Method for processing hydrogen chloride from isocyanate preparation

Abstract

A method for processing hydrogen chloride from isocyanate preparation comprises the steps: a) providing hydrogen chloride; b) purifying the hydrogen chloride provided; and furthermore step c) or step d): c) bringing the purified hydrogen chloride into contact with water and/or with hydrochloric acid which is not saturated with respect to uptake of hydrogen chloride, d) further processing the purified hydrogen chloride to chlorine by partial oxidation. The hydrogen chloride provided in step a) contains organic and/or nitrogen-containing impurities and in step b) the purification is carried out by bringing hydrogen chloride into contact with hydrochloric acid which is saturated to the extent of 90% with respect to uptake of hydrogen chloride at least in a first gas scrubber (10) and circulating this hydrochloric acid at least partially through the first gas scrubber (10).

Claims

1. A system for processing hydrogen chloride from isocyanate preparation, comprising: at least a first gas scrubber having a first pump assigned to it, wherein the gas scrubber brings into contact hydrogen chloride gas and hydrochloric acid which is saturated to the extent of 90% with respect to uptake of hydrogen chloride; wherein the first pump assigned to the particular gas scrubber allows hydrochloric acid which is saturated to the extent of 90% with respect to uptake of hydrogen chloride to circulate at least partially through the particular gas scrubber; and an absorption column or an oxidation reactor, wherein the absorption column brings hydrogen chloride removed from a gas scrubber of the system into contact with water and/or with hydrochloric acid which is not saturated with respect to uptake of hydrogen chloride; the oxidation reactor converts hydrogen chloride removed from a gas scrubber into chlorine by partial oxidation.

2. The system according to claim 1, wherein the hydrochloric acid which is saturated to the extent of 90% with respect to uptake of hydrogen chloride and the hydrogen chloride gas are led in counter-current to one another in the first gas scrubber.

3. The system according to claim 1, wherein the first gas scrubber is configured as a packed column.

4. The system according to claim 1, wherein a first hydrochloric acid tank is furthermore provided, in which hydrochloric acid which is saturated with respect to uptake of hydrogen chloride and which has passed through the first gas scrubber is collected and introduced on to the first gas scrubber again by means of the first pump.

5. The system according to claim 1, further comprising: a second gas scrubber having a second pump assigned to it, wherein the gas scrubber brings into contact hydrogen chloride gas and hydrochloric acid which is saturated to the extent of 90% with respect to uptake of hydrogen chloride; wherein the second pump assigned to the second gas scrubber allows hydrochloric acid which is saturated to the extent of 90% with respect to uptake of hydrogen chloride to circulate at least partially through the second gas scrubber; wherein the system passes hydrogen chloride removed from the first gas scrubber into the second gas scrubber.

Description

(1) The present invention is illustrated in more detail with reference to the following figures and examples, but without being fixed thereto. The figures show:

(2) FIG. 1 diagram of an installation for carrying out the method according to the invention

(3) FIG. 2 diagram of a further installation for carrying out the method according to the invention

(4) FIG. 3 the accumulation of the components causing the TN and TOC in Example 1

(5) FIG. 4 the accumulation of the components causing the TN and TOC in Example 2

(6) In FIG. 1 and FIG. 2 the designation HCl (aq) means hydrogen chloride gas dissolved in water, and therefore hydrochloric acid, without being limited to aqueous or dilute hydrochloric acid. This designation has been selected in order to be able to make a distinction from gaseous hydrogen chloride.

(7) FIG. 1 shows a diagram of an installation for carrying out the method according to the invention. The first gas scrubber 10 here is configured as a packed column. Hydrogen chloride gas is introduced at the base of the column via substance stream 100. Concentrated hydrochloric acid is present in a first reservoir tank 20. This is saturated with respect to uptake of hydrogen chloride (or will become saturated in the course of the gas scrubbing in column 10 before it leaves the column). The hydrochloric acid from the first reservoir tank 20 enters into the pump 30 as substance stream 300 and downstream of the pump 30 is introduced at the top of the column 10 as substance stream 400.

(8) After passing through the column 10, the concentrated hydrochloric acid leaves the gas scrubber 10 by means of substance stream 200. The hydrochloric acid collected in the first reservoir tank 20 can then be circulated again through the gas scrubber 10. During the gas scrubbing impurities become concentrated in the hydrochloric acid.

(9) It is provided for hydrochloric acid which is correspondingly concentrated and contains impurities to be sluiced out of the second hydrochloric acid tank 40, which is connected to the first hydrochloric acid tank 20. To compensate the volume removed, the first hydrochloric acid tank 20 can furthermore be topped up with hydrochloric acid and/or water, preferably hydrochloric acid.

(10) Purified hydrogen chloride gas is furthermore removed at the top of the first gas scrubber 10 by means of substance stream 500. This can then be used in a subsequent step for the preparation of hydrochloric acid having an increased purity. For this, the hydrogen chloride gas is passed into a column 50 in which it is brought into contact with water or with dilute hydrochloric acid, in each of the two cases by means of substance stream 700 at the top of the column. Waste gases can be fed to a disposal by means of substance stream 600 at the top of the column 50.

(11) It is furthermore possible for the hydrochloric acid draining out of the column 50 to be led over a stripping column 60 with substance stream 900. Steam is passed through this column in counter-current (substance stream 810) in order to remove remaining volatile organic substances from the acid. The loaded steam stream 800 removed at the top of the column is led to the bottom of the absorption column 50. The purified hydrochloric acid stream 1000 draining out of the stripping column 60 is cooled and led into a reservoir container 70. From there the acid is fed to further processing.

(12) FIG. 2 shows a diagram of a further installation for carrying out the method according to the invention. This figure shows how several gas scrubbing steps can be cascaded in order to be able to achieve an even higher purity of the hydrogen chloride gas. On the basis of the installation shown in FIG. One, which is not to be described again to avoid repetitions, a second purification installation is included downstream of the first gas scrubber 10, and receives prepurified hydrogen chloride gas from the first gas scrubber 10 by means of substance stream 100.

(13) The second gas scrubber 10 here is configured as a packed column. Hydrogen chloride gas is introduced at the base of the column via substance stream 100. Concentrated hydrochloric acid is present in a second reservoir tank 20. This is saturated with respect to uptake of hydrogen chloride (or will become saturated in the course of the gas scrubbing in column 10 before it leaves the column). The hydrochloric acid from the second reservoir tank 20 enters into the pump 30 as substance stream 300 and downstream of the pump 30 is introduced at the top of the column 10 as substance stream 400.

(14) After passing through the column 10 the concentrated hydrochloric acid leaves the gas scrubber 10 by means of substance stream 200. The hydrochloric acid collected in the first reservoir tank 20 can then be circulated again through the gas scrubber 10. During the gas scrubbing impurities become concentrated in the hydrochloric acid.

(15) It is provided for hydrochloric acid which is correspondingly concentrated and contains impurities to be sluiced out of the second hydrochloric acid tank 40, which is connected to the first hydrochloric acid tank 20. To compensate the volume removed, the first hydrochloric acid tank 20 can furthermore be topped up with hydrochloric acid and/or water, preferably hydrochloric acid.

(16) Purified hydrogen chloride gas is furthermore removed at the top of the first gas scrubber 10 by means of substance stream 500.

EXAMPLES

(17) Analysis Instructions

(18) 1) Scope of Use:

(19) This method applies to hydrochloric acids

(20) 2) Basis:

(21) TOC apparatus with connected TN unit

(22) The abbreviation TOC stands for total organic carbon, that is to say the total organic carbon content of a sample. In the case of hydrochloric acid, however, the TC (total carbon) content of the sample is determined. Nevertheless, in the context of the present invention the TOC content can be equated with the TC content since in particular in the phosgenation of amines no sources of inorganic or elemental carbon are present.

(23) The total inorganic (IC), organic (TOC) and elemental carbon is generally oxidised to CO.sub.2 (equation):

(24) $C_x{H}_x+n{\mathrm{NaHCO}}_3\stackrel{{\mathrm{Al}}_2{O}_3\text{/}\mathrm{Pt}\text{/}720C.}{.fwdarw.}\left(x+n\right){\mathrm{CO}}_2+x{H}_{2}O+n\mathrm{NaOH}$

(25) The CO.sub.2 formed is transferred into an NDIR detector (non-dispersive infrared) by means of a carrier gas stream and is measured quantitatively there.

(26) The TN value is understood as meaning the total nitrogen content of a sample. The method of combustion with subsequent chemoluminescence detection (CLD) is used for this. In this context, the sample is burned catalytically at 720 C. and nitrogen components are converted into NO:

(27) $4{\mathrm{NH}}_3+5O_2\stackrel{\mathrm{Cat}.\mathrm{Ox}.\text{/}720C.}{.fwdarw.}4\mathrm{NO}+6H_{2}O$

(28) The NO formed is reacted with ozone for the purpose of detection:
NO+O.sub.3.fwdarw.NO.sub.2*+O.sub.2

(29) During this reaction light quanta are released (chemoluminescence), which are measured by the detector:

(30) ${\mathrm{NO}}_2^{*}\stackrel{-\mathrm{hv}}{.fwdarw.}{\mathrm{NO}}_2$
3) Reagents:

(31) Millipore water is used as the water

(32) 4) Apparatuses:

(33) 4.1) Conventional Laboratory Apparatuses

(34) 4.2) TOC Apparatus with Connected TN Unit

(35) 5) Procedure:

(36) Introduce about 30 ml of Millipore water into a 100 ml volumetric flask and then pipette in 50 ml of the hydrochloric acid to be determined. After cooling to 20 C., fill the volumetric flask up to the mark with Millipore water.

(37) Measure the sample on the TOC apparatus with connected TN unit. If the dilution is set at 2, the total carbon and total nitrogen in mg/I are obtained as the result.

(38) 6) Calculation:

(39) The result is issued automatically by the apparatus as concentration by weight in mg/l.

Example 1

(40) In a first example a scrubbing column comprising two glass columns (scrubber 1 and scrubber 2) of 50 mm diameter with 500 mm of bulk material (4 mm Raschig rings) connected in series was installed. Underneath both glass columns was a reservoir container for the scrubbing liquid having a volume of 5 l. The circulation for the scrubbing liquid was removed from this container and led in parallel to the two glass columns. A volume stream of HCl gas to be purified of from 190 to 250 l/h was led through the scrubbing column. The concentrations of TOC and TN impurities in the HCl gas to be purified were each in the range of from 44 to 71 mg/kg. The volume stream of the scrubbing liquid was 30 to 45 l/h, which corresponds to a liquid load of about 23 m.sup.3/m.sup.2 h in the bulk material.

(41) A second scrubber unit was installed downstream of this scrubbing column in order to obtain information on the impurities remaining in the HCl gas stream. This comprised a glass column of 50 mm diameter with 500 mm of bulk material (4 mm Raschig rings). Underneath this glass column was a reservoir container having a volume of 2 l, from which the circulation for the scrubbing liquid was removed and led on to the glass column. The same volume stream of HCl gas to be purified of from 190 to 250 l/h was led through this downstream scrubbing column. The volume stream of the scrubbing liquid was likewise 30 to 45 l/h, which corresponds to a liquid load of about 23 m.sup.3/m.sup.2 h in the bulk material.

(42) A constant accumulation of the impurities in the reservoir of the first scrubbing column was found as the result of this first example. These impurities could be concentrated up to a content of 2 g/l without a decrease in the scrubbing action being observed. In the downstream second scrubber unit of virtually identical construction, on the other hand, only small amounts of impurities were found.

(43) The following table, which is also shown as a graph in FIG. 3, shows the accumulation of the components causing the TN and TOC in the bottom containers of the first and second scrubbing column, plotted against the gas volume passed through, for Example 1. A separation efficiency for the TN impurities of at least 98.5% and for the TOC impurities of at least 86.2% was found as the result.

(44) TABLE-US-00001 Amount of gas Scrubber 1 Scrubber 2 passed through TOC TN TOC TN [m.sup.3] [mg/l] [mg/l] [mg/l] [mg/l] 0 4.3 0.1 4.2 0.1 2.7 43 17.6 257 257 23 8.9 22.9 294 28.3 460 468 69 13 33.5 556 80 37.7 643 616 116 22 51.2 833 824 186 27 60.3 956 961 238 31 64.9 1036 1063 268 35 78.0 1281 1300 491 51 86.1 1446 1548 565 56 93.3 1618 1812 664 62 104.7 1825 1988 849 73

(45) In FIG. 3 the reference symbols of the curves have the following meanings:

(46) 1-TOC-TOC content scrubber 1

(47) 1-TN-1 TN content scrubber 1

(48) 1-TOC-2 TOC content scrubber 2

(49) 1-TN-2 TN content scrubber 2

Example 2

(50) In a second example a scrubbing column comprising two glass columns of 100 mm diameter with 500 mm of bulk material (12 mm Raschig rings) connected in series was installed. Underneath both glass columns was a reservoir container for the scrubbing liquid having a volume of 10 l. The circulation for the scrubbing liquid was removed from this container and led in parallel to the two glass columns. A volume stream of HCl gas to be purified of from 2,500 to 3,000 l/h was led through the scrubbing column. The concentrations of TOC and TN impurities in the HCl gas to be purified were each in the range of from 27 to 42 mg/kg. The volume stream of the scrubbing liquid was 150 to 200 l/h, which corresponds to a liquid load of about 26 m.sup.3/m.sup.2 h in the bulk material. A drop separator was installed at the gas outlet of the first scrubber unit in order to avoid the discharge of loaded scrubbing liquid with the purified HCl gas stream.

(51) A second scrubber unit was installed downstream of this scrubbing column in order to obtain information on the impurities remaining in the HCl gas stream. This comprised a glass column of 100 mm diameter with 500 mm of bulk material (12 mm Raschig rings). Underneath this glass column was a reservoir container having a volume of 5 l, from which the circulation for the scrubbing liquid was removed and led on to the glass column. The same volume stream of HCl gas to be purified of from 190 to 250 l/h was led through this downstream scrubbing column. The volume stream of the scrubbing liquid was likewise 30 to 45 l/h, which corresponds to a liquid load of about 23 m.sup.3/m.sup.2 h in the bulk material.

(52) An almost constant accumulation of the impurities in the reservoir of the first scrubbing column was likewise found as the result of this second configuration. These impurities could be concentrated up to a content of about 3 g/l without a decrease in the scrubbing action being observed. In the downstream second scrubber unit of virtually identical construction, on the other hand, only small amounts of impurities were found.

(53) The following table, which is also shown as a graph in FIG. 4, shows the accumulation of the components causing the TN and TOC in the bottom containers of the first and second scrubbing column, plotted against the gas volume passed through, for Example 2. A separation efficiency for the TN impurities of at least 95.2% and for the TOC impurities of at least 84.8% was found as the result.

(54) TABLE-US-00002 Amount of gas Scrubber 1 Scrubber 2 passed through TOC TN TOC TN [m.sup.3] [mg/l] [mg/l] [mg/l] [mg/l] 0 9.7 5.3 9.7 5.3 60 334.6 343.2 57.7 19.2 232.5 845 906.3 156 72.1 352.5 1244.8 1338.9 262.9 109.6 472.5 1836 1956 437 172 617.5 1325.7 2443 624.7 220.3 700 2775.6 3052.8 901.1 281.3

(55) In FIG. 4 the reference symbols of the curves have the following meanings:

(56) 2-TOC-1 TOC content scrubber 1

(57) 2-TN-1 TN content scrubber 1

(58) 2-TOC-2 TOC content scrubber 2

(59) 2-TN-2 TN content scrubber 2

Comparative Example

(60) Adsorption experiments were likewise carried out as the closest method for purification of aqueous HCl to remove organic compounds. In this context the most diverse commercially available active charcoals having different pore structures, base materials and surface treatments were brought into contact with aqueous HCl containing TOC and TN impurities. These experiments were carried out as shaking experiments at room temperature over 3 days. Nevertheless, no purification effect with respect to the TN impurities and TOC impurities present was found, as the following table shows:

(61) TABLE-US-00003 Amount of active TOC TN charcoal employed impurity impurity Active charcoal used [g/l] [mg/l] [mg/l] Untreated HCl (blank 12 12 sample) Hydraffin CC 12 40 10 12 12 spezial 50 10 12 Epibon X 12 40 50 14 15 spezial 50 11 12 Filtrasorb 300 50 11 11 CPG LF 10 12 12 50 11 12 Aquacarb 207C 12 30 50 25 12 Lewatit AF 5 10 12 12 50 10 12