ChElec 501 Sugar Engineering 2 Clarification-Phosphatation
PHOSPHATATION Separation mechanism is fundamentally different from carbonatation. In phosphatation,the principal mechanism is flocculation, while in carbonatation – mainly inclusion of impurities within the calcium carbonate crystals Phosphatation relies on the formation of flocs formed by the addition of flocculants, captures the fine colloidal matter and is separated by flotation with dissolved air. The basic reaction in phosphatation process:
8Ca(OH)2 + 6H3PO4 Ca8H2(PO4)6 +16H2O
SUGAR REFINERY PROCESS CLARIFICATION BY PHOSPHATATION RAW SUGAR
AFFINATION AFFINATION
MELTER
TALOFLOC / PHOSPHATATION
DEEP BED FILTRATION RECOVERY BOILING
ION EXCHANGE DECOLORIZATION
CRYSTALLIZATION / PURGING
3 STAGE SCUM DESWEETENING
FILTER CAKE
The “Talo-Phosphatation” Process
The “Talofloc” Phosphatation was developed by Tate and Lyle of England in the late 70”s. It is a combination of traditional Phosphatation process, modified equipment and newly developed synthetic processing chemicals.
Phosphatation Process Chemical Reaction: 8Ca(OH)2 + 6H3PO4 = Ca8H2(PO4)6 + 16H2O The octacalcium phosphate forms flocs which entraps most of the impurities and color bodies. The primary calcium phosphate flocs formed are very fine and takes time to form. Retention times in the Clarifiers takes as long as 45 minutes. The flocculants act as “bridges” to link the phosphate flocs and form bigger secondary flocs.
The Use Of Flocculants The flocculants act as “bridges” to link the phosphate flocs and form bigger secondary flocs. The flocculants are synthetic polyacrylamide of high molecular weight (up to 26 million). Their negative charge binds with the positive sites of the phosphate flocs. The secondary flocs formed are 1,000 x bigger than the primary flocs. The flocculant is added after the acid and lime are mixed.
The use of flocculants resulted to:
Reduced operating temp. to 82 oC. Increased brix to > 65 oBx. Reduced retention time from 45 to 25 minutes Reduced sugar losses More brilliant and clear liquor.
The Use Of Surfactants The positively charged calcium phosphate removes the negatively charged color and impurities by attracting them. Surfactants or color precipitants are synthetic chemicals which are a lot stronger than calcium phosphate, thus it attracts more impurities and color.
Chemical Reaction Model:
Color molecule
Surfactant
Highly soluble
Slightly soluble
Surfactant-Color Complex Insoluble
The formed flocs although numerous are very, very fine and takes time to settle. The addition of flocculant greatly increases the size of the flocs and reduces the flocculation time. When used as an adjunct to Phosphatation, the surfactants increases the % color removal up to 60 %.
The “Talo-Phosphatation” Process The “Talofloc” Phosphatation is composed of the following operations. Primary flocculation with lime and phosphoric acid. Aeration of the primary floc Secondary flocculation with the flocculant “Talofloc”. Precipitation of the color bodies and impurities with the surfactant “Taloflote”. Filtration of the clear liquor through a Deep Bed Filter (DBF).
The “Talofloc” Phosphatation consists of three main processes: 1) Clar Clarif ific icat atio ionn of of the the melt melt liquo liquorr 2) Filtra Filtratio tionn of the clarif clarified ied liquor liquor 3) Recove Recovery ry of of sugar sugar from from the the scum scum throu through gh dedesweetening
8Ca(OH)2 + 6H3PO4 Ca8H2(PO4)6 +16H2O
TALO ALOFLO FLOC C / PHO PHOSPH SPHA ATATIO TION N
PROCESS FLOW DIAGRAM LIME SUCRATE PREP. TANK
FLOCCULANT PREP. TANK
TALOFLOC DRUM HEATER TALOFLOC DOSING PUMP
LIME SUCRATE HOLDING TANK
FLOCCULANT HOLDING TANK
FLOCCULANT DOSING PUMP
CAVITATION AERATOR
PHOSPHORIC ACID
LIME SUCRATE DOSING PUMP
N O I T C K A N E A R T
ACID DOSING PUMP
N O I T K A R N E A A T
HEAT EXCHANGER
UNTREATED LIQUOR
CLARIFIER
SCUM
FLOW CONTROL MELT LIQUOR TANK
CLARIFIED LIQUOR TANK FEED PUMP
CLARIFIED LIQUOR
PHOSPHATATION Melt liquor heated in shell & tube HE to 87oC is feed to the stirred reaction tank , where phosphoric acid are automatically mixed in proportion to the liquor flow rate, and lime sucrate s ucrate is also added to adjust the pH to 7. (Lime sucrate is prepared by adding melt liquor to 7 oBe milk of lime in a ratio of 7:1). At this pH, the resultant calcium phosphate is in its most insoluble form and flocculates the anionic impurities (color, turbidity, suspended solids). This complex is known as the primary floc.
From the reaction tank, the liquor is gravity feed into the aeration tank where the microscopic particles of air dissolve into the liquor, and physico-chemically attach themselves to the primary floc. At this point a trace amount of flocculant TALOFLOTE is dosed, and this coagulates the primary flocs to form large, gelatinous, aerated secondary flocs. This secondary flocculation takes place in the circular TALO clarifier where the flocs rapidly rise to the surface to leave a sparkling, brilliant clarified liquor underneath.
CLARIFICATION
INLET
SCUMS OUTLET
The clear liquor is drawn –off over a weir with adjustable plate which control the clarifier liquid level and sent to the Deep Bed Filter for filtration, The scum is constantly removed by a rotating scraper blade and sent to the Scum Desweetening Process.
TALO FLOTATION CLARIFIER SCUM RAKE SCUM TROUGH
WEIR BOX SLEEVE
SYRUP OUTLET
FLOCCULATION CHAMBER FLOTATION CHAMBER
MIXING BAFFLES
SYRUP INLET
The “Talo” Clarifier
Scums
Rake
The “Talo” Clarifier Scum Discharge Trough
Scum Rake
Scum Layer
Talo-Phosphatation ” Process Controlled parameters: m3/hr. *Melt liquor brix – 65 Melt liquor flow rate – x m3/hr. max. Phosphoric acid dosage – 300 ppm P2O5 on liquors solids *Phosphoric acid assay – 85% Lime sucrate dosage – pH 6.8-7.2 (Prepared by mixing 100 kg CaO with 1 m3 cold water and 1 m3 raw melt liquor). Talofloc dosage – 300 ppm on solids ( Prepared by mixing 200 kgs of “Talofloc” with 6 m3 of raw melt liquor) Taloflote dosage – 10 ppm on solids (Prepared by mixing 1 kg of “ Taloflote” with 1 m3 of hot water).
Weir box setting – controls the retention time in the Clarifier (25 min. max) and the scum layer thickness (6”-8” deep). Speed of the scum rake – 1-2 rpm.
DEEP BED FILTER It is a column packed with layers of different sized particles decreasing in size from top to bottom of the filter. The layers of filter medium are carefully chosen both in terms of particle size and density. The clarified liquor is pumped via an orifice plate and percolates down thru the filter media from the top to bottom. The suspended solids are trapped in the interstitial spacing of the media particles.
When the concentration of the suspended particles reach a critical value ( DP = 15 psi, detected by the pressure sensor), back-flush sequence is automatically initiated. The filter is back-flushed in an upflow direction, with the largest particles having the lowest density, so that after backwashing the gradation of the filter medium size is preserved. During backwash air sparging is initiated to agitate the filter media and release the entrapped suspended solids.
Backwashing is done with the filtered liquor, so that no additional sweet water is generated. The Deep Bed Filter is fully automated. Operation is controlled by series of automatic valves, pumps, & blowers regulated by a central programmable controller, hence requires minimal supervision. The back-washings are returned to the process buffer tank where they are eventually removed by flotation in the main clarifier.
Deep Bed Filtration (DBF) Talo-Phosphatation comes with a specially designed filter called DBF. bone char It replaced the pressure filters sand sand which use filter aid. gravel It is constructed like a sand filter gravel gravel but uses carefully sized filter media. There are usually 5-6 layers of gravel, sand and bone char. The arrangement of the filter media ensures complete removal of suspended particulates from the liquor. Uses no filter cloths but the media is replaced periodically (every 2 years) Can be operated singly or in a battery of several units. Service cycle of up to 24 hours.
Deep Bed Filter PROCESS FLOW DIAGRAM
DEEP BED FILTE R
CLARIFIED LIQUOR
HIGH LEVEL
BACKWASH RECEIVING TANK
FILTER FEED TANK HIGH LEVEL LOW LEVEL
OVERFLOW FILTERED LIQUOR TANK
BACKWASH COMPARTMENT
FILTERED LIQUOR FEED
BACKWASH PUMP
AIR BLOWER
Deep Bed Filter
PHOSPHATATION PHOSPHATA TION / DEEP BED FILTER
PROCESS FLOW DIAGRAM LIME SUCRATE PREP. TANK
FLOCCULANT PREP. TANK
TALOFLOC DRUM HEATER TALOFLOC DOSING PUMP
LIME SUCRATE HOLDING TANK
FLOCCULANT HOLDING TANK
PHOSPHORIC ACID
LIME SUCRATE DOSING PUMP
FLOCCULANT DOSING PUMP
CAVITATION AERATOR
ACID DOSING PUMP
N O I T C K A N E A R T
N O I T K A R N E A A T
CLARIFIER
FLOWMETER
HEAT EXCHANGER
SCUM TO BE DESWEETENED
UNTREATED LIQUOR
FILTERED LIQUOR
85 C °
UNTREATED LIQUOR TANK
FLOW CONTROL
TREATED LIQUOR TANK
FILTERED LIQUOR TANK B/W
FEED PUMP FEED PUMP
DEEP BED FILTE R
AIR BLOWER
Scum De-sweetening The Scums are skimmed off the surface of the liquor and are sent to the Scum De-sweetening section. About 3-6% of incoming feed liquor is removed with the scum. After treatment, the scum going to waste should s hould contain <1% of sugar. The efficiency of the process rests on the following: Constant scum feed rate Constant dilution feed rate (usually 5:1 or as sweetwater is needed). Uniform flocculant dosage (1.5 ppm on liquor solids). Efficient aeration Correct scum withdrawal (scum thickness and rake speed)
3-STAGE SCUM DESWEETENING PROCESS It is a counter-current sucrose recovery recovery process. Hot condensate or hot pure water is metered via a positive displacement pump to the scum trough of the 2nd clarifier. The quantity of water is proportional to the ratio of water to scum (dilution ratio). This water transfer the scum from the 2 nd clarifier to the mixing tank where it is thoroughly mixed by a mechanical stirrer and gravity-feed to the 3rd clarifier where a trace of flocculant TALOFLOTE is added. A stream of aerated water is added to this which results in a flotation of scum and sweetwater.
This scum is discharged to drain while the low-brix low -brix sweetwater to the trough of the 1 st clarifier into the mixing tank of the 2 nd clarifier. A mechanical stirrer thoroughly mixes the scum s cum and the sweetwater and a trace of flocculant TALOFLOTE is added, simultaneously an aerated sweetwater stream is added & the mixture flows by gravity into the 2nd stage clarifier where separation takes place by flotation to produce scum on the surface and sweetwater underneath. The sweetwater is pumped to the mixing tank of the 1’st where it is mixed with the scum from the TALOFLOC phosphatation process.
Separation takes place in the 1 st stage clarifier, and the sweetwater if transferred to the melter of the affination station.
3 STAGE SCUM DESWEETENING
PROCESS FLOW DIAGRAM SCUM MIXING TANK 1
FLOCCULANT PREP. TANK
SCUM MIXING TANK 2
SCUM MIXING TANK 3
CLARIFIER 1
FLOCCULANT HOLDING TANK
CLARIFIER 2 AIR
FLOCCULANT DOSING PUMPS
CLARIFIER 3
AERATION PUMP 1
AIR
AERATION PUMP 2
AIR
AERATION PUMP 3
SWEETWATER TO REFINERY MELTER
SCUM FROM MAIN CLARIFIER
SWEET WATER TANK 2
HOLDING TANK
Figure 13
SWEET WATER TANK 3
DILUTION WATER TANK
DISCARDED SCUM
Table 2: Comparison of Carbonatation and Phosphatation Carbonatation
Phosphatation
Capital Cost
Capital intensive; Larger filter area required
Cheaper Equipment
Operating Cost
Low
High, due to cost of chemicals
Color removal
40-50 %; consistent
20-30 %;more dependent on sugar quality
Liquor quality
Excellent; includes filtration
Requires deep bed pressure filtration before decolorization
Higher lime usage precipitates more calcium salts
Little ash removal
Starch removal
Significant removal
Higher removal than carbonatation
Waste produced
Large quantity of cake produced
Solid waste quantity small
Maintenance
Gas pumps to maintain
Low maintenance
Ash removal
Acid floc
Removes floc-forming impurities impurities
Acid beverage floc formation possible
Comparison of Carbonatation and Phosphatation Carbonatation
Phosphatation
Capital cost
Capital intensive, Larger filter area required
Cheaper equipment
Operating cost
Low
High due to chemicals cost
Color removal
40-50% ; consistent
20-30%; dependent on sugar quality
Liquor quality
Excellent; includes filtration
Ash removal
Higher lime usage precipitaes precipitaes more calcium salts
Little ash removal
Starch removal
Significant removal
Higher removal than carbonatation
Waste produced
Large quantity of cake
Solid waste quantity small
Maintenance
Gas pumps to maintain
Low maintenance
Acid floc
Removes floc-forming impurities
Acid beverage formation possible
Requires deep bed pressure filtration
Carbonatation is technically the best choice for decolorization Destruction of Invert High pH in the first Carbonator ,eliminates most invert Phosphatation due low pH creates more invert Buffer Capacity/Sucrose Loss Carbonated Liquor pH of 7.8 – 8.2 Phosphate Liquor pH of 6.8 – 7.1 Stability of Process Carbonatation - excellent process stability Phosphatation – temperamental or unforgiving
Carbonatation is technically the best choice for decolorization
Quality of Sugar Product Carbonatation
Sparkling Much less sediment in final product
Phosphatation – lesser quality
Secondary floc Some particulates are not removed by Deep Bed Filter
Carbonatation is technically the best choice for decolorization
Additional Benefit of the Carbonatation Products contamination by pan scale due to sulfates will be practically eliminated. Acid washing of vacuum pan scale will be avoided. The operation of a Carbonation process generally is more consistent than other decolorization process.
Phosphatation Higher sucrose loss due due to low pH Higher energy consumption due to process instability. Lower initial capital cost. Flexibility if capacity expansion Low degree of color removal
