A new methodology for measurement of sludge residence time distribution in a paddle dryer using X-ray fluorescence analysis

Abstract

Drying is a necessary step before sewage sludge energetic valorization. Paddle dryers allow working with such a complex material. However, little is known about sludge flow in this kind of processes. This study intends to set up an original methodology for sludge residence time distribution (RTD) measurement in a continuous paddle dryer, based on the detection of mineral tracers by X-ray fluorescence. This accurate analytical technique offers a linear response to tracer concentration in dry sludge; the protocol leads to a good repeatability of RTD measurements. Its equivalence to RTD measurement by NaCl conductivity in sludge leachates is assessed. Moreover, it is shown that tracer solubility has no influence on RTD: liquid and solid phases have the same flow pattern. The application of this technique on sludge with different storage duration at 4 °C emphasizes the influence of this parameter on sludge RTD, and thus on paddle dryer performances: the mean residence time in a paddle dryer is almost doubled between 24 and 48 h of storage for identical operating conditions.

Modeling of Sewage Sludge Flow in a Continuous Paddle Dryer

“…Partially dried sludge was sampled at the outlet at a frequency of 5 min, dried at 105 C for 24 h and analyzed off-line using X-ray fluorescence spectroscopy (PANanlytical, Almelo, Netherlands), which give access to experimental E(t) curves. [29] As mentioned before, the dry solids throughput Q ds is known indirectly, from the inlet mass flow rate and moisture content. Thus, only the recirculation coefficient, R, and the solids hold-up, Hu, have to be identified in our model.…”

Modeling of a continuous sewage sludge paddle dryer by coupling Markov chains with penetration theory

“…More recently, Charlou et al developed a methodology for measuring experimental RTD with a good repeatability in a pilot-scale paddle dryer installation [17]. This study showed that water and dry solids have the same RTD, that is to say SS acts as a single phase during drying.…”

“…SS used in this experimental study was produced in the Albi municipal WWTP (France), operating for 60,000 population equivalent. Main characteristics for this sludge, which did not change significantly during the experimental campaign, can be found in [17].…”

“…The lab-scale pilot paddle dryer used in this work is represented in Figure 1; it has been described in previous works [17,27], so only its main features will be described here. It is a Ushaped dryer housing a single shaft, where the walls and the shaft are electrically heated.…”

Influence of Operating Parameters on Sewage Sludge Drying in a Paddle Dryer: Design of Experiments for the Determination of Hold-Up and Water Content Profiles

“…The first part of the graph illustrates a period of stability of both torque and sludge outlet flow rate; thanks to RTD experiments, we have observed that this behavior corresponds to a steady-state regime in the dryer. [15] After 4 h of operation, an increase in torque value is accompanied by a decrease in average flow rate and the reverse. This indicates that an accumulation of sludge occurred, followed by a partial purge of the material in the reactor.…”