Process Description
GFU Process Overview
The GFU process is designed to distill a roughly 50/50 weight percent mixture of ethylene glycol and water into a nearly pure water product and a highly concentrated ethylene glycol product. The distillation normally occurs under vacuum conditions (i.e., the operating pressure is less than atmospheric pressure). By using vacuum conditions, the separation of water and ethylene glycol can be made at lower temperatures. The lower temperatures in the distillation tower allow the use of steam to be used as a heat source for boiling mixtures of ethylene glycol and water. Medium pressure steam from the package boiler is generated at around 130 PSIG. The temperature of saturated steam at this pressure is around 355 DEG F.
Separation by distillation involves boiling a feed mixture in a series of trays or packed beds in a distillation tower. Distillation towers exploit differences in the volatilities (i.e., ability to vaporize) of the compounds in the mixture. The GFU unit’s Distillation Tower has 2 packed beds with the feed mixture entering between the two beds. Liquid falls through the packed bed to the bottom of the tower. Heat is input to the Vacuum Tower T-200 by the Reboiler E-280 connected to the bottom of the tower and boils the mixture. The Reboiler is a heat exchanger that uses steam from the Boiler B-275 as the heating medium. Vapor produced from boiling the bottom mixture travels up through the packing in the tower and heats and boils the feed mixture that flows down the packing.
Enough heat is applied to the Reboiler to result in boiling of some of the liquid entering the Vacuum Tower so that vapor flows up through the packed section of the Vacuum Tower above the feed tray. This vapor is mostly water with a very small concentration of ethylene glycol. This is because water is more volatile than ethylene glycol at the temperatures in the bottom of the Vacuum Tower. By counter currently contacting down-flowing liquid with rising vapor, the top vapor becomes more concentrated in the more volatile component (i.e., water) and the bottom liquid becomes concentrated in the less volatile component (i.e., ethylene glycol). All distillation towers operate on this principle.
The vapor leaving the top of the Vacuum Tower is condensed in Condenser E-230. The Condenser is a heat exchanger and uses circulating cooling water from the Cooling Tower CT-260 as the cooling fluid. Condensate is collected in the Overhead Receiver V-240 and some of the condensate is refluxed to the top of the Vacuum Tower via Overhead Pump P-240. Refluxing condensate to the tower further increases the separation of the feed mixture by washing down the less volatile component in the top section of the tower.
The pressure of the tower is controlled using Liquid Ring Compressor C-250 to achieve pressures significantly lower than atmospheric pressure. As discussed above, this helps achieve separation using medium pressure steam as the heat source. Cooling water is used in the Liquid Ring Compressor as the working liquid for compressing the overhead vapors from vacuum to atmospheric pressure.
Instrumentation
Overview
This section describes the controls and instruments of the Glycol Fractionation process.
Please note the following about the simulated instrumentation:
- All controllers are configured per the actual control system. No ratio controls are present. One cascade loop is present.
- All motors have switches representing its operating state. These can be adjusted from the schematics or from the Group/Trend displays. These are not found on the real DCS system.
- The Boiler has a main power switch and an AUTO/MAN switch for the BFW pump. There are also several indications and alarms related to the control and interlock systems for the unit. These may be different from the actual unit, but the main functions of pressure and level control are simulated.
- The following analog instruments are available on the schematics and/or Trend/Group displays of the simulator for providing more information about the process conditions:
AI-200 – T-200 bottoms glycol concentration in weight %
AI-210 – T-200 feed glycol concentration in weight %
AI-240 – Overhead distillate (V-240) glycol concentration in weight %
FI-210 – Overhead distillate flow returned to T-200
LI-275 – B-275 water level indicator
LC-260 – CT-260 basin level controller
LIC-270 – V-270 level controller
Vacuum Tower Section
T-200
The flow rate of bottoms from P-200 to the feed to Glycol Tower T-200 is controlled by FIC-200 which adjusts the position of control valve FV-200. Reflux flow from P-240/V-240 is controlled by FIC-240 which adjusts the position of control valve FV-240.
The top pressure of T-200 is indicated on PI-225. The differential pressure across the two packed sections of T-200 is indicated on PDI-220.
The temperature of the feed to T-200 is indicated on TI-210. The overhead temperature of T-200 is indicated on TI-225. The temperature at the midpoint feed section of T-200 is indicated on TI-215. The bottom temperature of T-200 is indicated on TI-200.
The level of the glycol in the base of T-200 is indicated on LI-210. This level will float depending on how much liquid is contained in Overhead Receiver V-240 and in the packed sections of T-200.
P-200
The motor for Bottoms Pump P-200 is operated by switch HS-200. This is a field switch but is presented on the schematics for the simulator.
E-280
The outlet temperature of Reboiler E-280 is controlled by TIC-205 which adjusts the setpoint of steam flow controller FIC-275. FIC-275 controls the steam flow from B-275 by adjustment of the position of control valve FV-275. The pressure of the steam side of E-280 is indicated on PI-280.
NOTE: The boiling temperature of essentially pure ethylene glycol will depend on the pressure in the bottom of T-200. Therefore, Reboiler outlet temperature controller is normally in manual mode and the setpoint of steam flow controller FIC-275 is adjusted by the operator.
Overhead Section
E-230
The outlet temperature of Condenser E-230 is indicated on TI-230.
V-240
The level of Overhead Receiver V-240 is controlled by LIC-240 which adjusts the flow of distillate from P-240 back to T-200 feed by adjustment of the position of control valve LV-240.
P-240
The motor for Overhead Pump P-240 is operated by switch HS-240. This is a field switch but is presented on the schematics for the simulator. The flow of overhead product through LV-240 is indicated on FI-210.
CT-260
The fan for Cooling Tower CT-260 is operated by switch HS-261. This is a field switch but is presented on the schematics for the simulator. The level of water in the basin of CT-260 is controlled by LC-260 which adjusts the position of makeup water control valve LV-260. This is a field controller but is presented on the schematics for the simulator.
P-260
The motor for Cooling Water Pump P-260 is operated by switch HS-260. This is a field switch but is presented on the schematics for the simulator.
Boiler Section
V-270
The level of V-270 is controlled by LIC-270 which adjusts the position of control valve LV-270 to add makeup water to the boiler/steam system as needed. This is a field controller but is presented on the schematics for the simulator. Normally no water is added. Water will be required if there is a leak, blowing of a safety valve on the Boiler B-275, or if the steam system/boiler is vented or boiler water is blown down.
P-275
The motor for BFW Pump P-275 is placed in auto-start (AUTO) or hand mode (MAN) using switch P-275AM. In AUTO mode, the motor hand switch HS-275 will be automatically turned on and off by LC-275 to add water to the boiler to keep the level of water in Boiler B-275 at a safe value. LC-275 is a local logic controller; the level of the water in the boiler is indicated on LI-275 on the simulator. This is a field instrument but is presented on the schematics for the simulator.
When P-275AM is In MAN mode, HS-275 is used to manually start and stop the motor. These switches are field switches but are presented on the schematics for the simulator.
B-275
The following simulated instruments are used to control the operation of B-275:
Simulator Tag Description
JS-275 Master power switch
LAL-275 Low level trip alarm (trip at 80%; auto-reset at 82%)
PAH-275 High pressure trip alarm (trip at 150 PSIG; autoreset at 140 PSIG)
XA-275 Boiler trip alarm
The level of water in B-275 is indicated on LI-275 which is the PV for LC-275. This is a local instrument but is presented on the schematics for the simulator. LC-275 starts and stops the motor of BFW Pump P-275 to regulate the level of water in the boiler. Note that switch P-275AM must be in the AUTO position for the level control to work.
The pressure of steam produced by B-275 is indicated on PI-275.
The blowdown valve for B-275 is operated by switch B275-V.
The pressure of the steam generated by B-275 is controlled by a local controller PC-275 which adjusts the duration of power to the boiler. The setpoint of PC-275 is 130 PSIG and cannot be changed by the operator.
Note: The trip logic for B-275 is described in the Interlocks Section below.
Compressor Section
C-250
The pressure of vapor leaving Overhead Receiver V-240 is controlled by PIC-250 which regulates the position of control valve PV-250. This controls the flow rate of air admitted from the vent line to the inlet of Liquid Ring Compressor C-250. The motor of C-250 is operated by switch HS-250. This is a field switch but is presented on the schematics for the simulator. The amount of gas pulled into C-250 is essentially constant for a given set of operating conditions, so PIC-250 controls the net vapor flow to the atmospheric vent.
The level of water in the liquid separator on the discharge of C-250 is indicated on LI-250. It is neither a DCS nor a local instrument but is presented on the simulator to understand the operation. The instrument tag is not shown on Schematic #6; however, a graphical bar to indicate LI-250 is provided on the schematic.
Interlocks
The only unit with an interlock is the Boiler B-275. Therefore, the operator must take care to monitor all instruments to ensure safe operation of all parts of the Glycol Fractionation process. Alarms will normally alert the operator to potentially unsafe operating conditions.
B-275 Interlock
Any one of the following conditions will cause a trip of the 480-volt electric power to B-275:
- Changing master electrical switch JS-275 to the OFF position
- LAL-275 is in the trip state (see Boiler instruments above)
- PAH-275 is in the trip state (see Boiler instruments above)
Note that LAL-275 and PAH-275 have auto-reset logic when conditions are returned to safe values (see Boiler instruments above) .
XA-275 will alarm, indicating power to Boiler B-275 has been tripped.
Once all conditions have been cleared, XA-275 will go to the OK state indicating electric power is being supplied to the heating elements of B-275.