentration unit where it is concen-trated at temperatures of from about 160C to 250C and under a pressure of fromabout 30 Torr to 100 Torr to a concentration of from 90% by weight to 98.3% by weight of sulphuric acid, and then delivered to a purification stage in which a temperature of from about 220C to 330C is maintained, and the acid is purifiedunder atmospheric pressure or reduced pressure.
The present invention also provides an apparatus for regenerating sul-phuric acid by preconcentrating the acid to 60% to 80% by weight and then concen-trating the acid to 90 to 98.3% by use of temperatures in the range of 160C to 250C and a pressure of 30 to 100 Torr, whereby the acid is purified, said appar-atus comprising heat exchangers of enamelled double-jacketed tubes heated by heat carriers and provided for heating th0 acid in a preconcentration unit; said pre-concentration unit including a first preconcentration stage comprising a first heat exchanger followed by a first evaporation vessel; a following, second pre-concentration stage comprising a second heat exchanger formed by double-jacketedtubes of enamelled steel and followed by a second evaporation vessel; and a final concentration stage following said preconcentration unit comprising a heat exchan-~ ~5'~Z8S
ger formed by double-jacketed tubes of enamelled steel and followed by an evapor-ation vessel; and a purification stage comprising a radiation-heated quartz glass tube, a heating zone for heating the acid, a following reaction zone and a follow-ing after-reaction zone. The first heat exchanger advantageously comprises a nest of tantalum tubes.
It has been found that, using enamel-coated installaticns, it is possi-ble to maintain the high temperatures mentioned above and, hence, to concentrate the acid to the azeotropic point. The acid is completely purified in an addi-tional step carried out in a separate apparatus, for example made of quartz or coated with enamel. This is carried out by heating the concentrated acid for a certain period (residence time) to 290 - 350C under reduced pressure or at s-pheric pressure, if necessary in the presence of an oxidising agent, preferably nitric acid.
Enamel coatings are applied to steel at elevated temperature. On acc-ount of the different thermal expansion coefficients of the steel support and the enamel coating, a compressive stress is developed in the enamel layer on cooling.
This stress is desirable and necessary for preventing the formation of cracks in the enamel layer. At an ambient temperature of 25C, the compression of the ena-mel layer corresponding to this compressive stress amounts to approximately 0.0015 m/lm. The compressive stress increases with decreasing temperature and decreases with increasing ~5;~2135 temperature~ so that it will normally be zero for an average temperature of the order of 400C. Accordingly, if it were to be assumed that a compressive stress must always be present in the enamel layer, it would be possîble under ideal conditions to use enamelled components up to a temperature of around 400C. Under normal service conditions, however, the effect of various factors in conventional evaporators is that, even at low ~ean temperatures, the compressive stre3s becomes zero or even negative, which almost inevitably results in unwanted hairline cracks. The~e factors include inter alia the transfer of heat from the steel side through the enamel layer to a medium to be treated.
Under the effect of the known temperature profile characterising the transfer of heat, the enamel layer has a lower mean temperature than the steel layer, with the result that the compressive stresses in the enamel layer are reduced in comparison with the stationary state. This phenomenon is further adver3ely affected by uneven heating or cooling of the enamel-coated wall, for e~ample due to the fact that the supply of heat fro~ the steel side is irregular or to the fact that the cooling effect on the enamel layer side is uneven on account of encrustation of this layer or on account Of irregular flow o~ the medium to be heated or its partial evaporation. A¢oording to the invention, therefore, the enamel layers are used in such a way that a residual compressive stress is still present in them. This residual stress according to the invention 3o prevents the formation of hairline cracks so that the enamel layer affords satisfactory protection against corrosion by the hot, highly concentrated acid. The residual compressive stress or rasidual compression ~S2285 according to the invention which should be maintained under all working conditions should be as high a possible under the particular workin~ conditions and preferably no less than 0.0003 m/lm. In very special cases, even lower residual compressive stresses are permissible, but only if there is no danger of deformation or other adverse effects on the enamel layer. In cases such as these, however, a residual compressive stress of approximately 10~, i.e. a compression of 0.00015 m/lm, should still be present in the enamel layer.
Another aspect of the invention concerns the overall temperature pro~ile, the acid automatically flowing past the enamel heat exchanger surfaces at such a predetermined speed that no solids can be deposited on those surfaces and, at the same time, in such a way that the acid i9 merely heated and not evaporated on the heat e~changer surfaces. For this reason, it is possible in accordance with the invention to circulate a multiple of the quantity of acid to be concentrated in the apparatus and, at the same time, to ensure that the static pressure prevailing at the outlet end o~ the last heat e~changer tube is high enough, taking the temperature of the acid into account, to avoid 25 evaporation. ~owever, this temperature profilé has to be established in con~unction with the reqidual compressive stress accordin~ to the invention. With ~low rates of the acid and the heat carrier of more than about 0.8 m/sec. and pre~erably in the range of Yrom 0.9 to 1.5 m/second, the residual compression according to the in~rention in the enamel layer does not fall below the minimum o~ 20~o or ~.0003 m/lm. In general, the rate o~ ~low of the acid in the heat ~L5Z2~35 exchanger tubes may be varied over a range o~ from about 0.4 to '~ m/second. However, it is particularly preferred to adjust the flow rate to a value of from 0.8 to 1.2 m/second. At thi~ level, any residues present remain in the suspension.
Although the high concentration stage may be operated with virtually any acid concentration Or more than 30~ by weight and preferably more than 40~0 by weight of E2~04 (input concentratio~), it is advantageous, particularly where highly dilute acids are used, for the high concentration stage to be preceded by a preliminary concentration stage which, for a high evaporation rate ? preferably operates in several stages at di~ferent pressures on the principle of vapour utilisation. The last evaporation ~tage of the preconcentration unit is also heated by heat carrier~. It is possible in this way to obtain high temperature gradients and, hence, small heating surfaces and economic operation By suitably selecting the
More particularly, the present invention provides a process for regen-erating contaminated sulphuric acid using indirect heating in apparatus of, or at least coated with, enamel which is characterised in that the acid is first con-centrated in an indirectly heated, single-stage or multiple-stage preconcentra-tion unit to an output concentration of from 60 to âO % by weight of sulphuric acid, subsequently introduced into a high concentration unit where it is concen-trated at temperatures of from about 160C to 250C and under a pressure of fromabout 30 Torr to 100 Torr to a concentration of from 90% by weight to 98.3% by weight of sulphuric acid, and then delivered to a purification stage in which a temperature of from about 220C to 330C is maintained, and the acid is purifiedunder atmospheric pressure or reduced pressure.
The present invention also provides an apparatus for regenerating sul-phuric acid by preconcentrating the acid to 60% to 80% by weight and then concen-trating the acid to 90 to 98.3% by use of temperatures in the range of 160C to 250C and a pressure of 30 to 100 Torr, whereby the acid is purified, said appar-atus comprising heat exchangers of enamelled double-jacketed tubes heated by heat carriers and provided for heating th0 acid in a preconcentration unit; said pre-concentration unit including a first preconcentration stage comprising a first heat exchanger followed by a first evaporation vessel; a following, second pre-concentration stage comprising a second heat exchanger formed by double-jacketedtubes of enamelled steel and followed by a second evaporation vessel; and a final concentration stage following said preconcentration unit comprising a heat exchan-~ ~5'~Z8S
ger formed by double-jacketed tubes of enamelled steel and followed by an evapor-ation vessel; and a purification stage comprising a radiation-heated quartz glass tube, a heating zone for heating the acid, a following reaction zone and a follow-ing after-reaction zone. The first heat exchanger advantageously comprises a nest of tantalum tubes.
It has been found that, using enamel-coated installaticns, it is possi-ble to maintain the high temperatures mentioned above and, hence, to concentrate the acid to the azeotropic point. The acid is completely purified in an addi-tional step carried out in a separate apparatus, for example made of quartz or coated with enamel. This is carried out by heating the concentrated acid for a certain period (residence time) to 290 - 350C under reduced pressure or at s-pheric pressure, if necessary in the presence of an oxidising agent, preferably nitric acid.
Enamel coatings are applied to steel at elevated temperature. On acc-ount of the different thermal expansion coefficients of the steel support and the enamel coating, a compressive stress is developed in the enamel layer on cooling.
This stress is desirable and necessary for preventing the formation of cracks in the enamel layer. At an ambient temperature of 25C, the compression of the ena-mel layer corresponding to this compressive stress amounts to approximately 0.0015 m/lm. The compressive stress increases with decreasing temperature and decreases with increasing ~5;~2135 temperature~ so that it will normally be zero for an average temperature of the order of 400C. Accordingly, if it were to be assumed that a compressive stress must always be present in the enamel layer, it would be possîble under ideal conditions to use enamelled components up to a temperature of around 400C. Under normal service conditions, however, the effect of various factors in conventional evaporators is that, even at low ~ean temperatures, the compressive stre3s becomes zero or even negative, which almost inevitably results in unwanted hairline cracks. The~e factors include inter alia the transfer of heat from the steel side through the enamel layer to a medium to be treated.
Under the effect of the known temperature profile characterising the transfer of heat, the enamel layer has a lower mean temperature than the steel layer, with the result that the compressive stresses in the enamel layer are reduced in comparison with the stationary state. This phenomenon is further adver3ely affected by uneven heating or cooling of the enamel-coated wall, for e~ample due to the fact that the supply of heat fro~ the steel side is irregular or to the fact that the cooling effect on the enamel layer side is uneven on account of encrustation of this layer or on account Of irregular flow o~ the medium to be heated or its partial evaporation. A¢oording to the invention, therefore, the enamel layers are used in such a way that a residual compressive stress is still present in them. This residual stress according to the invention 3o prevents the formation of hairline cracks so that the enamel layer affords satisfactory protection against corrosion by the hot, highly concentrated acid. The residual compressive stress or rasidual compression ~S2285 according to the invention which should be maintained under all working conditions should be as high a possible under the particular workin~ conditions and preferably no less than 0.0003 m/lm. In very special cases, even lower residual compressive stresses are permissible, but only if there is no danger of deformation or other adverse effects on the enamel layer. In cases such as these, however, a residual compressive stress of approximately 10~, i.e. a compression of 0.00015 m/lm, should still be present in the enamel layer.
Another aspect of the invention concerns the overall temperature pro~ile, the acid automatically flowing past the enamel heat exchanger surfaces at such a predetermined speed that no solids can be deposited on those surfaces and, at the same time, in such a way that the acid i9 merely heated and not evaporated on the heat e~changer surfaces. For this reason, it is possible in accordance with the invention to circulate a multiple of the quantity of acid to be concentrated in the apparatus and, at the same time, to ensure that the static pressure prevailing at the outlet end o~ the last heat e~changer tube is high enough, taking the temperature of the acid into account, to avoid 25 evaporation. ~owever, this temperature profilé has to be established in con~unction with the reqidual compressive stress accordin~ to the invention. With ~low rates of the acid and the heat carrier of more than about 0.8 m/sec. and pre~erably in the range of Yrom 0.9 to 1.5 m/second, the residual compression according to the in~rention in the enamel layer does not fall below the minimum o~ 20~o or ~.0003 m/lm. In general, the rate o~ ~low of the acid in the heat ~L5Z2~35 exchanger tubes may be varied over a range o~ from about 0.4 to '~ m/second. However, it is particularly preferred to adjust the flow rate to a value of from 0.8 to 1.2 m/second. At thi~ level, any residues present remain in the suspension.
Although the high concentration stage may be operated with virtually any acid concentration Or more than 30~ by weight and preferably more than 40~0 by weight of E2~04 (input concentratio~), it is advantageous, particularly where highly dilute acids are used, for the high concentration stage to be preceded by a preliminary concentration stage which, for a high evaporation rate ? preferably operates in several stages at di~ferent pressures on the principle of vapour utilisation. The last evaporation ~tage of the preconcentration unit is also heated by heat carrier~. It is possible in this way to obtain high temperature gradients and, hence, small heating surfaces and economic operation By suitably selecting the
2~ prooess steps, the type of heating and the construction of the apparatus according to the invention under the special conditions according to the invention, it is possible for the first time to use enamel materials in s~ch a way that it is possible in the la~t stage of the preconcentration unit and in the high concentration unit to dispense altogether with metallic material~
which have only a limited resistance to corrosion. At the same time, the speoial conditions according to the invention also ensure that s~fficiently high temperature differences are e~tablished between the individual evaporator stages to be able to utilise the heat of evaporation economically.
A suitable evaporator has proved to be the circulation-type flash evaporator in which the acid is ~52285 merely heated in the liquid phase in the heat e~changer itself, evaporation taking place in the evaporation vessel. This is particularly important when the acids to be concentrated contain impurities which have a tendency to crystallise out on the evaporation suri~aces This i9 the case, for example, with iron-containing acids. The solubility of iron sulphate in sulphuric acid rapidly decreases in the range beyond 90~0 by weight o~ H2S04 and there is a danger of iron sulphate settling on the evaporation surfaces. This danger does not e~ist in the circulation-type flash evaporator because no evaporation ta~es place on the heat exchanger surfaces and the iron precipitating can thus remain in suspension. Instead o~ circulation-type flash evaporators, however, it is also possible to use other types of evaporators, depending on the quality of the acid, for example falling-~ilm evaporators, heated evaporation ves~els, etc.
~o In the case of acids contaminated with organic constituents, the organic constituents, depending on their boiling temperature, actually distil o~f at least partly with the vapours in the various evaporator stages. In many cases, however, there is at least a residue o~ organic constituents which does not evaporate and which therefo.e has to be oxidised at elevated temperatures. Using the process and apparatus according to the in~ention, trinitrotoluene, for e~ample, may be completely degraded at 320C in the presence o~ nitric acid as oxidising agent. Even lower reaction temperatures may be maintained. In the case of acids which may be re-used i~or the same purpose, it is possible to accept, ~or e~ample, even lower ~egrees . . ..
~Z285 of reaction, and, hence~ working up because the impurities do not interfere with the preceding process.
It is known, for exa~ple, that the oxidation reaction may be carried out with nitric acid in a residence vessel of cast iron. ~owever, it has proved to be particularly ad~antageous to use a tube reactor o~ quartz or, optîonally, enamel in which the acid is initially brought to the reaction and oxidation temperature by indirect heating, ~or e~a~ple by radiation heating, in the heating zone and subsequently enters the reaction zone where the acid is contacted in countercurrent with an oxidising agent, preferably nitric acid, with the result that the organic constituents are o~idised. The nitrosyl sulphuric acid ~(H~OS04) partly formed reacts with the remaining organic constituents present in the ~ollowing a~ter-reactio~ zone, so that a highly pure acid may be run - o~ at the end of the purification stage.
The after-r~action zone i9 directly connected to 2~ the reaction zone. In the purification stage D, i.e.
in the reaction zone, the concentration of the acid leaving the high concentration unit may be reduced or increased to a certain extent or kept constant.
Where nitric acid is added to oxidise the organic constitue~ts, water i9 introduced into the already highly concentrated acid. Similarly, water is formed in small quantities during the o~ida~ion of the organic constituents. This minimal input of water is sufficient to reduce the concentration of the already 3o highly concentrated acid. In the case of acids contaminated with up to 2 _ 3do by weight or organic constituents, the reduction in concentration will amount to between about 2 and ~do by weight and, in ~2285 _ 9 exceptional cases, may even be higher, for eæample from 5 to 6~o by weight.
The preferred e~bodiment of the invention is c~aracterised in that the amount of water thus introducel is removed again by further heating in the reaction zone. In other words, the acid with the concentration adjusted in the high concentration unit is run off after the after-reaction zone. Another possibility i9 to carry out another high concentration in the reaction zone. I~here thiQ procedure is adopted, a highly pure and, at the same time, highly concentrated acid is obtained. ~owever, this procedure would appear to be of minor significance in most cases on economic grounds.
In the drawing, the heating zone and the following reaction zone and the after-reaction zone are accommodated in the same apparatus. This represents the preferred embodiment of the invention ~owever, this arrangement is by no Deans imperative. In one variant of the process and apparatus ~or axample, the reaction zone and heating zone consist of separate apparatus.
As already mentioned, t~e praconcentration unit consists of a single stage, but usually of at least two individual stages arranged one behind the other and operated ~t different pressures. The preconcentration unit will comprise fro~ one to three stages, depending on the concentration and composition of the acids to be worked up. It is only in special cases that it may if necessary be constructed with an even greater number of stages. If, in its preferred e~bodiment, the preconcentration unit is of multistage construction, the vapours are guided througho~t the entire pre-~152~85 concentration unit in such a way that the vapourscoming from the particular evaporator stage operating at higher pressure are used to heat the preceding stage (as seen fro~ the acid side) operating at lower pressure.
According to the invention, it has proved to be advantageous where the preconcentration unit has three stages to adjust the conditions in the various stages to values within the following ranges:
1st Stage Concentration of the acids to be worked up: 15 to 35 by weight of H2S04;
Concentration of the issuing acid: 20 to 42~ by weight f ~2S04;
~emperature range: 30 to 65C;
Pressure range: 30 to 100 Torr.
2nd Stage 2i~ Concentration o~ the acids to be worked up: 20 to 42~o by weight of H2S04;
Concentration of the issuing acids: 28 to 54~ by weight of H2S4;
Temperature range: 65 to 100C;
Pressure range: 150 to 500 Torr, 3rd Stage Concentration o~ the acids to be worked up: 28 to 54do by weight of H2S04;
Concentration of the issuing acids: 60 to 80~o by wsight of H2S04;
Temperature range: 120 to 125C;
Pressure range: 500 to 1500 ~orr.
~52285 l~ere the preconcentration unit comprises two stages, the following conditions are established in accordance with the invention:
5 ~
Concentration of the acids to be worked up: 25 to 50~p by weight of fI2S04;
Concentration of the issuing acids: 34 to 617ot by weight of H2S04;
lO Temperature range: 40 to 100C;
Pressure range: 30 to 200 Torr.
2nd Stage Concentration of the acids to be worked up: 34 to 61a~o 15 by weight of ~[2S04;
Concentration of the issuing acids: 60 to 80~o by weight ~ ~[2S4;
Temperature range: 125 to 225C;
Pressure range: 500 to 1500 Torr.
In special circumstances, for example when the acid actually accumulates with a sufficiently high concentratio:l, there may be no need for the pre-concentration unit to have several stages, i.e. it may be operated as a single-stage unit. In cases such as 25 the~e, the following condit-ions have proved to be effective in accordance with the invention:
Single Stage Concentratio~ of the acid to be worked up: 30 to 60do 3o by weight of H~S04;
Concentration of the issuing acid: 60 to ~Op by weight ~ ~2S4;
'remperature range: 60 to 160C;
13 5~2Z~35 Pressure range: 30 to 200 Torr.
A particularly advantageous embodiment of the process and of the apparatus required for carrying it out is described by way of exalnple in the following with reference to the accompanying drawings, in which:
Figure 1 is a general flow chart of the process using a two-stage preconcentration unit and diagrammatically illustrates the apparatus:
and Figure 2 diagrammatically illustrates one e~ample of an e~bodiment.
A 30~0 by weight crude sulphuric acid contaminated by organic constituents i9 to be regenerated to a 98.3~o ~2S04 in a two-stage preconcsntration unit, a high concentration unit and a following purification stage.
In Figure 1, the letters A and B denote two stages of the preconcentration unit, the letter C denotes the high concentration unit and the letter D denotes the purification stage with the reaction zone and after-reaction zone.
Crude acid (3333 kg/h, 4000 ~OC, 30~ ~2S04) is delivered at 1 by means of a rotary pu~p 2 to a heat exchanger 3 in which the crude acid is preheated by heat transfer from the product acid. The preheated ~47C) thin acid is then introduced at 4 into the circuit of the first stage A. The flow of crude acid is regulated by keeping the level constant in the
which have only a limited resistance to corrosion. At the same time, the speoial conditions according to the invention also ensure that s~fficiently high temperature differences are e~tablished between the individual evaporator stages to be able to utilise the heat of evaporation economically.
A suitable evaporator has proved to be the circulation-type flash evaporator in which the acid is ~52285 merely heated in the liquid phase in the heat e~changer itself, evaporation taking place in the evaporation vessel. This is particularly important when the acids to be concentrated contain impurities which have a tendency to crystallise out on the evaporation suri~aces This i9 the case, for example, with iron-containing acids. The solubility of iron sulphate in sulphuric acid rapidly decreases in the range beyond 90~0 by weight o~ H2S04 and there is a danger of iron sulphate settling on the evaporation surfaces. This danger does not e~ist in the circulation-type flash evaporator because no evaporation ta~es place on the heat exchanger surfaces and the iron precipitating can thus remain in suspension. Instead o~ circulation-type flash evaporators, however, it is also possible to use other types of evaporators, depending on the quality of the acid, for example falling-~ilm evaporators, heated evaporation ves~els, etc.
~o In the case of acids contaminated with organic constituents, the organic constituents, depending on their boiling temperature, actually distil o~f at least partly with the vapours in the various evaporator stages. In many cases, however, there is at least a residue o~ organic constituents which does not evaporate and which therefo.e has to be oxidised at elevated temperatures. Using the process and apparatus according to the in~ention, trinitrotoluene, for e~ample, may be completely degraded at 320C in the presence o~ nitric acid as oxidising agent. Even lower reaction temperatures may be maintained. In the case of acids which may be re-used i~or the same purpose, it is possible to accept, ~or e~ample, even lower ~egrees . . ..
~Z285 of reaction, and, hence~ working up because the impurities do not interfere with the preceding process.
It is known, for exa~ple, that the oxidation reaction may be carried out with nitric acid in a residence vessel of cast iron. ~owever, it has proved to be particularly ad~antageous to use a tube reactor o~ quartz or, optîonally, enamel in which the acid is initially brought to the reaction and oxidation temperature by indirect heating, ~or e~a~ple by radiation heating, in the heating zone and subsequently enters the reaction zone where the acid is contacted in countercurrent with an oxidising agent, preferably nitric acid, with the result that the organic constituents are o~idised. The nitrosyl sulphuric acid ~(H~OS04) partly formed reacts with the remaining organic constituents present in the ~ollowing a~ter-reactio~ zone, so that a highly pure acid may be run - o~ at the end of the purification stage.
The after-r~action zone i9 directly connected to 2~ the reaction zone. In the purification stage D, i.e.
in the reaction zone, the concentration of the acid leaving the high concentration unit may be reduced or increased to a certain extent or kept constant.
Where nitric acid is added to oxidise the organic constitue~ts, water i9 introduced into the already highly concentrated acid. Similarly, water is formed in small quantities during the o~ida~ion of the organic constituents. This minimal input of water is sufficient to reduce the concentration of the already 3o highly concentrated acid. In the case of acids contaminated with up to 2 _ 3do by weight or organic constituents, the reduction in concentration will amount to between about 2 and ~do by weight and, in ~2285 _ 9 exceptional cases, may even be higher, for eæample from 5 to 6~o by weight.
The preferred e~bodiment of the invention is c~aracterised in that the amount of water thus introducel is removed again by further heating in the reaction zone. In other words, the acid with the concentration adjusted in the high concentration unit is run off after the after-reaction zone. Another possibility i9 to carry out another high concentration in the reaction zone. I~here thiQ procedure is adopted, a highly pure and, at the same time, highly concentrated acid is obtained. ~owever, this procedure would appear to be of minor significance in most cases on economic grounds.
In the drawing, the heating zone and the following reaction zone and the after-reaction zone are accommodated in the same apparatus. This represents the preferred embodiment of the invention ~owever, this arrangement is by no Deans imperative. In one variant of the process and apparatus ~or axample, the reaction zone and heating zone consist of separate apparatus.
As already mentioned, t~e praconcentration unit consists of a single stage, but usually of at least two individual stages arranged one behind the other and operated ~t different pressures. The preconcentration unit will comprise fro~ one to three stages, depending on the concentration and composition of the acids to be worked up. It is only in special cases that it may if necessary be constructed with an even greater number of stages. If, in its preferred e~bodiment, the preconcentration unit is of multistage construction, the vapours are guided througho~t the entire pre-~152~85 concentration unit in such a way that the vapourscoming from the particular evaporator stage operating at higher pressure are used to heat the preceding stage (as seen fro~ the acid side) operating at lower pressure.
According to the invention, it has proved to be advantageous where the preconcentration unit has three stages to adjust the conditions in the various stages to values within the following ranges:
1st Stage Concentration of the acids to be worked up: 15 to 35 by weight of H2S04;
Concentration of the issuing acid: 20 to 42~ by weight f ~2S04;
~emperature range: 30 to 65C;
Pressure range: 30 to 100 Torr.
2nd Stage 2i~ Concentration o~ the acids to be worked up: 20 to 42~o by weight of H2S04;
Concentration of the issuing acids: 28 to 54~ by weight of H2S4;
Temperature range: 65 to 100C;
Pressure range: 150 to 500 Torr, 3rd Stage Concentration o~ the acids to be worked up: 28 to 54do by weight of H2S04;
Concentration of the issuing acids: 60 to 80~o by wsight of H2S04;
Temperature range: 120 to 125C;
Pressure range: 500 to 1500 ~orr.
~52285 l~ere the preconcentration unit comprises two stages, the following conditions are established in accordance with the invention:
5 ~
Concentration of the acids to be worked up: 25 to 50~p by weight of fI2S04;
Concentration of the issuing acids: 34 to 617ot by weight of H2S04;
lO Temperature range: 40 to 100C;
Pressure range: 30 to 200 Torr.
2nd Stage Concentration of the acids to be worked up: 34 to 61a~o 15 by weight of ~[2S04;
Concentration of the issuing acids: 60 to 80~o by weight ~ ~[2S4;
Temperature range: 125 to 225C;
Pressure range: 500 to 1500 Torr.
In special circumstances, for example when the acid actually accumulates with a sufficiently high concentratio:l, there may be no need for the pre-concentration unit to have several stages, i.e. it may be operated as a single-stage unit. In cases such as 25 the~e, the following condit-ions have proved to be effective in accordance with the invention:
Single Stage Concentratio~ of the acid to be worked up: 30 to 60do 3o by weight of H~S04;
Concentration of the issuing acid: 60 to ~Op by weight ~ ~2S4;
'remperature range: 60 to 160C;
13 5~2Z~35 Pressure range: 30 to 200 Torr.
A particularly advantageous embodiment of the process and of the apparatus required for carrying it out is described by way of exalnple in the following with reference to the accompanying drawings, in which:
Figure 1 is a general flow chart of the process using a two-stage preconcentration unit and diagrammatically illustrates the apparatus:
and Figure 2 diagrammatically illustrates one e~ample of an e~bodiment.
A 30~0 by weight crude sulphuric acid contaminated by organic constituents i9 to be regenerated to a 98.3~o ~2S04 in a two-stage preconcsntration unit, a high concentration unit and a following purification stage.
In Figure 1, the letters A and B denote two stages of the preconcentration unit, the letter C denotes the high concentration unit and the letter D denotes the purification stage with the reaction zone and after-reaction zone.
Crude acid (3333 kg/h, 4000 ~OC, 30~ ~2S04) is delivered at 1 by means of a rotary pu~p 2 to a heat exchanger 3 in which the crude acid is preheated by heat transfer from the product acid. The preheated ~47C) thin acid is then introduced at 4 into the circuit of the first stage A. The flow of crude acid is regulated by keeping the level constant in the
3~) evaporation vessel 6 of the first stage. The circulating sulphuric acid is heated in the heat exchanger 5 (for example, a nest of tantalum tllbes).
It is only in the evaporation vessel 6 that water is ~S2Z85 evaporated commensurately with the amount of heat absorbed, so that crusts are prevented from forming on the he~ting surfaces.
The first stage ~ is heated by the heat of condensation of the vapours (pipe 18) fro~ the second stage B. Stage A is operated in vacuo (50 ~orr, 41.5% of ~2~4~ 5~) The virtually acid-free vapours from the fir~t stage A (924 kg/h, 50C, 50 Torr) pass via a drop ~eparator 7, in which entrained drops of sulphuric acid are retained, through a pipe 8 into a mixing condenser 9 where the vapours are condensed by spraying in cooling water. The light organic constituents distilled off with the vapours are separated off as iar as possible from the remaining condensate in the following separation vessel 10.
Generation of the vacuum and extraction of the non-conde~sed organic vapours and the inert gasss is carried out, for exa~ple, by mea~s of a water ring vacuum pump 11.
Suitable construction materials for the circuit and the svaporation vessel are, for e~ample, glass-fibre-reinforced plastic materials, PVC, graphite, glas~ or enamel.
The first stage ~ may eve~ be formed by a falling-film evaporator, in which case the sulphuric acid is concentrated in a single passage. In both cases, the heat exchanger 5 i9 made of a metal, in the present case tantalum. Instead of using a metal, however, it is also possible to use graphite, glass or enamel.
~ he preconcentrated sulphuric acid flowing off from the first stage ~ (2409 kg/h, 50C, 41.5% H2S04) i9 introduced through a pipe 13 of a rotary pump 14 ~5ZZ8S
_ 14 -into the circuit of the second stage B. The flow volume is regulated by keeping the level constant in the evaporator 16 of the second stage B.
The sulphuric acid circulated by the rotary pump 19 is heated in the heat exchanger 15, consisting of enamelled double-jacketed tubes arranged onebehind the other, and then evaporated solely in the evaporation vessel 16 so that crusts are pravented ~rom forming on the enamelled heating ~ur~ace.
The heat exchanger 15 o~ the second stage B is heated by a heat carrier oil, the heat carrier being passed through the inter-jacket space o~ the enamelled double-jacketed tubes in countercurrent to the sulphuric acid. The concentration and pressure prevailing in the evaporation vessel 16 are selected in such a way that the concentration of acid in the vapours can be kept at substantially zero witho~t any additional column and the heat of condensation o~ the vapours can be utilised in the first stage A (75 ~ S04, 1 bar, 185C).
The vapours ~ormed in the evaporation vessel 16 (1076 kg/h, 1 bar, 100C3 pass via a drop separator 17, in which entrained drops of sulphuric acid are retained, through a pipe 18 into the heat exchanger 5 of the first stage A where they are condensed. Finally, the condensate formed is introduced into a separation vessel 20 at 21 (1076 kg/h, 1 bar, ~5C) In this separation ves~el, the organic constituents distilled o~f wlth the vapours (for example, all having a boiling point lower than 185C) and condensed in the heat e~changer 5 or crystallised may be separated ~rom the~conde~sate. The inert gases are removed through the exhaust system.
z~s The interme~iate produ^t acid lowing off from the second stage B is delivered by a rotary pump 23 through a pipe 22 to a rectification column 26 of the high concentration stage C. Providing the second and third stages A and B are suitably arranged, there is no need for the pu~p 23. The input (1333 ~g/h, 185C, 75~ H2S04) is regulated by keeping the level constant in the evaporation vessel 25 of the high concentration stage C
In the separation column 27, the liquid undergoes a material e~change with the vapours ascending from the rotary evaporator 25, the liquid becoming enriched with relativel~ high boiling constituents and the vapour with relatively low boiling constituents.
Providing the intermediate product concentration is correctly selected (75% ~2S04), the ~aterial and heat exchange results in complete absorption of the ~2S04 component in the vapours. If the final concentration of stage B is too high (for example higher than 75~
H2S04), the rectification column of stage C has to be supplemented b~v a concentration column which has to be operated with water.
The sulphuric acid circulated by the rotar-~ pump 29 is heated in the heat exchanger 24, consisting of enamelled double-jacketed tubes arranged one behind the other, and is then evaporated solely in the evaporation vessel 25, thereb~v preventing crusts from forming on the enamelled heating surface. The heat e~changer 24 is heated b~ a heat carrier oil, the heat carrier being 3o passed through the inter-jacket space of the enamelled double-jacketed tubes in countercurrent to the sulphuric acid.
It has surprisingly been found that enamelled ~52;~85 steel shows ~ery high che;nical resistance to sulphuric acid, particularlvv to very highly concentrated sulphuric acid (98.3%), under the conditions according to the invention. This is all the more surprising insofar as it is known that enamel develops hairline cracks at elevated temperatures, with the result that there is no alternative but to work under the special conditions a~cording to the invention. On the other hsnd, it is imperative for the temperatures or temperature differences not to fall below or exceed certain levels. For this reason, it has proved to be best to operate the high concentration sta~e C und~r reduced pressure. The following conditions have prove~
to be safe in operation: vacuum in the evaporator (25) 30 to 100 Torr, preferably 50 to 70 Torr, temperature 160 to 250C, boiling te3~perature of the 98.3~ acid 240C, heat carrier temperature at the end of the heat exchanger (24) 310 C, acid temperature at this point 2~0C, heat carrier temperature at the entrance to the heat exchanger (24) 290C and acid ~emperature at this point 2~0C. The vapours formed in the evaporation vessel 25 differ in their acid content according to the final concentration. This acid is exchanged in the rectification column 26 by material exchange with the inflowing sulphuric acid.
The vapours of stage C, whioh have an acid content corresponding to the equilibrium of the acLd introduced, flow via the drop separator 28, in which entrained drops of sulphuric acid are retained, through a pipe 30 into a miæing condenser 31 whera the vapours (313 kg!h, 50 Torr, 185C) are condensed by spraying in water. The organic constituents distilled off with the vapours (for example all those having a boilin,g point below ~!L15Z285 240C) and condensed in the mixing condenser 31 or crystallised are separated as far as possible from the remaining condensate in the following separation vessel 32. Generation of the vacuum and extraction o~ the non-condensed organic vapour~, the waste gases of the ~xidised organic constituents and the other inert gases, is carried out by means of the water ring vacuum pump 33.
Where the acid is contaminated by organic constituents, a reaction normally takes place between the organic constituents and the So3-gas present in the evaporation vessel. In order to avoid reduction of the S03 to S02 and the presence of undesirable sulphurous acid ~2S03 in the condensate, an oxidising agent, pre~erably HN03, i~ added.
In the case of iron-containing acids, the saturation state (approximately 20 ppm of Fe at 98.3 S04 and 240C) is generally e~ceeded in the high concentratio~ stagé C, resulting in the precipitation ~20 of iron (II) sulphate. By virtue oi the continuous circulation of the sulphuric acid in the circuit, the iron sulphate remains in suspension and does not have any opportunity to settle in the evaporator.
In order to protect the enamel against the abrasive effect o$ precipitated iron sulphates, the rate of circulation in the double-jacketed tubes and in the circuit pipes mny be limited to lm/s.
The high concentration stage i9 preierably made o~ enamelled steel. An alternative material for the 3o rectification column is glass. ~he circulation pump may also be made of a material other than enamelled steel, preferably cast silicon.
~he highly concentrated, but only partly purified, .
- .
~L5ZZ85 sulphuric acid issuing from the high concentration stage C (1020 kg/h, 98% H2S04, 240C, 20~0 ppm TOC) i~
delivered by a metering pump 35 through a pipe 3'~ to a vertical q~artz glass tube 36 which is filled with acid or through which acid trickle~. In thi~ quartz tube, the concentrated sulphuric acid is heated under normal pressure to a temperature close to its boiling point (for example ~rom 240C to 320C).
In o~e variant, reduced pressure is applied at the tube entrance (top of the quartz glass tube 36) and in a possible rectification column 41. Providing the reduced pressure is correctly selected, normal pressure prevails in the purification zone under the effect of the column of liquid in the quartz tube 36. Anot~er possibility is to introduce the sulphuric acid in the ~orm of a falling ~ilm in the upper part of the heating zone 38 in order to increase the transfer o~ heat.
The heat is transferred by radiation, i.e. the quartz tube 36 is surrounded by a concentric radiation jacket which is in turn surro~nded by an electrical resistance heating system ~7 (or possibly by another heat source, ~or e3ample smoke gases). The heat is transferre~ by radiation irom the heating system to the radiation jacket and1 ~rom there, the ra~iation emitted inwards is absorbed by the ~uartz and the acid.
In the heating zone 38~ a sulphuric acid l~hich has not been conc~ntrated to the azeotropio point (for example 90 to 97~0 by weight ~ ~23~) may i~ desired be concentrated to 98% by weight or to 98.3~o by weight.
In ca~es where strong vapours are given of~, it is advisable to provide the heating zone and~ in this case, the concentration zone standing beneath a column of liquid with a filling, most desirably in the form of a ~L5~Z85 packing.
The ~apours given off during concentration in the heating zone 38 difPer in their acid content according to the input concentration, pressure and temperature of the acid. This acid has to be eYchanged in a rectification column 41 by material exchange with a liquid to be intro-~uced. Water may be used as the liquid, in which case the outflowing washing water enriched with sulphuric acid may be returned to the p~rification stage or may even be introduced together with the thin acid into the high concentration stage.
The liquid to be introduced may even be formed by the thin acid (where the concentration i9 below 75~o by weight of ~2S04)-The ~apours given ofP during concentration pass through the rectification column 41, in which the above-mentioned exchange of material takes place, into a condenser 42. Generation of the vacuum and e~traction of the non-condense~ organic vapours, the waste gases of the oxidised organic constituents and the other inert gases is carried out by means of the vacuum pump 43.
The preheated, concentrated, but still contaminated sulphuric acid (1020 kg/h, 9710 by weight of ~2~4~
320C) ~lows from the heating zone 38 into the reaction 25 zone 39 The reaction zone, which consist~ oP the lower part of the quartz tube 36, ls surrounded by a co~centrlc radiation jacket which is in turn surrounded by an electrical resistance heating system 37 (or possibly 3~ by another heat source, for e~ample smoke gases). The heat is transferred by radiation from the heating system to the radiation jacket and, from there, the radiation emitted inwards i9 absorbed by the quartz and by the ~1~;2285 - 2~ -acid.
In the lower part of the quartz tube 36, the organic constituents are oxidised by means of an oxidising agent (for example 65~o nitric acid) which is added at 44 and which is introduced in vapour form in the lower part of t~e purification column, ~or example through a per~orated plate, to pro~uce small uniformly distributed vapour bubbles. In the case of highly contaminated su~phuric acids, it is advisable to provide the reaction zone 39 with a ~illing, most desirably with a packing. Ii necessary, the filling may be e~tended over the entire quartz tube. The sddition o~ nitric acid may even result in the partial formation of nitrosyl hydrogen sulphate which is unwanted in the product acid. This nitros-~l hydrogen sulphate is partly degraded again by reaction with the organic constituents in an after-reactor 40.
It has proved to be advantageous to arrange the heating zone and the possible concentration zone with the purification zone one be~ind the other. The S03-gase3 forme~ during concentration in the lower part o~
the heating zone 38 are re-absorbed by the colder acid in the upper part of the quartz glass tube 36. The non-reduced N0~-gasies ascending from the reaction zone are able ~urther to react with the organic const1tuents in the heating zone. According to the invention, there i9 no strict ssparatio~ between the heating zone and the reaction zone.
The inco~pletely reduced N0x-gases issuing ~rom 3o the puri~ic~tion stage D and the re~ction gases may be introduced through a pressure-reducing val~e in the pipe 45 into the evaporation vessel 25 of the high concentration stage C where the excess N0x-gas may react with the organic constituents and, at the same time, at least partly prevent the possible reduction of S03.
The hot concentrated sulphuric acid (320C, 98.3 of ~2S04) issuing ~rom the after-reactor 40 is cooled with secondary circuit acid of the same purity and concentration in a mixing condenser 46 arranged adjacent the after-reactor. Be~ore being introduced into the mixing condenser 46, the secondary circuit acid is cooled in the heat excha~ger 3 by the crude acid and by a ~ollowing water-cooled heat e~changer 47. The product acid is removed ~rom the secondary circuit and, if necessary, is introduced into a plate separator 54 where the suspended iron sulphate is separated off.
In the puri~ication stage ~, the sulphuric acid has an input concentration o~ from 90 to 93do by ~eight, an output concentration of from 90 to 93.3% by weight and a temperature of from 225 to 330C.
The heat e~changers 15 and 24 o~ the second and third stages B and C are heated by means of a central heating system consisting of a high temperature heater 48, a burner 49, a circulation pump 50, an air preheater 51, a combustion air fan 52 and a chimney 53.
This heating system has the a~vantage over smoke gas heating that eveIl sulphur-containing he~vy oil may be used as fuel. On the heat carrier side, the two heat e~changers 15 and 24 may be connected in series or in parallel.
3l~ It is also possible to reduce the ~O~-gases prasent in the waste gas in a sub-stoichiometric atmosphere in a spscial burner 49.
Accordingly, tbe described process carried out iiZZ8S
_ 22 -with the illust:rated apparatus gives satisfactorily purified sulphuric acid concentrated to the aze~tropic point.
2~
3o
It is only in the evaporation vessel 6 that water is ~S2Z85 evaporated commensurately with the amount of heat absorbed, so that crusts are prevented from forming on the he~ting surfaces.
The first stage ~ is heated by the heat of condensation of the vapours (pipe 18) fro~ the second stage B. Stage A is operated in vacuo (50 ~orr, 41.5% of ~2~4~ 5~) The virtually acid-free vapours from the fir~t stage A (924 kg/h, 50C, 50 Torr) pass via a drop ~eparator 7, in which entrained drops of sulphuric acid are retained, through a pipe 8 into a mixing condenser 9 where the vapours are condensed by spraying in cooling water. The light organic constituents distilled off with the vapours are separated off as iar as possible from the remaining condensate in the following separation vessel 10.
Generation of the vacuum and extraction of the non-conde~sed organic vapours and the inert gasss is carried out, for exa~ple, by mea~s of a water ring vacuum pump 11.
Suitable construction materials for the circuit and the svaporation vessel are, for e~ample, glass-fibre-reinforced plastic materials, PVC, graphite, glas~ or enamel.
The first stage ~ may eve~ be formed by a falling-film evaporator, in which case the sulphuric acid is concentrated in a single passage. In both cases, the heat exchanger 5 i9 made of a metal, in the present case tantalum. Instead of using a metal, however, it is also possible to use graphite, glass or enamel.
~ he preconcentrated sulphuric acid flowing off from the first stage ~ (2409 kg/h, 50C, 41.5% H2S04) i9 introduced through a pipe 13 of a rotary pump 14 ~5ZZ8S
_ 14 -into the circuit of the second stage B. The flow volume is regulated by keeping the level constant in the evaporator 16 of the second stage B.
The sulphuric acid circulated by the rotary pump 19 is heated in the heat exchanger 15, consisting of enamelled double-jacketed tubes arranged onebehind the other, and then evaporated solely in the evaporation vessel 16 so that crusts are pravented ~rom forming on the enamelled heating ~ur~ace.
The heat exchanger 15 o~ the second stage B is heated by a heat carrier oil, the heat carrier being passed through the inter-jacket space o~ the enamelled double-jacketed tubes in countercurrent to the sulphuric acid. The concentration and pressure prevailing in the evaporation vessel 16 are selected in such a way that the concentration of acid in the vapours can be kept at substantially zero witho~t any additional column and the heat of condensation o~ the vapours can be utilised in the first stage A (75 ~ S04, 1 bar, 185C).
The vapours ~ormed in the evaporation vessel 16 (1076 kg/h, 1 bar, 100C3 pass via a drop separator 17, in which entrained drops of sulphuric acid are retained, through a pipe 18 into the heat exchanger 5 of the first stage A where they are condensed. Finally, the condensate formed is introduced into a separation vessel 20 at 21 (1076 kg/h, 1 bar, ~5C) In this separation ves~el, the organic constituents distilled o~f wlth the vapours (for example, all having a boiling point lower than 185C) and condensed in the heat e~changer 5 or crystallised may be separated ~rom the~conde~sate. The inert gases are removed through the exhaust system.
z~s The interme~iate produ^t acid lowing off from the second stage B is delivered by a rotary pump 23 through a pipe 22 to a rectification column 26 of the high concentration stage C. Providing the second and third stages A and B are suitably arranged, there is no need for the pu~p 23. The input (1333 ~g/h, 185C, 75~ H2S04) is regulated by keeping the level constant in the evaporation vessel 25 of the high concentration stage C
In the separation column 27, the liquid undergoes a material e~change with the vapours ascending from the rotary evaporator 25, the liquid becoming enriched with relativel~ high boiling constituents and the vapour with relatively low boiling constituents.
Providing the intermediate product concentration is correctly selected (75% ~2S04), the ~aterial and heat exchange results in complete absorption of the ~2S04 component in the vapours. If the final concentration of stage B is too high (for example higher than 75~
H2S04), the rectification column of stage C has to be supplemented b~v a concentration column which has to be operated with water.
The sulphuric acid circulated by the rotar-~ pump 29 is heated in the heat exchanger 24, consisting of enamelled double-jacketed tubes arranged one behind the other, and is then evaporated solely in the evaporation vessel 25, thereb~v preventing crusts from forming on the enamelled heating surface. The heat e~changer 24 is heated b~ a heat carrier oil, the heat carrier being 3o passed through the inter-jacket space of the enamelled double-jacketed tubes in countercurrent to the sulphuric acid.
It has surprisingly been found that enamelled ~52;~85 steel shows ~ery high che;nical resistance to sulphuric acid, particularlvv to very highly concentrated sulphuric acid (98.3%), under the conditions according to the invention. This is all the more surprising insofar as it is known that enamel develops hairline cracks at elevated temperatures, with the result that there is no alternative but to work under the special conditions a~cording to the invention. On the other hsnd, it is imperative for the temperatures or temperature differences not to fall below or exceed certain levels. For this reason, it has proved to be best to operate the high concentration sta~e C und~r reduced pressure. The following conditions have prove~
to be safe in operation: vacuum in the evaporator (25) 30 to 100 Torr, preferably 50 to 70 Torr, temperature 160 to 250C, boiling te3~perature of the 98.3~ acid 240C, heat carrier temperature at the end of the heat exchanger (24) 310 C, acid temperature at this point 2~0C, heat carrier temperature at the entrance to the heat exchanger (24) 290C and acid ~emperature at this point 2~0C. The vapours formed in the evaporation vessel 25 differ in their acid content according to the final concentration. This acid is exchanged in the rectification column 26 by material exchange with the inflowing sulphuric acid.
The vapours of stage C, whioh have an acid content corresponding to the equilibrium of the acLd introduced, flow via the drop separator 28, in which entrained drops of sulphuric acid are retained, through a pipe 30 into a miæing condenser 31 whera the vapours (313 kg!h, 50 Torr, 185C) are condensed by spraying in water. The organic constituents distilled off with the vapours (for example all those having a boilin,g point below ~!L15Z285 240C) and condensed in the mixing condenser 31 or crystallised are separated as far as possible from the remaining condensate in the following separation vessel 32. Generation of the vacuum and extraction o~ the non-condensed organic vapour~, the waste gases of the ~xidised organic constituents and the other inert gases, is carried out by means of the water ring vacuum pump 33.
Where the acid is contaminated by organic constituents, a reaction normally takes place between the organic constituents and the So3-gas present in the evaporation vessel. In order to avoid reduction of the S03 to S02 and the presence of undesirable sulphurous acid ~2S03 in the condensate, an oxidising agent, pre~erably HN03, i~ added.
In the case of iron-containing acids, the saturation state (approximately 20 ppm of Fe at 98.3 S04 and 240C) is generally e~ceeded in the high concentratio~ stagé C, resulting in the precipitation ~20 of iron (II) sulphate. By virtue oi the continuous circulation of the sulphuric acid in the circuit, the iron sulphate remains in suspension and does not have any opportunity to settle in the evaporator.
In order to protect the enamel against the abrasive effect o$ precipitated iron sulphates, the rate of circulation in the double-jacketed tubes and in the circuit pipes mny be limited to lm/s.
The high concentration stage i9 preierably made o~ enamelled steel. An alternative material for the 3o rectification column is glass. ~he circulation pump may also be made of a material other than enamelled steel, preferably cast silicon.
~he highly concentrated, but only partly purified, .
- .
~L5ZZ85 sulphuric acid issuing from the high concentration stage C (1020 kg/h, 98% H2S04, 240C, 20~0 ppm TOC) i~
delivered by a metering pump 35 through a pipe 3'~ to a vertical q~artz glass tube 36 which is filled with acid or through which acid trickle~. In thi~ quartz tube, the concentrated sulphuric acid is heated under normal pressure to a temperature close to its boiling point (for example ~rom 240C to 320C).
In o~e variant, reduced pressure is applied at the tube entrance (top of the quartz glass tube 36) and in a possible rectification column 41. Providing the reduced pressure is correctly selected, normal pressure prevails in the purification zone under the effect of the column of liquid in the quartz tube 36. Anot~er possibility is to introduce the sulphuric acid in the ~orm of a falling ~ilm in the upper part of the heating zone 38 in order to increase the transfer o~ heat.
The heat is transferred by radiation, i.e. the quartz tube 36 is surrounded by a concentric radiation jacket which is in turn surro~nded by an electrical resistance heating system ~7 (or possibly by another heat source, ~or e3ample smoke gases). The heat is transferre~ by radiation irom the heating system to the radiation jacket and1 ~rom there, the ra~iation emitted inwards is absorbed by the ~uartz and the acid.
In the heating zone 38~ a sulphuric acid l~hich has not been conc~ntrated to the azeotropio point (for example 90 to 97~0 by weight ~ ~23~) may i~ desired be concentrated to 98% by weight or to 98.3~o by weight.
In ca~es where strong vapours are given of~, it is advisable to provide the heating zone and~ in this case, the concentration zone standing beneath a column of liquid with a filling, most desirably in the form of a ~L5~Z85 packing.
The ~apours given off during concentration in the heating zone 38 difPer in their acid content according to the input concentration, pressure and temperature of the acid. This acid has to be eYchanged in a rectification column 41 by material exchange with a liquid to be intro-~uced. Water may be used as the liquid, in which case the outflowing washing water enriched with sulphuric acid may be returned to the p~rification stage or may even be introduced together with the thin acid into the high concentration stage.
The liquid to be introduced may even be formed by the thin acid (where the concentration i9 below 75~o by weight of ~2S04)-The ~apours given ofP during concentration pass through the rectification column 41, in which the above-mentioned exchange of material takes place, into a condenser 42. Generation of the vacuum and e~traction of the non-condense~ organic vapours, the waste gases of the oxidised organic constituents and the other inert gases is carried out by means of the vacuum pump 43.
The preheated, concentrated, but still contaminated sulphuric acid (1020 kg/h, 9710 by weight of ~2~4~
320C) ~lows from the heating zone 38 into the reaction 25 zone 39 The reaction zone, which consist~ oP the lower part of the quartz tube 36, ls surrounded by a co~centrlc radiation jacket which is in turn surrounded by an electrical resistance heating system 37 (or possibly 3~ by another heat source, for e~ample smoke gases). The heat is transferred by radiation from the heating system to the radiation jacket and, from there, the radiation emitted inwards i9 absorbed by the quartz and by the ~1~;2285 - 2~ -acid.
In the lower part of the quartz tube 36, the organic constituents are oxidised by means of an oxidising agent (for example 65~o nitric acid) which is added at 44 and which is introduced in vapour form in the lower part of t~e purification column, ~or example through a per~orated plate, to pro~uce small uniformly distributed vapour bubbles. In the case of highly contaminated su~phuric acids, it is advisable to provide the reaction zone 39 with a ~illing, most desirably with a packing. Ii necessary, the filling may be e~tended over the entire quartz tube. The sddition o~ nitric acid may even result in the partial formation of nitrosyl hydrogen sulphate which is unwanted in the product acid. This nitros-~l hydrogen sulphate is partly degraded again by reaction with the organic constituents in an after-reactor 40.
It has proved to be advantageous to arrange the heating zone and the possible concentration zone with the purification zone one be~ind the other. The S03-gase3 forme~ during concentration in the lower part o~
the heating zone 38 are re-absorbed by the colder acid in the upper part of the quartz glass tube 36. The non-reduced N0~-gasies ascending from the reaction zone are able ~urther to react with the organic const1tuents in the heating zone. According to the invention, there i9 no strict ssparatio~ between the heating zone and the reaction zone.
The inco~pletely reduced N0x-gases issuing ~rom 3o the puri~ic~tion stage D and the re~ction gases may be introduced through a pressure-reducing val~e in the pipe 45 into the evaporation vessel 25 of the high concentration stage C where the excess N0x-gas may react with the organic constituents and, at the same time, at least partly prevent the possible reduction of S03.
The hot concentrated sulphuric acid (320C, 98.3 of ~2S04) issuing ~rom the after-reactor 40 is cooled with secondary circuit acid of the same purity and concentration in a mixing condenser 46 arranged adjacent the after-reactor. Be~ore being introduced into the mixing condenser 46, the secondary circuit acid is cooled in the heat excha~ger 3 by the crude acid and by a ~ollowing water-cooled heat e~changer 47. The product acid is removed ~rom the secondary circuit and, if necessary, is introduced into a plate separator 54 where the suspended iron sulphate is separated off.
In the puri~ication stage ~, the sulphuric acid has an input concentration o~ from 90 to 93do by ~eight, an output concentration of from 90 to 93.3% by weight and a temperature of from 225 to 330C.
The heat e~changers 15 and 24 o~ the second and third stages B and C are heated by means of a central heating system consisting of a high temperature heater 48, a burner 49, a circulation pump 50, an air preheater 51, a combustion air fan 52 and a chimney 53.
This heating system has the a~vantage over smoke gas heating that eveIl sulphur-containing he~vy oil may be used as fuel. On the heat carrier side, the two heat e~changers 15 and 24 may be connected in series or in parallel.
3l~ It is also possible to reduce the ~O~-gases prasent in the waste gas in a sub-stoichiometric atmosphere in a spscial burner 49.
Accordingly, tbe described process carried out iiZZ8S
_ 22 -with the illust:rated apparatus gives satisfactorily purified sulphuric acid concentrated to the aze~tropic point.
2~
3o
Claims (10)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A process for regenerating contaminated sulphuric acid using indirect heating in apparatus of, or at least coated with, enamel, characterised in that the acid is first concentrated to an output concentration of from 60 to 80% by weight of H2SO4 in an indirectly heated, single-stage or multistage preconcentra-tion unit; the acid is subsequently introduced into a high concentration unit where it is concentrated to a level of from 90% by weight to 98.3% by weight of H2SO4 at temperatures of from about 160°C to 250°C and under a pressure of from about 30 Torr to 100 Torr; the acid is then delivered to a purification stage in which a temperature of from 220°C to 350°C is maintained; and the acid is puri-fied under atmospheric pressure or reduced pressure.
2. A process as claimed in claim 1, characterised in that, where the pre-concentration unit has three stages, the acid is adjusted to an output concentra-tion of from about 20 to 42% by weight of H2SO4 in the first stage at tempera-tures of from 30°C to 65°C and under a pressure of from 30 Torr to 100 Torr, to an output concentration of from about 28 to about 54% by weight of H2SO4 in the second stage at temperatures of from about 65°C to 100°C and under a pressure of from 150 to 500 Torr, and to an output concentration of from about 60 to about 80% by weight of H2SO4 in the third stage at temperatures of from about 125 to 225°C and under a pressure of from 500 Torr to 1500 Torr.
3. A process as claimed in Claim 1, characterised in that, where the preconcentration unit has two stages, the acid is adjusted to an output concentration of from about 34 to 61% by weight of H2SO4 in a first stage at temperatures of from about 40 to about 100°C and under a pressure of from about 30 to 200 Torr, and to an output concentration of from about 60 to 80% by weight of H2SO4 in the second stage at temperatures of from about 125 to 225°C
and under a pressure of from about 500 to 1500 Torr.
and under a pressure of from about 500 to 1500 Torr.
4. A process as claimed in claim 1, 2 or 3, characterized in that, where the preconcentration unit is of multistage construction, the individual stages are operated at different pressures.
5. A process as claimed in claim 1, 2 or 3, characterized in that, where the preconcentration unit is of multistage construction, the vapours from the particular evaporator stage operating at relatively high pressure are used to heat the stage operating at low pressure.
6. A process as claimed in claim 1, 2 or 3, characterized in that the crude acid preheated with the concentrated acid is preconcentrated in two stages to about 75% by weight, the vapours from the second stage operated at atmospheric pressure being used to heat the first stage operated in vacuo, and the second preconcentration stage and the following high concentration unit are heated by a heat carrier circuit.
7. A process as claimed in claim 1, 2 or 3, characterized in that the high concentration unit is operated under such conditions that the acid is con-centrated to above 96% before it enters the purification stage.
8. An apparatus for regenerating sulphuric acid by preconcentrating the acid to 60% to 80% by weight and then concentrating the acid to 90 to 98.3% by use of temperatures in the range of 160°C to 250°C and a pressure of 30 to 100 Torr, whereby the acid is purified, said apparatus comprising heat exchangers of enamelled double-jacketed tubes heated by heat carriers and provided for heating the acid in a preconcentration unit; said preconcentration unit including a first preconcentration stage comprising a first heat exchanger followed by a first eva-poration vessel; a following, second preconcentration stage comprising a second heat exchanger formed by double-jacketed tubes of enamelled steel and followed by a second evaporation vessel; and a final concentration stage following said pre-concentration unit comprising a heat exchanger formed by double-jacketed tubes of enamelled steel and followed by an evaporation vessel; and a purification stage comprising a radiation-heated quartz glass tube, a heating zone for heating the acid, a following reaction zone and a following after-reaction zone.
9. An apparatus as claimed in claim 8, comprising in the purification zone a device through which an oxidizing agent may be delivered to the high concentra-tion stage in the form of small drops or bubbles in countercurrent to the liquid contained therein.
10. An apparatus as claimed in claim 8 or 9 wherein: said first heat exchanger comprises a nest of tantalum tubes.
Applications Claiming Priority (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CH551379A CH641126A5 (en) | 1979-06-13 | 1979-06-13 | Process and system for regenerating sulphuric acid |
| CH5513/79-0 | 1979-06-13 | ||
| DE19803018665 DE3018665A1 (en) | 1979-06-13 | 1980-05-16 | METHOD AND DEVICE FOR REGENERATING SULFURIC ACID |
| DEP3018665.3 | 1980-05-16 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1152285A true CA1152285A (en) | 1983-08-23 |
Family
ID=25697891
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000353787A Expired CA1152285A (en) | 1979-06-13 | 1980-06-11 | Process and apparatus for regenerating sulphuric acid |
Country Status (4)
| Country | Link |
|---|---|
| EP (1) | EP0022181B1 (en) |
| CA (1) | CA1152285A (en) |
| ES (1) | ES8104136A1 (en) |
| FI (1) | FI67355C (en) |
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4663490A (en) * | 1984-03-16 | 1987-05-05 | Bayer Aktiengesellschaft | Process for the production of dinitrotoluene |
| US4772757A (en) * | 1984-03-16 | 1988-09-20 | Bayer Aktiengesellschaft | Process for the production of nitrobenzene |
| US5228885A (en) * | 1990-09-20 | 1993-07-20 | Metallgesellschaft Aktiengesellschaft | Process of concentrating a dilute sulfuric acid in a three-stage forced-circulation vacuum evaporation plant |
Families Citing this family (12)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE3718675A1 (en) * | 1987-06-04 | 1988-12-15 | Bayer Ag | METHOD FOR EVAPORATING SULFURIC ACID |
| DE3935892C2 (en) * | 1989-10-27 | 1994-08-25 | Metallgesellschaft Ag | Method and device for concentrating a liquid containing sulfuric acid and water |
| DE3938915C1 (en) * | 1989-11-24 | 1991-05-08 | Bayer Ag, 5090 Leverkusen, De | |
| CH682398A5 (en) * | 1991-09-13 | 1993-09-15 | Escher Wyss Ag | A method for saving energy in the production of titanium dioxide. |
| DE4213324A1 (en) * | 1992-04-23 | 1993-10-28 | Bayer Ag | Process for concentrating and purifying sulfuric acid |
| DE4230099C2 (en) * | 1992-09-09 | 1998-09-24 | Bayer Ag | Process for regenerating used sulfuric acid |
| DE4403841C2 (en) * | 1994-02-08 | 1997-12-04 | Bayer Ag | Process for evaporating used sulfuric acid |
| DE19642328A1 (en) * | 1996-10-14 | 1998-04-16 | Bayer Ag | Process for the concentration of used sulfuric acids |
| DE19920695A1 (en) * | 1999-05-05 | 2000-11-16 | Metallgesellschaft Ag | Process for concentrating thin acid |
| WO2008003297A2 (en) * | 2006-07-03 | 2008-01-10 | Lailach Guenter | Method and system for concentrating metal sulfate-containing sulfuric acid |
| EP2272796B1 (en) * | 2009-07-07 | 2020-08-05 | Haldor Topsøe A/S | Process for production of concentrated sulphuric acid from off-gas |
| DE102020131827A1 (en) | 2020-12-01 | 2022-06-02 | Andreas Wilk | Device and method for concentrating corrosive liquids |
Family Cites Families (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE1074019B (en) * | 1960-01-28 | Carl Buschm^ Hamburg Bergedorf Willy | Methods for cleaning up and concentrating waste sulfuric acid | |
| FR1241856A (en) * | 1959-08-13 | 1960-09-23 | Kestner App Evaporateurs | High-grade sulfuric acid concentration process |
| DE2242055C3 (en) * | 1972-08-26 | 1979-05-31 | Industrie Chemie Thoma Gmbh, 6000 Frankfurt | Process for the concentration and purification of sulfuric acid contaminated by organic substances |
-
1980
- 1980-06-09 EP EP19800103187 patent/EP0022181B1/en not_active Expired
- 1980-06-11 CA CA000353787A patent/CA1152285A/en not_active Expired
- 1980-06-11 FI FI801880A patent/FI67355C/en not_active IP Right Cessation
- 1980-06-12 ES ES492361A patent/ES8104136A1/en not_active Expired
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4663490A (en) * | 1984-03-16 | 1987-05-05 | Bayer Aktiengesellschaft | Process for the production of dinitrotoluene |
| US4772757A (en) * | 1984-03-16 | 1988-09-20 | Bayer Aktiengesellschaft | Process for the production of nitrobenzene |
| US5228885A (en) * | 1990-09-20 | 1993-07-20 | Metallgesellschaft Aktiengesellschaft | Process of concentrating a dilute sulfuric acid in a three-stage forced-circulation vacuum evaporation plant |
Also Published As
| Publication number | Publication date |
|---|---|
| FI67355B (en) | 1984-11-30 |
| EP0022181A3 (en) | 1981-08-26 |
| FI67355C (en) | 1985-03-11 |
| ES492361A0 (en) | 1981-04-16 |
| ES8104136A1 (en) | 1981-04-16 |
| EP0022181B1 (en) | 1983-10-12 |
| FI801880A (en) | 1980-12-14 |
| EP0022181A2 (en) | 1981-01-14 |
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