Patent Application
20150211081
This invention relates to the refining of impure crystallised sucrose (raw sugar) by applying multiple effect falling film evaporation to concentrate a runoff produced by centrifugation of massecuite arising from a sucrose crystallisation process.
In this specification, the following terms have the following meanings.
Bagasse: Cane residue leaving mills after extraction of juice.
Boiling point elevation: Difference between the temperature of a boiling sugar solution and the temperature of boiling pure water, both measured at the same pressure.
Brix: Measure of dissolved solids in sugar, juice, liquor or syrup using a refractometer, otherwise referred to as refractometric dry solids. For solutions containing only sucrose and water, Brix=% sugar by mass.
Centrifugal: Centrifuge used to separate sugar crystals from mother liquor conventionally using a fine screen, allowing the mother liquor to pass through the screen whilst the crystals are retained on the screen.
Cooling crystallisation: Crystallisation by cooling of the massecuite. Massecuite exhibits direct solubility wherein the solubility threshold increases with temperature with the result that crystal growth results from simply cooling the solution. Conventionally the cooling is achieved by indirect heat transfer, i.e. heat transfer through a metal wall, to a colder cooling fluid stream. It is also possible to achieve the required cooling of the massecuite by flash cooling. This is achieved by reducing the pressure that the massecuite is exposed to, to the point where flash evaporation takes place and the massecuite cools as a result of the latent heat that is removed with the vapour so generated. The crystallisation that takes place is predominantly as a result of the resulting cooling of the massecuite, although a small proportion will be due to the increase in concentration resulting from the evaporation that takes place. In both instances this is crystallisation without heat input driven evaporation.
Crystallisation: Nucleation and/or growth of crystals.
Crystal yield for a stream: For a stream containing crystal, this is the mass of crystal expressed as a percent of the total mass of soluble solids in the stream (i.e. mass of crystal as a percent of the sum of mass of crystal and the mass of dissolved solids in the mother liquor).
Crystal yield for a process: For a process producing crystal, this is the mass of crystal produced, expressed as a percent of the total mass of soluble solids in the stream entering the process.
Evaporator effect: One of a system of evaporators operating in series as a multiple effect system (e.g., first effect, second effect). Condensates and vapours are labelled correspondingly (e.g., first condensate and vapour one: condensate and vapour from the first effect respectively).
Evaporative crystallisation: At an approximately constant temperature, achieved by holding the operating pressure constant, crystallisation occurs by increasing the solute concentration above the solubility threshold. To obtain this, the solute/solvent mass ratio is increased using evaporation that is driven by the addition of heat. The heat addition is conventionally indirect heating, i.e. heat transferred through a metal wall, using the latent heat that is released by the condensation of steam with a higher saturated vapour temperature than the temperature of the boiling solute. This is concentration with crystallisation
Falling film evaporator/evaporation: A heat exchanger where the evaporation takes place on vertical or substantially vertical surfaces, and the process fluid to be evaporated flows downwards by gravity as a continuous film. The fluid will create a film along the surfaces, progressing downwards (falling)—hence the name. Appropriately designed plates or tubes may be used as the surface for the fluid to flow over, but it will be appreciated that other embodiments of falling film evaporators are included in the ambit of this description. Falling film evaporators are very susceptible to solid fouling, for example, through crystallisation and are therefore inappropriate for use in evaporative crystallisation.
Liquor: A sugar syrup, a term generally used in sugar refining.
Magma: The mixture of crystals and liquor created my mixing these two products
Massecuite: The mixture of crystals and mother liquor resulting from the crystallisation process.
Melting: Another term for dissolving of sugar crystals.
Molasses: The mother liquor separated from the crystals by centrifuging.
Mother liquor: Liquid phase in the massecuite during crystallisation; refers to syrup or liquor in which the crystals are growing.
Multiple effect evaporation: Effective use of the heat from steam to evaporate water wherein water is boiled in a sequence of vessels, each held at a lower pressure than the last, Because the boiling temperature of water decreases as pressure decreases, the vapour boiled off in one vessel can be used to heat the next, and only the first vessel (at the highest pressure) requires an external source of heat.
Pan or vacuum pan: Vacuum evaporative crystalliser used in the sugar industry to crystallise sugar from liquor, syrup or molasses.
Polarization (or pal): The apparent sucrose content expressed as a mass percent measured by the optical rotation of polarized light passing through a sugar solution. This is accurate only for pure sucrose solutions.
Raw sugar: Brown sugar produced in a raw sugar mill generally destined for further processing to white sugar in a refinery.
Recovery house: A section of a sugar refinery that produces crystalline sugar that is not of sufficiently high enough purity to be classified as refined sugar, and in consequence is conventionally recycled as input to the refinery where it is treated as if it were raw sugar.
Refinery: A processing plant designed to refine raw sugar to produce refined sugar
Refined sugar: A very high purity sugar (sucrose) that is conventionally above 99.9% with specifications on the maximum levels of various impurity components, most importantly colour (e.g. less than 50 ICUMSA Colour Units)
Refining: Purification of sugar through chemical and physical methods, generally including some or all of dissolution, clarification, filtration, decolourisation and recrystallisation.
Runoff: General term for syrups or molasses produced on centrifuging a massecuite.
Saturation: A sugar solution at saturation will not dissolve any more crystals at the temperature of the solution.
Seeding: (a) Introducing crystal fragments to induce nucleation, as a means of initiating the crystallisation process; (b) introduction of fine crystals in the form of a slurry (similar to full seeding) to start crystallisation.
Seed: Suspension of fine crystals in saturated solution of alcohol, or the initial grain resulting from seeding in a vacuum pan.
Sucrose: Pure chemical compound C12H22O11 known as white sugar, generally measured by polarization in pure solution or by Gas Chromatography (GC) or High Performance Liquid Chromatography (HPLC) in impure solution. The chemical term is P-D-Fructofuranosyl α-D-glucopyranoside.
Sugar: Term for the disaccharide sucrose and products of the sugar industry, essentially composed of sucrose.
Supersaturation: The degree to which the sucrose content in solution is greater than the sucrose content in a saturated solution.
Syrup: The concentrated juice or sugar solution from the evaporators.
Vapour: Generally used to describe steam that is produced by the evaporation of a process stream, rather than steam produced by a boiler that evaporates high purity water. Vapour streams are often at pressures below atmospheric pressure.
Vapour bleeding: The process of withdrawing vapour from one or more of the vapour streams within a multiple effect evaporator station. By using this vapour in place of steam of the same type as that used to feed the multiple effect evaporator station, there is a saving of steam.
Vapour compression: A technique for improving energy efficiency by increasing the pressure and thereby the temperature of a vapour stream. This can be done using a venturi nozzle driven by an injection of high pressure steam (thermocompression) or by a mechanical compressor (vapour recompression).
Sucrose is a disaccharide compound used throughout the world in food-processing applications as a sweetener. Crystalline sucrose is primarily produced from the sugarcane plant which is cultivated in the tropical and semitropical regions of the earth. In this specification, the term sugar is considered to mean sucrose.
Throughout the world today, refined cane sugar from sugarcane has been accomplished in two steps: (a) the raw sugar process; and (b) the refinery process.
In the raw sugar process, sugar mills, located in or near the cane fields, convert the harvested sugarcane plant into a commodity of international commerce known as raw sugar (impure crystallised sucrose). The raw sugar is transported to sugar refineries, located in population centres throughout the world, where it is converted into its various refined end products. In contrast to the sugar mill, almost the entire output of the sugar refinery is intended, in one form or another, for human consumption.
In the production of raw sugar in the sugar mill, the sugarcane stalks are chopped into small pieces. Then, cane juice is extracted from the sugarcane, leaving behind a fibrous material called bagasse. The extracted juice is then clarified to yield a lighter-coloured raw sugar solution (juice). The clarified juice is then processed through a series of evaporators to eliminate water, which is approximately 85% of the cane juice, resulting in a concentrated sugar solution called syrup. The syrup is then put through a crystallization process, which generates sugar crystals and further separates impurities. Finally, centrifugation separates raw sugar from the syrup, now termed molasses. The molasses is usually processed more than once so that as much of the sugar as possible can be recovered from the syrup.
In the sugar refinery, the raw impure sugar crystals from the sugar mill are cleaned and then melted (dissolved in water). Then, the sugar solution is purified and (optionally) decolourised (including by way of carbonatation, phosphotation, ion exchange, carbon and combinations of these). The sugar solution is then passed through evaporators to remove some of the water and the remaining product is then passed to a vacuum pan for further evaporation and crystallization. Typically refinery crystallisation utilises vacuum evaporative crystallisation equipment. The crystallisations are carried out in batch evaporative crystallisers or continuous evaporative crystallisers. Evaporative crystallisers are commonly referred to the sugar industry as “pans”. The end product is then passed through centrifuges to separate the pure sucrose crystals from the mother liquor to yield refined sucrose crystals and runoff. Since the majority of the impurities are contained within the mother liquor it is important to remove even the smallest quantity of mother liquor adhering to the crystals. To achieve this it is conventional to wash the crystals with hot water and finally purge them with steam whilst the crystals are still retained within the centrifugal basket. Careful attention to both the method and quantity of water addition is necessary to maximise the purging of the mother liquor whilst minimising any crystal dissolution.
In the sugar mill (in which sugar is recovered from cane juice) the purification process is also via re-crystallisation, both by evaporative crystallisation and by cooling. Where cooling crystallisation is used the equipment is referred to simply as a “crystailiser”.
This basic process, raw sugar manufacturing followed by raw sugar refining, is the process commonly used throughout the world today to produce high-quality white refined cane sugar (sucrose) with a polarization (or, optically measured purity) of from about 99.90% to 99.99%. It is a two-step process which is employed even in locations where there is a sugar refinery near, or even within, a sugar mill. Even entities outside the sugar industry have arranged their business affairs to accommodate this state of the technology. Raw (impure) sugar is traded worldwide as a commodity on the New York and London stock exchanges.
Thus sugar mills produce crude sugar products, their main product being raw (impure) sugar whilst high-quality refined sugars demanded in major population centres come from the sugar refinery which is a technologically sophisticated operation that employs expensive equipment and numerous chemicals and consumes large amounts of energy in order to produce the refined sugar product (sucrose crystals).
Regardless of which upfront process is used to provide a decolourised feed to the refining process, the known processes in sugar refining have the following drawbacks arising from the crystallisation purification step.
The invention now makes it possible to produce high quality refined sugar at greatly reduced energy consumption. Thus, the invention facilitates the conservation of energy and material resources and minimizes environmental pollution. In particular there is (without limitation):
According to a first aspect to the present invention there is provided a process for refining impure crystallised sucrose, the process comprising the application of multiple effect falling film evaporation to concentrate without crystallisation a runoff produced on centrifugation of massecuite arising from a sucrose crystallisation process.
As such, it will be appreciated that concentration according to the present invention takes place by evaporation from a solely liquid phase stream, in the absence of any crystalline sucrose.
The sucrose crystallisation process may be a cooling crystallisation process, where the crystallisation takes place without any heat input driven evaporation.
The impure crystallised sucrose is derived from sugarcane and in one embodiment is obtained from a sugar mill.
In one embodiment the runoff has a Brix value of from approximately 66 to approximately 72 before the application of multiple effect falling film evaporation.
The runoff (syrup) may have a Brix value of from approximately 80 to approximately 83 after the application of multiple effect falling film evaporation.
In one embodiment of the present invention the multiple effect falling film evaporation is integrated into a single operation. The multiple effect evaporation may be a two effect, three effect, four effect or five effect evaporation.
The first effect of the multiple effect evaporation may be achieved by supplying steam to a first evaporator at a pressure of approximately 160 to approximately 220 kPa (kilopascal) absolute with corresponding vapour saturation temperatures of approximately 113.3 to approximately 123.3 degrees Centigrade.
The last effect of the multiple effect evaporators may run at a pressure of between approximately 10 and approximately 30 kPa absolute corresponding to saturated vapour temperatures of between approximately 45.8 and approximately 69.1 degrees Centigrade.
In one embodiment, the process includes bleeding steam for use outside the evaporator system. The process may also include bleeding or otherwise using clean water condensed from the steam used in the multiple effect evaporation.
The sugar crystals may be washed during or following centrifugation. In one embodiment the sugar crystals are washed using a liquor, for example a hot liquor, a secondary liquor and/or a fine liquor.
The process may include seeding the liquor or syrup prior to crystallisation. This comprises the addition of the requisite number of small crystals so that these seed crystals grow throughout the crystallisation process to provide product crystals of the required size (normally between 0.4 and 0.7 mm). The crystallisation process is carefully controlled to ensure that only the added seed crystals grow whilst no new seed crystals are allowed to arise by nucleation and then continue to grow.
As such, the process according to the invention may include the following steps:
The sugar crystals recovered may be washed with liquor, for example, the fine liquor of step a).
The cooling to derive sugar crystals is effected in a cooling crystalliser. The temperature difference in the cooling crystalliser between the syrup feed and the massecuite derived from the crystalliser may be from approximately 95 degrees Centigrade to approximately 25 degrees Centigrade.
Sucrose will crystallise from a hot concentrated solution (exhibiting no crystallisation) as the hot concentrated solution is cooled. This is illustrated in
The temperature of the fine liquor/syrups derived from the falling film evaporation steps may be from approximately 85 to approximately 100 degrees Centigrade.
Steps a) and e) may be effected in a single operation.
According to second aspect of the present invention there is provided a refined crystallised sucrose produced by a process as hereinbefore described.
The process according to the invention separates the evaporation and crystallisation steps so that each may be separately, and independently, optimised. This separation (decoupling) is achieved by:
The process according to the invention may use a hot (decolourised) liquor for washing the sugar crystal free of the mother liquor in the centrifugation step. This has the effect of improving steam economy in the following way:
The final runoff mother liquor is sent to a recovery house:
According to the present invention significant energy saving in sugar refining can be achieved (as compared with values taught in the art). Measuring the quantity of steam used, expressed relative the quantity of raw (impure) sugar melted reveals the following comparison.
The invention will now be described with reference to the following non-limiting figures in which:
In
The first syrup (J1C) is subject to cooling crystallisation to yield a second massecuite (M2E) which is centrifuged to yield a second sugar (S2) and a second runoff (J2). The second (cool) runoff (J2) is concentrated (without crystallisation) in a falling film evaporator to yield a second syrup (J2C).
The second syrup (J2C) is subject to cooling crystallisation to yield a third massecuite (M3E) which is centrifuged to yield a third sugar (S3) and a third runoff (J3). The third (cool) runoff (J3) is concentrated (without crystallisation) in a falling film evaporator to yield a third syrup (J3C).
The third syrup (J3C) is subject to cooling crystallisation to yield a fourth massecuite (M4E) which is centrifuged to yield a fourth sugar (S4) and a fourth runoff (J4). The fourth (cool) runoff (J4) is concentrated (without crystallisation) in a falling film evaporator to yield a fourth syrup (J4C).
The fourth syrup (J4C) is subject to cooling crystallisation to yield a fifth massecuite (M5E) which is centrifuged to yield a fifth sugar (S5) and a fifth runoff (J5). The fifth runoff (J5) is directed to a recovery house.
The falling film evaporators are multiple effect operated as a single unit to concentrate the fine liquor and the first four runoffs (without crystallisation).
It will be appreciated that (as a first approximation) in a multiple effect evaporator of 4 effects one kilogram (kg) of steam will evaporate 4 kg of water. In addition, if vapour is withdrawn (bled) from for example, the second effect of the evaporator and this vapour is used outside of the evaporator system in place of steam, the steam saving will be 2/4 times the quantity of steam used in this duty. These two important energy saving concepts are well known within the sugar industry as Rilleaux's Principles.
In
J1 is then heated and concentrated (without crystallisation) in a multiple effect falling film evaporator to derive a concentrated 1st runoff (J1C). The J1C is seeded with seed crystals SX2 which mixture (M2) is then subject to cooling crystallisation to derive a first massecuite (M2E) which is then centrifuged and washed with a wash liquor (W2) to yield a second sugar (S2) and a second runoff (J2).
This process is repeated three more times to finally yield a fifth sugar (S5) and fifth runoff (J5) which is directed to a recovery house (not shown).
The diagram in
The diagram in
The energy saving benefits of the proposed process for producing refined sugar (using cooling crystallisation and multiple effect evaporation) can be seen by a comparison of the mass balance data in
The data in
The steam efficiency of this conventional process can be improved by using multiple effect evaporation to concentrate the secondary liquor. If this is done in four effects, the steam requirement for concentrating the secondary liquor reduces to 3.5 tons/hr (13.8/4) and the total steam demand for the process then reduces to 47.8 tons/hr (44.3+3.5). Under these circumstances the steam efficiency of the process improves to 0.521 tons steam per ton sugar produced (47.8/91.7).
The data in
On this basis, the specific steam demand of the proposed refining process with quadruple effect evaporation is then only 26.5% of the demand for a conventional refining process that uses single effect liquor evaporation (0.168/0.634*100). Alternatively, the specific steam demand of the proposed refining process is only 32.2% of the demand for a conventional refining process that uses quadruple effect liquor evaporation (0.168/0.521*100).
Since there are diminishing returns for applying multiple effect evaporation to secondary liquor evaporation in the conventional refining process, an equitable comparison of the energy efficiency of the proposed refining process with that for a conventional refining process will likely lie between the two figures quoted above (i.e. between 26.5 and 32.2%).
In
Based on Rilleaux's first principle (previously described) that one ton of vapour will produce one ton of evaporation, a quadruple (four) effect evaporator can be expected to achieve the total 60 ton/hr evaporation requirement with a supply of only 15 ton/hr of steam (60/4). Whilst quadruple effect evaporators can be applied to each of the streams requiring evaporation to achieve this saving, it will be more cost effective to integrate all the evaporation requirements into a single multiple effect evaporator station. An example of how this can be achieved is shown in
As illustrated in
| Number | Date | Country | Kind |
|---|---|---|---|
| 1215261.7 | Aug 2012 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IB2013/058008 | 8/27/2013 | WO | 00 |
