POLYETHER POLYOL PRODUCTION Implementation of Inherently Safer Design
35030 – Chemical Chemical Engineering in Global Business Anissa Nurdiawati
13M53170
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INTRODUCTION
Process safety is a fundamental component of process design. From famous historic disasters such as Flixborough and Bhopal to recent events, the process safety in chemical process design becomes more important. Inherently safer design (ISD) is a philosophy for addressing safety issues in the design and operation of facilities that use or process hazardous chemicals. ISD should be considered throughout the process life cycle, from initial conception through R&D, plant design, construction, and operation. Polyether polyols serve as important raw materials in the urethane industry, which represents roughly 5% of the worldwide polymer consumption (Ionescu, 2005). A polyol is an alcohol containing multiple hydroxyl groups which can be classified as a polyether polyols and polyester polyols. Polyether polyols are used in the preparation of polyurethanes: flexible polyurethane foam, rigid polyurethane foam, and CASE (coatings, adhesives, sealants, elastomers). Polyether polyols are manufactured by ethoxylation/propoxylation of a polyhydric alcohol in the presence of a catalyst. Polyether polyols are produced by anionic ring opening addition polymerization of ethylene oxide or propylene oxide. The reaction is highly exothermic so that the temperature control becomes very critical. The raw material such as propylene oxide is a flammable substance containing hazard to the process. Moreover, the process is batch mode so that the operation procedure will be very important with regard to process safety. By applying for inherently safer concepts to this
process and procedur es, it’s expected to make less hazardous operations.
MAIN PROCESS DESCRIPTION - REACTIONS
The anionic polymerization of PO for the production of a polyether polyol involves the successive reaction of an organic oxide with an initiator compound containing active hydrogen atoms. This requires the addition of the alkylene oxide through anionic (basic) catalysis or cationic (acidic) to the initiator molecule. Commercial production is usually using a base such as KOH which catalyses the ring opening and oxide addition which is continued until a required molecular weight is achieved. The mechanism for the polymerization process is shown in Figure 1 and 2. Since the epoxide monomers and polyether polyols are easily oxidized, air is excluded from the manufacturing process. In the given case, polyether polyol is produced from initiators, which contains hydroxyl functions such as glycerin, sorbitol and sucrose, cauterized with alkaline materials such as alkaline metals (e.g. Sodium, Potassium hydroxide) amines with addition polymerization with propylene oxide at about o
115 C.
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Figure 1 Base Catalyst Production of Poly(propylene oxide)
Figure 2 Mechanism of Base Catalyzed Ring Opening Polymerization
PHYSICAL-CHEMICAL CHARACTERISTIC
Propylene oxide is a stable material that will not decompose under normal conditions of temperature and pressure. Propylene oxide is a colorless, low-boiling and highly volatile liquid with a sweet, ether-like odor. It is highly flammable and reactive, and storage and unloading areas must be appropriately designed and monitored. Table 1 shows the physical and chemical property of propylene oxide.
Table 1 Property of Propylene Oxide Property Physical State Color Boiling Pt. Molecular Wt. Vapor Density (Air =1.0) o Heat of Combustion, Liquid @25 C Flash Point Autoignition Temperature Upper Explosion Limit Lower Explosion Limit
Value Liquid Colorless o 34.2 C 58.08 2.0 -426.745 kcal/mol o -37.2 C o 449 C 42 vol% 1.6 vol% 3
By identifying the property of PO, it can be described that the hazards of processing propylene oxide include: -
Propylene oxide is an extremely flammable liquid
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A gas phase explosion hazard related to the flammability of PO in the presence of air or oxygen
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A runaway reaction hazard when PO is reacted with other chemicals in propoxylation processes, if PO is allowed to accumulate in reaction vessels, due to improper operating conditions. Also concentrated PO solutions may self-heat and runaway if the temperature is not controlled. Dow's Fire & Explosion Index Hazard Classification Guide 1987-published via the American Institute of Chemical Engineers in Appendix B Example problem 4 on page 58 suggests that 15 percent unreacted propylene oxide is a "worst case reaction mixture" for a polyol batch process reactor operating at a maximum reaction temperature of 120° C.
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A toxic hazard since PO is highly toxic and may cause cancer.
This hazard identification of related chemical substance then should be considered in the process design of polyether polyol production.
ANALYSIS OF EXISTING PROCESS
– POTENTIAL HAZARD IDENTIFICATION
Original procedure is 1. Charge pre-catalyzed initiator at about 5% of reactor volume 2. Charge PO with required amount (about 80% of reactor volume) 3. Mix well 4. Bring reactor temperature to the reaction temperature to react. 5. Cool reactor to remove heat of polymerization 6. Digestion 3 hours and transfer to the rundown tank for catalyst removal
Inert Gas Propylene Oxide
Reaction System
Polyether Polyols
Initiator
Heating/Cooling
o
PO, having a low boiling point (bp = 33.6 C), is volatilized spontaneously by the simple contact with o
the hot reaction mass at 100-125 C and generates a pressure. If all PO is reacted directly, without
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continuous cooling process, it is possible that it leads to a very strong exothermal reaction impossible to control (runway reaction). This process is inherently unsafe and experienced runaway batch with some unexpected problem takes place. To identify and evaluate possible hazard scenario in polyethers production, What-if analysis was conducted as presented in Table 2 below. What-If Analysis technique is prefered here (rather than o ther hazard evaluation technique) since it does not require detailed information of the plant design and has broad flexibility for identifying and evaluating hazards
Table 2 Potential Hazard Scenarios for Polyether Polyols Production What if …? There is no flow of gas inert
Initiating Cause Valve for inert gas fails open
There is no coolant circulation (loss of cooling)
Cooling water pump fails Recirculating pump fails Failure of heat exchanger
There is higher pressure in the reactor
Runaway reaction lead to higher temperature
There’s no agitation
External fire in the process area Gas inert regulator fails Agitator motor fails
There’s higher temperature in the reactor
EO accumulation
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Consequence In the presence of air and high temperature, oxidation of alcohol groups may be occured resulting in product degradation. Moreover, PO is flammable and in the presence of air is explosive mixture. Reactor overheating may lead to overpressure , possible reactor rupture, loss of product, and personnel injury due to exposure Leading to possible reactor rupture, release of flammable material to the atmosphere and personnel injury due to exposure
PO accumulation lead to runaway reactions may result in higher temperature and ovepressure. Leading to overpressure , possible reactor rupture, loss of product, and personnel injury due to exposure
INHERENTLY SAFER CONCEPT IMPLEMENTATION
Application of Inherently Safer Strategies to the Hazard and Process Design
It is extremely important to maintain reaction parameter constants, not only for the quality of the resulting polyethers, but for the safety as well. As an immediate consequence, inherently safer design is applied to the process, such as control of PO addition, automation of cooling system and the other process design and control which explained below . Table 3 Inherently Safer Design Choices for Design Application
Hazard Scenario Loss of inert gas flowing
Process Operation Operating a reactor under inert gas pressure
Loss of cooling (higher temperature)
Circulate coolant in jacket and external heat exchanger
Overpressure
Operating a reactor under inert gas pressure Filling a reactor with a pump
Potential Upset Case Failure of inert gas opening valve leading to product degradation and may lead to explosive condition. Failure of pump/heat exchanger leading to overheating and overpressure. Runaway reaction may lead to explosive condition. Failure of inlet gas regulator leading to overpressure. Overpressure by pump deadhead due to overfill
Inherently Safer Design Stop the flow of PO under this circumstances
Provide reactor with active safeguard (rupture disc, automatic water spray) Stop PO Flowing to maintain the amount of unreacted oxide
External Fire
Operating a reactor under inert gas
Overpressureand may lead to rupture of the reactor
No agitation
Mix the material inside the reactor
Underpressure
Draining an elevated process vessel by gravity
Failure of agitation/ loss of power supply causes PO to accumulate and leading to runaway reaction Blocked vent leading to vacuum pulled during draining
Filling PO to the reactor
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Stop PO Flowing to maintain the amount of unreacted oxide
Vessel design pressure greater than inert gas supply pressure Vessel design pressure greater than pump deadhead pressure
1.
Vessel designed for full vacuum 2. Liquid drain lined sized to be self venting Continuous feedingcontrolling PO rate
The improvement plans for achieving inherently safer design based on above analysis:
Reducing the amount of hazardous material present at any one time. Rather than charging all PO in a one time as in the initial procedure, the PO will continuously added to the reactor. Therefore, the process becomes semibatch mode. New proper operating procedure is explained in the next subchapter.
In the no inert gas flow, no agitation, and loss of cooling scenario, the hazard can be controlled and prevented by control the flow of PO. Reactor temperature, pressure or unreacted oxide in the reactor can be an indicator for maintaining a safe process. Therefore, adequate instrumentation and control system should be added to the process design. Active safeguard should be provided especially for medium to high level hazard.It will be further explained in the next subchapter.
In the overpressure and underpressure scenario, good engineering design for reactor, heat transfer equipment and the other process equipment are required to prevent those hazard possibilities.
Application of Inherently Safer Strategies to the Design of Layers of Protection
The maintenance of the reaction parameter constants is really important for t he process safety. There are two main control loops: 1. Pressure Control Loops In addition, the reactor is equipped with a high-high pressure switch, taking a signal from the rupture disk burst detector. The rupture disk is provided and set below the MAWP. 2. Temperature Control Loops To monitor the temperature and alert the operator if the temperature is not being controlled, the reactor has a temperature controller with a high temperature switch and audible alarm. In addition, the reactor is equipped with an independent temperature sensor and hgh-high temperature switch interlocked with an isolation valve in the PO feed line. This interlock will shut off the PO feed to the reactor in the event of a high-high temperature, and the heat of reaction will drop quickly. The cooling water supply line to the reactor jacket is backed up by an interconnection to the city water system, which can be manually turned on by the operator should the cooling water system fail.
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Piping and Instrumentation Diagram of Batch Reactor Initiator KOH
N2 Temperature Interlock
M
TI
TIC
FIC
PI
TT I F
Interlock System
PO
PSV
FT
PO Composition Interlock
LT
CI
T T
Hot/Cooling Fluid Supply LIC To Purification unit
Improved Semibatch Procedure for Polyether Polyol Production The process which implements inherently safer concept can be carried out by the following procedure. 1. Catalyst, initiator compound are introduced into a reaction vessel and then brought to the desired reaction temperature. 2. The reactor is purged with nitrogen to displace air and nitrogen as gas inert is continuously flow to the reactor. 3. Subsequently, the desired quantity of propylene oxide is dosed continuously into the reaction vessel by means of a controlled diaphragm pump. 4. The reactor jacket is switched from heating to cooling service. Cooling process is continuously maintained by the cooling jacket and more efficiently by external heat exchanger to remove heat of reaction. Temperature inside reactor is controlled within the required range by the flow rate of coolant and in emergency case (superior limit temperature) PO flow should be stopped and begun again when the parameter once again within the required value. 5. Agitation should be kept during the process to ensure that no overaccumulation unreacted oxide and improve heat exchanger effectiveness. In case of no agitation, the control loop will stop the PO addition to prevent the unreacted oxide over the set point. 6. At digestion step, the reaction of unreacted monomer is occurred by maintaining the reaction condition (temperature) under continuous stirring and recirculation of the reaction mass. Since the PO addition has been stopped at this stage, pressure will gradually decreased so that, the pressure is control during this process. Moreover, unreacted oxide should be analyzed and maintain within acceptable range to ensure the quality of the final product. To remove the last traces of unreacted oxide, degassing process may be introduced.
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7. In the final step, removal of KOH is required to purify the final product. Purification of the product can be performed by various methods such as neutralisation with acids, adsorption, extraction or ion exchange.
REFERENCES
Ionescu M. Chemistry and Technology of Polyols for Polyurethanes. Rapra Technology, 2005. Nie Y, Biegler L. Reactor Modeling and Recipe Optimization of Polyether Polyol Processes: Polypropylene Glycol. AIChE, 2013. Seay JR, Eden MR. Incorporating Risk Assessment and Inherently Safer Design Practices into Chemical Engineering Education. ASEE, 2008. U.S. Pat. 6,093,793. (Jul.25, 2000). Process for the production of polyether polyols. Hofmann J, Gupta P, Pielartzik H (to Bayer Aktiengesellschaft).
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