Experimental modeling to design a heat exchanger control strategy for a Fischer–Tropsch fluidized bed

Abstract

Fluidized beds are ideal contactors for high temperature Fischer–Tropsch (FT) synthesis because of their superior heat transfer and low pressure drop compared to fixed bed reactors. However, as the reaction is extremely exothermic, even in these systems, runaway reactions sinter catalyst when the heat removal is insufficient or the control strategy is deficient. Although FT technology has been well established for decades, new catalysts, process intensification, and modular designs require a cooling strategy and control system a priori to ensure the process operates safely through the development stages. Here, we designed a safe mode experiment for a pilot fluidized bed reactor (200mm in diameter) charged with 36kg of alumina powder with a vertical cooling coil bundle. We measured the heat transfer rate between hot water in the coils and the fluidized bed operating up to 300°C, 0.5-1MPa, and superficial gas velocities from 5 × to 10 × the minimum fluidization velocity (umf=2.4mms−1). The dynamics of the cooling was 35% faster at higher gas velocities and the preliminary estimates concluded two tube bundles with 4 vertical runs each with 3/4 in. Schedule 10 pipe was sufficient to remove the heat generated. The calculated convective heat transfer coefficients were comparable to several published models but they were lower than expectation. We compared heat transfer rates of a single bundle and a set of two bundles and designed a PI and a PID controller plus filter (PIDf) separately using the internal mode control approach. Based on the results, the performance of the PIDf controller is better than the PI, as it minimizes oscillations and responds faster.