Provide process engineering design support to reduce a food additives reactor cycle time.
REACTOR CYCLE TIME REDUCTION / COOLING AND NEUTRALIZATION TANK PROCESS DESIGN
Process Engineering Associates, LLC (PROCESS) was contracted by the client, a chemical manufacturer, to shorten cycle time and increase production from a reactor system at one of their U.S. production facilities. The reactor is operated as a component in a process that produces food additives. The reactor operates in a two-step process: reaction followed by neutralization. The total cycle takes several hours to complete; approximately 66% of the cycle time is for the reaction step (heating the reactor, completing the reaction chemistry, and cooling the reactor), and about 33% of the cycle time is for the neutralization step (pulling vacuum, heating the reactor, performing reaction products neutralization, and cooling reactor contents). The objective of this work was to shorten the reactor cycle time to increase the production rate by approximately thirty percent (+30%).
The client and PROCESS conceived a system consisting of two reactor vessels. The first vessel would be heated, perform the primary process chemistry, and then cooled as per current operations. The client would reuse the existing reactor for this process step. Cooled reactants would be transferred to a new, second vessel, where neutralization of the reaction step products would be performed. This second vessel would require support equipment: mix tank to generate a soda ash solution, vacuum system and overhead cooler, and final product cooling. Decoupling the two reaction steps by introducing a second vessel cuts the effective cycle time down to the extent that an effective thirty percent decrease in cycle time is achieved.
The first task of this work was to upgrade cooling of the reactor. The reactor is configured with multiple internal coils, each with a separate inlet nozzle that provides heating to half the coils and cooling service to half the coils. Heating of the reactor with hot oil is not problematic and requires no upgrade. Reactor cooling has been a troublesome issue since the reactor went into service. River Water (RW) use in the coils has resulted in catastrophic tube failure and baffle damage due to a combination of RW high chloride content and water hammer developed from flashing at the initiation of the cooling process (cold water, hot pipe). Use of RW is also not desirable as small pipe cracks and pin-holes in the internal coils allow for possible contamination of the food additive process fluids. A change over to City Water (CW) and installation of a downstream flash tank has mitigated process contamination but has not completely alleviated water hammer. Excessive water hammer has resulted in damage to the brackets and attachments that hold the internal reactor coils in place. The client desired a solution that alleviated process contamination and water hammer.
A system utilizing a food grade heat transfer fluid was developed for cooling the reactor. The cooling coils were reused with the heat transfer fluid in lieu of city water as the coolant. The heat transfer fluid is cooled via cooling tower water in an external plate and frame cooler. The heat transfer fluid is circulated from an expansion tank, through the plate and frame cooler, then routed to the reactor to provide coolant flow to the reactor coils.
Cooled product from the reactor is transferred to the second vessel, referred to as the Neutralization Tank. The Neutralization Tank is a vacuum-rated, agitated, and baffled vessel, with internal heating and cooling coils. PROCESS sized the tank, specified the length of heating and cooling coil required, and engineered the supporting process equipment associated with the Neutralization Tank. These items are discussed below.
Once reaction products from the reactor are transferred to the Neutralization Tank, an aqueous soda ash solution, produced in a separate agitated mix tank, is feed to the Neutralization Tank. A two-stage vacuum pump is used to provide near-total vacuum conditions. As the vacuum is developed, water “boils” off the Neutralization Tank. A condenser, cooled with cooling tower service water, is used to condense most of the water vapor and overheads from the tank. Condensate is drained into a vacuum-rated receiver for disposal. Uncondensed vapor exits the system through the vacuum pump.
Following development of vacuum conditions, the Neutralization Tank is heated at a controlled rate with condensing 140 psig steam. Heating the vessel finalizes neutralization and drying of the batch. After a specified time period, the Neutralization Tank is cooled with the food grade heat transfer fluid. The same cooling circuit used for the reactor is used on the Neutralization Tank. Processing times are staggered so there is no need for simultaneous cooling of both the reactor and Neutralization Tank. Cooled product is transferred from the Neutralization Tank to a third holding tank.
Heat transfer calculations on the reactor coils were performed using TankJkt from Chemengsoftware. This computer software calculates the heat transfer coefficient for batch systems using internal heating coils. Additional calculations were performed using an Excel spreadsheet that utilized the results from TankJkt to calculate reactor cooling as a function of time. The models were calibrated using empirical plant data before being used as a predictive tool. CHEMCAD was also used for modeling the remainder of the process.
PROCESS developed an FEL-2 process package consisting of cycle times, heat and material balance tables, process flow diagrams, P&IDs, as well as a Process Control Description. The package was delivered to the client. Some long lead time equipment was ordered. It is expected that PROCESS will remain active in the detail design and commissioning of the new configuration.
- Food Grade Production Manufacturing
- Batch cycle time improvement
- Process design
- Heat transfer