Method for purifying aqueous waste streams containing nitrobenzene - Patents.com

Abstract

The present invention relates to a method for purifying a wastewater stream AW1 contaminated with nitrobenzene, comprising: (I) stripping the wastewater stream AW1 with a stripping gas SG1 in a continuously operated stripping column to obtain a wastewater stream AW2 containing nitrobenzene at a reduced concentration (cNB,AW2) compared to AW1; and (II) further purifying the wastewater stream AW2 in a wastewater treatment plant, wherein a target value (cNB,AW2,SOLL) for the nitrobenzene concentration in the wastewater stream AW2 that is greater than zero and based on the wastewater treatment plant's requirements for a maximum nitrobenzene content in the wastewater stream fed to the wastewater treatment plant is specified, and wherein a series of linear mathematical relationships of the type mSG1=x·mAW1 are determined for at least one combination of given constraints: (a) the nitrobenzene concentration in AW1, (b) the temperature of AW1, and (c) the temperature of SG1, defining a range of nitrobenzene concentrations in AW2. the mathematical relationships (a) and (b) are stored in a database, and the set of linear mathematical relationships includes, in addition to the mathematical relationship (0) corresponding to the target values ​​cNB, AW2, and SOLL, at least a first linear mathematical relationship (1) for a first value of cNB and AW2 corresponding to 98% of the target values ​​cNB, AW2, and SOLL, and a second linear mathematical relationship (2) for a second value of cNB and AW2 corresponding to 102% of the target values ​​cNB, AW2, and SOLL; the flow rate of the stripping gas is adapted to the flow rate of the wastewater AW1 so that the flow rate of AW1 is within a range of values ​​(AB) defined by the first mathematical relationship (1) and the second mathematical relationship (2) for a specific flow rate of AW1; and when the actual measured value of the nitrobenzene concentration in AW2 is outside the interval of more than 98% and less than 102% of the target value, the flow rate of the stripping gas SG1 is adjusted accordingly, thereby controlling the nitrobenzene concentration (cNB, AW2) in AW2.

Description

[Technical field]

[0001] The present invention relates to a method for purifying a wastewater stream WW1 contaminated with nitrobenzene, comprising: (I) stripping the wastewater stream WW1 with a stripping gas SG1 in a continuously operated stripping column to obtain a reduced concentration (c NB,WW2 ) to obtain a wastewater stream WW2 containing nitrobenzene; and (II) further purifying the wastewater stream WW2 in a wastewater treatment plant, wherein a target value (c) for the concentration of nitrobenzene in the wastewater stream WW2 is set to a value greater than zero and taking into account the requirements of the wastewater treatment plant for a maximum nitrobenzene content in the wastewater stream fed to the wastewater treatment plant. NB,WW2,TARGET ) is specified, and for at least one combination of the specified boundary conditions (a) the concentration of nitrobenzene in WW1, (b) the temperature of WW1, and (c) the temperature of SG1, m SG1 =x m WW1 a series of linear mathematical relationships of the type defining a range of nitrobenzene concentrations in WW2 is stored in a database, and the series of linear mathematical relationships is adjusted to a target value c NB,WW2,TARGET In addition to the mathematical relationship (0) corresponding to the target value c NB,WW2,TARGET c corresponds to 98% of NB,WW2 A first mathematical relationship (1) for a first value of the target value c NB,WW2,TARGET c, which corresponds to 102% of NB,WW2 and a second linear mathematical relationship (2) for a second value of the second value of the first value of the ... NB,WW2 ) control method. [Background technology]

[0002] In many manufacturing processes, aqueous wastewater streams contaminated with organic compounds are generated. Typically, such wastewater streams are first pre-cleaned in the plant where they are generated and then sent to a wastewater treatment plant before being released into the environment. Even the wastewater streams fed to the wastewater treatment plant must meet minimum requirements for purity, in order to prevent microorganisms used, particularly in biological wastewater treatment plants, from being damaged or even killed by excessively high concentrations of organic compounds that are toxic to them. Therefore, operators of wastewater treatment plants are setting increasingly stringent specifications for the purity of the inflowing wastewater, with the result that on-site pre-cleaning plays an ever more important role. Such on-site pre-cleaning is often performed by stripping the wastewater in a stripping tower (also known as a wastewater stripper). The present invention relates to the operation of such a stripping tower.

[0003] Patent document 1 describes a process for treating alkaline wastewater formed in the washing of crude nitrobenzene obtained by the nitration of benzene, which comprises (i) heating the alkaline wastewater in the absence of oxygen and under superatmospheric pressure to a temperature between 150° C. and 500° C., (ii) mixing the wastewater obtained in (i) with a base, and (iii) further purifying the wastewater obtained in (ii) by stripping with a stripping gas, followed by cooling the impurity-containing stripping gas stream to a temperature between 10° C. and 60° C., but the control of stripping is not detailed in this application.

[0004] US Pat. No. 5,399,633 describes a process for the continuous treatment of industrial wastewater, which may contain various impurities. The only specific example of a suitable industrial wastewater is wastewater from a refinery. The wastewater is introduced into the top of a stripping tower (8) through at least one feed conduit (7, 13) and flows downwards there. A steam stream is injected into the tower (8) at a height (14) such that the wastewater and the steam flow in countercurrent in said tower (8). The gases removed by stripping the wastewater with said steam are recovered at the top (15) of the tower and the treated water is discharged at the bottom (16) of the stripping tower (8). The process includes the feature of determining online at least a part of the ultraviolet spectrum of impurities, in particular the group formed by sulfides, ammonia and phenols. Measurements are carried out in the conduits (7, 13) for feeding the wastewater or in the conduit (16) for discharging the treated water of the tower (8) and, using mathematical processing of the measured intensities, at least one impurity product present in the recovered sample is determined. As a function of the results thus obtained and by comparison with predetermined settings, this information is expressed in the form of an electrical signal which controls the waste water and steam feed throughput in the stripping column. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2014 / 170309 [Patent Document 2] U.S. Patent Application No. 7,402,192 Summary of the Invention [Problem to be solved by the invention]

[0006] From an economic point of view, it is very important to operate such a stripping column in the most energy-saving manner possible while achieving the prescribed requirements for the content of organic matter in the purified wastewater, in particular organic matter that is toxic to microorganisms.It is a further object to configure the operation of the stripping column in a very convenient manner, i.e. in a manner that requires little or no manual intervention by the operating personnel, and ideally no complicated interventions, if any. [Means for solving the problem]

[0007] To achieve these objectives, stripping towers used in in-situ purification need to be efficiently controlled. Accordingly, the present invention provides:

[0008] Nitrobenzene (NB) at concentration c NB,WW1 A method for purifying a wastewater stream WW1 contaminated with nitrobenzene, comprising: (I) In a continuously operated stripping tower, a wastewater stream WW1 is stripped with a stripping gas SG1 to remove nitrobenzene to a concentration of c NB,WW2 (c NB,WW1 obtaining a wastewater stream WW2 containing less than 1000 mg of nitrobenzene and a stripping gas SG2 containing nitrobenzene; Here, the wastewater flow WW1 is defined as the flow rate m WW1 , temperature T WW1 and the stripping gas SG1 is fed to the stripping column at a flow rate m SG1 , temperature T SG1 The nitrobenzene concentration in the wastewater stream WW2 is c NB,WW2 at the measuring points continuously or at intervals (in particular, at least WW1 (when changes occur); (II) Purifying wastewater stream WW2 in a wastewater treatment plant to reduce nitrobenzene to a concentration of c NB,WW3 (c NB,WW2 obtaining a purified wastewater stream WW3 containing less than Here, the nitrobenzene concentration in the wastewater stream WW2 fed to the wastewater treatment plant is determined to be equal to or greater than a predetermined maximum value c NB,WW2,MAXThe nitrobenzene concentration in the wastewater stream WW2 must not exceed c NB,WW2 A target value c is specified that is greater than zero and within a target range defined by the following conditions: NB,WW2,TARGET is prescribed. 0.50·c NB,WW2,MAX ≦c NB,WW2,TARGET ≦0.95·c NB,WW2,MAX ; Including, The operation of the stripping tower (i) continuously feeding a stripping gas SG1 and a wastewater stream WW1 to a stripping column; Here, the prescribed boundary conditions (a)c NB,WW1 , (b) temperature T WW1 and (c) temperature T SG1 For at least one combination of m SG1 =x m WW1 each of the series of linear mathematical relationships has a certain concentration c NB,WW2 , the set of linear mathematical relationships corresponds to the concentration c NB,WW2 A set of linear mathematical relationships is stored in a database that defines a range of the target value c NB,WW2,TARGET In addition to the mathematical relationship (0) corresponding to the target value c NB,WW2,TARGET c corresponds to 98% of NB,WW2 A first linear mathematical relationship (1) for a first value of the target value c NB,WW2,TARGET c, which corresponds to 102% of NB,WW2 and a second linear mathematical relationship (2) for a second value of flow rate m SG1 At time t i Flow rate m occurring at WW1 (t i ) for the flow rate m SG1 But at time t i Flow rate m occurring at WW1 (t i ) is selected to be within a range of values ​​(AB) generated by the first mathematical relationship (1) and the second mathematical relationship (2) (flow rate m SG1 Flow rate m WW1 (ti ) to fit; (ii) The actual concentration of nitrobenzene in wastewater WW2 measured at the measurement point c NB,WW2,ACTUAL is 0.98·c NB,WW2,TARGET When the flow rate of stripping gas SG1 becomes SG1 and The actual concentration of nitrobenzene in wastewater WW2 measured at the measurement point c NB,WW2,ACTUAL is 1.02·c NB,WW2,TARGET When this is the case, the flow rate m of stripping gas SG1 SG1 By increasing the concentration c NB,WW2 and A method comprising:

[0009] Surprisingly, the purity achieved is “as high as possible” (i.e., c NB,WW2 As opposed to controlling the stripping column so that c is as low as possible, ideally below the detection limit, it is controlled so that c is greater than "zero" (i.e., in this context, above the detection limit). NB,WW2It has been found that the above-mentioned objective can be achieved, or at least approximately achieved, by deliberately allowing a value of up to 95% of the maximum permissible value (also greater than zero) specified by the operator of the wastewater treatment plant, controlling this value by continuous or interval measurements and, if necessary, varying the feed rate of stripping gas per unit time derived therefrom to the stripping column. From the point of view of environmental protection, what is ultimately important is that the wastewater discharged into the environment, i.e. the wastewater that normally leaves the wastewater plant, has the highest possible purity and at least meets the legal requirements applicable in the individual case. The question of where exactly this purity is achieved, whether in an on-site pre-purification or in the wastewater treatment plant itself, is generally of little or even minor importance in this context. However, in the context of the present invention, it has been found that from an economic point of view, it can be very important to divide in a specific way the degree of purification achieved overall over the various purification stages (observing, of course, applicable boundary conditions such as the above-mentioned requirements by the operator of the wastewater treatment plant). The method according to the invention takes the above observations into account by providing controls that allow this in an efficient manner.

[0010] Of course, the wastewater stream WW1 to be purified that is contaminated with "nitrobenzene" may also contain one or more further organic compounds, without departing from the scope of the invention. In the case of aqueous wastewater streams derived from nitrobenzene production processes (especially acidic and alkaline wastewaters, see below), it is advantageous to focus on the control of nitrobenzene, since nitrobenzene is often present in relatively high concentrations relative to other organic impurities such as benzene. [Brief description of the drawings]

[0011] [Figure 1] FIG. 1 is a schematic diagram of a stripping tower that can be employed in accordance with the present invention. [Diagram 2] 13 is a graph showing a schematic diagram of a change in mWW1 over a conceptual period t0 to t4 including an intermediate period t0 to t2 and an intermediate period t2 to t4. [Diagram 3] 13 is a graph showing a schematic diagram of the relationship between the flow rates of SG1 and WW1 and cNB, WW2, and TARGET over a period from t0 to t2. [Figure 4] 13 is a graph showing a schematic diagram of the relationship between the flow rates of SG1 and WW1 and cNB, WW2, and TARGET over a period from t2 to t4. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] First, an overview of various possible embodiments is given below.

[0013] In a first embodiment of the invention, which can be combined with all other embodiments, the stripping gas employed is steam and / or nitrogen.

[0014] In a second embodiment of the present invention, which can be combined with all other embodiments, NB,WW2,ACTUAL is determined by near infrared spectroscopy or gas chromatography.

[0015] In a third embodiment of the present invention, which is a specific configuration of the second embodiment, NB,WW2,ACTUAL is determined by near-infrared spectroscopy.

[0016] In a fourth embodiment of the present invention, which can be combined with all other embodiments, NB,WW2 The value of is determined independently at two or more (particularly two) measurement points, and the obtained values ​​are used to determine c NB,WW2,ACTUAL Form an average to use as the value of

[0017] In a fifth embodiment of the invention, which is a specific configuration of the sixth embodiment, if one or more, but not all, of the two or more measurement points fail, only at least one functioning measurement point is used, and the method includes at least one such failure during continuous stripping.

[0018] In a sixth embodiment of the present invention, which can be combined with all other embodiments, provided that it does not include at least one measuring point functioning as in the fifth embodiment, when all measuring points fail, the flow rate m of the stripping gas SG1 fed to the stripping column, which is determined from the measured values ​​of the measuring points before the failure, is reduced or increased according to the control of step (ii). SG1 Increase the last value of by 5%-10% and maintain it until at least one of the measurement points is functional again.

[0019] In a seventh embodiment of the present invention, which can be combined with all other embodiments, in step (i), a flow rate m SG1 But at time t i Flow rate m occurring at WW1 (t i ) and the target value c NB,WW2,TARGET and a second mathematical relationship (2) corresponding to the flow rate m SG1 At time t i Flow rate m occurring at WW1 (t i ) for (flow rate m SG1 Flow rate m WW1 (t i ) to fit.

[0020] In an eighth embodiment of the invention, which represents an alternative to the seventh embodiment, in step (i) a flow rate m SG1 But at time t i Flow rate m occurring at WW1 (t i ) and the target value c NB,WW2,TARGET and the first mathematical relationship (1) so that the flow rate m SG1 At time t i Flow rate m occurring at WW1 (t i ) for (flow rate m SG1 Flow rate m WW1 (t i ) to fit.

[0021] In a ninth embodiment of the invention, which represents a further alternative to the seventh embodiment, a series of linear mathematical relationships are NB,WW2,TARGET c corresponds to 99% of NB,WW2 a third linear mathematical relationship for a third value of the target value c NB,WW2,TARGET c corresponds to 101% of NB,WW2 and a fourth linear mathematical relationship for a fourth value of the flow rate m SG1 But at time t i Flow rate m occurring at WW1 (t i ) within a range of values ​​generated by the third mathematical relationship and the fourth mathematical relationship. SG1 At time t i Flow rate m occurring at WW1 (t i ) for (flow rate m SG1 Flow rate m WW1 (t i ) to fit.

[0022] In a tenth embodiment of the present invention, which can be combined with all other embodiments, the boundary condition (a)c NB,WW1 , (b) temperature T WW1 and (c) temperature T SG1 and storing a set of linear mathematical relationships for each of at least two combinations of at least temperature T SG1 are different for at least two combinations of boundary conditions.

[0023] In an eleventh embodiment of the present invention, which can be combined with all other embodiments, the wastewater stream to be purified is obtained by washing a nitrobenzene phase obtained by nitration of benzene with nitric acid in the presence of sulfuric acid and subsequent separation of the aqueous sulfuric acid phase.

[0024] In a twelfth embodiment of the invention, which is a particular configuration of the eleventh embodiment, the washing of the nitrobenzene phase comprises three stages, a first stage comprising washing with water, a second stage comprising washing with an aqueous base solution (in particular aqueous sodium hydroxide solution) and a third stage comprising washing with water, each of the three stages generating wastewaters, at least one of which is purified in a stripping column as wastewater stream WW1.

[0025] In a thirteenth embodiment of the invention, which is a particular configuration of the eleventh embodiment, wastewater employed as a constituent of the wash water employed in the second and / or first stage is generated in a third stage, and the wastewater generated in the first and / or second stage is purified in a stripping tower as wastewater stream WW1.

[0026] In a fourteenth embodiment of the present invention, which can be combined with all other embodiments, the maximum value c NB,WW2,MAX is in the range of 2.0 ppm to 10 ppm relative to the total mass of WW2.

[0027] In a fifteenth embodiment of the present invention, which can be combined with all other embodiments, NB,WW2 is measured continuously or at intervals of 6 hours or less, preferably 3 hours or less, and particularly preferably 1 hour or less.

[0028] The above outlined embodiments and further possible configurations of the invention are explained in more detail below. All embodiments and further configurations can be combined with each other as required, unless otherwise stated or clear from the context.

[0029] The actual stripping of the wastewater stream WW1 by the stripping gas SG1 in step (I) can in principle be carried out as known to the person skilled in the art. The stripping gas used is preferably steam and / or nitrogen, particularly preferably steam. As shown in FIG. 1, the wastewater to be purified (WW1 in the terminology of the present invention; flow rate m WW1 , nitrobenzene concentration c NB isNB,WW1 ) is fed in particular from the top of the column, and a suitable stripping gas (in the terminology of the present invention, SG1; at a flow rate m SG1 ) passes countercurrently from the bottom of the stripping column. Nitrobenzene-depleted water (WW2 in the present terminology; flow rate m WW2 , nitrobenzene concentration c NB,WW2 ) is withdrawn from the bottom of the column. A stripping gas stream enriched in nitrobenzene (SG2 in the present terminology) is obtained at the top of the column. This stripping gas stream SG2 is preferably condensed and sent for further use. The further use can be of a chemical nature, feeding the condensate obtained in the condensation to a chemical process, but it can also be combusted, since the condensate obtained has a significantly higher organic matter content than WW1, and the released heat can be employed as an energy carrier, for example for manufacturing processes.

[0030] concentration c NB,WW2 can in principle be determined by all methods known to the skilled artisan for the purpose of detecting nitrobenzene in aqueous solutions, in particular by near-infrared spectroscopy or gas chromatography, with near-infrared spectroscopy being preferred. Normally, all methods provide consistent results within the context of the precision required for the purposes of the present invention. In the unlikely event of significant discrepancies between the different measurement methods, the value determined by near-infrared spectroscopy is decisive for the purposes of the present invention. NB,WW2 The measurements are in particular carried out continuously or at intervals of not more than 6 hours, preferably not more than 3 hours, particularly preferably not more than 1 hour.

[0031] concentration c NB,WW2 To determine c, it is possible to establish two or more measurement points, in particular (strictly speaking) two measurement points. Usually, it is not necessary to use three or more measurement points, but it is of course possible to do so. It is possible to obtain measurements at two or more measurement points that are independent of each other, calculate the average from these measurements, and NB,WW2,ACTUALas the value of . This provides high accuracy and also allows the continued operation of the process even if one (but not all) of the measuring points fails as a result of a malfunction. In such a case, at least one measuring point that is still functioning can simply continue to be used. It goes without saying that if only one measuring point is actually left usable, averaging is no longer possible during the period of the malfunction.

[0032] Regardless of whether exactly one or more measurement points are available, if no measurement points are available temporarily due to a failure, the c at the measurement points before the failure NB,WW2 The flow rate m of the stripping gas SG1 supplied to the stripping column according to the control of step (ii), determined from the measured value SG1 It is preferable to increase the final value of by 5% to 10% and maintain it until at least one of the measurement points is functional again and able to do its job.

[0033] For a given stripping column, the flow rate of stripping gas SG1 required to maintain a desired purity of wastewater WW2 depends on various factors, in particular: 1. The magnitude of flow during WW1 2. Nitrobenzene concentration during WW1 3. Temperature in WW1 4. Temperature of stripping gas SG1

[0034] For a given process, factors 2 through 4 typically have at most small variations, so the main focus of control is on the magnitude of the WW1 flow rate. The change in flow rate is also called a load variation. A schematic diagram of such a load variation is shown in Figure 2, which shows a plot of the flow rate of wastewater WW1 against time t. Here, the flow rate of WW1 feeding the wastewater stripping tower (at time t 2 t) significantly increases 0 ~t 4 Consider the period of low load (t 0 ~t 2 ), at time t 1 Mark the high load period (t2 ~t 4 , in which case t 3 The same is true for time t 2 If the load is increased instantaneously at t, the high load period is strictly t 2 For simplicity, the high-load period begins just after t 2 ~t 4 " or "t 2 -t 4 " (hereinafter referred to as "the "

[0035] Against this background, in the context of the present invention, the prescribed boundary conditions (a)c NB,WW1 , (b) temperature T WW1 and (c) temperature T SG1 (b) store a set of linear mathematical relationships for at least one combination of T WW1 Combined with the value of T SG1 (a) c in combination with the value of NB,WW1 It means storing a set of linear mathematical relationships for at least one value of (a)c NB,WW1 , (b) T WW1 and (c) T SG1 If it becomes apparent in practical operation that the stripping column may be advantageously operated with two or more combinations of values ​​of WW1, then it is preferred to store a set of linear mathematical relationships for each of these combinations of values. This is particularly true when the temperature of the wastewater stream is variable, i.e., when the wastewater arrives at the feed inlet of the stripping column at a first temperature for a first period of time and at a second temperature different from the first temperature for a second period of time. In such a case, one simply employs a second set of linear mathematical relationships corresponding to the second temperature of WW1.

[0036] Therefore, under these boundary conditions, m SG1 =x m WW1 Each of the linear mathematical relationships of the type NB,WW2In other words, under these boundary conditions, the flow rate of stripping gas required for the purification of wastewater WW1 has a linear dependence on the amount of wastewater WW1 introduced, and the slope of the line corresponds to the linear mathematical relationship m SG1 =x m WW1 This corresponds to the coefficient x in NB,WW2 It is possible to establish a series of such linear mathematical relationships that correspond to a range of

[0037] FIG. 3 is a graph showing a typical dependence of the required flow rate of the stripping gas SG1 on the flow rate of the wastewater WW1 fed to the stripping tower. WW1 _MIN indicates the minimum wastewater flow rate at which the stripping column can still be operated profitably. Time t 1 (See Figure 2) is the flow rate of wastewater entering the stripping tower (m WW1 (t 1 )) is shown. 0 ~t 2 WW1 flow rate (m WW1 (t 0 -t 2 ) is constant, so m WW1 (t 1 )=m WW1 (t 0 -t 2 ).

[0038] The data required to create such a graph are derived from operating experience (selected boundary conditions (a) c NB,WW1 , (b) T WW1 and (und)(c)T SG1 ) and / or engineering calculations known to those skilled in the art. The dependence is linear, i.e., the function m SG1 (m WW1 ) is a line whose slope corresponds to the value x in the linear mathematical relationship. In accordance with the present invention, the set of linear mathematical relationships includes at least three lines, labeled (0), (1) and (2) as shown in FIG.

[0039] The line labeled (0) corresponds to the mWW1 At the value of m, the content of organic compounds in the wastewater WW2 leaving the stripping column corresponds to the desired target value. SG1 corresponds to the value of The line labeled (1) corresponds to the m WW1 At this value, the content of organic compounds in the wastewater WW2 leaving the stripping tower is 2% lower than the desired target value. SG1 corresponds to the value of The line labeled (2) corresponds to the m WW1 At a value of m, the content of organic compounds in the wastewater WW2 leaving the stripping tower is 2% higher than the desired target value. SG1 corresponds to the value of

[0040] Lines (1) and (2) define a frustum of a cone, which is indicated by hatching.

[0041] Thus, the information on which a graph such as that shown in Figure 3 is based is stored in a database as a series of linear mathematical relationships. The database itself is integrated with the process control system that operates the stripping tower.

[0042] Here, time t=t 1 The flow rate of the stripping gas SG1 is m SG1 (t 0 -t 2 )_MIN~m SG1 (t 0 -t 2 )_MAX. These two values ​​are the minimum (m SG1 (t 0 -t 2 )_MIN) and Max(m SG1 (t 0 -t 2 )_MAX) stripping gas flow rate, which allows the wastewater volume m WW1 (t 1 )=m WW1 (t 0 -t 2), the nitrobenzene concentration in the wastewater WW2 can be maintained within ±2% of the target value. 1 About m SG1 At time t 1 (In general terms, time t i ) defines the end points A and B of the range AB of values ​​that can be selected. This defines the flow rate m SG1 But at time t i Flow rate m occurring at WW1 (t i ) within a range of values ​​(AB) generated by the first mathematical relationship (1) and the second mathematical relationship (2). SG1 At time t i Flow rate m occurring at WW1 (t i ) for (flow rate m SG1 Flow rate m WW1 (t i This is what is meant by the requirement according to the invention that

[0043] If the introduction of stripping gas tends to be too high (i.e., in the upper region of the truncated cone in FIG. 3), the control according to step (ii) tends to include reducing the flow rate of SG1. WW1 (t i ) can be selected. However, if the amount of stripping gas introduced tends to be too small (i.e., in the lower region of the truncated cone in FIG. 3), the flow rate m WW1 (t i ) can be selected. Finally, when the amount of stripping gas introduced is in the range above and below the ideal straight line (0), i.e. in the central region of the truncated cone of FIG. 3, the flow rate m is adjusted so that the control according to step (ii) tends to involve reducing or increasing the flow rate of SG1 depending on the results of measurements taken continuously or at intervals. WW1 (t i ) can be selected.

[0044] From a convenience standpoint, the first two options are preferred.

[0045] It goes without saying that the boundary conditions associated with existing equipment must be observed. For example, in the context of the control according to step (ii), the amount of stripping gas SG1 cannot usually be varied to any desired extent, but only within a certain defined range, for example by not more than 20%. This is because the flow rate m WW1 (t i ) should be taken into account when selecting the flow rate m WW1 (t i ) should be selected, for example, when using the second of the above mentioned options, in the example given above of limiting the variability of the stripping gas flow rate to 20%, so that in step (i) the flow rate m WW1 (t i ) must be selected.

[0046] Figure 4 shows the high load period (t 2 ~t 4 ) at time t 3 4 is a graph similar to that of FIG. 3 The corresponding wastewater flow rate is m WW1 (t 3 ) as t 2 ~t 4 Since the flow rate of wastewater WW1 is constant during the period, it can be inferred from Figure 3 that WW1 (t 3 )=m WW1 (t 2 -t 4 ) is applied. In this figure, m WW1 (t 3 The minimum value (m for the stripping gas flow rate (range AB) corresponding to the value SG1 (t 2 -t 4 )_MIN) and maximum value (mSG1 (t 2 -t 4 )_MAX) exists.

[0047] The process according to the invention is suitable for the purification of aqueous wastewater streams resulting 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 stoichiometric excess over nitric acid, especially in the adiabatically operated nitration processes customary today, and must therefore be removed in the course of work-up of the nitration product. Work-up of the nitration product is typically achieved by separating the reaction mixture (nitration product) present after the nitration, which comprises nitrobenzene, unconverted benzene and sulfuric acid, into an aqueous sulfuric acid phase and an organic nitrobenzene phase in a first step, followed by a second step of washing the nitrobenzene phase in one or more stages, followed by a third step of separating the excess benzene from the washed nitrobenzene phase. The excess benzene is typically recycled to the nitration.

[0048] The second washing step preferably comprises three stages, the so-called acid wash, the so-called alkaline wash and the so-called neutral wash. In the first stage, the nitrobenzene phase obtained in the phase separation is washed with water to wash away as much entrained or dissolved acid residues as possible. In the second stage, the organic wash product from the first stage obtained after phase separation is washed with an aqueous base solution, in particular with sodium hydroxide solution, to neutralize any acid residues remaining in the nitrobenzene. In the third stage, the organic wash product from the second stage obtained after phase separation is finally washed with water. In each stage, from the phase separation, not only the respective organic wash product is generated, but also waste water, which can be purified according to the invention as waste water stream WW1.

[0049] The washing is preferably carried out in countercurrent so that the wastewater obtained in the third stage is used as a constituent of the washing liquid in the second and / or first stage washing. The process according to the invention is particularly suitable for the purification of wastewater obtained in the first and / or second stage.

[0050] As already mentioned above, the purification method according to the invention is used for prepurifying a wastewater stream WW1, so that the resulting wastewater stream WW2 can be fed to a (particularly biological) wastewater treatment plant, while complying with the purity requirements laid down by the operator of the plant for the wastewater stream WW2. The invention has the advantage that it makes it possible to control the stripping column in such a way that the content of nitrobenzene in the wastewater WW2 is not necessarily as low as possible, but is relatively constant and always well below the specification limits laid down by the operator of the wastewater treatment plant. The maximum value c NB,WW2,MAX is, for example, in the range of 2.0 ppm to 10 ppm with respect to the total mass of WW2.

[0051] The present invention as described above has the characteristic of optimizing steam consumption, and thus improves convenience.

Claims

1. Nitrobenzene at concentration c ＮＢ，ＷＷ１ 1. A method for purifying a wastewater stream WW1 contaminated with nitrobenzene, comprising: (I) In a continuously operated stripping tower, the wastewater stream WW1 is stripped with a stripping gas SG1 to remove nitrobenzene to a concentration of c ＮＢ，ＷＷ２ <c ＮＢ，ＷＷ１ and a stripping gas SG2 containing nitrobenzene; Here, the wastewater flow WW1 is defined as a flow rate m ＷＷ１ , temperature T ＷＷ１ and the stripping gas SG1 is supplied to the stripping tower at a flow rate m ＳＧ１ , temperature T ＳＧ１ to the stripping tower, and the nitrobenzene concentration in the wastewater stream WW2 is c ＮＢ，ＷＷ２ is measured continuously or at intervals at the measurement points; (II) purifying said wastewater stream WW2 in a wastewater treatment plant to reduce nitrobenzene to a concentration of c ＮＢ，ＷＷ３ <c ＮＢ，ＷＷ２ obtaining a purified wastewater stream WW3 containing Here, the nitrobenzene concentration in the wastewater stream WW2 supplied to the wastewater treatment plant is a predetermined maximum value c ＮＢ，ＷＷ２，ＭＡＸ The nitrobenzene concentration in the wastewater stream WW2 must not exceed ＮＢ，ＷＷ２ , a target value c that is greater than zero and is defined to be within a target range defined by the following conditions: ＮＢ，ＷＷ２，ＴＡＲＧＥＴ is prescribed. 0.50・c ＮＢ，ＷＷ２，ＭＡＸ ≦c ＮＢ，ＷＷ２，ＴＡＲＧＥＴ ≦0.95・c ＮＢ，ＷＷ２，ＭＡＸ ; Including, The operation of the stripping tower comprises: (i) continuously feeding the stripping gas SG1 and the wastewater stream WW1 to the stripping tower; Here, the boundary conditions (a)c ＮＢ，ＷＷ１ , (b) the temperature T ＷＷ１ and (c) the temperature T ＳＧ１ For at least one combination of ＳＧ１ = x m ＷＷ１ Each of the series of linear mathematical relationships has a certain concentration c ＮＢ，ＷＷ２ , the set of linear mathematical relationships corresponds to the concentration c ＮＢ，ＷＷ２ A series of linear mathematical relationships is stored in a database, the series of linear mathematical relationships defining a range of the target value c ＮＢ，ＷＷ２，ＴＡＲＧＥＴ In addition to the mathematical relationship (0) corresponding to the target value c ＮＢ，ＷＷ２，ＴＡＲＧＥＴ c corresponds to 98% of ＮＢ，ＷＷ２ a first linear mathematical relationship (1) for a first value of the target value c ＮＢ，ＷＷ２，ＴＡＲＧＥＴ c, which corresponds to 102% of ＮＢ，ＷＷ２ and a second linear mathematical relationship (2) for a second value of The flow rate m ＳＧ１ At time t ｉ Flow rate m generated ＷＷ１ (t ｉ ) for the flow rate m ＳＧ１ However, the time t ｉ The flow rate m ＷＷ１ (t ｉ ) to be within a range of values ​​(AB) generated by said first mathematical relationship (1) and said second mathematical relationship (2); (ii) The actual concentration c of nitrobenzene in the wastewater WW2 measured at the measurement point ＮＢ，ＷＷ２，ＡＣＴＵＡＬ is 0.98・c ＮＢ，ＷＷ２，ＴＡＲＧＥＴ When the flow rate m of the stripping gas SG1 becomes equal to or less than the above, ＳＧ１ and The actual concentration c of nitrobenzene in the wastewater WW2 measured at the measurement point ＮＢ，ＷＷ２，ＡＣＴＵＡＬ is 1.02・c ＮＢ，ＷＷ２，ＴＡＲＧＥＴ When the flow rate m of the stripping gas SG1 becomes equal to or less than the above, ＳＧ１ By increasing the concentration c ＮＢ，ＷＷ２ and A method comprising:

2. When all the measuring points fail, the flow rate m of the stripping gas SG1 supplied to the stripping column, which is reduced or increased according to the control in step (ii), determined from the measured values ​​of the measuring points before the failure. ＳＧ１ 2. The method of claim 1, wherein the final value of is increased by 5% to 10% and maintained until at least one of the measurement points is functional again.

3. In step (i), the flow rate m ＳＧ１ The time t ｉ The flow rate m ＷＷ１ (t ｉ ) for the flow rate m ＳＧ１ However, the time t ｉ The flow rate m ＷＷ１ (t ｉ ) and the target value c ＮＢ，ＷＷ２，ＴＡＲＧＥＴ 3. The method of claim 1, wherein the first mathematical relationship (0) corresponds to:

4. In step (i), the flow rate m ＳＧ１ The time t ｉ The flow rate m ＷＷ１ (t ｉ ) for the flow rate m ＳＧ１ However, the time t ｉ The flow rate m ＷＷ１ (t ｉ ) and the target value c ＮＢ，ＷＷ２，ＴＡＲＧＥＴ 3. The method of claim 1, wherein the first mathematical relationship (1) is selected to be within a range of values ​​generated by the mathematical relationship (0) corresponding to:

5. The set of linear mathematical relationships is ＮＢ，ＷＷ２，ＴＡＲＧＥＴ c corresponds to 99% of ＮＢ，ＷＷ２ a third linear mathematical relationship for a third value of said target value c ＮＢ，ＷＷ２，ＴＡＲＧＥＴ c, which corresponds to 101% of ＮＢ，ＷＷ２ and a fourth linear mathematical relationship for a fourth value of the flow rate m ＳＧ１ The time t ｉ The flow rate m ＷＷ１ (t ｉ ) for the flow rate m ＳＧ１ However, the time t ｉ The flow rate m ＷＷ１ (t ｉ 3. The method of claim 1, wherein the first mathematical relationship is selected to be within a range of values ​​generated by the third mathematical relationship and the fourth mathematical relationship.

6. Boundary conditions (a)c ＮＢ，ＷＷ１ , (b) the temperature T ＷＷ１ and (c) the temperature T ＳＧ１ and storing a series of linear mathematical relationships for each of at least two combinations of the temperature T ＳＧ１ The method according to any one of claims 1 to 5, wherein x, y, y, and z are different for at least two combinations of the boundary conditions.

7. The maximum value c ＮＢ，ＷＷ２，ＭＡＸ The method according to any one of claims 1 to 6, wherein the concentration of WW2 is in the range of 2.0 ppm to 10 ppm, based on the total mass of the wastewater stream WW2.

8. c ＮＢ，ＷＷ２ The method according to any one of claims 1 to 7, wherein the measurement is performed continuously or at intervals of not more than 6 hours.

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