METHOD FOR THE PURIFICATION OF AQUATIC WASTEWATER FLOWS CONTAMINATED WITH NITROBENZOL

Description

[0001] The present invention relates to a method for cleaning a wastewater stream AW1 contaminated with nitrobenzene, comprising (I) stripping of the wastewater stream AW1 with a stripping gas SG1 in a continuously operated stripper column to obtain a wastewater stream AW2 containing nitrobenzene in a reduced concentration compared to AW1 (c NB,AW2 ), (II) a further purification of the wastewater stream AW2 in a wastewater treatment plant, wherein a target value (c NB,AW2,SOLL ) is set for the concentration of nitrobenzene in the wastewater stream AW2, which is greater than zero but is based on the wastewater treatment plant's requirements for the maximum nitrobenzene content of the wastewater streams supplied to it, wherein a database for at least one combination of given boundary conditions of (a) Nitrobenzene concentration in AW1, (b)(c) the temperature of AW1 and (c) the temperature of SG1, a family of linear mathematical relationships of the form ṁ SG1 = x · ṁ AW1 is defined, which define a range of concentrations of nitrobenzene in AW2, wherein the family includes, in addition to a mathematical relationship (0) corresponding to the target value c NB,AW2,SOLL, at least a first linear mathematical relationship (1) for a first value of c NB,AW2, which corresponds to 98% of the target value c NB,AW2,SOLL, and a second linear mathematical relationship (2) for a second value of c NB,AW2, which corresponds to 102% of the target value c NB,AW2,SOLL, and wherein the flow rate of the stripping gas is matched to the flow rate of the wastewater AW1 such that the flow rate of AW1 lies within a range of values ​​(AB) defined by the first mathematical relationship (1) and the second mathematical relationship (2) for the respective flow rate of AW1 is set out, and rules of the concentration of nitrobenzene in AW2 (c NB,AW2), by adjusting the flow rate of stripping gas SG1 accordingly when the measured actual value of this concentration lies outside a window of > 98% to < 102% of the target value.

[0002] Many production processes generate aqueous wastewater streams contaminated with organic compounds. Typically, these streams are first pre-treated at the point of origin and then fed into a wastewater treatment plant (sewage treatment plant) before being released into the environment. Even the wastewater streams fed into the treatment plant must meet minimum purity requirements, particularly to prevent the microorganisms used in biological treatment plants from being damaged or even killed by excessively high concentrations of toxic organic compounds. Therefore, operators of wastewater treatment plants are increasingly establishing stringent specifications for the purity of the wastewater they process. to be removedWastewater, which is why in-house pretreatment is playing an increasingly important role. In many cases, such in-house pretreatment is carried out by stripping the wastewater in a stripper column (also called a wastewater stripper). The present invention relates to the operation of such a stripper column.

[0003] WO 2014 / 170309 Al describes a process for the treatment of alkaline wastewater generated during the washing of crude nitrobenzene obtained by the nitration of benzene, wherein (i) the alkaline wastewater is heated to a temperature of 150 °C to 500 °C under an oxygen-free environment and at a pressure higher than atmospheric pressure; (ii) the wastewater obtained in (i) is treated with a base; (iii) the wastewater obtained in (ii) is further purified by stripping with a stripping gas, and the impurity-laden stripping gas stream is subsequently cooled to a temperature of 10 °C to 60 °C. The application does not address control engineering details of the stripping process.

[0004] US 7,402,192 B2 describes a process for the continuous treatment of industrial wastewater, which may contain various contaminants. The only specific example of suitable industrial wastewater is refinery wastewater. The wastewater is introduced via at least one feed line (7, 13) into the upper part of a stripping column (8), where it flows downwards. A stream of steam is injected into this column (8) at a height (at 14) such that the wastewater and steam flow countercurrently within the column (8). The gases extracted by this steam during the stripping of the wastewater are recovered at the top of the column (at 15), and the treated water is discharged into the bottom of the stripping column (8) (at 16).The method is characterized by the online determination of at least one section of the ultraviolet spectrum of impurities, particularly the group formed by sulfides, ammonia, and phenols. The measurement is performed in the lines for supplying (7, 13) the wastewater or in the line for draining the treated water (16) from the column (8). By mathematically processing the measured intensities, at least one impurity product contained in the samples is determined. This information, based on the results obtained and by comparison with predetermined setpoints, is represented as electrical signals that control the feed rates of wastewater and steam to the stripper column.

[0005] EP 1 019 324 B1 (also published as ES 2 212 351 T3) describes a process for wastewater treatment by mixing and evaporating the wastewater to be treated in a stream of slurry gas, utilizing the heat from the purified, outgoing, and compressed slurry gas-steam mixture to heat the slurry gas-wastewater mixture and remove the residue formed by superheating the steam. The document further describes an arrangement for carrying out this process.

[0006] From an economic perspective, it is crucial to operate such a stripper column as energy-efficiently as possible without compromising the specified requirements for the content of organic substances (especially those toxic to microorganisms) in the treated wastewater. A further objective is to design the operation of a stripper column to be as user-friendly as possible, i.e., in a way that minimizes the need for manual intervention by operating personnel and, when required, avoids the need to perform complex procedures.

[0007] To achieve these goals, efficient control of the stripper column used for in-house cleaning is required. Accordingly, the present invention provides the following: A Method for cleaning a wastewater stream contaminated with nitrobenzene A W1, containing nitrobenzene (NB) in a concentration c NB,AW1, comprising: (I) Stripping the wastewater stream AW1 with a stripping gas SG1 in a continuously operated stripper column to obtain a wastewater stream AW2 containing nitrobenzene at a concentration c NB,AW2 (which is less than c NB,AW1) and a stripping gas stream SG2 loaded with nitrobenzene, wherein the wastewater stream AW1 is fed to the stripper column at a flow rate ṁ AW1 and a temperature T AW1 and the stripping gas SG1 is fed to the stripper column at a flow rate ṁ SG1 and a temperature T SG1 and wherein the concentration of nitrobenzene in the wastewater stream AW2 c NB,AW2 is measured continuously or at intervals (but in particular at least when the flow rate ṁ AW1 changes) at a measuring point;and (II) purifying the wastewater stream AW2 in a wastewater treatment plant to obtain a purified wastewater stream AW3 containing nitrobenzene at a concentration c NB,AW3 (which is less than c NB,AW2 ), wherein the concentration of nitrobenzene in the wastewater stream AW2 supplied to the wastewater treatment plant must not exceed a predetermined maximum value c NB,AW2,MAX and a target value c NB,AW2,SOLL is set for the concentration of nitrobenzene in the wastewater stream AW2 c NB,AW2 which is greater than zero and is set to a value within a range defined by the condition ; 0 , 50 ⋅ c NB , AW 2 , MAX ≤ c NB , AW 2 , SOLL ≤ 0 , 95 ⋅ c NB , AW 2 , MAX defined target area is determined; the operation of the stripper crew includes: (i) Continuous feeding of the stripping gas SG1 and the wastewater stream AW1 into the stripper column, wherein in a database for at least one combination of given boundary conditions of (a) c NB,AW1 , (b) the temperature T AW1 and (c) the temperature T SG1 a family of linear mathematical relationships of the form m ˙ SG 1 = x ⋅ m ˙ AW 1 is stored, each of which corresponds to a concentration c NB,AW2, such that the set of linear mathematical relationships defines a range of concentrations c NB,AW2, wherein the set of linear mathematical relationships includes, in addition to a mathematical relationship (0) corresponding to the target value c NB,AW2,SOLL, at least a first linear mathematical relationship (1) for a first value of c NB,AW2 corresponding to 98% of the target value c NB,AW2,SOLL, and a second linear mathematical relationship (2) for a second value of c NB,AW2 corresponding to 102% of the target value c NB,AW2,SOLL, and wherein the quantity flow ṁ SG1 for a quantity flow ṁ AW1 (ti ) given at a time ti is chosen such (where the quantity flow ṁ SG1 is matched to the quantity flow ṁ AW1 (ti )) that the quantity flow ṁ SG1 within a Value range (AB) lies,which is defined by the first mathematical relationship (1) and the second mathematical relationship (2) for the flow rate ṁ AW1 (ti) given at time ti; (ii) control of the concentration c NB,AW2 , by reducing the flow rate of stripping gas SG1 ṁ SG1 when the actual concentration c NB,AW2,IST of nitrobenzene measured at the measuring point is equal to or less than 0.98 • c NB,AW2,SOLL , and by increasing the flow rate of stripping gas SG1 ṁ SG1 when the actual concentration c NB,AW2,IST of nitrobenzene measured at the measuring point is equal to or greater than 1.02 • c NB,AW2,SOLL . ,

[0008] Surprisingly, it was found that the aforementioned goals can be achieved, or at least approached, if the stripping column is not regulated in such a way that the purity to be achieved is "as high as possible" (i.e., c NB,AW2 is as low as possible, ideally below the measurement threshold), but rather that a value for c NB,AW2 is deliberately allowed that is greater than "zero" (i.e., in this context: which is above the measurement threshold ) and can even be up to 95% of the maximum permissible value specified by the operator of the wastewater treatment plant (which is also greater than zero), and this value is regulated by means of continuous or interval measurements and, if necessary, adjustments to the amount of stripping gas supplied to the stripper column per unit of time for a given wastewater flow rate. From an environmental protection perspective, the only thing that ultimately matters is that the into the environmentdischarged wastewater, which is usually a wastewater treatment plant leaving Wastewater has the highest possible purity, but at least meets the applicable legal requirements in each individual case; where exactly this purity is achieved, whether largely in in-house pre-treatment or in the wastewater treatment plant itself, is in this context incidental or even irrelevant. However, within the scope of the present invention, it was found that under economic aspects It can be of crucial importance to distribute the overall degree of purification achieved among the various purification stages in a specific manner (naturally taking into account the applicable boundary conditions, such as the aforementioned requirements of wastewater treatment plant operators). The inventive method takes this into account by providing a control system that enables this in an efficient manner.

[0009] The material to be cleaned in the inventive method "with nitrobenzene" The contaminated wastewater stream AW1 can, of course, contain one or more other organic compounds besides nitrobenzene; this does not exceed the scope of the present invention. In aqueous wastewater streams from a process for the production of nitrobenzene (here, in particular, acidic and alkaline wastewater; see the explanations below), nitrobenzene is regularly present in a comparatively high concentration relative to other organic contaminants such as benzene, so it is advantageous to base the control system on nitrobenzene.

[0010] In the attached Drawings show FIG. 1 a schematic representation of a stripper column that can be used in the process according to the invention; FIG. 2a schematic representation of the change of ṁ AW1 in a fictitious period t 0 to t 4 with the intermediate periods t 0 to t 2 and t 2 to t 4 . FIG. 3 a schematic representation of the relationships between the quantity flows of SG1, AW1 and c NB,AW2,SOLL in the period t 0 to t 2 ; and FIG. 4 a schematic representation of the relationships between the quantity flows of SG1, AW1 and c NB,AW2,SOLL in the period t 2 to t 4 .

[0011] First, a Summary various possible designs.

[0012] In one first embodiment The invention, which can be combined with all other embodiments, uses steam and / or nitrogen as the stripping gas.

[0013] In one second embodiment of the invention, which can be combined with all other embodiments, c NB,AW2,IST is determined by near-infrared spectroscopy or gas chromatography.

[0014] In one third embodimentof the invention, which is a special embodiment of the second embodiment, c NB,AW2,IST is determined by near-infrared spectroscopy.

[0015] In one fourth embodiment According to the invention, which can be combined with all other embodiments, a value for c NB,AW2 is determined independently at two or more (in particular at two) measuring points, and the mean value is formed from the obtained measured values, which is used as the value for c NB,AW2,IST.

[0016] In one fifth embodiment According to the invention, which is a particular embodiment of the sixth embodiment, in the event of the failure of one or more of the two or more measuring points, but not all measuring points, only the at least one measuring point that remains functional is used, wherein the method includes at least one such failure during the continuously performed stripping.

[0017] In one sixth embodimentAccording to the invention, which can be combined with all other embodiments, unless these, like the fifth embodiment, include the continued functionality of at least one measuring point, in the event of the failure of all measuring points, the last value determined from the measured value of a measuring point before the failure of that measuring point for the reduced or increased flow rate of stripping gas SG1 ṁ SG1 to be supplied to the stripper column according to the control of step (ii) is increased by 5% to 10% and maintained until at least one of the measuring points is functional again.

[0018] In one seventh embodimentIn the invention, which can be combined with all other embodiments, in step (i) the flow rate ṁ SG1 is selected for the flow rate ṁ AW1 (ti ) given at time ti such that the flow rate ṁ SG1 lies within a range of values ​​defined by the mathematical relationship (0), which corresponds to the target value c NB,AW2,SOLL, and the second mathematical relationship (2) for the flow rate ṁ AW1 (ti ) given at time ti.

[0019] In one note designIn the invention, which represents an alternative to the seventh embodiment, in step (i) the quantity flow ṁ SG1 is selected for the quantity flow ṁ AW1 (ti ) given at time ti such that the quantity flow ṁ SG1 lies within a range of values ​​defined by the mathematical relationship (0), which corresponds to the target value c NB,AW2,SOLL, and the first mathematical relationship (1) for the quantity flow ṁ AW1 (ti ) given at time ti.

[0020] In one ninth embodimentThe invention, which represents a further alternative to the seventh embodiment, comprises a set of linear mathematical relationships comprising a third linear mathematical relationship for a third value of c NB,AW2, which corresponds to 99% of the target value c NB,AW2,SOLL, and a fourth linear mathematical relationship for a fourth value of c NB,AW2, which corresponds to 101% of the target value c NB,AW2,SOLL, wherein in step (i) the flow rate ṁ SG1 for the flow rate ṁ AW1 (ti ) given at time ti is selected such (whereby the flow rate ṁ SG1 is matched to the flow rate ṁ AW1 (ti )) that the flow rate ṁ SG1 lies within a range of values ​​spanned by the third mathematical relationship and the fourth mathematical relationship for the flow rate ṁ AW1 (ti ) given at time ti.

[0021] In one tenth embodimentThe invention, which can be combined with all other embodiments, provides a set of linear mathematical relationships for at least two combinations of boundary conditions of (a) c NB,AW1 , (b) the temperature T AW1 and (c) the temperature T SG1, wherein at least the temperature T SG1 is different in the at least two combinations of boundary conditions.

[0022] In one eleventh embodiment According to the invention, which can be combined with all other embodiments, the wastewater stream to be cleaned is generated in the washing of a nitrobenzene phase, wherein the nitrobenzene phase is obtained by nitration of benzene with nitric acid in the presence of sulfuric acid followed by separation of an aqueous sulfuric acid phase.

[0023] In one twelfth embodimentThe invention, which is a particular embodiment of the eleventh embodiment, comprises the washing of the nitrobenzene phase in three stages, wherein washing is carried out in the first stage with water, in the second stage with an aqueous base solution (in particular sodium hydroxide) and in the third stage with water, wherein wastewater is generated in each of the three stages and at least one of it is purified as wastewater stream AW1 in the stripper column.

[0024] In one thirteenth embodiment According to the invention, which is a particular embodiment of the twelfth embodiment, wastewater is generated in the third stage, which is used as a component of wash water used in the second and / or first stage, wherein wastewater is generated in the first and / or second stage, which is purified as wastewater stream AW1 in the stripper column.

[0025] In one fourteenth embodimentAccording to the invention, which can be combined with all other embodiments, the maximum value c NB,AW2,MAX is in the range of 2.0 ppm to 10 ppm, based on the total mass of AW2.

[0026] In one fifteenth embodiment According to the invention, which can be combined with all other embodiments, c NB,AW2 is measured continuously or at intervals of a maximum of 6 h, preferably a maximum of 3 h, particularly preferably a maximum of 1 h.

[0027] The embodiments and further possible configurations of the invention briefly described above will be discussed below. explained in more detail. All embodiments and further configurations can be combined with one another as desired, unless expressly stated otherwise or the context clearly indicates otherwise.

[0028] The actual stripping of the wastewater stream AW1 with a stripping gas SG1 in Step (I)The process can generally be carried out as known to those skilled in the art. Water vapor and / or nitrogen, particularly water vapor, are preferably used as the stripping gas. The stripper column is supplied with, in particular, the following: FIG. 1 shown, the wastewater to be treated ( AW1 in the terminology of the present invention; mass flow ṁ AW1 , concentration of nitrobenzene, c NB , is supplied at the top, and a suitable stripping gas is supplied countercurrently from below ( SG1 In the terminology of the present invention; the mass flow ṁ SG1 ) is passed through the stripper column. In the bottom of the column, the nitrobenzene-depleted wastewater is collected. (AW2 in the terminology of the present invention; mass flow ṁ AW2 , concentration of nitrobenzene c NB,AW2 ) is withdrawn. At the top of the column, a stripping gas stream enriched with nitrobenzene ( SG2(in the terminology of the present invention). This current SG2 is preferably condensed and put to further use. This further use can be of a material nature, in that the condensate obtained during condensation is fed into a chemical process. However, it is also possible to combust the condensate obtained, whose organic content is significantly higher than that of AW1, and to use the released heat as an energy carrier, for example, for production processes.

[0029] The concentration cNB,AW2 can, in principle, be determined by all methods known to those skilled in the art for the purpose of detecting nitrobenzene in aqueous solution, in particular by near-infrared spectroscopy or gas chromatography, with near-infrared spectroscopy being preferred. Generally, all methods provide consistent results with sufficient accuracy for the purposes of the present invention. In the unlikely event of a significant discrepancy between different measurement methods, the value determined by near-infrared spectroscopy shall be decisive for the purposes of the present invention. The measurement of cNB,AW2 is carried out, in particular, continuously or at intervals of a maximum of 6 hours, preferably a maximum of 3 hours, and most preferably a maximum of 1 hour.

[0030] It is possible to use more than one measuring point, in particular (exactly) two measuring points, to determine the concentration c NB,AW2. While the use of more than two measuring points is generally not necessary, it is of course possible. The multiple measuring points independently determine measured values, from which the average is calculated and used as the value for c NB,AW2,IST. This increases the accuracy and allows the procedure to continue even if one of the measuring points (not all) fails due to a malfunction. In such a case, the remaining functioning measuring point can simply be used. If only a single measuring point is available, then, of course, no average can be calculated for the period of the malfunction.

[0031] If, regardless of whether exactly one or more measuring points are used, no measuring point is available temporarily due to a malfunction, the last value determined from the measured value for c NB,AW2 before the failure of a measuring point for the quantity flow of stripping gas SG1 ṁ SG1 to be supplied to the stripper column according to the control of step (ii) is preferably increased by 5% to 10% and maintained until at least one of the measuring points is functional again and can fulfill its task.

[0032] The required flow rate of stripping gas SG1 to maintain the desired purity of the wastewater AW2 depends on various factors for a given stripper column, including in particular: 1. Quantity of the mass flow AW1; 2. Concentration of nitrobenzene in AW1; 3. Temperature of AW1; 4. Temperature of the stripping gas SG1.

[0033] For a given process, factors 2 to 4 generally exhibit only minor fluctuations, so the main focus for control should be on the size of the flow rate AW1. Changes in this flow rate are also referred to as load change such a load change is in FIG. 2 The diagram is schematically represented. It shows a plot of the wastewater flow rate (AW1) over time t. The period considered is from t0 to t4, during which (at time t2) the flow rate of AW1 fed to the wastewater stripping column increases significantly. Within the low-load period (t0 to t2), time t1 is highlighted, which will be discussed in more detail later. The same applies to the high-load period (t2 to t4, specifically t3). (Strictly speaking, the high-load period begins immediately.) aftert2, if the load is suddenly increased at time t2. For the sake of simplicity, the period of higher load will nevertheless be referred to in this discussion as "t2 to t4" or "t2 - t4".

[0034] Against this background, the present invention presents a family of linear mathematical relationships for at least one combination of given boundary conditions of (a) c NB,AW1 , (b) the temperature T AW1 and (c) the temperature T SG1 stored. This means that for at least one value for (a) c NB,AW1 combined with a value for (b) T AW1 and combined withA set of linear mathematical relationships is assigned to a value for (c) T SG1. If, in practice, it turns out that the stripper column can be operated effectively with several combinations of values ​​for (a) c NB,AW1, (b) T AW1, and (c) T SG1, then preferably a set of linear mathematical relationships is assigned to each of these combinations. This applies particularly if the temperature of the wastewater stream is variable, for example, if the wastewater AW1 arrives at the stripper column inlet at a certain temperature during a first period and at a different temperature during a second period. In such a case, a different set of linear mathematical relationships corresponding to the second temperature of AW1 is simply used.

[0035] Under these boundary conditions, every linear mathematical relationship corresponds to the form SG1 = x • ṁ AW1 a specific concentration of nitrobenzene in the wastewater leaving the stripper column, c NB,AW2. In other words: Under these boundary conditions, the dependence of the quantity of stripping gas required for the purification of wastewater AW1 on the quantity of wastewater AW1 introduced is linear, where the slope of the line corresponds to the factor x in the linear mathematical relationship ṁ SG1 = x • ṁ AW1. In this way, it is possible to establish a family of such linear mathematical relationships which an area at concentrations c NB,AW2 corresponds.

[0036] FIG. 3Figure 1 shows a schematic representation of a typical dependence of the required flow rate of stripping gas SG1 on the flow rate of wastewater AW1 supplied to the stripper column. In this figure, ṁ AW1_ MIN denotes the minimum wastewater flow rate with which the stripper column can still be operated efficiently. The wastewater flow rate at time t 1 (see Figure 1) is 1. FIG. 2 ) is led into the stripper column ( ṁ AW1 (t 1 ) ) is shown. Since the quantity flow to AW1 from t 0 to t 2 ( ṁ AW1 (t 0 -t 2 ) ) is constant, ṁ AW1 (t 1 ) = ṁ AW1 (t 0 -t 2 ).

[0037] The data required for creating such a graph can easily be obtained through operational experience (preliminary tests under the selected boundary conditions of (a) c NB,AW1 , (b) T AW1 and (c) T SG1 ) and / or engineering calculations known to those skilled in the art. The relationship is linear, i.e., the function ṁ SG1 (ṁ AW1 ) is a straight line whose slope corresponds to the value x in the linear mathematical relationship. According to the invention, the family of linear mathematical relationships comprises at least the three in FIG. 1 Lines labelled (0), (1) and (2). The line labeled (0) corresponds to those values ​​of ṁ SG1 which, given the corresponding values ​​of ṁ AW1 (under the given boundary conditions), result in an organic compound content in the wastewater AW2 leaving the stripper column that meets the target value. The line labeled (1) corresponds to those values ​​of ṁ SG1 which, given the corresponding values ​​of ṁ AW1 (under the given boundary conditions), result in an organic compound content in the wastewater AW2 leaving the stripper column that is 2% below the target value. The line labeled (2) corresponds to those values ​​of ṁ SG1 which, given the corresponding values ​​of ṁ AW1 (under the given boundary conditions), result in an organic compound content in the wastewater AW2 leaving the stripper column that is 2% above the target value.

[0038] The lines (1) and (2) span a truncated cone, which is indicated by the hatching.

[0039] The knowledge that a graphic like the one in FIG. 3 The underlying principle shown is therefore, within the scope of the present invention, considered family of linear mathematical relationships in one database stored. The database, in turn, is integrated into a process control system that manages the stripping column.

[0040] Let us now consider the time t = t 1: The quantity flow of stripping gas SG1 can be in the range ṁ SG1 (t 0 -t 2 )_MIN until ṁ SG1 (t 0 -t 2 )_MAXThese two values ​​denote the minimum (ṁ SG1 (t 0 -t 2 )_MIN) and maximum (ṁ SG1 (t 0 -t 2 )_MAX) flow rate of stripping gas with which, for a wastewater volume ṁ AW1 (t 1 ) = ṁ AW1 (t 0 -t 2 ), the concentration of nitrobenzene in the wastewater AW2 can be maintained within a range of ± 2% around the target value, according to the knowledge stored in the database. In this way, the endpoints A and B of a value range AB are defined for time t 1, within which ṁ SG1 can be selected at time t 1 (generally speaking: at time ti ). This fulfills the requirement of the invention, according to which the quantity flow ṁ SG1 for a quantity flow ṁ AW1 (ti ) given at a time ti is chosen such (where the quantity flow ṁ SG1 is matched to the quantity flow ṁ AW1 (ti )) that the quantity flow ṁ SG1 lies within a range of values ​​(AB) that is spanned by the first mathematical relationship (1) and the second mathematical relationship (2) for the quantity flow ṁ AW1 (ti ) given at time ti, meant. The flow rate ṁ AW1 (ti ) can be chosen such that the amount of stripping gas introduced tends to be too high (i.e., it is located in the upper region of the truncated cone). FIG. 3moved), which means that the regulation according to step (ii) tends to involve a reduction in the flow rate to SG1. However, the flow rate ṁ AW1 (ti ) can also be chosen such that the registered stripping gas quantity tends to be too low (i.e., located in the lower part of the truncated cone). FIG. 3 (moves), which means that the regulation according to step (ii) tends to involve an increase in the flow rate at SG1. Finally, the flow rate ṁ AW1 (ti ) can also be chosen such that the applied stripping gas quantity moves in a range above and below the ideal straight line (0), i.e., in a middle region of the truncated cone. FIG. 3 This results in the regulation according to step (ii) tending to involve a reduction or an increase in the quantity flow to SG1, depending on the result of the measurements carried out continuously or at intervals.

[0041] From the point of view of user-friendliness, the first two options are preferred.

[0042] It goes without saying that the existing technical constraints of the equipment must be taken into account. For example, it will regularly be the case that the stripping gas quantity of SG1 cannot be varied arbitrarily within the framework of the control according to step (ii), but only within a certain given range, for example by a maximum of 20%. This must be taken into account when selecting the flow rate ṁ AW1 (ti ) according to step (i), i.e., the flow rate ṁ AW1 (ti ) must be selected such that any potential deviation from the corridor of ± 2% around the target value for the concentration of nitrobenzene in AW2 can also be compensated for.This means, for example, that when using the second of the aforementioned options, the quantity flow ṁ AW1 (ti ) in step (i) must be chosen so that - for the example mentioned of limiting the variability of the stripping gas quantity flow by up to 20% - in step (ii) an increase in the quantity flow of SG1 by up to 20% is actually sufficient to ensure the required correction.

[0043] FIG. 4 is a to FIG. 3 Analogous representation for time t3, which lies within the period of higher load (t2 to t4). The wastewater flow rate corresponding to time t3 is shown as ṁAW1 (t3). Since the wastewater flow rate AW1 is constant during the period t2 to t4, the following applies analogously: FIG. 3 ṁ AW1 (t 3 ) = ṁ AW1 (t 2 -t 4 ). Here too, there is a corresponding minimum value for ṁ AW1 (t 3 ). (ṁ SG1 (t 2 -t 4 )_MIN) and maximum values (ṁ SG1 (t 2 -t 4 )_MAX) for the stripping gas flow rate (value range AB).

[0044] The process according to the invention is suitable for the purification of aqueous wastewater streams from a process for the production of nitrobenzene by nitration of benzene with nitric acid in the presence of sulfuric acid. Benzene is typically used in a stoichiometric excess relative to nitric acid, particularly in the adiabatic nitration processes commonly used today, so that it must be separated during the work-up of the nitrated product.The work-up of the nitration product is typically carried out by first separating the reaction mixture (nitration product) present after nitration, containing nitrobenzene, unreacted benzene, and sulfuric acid, into an aqueous sulfuric acid phase and an organic nitrobenzene phase. This is followed by a second step of one- or multi-stage washing of the nitrobenzene phase, and finally, a third step in which the excess benzene is removed from the washed nitrobenzene phase. The excess benzene is usually recycled in the nitration process.

[0045] The second step of the washing process preferably comprises three stages, a so-called... acidic laundry a so-called alkaline laundry and a so-called neutral laundry.In the first stage, the nitrobenzene phase obtained from the phase separation is washed with water to remove as many entrained or dissolved acid residues as possible. In the second stage, the organic washing product obtained after phase separation from the first stage is washed with an aqueous base solution, in particular sodium hydroxide, to neutralize any remaining acid residues in the nitrobenzene. Finally, in the third stage, the organic washing product obtained after phase separation from the second stage is washed with water. In each stage, in addition to the respective organic washing product, wastewater is also generated after phase separation, which can be purified according to the invention.

[0046] Preferably, the washing process is carried out in countercurrent flow such that the wastewater obtained in the third stage is used as a component of the washing liquid in the second and / or first stage. The process according to the invention is particularly suitable for purifying the wastewater generated in the first and / or second stage.

[0047] As mentioned at the outset, the purification process according to the invention serves to pre-purify the wastewater stream AW1 such that the resulting wastewater stream AW2 can be fed into a (particularly biological) wastewater treatment plant, while complying with the purity requirements for the wastewater stream AW2 specified by the operator of the plant. The present invention offers the advantage of enabling the stripper column to be controlled in such a way that the nitrobenzene content in the wastewater AW2 is not necessarily always minimal, but rather relatively constant and always sufficiently far below the specification limit of the wastewater treatment plant operator. The maximum value cNB,AW2,MAX specified by the operator of the wastewater treatment plant is, for example, in the range of 2.0 ppm to 10 ppm, based on the total mass of AW2.

[0048] The invention described above is characterized by an optimization of steam consumption and increases user-friendliness.

Claims

1. Process for purifying a wastewater stream WW1 contaminated with nitrobenzene which contains nitrobenzene in a concentration cNB,WW1, comprising: (I) stripping the wastewater stream WW1 with a stripping gas SG1 in a continuously operated stripping column to obtain a wastewater stream WW2 which contains nitrobenzene in a concentration cNB,WW2 < cNB,WW1 and a stripping gas stream SG2 laden with nitrobenzene, wherein the wastewater stream WW1 is supplied to the stripping column at a flow rate ṁWW1 and a temperature TWW1 and the stripping gas SG1 is supplied to the stripping column at a flow rate ṁSG1 and a temperature TSG1, and wherein the concentration of nitrobenzene in the wastewater stream WW2 cNB,WW2 is measured at a measuring point continuously or at intervals; and (II) purifying the wastewater stream WW2 in a wastewater treatment plant to obtain a purified wastewater stream WW3 which contains nitrobenzene in a concentration cNB,WW3 < cNB,WW2, wherein the concentration of nitrobenzene in the wastewater stream WW2 supplied to the wastewater treatment plant must not exceed a predetermined maximum value cNB,WW2,MAX and a target value cNB,WW2,TARGET is specified for the concentration of nitrobenzene in the wastewater stream WW2 cNB,WW2 which is greater than zero and is specified to a value within a target range defined by the condition 0.50 ⋅ c NB , WW 2 , MAX ≤ c NB , WW 2 , TARGET ≤ 0.95 ⋅ c NB , WW 2 , MAX ; wherein the operation of the stripping column comprises: (i) continuously supplying the stripping gas SG1 and the wastewater stream WW1 to the stripping column, wherein for at least one combination of boundary conditions (a) cNB,WW1, (b) the temperature TWW1, and (c) the temperature TSG1, a set of linear mathematical relationships of the type m ˙ SG 1 = x ⋅ m ˙ WW 1 is stored in a database, each of which linear mathematical relationships corresponds to a concentration cNB,WW2, so that the set of linear mathematical relationships defines a range of concentrations cNB,WW2, wherein the set of linear mathematical relationships comprises, in addition to a mathematical relationship (0) which corresponds to the target value cNB,WW2,TARGET, at least a first linear mathematical relationship (1) for a first value of cNB,WW2, which corresponds to 98% of the target value cNB,WW2,TARGET, and a second linear mathematical relationship (2) for a second value of cNB,WW2, which corresponds to 102% of the target value cNB, WW2, TARGET, and wherein the flow rate ṁSG1 is selected for a flow rate ṁWW1 (ti) occurring at a time ti in such a way that the flow rate ṁSG1 is within a range of values (AB) that is generated by the first mathematical relationship (1) and the second mathematical relationship (2) at the flow rate ṁWW1 (ti) occurring at the time ti; (ii) controlling the concentration cNB,WW2 by reducing the flow rate of stripping gas SG1 ṁSG1 in the event that an actual concentration cNB,WW2,ACTUAL of nitrobenzene in the wastewater WW2 measured at the measuring point is equal to or less than 0.98 • cNB, WW2,TARGET and by increasing the flow rate of stripping gas SG1 ṁSG1 in the event that an actual concentration cNB,WW2,ACTUAL of nitrobenzene in the wastewater WW2 measured at the measuring point is equal to or less than 1.02 • cNB,WW2,TARGET.

2. Process according to Claim 1, wherein steam and / or nitrogen is used as the stripping gas.

3. Process according to either of the preceding claims, wherein a value for cNB,WW2 is determined independently at two or more measuring points and the obtained measured values are used to form an average which is used as the value for cNB,WW2,ACTUAL.

4. Process according to Claim 3, wherein in the event of failure of one or more of the two or more measuring points, but not of all of the measuring points, only the at least one measuring point remaining functional is used, wherein the process comprises at least one such failure during the continuously performed stripping.

5. Process according to any of Claims 1 to 3, wherein in the event of failure of all measuring points, the last value for the reduced or increased flow rate of stripping gas SG1 ṁSG1 to be supplied to the stripping column according to the controlling of step (ii) that was determined from the measured value of a measuring point before its failure, is increased by 5% to 10% and maintained until at least one of the measuring points is functional again.

6. Process according to any of the preceding claims, wherein in step (i) the flow rate ṁSG1 is selected for the flow rate ṁWW1 (ti) occurring at the time ti in such a way that the flow rate ṁSG1 is within a range of values that is generated by the mathematical relationship (0), which corresponds to the target value cNB,WW2,TARGET, and the second mathematical relationship (2) at the flow rate ṁWW1 (ti) occurring at the time ti.

7. Process according to any of Claims 1 to 5, wherein in step (i) the flow rate ṁSG1 is selected for the flow rate ṁWW1 (ti) occurring at the time ti in such a way that the flow rate ṁSG1 is within a range of values that is generated by the mathematical relationship (0), which corresponds to the target value cNB,WW2,TARGET, and the first mathematical relationship (1) at the flow rate ṁWW1 (ti) occurring at the time ti.

8. Process according to any of Claims 1 to 5, wherein the set of linear mathematical relationships includes a third linear mathematical relationship for a third value of cNB,WW2, which corresponds to 99% of the target value cNB,WW2,TARGET, and a fourth linear mathematical relationship for a fourth value of cNB,WW2, which corresponds to 101% of the target value cNB,WW2,TARGET, wherein in step (i) the flow rate ṁSG1 is selected for the flow rate ṁWW1 (ti) occurring at the time ti in such a way that the flow rate ṁSG1 is within a range of values that is generated by the third mathematical relationship and the fourth mathematical relationship at the flow rate ṁWW1 (ti) occurring at the time ti.

9. Process according to any of the preceding claims, wherein a set of linear mathematical relationships is stored for each of at least two combinations of boundary conditions of (a) cNB,WW1, (b) the temperature TWW1 and (c) the temperature TSG1, wherein at least the temperature TSG1 differs in the at least two combinations of boundary conditions.

10. Process according to any of the preceding claims, wherein the wastewater stream to be purified is obtained in the washing of a nitrobenzene phase, wherein the nitrobenzene phase is obtained by nitration of benzene with nitric acid in the presence of sulfuric acid followed by separation of an aqueous sulfuric acid phase.

11. Process according to Claim 10, wherein the washing of the nitrobenzene phase comprises three stages, wherein the first stage comprises washing with water, the second stage comprises washing with an aqueous base solution and the third stage comprises washing with water, wherein wastewater is generated in each of the three stages and at least one of these is purified in the stripping column as wastewater stream WW1.

12. Process according to any of the preceding claims, wherein the maximum value cNB,WW2,MAX is in the range from 2.0 ppm to 10 ppm based on the total mass of the wastewater stream WW2.

13. Process according to any of the preceding claims, wherein cNB,WW2 is measured continuously or at intervals of not more than 6 h.