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Thread: Particle Size Degradation - Sugar

  1. #1

    Particle Size Degradation - Sugar

    G'day,

    We're having problems with particle size breakdown when conveying caster sugar pneumatically (pressure driven, about 60kPag). The system is set up with a dense phase bypass, which stops the blockages, but doesn't reduce the breakdown of sugar. It is running as close to dense phase as we can get it, through horizontal pipes (70m) and only 1 incline (12m vertical). It has five 90 degree bends and one 30 degree bend.

    Any ideas on how to minimise the breakdown through the system?? What are the effects of leaning it out further, or increasing the pot pressure before discharge??

    Thanks

    Daniel Fitzgerald
    Sugar Australia

  2. #2
    Dennis Hauch - Freeport, TX, USA Guest

    Sugar Conveying

    Daniel,

    Ive has some experience conveying cane sugar here in Freeport Texas so perhaps I can help.

    Particle degradation is a common problem with sugar and it is velocity related. The fact that you are operating your system at 0.6 bar argues loudly that the system is operating in a high-velocity dilute-phase conveying regime. Determine the conveying gas flow and calculate the terminal velocity. A properly designed dense-phase system will have a terminal velocity in the range of, say 6 to 8 m/s. Id bet a cold one that yours is closer to 30 m/s.

    An inclined conveying pipe is anathema to any pneumatic conveying system and is most assuredly a factor in the degradation of the sugar. Replace the inclined section with a straight / horizontal section.

    You mentioned that yours is a bypass type of system, this is a desirable feature in dense-phase sugar handling systems. Make sure, however, that it is truly designed to bypass the conveying gas as opposed to injecting additional conveying gas. When conveying gas is added willy-nilly to a dense-phase system it can easily become a high-velocity dilute-phase system.

    Leaning out the system further is the wrong direction from a degradation standpoint. Replace the inclined section and reduce the gas flow to 2 to 4 m/s at pickup. The conveying pressure will rise significantly but you should be in good shape with a supply pressure of 6 to 10 bars.

    Lastly, moisture in the conveying gas can pose problems in sugar handling systems. A dewpoint of minus 40 F would be desirable, you might even consider using nitrogen.

    I trust this input is good for you.

    Regards,

    Dennis Hauch

  3. #3
    Dennis,

    Firstly thanks for your input. I've had a look at the things you've outlined in the post, and a little underlying theory behind dense phase transport.

    Our system runs at a relatively low pressure, but from the characteristics of the flow (by listening) and from what the commisioning engineers have told me, our system seems to be running at around dune flow. It does however form slugs near the first 90 degree horizontal bend, and these can be heard travelling through and belting against the pipe fittings and any other obstructions. By my estimates the slugs seem to be moving at about 10-15m/s. I assume that this is higher than the average sugar flow due to the buildup of pressure behind them (up to 100kPa above operating pressure). From the background info i have read, the slower the flow, the less breakdown. However from talking to the guys that set it up, there is a point where the flow is small enough to allow the slugs to form, and consequently block the system. The other problem we have is that our maximum realistic operating pressure is about 2bar gauge due to the designs of the pipes.

    I'm thinking about putting a Variable speed drive on the blower, to slow it down, and raise the pressure up to about 2 bar g. The other, cheaper, option is to add a relief valve on the blower outlet to slow the system, and rely on the injection points to keep aeration. I'm unsure as to the effect this will have on the formation of slugs, which I assume cause a great deal of the breakdown, or maybe even blockages.

    What tests can be done on the injection ports to test their effectiveness, or to see if they're injecting too much air??

    Look forward to hearing from you.

    Daniel Fitzgerald
    Sugar Australia

  4. #4
    Dennis Hauch - Freeport, TX, USA Guest

    Sugar Degradation

    Daniel,

    I have evaluated the additional information that you provided. I see two core issues here:

    1) Before anything else is done the inclined pipe section must be replaced with horizontal / vertical elements. The inclined pipe is a virtual guarantee that the sugar will be degraded. It is also a key factor in unstable system operation and line blockages.

    2) It is imperative that the blower flow and injector flows be determined so the conveying velocities can be calculated. A blower curve and injector flow measurements are required.

    A dense-phase system can be optimized by reducing the gas flow (conveying velocity) as long as stable conveying is maintained or until the system pressure approaches that of the pressure source. The line injectors on dense-phase systems normally do not add conveying gas but simply bypass a part of the conveying gas to break up line blockages. In this case the line injectors are normally incrementally spaced along the full length of the system.

    A dilute-phase system can be optimized by reducing the gas flow (conveying velocity) until the point of minimum conveying pressure is reached (where the sugar begins to salt out of the conveying stream) and then by adding gas flow until the operating point is shifted just to the right of this minimum point on the pneumatic conveying diagram. Line injectors on dilute-phase systems are not as common but when provided should not add taken together more than 10% to the conveying gas stream. In this case the line injectors are located only at bends or where blockages are more likely to form.

    I trust this helps.

    Best regards,

    Dennis Hauch

  5. #5
    Daniel,

    From what you have told it appears to me that your conveying system is not running in true phase mode. Dense phase means a low velocity system, both at the pick up point and at the terminal point. In general, the pick up velocity should be about 2 meters/sec and the terminal velocity less than 8 meters/sec. You should calculate what these velocities are in your case. Line injectors (or boosters) are necessary only if you have line plugging problem. You should check the gas flow from each booster. Most vendors have flow indicators at each booster so that the gas flow can be monitored. In dense phase, inclined lines can be used as long as the slope of the line is less than the angle of wall friction, in general less than 20 degrees from the horizontal. Do you have long radius bends or some other kind of bends?

    Regards,

    Amrit Agarwal
    Pneumatic Conveying Systems
    Please visit our web page if you need more information.

  6. #6
    If you are not already using nitrogen to convey degraded caster sugar the you certainly ought to consider doing so on the grounds of reduced explosion risk.

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