Modern Procedure for Crude Oil Temperature Control with Programmable Logic Controller

Transcription

1 Modern Procedure for Crude Oil Temperature Control with Programmable Logic Controller CRISTINA POPESCU*, GABRIELA BUCUR, ADRIAN GEORGE MOISE, OTILIA CANGEA Petroleum Gas University of Ploiesti, 39 Bucuresti Blvd., , Ploiesti, Romania This paper describes and proposes a modern solution for controlling the temperature of crude oil atmospheric distillation in which, a programmable logic controller (PLC) was used instead of the classic proportionalintegral-derivative (PID) controller. The authors present in details the design of PLC programme together with simulation diagrams that show how the designed control system works. Programming the PLC is done by using a function block diagrams method. Keywords: programmable logic controller (PLC), control system,function block diagrams, crude oil Crude oil is a complex mixture of hydrocarbons with close boiling points and can be separated into two or more components with narrow boiling range, which are called oil fractions. By the distillation of crude oil using a process that involves one or more columns, some oil fractions with well specified distillation temperature (gasoline, white spirit, petroleum, diesel) are obtained, as well as naphtha as residual. Since the fractional distillation is achieved at a pressure close to the atmospheric pressure, the process is called atmospheric distillation (AD). Being the first from a long series of physical and chemical transformations to which crude oil is processed, it is also called primary distillation. For this purpose, the heating of crude oil in tubular furnaces up to temperatures of C is required [1-3]. Atmospheric distillation of crude oil is made in various fractional columns types, denoted as U, A or R types columns. In crude oil distillation plants either a single column with a vaporizer or multiple columns are used taking into account the light fractions and corrosive content of the crude oil (mostly sulfur compounds). By using a single column for continuous atmospheric distillation the crude oil coming from the electrical desalter unit is preheated by heat exchange with the side fractions extracted from the column. Then, it is sent into the tubular furnace where is still heated up and partially vaporized. After that, the oil enters the vaporization section of the atmospheric distillation column. The main components of an atmospheric distillation plant are (fig. 1) [4] the fractional column, the tubular furnace, the condensing and heat exchange section, strippers and pumps [4, 5]. In a U-type column, the taking over of the heat released by oil products is done in order to cool down the temperature from the input to the output of column with a cold reflux introduced above the first tray in the top of the column. The column is supplied with crude oil that is partially vaporized in the vaporization section. The lateral products extracted from the trays in the liquid state are passed through the strippers in order to remove light products involved in extracting the fraction out of the column. The light fraction extracted from the strippersis reintroduced into the column at a tray higher than the extraction tray. After cooling down the stripped products are sent into the tanks by exchanging heat with crude oil. Fig. 1. A classical continuous distillation plant. [4] TC-temperature controller, PC-pressure controller, LC-level controller, FC-flow controller, TI-temperature indicator * cristinap@upg-ploiesti.ro REV.CHIM.(Bucharest) 67 No

2 Out of the stripping section of the column, naphtha is obtained [6, 7]. In the classical process automation, the output temperature of the product heated in the tubular furnace is measured at the input of the AD column with a basic sensor, an adapting/converter/transmitter element and a display to show the measurement result. Usually, thermocouple sensors are used but thermo-resistors with a measurement range of 200 to 600 o Ccan be used as well. Controlling the output temperature of the product is usually done with closed loop systems based on calculation of the error value between the reference temperature and the current temperature. Some disturbance consequences (such as flow variation or heated product temperature variation) are compensated by using open loop systems that act on certain disturbances [8]. Temperature control systems can be simple, either with a single electrical circuit or they can include two or more circuits. In figure 2 a version of a closed loop control system with one circuit is shown. The product temperature control at the furnace output is done based on fuel flow modification. Fig. 2. A closed loop control system for measuring the product temperature at the tubular furnace output TT-temperature transducer, TRC-temperature controller and recorder, IP/Y-current-pressure converter, Top-product output temperature This structureis chosen for tubular furnaces working with a constant load, without any disturbances from the furnace focus or the product to be heated. As actuators, pneumatic control valves are used which include closed loop position control systems. As controllers, PI or PID electronic controllers are used [9, 10]. Due to the special significance of the energy economy, the most important optimization criterion for heating and vaporization furnaces without chemical reactions is the maximum furnace efficiency. Therefore some operating restrictions concerning the flow rate, temperature, pressure or vaporization fraction have to be added. All these parameters should be maintained in certain limits. For instance, the monitored values ofproduct output temperature T op and product flow rate Q p can be expressed as: T op = Q op ref ± T (1) Q p = Q p ref ± Q (2) where T op ref and Q p ref are the reference values of the heated product output temperature and product flow rate, while ÄT and ÄQ are the errors of these variables, respectively [9,11,12]. Experimental analysis of different types of tubular furnaces shows that dynamically they behave similarly,and in consequence their forms and dimensions affect only quantitatively the response commands and disturbances. The time duration for the transient processes varies from 5 min to 50 min, depending on the furnace winding length and mass and, somehow, on the area and weight of the refractory lining. The programmable logic controller (PLC) is specially designed to be used in process control. PLCs are frequently used in industrial process control [13] because they are easy to set up and programmed, their behaviour is predictable and they are robust enough to work in bad environmental conditions as the presence of dust, moisture, electric noise etc. (fig. 3). In general, the PLC controller is programmed by using a user-friendly method and combining some special and dedicated function blocks. The current task is divided into stages that can be represented by a number of function blocks. Function blocks programming simplifies the application representation but also ensures a complete control of processes. The algorithm can be implemented in a few simple steps, and even complex tasks can be represented. In order to easy use of function blocks, the PLC has been pre-programmed to perform certain tasks, offering flexibility and the possibility to be improved [13]. It is relatively easy for a user to build a complex circuit. The PLC will acquire and process information and generate the necessary control signals for the current application according to programmed algorithm. Each function block gives the user the possibility to access the specific controls and to customize each program for different applications. The function blocks are assembled together to form a circuit, by using the function blocks diagram (FBD). This paper proposes a modern procedure for controlling the product temperature at the output of a tubular furnace, T ep, by using in the electrical scheme a programmable controller instead of a classic controller (fig. 3). The programmable controller, although simple, has the advantages of multiple combinations that can be created between functions blocks and easy to be handled and interconnected to design the program. The purpose of this application is to develop a system for measuring and controlling the temperature of crude oil, following a previously established diagram. Fig. 3. PLC connection into the processing work [13] REV.CHIM.(Bucharest) 67 No

3 Fig. 5.The used electrical diagram with PLC Fig. 4. The desired temperature profile introduced in PLC during monitoring Experimental part For the purpose of this paper, the U type column was chosen.the desired temperature time profile is shown in figure 4. The general electrical scheme is presented in figure 5. In this electrical connection, T ref is the oil reference temperature, AC is the actuator (control valve) and Pt100 is the thermo-resistance temperature sensor.the temperature sensor and the actuator are connected to the PLC via an analog input module and an analogue output module, respectively. The corresponding electrical diagramsare presented in figures 6 and 7, where CPU is Central Processing Unit., In 1 is the input number 1 and Out 4 is the output number 4. In our experiments a MITSUBISHI AL2-14MR-D type programmable logic controller is used.the operation logic is sustained by a program implemented in a PLC which generates control signals for obtaining, maintaining and monitoring the temperature. The program was built acording to the following rules: (a) when the controller starts the current status is displayed (ambient temperature); (b) if the controller is switched into the RUN mode, the program starts running, and constantly compares the enclosure temperature (the signal received from the transducer) with the reference temperature, which is the first level of the required temperature. The message heating is displayed during this operation. When the first level of the prescribed temperature is achieved, the controller disconnects the heater and maintains the temperature at the specified value. The system has high thermal inertia (high transient regime) which means that, after reaching the set temperature and after disconnecting the heater, the temperature will continue to increase; that is why the cooling system mentioned above is necessary. The first level temperature is maintained within a well defined period of time (15 min). Then, the program takes into consideration the second reference. The second temperature level is maintained for the same period (determined by the program) after which the controller switches back to the initial state (standby) waiting to receive new commands to perform a new heating cycle or to go into the STOP state. In case of an equipment failure, the controller can use the program functions WARNING or AUTO-OFF or, the program can be interrupted by an operator by switching it into the STOP state. In order to programming this application, the authors of this paper developed and used the control diagram shown in figure 8. The output Out 1 for the heating resistance command circuit is relay type. The power supply provides a 24 Vcc stabilized output voltage required to supply the controller and the adapter used. Terminal A is connected to the minus terminal, which leads to an entry connection of PNP type or voltage difference to 0. A converting resistor CR is required to convert the signal from 4-20mA to a voltage of 1-5V, required to the analogic gate input (IN).This signal is generated by intelligent adapter Conv R-I type Rosemount HART 248 which converts the variable resistance generated by Pt 100 sensor into unified signal. To achieve voltage requirements for the PLC, a stabilized voltage source Alpha Power was used. The sensor waspt 100 ROSEMOUNT type. For conditioning the sensor signal a large variety of equipments is used. For this application a 248 HART type R-I ROSEMONT adaptor has been used to convert physical variables into conditioned and standardized output signal. Analog signal of 4-20 ma amplitude allows the detection of stoppage situation Fig. 6.Connecting an analog input module using three wires. Only one channel is shown, this one corresponding to a Pt100 sensor. Fig. 7.Connecting an analog output module. Only one channel is shown, this one used to control the actuator REV.CHIM.(Bucharest) 67 No

4 Fig. 8. Wiring diagram control application. SVS stabilized voltage source; PLC programmable logic controller; Conv R to I resistance to current converter; CR conversion resistance, from 4-20 ma to 1-5V; Pt100 temperature sensor Fig. 9. The experimental stand for testing the monitoring and control Fig. 9.The experimental stand for testing the monitoring and control (missing current or a current below 4 ma).the DC transmission, both for analog and digital signals, allows an increased immunity level at disturbance. Information at the receiver input is not affected by the loss of voltage on cables and connectors, nor the parasite thermocouples distributed on different points on the signal path. For analog signals, transmissions over a distance of maximum 600 m can be realized. The power supply is connected through a pair of wires used also to transmit the measurement result, thus creating the current loop. The lowest value of the current is 4 ma and the head of scale value is 20 ma, so that a variation from 0 to 100% of the measured parameter corresponds to a 16 ma variation of the current. This device is also known in technical terms as adapter, or alternative as adapter with two-wires conduction, resistance-to-current converter, voltage-tocurrent convertor etc. The ROSEMOUNT 248H temperature adapter (transmitter) is an universal intelligent adapter connected by a two wires HART protocol to a communicator from the same HART series or a PC computer, and is built to be assembled directly on top of the Pt 100 ROSEMOUNT sensor. The supply voltage for the adapter is between V d.c. voltage. It ensures a linear characteristic of the output if it is maintained in this range and makes possible the HART protocol adapter configuration when it is connected to the HART Communicator. The dedicated communicator with ROSEMOUNT 248H adapter uses a specialized software; it can set different ranges for operation temperature, voltages and output levels, all of which being possible using only two wires (the supply ones), the adapter generating a digital signal overlapped with the supply voltage. For testing the proposed control system, an experimental platform was built at the Petroleum-Gas University of Ploiesti. It has the same configuration as the one described in this paper with only one exception: the process tubular furnace was replaced by a thermal enclosure. Figure 9 shows the component of the experimental platform. Fig. 10. Window screen used to setup controller properties Results and discussions The PLC program was realized by following the described algorithm and the values for the temperature profile were introduced according to figure 4. The use of logical function blocks is the easiest way during program running, because certain parameters can be set up when implementing the multitude of logical function block instead of using a specialized regulator. Programming the application by using function blocks diagrams (FBD) When the executable AL-PCS/WIN-E is launched, the user can choose options in order to realize a new program, to connect the PLC and check its functionality. Some icons are disabled, but they will become active in simulation mode or when MITSUBISHI 14MR-AL2-D programmable logic controller will be connected to the PC. The effective PLC programming is done as follows: - open the File menu and choose the option New; - the display shows the controller type selection window; check AL2 Series together with 8 Input and 6 Output (fig. 10), then press OK; - a new window which represents the workspace for the new program configuration will appear. The FDB basis provides the platform on which the alpha series program will be built. This FBD basis is constituted from an enlarged green screen used for cabling devices, a title box and, along the right and left hand sides, input/output vertical rectangles. Programmed components are located in the cabling area or in rectangles and they are connected; - after that, from the new main menu bar which will be turned the Com function has to be chosen, in order to select the port connection between the controller and the PC; for our purpose, Com 1 was chosen; - the selected items required to build the program are dropped on the displayed work space as follows: analog input must be selected from the Input group and dropped on the I01. In order to configure the program, the necessary functions must be chosen: GAIN, COMPARE, ADD, SUB, SET RESET, DELAY, DISPLAY, OR, AND, DISPLAY FB CONTROLER, HEATER and dropped on the FBD base, as shown in figure 11; REV.CHIM.(Bucharest) 67 No

5 Fig. 12. The final form of the simulation program for PLC running Fig. 11. The FBD basis with all the functions selected and connected - the wiring analyzer is used to connect the components in the FBD basis space; - after connecting all the function blocks and choosing the appropriate parameters for them (parameters resulting out of the controlled process characteristics), the program is ready to be run and to simulate the process (fig. 12). - after checking out the accurate program functioning and making the corrections of possible errors, the program is transfered into the PLC and saved. The running of the program can be described indicating in particular the role of each block or group of blocks that interact (fig. 12): - Using analog input gate I01, the signal received from the temperature transducer is continuously monitored and applied to an amplifier block, which can adjust the signal within certain limits; - The processed signal is applied to a comparator and to a display where the current temperature value appears; the comparator also receives the reference (a fixed value) and compares it permanently with the value received from the transducer: - if Tref > Tcrt, the O01 output (a relay type) is connected, and HEATING is displayed; - if Tref = Tcrt, the O01 output is switched off, and the delay function which maintains the reference set for 180 swill automatically starts; - if Tcrt is with 1 o C higher than Tref, the cooling down is ordered using O02 output by startingthe pump and solenoid valve, and the command is maintained until the following equality Tcrt = Tref -1 is reached; - the trigger block setto: ON = Tref + 1 and OFF = Tref- 1 is used to ensure a hysteresis around Tref = 2 o C; - when 180 stime has elapsed during the run, the reference 1 is disconnected using SET/RESET function and the second reference generated is joined to the another Compare block, where the signal is compared permanently with the current value generated by the transducer. Conclusions In this paper the authors have succeeded to build a performant temperature closed loop control system useful for industrial distillation processes in oil and petrochemical industries. In order to optimize the control of temperature, a programmable logic controller (PLC)was chosen. The temperature limits can be established and implemented in a program written with dedicated software. With the help of the controller, it was practically proved that the considered process is fully automatized, human intervention being necessary only for program supervision, start-stop and tracking. Heating temperature values can be changed simply by using the dedicated keys via software, while the holding times for these temperatures can be established by a suitable setup in the implemented software. The main advantages of the used PLC are its small dimensions and programming simplicity, as well as its facilities. REV.CHIM.(Bucharest) 67 No

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