Washable Baghouse Operation and Design as Applied to Milk Powder Production
Thesis DisciplineChemical Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
The use of washable baghouses for fines collection in milk powder plants has been investigated. The main aim of this study was to increase the fundamental understanding of both operation and design of washable baghouses for application in milk powder plants. This work has focussed on the industrial scale. Industrial plant operating data has been collected, plant designs compared and analyses conducted on powder produced at the industrial scale. The amount of powder that becomes fines, the small size fraction of powder entrained in spray dryer outlet air streams, has been shown to be significantly greater than the traditionally vague estimate of 10 % to 20 %. The ratio of fines flows to total powder flows ranged from 49 ± 8 % to 86 ± 2 % depending on the powder type and plant operating conditions. A simple yet reliable method was developed to quantify fines flows based on measured powder size distributions of samples taken from around the plant. These estimates were supported by readings from an online optical scintillation instrument, which was shown to be capable of measuring fines flows at concentrations approximately four times the supposed maximum stated by the instrument’s manufacturer. Observations in another part of this work supported previous Fonterra observations showing that the amount of bulk fat in skim milk powder (SMP) has a large influence on the baghouse differential pressure. Fines flows measured by the optical scintillation instrument and analysis of other plant operating data showed that a change in bulk fat in SMP does not appear to cause any change in fines concentration. Observations of the surface of SMP by scanning electron microscopy, and electron spectroscopy for chemical analysis, both showed that fat is over-represented on the surface of the particles, and that only small increases in the bulk fat content are required to cause large increases in the surface fat coverage. It is hypothesised that increased fat on the surface of particles increases the clumping of SMP before deposition on the bags. Consequently, the powder forms more porous cakes and is less likely to penetrate into the interior of the filter bags, which also makes it easier to pulse clean powder from the filter bags. Therefore, the baghouse differential pressure is reduced. The design of pulse-jet baghouses from the literature was found to rely heavily on the authors past experience and approach, giving rise to large variation in recommended values of the key design parameters. A procedure for determining the optimal combination of these parameters was developed. This procedure showed that the main Fonterra washable baghouses are far from optimal because of their high air-to-cloth ratios, long bags and high elutriation and annular velocities. This procedure also showed that the Fonterra vibrating fluid bed washable baghouses are much closer to the optimum, which is the probable reason these washable baghouses have had almost no operational issues. Observations of the movement of the bags from below showed significant movement for bags near the inlet of the baghouse, indicating that this was the probable cause of the high bag damage in this zone. It is suggested that increasing the outer gap (distance between the baghouse wall and the bag on the edge of the bag bundle) be investigated further in an attempt to slow the annular velocity around the edge of the bag bundle and reduce bag movement. It is also recommended that stainless steel inspection hatches installed in the wall of a baghouse for this research, be included in all current and future washable baghouses because use of these hatches reduced the overall clean-in-place turn around time by 20 %. Computational fluid dynamics simulations of the air flow patterns within the Clandeboye Dryer 2 chamber were carried out using a commercial code CFX10.0. These simulations are possibly the first to include the influence of a spray dryer’s internal fluid bed airflow on the flow patterns within a spray dryer. As expected, the simulations showed the main air jet oscillated and precessed about the central axis with no apparent distinct frequency. In turn, the recirculation zones between the main jet and the chamber walls fluctuated in size. Different fluid bed flows within the industrial range had only a local influence on the air only flow field by reducing the length of the main jet. A different outlet boundary condition (including a flow resistance representing the baghouse) also appeared to have little influence on the overall flow field. Good agreement was found between the movements of the main jet via simulations and from telltale tufts installed in the plant dryer. This supported other indications that the simulations were an accurate representation of the actual flows. It was concluded that this project achieved its main aim of improving the fundamental understanding of washable baghouse operation and design, especially for application in milk powder plants. Also this project, as well as a change in production schedules, has helped to reduce downtime associated with the washable baghouses in the Fonterra Clandeboye Dryer 2 plant by an estimated 50 hours per annum.