Design a temperature control unit for a food additive product manufacturing skid process.
PROCESS ENGINEERING FOR REACTOR TEMPERATURE CONTROL UNIT
Process Engineering Associates, LLC (PROCESS) was contracted by the client, a chemical manufacturer, to engineer a Temperature Control Unit (TCU) for their reactor skid. The reactor is operated as a component in a process that produces food additives. The reactor is configured with multiple internal coils, each with a separate inlet nozzle that provides heating and cooling 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 could allow for 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.
PROCESS proposed a TCU using circulating food-grade oil as the working fluid. The TCU oil circulates in a closed loop between the reactor and a new expansion tank. The expansion tank provides head to a circulating pump as well as a reservoir of cool oil that is used to temper the oil system. Duty for reactor heating will be provided through a plate and frame exchanger that heats the oil as warranted. Reactor cooling is provided by routing oil through an air cooler and a second plate and frame exchanger (arranged in series). A temperature controller will modulate flow between the exchangers.
Duty for oil heating in the first plate and frame is provided by circulating hot oil heated in a natural gas-fired furnace (not part of the TCU scope described here). This hot oil is in a separate closed loop that provides thermal input to the reactor as described herein as well as a Deodorizer. Cooling to the second plate and frame is to be provided by Cooling Water Service (CWS) from the existing cooling tower.
Heat transfer calculations on the reactor were performed using TankJkt from Chemengsoftware. This computer software calculates the heat transfer coefficient for batch systems that use internal heating coils. Additional calculations were performed using an Excel spreadsheet that utilized the results from TankJkt to calculate vessel cooling as a function of time. The models were calibrated using empirical plant data before being used as a predictive tool.
Various configurations were evaluated along with several food-grade heat transfer fluids. Each of the approaches were hampered by lengthy cool-down times resulting from poor overall heat transfer coefficients associated with use of the heat transfer fluid (as compared to River Water or Cooling Water). A low heat transfer coefficient (compared to that of water) resulted as the food grade thermal liquids (mineral oils) have high viscosities at the low temperatures where they were being applied.
Low overall heat transfer coefficients resulted in unacceptably lengthy cool-down times. As it became clear that a workable solution could not be developed, PROCESS suggested several alternatives. One of the alternatives suggested by PROCESS involved external cooling of the reactor contents in a fin-fan exchanger. The client and PROCESS further developed this idea to include a second reaction vessel. The resulting configuration could provide a forty-percent (40%) increase in production of the reactor product as well as mitigate water hammer. This concept was further developed, and a Process Description was generated. This was presented to the client and a new engineering effort was proposed by PROCESS and accepted by the client. Engineering and other development work for the two-reactor approach will be performed under a new project contract.
- Food Grade Material Production
- Process design
- Heat transfer evaluation
- Process alternatives evaluation