Control system can be apply to meet the needs of 3 groups, namely:
- Pressing the influence of external disturbance.
- Ensure the stability of a chemical process
- Performance optimization of a chemical process
The variables involve in the process of factory operation is F (flow rate), T (temperature), P (pressure) and C (concentration). These variables can be categorize into two groups, namely the input variables and output variables. The input variable is a variable that is mark the environmental effects on the chemical process of the intend recipients. This variable is also classify into two categories, namely:
- The manipulate variable (adjustable), if the price of these variables can be set freely by the operator or the mechanisms controlling.
- Disturbance variable, if not price can be set by the operator or control system, but it is a distraction.
While the output variable is the variable that indicates the effects of chemical processes on the environment are classify in two groups:
- The output variables measure, if the variable can be known with direct measurement
- Output unmeasured variables, if the variable cannot be known with direct measurement
The Design Elements of the Process Controller
Design of the control element should correspond to the needs of the desire control and working within the process control of the factory. To achieve that goal to note the steps in designing control systems in the business of designing a control system that can meet the needs of a chemical process, there are several essential elements and basic considerations to be aware of. These elements are:
- Definition and objectives goal setting/control (control objective definition)
- Determination of variables should be measured (measurement selection)
- What determination of variables that will be manipulated (the manipulated variables selection)?
- The selection of configuration control (configuration control selection)
- Design of control system (controller design)
The Definition of Control Purposes
In defining the purpose of control is to note a few things petting which is a basic implementation of principle process control at the factory. The main principle of the application of process control in the factory is to ensure the performance of a chemical process, ensuring the stability of a chemical process, and suppress the external interference. This basic principle should be include in the definition of the purpose of controlling either one or a combination of all three.
At the beginning of the design, target control (control objectives) is define qualitatively, further this goal be quantify in the form of the output variable. For example, for CSTR reactor system one controller is perform with the purpose of usage control (control objectives) as follows:
- Qualitative: guarantee the stability of the temperature inside the reactor (assume to be the same as the temperature of the reactor output) in a State of steady state unstable.
- Quantitatively: keep the temperatures (variable output) does not fluctuate more than 5% of the price of the face.
The selection of the variables should be measure
Some of the measurement variable should be done so that the performance of the operation of the plant can be monitor, there are several types of measurement variables that can be apply to the process control:
Primary Measurement
Whenever possible preferably on process control variable price into objective control should be measure/monitor. How to become a process variable measurement control objective control directly is call a primary measurement? For example, in the system mixer tank stirred control objective is maintaining the T and h liquid in the tank at the price of T = Tsp (sp = set point) and h = hsp. Therefore, the first attempt to do is to install gauges to be able to observe the value of T and h liquid in the tank directly, i.e. with use it thermocouples for measurement of differential pressure and T cell to measure h.
Secondary Measurement
In certain cases, a variable which is a control objective cannot be measure directly (unmeasured output). In cases with a control objective that cannot be measure directly, the other variables should be measure which belongs to the measure variable and can be correlated through a certain mathematical relationship with unmeasured output who want to control.
Measurement of external disturbance
Disturbance Measurements before the variables enter into the process can be very profitable, because the measuring results can provide information about the behavior of a process that will case. Such information can be use to specify the action to be taken when control using feed forward control systems.
The selection of variables that are Manipulated
In chemical processes, generally there are several input variables that can be set freely. To select which variables will be manipulated, to consider the effect of action taken on quality control. As an example of the control of height liquid in the tank, reactor, or column distillation can be done by setting the flow rate of the incoming and outgoing flow rate of the fluid.
The selection of Configuration Control
is a control structure of the Configuration information that is used for connecting the variables measured variables that will be manipulated? For example, the temperature control and the height of the liquid column distillation, reactors, mixers, and other devices have several alternative governing system configurations. Difference that can be observed in the temperature control system and control system of liquid height occur because (1) there may be differences of the variables measured, but the results of the measurements are used to manipulate the same variable, or (2) the variables are measured the same, but the results of these measurements are used to manipulate different variables.
There are 3 types of configuration control, among other things:
Feedback control configuration
this configuration is directly measured variables controlled to set the variable price are manipulated. The purpose of this control is to maintain a variable that is controlled at the desired level (set point). Most of the instrumentation on the making process of formaldehyde and hydrogen peroxide-based methanol with enzymatic reactions using feedback control configuration, ranging from temperature control, altitude control, control the difference in pressure, and pressure control.
Feed forward control configuration
the configuration control system utilizing direct measurement of feed forward on disturbance to set variable price will be manipulated. The purpose of control is to maintain the output variable controlled on expected value.
The Inferential Control Configuration
The configuration of inferential control system utilizing a secondary output measurement results data (secondary measurement) to set the variable price will manipulate. This is done because output will be controlled variable cannot be measured directly. The purpose of this control is unmeasured variables maintain the output at a rate/price set at set point. Measuring instrument using the measured variable price (measured output) were detected in the equation of mass and energy balance sheets that can represent the process into a specific mathematical equation can be calculated by computer output unmeasured variables that would like to control. The results of these calculations by instrumentation control is then used to set the variable prices are manipulated.
Inferential control configuration is used in the control of the composition of output flow on distillation column and every tank mixer stirred. Composition analysis is not done directly with composition analyzer because the price of the tool, and the tool is very analytical so that its capacity is small and does not fit the scale applied in the factory. The composition of the distillation columns and mixer flow should always be controlled due to the very tight market demand products formaldehyde and hydrogen peroxide according to specifications.
Design of control system
control system (controller) is the active element in the control systems which receive information from measurements and make the appropriate actions to set the price of the manipulated variables. Manipulated variables settings rely heavily on control law applied automatically on the controller. Some common control law applied in control systems:
- Use of proportional controller (P-controller) where the value of the output of the P-controller will be comparable against error.
- The use of proportional-integral controller (PI-controller) where the value of the output of the PI-controller will be comparable against error plus a factor multiplied by the value of the integration error as a function of time.
- Proportional-integral-derivative controller where the value of the output of the PID-controller will be determined by a constant of proportionality constant linking error against output plus a factor multiplied by the value of the integration error as a function of time and then added a factor multiplied by the value of the differential (gradient/slope) error as a function of time.
The use of the Digital computer in the Control Process
in the aspect of controlling the entire factory not only involves a single process unit, such as CSTR, tank stirred, column distillation. In fact, the production process of the product from feed stock with a certain reaction is compose of many units sailing is associate with the presence of the flow of materials (material) and energy from one unit to another unit. On the chemical processes will develop the characteristics of things do not happen on an operation single unit process only. Computer technology advances very rapidly with the increasingly affordable price make this device widely use for control in chemical process. Instrument control on large, modern factories are generally design using computer controllers. Some specific applications of computers for process control are as follows:
The Direct Digital Control (DDC)
digital Computers can be use to simultaneously control multiple outputs. On the main control system (supervisor controller) there is one computer processors for controlling and operating processes. So all the data is collect in a single computer unit. The computer is use to change the value of the set point according to the price parameter local controller. Local controller serves as the digital signal is apply to the Direct Digital Controller (DCC). Inter phase input/output will generate information to supervisors in the form of a computer on the local control loop used computers to generate the value of the set point on the local control loop. Computer minim measurement results directly from the process, and then calculating the manipulated variables based on control law which has been program and store in its memory.
Manipulated variables are then apply back into the process by using the elements of the final controller such as shellfish, pump, compressor, switch, and so on. Thus the computer and process bridge by device-a hardware device use to obtain good communication between computer processes. DDC is generally use for units in such a limit scale to one production unit, or use for a unit operation with a production unit.
Distributed Control System (DCS)
use of control systems by using one computer to control a unit operation will be more easily apply. However, the system control supervisor will experience difficulties if apply on a complex because units will be produce of a control and the operation of a very complex and complicate. The latest process control method in the factory is a Distribute Control System (DCS) that immediate success is sought after for commercial-scale when it is first launch.
DCS consists of some microprocessor that are all in a digital communication network often known with the data highway. Process control objective is get the optimum process performance. The human operator is often difficult or cannot find the best plant operation setting so that the operating costs can be press is minimal as possible. This is due to the high complexity of the chemical plant that will be controlled. In this case program intelligence of the computer can be use to analyze the situation and make a proposal for the setting of process operation at best. On supervisory control of this computer, coordinate the activities of several loop DCC.
In this system a single main computer (the supervisor computer) divides the work of controllability on some computers working as local DDC. DCS system advantages compare to DDC is the system DCS allows the work area or a local one DDC still work and can be controlled even though there is a certain location or the unit does not operate. Conversely, a lack of unit DCS than DDC system investment costs is very large because the DCS need computer controllers.
Scheduling Computer Control (SCC)
the possibilities of computer use is to set the scheduling of operation of a chemical factory. Market conditions change from time to time will cause management need to be continuously changing operational scheduling. The factory, such as reducing the production time to prevent stack up products (over stock). The addition of production while increasing needs, and others. Inform decisions can be taken with the help of the digital computer. Which will then be communicating these decisions decision with supervisory computer controller. Who then implement those decisions through DDC-DDC that relate directly to the process.
Process Control System Hardware
System configuration on each controller can be distinguish each hardware element as follows:
Chemical process
chemical process equipment represent the process use and the processes/ operation either in chemistry or physics that occur in the equipment.Measuring Instruments or Sensors, Measuring Equipment
Sensors use to measure disturbance, measuring the control output variables, and measure the secondary output variables. Measuring equipment/sensors is a source of information to identify the things that are going on in the process. One key requirement in the selection of the sensor is a sensor measurement results must be transmitted easily. Examples of control instrument use in factory formaldehyde and hydrogen peroxide. It is the thermo couple, venture meters, and composition analyzer.Transducers
Some measurement results cannot be use uptick control purposes before being convert into physical quantities that can be easily transmitted as electrical voltage. Transducer is a device us to convert measurement results into quantities which are transmitted.Amplifier transmission lines and
Transmission lines is a medium to carry the signal of the measurement results of the measuring instrument to the controller. In many cases the resulting gauge signals are too weak to be transmitted so that the signal must be amplify beforehand with the amplifier.Governing Element
Element controller is hardware that has intelligence. This device receives information from measuring instrument and decide what to do.The elements of the Final
Control element of the final controller is a hardware device that carries out an action that instruct the controller. Elements of the final controller apply in the design of this plant is the control valve. That opens and closes to a certain degree by a decree of the controller.Element note taker
Note taker Element is part of the control system which keeps track of all the variables. So that ongoing process behavior can be demonstrate visually.
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