Abstract:
A process for monitoring and/or controlling a nitrating process, having the following steps:  
     measuring inline infrared spectra of nitric acid content in a reaction mixture stream downstream of the nitration reaction, preferably near-infrared spectra,  
     evaluating the measured spectra by means of a computer-assisted, matrix-specific calibration model for the purpose of determining the content of nitric acid,  
     transmitting the results of spectrometric examination to a process control system,  
     inputting the results of spectrometric examination for the purpose of specifying the content of nitric acid in the acid phase into a regulator ( 224 ) for control of the metering ( 207, 217 ) of nitric acid into a nitrating reactor.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to a process for monitoring and for controlling a nitrating process, in particular for the nitration of toluene, with the aid of a computer-assisted, matrix-specific calibration model and a process model.  
           [0002]    Various nitration processes are known in the prior art. For example, toluene is nitrated with nitric acid to yield nitrotoluidines by way of intermediate dye products. It is also known to nitrate toluene with mixed acids to yield nitrotoluene and dinitrotoluene. Dinitrotoluene is, for example, processed further to yield diamines, diisocyanates, trinitrotoluene or phloroglucinol.  
           [0003]    For economic reasons, the aim in the course of nitration is to conduct the nitration with as small an excess of acid as possible. To this end, it is known from the state of the art to take samples from the process manually and to examine them analytically in the laboratory. The process is then readjusted manually when required.  
           [0004]    One disadvantage of such manual sampling and adjustment is the high cost of labor for the sampling and for the laboratory analysis. Another disadvantage is that the effort increases linearly with the number of measuring-points. Furthermore, manual sampling is problematic from the point of view of industrial safety, since, particularly in the case of 2-nitrotoluene, it is a question of working with a substance that is detrimental to health. Therefore in the course of handling 2-nitrotoluene, the wearing of respiratory protection at all times as a precaution is prescribed.  
           [0005]    Another disadvantage of manual sampling with subsequent laboratory analysis is the fact that readjustment of the process can only be effected irregularly and after relatively long time-intervals. This may result in use of a relatively large excess of acid; the plant cannot then be operated in optimal manner, either technically or economically.  
         SUMMARY OF THE INVENTION  
         [0006]    An object of the present invention is therefore to create an improved process for monitoring and controlling nitrating processes that enables a diminution of the excess of acid. Further objects underlying the invention are to create an online method of measurement with a computer-assisted process model and to create an appropriate production process for the nitration.  
           [0007]    The objects of the present invention are achieved by online spectrometric measurement of the acid phase from the reaction mixture and transmission of that data to a process control system. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    [0008]FIG. 1 is a flow chart for the process of the present invention for improved monitoring and control of nitrating processes by virtue of the online measurement of the acid phase and the improved metering of nitric acid.  
         [0009]    [0009]FIG. 2 is a block diagram of a two-stage nitration process according to the invention.  
         [0010]    [0010]FIG. 3 shows various NIR spectra for various concentrations of nitric acid.  
         [0011]    [0011]FIG. 4 is a flow chart for the process of creating the matrix-specific calibration model, the validation and enhancement thereof.  
         [0012]    [0012]FIG. 5 is a schematic representation of a bypass with a measuring cell. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0013]    In accordance with the present invention, the acid phase recovered from the nitration reaction mixture is spectrometrically examined online. This is preferably done by infrared spectrometry. Such measurements are also designated as infrared spectroscopy. Appropriate IR spectrometers, in particular for the near-infrared range (NIR) are commercially available, for example from Polytec GmbH and other manufacturers.  
         [0014]    In a preferred embodiment of the invention, the content of nitric acid in the acid phase is determined online after nitration by means of an NIR spectrometer and a suitable computer-assisted calibration model. Data for quantifying the content of nitric acid are then transmitted from the NIR spectrometer to the process control system, for example via a field bus. On the basis of the nitric-acid content in the acid phase that is determined after nitration, regulation by the process control system for the supply of nitric acid is possible. The online control of a production plant for the purpose of regulating various polymerization parameters is disclosed in U.S. Pat. No. 5,121,337 and EP-0 948 761 B1 [sic]. In these disclosed processes, a predictive model created on the basis of measured spectra is used.  
         [0015]    In a particularly preferred embodiment of the invention, the measured NIR spectrum is evaluated with the aid of a matrix-specific calibration model. The physical matrix is predetermined by the nitrating process and is dependent on the parameters of the process. With the aid of chemometric methods, the measured spectra are referenced against results obtained from laboratory examinations.  
         [0016]    This is effected in such a way that the same sample for which an NIR spectrum was determined online is also analyzed in the laboratory with the aid of titration measurements. By virtue of the examination and the comparison of a suitable number of varying samples, it is possible to create a matrix-specific calibration model with the aid of chemometry. This matrix-specific calibration model is stored on a computer that is programmed to control the recording of the spectrum and to evaluate the measured spectrum online with the aid of the calibration model, so that the nitric acid content is available to the process control system online.  
         [0017]    The nitration of toluene to yield dinitrotoluene (DNT) is generally conducted in two stages. The spectrometric examination of the acid phase is undertaken at least after the second nitrating stage, in order to readjust the supply of nitric acid to the first and/or the second nitrating stage.  
         [0018]    In another preferred embodiment of the invention, the NIR spectrometer is connected to several measuring-points. The NIR spectrometer is multiplexed, in order to carry out spectrometric measurements in succession at the various measuring-points. By reason of this multiplex operation of the NIR spectrometer, the measurement effort increases degressively with the number of measuring-points.  
         [0019]    The invention is particularly advantageous because it enables distinctly improved process control. In particular, the invention enables the production plant to be operated continuously, close to the technical and economic optimum. Another advantage is the improvement in industrial safety.  
         [0020]    Preferred embodiments of the invention will be elucidated in greater detail below with reference to the drawings.  
         [0021]    [0021]FIG. 1 is a flow chart for a process according to the invention for monitoring and controlling nitrating processes. In step  100 , nitric acid (HNO 3 ) is supplied continuously to a nitrating stage. The product of the nitrating stage is a two-phase system composed of nitrated organic phase and acid phase. An NIR spectrum of the acid phase is recorded by means of a suitable measuring cell and an NIR spectrometer. This is undertaken in step  102 . In step  104 , the HNO 3  content in the acid phase is determined by evaluation of the NIR spectrum by means of a matrix-specific calibration model. This measurement is preferably undertaken inline or online, i.e. in the current product stream.  
         [0022]    In step  106 , the HNO 3  content that has been ascertained is transmitted to a process control system. In step  108 , the quantity of continuously supplied HNO 3  is readjusted manually or by the process control system, in order to reduce the HNO 3  content in the acid phase if necessary.  
         [0023]    [0023]FIG. 2 is a block diagram of an embodiment of an appropriate plant. The plant has a reactor  200  for the purpose of realizing a first nitrating stage (mononitration MNT). The feed materials are toluene  201 , sulfuric acid  202  and nitric acid  203 . The product of the first nitrating stage is a two-phase system which is separated, in the separator  208  connected downstream, into the organic phase  211  and the acid phase  209 .  
         [0024]    A measuring-point  205  for recording an NIR spectrum of the acid phase may be provided downstream of the reactor  200 . To this end, an NIR spectrometer  220  may be connected to the measuring-point  205  via an optical waveguide  206 .  
         [0025]    The separator  208  is followed by a further reactor  210  for the purpose of realizing the second nitrating stage (dinitration DNT). The feed materials are MNT  211 , sulfuric acid  212  and nitric acid  213 . The product of the second nitrating stage is a two-phase system which is separated, in the separator  218  connected downstream, into the organic phase  230  and the acid phase  219 .  
         [0026]    A measuring-point  215  is preferably arranged downstream of the output of the reactor  210 . The NIR spectrometer  220  is connected to the measuring-point  215  via an optical waveguide  216 . As a result, NIR spectra for the acid phase can be recorded.  
         [0027]    The measuring-points  205  and  215  may each be operated with their own spectrometer; however, they are preferably operated with a single spectrometer  220  which switches between the measuring-points  205  and  215 .  
         [0028]    The NIR spectrometer  220  passes on the measured NR spectra for evaluation by means of the matrix-specific computer-assisted calibration model  222 . The computer with the matrix-specific calibration model  222  passes on its results for the content of nitric acid to the process control system  224 . The subsequent regulation (manual or automated) of the metering  207  or  217  of the first  200  and/or the second  210  nitrating stage, respectively, permits improved monitoring of the process and improved process control for the content of HNO 3  in the acid phase within the range 0-5%, in particular close to 0%, preferably within the range from 0% to 0.3%.  
         [0029]    In a preferred embodiment of the invention, the HNO 3  content is determined only at measuring-point  215  and not at measuring-point  205 . The measurement of the HNO 3  content at measuring-point  215  after the second nitrating stage is generally sufficient for regulation of the production of dinitrotoluene.  
         [0030]    In the case where production of dinitrotoluene is undertaken in multiple lines, several parallel measuring-points  215  may be provided. All the measuring-points  215  are then preferably connected to the same NIR spectrometer  220 , which operates in multiplex mode. The NIR spectrometer  220  accordingly measures the spectra at the measuring-points  215  in succession, in cyclic sequence. By virtue of the multiplexing of the NIR spectrometer  220 , it is possible for the instrumentation effort for implementation of the HNO 3  measurements to be optimized.  
         [0031]    [0031]FIG. 3 shows the spectra  300 ,  302  and  304 . Spectrum  300  has been recorded for 75% sulfuric acid without nitric-acid content. Spectrum  302  has been recorded for 75% sulfuric acid with 1% nitric-acid content. Spectrum  304  has been recorded for 75% sulfuric acid with 5% nitric-acid content.  
         [0032]    The measured NIR spectra  300 ,  302  and  304  accordingly differ distinctly, depending on the percentage content of nitric acid in the acid phase. In corresponding manner it is possible for the nitric-acid content in the acid phase to be determined by measurement of the NIR spectrum. To this end, a matrix-specific calibration model based on comparative titration measurements is preferably used.  
         [0033]    [0033]FIG. 4 illustrates the procedure for obtaining a database for the generation of a matrix-specific calibration model. Step  400  illustrates the creation of such a calibration model; step  420  illustrates the validation of this model.  
         [0034]    Step  402  represents the physical matrix, which is process-specific and dependent on the process parameters with regard to its special composition.  
         [0035]    The nitric-acid content, which is ascertained by means of manual sampling  406  with subsequent titration  408 , is used for creation of the calibration model. Sampling and titration may also be undertaken in automated manner and online, or manually and offline. In parallel, a measuring cell, with which the NIR spectra pertaining to the samples can be recorded, is installed in the process flow. This is undertaken in step  404 .  
         [0036]    In step  410 , the results from the titration determinations are compared and are correlated with the respective NIR spectra with the aid of chemometric methods.  
         [0037]    In step  412 , all of the comparisons between all of the NIR spectra and all of the titration results are combined and are correlated in a model. The parameters of the model are adapted and adjusted in such a way that the content of nitric acid for the existing substance system and the existing process parameters are reproduced optimally. Once the model has been adapted and optimized, the matrix-specific calibration model is available at the end of step  412 .  
         [0038]    Subsequently, in step  420 , validation of the model is undertaken in respect of the current process. Whenever titration results are available in a manner temporally appropriate to the spectra arising from the process, said results can be integrated in accordance with step  400  for the purpose of successive enhancement of the model (step  422 ).  
         [0039]    [0039]FIG. 5 shows an embodiment of the invention with a measuring-point (for example, measuring-point  215  of FIG. 2). The product stream of the current production flows through the line  500 . A bypass  502  is located on the line  500 . The bypass  502  has a measuring cell  504 . Located upstream and downstream of the measuring cell  504  in the direction of flow is a shut-off device  506  and  508 , respectively. The shut-off devices enable access to the measuring cell while the product stream is running.  
         [0040]    List of Reference Symbols  
         [0041]    reactor  200   
         [0042]    toluene  201   
         [0043]    sulfuric acid  202   
         [0044]    nitric acid  203   
         [0045]    measuring-point  205   
         [0046]    optical waveguide  206   
         [0047]    regulation of metering  207   
         [0048]    separator  208   
         [0049]    acid phase  209   
         [0050]    reactor  210   
         [0051]    mononitrotoluene (MNT)  211   
         [0052]    sulfuric acid  212   
         [0053]    nitric acid  213   
         [0054]    measuring-point  215   
         [0055]    optical waveguide  216   
         [0056]    regulation of metering  217   
         [0057]    separator  218   
         [0058]    acid phase  219   
         [0059]    NIR spectrometer  220   
         [0060]    calibration model  222   
         [0061]    process control system  224   
         [0062]    organic phase  230   
         [0063]    spectrum  300   
         [0064]    spectrum  302   
         [0065]    spectrum  304   
         [0066]    line  500   
         [0067]    bypass  502   
         [0068]    measuring cell  504   
         [0069]    shut-off device  506   
         [0070]    shut-off device  508   
         [0071]    Although the invention has been described in detail in the foregoing for the purpose of illustration, it is to be understood that such detail is solely for that purpose and that variations can be made therein by those skilled in the art without departing from the spirit and scope of the invention except as it may be limited by the claims.