The present invention relates to a process for preparing at least one aromatic isocyanate comprising a specific starting-up, and to a chemical unit U for carrying out said process.

Aromatic isocyanates are often prepared via a continuous or discontinuous process in suitable plant sections or plants. Since such a process comprises several sub-processes, a plant section or plant comprises one or more chemical processing sub-units for processing the subprocesses. It is also possible to carry out a sub-process in parallel in several chemical processing sub-units. Considering that preparing an aromatic isocyanate usually includes nitration, hydrogenation and phosgenation of an aromatic compound, as for example in the preparation of toluene diisocyanate starting from toluene, a process for preparing at least one aromatic isocyanate includes several sub-processes wherein a sub-process is carried out in a chemical processing sub-unit. The product compositions obtained from the chemical processing units are then fed to further processing in subsequent chemical processing sub-units until a desired product composition is obtained.

During a regular operating mode of a sub-unit, the sub-process being carried out in said subunit thus comprises feeding an educt composition comprising at least one aromatic compound into said sub-unit and processing said composition in said sub-unit for obtaining a product composition comprising at least one aromatic compound, such that eventually at least one aromatic isocyanate is obtained from the latest chemical processing sub-unit. As used herein, an educt composition is equivalent to a reactant composition. Likewise the term educt, also spelled Edukt in the German language means reactant as defined by Baerns, M. et al in “Technische Chernik, Wiley, 2014, page 697. Therefore, as defined in the present invention, an educt composition is equivalent to a reactant composition.

In order to ensure a high performance of the one or more chemical processing units and chemical processing sub-units of a plant section or plant, regular maintenance, cleaning and optionally repairing of said chemical processing units or chemical processing sub-units is required.

For doing so, plant sections or plants comprising one or more chemical processing units and chemical processing sub-units have to be shut down in parts or completely. Parts of a plant section or plant not affected by maintenance, cleaning or repairing can stay in steady state, in particular if a return of a stream comprising a product composition or another circulating stream can be ensured. For instance, WO 2015/197522 A1 discloses a method in which plant sections not affected by maintenance, cleaning or repairing are operated in a so-called recirculation mode. It is disclosed therein that the product of such a plant section or of a further downstream plant section is fed into this plant section to maintain a circulating flow. In WO 2017/050776 A1 a similar method is disclosed in which plant sections not affected by maintenance, cleaning or repairing are operated under a circulating flow.

On the other hand, those parts of a plant section or plant which have been affected by maintenance have to be re-started. However, starting-up of a process carried out in one or more chemical processing units and chemical processing sub-units is an expensive procedure, in particular considering resource efficiency and energy costs. Resource efficiency relates to the influence of process parameters different to target process parameters on process performance. If for example an educt composition differs from a target educt composition having an optimized composition for further processing, an inferior conversion may be expected making recycling or wasting of excess starting material necessary. Energy costs include costs for heating or cooling of chemical processing units, chemical processing sub-units, separation devices, and other processing devices. Additionally, starting-up of a process can be time consuming. In the context of the present invention starting-up of a process is terminated in particular when the product composition meets the desired specifications, inter alia allowing further processing thereof in a subsequent process or process step.

Therefore, it was an object of the present invention to provide a novel process for preparing at least one aromatic isocyanate comprising a specific starting-up process. In particular, it was an object of the present invention to provide an improved process for preparing at least one aromatic isocyanate comprising a specific starting-up concerning resource efficiency.

Therefore, the present invention relates to a process for preparing at least one aromatic isocyanate, said process comprising n chemical sub-processes sp(i), wherein a sub-process sp(i) is carried out in a chemical processing sub-unit U(i), i=1 ...n, n>1 , wherein during a regular operating mode of a sub-unit U(i), the sub-process sp(i) being carried out in said sub-unit U(i) comprises feeding an educt composition E(i) comprising at least one aromatic compound EO), j^1 , into said sub-unit U(i), processing said composition E(i) in said sub-unit U(i) and obtaining a product composition P(i) comprising at least one aromatic compound Z ^k p(i), k>1 , wherein the chemical process comprises starting up said sub-process sp(i) in said sub-unit U(i), wherein said starting-up comprises processing a chemical start-up composition S(i) in said subunit U(i), wherein said S(i) * E(i) and wherein said S(i) comprises at least one of the aromatic compounds Z ^k p(i).

As mentioned above plant sections or plants comprising one or more chemical processing units and chemical processing sub-units have to be shut down in parts or completely if maintenance, cleaning or repairing needs to be carried out. Thus, it is preferred that the process according to the present invention further comprises, prior to starting up the sub-process sp(i) in the sub-unit U(i), shutting down the sub-process sp(i) in the sub-unit U(i) and keeping the sub-unit U(i) in a non-operation mode for a period of time At(i).

For starting up the sub-process sp(i) of the process according to the present invention, it is preferred that said starting up comprises feeding the chemical composition S(i) into the sub-unit U(i).

As defined above, the process according to the present invention comprises n chemical subprocesses sp(i), wherein a sub-process sp(i) is carried out in a chemical processing sub-unit U(i), i=1…n, n≥1. In the case where n=1 the process comprises only one sub-process being carried out in a single sub-unit which can be considered as a chemical unit U. In the case where n≥2, at least 2 sub-processes are comprised by the process of the present invention each being carried out in a chemical processing sub-unit. If n≥2, thus it n is at least 2, it is preferred that all sub-units U(i) are part of a chemical unit U. In the case where all sub-units U(i) are part of a chemical unit U, it is preferred that U further comprises, in addition to the n chemical processing sub-units U(i), r storing devices D(m), m=1…r, r≥1. If the chemical unit U further comprises, in addition to the n chemical processing sub-units U(i), r storing devices D(m), m=1…r, r≥1, it is preferred that the process according to the present invention further comprises, during a regular operating mode of a sub-unit U(i), re- moving the product composition P(i) from said sub-unit U(i) and transferring a portion of said product composition P(i) removed from said sub-unit U(i) into a storing device D(m). In the case where the product composition P(i) is removed from a sub-unit U(i) during a regular operating mode of said sub-unit U(i) and where a portion of said product composition P(i) re- moved from said sub-unit U(i) is transferred into a storing device D(m), it is preferred that, dur- ing starting up the sub-process sp(i), at least a portion of P(i), stored in D(m) during the regular operating mode of the sub-unit U(i), is fed as at least a portion of the chemical composition S(i) into the sub-unit U(i). According to the present invention a chemical sub-process sp(i) is carried out in a chemical pro- cessing sub-unit U(i). Thus, a chemical processing sub-unit U(i) can be any conceivable means suitable for carrying out a chemical sub-process sp(i). It is preferred that the sub-unit U(i) is se- lected from the group consisting of a chemical reactor, a distillation column, a rectification col- umn, a tank, a phase separator, a washing device, and a combination of two or more thereof. According to the present invention the process for preparing at least one aromatic isocyanate comprises n chemical sub-processes sp(i). It is preferred that at least one sub-process sp(i) comprises a nitration, a hydrogenation, a phosgenation, washing, a phase separation, or a dis- tillation. In the case where a sub-process sp(i) comprises a nitration, this can be indicated by an index N. Thus, a sub-process comprising a nitration can be designated as spN(i). Similarly, a sub- process comprising a hydrogenation can be designated as sp _H (i), a sub-process comprising a phosgenation as sp _P (i), a sub-process comprising a washing as sp _W (i), a sub-process compris- ing a separation as spS(i), and a sub-process comprising a distillation as spD(i). The same ap- plies to a sub-unit U(i), an educt composition E(i), a storing device D(m), a product composition P(i) or a chemical start-up composition relating to a sub-process as defined above. It is preferred that at least one sub-process sp(i) comprises a nitration and at least one further sub-process sp(i) comprises a hydrogenation. Also, it is preferred that at least one further sub- process sp(i) comprises a phosgenation. Thus, it is particularly preferred that at least one sub- process sp(i) comprises a nitration, that at least one further sub-process sp(i) comprises a hy- drogenation, and that at least one further sub-process sp(i) comprises a phosgenation. It is preferred that the at least one aromatic isocyanate comprises an aromatic diisocyanate, preferably a toluene diisocyanate, more preferably one or more of 2,4-toluene diisocyanate and 2,6-toluene diisocyanate, or a methylene diphenyl diisocyanate, preferably one or more of methylene diphenyl 2,2’-diisocyanate, methylene diphenyl 2,4’-diisocyanate, and methylene diphenyl 4,4’-diisocyanate. It is particularly preferred that at least one sub-process sp(i) comprises a nitration, a hydrogena- tion, or a phosgenation, more preferably at least one sub-process sp(i) comprises a nitration, at least one further sub-process sp(i) comprises a hydrogenation, and at least one further sub- process sp(i) comprises a phosgenation, wherein the at least one aromatic isocyanate compris- es an aromatic diisocyanate, preferably a toluene diisocyanate, more preferably one or more of 2,4-toluene diisocyanate and 2,6-toluene diisocyanate, or a methylene diphenyl diisocyanate, preferably one or more of methylene diphenyl 2,2’-diisocyanate, methylene diphenyl 2,4’- diisocyanate, and methylene diphenyl 4,4’-diisocyanate. Further, it is particularly preferred that the at least one aromatic isocyanate comprises, preferably is, one or more of 2,4-toluene diiso- cyanate and 2,6-toluene diisocyanate. In accordance with the above, it is preferred that one or more sub-processes sp(i) comprise a nitration, sp _N (i). In accordance with the above, a sub-unit U(i) in which a sub-process sp _N (i) is carried out is also designated herein as UN(i), an educt composition relating to a sub-process sp _N (i) is designated herein as E _N (i), and a product composition P(i) obtained from a sub-unit U _N (i) is designated herein as P _N (i). Thus, it is preferred that one or more sub-processes sp(i) comprise a nitration, sp _N (i), wherein each sub-unit U _N (i) in which a sub-process sp _N (i) is carried out comprises a nitration reactor, wherein an educt composition EN(i), being fed into a sub-unit U _N (i), comprises at least one nitrating agent, preferably nitric acid, and preferably further com- prises at least one water scavenger, preferably sulfuric acid. More preferably, the process according to the present invention comprises two or more sub- processes sp _N (i) which are carried out in sequence, wherein at last a portion of the product composition P _N (i) obtained from a sub-unit U _N (i) is fed, optionally after an intermediate treatment sub-process, as educt composition EN(i+1) into the sub-unit UN(i+1) downstream of UN(i) during a regular operating mode of U _N (i+1), wherein the educt composition E _N (1) being fed into the first, most upstream sub-unit U _N (1) comprises toluene as Z ¹ _E,N (1), wherein more preferably, j=1. Further, it is preferred that during a regular operating mode of a sub-unit U _N (i), a portion of P _N (i) is stored in at least one storing device D _N (m). As defined hereinabove, r storing devices D(m), m=1…r, r≥1, can be comprised in the chemical unit U in addition to the n chemical processing sub-units U(i). Thus, it is particularly preferred that that the process according to the present invention further comprises, during a regular operating mode of a sub-unit U _N (i), removing the product composition P _N (i) from said sub-unit U _N (i) and transferring a portion of said product composition PN(i) removed from said sub-unit UN(i) into a storing device DN(m). In the case where the process according to the present invention further comprises, during a regular operating mode of a sub-unit U _N (i), removing the product composition P _N (i) from said sub-unit UN(i) and transferring a portion of said product composition PN(i) removed from said sub-unit U _N (i) into a storing device D _N (m), it is preferred that said process comprises starting up a sub-process sp _N (i) in a sub-unit U _N (i), wherein said starting up comprises feeding at least a portion of PN(i), obtained from said UN(i) during regular operation mode of UN(i), and stored in the at least one storing device DN(m), as SN(i) into UN(i). In the case where one or more sub-processes sp(i) comprise a nitration, sp _N (i), wherein each sub-unit UN(i) in which a sub-process spN(i) is carried out comprises a nitration reactor, wherein an educt composition E _N (i), being fed into a sub-unit U _N (i), comprises at least one nitrating agent, it is preferred that a product composition P _N (i), obtained from a sub-unit U _N (i), comprises one or more of 2,4-dinitrotoluene as Z ¹ P,N(i), 2,6-dinitrotoluene as Z ² P,N(i), and 2-nitrotoluene as Z ³ P,N(i). Further in the case where one or more sub-processes sp(i) comprise a nitration, sp _N (i), wherein each sub-unit UN(i) in which a sub-process spN(i) is carried out comprises a nitration reactor, wherein an educt composition E _N (i), being fed into a sub-unit U _N (i), comprises at least one nitrat- ing agent, it is preferred that the process further comprises at least one first intermediate treat- ment sub-process, wherein a first intermediate treatment sub-process comprises, preferably consists of, a phase separation sub-process sp _S (i). Said phase separation sub-process sp _S (i) is carried out in a sub-unit which is also designated herein as U _S (i), wherein the product composi- tion P _N (i) obtained from U _N (i), prior to being fed to the sub-unit U _N (i+1), is subjected as educt composition ES(i) to separation and is separated during spS(i) in an organic phase and an aque- ous phase, wherein the organic phase, which is also designated herein as P _S (i), is fed as educt composition E _N (i+1) into the sub-unit U _N (i+1), wherein the sub-unit U _S (i) in which sp _S (i) is carried out comprises a phase separator. If the process according to the present invention comprises at least one first intermediate treat- ment sub-process as defined above, it is preferred that a portion of P _S (i) is stored in at least one storing device DS(m) during a regular operating mode of a sub-unit US(i). As defined here- inabove, r storing devices D(m), m=1…r, r≥1, can be comprised in the chemical unit U in addi- tion to the n chemical processing sub-units U(i). Thus, it is particularly preferred that the process according to the present invention further comprises, during a regular operating mode of a sub- unit U _S (i), removing the product composition P _S (i) from said sub-unit U _S (i) and transferring a portion of said product composition P _S (i) removed from said sub-unit U _S (i) into a storing device DS(m). In the case where the process of the present invention comprises storing a portion of P _S (i) in at least one storing device D _S (m) during a regular operating mode of a sub-unit U _S (i), it is preferred that the process comprises starting up the sub-process sp _S (i) in the sub-unit U _S (i), wherein said starting up comprises feeding at least a portion of PS(i), obtained from said US(i) during a regular operating mode of US(i) and stored in the at least one storing device DS(m), as chemical start-up composition S _S (i) into U _S (i). Further in the case where the process of the present invention comprises storing a portion of P _S (i) in at least one storing device D _S (m) during a regular operating mode of a sub-unit U _S (i), it is preferred that the process comprises starting up the sub-process sp _N (i) in a sub-unit U _N (i), wherein said starting up comprises feeding at least a portion of PS(i), obtained from said US(i) and stored in the at least one storing device DN(m), as chemical start-up composition SN(i) into U _N (i). In the case where the process further comprises at least one first intermediate treatment sub- process, wherein a first intermediate treatment sub-process comprises, preferably consists of, a phase separation sub-process sp _S (i) as defined above, it is preferred that a product composition PS(i), obtained from a sub-unit US(i), comprises one or more of 2,4-dinitrotoluene as Z ¹ P,S(i), 2,6- dinitrotoluene as Z ² P,S(i), and 2-nitrotoluene as Z ³ P,S(i). Further in the case where the process further comprises at least one first intermediate treatment sub-process, wherein a first intermediate treatment sub-process comprises, preferably consists of, a phase separation sub-process sp _S (i) as defined above, it is preferred that the product com- position P _S (i), obtained from the most downstream sub-unit U _S (i), comprises one or more of 2,4- dinitrotoluene as Z ¹ P,S(i) and 2,6-dinitrotoluene as Z ² P,S(i), wherein at least 75 mol-%, preferably at least 80 mol-%, more preferably at least 85 mol-%, more preferably at least 90 mol-% of all Z ^k _P,S (i) comprised in the product composition P _S (i) obtained from the most downstream sub-unit U _S (i) consist of one or more of 2,4-dinitrotoluene as Z ¹ _P,S (i) and 2,6-dinitrotoluene as Z ² _P,S (i). As noted above, it is preferred that one or more sub-processes sp(i) comprise a nitration, sp _N (i), wherein each sub-unit U _N (i) in which a sub-process sp _N (i) is carried out comprises a nitration reactor. In this connection, it is further preferred that an educt composition EN(i), being fed into a sub-unit U _N (i), comprises at least one nitrating agent and preferably further comprises at least one water scavenger. In such a case, it is preferred that the process further comprises at least one second intermediate treatment sub-process, wherein a second intermediate treatment sub- process comprises, preferably consists of a washing sub-process, spW(i), which is carried out in a sub-unit U _W (i), wherein a product composition P _N (i) obtained from a sub-unit U _N (i), or a prod- uct composition P _S (i) obtained from a sub-unit U _S (i), is subjected as educt composition E _W (i) to washing, obtaining a product composition PW(i), wherein the sub-unit UW(i) in which spW(i) is carried out comprises a washing device. In the case where the process further comprises at least one second intermediate treatment sub-process as defined above, it is preferred that the process comprises one sub-process sp _W (1), wherein sp _W (1) is carried out directly downstream of the most downstream sub-process sp _N (i), or directly downstream of the most downstream sub-process sp _S (i), or directly down- stream of the most downstream sub-process sp _N (i) and directly downstream of the most down- stream sub-process spS(i), preferably directly downstream of the most downstream sub-process spS(i). Further in the case where the process further comprises at least one second intermediate treatment sub-process as defined above, it is preferred that the process comprises, during a regular operating mode of a sub-unit U _W (i), storing a portion of P _W (i) in at least one storing de- vice D _W (m). As defined hereinabove, r storing devices D(m), m=1…r, r≥1, can be comprised in the chemical unit U in addition to the n chemical processing sub-units U(i). If a portion of P _W (i) is stored in at least one storing device D _W (m) during a regular operating mode of a sub-unit U _W (i), it is preferred that the process comprises starting up a sub-process spW(i) in a sub-unit UW(i), wherein said starting up comprises feeding at least a portion of PW(i), obtained from said U _W (i) and stored in the at least one storing device D _W (m), as S _W (i) into U _W (i). Further in the case where the process further comprises at least one second intermediate treatment sub-process as defined above, it is preferred that a product composition PW(i), ob- tained from a sub-unit U _W (i), comprises one or more of 2,4-dinitrotoluene as Z ¹ _P,W (i), 2,6- dinitrotoluene as Z ² _P,W (i), and 2-nitrotoluene as Z ³ _P,W (i). Further in the case where the process further comprises at least one second intermediate treatment sub-process as defined above, it is preferred that the product composition P _W (i), ob- tained from the most downstream sub-unit UW(i), comprises one or more of 2,4-dinitrotoluene as Z ¹ _P,W (i) and 2,6-dinitrotoluene as Z ² _P,W (i), wherein at least 75 mol-%, preferably at least 80 mol- %, more preferably at least 85 mol-%, more preferably at least 90 mol-% of all Z ^k _P,W (i) com- prised in the product composition P _W (i) obtained from the most downstream sub-unit U _W (i) con- sist of one or more of 2,4-dinitrotoluene as Z ¹ P,W(i) and 2,6-dinitrotoluene as Z ² P,W(i). It is preferred that one or more sub-processes sp(i) comprise a hydrogenation, sp _H (i), wherein each sub-unit UH(i) in which a sub-process spH(i) is carried out comprises a hydrogenation reac- tor, wherein an educt composition E _H (i), being fed into a sub-unit U _H (i), comprises at least one hydrogenation agent. It is further preferred that one or more sub-processes sp(i) comprise a nitration, spN(i), as de- fined herein, and that one or more sub-processes sp(i) comprise a hydrogenation, sp _H (i), where- in each sub-unit U _H (i) in which a sub-process sp _H (i) is carried out comprises a hydrogenation reactor, wherein an educt composition EH(i), being fed into a sub-unit UH(i), comprises at least one hydrogenation agent. It is particularly preferred that one or more sub-processes sp(i) comprise a nitration, spN(i), as defined herein, that the process comprises at least one first intermediate treatment sub-process as defined herein, and that one or more sub-processes sp(i) comprise a hydrogenation, sp _H (i), wherein each sub-unit U _H (i) in which a sub-process sp _H (i) is carried out comprises a hydrogena- tion reactor, wherein an educt composition E _H (i), being fed into a sub-unit U _H (i), comprises at least one hydrogenation agent. It is more particularly preferred that one or more sub-processes sp(i) comprise a nitration, sp _N (i), as defined herein, that the process comprises at least one first intermediate treatment sub- process as defined herein, that the process comprises at least one second intermediate treat- ment sub-process as defined herein, and that one or more sub-processes sp(i) comprise a hy- drogenation, sp _H (i), wherein each sub-unit U _H (i) in which a sub-process sp _H (i) is carried out comprises a hydrogenation reactor, wherein an educt composition EH(i), being fed into a sub- unit UH(i), comprises at least one hydrogenation agent. In the case where one or more sub-processes sp(i) comprise a hydrogenation, sp _H (i), as defined herein, it is preferred that the process comprises one sub-process spH(1). Further in the case where one or more sub-processes sp(i) comprise a hydrogenation, sp _H (i), as defined herein, it is preferred that the at least one aromatic compound Z ^j E.H(i) comprised in EH(i) is one or more of 2,4-dinitrotoluene as Z ¹ E,H(i), 2,6-dinitrotoluene as Z ² E,H(i), 2-nitrotoluene as Z ³ _E,H (i) and 6-nitrotoluene as Z ⁴ _E,H (i). Further in the case where one or more sub-processes sp(i) comprise a hydrogenation, spH(i), as defined herein, it is preferred that E _H (i) comprises one or more of a product composition P _N (i) as defined in any one of the embodiments disclosed herein, a product composition P _S (i) as defined in any one of the embodiments disclosed herein, and a product composition PW(i) as defined in any one of the embodiments disclosed herein, preferably a product composition P _W (i) obtained from washing as defined in any one of the embodiments disclosed herein, preferably a product composition P _W (i) obtained from the most downstream sub-unit U _W (i), and comprising one or more of 2,4-dinitrotoluene as Z ¹ P,W(i) and 2,6-dinitrotoluene as Z ² P,W(i), wherein at least 75 mol- %, preferably at least 80 mol-%, more preferably at least 85 mol-%, more preferably at least 90 mol-% of all Z ^k _P,W (i) comprised in the product composition P _W (i) obtained from the most down- stream sub-unit UW(i) consist of one or more of 2,4-dinitrotoluene as Z ¹ P,W(i) and 2,6- dinitrotoluene as Z ² _P,W (i). Further in the case where one or more sub-processes sp(i) comprise a hydrogenation, sp _H (i), as defined herein, it is preferred that the process comprises a portion of PH(i) is stored in at least one storing device D _H (m) during a regular operating mode of a sub-unit U _H (i). As defined here- inabove, r storing devices D(m), m=1…r, r≥1, can be comprised in the chemical unit U in addi- tion to the n chemical processing sub-units U(i). If a portion of P _H (i) is stored in at least one storing device D _H (m) during a regular operating mode of a sub-unit UH(i), it is preferred that the process comprises starting up a sub-process spH(i) in a sub-unit UH(i), wherein said starting up comprises feeding at least a portion of PH(i), obtained from said U _H (i) and stored in the at least one storing device D _H (m), as S _H (i) into U _H (i). Further in the case where one or more sub-processes sp(i) comprise a hydrogenation, sp _H (i), as defined herein, it is preferred that a product composition PH(i), obtained from a sub-unit UH(i), comprises one or more of 2,4-diaminotoluene as Z ¹ P,H(i), 2,6-diaminotoluene as Z ² P,H(i), and 2- aminotoluene as Z ³ _P,H (i). Further in the case where one or more sub-processes sp(i) comprise a hydrogenation, spH(i), as defined herein, it is preferred that the process comprises starting up a sub-process sp _H (i) in a sub-unit U _H (i), said starting up comprising feeding at least a portion of P _D (i), obtained from U _D (i) and stored in at least one storing device DD,P(m) as defined hereinbelow, as SH(i) into UH(i). Further in the case where one or more sub-processes sp(i) comprise a hydrogenation, sp _H (i), as defined herein, it is preferred that the process comprises starting up a sub-process sp _H (i) in a sub-unit UH(i), wherein said starting up comprises feeding at least a portion of UD(i), obtained from U _D (i) and stored in at least one storing device D _D,C (m) as defined hereinbelow, as S _H (i) into U _H (i). Further in the case where one or more sub-processes sp(i) comprise a hydrogenation, spH(i), as defined herein, it is preferred that the process further comprises at least one third intermediate treatment sub-process, wherein a third intermediate treatment sub-process comprises, prefera- bly consists of, a distillation sub-process, spD(i), which is carried out in a sub-unit UD(i), wherein preferably, a product composition P _H (i) obtained from a sub-unit U _H (i) is subjected as educt composition E _D (i) to distillation, obtaining a product composition P _D (i), wherein the sub-unit U _D (i) in which spD(i) is carried out comprises a distillation device and wherein, in UD(i), ED(i) is sepa- rated in P _D (i) and at least one further distillation composition C _D (i). If the process further comprises at least one third intermediate treatment sub-process as de- fined above, it is preferred that a portion of PD(i) is stored in at least one storing device DD,P(m) during a regular operating mode of a sub-unit U _D (i). As defined hereinabove, r storing devices D(m), m=1…r, r≥1, can be comprised in the chemical unit U in addition to the n chemical pro- cessing sub-units U(i). In the case where a portion of P _D (i) is stored in at least one storing device D _D,P (m) during a regular operating mode of a sub-unit U _D (i), it is preferred that the process comprises starting up a sub-process spD(i) in a sub-unit UD(i), wherein said starting up comprises feeding at least a portion of P _D (i), obtained from said U _D (i) and stored in the at least one storing device D _D,P (m), as S _D (i) into U _D (i). If the process comprises starting up a sub-process sp _H (i) in a sub-unit U _H (i), wherein said start- ing up comprises feeding at least a portion of P _D (i), obtained from U _D (i) and stored in at least one storing device DD,P(m) as defined hereinabove, as SH(i) into UH(i), it is preferred that a por- tion of CD(i) is stored in at least one storing device DD,C(m) during a regular operating mode of a sub-unit U _D (i). As defined hereinabove, r storing devices D(m), m=1…r, r≥1, can be comprised in the chemical unit U in addition to the n chemical processing sub-units U(i). In the case where a portion of CD(i) is stored in at least one storing device DD,C(m) during a reg- ular operating mode of a sub-unit UD(i), it is preferred that the process comprises starting up a sub-process sp _D (i) in a sub-unit U _D (i), wherein said starting up comprises feeding at least a por- tion of C _D (i), obtained from said U _D (i) and stored in the at least one storing device D _D,C (m), as SD(i) into UD(i). If the process comprises starting up a sub-process sp _H (i) in a sub-unit U _H (i), wherein said start- ing up comprises feeding at least a portion of PD(i), obtained from UD(i) and stored in at least one storing device DD,P(m) as defined hereinabove, as SH(i) into UH(i), it is preferred that a prod- uct composition P _D (i), obtained from a sub-unit U _D (i), comprises one or more of 2,4- diaminotoluene as Z ¹ _P,D (i), 2,6-diaminotoluene as Z ² _P,D (i), and 2-aminotoluene as Z ³ _P,D (i). It is preferred that the weight ratio of the total weight of 2,4-diaminotoluene as Z ¹ _P,D (i), 2,6- diaminotoluene as Z ² _P,D (i), and 2-aminotoluene as Z ³ _P,D (i) comprised in P _D (i) relative to the total weight of 2,4-diaminotoluene as Z ¹ P,H(i), 2,6-diaminotoluene as Z ² P,H(i), and 2-aminotoluene as Z ³ P,H(i) comprised in PH(i) is greater than 1:1, more preferably in the range of from 1.05:1 to 2.5:1, more preferably in the range of from 1.3:1 to 2.2:1, more preferably in the range of from 1.5:1 to 2.1:1. It is preferred that the weight ratio of the product composition P _D (i) relative to the at least one further distillation composition C _D (i) is in the range of from 25:1 to 2:1, more preferably in the range of from 20:1 to 5:1. Further in the case where the process comprises starting up a sub-process sp _H (i) in a sub-unit U _H (i), wherein said starting up comprises feeding at least a portion of P _D (i), obtained from U _D (i) and stored in at least one storing device DD,P(m) as defined hereinabove, as SH(i) into UH(i), it is preferred that the process comprises starting up a sub-process sp _D (i) in a sub-unit U _D (i), where- in said starting up comprises feeding at least a portion of P _H (i), obtained from U _H (i) and stored in the at least one storing device DH(m) as defined herein, as SD(i) into UD(i). As defined above, it is preferred that one or more sub-processes sp(i) comprise a nitration, spN(i), wherein each sub-unit UN(i) in which a sub-process spN(i) is carried out comprises a nitra- tion reactor, wherein an educt composition E _N (i), being fed into a sub-unit U _N (i), comprises at least one nitrating agent, preferably nitric acid, and preferably further comprises at least one water scavenger, preferably sulfuric acid. In such a case, it is preferred that one or more sub- processes sp(i) comprise a phosgenation, sp _P (i), wherein each sub-unit U _P (i) in which a sub- process sp _P (i) is carried out comprises a phosgenation reactor, wherein an educt composition EP(i), being fed into a sub-unit UP(i), comprises at least one phosgenation agent. As defined herein, it is preferred that one or more sub-processes sp(i) comprise a nitration, sp _N (i). In such a case, it is preferred that one or more sub-processes sp(i) comprise a phos- genation, sp _P (i), wherein each sub-unit U _P (i) in which a sub-process sp _P (i) is carried out com- prises a phosgenation reactor, wherein an educt composition EP(i), being fed into a sub-unit UP(i), comprises at least one phosgenation agent. It is further preferred that one or more sub-processes sp(i) comprise a nitration, sp _N (i), as de- fined herein, that the process comprises at least one first intermediate treatment sub-process as defined herein, and that one or more sub-processes sp(i) comprise a phosgenation, sp _P (i), wherein each sub-unit U _P (i) in which a sub-process sp _P (i) is carried out comprises a phosgena- tion reactor, wherein an educt composition EP(i), being fed into a sub-unit UP(i), comprises at least one phosgenation agent. It is further preferred that one or more sub-processes sp(i) comprise a nitration, sp _N (i), as de- fined herein, that the process comprises at least one first intermediate treatment sub-process as defined herein, that the process comprises at least one second intermediate treatment sub- process as defined herein, and that one or more sub-processes sp(i) comprise a phosgenation, spP(i), wherein each sub-unit UP(i) in which a sub-process spP(i) is carried out comprises a phosgenation reactor, wherein an educt composition EP(i), being fed into a sub-unit UP(i), com- prises at least one phosgenation agent. It is further preferred that one or more sub-processes sp(i) comprise a nitration, spN(i), as de- fined herein, that the process comprises at least one first intermediate treatment sub-process as defined herein, that the process comprises at least one second intermediate treatment sub- process as defined herein, that one or more sub-processes sp(i) comprise a hydrogenation, sp _H (i), as defined herein, and that one or more sub-processes sp(i) comprise a phosgenation, sp _P (i), wherein each sub-unit U _P (i) in which a sub-process sp _P (i) is carried out comprises a phosgenation reactor, wherein an educt composition E _P (i), being fed into a sub-unit U _P (i), com- prises at least one phosgenation agent. It is further preferred that one or more sub-processes sp(i) comprise a nitration, sp _N (i), as de- fined herein, that the process comprises at least one first intermediate treatment sub-process as defined herein, that the process comprises at least one second intermediate treatment sub- process as defined herein, that one or more sub-processes sp(i) comprise a hydrogenation, sp _H (i), as defined herein, that the process further comprises at least one third intermediate treatment sub-process as defined herein, and that one or more sub-processes sp(i) comprise a phosgenation, sp _P (i), wherein each sub-unit U _P (i) in which a sub-process sp _P (i) is carried out comprises a phosgenation reactor, wherein an educt composition E _P (i), being fed into a sub-unit UP(i), comprises at least one phosgenation agent. If one or more sub-processes sp(i) comprise a phosgenation, sp _P (i), as defined herein, it is pre- ferred that the process comprises one sub-process spP(1). Further in the case where one or more sub-processes sp(i) comprise a phosgenation, sp _P (i), as defined herein, it is preferred that the at least one aromatic compound Z ^j _E.P (i) comprised in E _P (i) is one or more of 2,4-diaminotoluene as Z ¹ _E,P (i), 2,6-diaminotoluene as Z ² _E,P (i), 2-aminotoluene as Z ³ E,P(i) and 6-aminotoluene as Z ⁴ E,P(i). Further in the case where one or more sub-processes sp(i) comprise a phosgenation, sp _P (i), as defined herein, it is preferred that E _P (i) comprises one or more of a product composition P _H (i) as defined in any one of the embodiments disclosed herein, a product composition PD(i) as defined in any one of the embodiments disclosed herein, a product composition P _D (i) as defined in any one of the embodiments disclosed herein, a distillation composition C _D (i) as defined in any one of the embodiments disclosed herein, preferably a product composition PD(i) obtained from a sub-unit UD(i) as defined herein or a product composition PD(i) obtained from the most down- stream sub-unit U _D (i) as defined herein, more preferably a product composition P _D (i) obtained from the most downstream sub-unit U _D (i) as defined herein. In the case where one or more sub-processes sp(i) comprise a phosgenation, sp _P (i), as defined herein, it is preferred that a portion of P _P (i) is stored in at least one storing device D _P (m) during a regular operating mode of a sub-unit UP(i). As defined hereinabove, r storing devices D(m), m=1…r, r≥1, can be comprised in the chemical unit U in addition to the n chemical processing sub-units U(i). Further in the case where one or more sub-processes sp(i) comprise a phosgenation, spP(i), as defined herein, it is preferred that a portion of P _P (i) is stored in at least one storing device D _P (m) during a regular operating mode of a sub-unit U _P (i). As defined hereinabove, r storing devices D(m), m=1…r, r≥1, can be comprised in the chemical unit U in addition to the n chemical pro- cessing sub-units U(i). If a portion of P _P (i) is stored in at least one storing device D _P (m) during a regular operating mode of a sub-unit UP(i), it is preferred that the process comprises starting up a sub-process sp _P (i) in a sub-unit U _P (i), wherein said starting up comprises feeding at least a portion of P _P (i), obtained from said U _P (i) and stored in the at least one storing device D _P (m), as S _P (i) into U _P (i). Further in the case where one or more sub-processes sp(i) comprise a phosgenation, sp _P (i), as defined herein, it is preferred that a product composition P _P (i), obtained from a sub-unit U _P (i), comprises one or more of 2,4-toluene diisocyanate as Z ¹ _P,P (i), 2,6-toluene diisocyanate as Z ² P,P(i), and 2-toluene isocyanate as Z ³ P,P(i). It is preferred that the abovementioned process is a continuous process or discontinuous pro- cess, more preferably a continuous process. Further, the present invention relates to a chemical unit U for carrying out the process according to any one of the embodiments defined herein. It is preferred that the chemical unit comprises (i.1) one or more sub-units U _N (i), preferably two sub-units U _N (1) and U _N (2); (i.2) one or more sub-units U _S (i), preferably two sub-units U _S (1) and U _S (2); (i.3) one or more sub-units UW(i), preferably one sub-unit UW(1); (i.4) one or more storing devices DN(m), preferably two storing devices DN(1) and DN(2); wherein U _N (1) is arranged upstream of U _S (1), U _S (1) is arranged upstream U _N (2), U _N (2) is arranged upstream U _S (2), and U _S (2) is arranged upstream of U _W (1); (ii.1) one or more sub-units UH(i), preferably one sub-unit UH(1); (ii.2) one or more sub-units U _D (i), preferably one sub-unit U _D (1); (ii.3) one or more storing devices D _H (m), preferably one storing device D _H (1); (ii.4) one or more storing devices DD(m), preferably two storing devices DD,P(1) and DD,C(1); wherein UH(1) is arranged downstream of UW(1) and upstream UD(1); (iii.1) one or more sub-units U _P (i), preferably one sub-unit U _P (1); wherein U _P (1) is arranged downstream of U _D (1). The present invention is further illustrated by the following set of embodiments and combina- tions of embodiments resulting from the dependencies and back-references as indicated. In particular, it is noted that in each instance where a range of embodiments is mentioned, for ex- ample in the context of a term such as "The chemical process of any one of embodiments 1 to 4", every embodiment in this range is meant to be explicitly disclosed for the skilled person, i.e. the wording of this term is to be understood by the skilled person as being synonymous to "The chemical process of any one of embodiments 1, 2, 3 and 4". Further, it is explicitly noted that the following set of embodiments is not the set of claims determining the extent of protection, but represents a suitably structured part of the description directed to general and preferred aspects of the present invention. 1. A chemical process for preparing at least one aromatic isocyanate, said process compris- ing n chemical sub-processes sp(i), wherein a sub-process sp(i) is carried out in a chemi- cal processing sub-unit U(i), i=1…n, n≥1, wherein during a regular operating mode of a sub-unit U(i), the sub-process sp(i) being carried out in said sub-unit U(i) comprises feeding an educt composition E(i) comprising at least one aromatic compound Z ^j E(i), j≥1, into said sub-unit U(i), processing said composi- tion E(i) in said sub-unit U(i) and obtaining a product composition P(i) comprising at least one aromatic compound Z ^k _P (i), k≥1, wherein the chemical process comprises starting up said sub-process sp(i) in said sub- unit U(i), wherein said starting-up comprises processing a chemical start-up composition S(i) in said sub-unit U(i), wherein said S(i) ≠ E(i) and wherein said S(i) comprises at least one of the aromatic compounds Z ^k _P (i). 2. The process of embodiment 1, further comprising, prior to starting up the sub-process sp(i) in the sub-unit U(i), shutting down the sub-process sp(i) in the sub-unit U(i) and keep- ing the sub-unit U(i) in a non-operation mode for a period of time Δt(i). 3. The process of embodiment 1 or 2, wherein starting up the sub-process sp(i) comprises feeding the chemical composition S(i) into the sub-unit U(i). 4. The process of any one of embodiments 1 to 3, wherein, if n≥2, at least 2, preferably all sub-units U(i) are part of a chemical unit U. 5. The process of embodiment 4, wherein U further comprises, in addition to the n chemical processing sub-units U(i), r storing devices D(m), m=1…r, r≥1. 6. The process of embodiment 5, further comprising, during a regular operating mode of a sub-unit U(i), removing the product composition P(i) from said sub-unit U(i) and transfer- ring a portion of said product composition P(i) removed from said sub-unit U(i) into a stor- ing device D(m). 7. The process of embodiment 6, further comprising, during starting up the sub-process sp(i), feeding at least a portion of P(i), stored in D(m) during the regular operating mode of the sub-unit U(i), as at least a portion of the chemical composition S(i) into the sub-unit U(i). 8. The process of any one of embodiments 1 to 7, wherein the sub-unit U(i) is selected from the group consisting of a chemical reactor, a distillation column, a rectification column, a tank, a phase separator, a washing device, and a combination of two or more thereof. 9. The process of any one of embodiments 1 to 8, wherein at least one sub-process sp(i) comprises a nitration and at least one further sub-process sp(i) comprises a hydrogena- tion. 10. The process of any one of embodiments 1 to 9, preferably of embodiment 9, wherein at least one further sub-process sp(i) comprises a phosgenation. 11. The process of any one of embodiments 1 to 10, preferably of any one of embodiments 8 to 10, wherein the at least one aromatic isocyanate comprises an aromatic diisocyanate, preferably a toluene diisocyanate, more preferably one or more of 2,4-toluene diisocya- nate and 2,6-toluene diisocyanate, or a methylene diphenyl diisocyanate, preferably one or more of methylene diphenyl 2,2’-diisocyanate, methylene diphenyl 2,4’-diisocyanate, and methylene diphenyl 4,4’-diisocyanate. 12. The process of embodiment 11, wherein the at least one aromatic isocyanate comprises, preferably is, one or more of 2,4-toluene diisocyanate and 2,6-toluene diisocyanate. 13. The process of any one of embodiments 1 to 12, preferably of any one of embodiments 8 to 12, wherein one or more sub-processes sp(i) comprise a nitration, sp _N (i), wherein each sub-unit UN(i) in which a sub-process spN(i) is carried out comprises a nitration reactor, wherein an educt composition EN(i), being fed into a sub-unit UN(i), comprises at least one nitrating agent, preferably nitric acid, and preferably further comprises at least one water scavenger, preferably sulfuric acid; wherein said process preferably comprises two or more sub-processes sp _N (i) which are carried out in sequence, wherein at last a portion of the product composition PN(i) ob- tained from a sub-unit UN(i) is fed, optionally after an intermediate treatment sub-process, as educt composition E _N (i+1) into the sub-unit U _N (i+1) downstream of U _N (i) during a regu- lar operating mode of U _N (i+1); wherein the educt composition EN(1) being fed into the first, most upstream sub-unit UN(1) comprises toluene as Z ¹ _E,N (1), wherein more preferably, j=1. 14. The process of embodiment 13, comprising, during a regular operating mode of a sub-unit UN(i), storing a portion of PN(i) in at least one storing device DN(m) as defined in embodi- ment 5. 15. The process of embodiment 14, comprising starting up a sub-process spN(i) in a sub-unit U _N (i), said starting up comprising feeding at least a portion of P _N (i), obtained from said U _N (i) during regular operation mode of U _N (i), and stored in the at least one storing device DN(m), as SN(i) into UN(i). 16. The process of any one of embodiments 13 to 15, wherein a product composition P _N (i), obtained from a sub-unit U _N (i), comprises one or more of 2,4-dinitrotoluene as Z ¹ _P,N (i), 2,6- dinitrotoluene as Z ² P,N(i), and 2-nitrotoluene as Z ³ P,N(i). 17. The process of embodiment 16, wherein a product composition P _N (i), obtained from a sub-unit UN(i), comprises 2,4-dinitrotoluene as Z ¹ P,N(i), 2,6-dinitrotoluene as Z ² P,N(i), and 2- nitrotoluene as Z ³ _P,N (i). 18. The process of any one of embodiments 13 to 17, further comprising at least one first in- termediate treatment sub-process, wherein a first intermediate treatment sub-process comprises, preferably consists of a phase separation sub-process, sp _S (i), which is carried out in a sub-unit U _S (i), wherein the product composition P _N (i) obtained from U _N (i), prior to being fed to the sub-unit UN(i+1), is subjected as educt composition ES(i) to separation and is separated during sp _S (i) in an organic phase and an aqueous phase, wherein the organic phase, P _S (i), is fed as educt composition E _N (i+1) into the sub-unit U _N (i+1), wherein the sub-unit U _S (i) in which sp _S (i) is carried out comprises a phase separator. 19. The process of embodiment 18, comprising, during a regular operating mode of a sub-unit U _S (i), storing a portion of P _S (i) in at least one storing device D _S (m) as defined in embodi- ment 5. 20. The process of embodiment 19, comprising starting up the sub-process sp _S (i) in the sub- unit US(i), said starting up comprising feeding at least a portion of PS(i), obtained from said US(i) during a regular operating mode of US(i) and stored in the at least one storing device D _S (m), as S _S (i) into U _S (i). 21. The process of embodiment 19 or 20, comprising starting up the sub-process sp _N (i) in a sub-unit UN(i), said starting up comprising feeding at least a portion of PS(i), obtained from said US(i) and stored in the at least one storing device DN(m), as SN(i) into UN(i). 22. The process of any one of embodiments 18 to 21, wherein a product composition P _S (i), obtained from a sub-unit US(i), comprises one or more of 2,4-dinitrotoluene as Z ¹ P,S(i), 2,6- dinitrotoluene as Z ² _P,S (i), and 2-nitrotoluene as Z ³ _P,S (i). 23. The process of any one of embodiments 18 to 22, wherein the product composition PS(i), obtained from the most downstream sub-unit US(i), comprises one or more of 2,4- dinitrotoluene as Z ¹ _P,S (i) and 2,6-dinitrotoluene as Z ² _P,S (i), wherein at least 75 mol-%, pref- erably at least 80 mol-%, more preferably at least 85 mol-%, more preferably at least 90 mol-% of all Z ^k P,S(i) comprised in the product composition PS(i) obtained from the most downstream sub-unit U _S (i) consist of one or more of 2,4-dinitrotoluene as Z ¹ _P,S (i) and 2,6- dinitrotoluene as Z ² _P,S (i). 24. The process of any one of embodiments 13 to 23, preferably of any one of embodiments 18 to 23, further comprising at least one second intermediate treatment sub-process, wherein a second intermediate treatment sub-process comprises, preferably consists of a washing sub-process, spW(i), which is carried out in a sub-unit UW(i), wherein a product composition P _N (i) obtained from a sub-unit U _N (i), or a product composition P _S (i) obtained from a sub-unit U _S (i), is subjected as educt composition E _W (i) to washing, obtaining a product composition PW(i), wherein the sub-unit UW(i) in which spW(i) is carried out com- prises a washing device. 25. The process of embodiment 24, comprising one sub-process sp _W (1), wherein sp _W (1) is carried out directly downstream of the most downstream sub-process spN(i), or directly downstream of the most downstream sub-process sp _S (i), or directly downstream of the most downstream sub-process sp _N (i) and directly downstream of the most downstream sub-process spS(i), preferably directly downstream of the most downstream sub-process sp _S (i). 26. The process of embodiment 24 or 25, comprising, during a regular operating mode of a sub-unit UW(i), storing a portion of PW(i) in at least one storing device DW(m) as defined in embodiment 5. 27. The process of embodiment 26, comprising starting up a sub-process spW(i) in a sub-unit U _W (i), said starting up comprising feeding at least a portion of P _W (i), obtained from said U _W (i) and stored in the at least one storing device D _W (m), as S _W (i) into U _W (i). 28. The process of any one of embodiments 24 to 27, wherein a product composition PW(i), obtained from a sub-unit U _W (i), comprises one or more of 2,4-dinitrotoluene as Z ¹ _P,W (i), 2,6-dinitrotoluene as Z ² _P,W (i), and 2-nitrotoluene as Z ³ _P,W (i). 29. The process of any one of embodiments 24 to 28, wherein the product composition P _W (i), obtained from the most downstream sub-unit UW(i), comprises one or more of 2,4- dinitrotoluene as Z ¹ P,W(i) and 2,6-dinitrotoluene as Z ² P,W(i), wherein at least 75 mol-%, preferably at least 80 mol-%, more preferably at least 85 mol-%, more preferably at least 90 mol-% of all Z ^k _P,W (i) comprised in the product composition P _W (i) obtained from the most downstream sub-unit UW(i) consist of one or more of 2,4-dinitrotoluene as Z ¹ P,W(i) and 2,6- dinitrotoluene as Z ² _P,W (i). 30. The process of any one of embodiments 1 to 29, preferably of any one of embodiments 13 to 29, more preferably of any one of embodiments 18 to 29, more preferably of any one of embodiments 24 to 29, wherein one or more sub-processes sp(i) comprise a hydrogena- tion, sp _H (i), wherein each sub-unit U _H (i) in which a sub-process sp _H (i) is carried out com- prises a hydrogenation reactor, wherein an educt composition EH(i), being fed into a sub- unit U _H (i), comprises at least one hydrogenation agent. 31. The process of embodiment 30, comprising one sub-process spH(1). 32. The process of embodiment 30 or 31, wherein the at least one aromatic compound Z ^j _E.H (i) comprised in E _H (i) is one or more of 2,4-dinitrotoluene as Z ¹ _E,H (i), 2,6-dinitrotoluene as Z ² E,H(i), 2-nitrotoluene as Z ³ E,H(i) and 6-nitrotoluene as Z ⁴ E,H(i). 33. The process of any one of embodiments 30 to 32, wherein E _H (i) comprises one or more of a product composition PN(i) as defined in any one of embodiments 13 to 17, a product composition P _S (i) as defined in any one of embodiments 18 to 23, and a product composi- tion P _W (i) as defined in any one of embodiments 24 to 29, preferably a product composi- tion P _W (i) as defined in any one of embodiments 24 to 28, more preferably the product composition PW(i) as defined in embodiment 29. 34. The process of any one of embodiment 30 to 33, comprising, during a regular operating mode of a sub-unit UH(i), storing a portion of PH(i) in at least one storing device DH(m) as defined in embodiment 5. 35. The process of embodiment 34, comprising starting up a sub-process sp _H (i) in a sub-unit UH(i), said starting up comprising feeding at least a portion of PH(i), obtained from said U _H (i) and stored in the at least one storing device D _H (m), as S _H (i) into U _H (i). 36. The process of any one of embodiments 30 to 35, wherein a product composition PH(i), obtained from a sub-unit U _H (i), comprises one or more of 2,4-diaminotoluene as Z ¹ _P,H (i), 2,6-diaminotoluene as Z ² _P,H (i), and 2-aminotoluene as Z ³ _P,H (i). 37. The process of embodiment 36, wherein a product composition PH(i), obtained from a sub- unit UH(i), comprises 2,4-diaminotoluene as Z ¹ P,H(i), 2,6-diaminotoluene as Z ² P,H(i), and 2- aminotoluene as Z ³ _P,H (i). 38. The process of any one of embodiments 30 to 37, comprising starting up a sub-process spH(i) in a sub-unit UH(i), said starting up comprising feeding at least a portion of PD(i), ob- tained from UD(i) and stored in at least one storing device DD,P(m) as defined in embodi- ment 41, as S _H (i) into U _H (i). 39. The process of any one of embodiments 30 to 38, comprising starting up a sub-process sp _H (i) in a sub-unit U _H (i), said starting up comprising feeding at least a portion of U _D (i), ob- tained from U _D (i) and stored in at least one storing device D _D,C (m) as defined in embodi- ment 43, as SH(i) into UH(i). 40. The process of any one of embodiments 30 to 39, further comprising at least one third intermediate treatment sub-process, wherein a third intermediate treatment sub-process comprises, preferably consists of a distillation sub-process, spD(i), which is carried out in a sub-unit U _D (i), wherein preferably, a product composition P _H (i) obtained from a sub-unit U _H (i) is subjected as educt composition E _D (i) to distillation, obtaining a product composi- tion PD(i), wherein the sub-unit UD(i) in which spD(i) is carried out comprises a distillation device and wherein in UD(i), ED(i) is separated in PD(i) and at least one further distillation composition C _D (i). 41. The process of embodiment 40, comprising, during a regular operating mode of a sub-unit U _D (i), storing a portion of P _D (i) in at least one storing device D _D,P (m) as defined in embod- iment 5. 42. The process of embodiment 41, comprising starting up a sub-process sp _D (i) in a sub-unit U _D (i), said starting up comprising feeding at least a portion of P _D (i), obtained from said U _D (i) and stored in the at least one storing device D _D,P (m), as S _D (i) into U _D (i). 43. The process of any one of embodiments 38 to 42, comprising, during a regular operating mode of a sub-unit U _D (i), storing a portion of C _D (i) in at least one storing device D _D,C (m) as defined in embodiment 5. 44. The process of embodiment 43, comprising starting up a sub-process sp _D (i) in a sub-unit U _D (i), said starting up comprising feeding at least a portion of C _D (i), obtained from said UD(i) and stored in the at least one storing device DD,C(m), as SD(i) into UD(i). 45. The process of any one of embodiments 38 to 44, wherein a product composition P _D (i), obtained from a sub-unit UD(i), comprises one or more of 2,4-diaminotoluene as Z ¹ P,D(i), 2,6-diaminotoluene as Z ² _P,D (i), and 2-aminotoluene as Z ³ _P,D (i). 46. The process of embodiment 45, wherein a product composition PD(i), obtained from a sub- unit UD(i), comprises 2,4-diaminotoluene as Z ¹ P,D(i), 2,6-diaminotoluene as Z ² P,D(i), and 2- aminotoluene as Z ³ _P,D (i). 47. The process of any one of embodiments 40 to 46, the weight ratio of the total weight of 2,4-diaminotoluene as Z ¹ P,D(i), 2,6-diaminotoluene as Z ² P,D(i), and 2-aminotoluene as Z ³ P,D(i) comprised in PD(i) relative to the total weight of 2,4-diaminotoluene as Z ¹ P,H(i), 2,6- diaminotoluene as Z ² _P,H (i), and 2-aminotoluene as Z ³ _P,H (i) comprised in P _H (i) is greater than 1:1, more preferably in the range of from 1.05:1 to 2.5:1, more preferably in the range of from 1.3:1 to 2.2:1, more preferably in the range of from 1.5:1 to 2.1:1. 48. The process of any one of embodiments 40 to 46, wherein the weight ratio of the product composition PD(i) relative to the at least one further distillation composition CD(i) is in the range of from 25:1 to 2:1, more preferably in the range of from 20:1 to 5:1. 49. The process of any one of embodiments 38 to 48, comprising starting up a sub-process spD(i) in a sub-unit UD(i), said starting up comprising feeding at least a portion of PH(i), ob- tained from U _H (i) and stored in the at least one storing device D _H (m) as defined in embod- iment 35, as S _D (i) into U _D (i). 50. The process of any one of embodiments 13 to 49, preferably of any one of embodiments 18 to 47, more preferably of any one of embodiments 24 to 49, more preferably of any one of embodiments 30 to 47, more preferably of any one of embodiments 40 to 49, wherein one or more sub-processes sp(i) comprise a phosgenation, spP(i), wherein each sub-unit U _P (i) in which a sub-process sp _P (i) is carried out comprises a phosgenation reactor, wherein an educt composition E _P (i), being fed into a sub-unit U _P (i), comprises at least one phosgenation agent. 51. The process of embodiment 50, comprising one sub-process sp _P (1). 52. The process of embodiment 50 or 51, wherein the at least one aromatic compound Z ^j E.P(i) comprised in E _P (i) is one or more of 2,4-diaminotoluene as Z ¹ _E,P (i), 2,6-diaminotoluene as Z ² _E,P (i), 2-aminotoluene as Z ³ _E,P (i) and 6-aminotoluene as Z ⁴ _E,P (i). 53. The process of any one of embodiments 50 to 52, wherein E _P (i) comprises one or more of a product composition P _H (i) as defined in embodiment 36 or 37, a product composition P _D (i) as defined in any one of embodiments 45 to 48, and a distillation composition C _D (i) as defined in embodiment 40, preferably a product composition PD(i) as defined in any one of embodiments 45 to 48, more preferably the product composition P _D (i) as defined in any one of embodiments 46 to 48. 54. The process of any one of embodiment 50 to 53, comprising, during a regular operating mode of a sub-unit U _P (i), storing a portion of P _P (i) in at least one storing device D _P (m) as defined in embodiment 5. 55. The process of embodiment 54, comprising starting up a sub-process sp _P (i) in a sub-unit UP(i), said starting up comprising feeding at least a portion of PP(i), obtained from said UP(i) and stored in the at least one storing device DP(m), as SP(i) into UP(i). 56. The process of any one of embodiments 50 to 55, wherein a product composition P _P (i), obtained from a sub-unit UP(i), comprises one or more of 2,4-toluene diisocyanate as Z ¹ _P,P (i), 2,6-toluene diisocyanate as Z ² _P,P (i), and 2-toluene isocyanate as Z ³ _P,P (i). 57. The process of embodiment 56, wherein a product composition PP(i), obtained from a sub- unit UP(i), comprises 2,4-toluene diisocyanate as Z ¹ P,P(i), 2,6-toluene diisocyanate as Z ² _P,P (i), and 2-toluene isocyanate as Z ³ _P,P (i). 58. The process of any one of embodiments 1 to 57, wherein the process is a continuous or discontinuous process, preferably a continuous process. 59. A chemical unit U for carrying out the process according to any one of embodiments 1 to 58. 60. The chemical unit of embodiment 59, comprising (i.1) one or more sub-units UN(i), preferably two sub-units UN(1) and UN(2); (i.2) one or more sub-units U _S (i), preferably two sub-units U _S (1) and U _S (2); (i.3) one or more sub-units U _W (i), preferably one sub-unit U _W (1); (i.4) one or more storing devices DN(m), preferably two storing devices DN(1) and DN(2); wherein U _N (1) is arranged upstream of U _S (1), U _S (1) is arranged upstream U _N (2), U _N (2) is arranged upstream U _S (2), and U _S (2) is arranged upstream of U _W (1); (ii.1) one or more sub-units U _H (i), preferably one sub-unit U _H (1); (ii.2) one or more sub-units UD(i), preferably one sub-unit UD(1); (ii.3) one or more storing devices D _H (m), preferably one storing device D _H (1); (ii.4) one or more storing devices D _D (m), preferably two storing devices D _D,P (1) and DD,C(1); wherein U _H (1) is arranged downstream of U _W (1) and upstream U _D (1); (iii.1) one or more sub-units U _P (i), preferably one sub-unit U _P (1); wherein U _P (1) is arranged downstream of U _D (1). Figure 1 shows a flow diagram according to the process of the present invention. Depicted is a process comprising n chemical sub-processes sp(i). The sub-process sp(i) is carried out in a chemical processing sub-unit U(i) where i=1…n, with n≥1. For example, when n=1, during regular operation mode of a sub-unit U _N (1) for nitration, the sub- process spN(1) being carried out in said sub-unit UN(1) comprises feeding an educt composition EN(1) comprising at least one aromatic compound Z ^j E(1), such as toluene, into said sub-unit U _N (1). The composition E _N (1) comprising at least one aromatic compound Z ^j _E (1) is processed in said sub-unit to obtain a product composition P _N (1) comprising at least one aromatic compound Z ^k P(1), k≥1 being dinitrotoluene. For example, when n=2, for nitration, two sub-units U _N (1) and U _N (2), the sub-units U _N (1) and U _N (2), are part of a chemical unit U, wherein U further comprises, in addition to the 2 chemical processing sub-units UN(1) and UN(2), one storing device DN(1). During regular operation mode of the sub-unit U _N (1), a portion of the product P _N (1) obtained from U _N (1) and comprising dinitro- toluene is stored in a storing device D _N (1), and the remaining portion of the product P _N (1) is re- moved from UN(1) and fed to either UN(2) or to a further downstream sub-unit. Sub-unit UN(1) is shut down for maintenance and regular operation is stopped. After a maintenance routine of U _N (1) is completed, in the starting up process of the sub-process sp _N (1) in sub-unit U _N (1), a por- tion of P _N (1) (dinitrotoluene), obtained from said U _N (1) during regular operation mode of U _N (1) and UN(2), and stored in the storing device DN(1), is fed back into the nitration sub-unit UN(1) as a start-up composition S _N (1). Accordingly, the start-up composition S _N (1) ≠ educt composition E _N (1). Depicted as well are optional intermediate processes and units symbolized as a broken lines between the two sub-units. Cited literature - WO 2015/197522 A1 - WO 2017/050776 A1