REV.CHIM.(Bucharest)♦ 67♦ No. 4 ♦ 2016 http://www.revistadechimie.ro 785

Modern Procedure for Crude Oil Temperature Controlwith Programmable Logic Controller

CRISTINA POPESCU*, GABRIELA BUCUR, ADRIAN GEORGE MOISE, OTILIA CANGEAPetroleum Gas University of Ploiesti, 39 Bucuresti Blvd., 100680, Ploiesti, Romania

This paper describes and proposes a modern solution for controlling the temperature of crude oil atmosphericdistillation in which, a programmable logic controller (PLC) was used instead of the classic proportional-integral-derivative (PID) controller. The authors present in details the design of PLC programme togetherwith simulation diagrams that show how the designed control system works. Programming the PLC is doneby using a function block diagrams method.

Keywords: programmable logic controller (PLC), control system,function block diagrams, crude oil

* email:cristinap@upg-ploiesti.ro

Crude oil is a complex mixture of hydrocarbons withclose boiling points and can be separated into two or morecomponents with narrow boiling range, which are calledoil fractions. By the distillation of crude oil using a processthat involves one or more columns, some oil fractions withwell specified distillation temperature (gasoline, whitespirit, petroleum, diesel) are obtained, as well as naphthaas residual.

Since the fractional distillation is achieved at a pressureclose to the atmospheric pressure, the process is calledatmospheric distillation (AD). Being the first from a longseries of physical and chemical transformations to whichcrude oil is processed, it is also called primary distillation.For this purpose, the heating of crude oil in tubular furnacesup to temperatures of 280-330°C is required [1-3].Atmospheric distillation of crude oil is made in variousfractional columns types, denoted as U, A or R – typescolumns.

In crude oil distillation plants either a single column witha vaporizer or multiple columns are used taking intoaccount the light fractions and corrosive content of thecrude oil (mostly sulfur compounds). By using a singlecolumn for continuous atmospheric distillation the crudeoil coming from the electrical desalter unit is preheated by

heat exchange with the side fractions extracted from thecolumn. Then, it is sent into the tubular furnace where isstill heated up and partially vaporized. After that, the oilenters the vaporization section of the atmosphericdistillation column.

The main components of an atmospheric distillationplant are (fig. 1) [4] the fractional column, the tubularfurnace, the condensing and heat exchange section,strippers and pumps [4, 5].

In a U-type column, the taking over of the heat releasedby oil products is done in order to cool down thetemperature from the input to the output of column with acold reflux introduced above the first tray in the top of thecolumn. The column is supplied with crude oil that ispartially vaporized in the vaporization section. The lateralproducts extracted from the trays in the liquid state arepassed through the strippers in order to remove lightproducts involved in extracting the fraction out of thecolumn.

The light fraction extracted from the strippersis re-introduced into the column at a tray higher than theextraction tray. After cooling down the stripped productsare 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-flowcontroller, TI-temperature indicator

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Out of the stripping section of the column, naphtha isobtained [6, 7].

In the classical process automation, the outputtemperature of the product heated in the tubular furnace ismeasured at the input of the AD column with a basic sensor,an adapting/converter/transmitter element and a displayto show the measurement result. Usually, thermocouplesensors are used but thermo-resistors with a measurementrange of 200 to 600oCcan be used as well. Controlling theoutput temperature of the product is usually done withclosed loop systems based on calculation of the error valuebetween the reference temperature and the currenttemperature. Some disturbance consequences (such asflow variation or heated product temperature variation) arecompensated by using open loop systems that act oncertain disturbances [8].

Temperature control systems can be simple, either witha single electrical circuit or they can include two or morecircuits. In figure 2 a version of a closed loop control systemwith one circuit is shown. The product temperature controlat the furnace output is done based on fuel flowmodification.

where Top ref and Qp ref are the reference values of the heatedproduct output temperature and product flow rate, whileÄT and ÄQ are the errors of these variables, respectively[9,11,12].

Experimental analysis of different types of tubularfurnaces shows that dynamically they behave similarly,andin consequence their forms and dimensions affect onlyquantitatively the response commands and disturbances.The time duration for the transient processes varies from 5min to 50 min, depending on the furnace winding lengthand mass and, somehow, on the area and weight of therefractory lining.

The programmable logic controller (PLC) is speciallydesigned to be used in process control. PLCs are frequentlyused in industrial process control [13] because they areeasy to set up and programmed, their behaviour ispredictable and they are robust enough to work in badenvironmental conditions as the presence of dust,moisture, electric noise etc. (fig. 3). In general, the PLCcontroller is programmed by using a user-friendly methodand combining some special and dedicated functionblocks. The current task is divided into stages that can berepresented by a number of function blocks. Functionblocks programming simplifies the applicationrepresentation but also ensures a complete control ofprocesses. The algorithm can be implemented in a fewsimple steps, and even complex tasks can be represented.In order to easy use of function blocks, the PLC has beenpre-programmed to perform certain tasks, offeringflexibility 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 generatethe necessary control signals for the current applicationaccording to programmed algorithm. Each function blockgives the user the possibility to access the specific controlsand to customize each program for different applications.The function blocks are assembled together to form acircuit, by using the function blocks diagram (FBD).

This paper proposes a modern procedure for controllingthe product temperature at the output of a tubular furnace,Tep, by using in the electrical scheme a programmablecontroller instead of a classic controller (fig. 3). Theprogrammable controller, although simple, has theadvantages of multiple combinations that can be createdbetween functions blocks and easy to be handled andinterconnected to design the program.

The purpose of this application is to develop a systemfor measuring and controlling the temperature of crude oil,following a previously established diagram.

Fig. 2. A closed loop controlsystem for measuring the

product temperature at thetubular furnace output

TT-temperature transducer,TRC-temperature controller andrecorder, IP/Y-current-pressureconverter, Top-product output

temperature

This structureis chosen for tubular furnaces working witha constant load, without any disturbances from the furnacefocus or the product to be heated. As actuators, pneumaticcontrol valves are used which include closed loop positioncontrol systems. As controllers, PI or PID electroniccontrollers are used [9, 10].

Due to the special significance of the energy economy,the most important optimization criterion for heating andvaporization furnaces without chemical reactions is themaximum furnace efficiency. Therefore some operatingrestrictions concerning the flow rate, temperature, pressureor vaporization fraction have to be added. All theseparameters should be maintained in certain limits. Forinstance, the monitored values ofproduct outputtemperature Topand product flow rate Qp can be expressedas:

Top = Qop ref ± ∆T (1) Qp = Qp ref ± ∆Q (2)

Fig. 3. PLC connection into the processingwork [13]

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Experimental partFor the purpose of this paper, the U type column was

chosen.The desired temperature – time profile is shown infigure 4. The general electrical scheme is presented infigure 5.

In this electrical connection, Tref is the oil referencetemperature, AC is the actuator (control valve) and Pt100is the thermo-resistance temperature sensor.Thetemperature sensor and the actuator are connected to thePLC via an analog input module and an analogue outputmodule, respectively. The corresponding electricaldiagramsare presented in figures 6 and 7, where CPU isCentral Processing Unit., In 1 is the input number 1 and Out4 is the output number 4.

In our experiments a MITSUBISHI AL2-14MR-D typeprogrammable logic controller is used.The operation logicis sustained by a program implemented in a PLC whichgenerates control signals for obtaining, maintaining andmonitoring the temperature. The program was builtacording to the following rules:

(a) when the controller starts the current status isdisplayed (ambient temperature);

(b) if the controller is switched into the RUN mode, theprogram starts running, and constantly compares theenclosure temperature (the signal received from thetransducer) with the reference temperature, which is thefirst level of the required temperature.

The message heating is displayed during this operation.When the first level of the prescribed temperature isachieved, the controller disconnects the heater andmaintains the temperature at the specified value. Thesystem has high thermal inertia (high transient regime)

which means that, after reaching the set temperature andafter disconnecting the heater, the temperature willcontinue to increase; that is why the cooling systemmentioned above is necessary.

The first level temperature is maintained within a welldefined period of time (15 min). Then, the program takesinto consideration the second reference. The secondtemperature level is maintained for the same period(determined by the program) after which the controllerswitches back to the initial state (standby) waiting toreceive new commands to perform a new heating cycleor to go into the STOP state.

In case of an equipment failure, the controller can usethe program functions WARNING or AUTO-OFF or, theprogram can be interrupted by an operator by switching itinto the STOP state. In order to programming thisapplication, the authors of this paper developed and usedthe control diagram shown in figure 8.

The output Out 1 for the heating resistance commandcircuit is relay type. The power supply provides a 24 Vccstabilized output voltage required to supply the controllerand the adapter used. Terminal A is connected to the minusterminal, which leads to an entry connection of PNP typeor voltage difference to 0.

A converting resistor CR is required to convert the signalfrom 4-20mA to a voltage of 1-5V, required to the analogicgate input (IN).This signal is generated by intelligent adapterConv R-I type Rosemount HART 248 which converts thevariable resistance generated by Pt 100 sensor into unifiedsignal.

To achieve voltage requirements for the PLC, a stabilizedvoltage source Alpha Power 24-0.75 was used. The sensorwasPt 100 ROSEMOUNT type. For conditioning the sensorsignal a large variety of equipments is used. For thisapplication a 248 HART type R-I ROSEMONT adaptor hasbeen used to convert physical variables into conditionedand standardized output signal. Analog signal of 4-20 mAamplitude allows the detection of stoppage situation

Fig. 4. The desired temperature profile introduced in PLC duringmonitoring

Fig. 5.The used electrical diagram with PLC

Fig. 6.Connecting an analog input module usingthree wires. Only one channel is shown, this one

corresponding to a Pt100 sensor.

Fig. 7.Connecting an analog output module. Onlyone channel is shown, this one used to control

the actuator

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(missing current or a current below 4 mA).The DCtransmission, both for analog and digital signals, allows anincreased immunity level at disturbance. Information atthe receiver input is not affected by the loss of voltage oncables and connectors, nor the parasite thermocouplesdistributed on different points on the signal path.

For analog signals, transmissions over a distance ofmaximum 600 m can be realized. The power supply isconnected through a pair of wires used also to transmitthe measurement result, thus creating the current loop.The lowest value of the current is 4 mA and the head ofscale value is 20 mA, so that a variation from 0 to 100% ofthe measured parameter corresponds to a 16 mA variationof the current. This device is also known in technical termsas adapter, or alternative as adapter with two-wiresconduction, resistance-to-current converter, voltage-to-current convertor etc. The ROSEMOUNT 248H temperatureadapter (transmitter) is an universal intelligent adapterconnected by a two wires HART protocol to acommunicator from the same HART series or a PCcomputer, and is built to be assembled directly on top ofthe Pt 100 ROSEMOUNT sensor.

The supply voltage for the adapter is between 12-42 Vd.c. voltage. It ensures a linear characteristic of the outputif it is maintained in this range and makes possible theHART protocol adapter configuration when it is connectedto the HART Communicator. The dedicated communicatorwith ROSEMOUNT 248H adapter uses a specializedsoftware; it can set different ranges for operationtemperature, voltages and output levels, all of which beingpossible using only two wires (the supply ones), the adaptergenerating a digital signal overlapped with the supplyvoltage.

For testing the proposed control system, an experimentalplatform was built at the Petroleum-Gas University ofPloiesti. It has the same configuration as the one describedin this paper with only one exception: the process tubularfurnace was replaced by a thermal enclosure. Figure 9shows the component of the experimental platform.

Fig. 8. Wiring diagram control application.SVS – stabilized voltage source; PLC – programmable logiccontroller; Conv R to I – resistance to current converter;

CR –conversion resistance, from 4-20 mA to 1-5V;Pt100 – temperature sensor

Fig. 10. Window screen used to setup controller properties

Fig. 9. The experimental stand for testing themonitoring and control

Fig. 9.The experimental stand for testing the monitoring and control

Results and discussionsThe PLC program was realized by following the

described algorithm and the values for the temperatureprofile were introduced according to figure 4. The use oflogical function blocks is the easiest way during programrunning, because certain parameters can be set up whenimplementing the multitude of logical function blockinstead of using a specialized regulator.Programming the application by using function blocksdiagrams (FBD)

When the executable AL-PCS/WIN-E is launched, theuser can choose options in order to realize a new program,to connect the PLC and check its functionality. Some iconsare disabled, but they will become active in simulationmode or when MITSUBISHI 14MR-AL2-D programmablelogic controller will be connected to the PC. The effectivePLC 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 forthe new program configuration will appear. The FDB basisprovides the platform on which the alpha series programwill be built. This FBD basis is constituted from an enlargedgreen screen used for cabling devices, a title box and, alongthe right and left hand sides, input/output verticalrectangles. Programmed components are located in thecabling area or in rectangles and they are connected;

- after that, from the new main menu bar which will beturned the Com function has to be chosen, in order to selectthe port connection between the controller and the PC; forour purpose, Com 1 was chosen;

- the selected items required to build the program aredropped on the displayed work space as follows: analoginput must be selected from the Input group and droppedon the I01. In order to configure the program, the necessaryfunctions must be chosen: GAIN, COMPARE, ADD, SUB,SET RESET, DELAY, DISPLAY, OR, AND, DISPLAY FBCONTROLER, HEATER and dropped on the FBD base, asshown in figure 11;

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- the wiring analyzer is used to connect the componentsin the FBD basis space;

- after connecting all the function blocks and choosingthe appropriate parameters for them (parameters resultingout of the controlled process characteristics), the programis ready to be run and to simulate the process (fig. 12).

- after checking out the accurate program functioningand making the corrections of possible errors, the programis transfered into the PLC and saved.

The running of the program can be described indicatingin particular the role of each block or group of blocks thatinteract (fig. 12):

- Using analog input gate I01, the signal received fromthe temperature transducer is continuously monitored andapplied to an amplifier block, which can adjust the signalwithin certain limits;

- The processed signal is applied to a comparator andto 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 fromthe transducer:

- if Tref > Tcrt, the O01 output (a relay type) isconnected, and HEATING is displayed;

- if Tref = Tcrt, the O01 output is switched off, and thedelay function which maintains the reference set for 180swill automatically starts;

- if Tcrt is with 1oC higher than Tref, the cooling down isordered using O02 output by startingthe pump and solenoid

Fig. 11. The FBD basis with all the functions selected and connected

Fig. 12. The final form of the simulation program for PLCrunning

valve, and the command is maintained until the followingequality Tcrt = Tref -1 is reached;

- the trigger block setto: ON = Tref + 1 and OFF = Tref-1 is used to ensure a hysteresis around Tref = 2oC;

- when 180 stime has elapsed during the run, thereference 1 is disconnected using SET/RESET function andthe second reference generated is joined to the anotherCompare block, where the signal is compared permanentlywith the current value generated by the transducer.

ConclusionsIn this paper the authors have succeeded to build a

performant temperature closed loop control system usefulfor industrial distillation processes in oil and petrochemicalindustries. In order to optimize the control of temperature,a programmable logic controller (PLC)was chosen.The temperature limits can be established andimplemented in a program written with dedicatedsoftware. With the help of the controller, it was practicallyproved that the considered process is fully automatized,human intervention being necessary only for programsupervision, start-stop and tracking. Heating temperaturevalues can be changed simply by using the dedicated keysvia software, while the holding times for thesetemperatures can be established by a suitable setup in theimplemented software. The main advantages of the usedPLC are its small dimensions and programming simplicity,as well as its facilities.

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Manuscript received: 17.02.2016