What are the Selection criteria for vacuum system? Generally multiple pick up we prefer vacuum system. ■

What are the safety points which should be consider in vaccum system as compared to pressure system? ■

Originally Posted by Teus Tuinenburg

What do you mean by ?

The installation must work and stays intact.

What should be safety controls should be in consideration?what interlock should be necessary for control panel? ■

My first posting - not sure whether this should be a reply to this thread or a new separate posting.

Are you aware of any vacuum ship unloading and pneumatic conveying systems operating on raw sugar cargoes in 40,000 - 50,000 DWT bulk carriers. I have been requested to consider a vacuum ship unloader for the inbound raw sugar carried in Handymax or Supramax dry bulk carriers. While I have many years of experience in designing and consulting on raw sugar refineries we have no particular experience of vacuum ship unloaders and the feasibility of using them on raw sugar.

Raw sugar is generally categorised as a non-caking, free flowing material. However depending on the source of raw sugar and the pre-shipment storage conditions, the quality of the raw sugar can vary greatly. It can be prone to caking and compression at the base of a pile. The migration of moisture form the crystals can increase the "stickiness" of the sugar. Some cargoes can have a higher moisture content depending on the level of molasses on the surface of the crystal. This variability does not normally cause a problem with the traditional method of unloading and transferring raw sugar using clam grabs, hoppers and belt conveyors.

In terms of vacuum conveying I understand that the compressed air temperature can be elvated (as high as 175 decC) which would be problematic as it could cause degradation of the sugar inducing colour.

I would be grateful to hear of anyone's experience on raw sugar.

Thanks ■

Originally Posted by Teus Tuinenburg

Dear John Atkins,

I worked in the Port of Rotterdam for many years but was never aware that sugar was transported in 40.000 dwt to 50.000 dwt ships in bulk.

Vacuum unloading of non-caking, free flowing sugar is certainly a viable option.

As soon as the other properties are playing a role, it becomes at least a bit uncertain if it is feasible.

Laboratory tests are advised for the sticky and moist varieties.

The disadvantage of a pneumatic conveying system for sugar can be (probably will) particle size degradation and forming sugar dust, with an explosion risk.

Conveying temperatures in vacuum conveying is around the ambient temperature and the sugar temperature. That is no problem.

Water condensation is no problem either, because the conveying air reduces in pressure along the pipeline, causing the conveying air getting dryer.

For a pressure system, the compressed air must be cooled, and the formed condensation must be separated.

This is the only way to prevent water condensation at the mixing point and along the pipeline.

Also, the conveying gas must be cooled to prevent caramelization.

In food factories, pneumatic conveying is used for low capacity transport.

A pneumatic vacuum unloader for sugar for handymax vessels is not a small unit.

Dear Teus

Many thanks for your thoughtful reply. To expand on some of your points I can add the following.

It is true that historically most cane sugar refineries in Europe were riverside and probably that limited vessel size for incoming raw sugar shipments. Also mainland Europe sugar industry was predominantly based on regionally grown sugar beet, rather than imported raws, whihc mainly came from ex-colonial countries with declining cane sugar industries. As more countries around the world started their own sugar refinery industry seaport side locations were favoured and the capacity of these new refineries has steadily grown several fold. Raw sugar was imported in large quantities from extremely large cane sugar industries in such countries as Brazil, Australia, Thailand, India, Guatamala and South Africa. THe trend has been to larger cargo sizes and 50,000 DWT is now probably on the smaller size for these shipments.

You are correct that pneumatic conveying would cause crystal degradation but this is not critical as the first step in the refining process is to melt the raws. It probably only represents a housekeeping problem - which leads me to dust. As to dust, it is extremely rare for raw sugar dust to present an explosion risk owing to its the high moisture content. large particle size and low concentration in air. Almost without exception the explosion risk is with refined sugar dust - where it is real problem that has caused fatalities and catastrophic damage. High temperatures would increase the colour of the raw sugar making refining process more challenging and also increase the propensity of the raw sugar to cake & build-up.

Your point about the size of the unit is important. In the project which is the subject of the feasibility study, the refinery capacity is small and the number of 50,000 DWT vessels to be discharged annually is probably no more than 10 to 12. Therefore the overall utilisation of this plant would be low and I suspect that the capital cost of the investment is not justified, even if a supplier was interested in investigating the suitability of raw sugar for pneumatic conveying.

Do you have an indicative cost ex factory for, say, a 500 t/h vacuum ship unloader only (without pneumatic conveying)?

As the basis of a feasibility study, it appears that the more traditional grab unloader-transfer hopper-belt conveyor is a better option.

I do have one final question - I read on another post your formula for estimating average unloading capacity based on DWT (Qave=120*DWT^0.35). Using this for 50,000DWT gives an average rate of 5300 t/d. Assuming, say, 67% unloader efficiency (is this too high for grab unloader system?), this would indicate a grab unloader design capacity of approx 500t/h for 16h day and 330 t/h for 24h day. This also implies a discharge time of 50,000/5300 = 9.5 days. Not being a shipping expert I wondered whether this would be considered a reasonable laytime for a handymax vessel?

I have contacted a number of companies manufacturing vacuum ship unloaders and so far none has replied positively.

It appears that in semi-retirement after 30 years in the sugar industry there is still more to learn! ■

Dear Teus

Yes you are right there would not have been many large shipments of bulk raw sugar through Rotterdam. Most refined sugar in Europe is from locally grown sugar beet. What raw sugar that was imported into Europe at that time probably came from former colonial countries with relatively small cane raw sugar industries and the European refineries would have been based on river/sea ports such as in London, Liverpool, Glasgow., Lisbon and Marseille. In recent years there has been a rapid expansion of refineries in countries in eg Asia, Middle East and North Africa. These are large capacity refineries usually close to a seaport or wharf frontage importing large quantities of raw sugar from major exporters such as Brazil, Australia, Thailand, India, Guatemala and South Africa. To keep freight costs down the Handymax/Supramax bulk carriers are probably now amongst the smaller sized vessels for raw sugar trade.

Pneumatic conveying of raw sugar would cause crystal degradation but this is probably not critical as the initial stage of the refining process is to melt the raw sugar in water. It probably only represents a housekeeping challenge to avoid clogging and caking of conveying equipment. Sugar dust is problematic but dust from raw sugar does not present an major explosion risk (although it is a serious fire risk) owing to its moisture content, particle size and concentration in air. Dust from refined sugar is however a major explosion risk and has caused catastrophic damage and fatalities in the industry.

High temperatures in a pressure system would be problematic as it increases the colour of the sugar (even before caramelisation) so cooling would be essential.

Your comments about pneumatic conveying in food industry used for low capacity are pertinent as we would be looking for unloading rates around 500 t/h (see below) and conveying the raw sugar some 400 m to the store. In addition, the proposed size of the refinery would indicate that it would receive only eight to ten 40,000-50,000 DWT raw sugar shipments annually resulting in a low overall utilisation of the system. As you indicate that a vacuum unloader for a Handymax vessel would not be a small unit I suspect that the capital cost would not be justified. The option for a vacuum unlaoding system must be questionable given the above and the variability of the raw sugar physical characteristics leading to technical uncertainty on the feasibility of designing a raw sugar vacuum unloading/pneumatic conveying system. I believe the feasibility study at this stage would be better based on a traditional grab unloader, transfer hopper and belt conveyor system. To support this view I have contacted various vacuum unloader manufacturers and none has yet confirmed its suitability to handle raw sugar.

Would you have a ball park estimate of the capital cost ex factory of a 500 t/h rail-mounted vacuum unloader.

If I may raise one final point - in another of your posts I came across a formula to estimate average unloading rates (Qave=120*(DWT^0.35)). For a 50000 DWT vessel this gives Qave=5300 t/d which equates to nearly 500 t/h (assuming a unloading efficiency of 67%). Given that the port is small and bulk cargo unloading is not commonly practised would 67% efficiency be too high for a grab unloader system. Even assuming average 5300 t/d, this would represent a laytime of just over 9 days. Is there a typical laytime for Handymax bulk carriers in small ports?

In semi-retirement after 30+ years in the sugar industry and I still find something new to learn! ■

Thanks, Teus - very useful data which will help with my research.

Stay safe in these unusual times

John ■