Patent Publication Number: US-2011076200-A1

Title: Chemical plant

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a chemical plant for scaling up the production throughput of a reactor (hereafter, referred to as microreactor) utilizing a minute flow path. 
     In recent years, research and development have been increasingly promoted on microreactors for actively applying micromachining technologies or the advantages of microscale to chemical processes. Various advantages are expected from microreactors and one of such advantages is that they make it easy to scale up a throughput. In research and development and manufacture of chemical products, usually, a production is scaled up stepwise from research at the laboratory level to mass production plant by way of product development and test plant. 
     In general, conditions under which chemical reaction is carried out are different between chemical reaction for a small quantity at the beaker level and chemical reaction for a large quantity in a reaction vessel for production. Therefore, a technical problem often arises in scale-up from the laboratory level to the mass production level. To avoid this problem as much as possible, a strategy has been proposed with respect to microreactors. In this strategy, multiple microreactors for small quantity processing developed and applied at the laboratory level are equipped in parallel in accordance with a required quantity at the mass production level and a throughput is thereby scaled up in a stroke. This throughput scaling-up technique in which multiple microreactors each of which is low in throughput are equipped in parallel in plant and a throughput is thereby increased may be designated as numbering-up sometimes. 
     In scale-up from a throughput at the laboratory level using a microreactor to a throughput at the mass production level at plant, the following is important to obtain a high-quality product: how a raw material should be uniformly distributed to each microreactor to reproduce the processing state at the laboratory level in concrete numbering-up. 
     In scale-up from the laboratory level to the plant level by numbering-up, the following method is often adopted as disclosed in Japanese Unexamined Patent Publication No. 2008-80306 and Japanese Unexamined Patent Publication No. 2004-344877: a method of connecting microreactors in parallel to increase the number of equipped microreactors. The greatest benefit of this parallel connected piping is that: raw material is branched from one liquid sending means (pump, header tank) and thus the cost of liquid sending facilities can be suppressed. 
     The following is a description of a disadvantage. A branched minute flow path may be choked due to the following: the adhesion or residue of an air bubble due to the influence of a capillary phenomenon, the deposition of a reactant in a microreactor on a flow path wall, or the like. In this case, this imbalance of flow path resistance has influence on all the other branched flow paths and a desired quantity of liquid sent to each microreactor is disturbed. In priming, that is, in an initial state in which operation is started with the raw material lines and microreactors of a plant empty to substitute raw materials for the contents of the flow paths, substitution failure may occur. In substitution failure, that is, the residue of a vapor phase in a minute flow path, there is high uncertainty with respect to when the air bubble is detached and flows downstream. This can cause degradation in the performance of chemical operation in a minute space in a microreactor. 
     As a means for solving problems of the deposition of a reactant in a microreactor on a flow path wall and the like, Japanese Unexamined Patent Publication No. 2008-80306 discloses a configuration in which the following is implemented: the state of sending of raw material flowing through each flow path is monitored; and based on information obtained by this monitoring, a valve and a liquid sending means are controlled to keep the flow rate of raw material flowing through each flow path at a desired value. However, when a large number of microreactors are branched and piped and connected in parallel, a change in the flow rate in some microreactor has influence on the flow rates in the other microreactors and this complicates control. The above patent document does not clearly describe a concrete control law therefor. Japanese Unexamined Patent Publication No. 2004-344877 also describes an embodiment of parallel connected piping; however, it does not give sufficient consideration to the above-mentioned adhesion or residue of an air bubble due to the influence of a capillary phenomenon. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of the invention to solve the above problems associated with each piping system to achieve a stable state of liquid sending operation in chemical plant subjected to numbering-up by piping microreactors in parallel. 
     The invention is a chemical plant including multiple raw material tanks for storing raw materials and multiple reactors connected in parallel by branch lines from the individual raw material tanks. Each of branched pipes includes: a distribution flow rate detecting means for detecting the flow rate of raw material flowing through the pipe; and a distribution flow rate adjusting means for adjusting the flow rate of raw material flowing through the pipe. The chemical plant further includes a control apparatus that controls the setting of each distribution flow rate adjusting means to a target flow rate using the flow rate in the corresponding pipe detected by the corresponding distribution flow rate detecting means. This control apparatus is provided with a non-interference property providing device that renders a multiple-input multiple-output system apparently of non-interference type. The multiple-input multiple-output system is inputted with a flow rate detected by a distribution flow rate detecting means and outputs the opening of a distribution flow rate adjusting means. 
     It is desirable that this non-interference property providing device should have a transfer characteristic matrix expressed as the product of the following: an inverse matrix of a transfer characteristic matrix representing the relation between the input of raw material to each reactor and a distribution flow rate detected by a distribution flow rate detecting means and a diagonal matrix. It is desirable that the control apparatus should use this non-interference property providing device to provide a non-interference property with respect to each reactor and independently control the distribution flow rate adjusting means. It is desirable that the following should be provided between a bifurcation in a branch line connected to each of the raw material tanks and the raw material tank: multiple pumps that send raw material to multiple reactors; a raw material flow rate detecting means for detecting the flow rate of raw material; and a raw material flow rate adjusting means for adjusting the flow rate of raw material. It is desirable that the control apparatus should control a raw material flow rate adjusting means so as to adjust the flow rate of raw material based on a detection value detected by a flow rate detecting means. 
     A branch tank may be provided at a bifurcation of a branch line connected to each of the multiple raw material tanks; and multiple pipes may be connected to the branch tank and multiple reactors may be connected in parallel. A raw material tank, a pump, a branch tank, a raw material flow rate adjusting means, and a raw material flow rate detecting means may be connected in series. The chemical plant may be provided with a system adjustment unit. When the flow rate of raw material to each reactor is inputted, the system adjustment unit automatically measures a corresponding distribution flow rate. It then determines a transfer characteristic matrix from their cause and effect relationship and adjusts a control law in accordance with this transfer characteristic. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         FIG. 1  is a drawing of a chemical plant in an embodiment of the invention and illustrates the basic configuration thereof; 
         FIG. 2A  is a drawing illustrating the operation of a branch tank provided in the chemical plant illustrated in  FIG. 1 ; 
         FIG. 2B  is another drawing illustrating the operation of the branch tank provided in the chemical plant illustrated in  FIG. 1 ; 
         FIG. 2C  is another drawing illustrating the operation of the branch tank provided in the chemical plant illustrated in  FIG. 1 ; 
         FIG. 3A  is a drawing illustrating a plant required for designing the control apparatus provided in the chemical plant illustrated in  FIG. 1 ; 
         FIG. 3B  is a drawing illustrating a transfer characteristic corresponding to the plant in  FIG. 3A ; 
         FIG. 3C  is a drawing illustrating the plant required for designing the control apparatus provided in the chemical plant illustrated in  FIG. 1 ; 
         FIG. 3D  is another drawing illustrating a transfer characteristic of the non-interference property providing device in  FIG. 3C ; 
         FIG. 4A  is a drawing illustrating effect obtained when a non-interference property providing device is connected to the chemical plant illustrated in  FIG. 1 ; 
         FIG. 4B  is another drawing illustrating effect obtained when the non-interference property providing device is connected to the chemical plant illustrated in  FIG. 1 ; 
         FIG. 4C  is another drawing illustrating effect obtained when the non-interference property providing device is connected to the chemical plant illustrated in  FIG. 1 ; 
         FIG. 4D  is another drawing illustrating effect obtained when the non-interference property providing device is connected to the chemical plant illustrated in  FIG. 1 ; 
         FIG. 4E  is another drawing illustrating effect obtained when the non-interference property providing device is connected to the chemical plant illustrated in  FIG. 1 ; 
         FIG. 4F  is another drawing illustrating effect obtained when the non-interference property providing device is connected to the chemical plant illustrated in  FIG. 1 ; 
         FIG. 5A  is a drawing illustrating a plant control system for the chemical plant illustrated in  FIG. 1 ; 
         FIG. 5B  is another drawing illustrating a plant control system for the chemical plant illustrated in  FIG. 1 ; and 
         FIG. 6  is a drawing illustrating a method for identifying a transfer characteristic of a plant in another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereafter, description will be given to chemical plants in some embodiments of the invention with reference to the drawings. 
     First Embodiment 
       FIG. 1  illustrates the basic configuration of the piping and raw material sending control in the first embodiment of the invention. As an example of this embodiment, the following process will be taken: a two-raw material one-step reaction or mixture process in which raw material A and raw material B are mixed or reacted with each other in microreactors (reactors) to obtain product C. 
     In  FIG. 1 , liquid raw material A and liquid raw material B are respectively stored in raw material tanks  101  and  102  and respectively driven and sent by a pump  103  and a pump  104 . The pumps  103 ,  104  respectively distribute raw material A and raw material B to each reactor  107  through branch tanks  105 ,  106  and liquid sending lines  115 ,  116  branched therefrom. The distributed raw material A and raw material B are mixed or reacted with each other in each reactor  107  and are recovered as product C into a recovery tank  108 . Aside from the distributed liquid sending lines  115 ,  116 , the branch tanks  105 ,  106  are provided with lines  109 ,  110  through which the raw materials can be returned to the respective raw material tanks  101 ,  102 . Raw materials A, B are let to flow back through these lines. 
     In these lines  109 ,  110 , there are installed raw material flow rate detecting means  111 ,  112  for detecting the flow rate of raw material and flow rate control valves  113 ,  114  as raw material flow rate adjusting means that can adjust or block this flow. Also in each liquid sending line  115 ,  116  connected to each reactor  107 , there are installed distribution flow rate detecting means  117  for detecting a distribution flow rate and flow rate control valves  118  as distribution flow rate adjusting means for adjusting this flow rate. Information from each flow rate detecting means  111 ,  112 ,  117  is sent to the control apparatus  119  and the control apparatus  119  properly controls the opening of each control valve  113 ,  114 ,  118  in accordance with a request from a user console  120 . 
     Description will be given to the operation of the fluid control system illustrated in  FIG. 1 . First, description will be given to priming carried out at the start of operation. At the start of operation, raw material has not been passed through each liquid sending line  115 ,  116 ; therefore, the flow paths are filled with air or any other gas. To completely substitute liquid raw material for the interior of the liquid sending lines in this state, this embodiment with the above-mentioned piping configuration adopts priming mode comprised of a two-stage operation pattern. 
     First, all the control valves  118  in the liquid sending lines to which the multiple microreactors  107  are connected are closed and the control valves  113 ,  114  in the lines communicating with the respective raw material tanks  101 ,  102  are opened. Then the pumps are operated at an appropriate flow rate. Description will be given to the operation with the branch tank  105  at this time taken as an example.  FIG. 2A  illustrates a state in which liquid raw material has not been filled in the tank before priming is started. It will be assumed that gravity acts downward as viewed in the plane of the drawing. The raw material is sent from the branch tank  105  through a liquid sending line  203  by the pump  103 . 
     Though not shown in  FIG. 2A , the following lines are provided with the control valves  113 ,  114 ,  118  illustrated in  FIG. 1 : liquid sending lines  201  (equivalent to reference numerals  115  and  116  in  FIG. 1 ) connected to the microreactors and a line  202  (equivalent to reference numerals  109  and  110  in  FIG. 1 ) communicating with the raw material tank  101 . 
     The following takes place by closing the liquid sending lines  201  connected to the microreactors and opening the liquid sending line  202  communicating with the raw material tank as mentioned above: the raw material is sent so as to carry away air to the raw material tank as illustrated in  FIG. 2B . At this time, the raw materials flow in the loop indicated by arrows  121 ,  122  in  FIG. 1 . Whether the contents of these loop-like liquid sending lines have been completely replaced with the raw materials is detected by the raw material flow rate detecting means  111 ,  112  for detecting the flows of raw materials. 
     When the content of the branch tank  105  is replaced with the liquid raw material (the content of the branch tank  106  is similarly replaced with the liquid raw material), the following procedure is taken in turn: all the control valves  118  in the liquid sending lines  201  to which the microreactors are connected are opened and all the control valves  113 ,  114  in the liquid sending lines  202  communicating with the raw material tanks are closed. As illustrated in  FIG. 2C , the raw material in the branch tank  105  is sent to the microreactors by this operation. 
     When a certain flow rate is given in this operation in the second stage, the contents of the branch tanks  105 ,  106  and the lines up to the microreactors  107  and the recovery tank  108  are also replaced with liquid raw material. In the operation in the second stage, each raw material must be sent to each microreactor at a predetermined flow rate for substitution. If this flow rate is low, it is suspected that the content of any liquid sending line is not completely replaced with liquid raw material. In this case, the content of which liquid sending line has not replaced is detected by the distribution flow rate detecting means  117  for monitoring flow rates provided in the respective liquid sending lines  115 ,  116 . When it is detected that the content of one liquid sending line has not been replaced, the control valve in that liquid sending line is kept open and all the other control valves are closed. Thus the pumping quantities of the pumps are reduced to a quantity equivalent to one line and the liquids are sent. The content of the liquid sending line is thereby replaced with liquid raw material. When there are multiple lines the contents of which could not be replaced with liquid raw material, the above operation is repeated line by line. 
     Description will be given to the stabilization of liquid sending state in a reaction operation mode in which reaction operation is carried out by the microreactors after the contents of the liquid sending lines are completely replaced with liquid raw material. If the individual liquid sending lines  115 ,  116  are ideally fabricated in the same shape, every liquid sending line has the same flow path resistance; therefore, raw materials sent from the pumps are equally distributed. In reality, however, it is almost impossible to achieve this state. In the present chemical plant (hereafter, referred to as plant), different kinds of raw materials are mixed together to produce a substance different from the original raw materials. Therefore, the physical properties of liquid may change in a microreactor and it is difficult to manage distribution at an equal flow rate and chemical reaction in each reactor only by the geometrical conditions of the flow paths. Further, there is a possibility that a reaction product is deposited and accumulates in a flow path and it causes change in the flow path resistance of the liquid sending line. To cope with cases where imbalance of flow path resistance is produced among piping flow paths from any cause as mentioned above, this embodiment adopts the control law for parallel piping system flow rate described below. 
     When it is desired to equally send raw material to each microreactor in the plant control system illustrated in  FIG. 1 , the quantity of liquid sent from a pump is determined by the expression of (a desired flow rate to be given to each liquid sending line)×(the number of microreactors arranged in parallel). If each liquid sending line is perfectly equal in flow path resistance, as mentioned above, raw material can be equally sent to each microreactor just by controlling this quantity of liquid sent from a pump. However, when imbalance is produced in this flow path resistance from any cause, it is required to take the following measure: the control valve provided in each liquid sending line is adjusted to control the flow rate of raw material flowing through the liquid sending line to a desired flow rate d. 
     Letting the number of microreactors arranged in parallel be n, the plant can be handled as a multiple input/output system having n inputs (the aperture of each control valve) and n outputs (the flow rate of each liquid sending line). Evidently the plant is an interference multiple input/output system. That is, the plant is a system in which the operation of some control valve has influence on not only the corresponding liquid sending line but also the flow rates of the other liquid sending lines. Therefore, if imbalance in flow path resistance is produced among the piping lines from any cause, it becomes more difficult to adjust each control valve so as to compensate this imbalance with increase in the number of the microreactors arranged in parallel. 
     With respect to this embodiment, to cope with this, the following control law has been devised: a control law in which this multiple input/output system is rendered of apparent non-interference type and further a feedback control system is built in each flow path. 
     Letting the aperture of the n control valves in the plant be u (u 1 , u 2 , . . . , u n ) and the resultant flow rate of each liquid sending line be q (q 1 , q 2 , . . . , q n ), the block diagram of this input/output system is expressed as in  FIG. 3A . The transfer characteristic P(s) corresponding to the system represented by this block diagram can be generally represented in the form of such transfer matrix as indicated by  FIG. 3B . Though the element p i,j  of a transfer matrix may be a real number sometimes, in general, it is represented by a transfer function. 
     To render the plant of an interference n-input/output system having the input/output characteristics shown in  FIG. 3A  of apparent non-interference type, such an n-input/output non-interference property providing device  301  as shown in  FIG. 3C  is designed. The transfer characteristic of this non-interference property providing device  301  is given by such a transfer matrix shown in  FIG. 3D . This non-interference property providing device  301  is characterized in that: the elements of a matrix is a design parameter of the non-interference property providing device  301  and it is designed according to a characteristic of a plant desired to be rendered of non-interference type, that is, a P(s) matrix. This design method is a characteristic point of this embodiment. 
     When the non-interference property providing device  301  is connected to the input side of the plant as shown in  FIG. 4A , the output q 1 , q 2 , . . . , q n  to its input x 1 , X 2 , . . . , x n  is described by the relation between the left side and the second side of the matrix equation shown in  FIG. 4B . Reference numeral  401  denotes the multiple input/output system with the non-interference property providing device  301  is connected in series on the input side of the plant. 
     Here, a requirement that inputs x 1 , x 2 , . . . , x n  should respectively have influence only on corresponding outputs q 1 , q 2 , . . . , q n  is imposed. (A condition that x 1  should have influence only on q 1 , x 2  should have influence only on q 2 , . . . , x n  should have influence only on q n  is imposed.) That is, a requirement (condition) that q i =p ii ×x i  (i=1, 2, . . . n) is imposed. In this case, it is required to satisfy the relation of diagonal matrix and equal sign indicated as the relation between the second side and the right side in the matrix equation shown in  FIG. 4B . The relation between inputs x 1 , x 2 , . . . , x n  and apertures u 1 , u 2 , . . . , u n  is of interference multiple input/output. However, that between inputs x 1 , x 2 , . . . , x n  and outputs q 1 , q 2 , q n  is of apparent non-interference type. 
     When the relation between the second side and the right side of the matrix equation shown in  FIG. 4B  is represented by matrix signs P and G and input vector x, the equation shown in  FIG. 4C  is obtained. When both sides of this equation are multiplied by the inverse matrix of P from the left side, the equation is transformed into the equation shown in  FIG. 4D . To make the equation in  FIG. 4D  identically hold to an arbitrary input (x vector), the coefficient matrixes on both sides of the x vector must be equal, that is, the relation shown in  FIG. 4E  must hold. The transfer characteristic matrix (G) in  FIG. 4E  is comprised of the product (right side) of the following: the inverse matrix (P −1 ) of the transfer characteristic matrix of the plant that indicates the relation between the amount of adjustment of raw material to each reactor and a distribution flow rate; and an appropriate (proper) diagonal matrix. 
     That is, the right side of the equation in  FIG. 4E  is comprised of the product of the inverse matrix of P(s) matrix and a diagonal matrix and both relate to the transfer characteristic of the plant. For this reason, a concrete transfer characteristic of the plant desired to be rendered of non-inference type is obtained. Further, an appropriate control condition pertaining to control specification is given and the inverse matrix of the P(s) matrix is uniquely determined. Thus the non-interference property providing device  301  for rendering the input/output characteristics of that plant of non-interference type is designed by the equation in  FIG. 4E . 
     The multiple input/output system  401  of the plant in  FIG. 4A  with this non-interference property providing device  301  connected thereto is equivalent to  FIG. 4F  in transfer characteristic. Thus a non-interference multiple input/output system  402  in which inputs x 1 , x 2 , . . . , x n  respectively have influence only on corresponding outputs q 1 , q 2 , . . . , q n  is obtained. 
     A feedback control law is applied to the plant rendered of non-interference type by connecting the non-interference property providing device as mentioned above. Thus the automatic control system in  FIG. 5A  is obtained. When a target flow rate r 1 , r 2 , . . . , r n  of each liquid sending line is determined, in this automatic control system, they are automatically followed. A target flow rate r of each liquid sending line is issued as a request from the user console  120  illustrated in  FIG. 1 . 
     Zone  501  encircled with broken line indicates the input/output characteristics of the plant rendered of non-interference type by the non-interference property providing device  301  described with reference to  FIGS. 4A to 4F . On the input side, there are connected controllers  502  for apparently independently feedback-controlling each line rendered of non-interference type and each flow rate q 1 , q 2 , . . . , q n  as output is fed back. Then the plant is controlled by the controllers  502  based on the differences between them and the target flow rates r 1 , r 2 , . . . , r n . 
     Therefore, a stable control system is implemented by properly designing the controller  502  according to the characteristics  501  of the plant rendered of non-interference type. In this stable control system, even though some sort of disturbance is produced in the flow rate of each microreactor, its target flow rate r 1 , r 2 , . . . , r n  is automatically caused to follow a target value. When this plant control system is depicted in the form of block diagram using the original transfer characteristic P(s) of the plant, it is depicted as in  FIG. 5B . The zone  503  encircled with alternate long and short dash line is equivalent to the control device  119  of this plant control system. The control device  119  in  FIG. 1  is provided therein with the calculation function indicated by reference numeral  503  in  FIG. 5B . 
     For this plant control system, the following is important: how accurately the transfer characteristics of the plant to be controlled in output response (the flow rate of each liquid sending line) q to input (the aperture of each control valve) u should be modeled and described in the form of transfer matrix P(s). When this transfer matrix P(s) can be accurately obtained form experiment or the like, the proper control system is built by the procedure described with reference to  FIG. 4A  to  FIG. 5B . 
     However, when a plant is continuously operated for a long time, its input/output characteristics are varied with deterioration or aging of the equipment (also including microreactors, control valves, piping, and the like) comprising the plant. This turns it into an interference multiple input/output system. This situation cannot be designated as disturbance anymore and it must be coped with as expected input/output system parameter fluctuation in the plant. To cope with this, the control apparatus  119  of this plant control system illustrated in  FIG. 1  is provided with a system adjustment unit  119   a , described next in relation to a second embodiment, having a system identification function for the plant to be controlled. 
     Second Embodiment 
     Description will be given to an example of system identification by a system identification unit  119   a  with reference to  FIG. 6 . In the second embodiment illustrated in  FIG. 6 , the control valves  118  on the input side of the individual liquid sending lines are sequentially (in a time sharing manner) opened (fully opened) only for a short time one by one to send liquid. All the responses of the flow rates q of the individual liquid sending lines are simultaneously measured by the distribution flow rate detecting means  117 . This measurement is periodically carried out. In  FIG. 6 , reference codes u* i  to u* n  indicate an open signal of the input of each liquid sending line and q* 1  to q* n  indicate the flow rate of each liquid sending line. It is understood from the result of measurement shown in  FIG. 6  that liquid sending through each liquid sending line has influence on the flow rate q of each of the other liquid sending lines and this plant has been turned into an interference multiple input/output system by deterioration with age. 
     In this embodiment, a new parameter of P(s) is identified from the input/output characteristics of the system P(s) indicated by the values of opening u and flow rate q obtained by the above measurement by proper arithmetic processing. Then the control system is updated according to the obtained parameter by the procedure with reference to  FIG. 4A  to  FIG. 5B  and the updated input/output characteristics (new transfer characteristic matrix) is set in the control device  19 . Thus the optimum plant control system is implemented against the above-mentioned parameter fluctuation. The foregoing is implemented by the system adjustment unit  119   a  in the control device  19 . 
     This system identification function is also applicable to automatic anomaly and failure diagnoses on a plant. The above system identification is carried out at an appropriate time even when the plant is in operation. If a parameter has extremely fluctuated as compared with past system parameters, it is handled as an anomaly or a failure of the plant. A criterion for determining a plant to be normal when the amount of fluctuation in parameter is within some range and to be anomalous or faulty when it is out of the range must be established in some way by the designer of the plant, needless to add. 
     Up to this point, embodiments of the invention have been described with the simplest system in which two different kinds of raw materials are mixed or reacted with each other in one step taken as an example. The idea for parallel piping flow rate control is basically identical even in a more complicated plant system in which multiple kinds of raw materials are mixed or reacted with one another in multiple steps and the invention can be applied to such plant systems. 
     According to the invention, as mentioned up to this point, the following is implemented in a chemical plant whose throughput has been scaled up by numbering-up (parallel piping connection) of microreactors: disturbance (change) in the raw material flow rate of a microreactor can be prevented from having influence on the other microreactors; therefore, the flow rate control on each microreactor can be easily stabilized. An unmanned stable chemical manufacturing process can be achieved by providing the following function: a function of automatically detecting disturbance (change) in raw material flow rate due to the adhesion of an air bubble or the deposit of a reaction product in a microreactor arising from deterioration with age and automatically restoring or adjusting the state of liquid sending. 
     The invention is not limited to the above-mentioned examples and it will be understood by those skilled in the art that the invention can be variously modified without departing from the scope of the invention described in the claims.