Factory: Engineering CONTINUOUS CENTRIFUGAL …

1 factory : Engineering CONTINUOUS CENTRIFUGAL FOR high -GRADE SUGARS Greigl & A. Belotti2 STG Group, Australia PCB, Prance ABSTRACT For several decades technologists throughout the world have sought to design a CONTINUOUS CENTRIFUGAL capable of producing highquahty crystal. Both raw sugar (for feeding the refinery or direct consumption) and white sugar have stringent quahty specifications set by customers. Batch machines have traditionally been preferred for such applications because they provide higher purity and unbroken crystal. CONTINUOUS machines tend to produce lower purity sugar and crystals are broken due to highenergy impacts with the casing.

2 In addition problems occur with wet, lumpy sugar. In 1993 STG Engineering constructed a prototype machine for CSR Ltd. to work with Australian A & B strike massecuites, which produce high -grade sugar for feed stock for international refineries. This machine produced high -quahty sugar with none of the quality disadvantages that had previously been experienced. Soon afterwm, a white sugar prototype was built for Manildra Harwood Sugars Refinery for use on first, second and third strike white massecuites to produce high -@ty white refined sugars. Keywords: Sugarcane, CONTINUOUS CENTRIFUGAL , crystal breakage, white sugar.

3 INTRODUCTION The STG Group became involved in designing and constructing centrifugals for high -grade sugar processing in 1989. The former resulted from the application of CONTINUOUS centrifugals to affination in a refinery constructed and operated by Manildra Hatwood Sugars in Australia. CONTINUOUS centrifugation was adopted for the affination process by installing mded low-grade machines from a traditional commercial supplier. However, inadequate performance necessitated a redesign and-retrofit, which was carried out by STG in association with Manildra Harwood Sugars. This successful modification has been described by Greig et al(1992).

4 When affination is practiced, crystal breakage, lump formation and moisture content in the product sugar are not problematic as the sugar is melted. However, these criteria are all crucial to the production of c~ and "shipmentN-quality sugars. In 1993 after several Mer years ,off developmeqt, STG installed a prototype conthuous CENTRIFUGAL in CSR's , Invicta factory , processing A & B strikes-both of wbich are marketed as "shipment" sugar for export. Following a short period of development work, this machine was accepted by the customer as fulfilling all the requirements of a high -grade CENTRIFUGAL that are traditionally fulfilled by batch-type machines.

5 Very soon aRer the successful demonstration of the raw sygar machine in Australia, Manildra Hardwood Sugars decided to investigate the feasibility of using this technology on white refined sugar. The feasibility study suggested there would be substantial benefits in cost, delivery time impact; thus they placed an order for a CONTINUOUS CENTRIFUGAL with STG. Again after a short development period, the CONTINUOUS CENTRIFUGAL was found to satisfy all the refinery's objectives and has now been in routine operation since 1993. This paper outlines various design feature of the machine, its performance and the implications for sugar producers.

6 The results and conclusions reached to date have been established over 3 yr on affination, 2 milling campaigns on raw sugar and 1 yr on refined sugar. REVIEW OF PREVIOUS WORK Goodacre et a1 (1984) presented a concise review of the various studies and programs to develop a viable CONTINUOUS alternative to batch centrifugals for pgh-purity applications. These same authors also reported on their own efforts to achieve that objective. They achieved positive results for affination, where crystal quality (size, shape and moistm) is of no concern. Mitchell & Smith (1987) and Greig et al(1992) reported similar success with the latter claiming to match and exceed the performance of batch machines.

7 Greig et al(1992) were not able to compare batch and CONTINUOUS equipment directly in side-by-side tests, but they dexreloped a novel method for generalized comparisons based on the "residual mother syrup" adhering to the mysd. The ultimate challenge-developing a CONTINUOUS machine for shipment raw and direct consumption sugars-had * Greig & A. Belotti not met with success. A number of patents claiming to eliminate breakage were listed by Goodacre et al (1984). Until then, however, no CONTINUOUS machine had been accepted as a truly viable alternative to the traditional batch machine.

8 Atherton et al(1990) delivered the most positive report on the performance of the "Superfugal" in processing Austra- lian raw sugar rnassecuites, but acceptance by industry appears to bave been limited. For refinery and factory output products, yield, crystal breakage, lump formation and excessive product moisture remained as significant barriers. DESIGN FEATURES OF THE STG CONTINUOUS CENTRIFUGAL Given the present nature of current patent applications, full details of design feature are not revealed here. The focus of the design effort was directed at: Controlling the trajectory of crystals after leaving the basket so as to decelerate and deliver them to subsequent processes without breakage Creating a self-cleaning impact surface to prevent lump buildup The separation efficiency gains were established on the basis of the work report'ed by Greig et al(1992).

9 A 25- degree basket angle (also recommended by Goodacre et a1 1984) was adopted. Low-impact sprays were employed to retain a CONTINUOUS , uniform crystal layer on the screen. Low-pressure steam applied to the basket aided purging as well. A basket speed of approx. 800 rpm provides optimal purging and washing without incurring any detectable grain fracture. This low speed also results in low power consumption (Greig et a1 1986). A key feature& the complete automation of the machine with remote, sequenced startlstopping, automatic control of the feed valve to provide steady load atxl automatic control of the wash water flows as a function of actual feed rate.

10 This gained acceptance by operators when the machines were commercially introduced. Dimensions of the machine are shown in Figure 1. GENERAL PERFORMANCE FEATURES OF THE STG CONTINUOUS CENTRIFUGAL Crystal quality Crystal @ty problems had previously militated against the application of CONTINUOUS centrifugals for high -grade sugar processing. Experience throughout the world on the use of CONTINUOUS centrifugals in high -grade applications has underlined three major crystal quality issues: Crystal breakage, moisture content and lumps. Experience throughout the Australian raw sugar industry up to 1993 has been that crystal breakage can be avoided by the use of static deflector baffles to decelerate the crystals.