Sample Batch Reactor for Time Estimating

By Dave Harrold, CONTROL ENGINEERING April 1, 1999
This sample was developed by Control Engineering and sent to 14 control system supplier companies with batch product focus requesting participation in this article.

The sample application was developed from material provided in Guidelines for Safe Automation of Chemical Processes published by Center for Chemical Process Safety of the American Institute of Chemical Engineers (New York, N.Y.). This sample was written and presented in a manner similar to what many service estimators receive from users. Using the drawings and written information, please prepare a

The unrelated detail often shown on Process Flow Diagrams (PFDs) and Piping and Instrumentation Diagrams (P&IDs) has not been included in this sample application. The purpose of this example is to develop time estimates to implement the process control shown. There is no assurance that this batch unit could produce PVC (polyvinyl chloride).

Simplified process description
The manufacturer of PVC from monomer is a relatively straightforward application. The reactor is the heart of the process. A batch requires about 10 hours. While the contents are agitated, the circulation of cooling water through the reactor jacket removes heat created by the exothermic reaction. To provide a continuous feed of PVC from the facility, four identical reactors are present. To minimize control software support and maintenance, the software should be developed so it can run on any of the four reactors without modification.

If the reactor has been opened for maintenance, it must be evacuated to remove any residual air (oxygen) to minimize oxidation reaction of monomer which may lead to stress corrosion damage to the reactor and/or poor product quality. Otherwise, the first step is to treat the reactor with antifoulant solution to prevent polymerization on the reactor walls. The antifoulant charge is followed with a demineralized water and surfactants charge. Liquid vinyl chloride monomer (VCM) is added at its vapor pressure (about 56 psig (3.86 bars) at 70°F (21°C)). The reaction initiator is liquid peroxide that is dissolved in a solvent. Because it is fairly active, it is stored at cold temperatures in a special bunker. Small quantities are removed for daily process use and are kept in a freezer. To ensure correct quantity additions of initiator, a small charge pot is manually filled.

Following the initiator, steam-heated water is applied to the reactor jacket to raise the temperature to about 130-140°F (54-60°C) (depending on the batch recipe for the particular grade of product). Agitation is necessary to suspend and control particle size of the VCM in the water, improve heat transfer throughout the batch, and produce a uniform product. Because the reaction is exothermic, once the reaction begins, the steam-heated water must be shutoff and cooling water circulated through the reactor jacket to control reactor temperature. The reaction is complete when reactor pressure decreases. Reacted polymer is dumped from the reactor and sent to downstream process units for residual VCM recovery, stripping, dewatering, and drying.

Pre-evacuation of air: If the reactors have been opened for maintenance, oxygen must be removed from the system for quality and metallurgical integrity reasons. This is done using steam ejectors to pull a vacuum.

Reactor preparation: The empty reactor is high-pressure water rinsed, leak tested, and treated with antifoulant.

Demineralized water charging: A controlled charge of water is added. An overcharge might lead to a hydraulic overfill; and undercharge may cause quality problems and potential reaction runaway. Any surfactants or other additives are introduced.

VCM charging: An accurate charge of VCM is added to the reactor.

Reactor heatup: The initiator is added from the charge pot, and cooling water is steam heated until it reaches 130-140°F (54-60°C) (10°F below steady-state reaction temperature).

Reaction: The steam system is isolated and cooling water is circulated to remove the heat created by the formation of the polymer. Water flow must be continuously adjusted to maintain the polymer at about 125°F (51°C).

Termination: When the reactor pressure starts to decrease, it is assumed most of the VCM has been consumed by the polymerization. The batch is ready to dump.

Reactor discharge: The reactor contents are dumped under pressure to a downstream holding facility where the product is degassed for subsequent stripping and drying. To prevent resin settling in the reactor, the agitator operates during the dumping process.

Two emergency processing steps are required.

Shortstop (chain stopper): Polymerization is stopped when the shortstop chemical is dumped into the reactor. The shortstop chemical is manually loaded into the shortstop vessel and pressurized with N 2 . When the shortstop chemical is dumped into the reactor, the agitator should continue to operate. If the agitator has failed, the shortstop chemical must be dumped into the reactor within one minute. If the shortstop chemical cannot be added, ‘burping’ the reactor- dropping the reactor pressure to form bubbles within the bulk liquid mass can stop polymerization.

Uncontrolled reaction: If cooling water flow is insufficient to remove the reaction heat and the liquid polymer temperature is 10°F (operator adjustable) above the temperature control setpoint, it is assumed a run-away reaction is occurring. Depressurizing the reactor will safely limit the reaction.

Process in S88 terminology

Control Modules
Tag Inputs Outputs Service
XCV1 2 DIs 1 Latching-DO Steam addition valve
XCV2 2 DIs 1 Latching-DO High pressure water addition valve
XCV3 2 DIs 1 Latching-DO Shortstop chemical addition valve
XCV4 2 DIs 1 Latching-DO Reactor bottom valve
XCV5 2 DIs 1 Latching-DO Liquid VCM addition valve
XCV6 2 DIs 1 Latching-DO Recycled VCM addition valve
XCV7 2 DIs 1 Latching-DO Depressurization valve
XCV8 2 DIs 1 Latching-DO Pre-evacuation valve
XCV9 2 DIs 1 Latching-DO Post-evacuation valve
XCV10 2 DIs 1 Latching-DO Additive 1 addition valve
XCV11 2 DIs 1 Latching-DO Additive 2 addition valve
XCV12 2 DIs 1 Latching-DO Flush water addition valve
XCV13 2 DIs 1 Latching-DO Initiator addition valve
PI1 1 AI Shortstop vessel pressure transmitter
PI2 1 AI Reactor pressure transmitter
PI3 1 AI Cooling water pressure
PMP1 1 DI 2 Momentary-DO Cooling water pump
MTR1 1 DI 2 Momentary-DO Reactor agitator
FQI1 1 AI Cooling water flow transmitter. Totalization to occur in software.
LSL1 1 DI Reactor low level switch
LSH1 1 DI Reactor high level switch
TI1 1 AI Reactor cooling water discharge temperature (RTD)
WQI1 1 AI Reactor weight (load cells). Totalization to occur in software.
TIC1 1 AI (RTD) Reactor product temperature. Loop is primary in cascade control with FIC1.
FIC1 FQI1 1 AO Cooling water flow control. Loop is secondary in cascade control with TIC1.

Equipment Modules
Name Control Modules Operational Description
HEADER1 XCV7

XCV8
XCV9

Interface provided with four setpoints of: ALL CLOSED, DEGAS, PRE-EVACUATE, POST-EVACUATE.

ALL CLOSED: All valves are commanded and verified OFF. If any valve fails to verify OFF, a unique alarm should be generated.
DEGAS: All valves are commanded and verified OFF. Only after all valves have verified OFF, XCV7 is commanded and verified ON. If XCV8 or XCV9 fails to verify OFF, or XCV7 fails to verify ON, a unique alarm should be generated and all valves commanded OFF.
PRE-EVACUATE: All valves are commanded and verified OFF. Only after all valves have verified OFF, XCV8 is commanded and verified ON. If XCV7 or XCV9 fails to verify OFF, or XCV8 fails to verify ON, a unique alarm should be generated and all valves commanded OFF.
POST-EVACUATE: All valves are commanded and verified OFF. Only after all valves have verified OFF, XCV9 is commanded and verified ON. If XCV7 or XCV8 fails to verify OFF, or XCV9 fails to verify ON, a unique alarm should be generated and all valves commanded OFF.

HEADER2 XCV10

XCV11
XCV12
XCV13

Interface provided with four setpoints of: ALL CLOSED, ADD 1, ADD 2, WATER FLUSH, INITIATOR.

ALL CLOSED: All valves are commanded and verified OFF. If any valve fails to verify OFF, a unique alarm should be generated.
ADD 1: All valves are commanded and verified OFF. Only after all valves have verified OFF, XCV10 is commanded and verified ON. If XCV11, XCV12, or XCV13 fails to verify OFF, or XCV10 fails to verify ON, a unique alarm should be generated and all valves commanded OFF.
ADD 2: All valves are commanded and verified OFF. Only after all valves have verified OFF, XCV11 is commanded and verified ON. If XCV10, XCV12, or XCV13 fails to verify OFF, or XCV11 fails to verify ON, a unique alarm should be generated and all valves commanded OFF.
WATER FLUSH: All valves are commanded and verified OFF. Only after all valves have verified OFF, XCV12 is commanded and verified ON. If XCV10, XCV11, or XCV13 fails to verify OFF, or XCV12 fails to verify ON, a unique alarm should be generated and all valves commanded OFF.
INITIATOR: All valves are commanded and verified OFF. Only after all valves have verified OFF, XCV13 is commanded and verified ON. If XCV10, XCV11, or XCV12 fails to verify OFF, or XCV13 fails to verify ON, a unique alarm should be generated and all valves commanded OFF.

Additional Information for Purposes of Estimating:

  1. Assume there are three additional batch reactor units (four total) as described above.

  2. Assume each reactor requires one main graphic and one support graphic.

  3. Assume there will be four product recipes, with three product grades per recipe, and each grade contains 15 variables that can be adjusted by the operator prior to starting a batch. Also, assume recipes operate only on the batch reactors. (e.g., Donuts = one product recipe. Apple jelly filled, grape jelly filled, and cream filled = three product grades.)

  4. Assume all other equipment shown on the PFD is outside the scope of this estimate.

  5. Assume each batch requires automatic generation of a batch-end report showing start and end times for the batch; start and end time for each phase, and start and end time each time an operator HOLDS and RELEASES a phase along with operator entered comments on why the HOLD was initiated.

  6. Assume each batch-end report includes an alarm history including when the alarm occurred and when it clears.

  7. Assume each batch report can be requested by the operator anytime during the batch (partial report) without compromising data for the final batch-end report. (No matter how many partial batch reports are requested, all data must be retained for inclusion in the final batch-end report.)

This concludes the information provided for purposes of estimating.

The following table indicates the information required by different control system vendors to prepare an estimate.

Requirements for Estimate Preparation
Vendor
A B C D E F G H I J K
Hardware/software platform X X
# & type of foreign device interfaces X X X
Total # of graphics X X X X X
PFDs X X X X X X
P&IDs X X X
Site/Bldg. plan X
Process description X X X X X X X
Recipes X X X X X X X
Standard operating procedures X
Reporting requirements X X X X
Information management requirements X
Total I/O count X X X X X X X X X
# & type of equipment and control modules per unit X X X X X
# & type of units X X X X X X
# of type process cells X X X X
# & type of operations X X X X X X
# of control modules per operation X X X X
# of unit procedures X X X X
# of operations per unit procedure X X X X X
Testing requirements X
Schedule X
Project support requirements X X X X
Additional requirements for
Hardware/software platform X X X
P&IDs X X
# & type of graphics X X
Phase logic descriptions including abnormal situation handling X X X X X X X X
Flow charts of recipes X X X
Foreign device interfaces X
Complexity of basic control (i.e., SAMA, Cause & effect diagrams, etc.) X X X X X X
Detail batch reporting requirements X X X X X
Detailed functional specification X X X
Required support activities (i.e., training, startup, etc.) X X X X X