By its very nature, the process industries are dynamic. Dynamic in that process plants rarely run at a steady state condition. External disturbances, changes in ambient conditions, equipment vibrations, and degrading equipment performance continually affect the smooth running of a process operation. Dynamic in that changing market conditions - raw material costs, product prices, government regulations and consumer demands continually drive companies to make process modifications to get more out of their plants.
Through dynamic simulation analyses users are able to effectively study the impacts that changing operating conditions and design modifications have on the operation of a process. Such analyses are value-added. Process configurations and control system designs can be evaluated to ensure that they will meet corporate manufacturing objectives regardless of changing process and market conditions.
What is Dynamic Simulation?
Dynamic simulation is a process engineering design tool that predicts how a process and its controls respond to various upsets as a function of time.
simulation can be used to evaluate equipment configurations and control schemes
and to determine the reliability and safety of a design before capital is committed
to the project.
For grassroots and revamp projects, dynamic simulation can be used to accurately assess transient conditions that determine process design temperatures and pressures. In many cases, unnecessary capital expenditures can be avoided using dynamic simulation.
Dynamic simulation during process design leads to benefits during plant start-up. Expensive field changes, which impact schedule, can often be minimised if the equipment and control system is validated using dynamic simulation. Start-up and shutdown sequences can be tested using dynamic simulation. Dynamic simulation also provides controller-tuning parameters for use during start-up. In many cases, accurate controller settings can prevent expensive shutdowns and accelerate plant start-up.
Dynamic simulation models used for process design are not based on transfer functions as normally found in operator training simulators, but on fundamental engineering principles and actual physical equations governing the process. When used for process design, dynamic simulation models include:
· Equipment models that include mass and energy inventory from differential balances
· Rigorous thermodynamics based on property correlations, equations of state, and steam tables
· Actual piping, valve, distillation tray, and equipment hydraulics for incompressible, compressible, and critical flow
· Detailed controller models to duplicate modern distributed control systems (DCS).
These models are so detailed that the results can influence engineering design decisions and ensure a realistic prediction of the process and the control system's interaction to assess control system stability.
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The SACDA TRAINER Dynamic Simulation System
The SACDA TRAINER System is a dynamic training simulator specifically designed for training operators and maintenance personnel in the process industries. It is similar in principle to the flight simulators used in the airline industry. The simulation model is the mathematical representation of a process, its controls and logic; for example, a distillation unit, or a hydrocracking unit, etc.
In this set-up, the system modelled a processing plant consisting of two distillation towers. The plant is complete with full instrumentation and controls, alarms and related interlock sequences. Students can interface with the simulated process via the keyboard and mouse provided.
The plant separates a blended feed using two distillation towers (a debutanizer and a depropanizer) to produce the following products:
C5 and heavier hydrocarbons
The following sections describe the simulated process equipment and process flow.
to the process flow diagrams for the two distillation
plants for more information..
The following Table 1 describes the process equipment used in the plant:
The following Table 2 lists design operating conditions, including cold start initial conditions and normal conditions. When starting the plant from a cold start condition, assume the plant has been checked for mechanical completeness. All control valves have been stroked and left in the closed position for fail-closed valves and open position for fail-open valves. The plant has been steam or natural purged and left under a blanket of natural gas at 20 oC (68 oF) and close to atmospheric pressure. The normal condition reflects the 100% operating values.
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A heavy and a light feed available from separate sources at a fixed pressure of 1675 kPag (243.0 psig) are blended in desired ratios and fed to the debutanizer T-100.
The temperature of the heavy feed is 121 oC (250 oF), and can be varied in the range 20-150 oC (68-300 oF). The temperature of the light feed is fixed at 27 oC (80 oF).
The heavy feed contains almost 60 mole % of C5 and heavier hydrocarbons. The light feed contains almost 90 mole % C3+ C4. The following Table 3 lists the composition, temperature, pressure, and flow control of both feeds.
The debutanizer has forty-two trays and operates at 1275 kPag (185.0 psig). The tower operating objective is to remove butanes (C4) and lighter hydrocarbons from the feed and make on specification an overhead product.
The overhead product of the debutanizer, which is the feed for the depropanizer, has a maximum composition specification of 1 mole % C5's.
The blended feed enters the debutanizer on tray 23.
E-110, the debutanizer condenser, condenses overhead vapours using eight fans. D-100, the reflux drum, holds the condensed hydrocarbons.
P-62 or P-63 pumps the hydrocarbons as reflux to the debutanizer. The overheads product analyser (%C5) resets the reflux flow.
P-68 or P-69 pumps the debutanizer overhead product to T-200, the depropanizer, via E-250, the depropanizer feed/bottoms exchanger.
E-100, the thermosiphon debutanizer reboiler, provides reboil for the debutanizer using high pressure steam of 2585 kPag (375.0 psig). The reboiler takes total flow from tray 1 with reboiler return to the tower bottoms. The temperature of tray 2 controls the reboiler steam flow.
The debutanizer bottoms product is rundown to storage.
A split range pressure controller controls the pressure in the D-100 reflux drum. The pressure controller can import non?condensable natural gas from the compressor interstage drum at 1100 kPag (160 psig) during start-up via the bypass (HC44). The pressure controller can also release excess pressure to either flare or the compressor interstage drum.
The depropanizer produces an overhead C3 product with less than 1 mole % C4's. The tower has forty trays and operates at 1925 kPag (280.0 psig).
The tower feed stream is the overhead product from the debutanizer and contains mostly C3's and C4's.
After the feed is preheated with the depropanizer bottoms in E-250, the feed enters the tower on tray 20.
E-210 condenses the tower overheads using six fans.
P-66 or P-67 pumps reflux back to the tower. The reflux flow is reset by the overhead product analyser controller (mole % C4's).
E-230 uses water to cool the overhead C3 product flow. The product is rundown to storage.
E-220, the thermosiphon depropanizer reboiler, provides reboil for the depropanizer. The reboiler uses medium pressure steam of 1035 kPag (150.0 psig). The temperature of tray 3 controls the steam flow.
Two on-line analysers measure the tower bottom C3 and C4 composition.
The C4 product is cooled in E-250, the feed/bottoms exchanger, and further cooled with water in E-240, the C4 product cooler. The C4 product is stored outside the boundary of the model.
A split-range pressure controller controls the pressure in D-200, the reflux drum, by importing non?condensable natural gas from the recontact drum (from elsewhere outside the boundary of the plant) at start-up and releasing excess pressure to the recontact drum.
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INSTRUMENTATION & CONTROL SYSTEM
The following sections describe some of the control loops and the instrumentations used in the distillation process.
Some of the more complex cascaded control loops are discussed below.
(a) T-100 Bottoms Level Control
FC17 regulates the flow of tower bottoms sent to storage which in turn controls the liquid level in the bottom of the debutanizer.
Normally, FC17 is in cascade mode. FC17 receives its setpoint from LC14. When the level of the debutanizer bottoms falls below 1 %, LSD15, the level shutdown interlock trips the shut off valve upstream of the FC17 and FC18 flow control valves.
Use HS39 to reset the shutdown valve after the LSD15 has a normal status (when the level on LC14 is greater than 1%).
FC18 regulates the flow to the off specification storage tank.
(b) Reflux Drum Pressure Control
Both reflux drums D-100 and D-200 on the two towers have pressure controllers operating on similar principles.
PC16 controls the pressure on D-100 by split range action on two valves. PV16A and PV19A valves are reverse acting and PV16B and PB19B valves are direct acting.
following table illustrates how valves A and B open with the split range signal.
Valve A is the valve on the line bypassing the debutanizer overhead condenser
E-110. Valve B is on the line connecting D-100 to the compressor interstage drum
which provides a backpressure of 240 kPag (35 psig). The flare has a backpressure
of 14 kPag (2.0 psig).
PC19 operates similarly for the depropanizer. PC19 releases excess pressure to the recontact drum which has a backpressure of 240 kPag (35 psig).
(c) Composition Control
AC12, the composition controller on the debutanizer, has an analysis time of 300 seconds. Al16A, Al16B and AC17, the composition analysers on the depropanizer, have an analysis time of 150 seconds. These analyser controllers control the composition of tower overhead streams by resetting the reflux flow controllers of each tower.
The following Table 4 lists the tag names and ranges for each instrument used in the plant.
NOTE: AC12 has an analysis time of 300 seconds, AC17, AI16A, AI16B each have an analysis time of 150 seconds
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