Left to Right - Prof Alan Reed,
    Prof Ugur Tüzün, Keith Sinclair,
    Prof Mark Cross

QPM - the way forward
Industries which go to great lengths to manufacture
sound particulate materials often find their efforts
hampered by the effects of degradation, segregation
and caking - factors that adversely affect operational
reliability of plant, compromising the quality of the
final product and ultimately its value.
  Over the last four years, a team from the
University of Greenwich, London and the
University of Surrey in conjunction with six major UK-based industrial partners has collaborated to try and determine the key factors that reduce product quality and provide industry with a set of tools to predict and subsequently avoid these problems.
   Around £2.5m and 20 man-years have been invested in the QPM (Quality in Particulate Manufacturing) programme with financial support provided by the EPSRC (Engineering and Physical Sciences Research Council) and the project’s industrial partners. QPM has combined a number of disparate research areas to provide an integrated approach to the design and analysis of particulate based manufacturing and handling systems. The prime objective is to integrate analytical and process modelling tools with laboratory based experiments to provide a procedure for designing out the key problems and embedding the project’s outputs into the everyday activities of the industrial partners. The team’s work and its conclusions were presented to an invited audience at a recent seminar held at the University of Greenwich’s campus.
   One of the key speakers, Dr Mike Bradley of the Wolfson Centre, said that the object of QPM was not merely to solve a particular degradation, segregation or caking problem, but to put in place a set of strategies for getting the plant design right first time by knowing which particular technology to use in a particular application, how far to go (in monetary terms) with a particular line of thinking and how to predict the possible effects of any action taken - areas which industry seems to consistently fall down on.
   One of the QPM workgroups led by Prof Mark Cross, director of the Centre for Numerical Modelling and Process Analysis at the University of Greenwich, has its background in the development of techniques and software tools for modelling purposes. The group's work is based on the application of a software framework called Physica, a computational modelling package for engineering processes that involve complex interaction phenomena and with a specific focus on flow behaviour.
   Another group under Prof Ugur Tüzün, head of the Department of Chemical and Process Engineering at University of Surrey, puts a different emphasis on the problem, specialising in the mechanics of particulate and multi-phase flow systems. Over the past decade, the group has developed a significant number of mathematical models (and software), using novel computational simulation techniques to relate particle properties to the assembly behaviour of granular materials. A third group led by Prof Alan Reed, head of the school of engineering and director of the Wolfson Centre for Bulk Solids Handling Technology at the University of Greenwich, draws on the expertise of the centre in solving a wide range of materials handling problems.
   Dr Mike Bradley explained how, at the start of the project, 14 plant case studies had been undertaken by the team at the sites of the industrial partners to identify what the specific problems were. It was at this stage that degradation, segregation and caking were identified as being the most influential and of most concern to the industrial partners, and the areas which the project groups would concentrate on. Particular problem areas that were identified included: particle breakage in pneumatic conveying systems leading to the generation of fines, dust and poor flow; segregation during the filling and discharging of hoppers and silos; and caking during transport and storage as a result of moisture migration.
   Dr John Baxter, University of Surrey, outlined the work which had been carried out by the team on degradation, a problem that seems to affect most industries at one stage or another. It has the potential to do untold damage when a product is likely to impact a hard surface within say a pneumatic conveying system or silo, possibly reducing particle size and modifying particle shape.
  To study this phenomena in an efficient, repeatable and convenient manner on site, the team developed a portable degradation tester, enabling particle speed and angle of impact to be carefully controlled when testing small quantities of material. The prototype toolkit was used to predict degradation in lean phase pneumatic conveying systems with results checked on full size conveying test rigs at the Wolfson Centre. Information on particle size distribution behaviour from the tests was inputted in spreadsheet form for analysis by the Physica software, enabling material degradation based on specific parameters to be predicted.
   Dr Mayur Patel of the University of Greenwich outlined the work undertaken on segregation, a key problem area associated with filling and discharge processes. The segregation toolkit developed by the team consists of a segregation tester which allows variables such as heap length, angle of repose and free fall height to be controlled. During the project a direct link was established between the DEM (Discrete element method) studies at Surrey and the continuum (Physica) modelling effort at Greenwich for predicting segregation in granular flows.
   Equations in the Physica framework reflect the three principal mechanisms for size segregation: kinetic sieving, where particles (mostly coarse) migrate across gradients of bulk strain; diffusion; and percolation. Work so far has allowed models to be developed for the segregation tester itself, as well as mass flow hoppers and core flow hoppers.
  One of the least understood areas which the team studied was caking, a problem that usually manifests itself during the transport and storage of particulate materials. This problem occurs as a result of moisture migration when there are changes in temperature and relative humidity.
   The team studied moisture migration in IBCs, a problem that occurs when the bag surface cools. The lower interstitial air temperature that results causes an increase in relative humidity, leading to the formation of liquid bridges between particles which solidify as the air temperature rises again and the relative humidity drops. With each cycle, these bridges increase in size until agglomerates form, compromising product quality. The QPM study examined how temperature varies within bulk solid systems, both in IBCs and within a caking rig developed at the Wolfson Centre.
   Moisture migration caking was studied through sets of experiments performed using the Wolfson Centre’s humidity facility to determine equilibrium sorption-desorption curves of materials and their crystalline strength - information which is essential for the Physica software to parameterise the caking process. Caking studies also measured tensile strength to try and link this to the amount of cycling.
  By relating these results to the moisture migration model, an engineering model could be constructed linking temperature cycling, humidity, moisture sorption characteristics, particle size and packing arrangement to the tensile strength of the cake formed. This then allows the location and degree of caking likely from changes in ambient conditions to be predicted. Potato starch in silos, pneumatic conveying of sugar and fertiliser storage were all areas studied by the team.
   Overall, QPM provided a number of tangible benefits. The characterisation equipment that has been developed will be available for sale in due course and consultancy services, using the full toolkit, will also be offered, although the full toolkits will not be available for sale until late 2005, giving the industrial partners adequate time to take full advantage of their investment.
   Summing up the work, team member Dr Hadi Abou-Chakra of the University of Surrey said that towards the end of the project, it was evident that the original objectives had been achieved, namely that: process models based on experimental work at the partners sites had been validated in new software programmes; there was an improved understanding of design tools for solids handling plant; and new test equipment to assess segregation, degradation and caking had been developed. These successes were underlined by the fact that the principal industrial partners had remained enthusiastic throughout the work, with others joining in at the halfway stage and agreeing to participate in a further year of technology transfer. This has five months to run and is designed to ensure that the outputs of the QPM Project are fully integrated into design and planning procedures.
   In their efforts to take the work forward, the QPM team has applied to the EPSRC to contribute to one of the new IMRCs (Innovative Manufacturing Research Centres) in the area of ‘Manufacturing for product effect and function, and the exploitation of nano science and technology’. If this application is successful, it will be of major benefit to researchers at Greenwich and Surrey in terms of funding in developing a possible ‘Son of QPM’.
www.qpm.org.uk