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Title: An investigation into the fouling and cleaning behaviour of dairy deposits
Author: Changani, Sushilla
Awarding Body: University of Birmingham
Current Institution: University of Birmingham
Date of Award: 1999
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This thesis has investigated the fouling and cleaning behaviour of dairy fluids in a pilot scale plate heat exchanger (PHE). Commissioning of the pilot scale PHE showed that accurate and repeatable data was obtainable. The fouling and cleaning behaviour of two types of dairy fluid, whey protein concentrate (WPC) and WPC with minerals added (WPC m ), were investigated and compared to that of milk. The model fluid, WPC m , was developed by adding calcium and phosphorus salts to WPC to the same levels found in milk. Comparison of WPC m and WPC fouling behaviour revealed two differences: (i) fouling rate declined for WPC m relative to WPC, (ii) deposit distribution in the PHE changed to resemble milk behaviour. Addition of calcium and phosphorus salts also resulted in a decline in the pH of the WPC solution from 7.01 to 5.57. Decline in pH could explain the reduction of the extent of fouling. Addition of calcium ions could be responsible for deposition earlier on in the PHE at lower temperatures due to increased aggregation. Distribution of deposit across sections of the PHE and the type of deposit produced had important influences on the cleaning behaviour. For the first time observations of changes in overall heat transfer coefficient (U) and pressure drop (AP) across different sections of the PHE were made during cleaning. The former indicated surface cleanliness and the latter indicated complete removal of deposit from the system. Differences were noted when cleaning WPC and WPC m deposits. WPC deposit was removed from PHE surfaces as a whole very quickly. Although WPC m deposit was also removed from PHE surfaces quickly, it took considerably longer for the cleaning fluid containing the deposit to be removed from the PHE. This was indicated by secondary pressure peaks observed in all PHE sections for WPC m only. Cleaning end-points occurred successively in each section of the PHE: the pasteuriser cleaned first, then the intermediate and finally the UHT section. Distribution of deposit in the PHE was critical in its final removal and thus WPC m was most appropriate as a milk simulant. Removal may depend on: (a) mass transfer of the cleaning chemical into the deposit to dissolve or break it up, (b) shear on the deposit from the fluid, or (c) formation of a fluid of high viscosity which is difficult to remove, for example close to the surface of the deposit. Thermal recovery was an incomplete measure of cleaning: pressure drop measured the removal both of surface-bound deposit and of the fluid which has dissolved the deposit. Cleaning times and cleaning rates were assessed for the pasteuriser section and the PHE as a whole. They were defined as the time taken for AP and U to recover to a percentage of their clean values. The effects of varying the three process variables, concentration, flowrate and temperature of cleaning chemical (single-stage cleaner, LQ32), were assessed for their effects on cleaning behaviour individually. In general, an optimum chemical concentration of 0.7wt% with respect to NaOH and cleaning temperature of 80-85°C was found, increasing flowrate increased cleaning rate and decreased cleaning times. This work was extended by the development of empirical relationships to account for interactive effects of these process variables using Response Surface Methodology. Heat transfer measurements were better correlated than pressure drops. Flowrate emerged as the most important factor in cleaning. The experiments showed that the chemistry of cleaning can be less important than the engineering and that the key step is the removal of the deposit from the exchanger.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available