The application of the six sigma quality concept to improve process performance in a continuous processing plant

[摘要] ENGLISH ABSTRACT:This report presents the application of the six sigma quality concept in solving a truebusiness problem. Six sigma is a quality improvement and business strategy/tooldeveloped by Motorola in the mid 1980s. It aims at delivering products and services thatapproach levels of near perfection. To achieve this objective a six sigma process must notproduce more than 3.4 defects per million opportunities, meaning the process should beat least 99.9997% perfect [Berdebes, 2003]. Motorola's success with six sigmapopularised the concept and it has now been adopted by many of the world's topcompames e.g. General Electric, Allied Signal-Honeywell, etc. All the six sigmacompanies report big financial returns as a result of increased quality levels due to thereduction in the number of defects. 'General Electric reports annual benefits of over $2.5billion across the organisation from six sigma' [Huag, 2003].The six sigma concept follows a five step problem-solving methodology known asDMAIC (Define, Measure, Analyse, Improve, Control) to improve existing processes.Each of these steps makes use of a range of tools, which include quality, statistical,engineering, and business tools.This report first gives a theoretical presentation on quality and six sigma, attempting toanswer the question 'What is six sigma'. A step-by-step guide on how to go through theDMAIC problem solving cycle is also presented.The six sigma concept was demonstrated by application to the colour removal process ofa continuous processing plant manufacturing refined sugar. Colour removal is a veryimportant process in sugar refining since the purpose of a refinery is to remove colourand other impurities from the raw sugar crystals. The colour removal process consists ofthree unit operations; liming, carbonation and sulphitation. Liming involves the additionof lime (calcium hydroxide) required for the formation of a calcium precipitate in thenext unit operations. Carbonation is carried out in two stages; primary and secondarycarbonation. Both stages involve the formation of a calcium carbonate precipitate, which traps colour bodies and other impurities. Sulphitation occurs in a single step and involvethe formation of a calcium sulphite precipitate which also traps impurities. The pH andcolour are the main variables that are being monitored throughout the colour removalprocess. Colour removal processRaw sugarMelting Carbonation Crystalli~ Liming ~ c::J Secondary f+ Sulphitation ..SugarsationFigure 1: Colour removal processThe pH control of the two colour removal unit operations; carbonation and sulphitation,is very poor and as a result the colour removal achieved is below expectation. Thiscompromises the final refined sugar quality since colour not removed in the colourremoval processes ends up in the sugar. The first carbonation stage (primary) fails tolower the pH to the required specification and the second carbonation stage (secondary)is highly erratic, the pH fluctuating between too high and too low. The sulphitationprocess adds more sulphur dioxide than required and hence the pH is lowered below thelower specification limit.The six sigma DMAIC cycle was implemented in order to solve the problem of poor pHcontrol. The Define phase defined the project and identified the process to be improved.The Measure phase measured the current performance of the process by collecting pastlaboratory data with the corresponding field instruments data. The data was used to drawfrequency distribution plots that displayed the actual variation of the process relative tothe natural variation of the process (specification width) and to calculate processcapability indices. The Analyse phase analysed the data so as to determine the keysources of variation. The Improve phase used the findings of the analyse phase to propose solutions to improve the colour removal processes. The Control phase proposed a controlplan so as to monitor and sustain the improvement gained.The key findings of the study are presented below:• Failure of the first carbonation stage to lower the pH to the required level is due toinsufficient carbon dioxide gas supply.• The second carbonation reaction occurs very fast hence poor control will result inhigh variability.• The amount of colour removed is dependent on the input raw melt colour.• The histograms of the colour removal unit operations are off-centered and display aprocess variation greater than the specification width and hence a large proportion ofthe data falls outside the specification limits.• The % CaO and CO2 gas addition were found to be the key variables that control theprocesses centering on target. The % CaO having a stronger effect in the limingprocess and CO2 gas addition on the carbonation process.• The variation between the field instrument's pH and laboratory pH is the key variablethat control the processes spread (standard deviation of the processes).• The processes Cpk values are less than C, (Cpk

[发布机构] Stellenbosch University