Liquid purification or separation – Processes – Separating
Reexamination Certificate
1999-06-01
2001-07-31
Reifsnyder, David A. (Department: 1723)
Liquid purification or separation
Processes
Separating
C210S787000, C210S360100, C210S369000, C210S377000, C210S380100, C127S019000, C494S036000, C494S037000, C494S056000, C494S057000
Reexamination Certificate
active
06267899
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to centrifugal separation apparatus.
This invention has particular but not exclusive application to a continuous centrifuge for separating sugar crystals from a sugar solution, or massecuites and it will therefore be convenient to hereinafter refer to this example application. However It is to be clearly understood that this invention could be used in other applications for separating other solid/liquid mixtures.
2. Brief Description of the Art
In the processing of sugar crystals, a filtration centrifugation separation process, eg a centrifuge, is frequently employed to separate sugar crystals from a sugar solution. This is especially so for highly pure crystal sugar for consumption or refining where crystal quality is important.
In most known industrial applications, the separation of sugar crystals from massecuite and washings is carded out in batch processes using batch centrifuges. In a batch centrifuge, a predetermined charge of massecuite is delivered to a centrifuge basket. The basket is then accelerated to a predetermined angular velocity and held for a predetermined spin time to achieve a desired level of syrup removal before wash water and/or an alternative liquid is sprayed onto the exposed crystal layer for a predetermined “wash time”. The basket is then hold at the aforementioned, or an alternative, angular velocity before decelerating the basket and discharging the washed crystals therefrom.
Some advantages of the batch centrifuge are that crystal size and shape are preserved since the crystals do not undergo any high velocity impacts and that by appropriate selection of spin times and wash times, the required level of residual impurities can be controlled. However these machines require time to be charged with feedstock and emptied of product and also require time to accelerate and decelerate thus reducing their effective capacity, Thus the batch operation imposes a considerable down time thereby requiring a relatively larger number of filtration centrifugation units to satisfy the throughput of a sugar crystallization plant.
Maintenance requirements are significant due to the cyclic operation of the equipment and power consumption is increased due to the inefficiency of the cyclic operation.
Attempts have been made to develop continuous centrifuges suitable for producing high quality sugar crystals. However these attempts have not produced very satisfactory results. A major problem with known continuous centrifuges is crystal breakage caused by high speed impacts between crystals leaving the conical basket and the walls of a sugar chamber and also crystal impacts with other crystals. In addition, wet lumps of sugar may agglomerate and these naturally adversely affect sugar crystal quality.
As a result continuous centrifuges have not been widely used up to now for high purity product sugar separation. However in view of the obvious shortcomings of batch centrifuges as articulated above, It would clearly be advantageous if a continuous centrifuge could be devised which preserved a good sugar crystal quality, eg suitable for consumption and use in refining.
OBJECTS AND SUMMARY OF THE INVENTION
According to one aspect of this invention there is provided a centrifugal separation apparatus for separating sugar crystals from a sugar syrup, the apparatus including:
an open topped basket having a discharge kip of curved convex configuration towards an upper end thereof over which sugar crystals pass when they are discharged from the basket; and
a deflector spaced radially outwardly of the lip for deflecting sugar crystals discharged from the basket downwardly into a sugar chamber.
Preferably said convex discharge Up has a radius greater than or equal to the radius calculated by the following formula:
V
2
/&dgr;=9.8 cos &thgr;+
Rw
2
sin &thgr;
where
V is the relative velocity between the crystals and the centrifuge;
&thgr; is the angle from a horizontal axis at which free solids leave the centrifuge and is less than or equal to 2;
R is the basket radius of the centrifuge at &thgr;;
w is the rotation speed in radians per second of the centrifuge;
&dgr; is the lip radius at the top of the centrifuge.
V is difficult to determine but is typically in the range 0.5 to 5 ms
−1
and for high grade sugar applications close to 1 ms
−1
Typically, &thgr; would be selected at or near 1° as shown in FIG.
5
.
Advantageously the curved convex discharge lip causes the trajectory of the sugar crystals to be independent of their size and weight and to make the initial impact at broadly the same height on the deflector.
According to another aspect of this invention there is provided a process for continuously centrifuging sugar crystals from a mixture of crystals and syrup, the process including:
continuously feeding the mixture into a centrifuge as described above with respect to the first aspect of the invention through the inlet thereof, centrifuging the massecuites in the basket of the centrifuge so that liquid passes through openings in the wall of the basket and the crystals move progressively up the wall of the basket;
discharging crystals from the basket by passing them over said discharge lip;
and deflecting the crystals off the deflector downwardly towards the solids outlet
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Greig Christopher Roy
Kirk Russell
Tait Peter James
Burns Doane , Swecker, Mathis LLP
Reifsnyder David A.
STG-FCB Holdings PTY Ltd.
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