Crystal separation and drying devices.

Crystals and mother liquor are separated in basket-type centrifuges (Figure 37). Continuous machines are used for C and sometimes B sugars, but batch machines are best for A sugars because of the crystal breakage that takes place in continuous centrifuges. Mother liquor is spun off the crystals, and a fine jet of water is sprayed on the sugar pressed against the wall of the centrifugal basket, reducing the syrup coating on each crystal. In modern factories, the washing process is quite extensive in an effort to produce high-purity raw sugar. Overall recovery of sugar from cane juice averages between 70 and 80 percent.

The washed sugar, dumped from the baskets onto moving belts, dries and cools on the belts as it moves to bulk storage. At this point it is pale brown to golden yellow, with a sucrose content of 97-99 percent and a moisture content of 0.5 percent. This raw sugar, the sugar of commerce, is stored in bags in countries where labour is abundant and cheap. Generally, however, it is stored in bulk and shipped loose, like grain, in dry-bulk ships to areas where it will be refined.

Figure 37. Basket-type centrifuge.

Exercise 2. Translate the following word-combinations.

Overall recovery of sugar from cane juice, against the wall of the centrifugal basket, the crystal breakage, another set of evaporators, nonsugars deposit, a fine jet of water, to produce high-purity raw sugar, to be sent to vacuum pans, to reduce the efficiency of heat transfer.

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Exercise 3. Checking facts and ideas.

Decide if the following statements are true or false.

1. Sugar crystallization is a batch process.

2. The multiple-effect evaporators were developed in the 18th century.

3. The cane sugar is white in colour.

4. A-sugar crystals and mother liquor are separated in continuous basket-type centrifuges. 5. The sugar "mother liquor" contains about 50 percent by weight crystalline sugar.

Exercise 4. Complete each sentence with a word or phrase from the box.

washing series of evaporators scale deposits overall recovery continuous centrifuges

1. The crystal breakage usually takes place in ___. 2. Sugar concentration is carried out in a ___.

3. ___ on the walls and tubes reduce the efficiency of heat transfer in the evaporator. 4. ___ of sugar from cane juice at modern sugar plants is about 80 percent.

5. ___ is a very important stage in the process of high-purity raw sugar manufacture.

Exercise 5. Give all the meanings of the following words. Illustrate them in sentences. Grade, scale, fine, roll, plant, seed, bulk, belt, raw, point, recovery, mother, break, jet.

Exercise 6. Complete the table concerning the stages of cane sugar production using the information from the texts above and your own knowledge.

Process Purpose Equipment 1. Weighing

2. Loading 3. Cutting

4. Juice extraction 5. Clarification

6. Concentration 7. Crystallization 8. Separation

Exercise 7. Translate in the written form.

The Force-Fed Lotus Mill (Figure 38) is a 4-roll mill having a Unice Chokeless Lotus Roll as top roll, a front roll, a back roll, and a force-feed roll of large diameter -all disposed as to form a so-called pressure chamber 4 and three nips, namely, a pressure feeder nip 1, a mill front nip 2 and a mill discharge nip 3 - with a chute for the purpose of feeding well prepared cane through nip 1, as shown.

The cane passing through a Force-Fed Lotus Mill undergoes three distinct processes, namely:

· The compacting process (to increase the density of the prepared cane prior to the latter entering nip 1);

· The pre-drying process (to remove a portion of the juice from the cane prior to the latter entering nip 2);

· The milling process (to extract a maximum amount of the juice remaining in the cane).

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Figure 38. Force-Fed Lotus Mill.

Дата добавления: 2015-11-16