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Cleaning regimes used in falling film plate evaporators Mark ......The falling film plate evaporator...

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Cleaning regimes used in falling film plate evaporators Authors Mark Suhr, MS PROCESSES INTL, LLC, 410 Campbell Ln, Hutchinson, MN 55350, Dr.-Ing. Bernd-Christoph Schulze, Engineering Services & Consulting GmbH, Schlossstrasse 48 a D - 12165 Berlin Germany Abstract The development and use of more efficient equipment is part of bli"siness. The falling film plate evaporator is a development that allows the sugar industryl to use a compact design with higher heat transfer rates and lower wetting rates to achieve reductions in color development, sucrose losses and utilization of lower vapor pressures. The principles of the design have allowed it to achieve very favorable results. However, precipitation of organic and inorganic substances as well as potentially oxidizing sucrose breakdown products adhering to the heat transfer surfaces will occur and cleaning regimes are discussed relevant to this design. The evaporator's design for juice entering and leaving complicates successful cleaning. The complications of wetting the heat transfer surfaces with cleaning chemicals and the choices of cleaning chemicals are discussed as they relate to cost effectiveness and successfulness. Results of removing layers of different plated out materials is also covered. Introduction The general application of concentrating sugar solutions is the same for the falling film plate evaporator as it is for the Robert, rising film plate, Kestner and/or falling film tubular evaporators. Therefore a first assumption may be that the scaling tendencies of the sugar solutions and the scaling compounds will be the same for this style of evaporator. However the design details and operating results need to be reviewed to confirm that scaling tendencies are similar. Also in reviewing the details, the mechanisms to most effectively clean fouled evaporators of this type will be better understood. Reducing severe fouling and implementing safe operating and cleaning procedures will help maintain this highly efficient design as the evaporator of choice. The falling film plate evaporator combines a compact, thin, rigid plate design of high density heating surface area producing excellent heat-transfer rates along with a low wetting rate requirement and small juice volumes. This has allowed the evaporator to be the ideal choice for the modern large throughput, low energy conswning factories being designed and operated today. These advantageous design elements also have made cleaning this style evaporator more complicated and difficult than the more conventional styles. The basic design of the falling film plate evaporator is to stack heat-transfer sections of varying length and width upon each other to achieve the required heating surface desired. The depth of each section is approximately 13 inches. Groups of these sections are fabricated as a unit to facilitate installation. The gap between sections is approximately 1.5 inches and between groups of sections approximately 4 inches. This flexible feature allows for evaporators of over 60,000 square feet to be installed within a 15 foot diameter by 50 foot high vessel. The juice enters the heat-transfer area at the top and travels downward. There are a number of different inlet configurations that are in current use. All the inlet designs include multiple trays to facilitate the design criteria of wetting the 135
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Page 1: Cleaning regimes used in falling film plate evaporators Mark ......The falling film plate evaporator combines a compact, thin, rigid plate design of high density heating surface area

Cleaning regimes used in falling film plate evaporators

Authors Mark Suhr, MS PROCESSES INTL, LLC, 410 Campbell Ln, Hutchinson, MN 55350, Dr.-Ing. Bernd-Christoph Schulze, Engineering Services & Consulting GmbH, Schlossstrasse 48 a D - 12165 Berlin Germany

Abstract The development and use of more efficient equipment is part of bli"siness. The falling film plate evaporator is a development that allows the sugar industryl to use a compact design with higher heat transfer rates and lower wetting rates to achieve reductions in color development, sucrose losses and utilization of lower vapor pressures. The principles of the design have allowed it to achieve very favorable results. However, precipitation of organic and inorganic substances as well as potentially oxidizing sucrose breakdown products adhering to the heat transfer surfaces will occur and cleaning regimes are discussed relevant to this design. The evaporator's design for juice entering and leaving complicates successful cleaning. The complications of wetting the heat transfer surfaces with cleaning chemicals and the choices of cleaning chemicals are discussed as they relate to cost effectiveness and successfulness. Results of removing layers of different plated out materials is also covered.

Introduction The general application of concentrating sugar solutions is the same for the falling film plate evaporator as it is for the Robert, rising film plate, Kestner and/or falling film tubular evaporators. Therefore a first assumption may be that the scaling tendencies of the sugar solutions and the scaling compounds will be the same for this style of evaporator. However the design details and operating results need to be reviewed to confirm that scaling tendencies are similar. Also in reviewing the details, the mechanisms to most effectively clean fouled evaporators of this type will be better understood. Reducing severe fouling and implementing safe operating and cleaning procedures will help maintain this highly efficient design as the evaporator of choice.

The falling film plate evaporator combines a compact, thin, rigid plate design of high density heating surface area producing excellent heat-transfer rates along with a low wetting rate requirement and small juice volumes. This has allowed the evaporator to be the ideal choice for the modern large throughput, low energy conswning factories being designed and operated today. These advantageous design elements also have made cleaning this style evaporator more complicated and difficult than the more conventional styles.

The basic design of the falling film plate evaporator is to stack heat-transfer sections of varying length and width upon each other to achieve the required heating surface desired. The depth of each section is approximately 13 inches. Groups of these sections are fabricated as a unit to facilitate installation. The gap between sections is approximately 1.5 inches and between groups of sections approximately 4 inches. This flexible feature allows for evaporators of over 60,000 square feet to be installed within a 15 foot diameter by 50 foot high vessel. The juice enters the heat-transfer area at the top and travels downward. There are a number of different inlet configurations that are in current use. All the inlet designs include multiple trays to facilitate the design criteria of wetting the

135

Page 2: Cleaning regimes used in falling film plate evaporators Mark ......The falling film plate evaporator combines a compact, thin, rigid plate design of high density heating surface area

entire heat-transfer surfaces. The outlet from the plate packs has generally consisted of a collection tray with an outlet to the next evaporator and simple overflow of the tray for recycle. This design correctly attempts to minimize recycle but has the potential to cause failure of the tray if a rising film cleaning regime or a higher than normal recycle level is maintained.

Figure 1 is a general concept drawing of the main principles of a falling film plate type evaporator as compared to a Roberts' type evaporator. "

Plate evaporatorRobert evaporator

Vapour outletVapour outler

upour

---i --t-- ,---t====:1.-,.:ce inlet

Juice distribution

Condensate ~1!IHt~~~~~~~ outlet

Juice outlet

.. -

Juice inlet

~!5-t----

Juice outlet

Vapour duct

Juice dislributior

Vapour duct

Figure 1: Comparison of the Robert to falling film plate evaporator

The next pictures show the juice inlet distributor and the plate pack construction:

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Page 3: Cleaning regimes used in falling film plate evaporators Mark ......The falling film plate evaporator combines a compact, thin, rigid plate design of high density heating surface area

Figure 2: Inlet distributor with a removable section Figure 3: Plate pack; note the undulated channels for the cross flow of stearn/vapor

-

Figure 3 Juice side section of a plate pack, elliptical-like openings

There are several key elements needed to successfully clean the evaporator. The understanding of the scale type and formation is imperative but just as important is how to contact the scale with the cleaning devise or reagent.

There are two general classifications of cleaning regimes in all evaporators, mechanical and chemical. In the plate style, a third option is to remove the scale by thermal means, arguably a subset of either or both of the other two major classifications.

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Page 4: Cleaning regimes used in falling film plate evaporators Mark ......The falling film plate evaporator combines a compact, thin, rigid plate design of high density heating surface area

Previous papers have covered the subject of scale type and fonnation as well as various means of prevention in evaporators.2, J The nature and current basic design of the falling film evaporator creates potential changes in chemistry and opportunity for scale fonnation. The use of gravity and two-phase flow for concentrating a sugar solution as that solution moves through the plate pack is essential to understanding the potential changes occurring within sections of the plate pack. Also the wetting number is decreasing as the juice concentrates down the plate pack. The expectation is this will produce more scale at the bottom of the plate pack. However the vapor generated as the juice travels downward helps to stabilize the juice film and this stabilized film is subjected to a lower temperature resulting in even scale formation. The lower temperature is due to the multiple pass~s of the steam from top to bottom. Nonnal operation has confinned the scaling is generally unifonn. The real concern is uneven distribution or higher pressure differentials between the supply vapor and the leaving vapor that results in excessive foam generation. The result of foam can be the loss of movement of the juice through the plate pack or for the juice to go into full or partial rising film mode. In the foamed environment, the concentrations and chemistries are changing more rapidly than in the bulk liquid of sugar solutions. Investigative research has shown that foaming juice on the evaporator surface changes the film velocity and consequently the film thickness. In the upper part of the evaporator, the film is expanded, because the film-surface is curbed by the rising foam. The hydrostatic pressure of the sucrose solution on the foam pads in the lower part of the evaporator causes a higher fluid velocity and a thinner film. This involves the danger of breaking the film and reaching a dry out point. These conditions are difficult to model. However, mathematical descriptions for technical sucrose solutions are given in the literature4

• Figures four through six depict the bubble fonnation in the falling film plate evaporator and the potential for cha~gi~g the juice flow,pattern.

Figure 4: Bubble fonnation in falling film plate evaporator at an effective temperature

difference of 2 °C

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Page 5: Cleaning regimes used in falling film plate evaporators Mark ......The falling film plate evaporator combines a compact, thin, rigid plate design of high density heating surface area

-t/' ~-""'

c)

Fluidfilm

I I I I

~ I

Figure 5: secondary flows due to foam pad formation

Figure 6: foam formation of technical sucrose solution in falling film plate evaporators

Experience has shown that if a foam condition or partial rising film is established, heat­transfer goes down during this period and does not recover if allowed to exist over an extended time period. This time frame is not established but the condition has known to exist for several hours before the plug of foam was cleared with limited long term effect on the heat transfer coefficient. The scaling during these upset conditions has generally been found to include more caramel products mixed with the typical fouling products of sugar solutions. The increase in the amount oforganic substances is shown from composite analysis of scale after juice was subjected to prolong high heat as a foam plug in an evaporator. Three samples were analyzed from the scale found in this first effect evaporator and are shown in table 1. The evaporator operated at 40 psig inlet pressure with steam temperatures varying from 142 to 280°C. Table 2 lists the results of scale analysis from the fourth and fifth effect.

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Page 6: Cleaning regimes used in falling film plate evaporators Mark ......The falling film plate evaporator combines a compact, thin, rigid plate design of high density heating surface area

% composite from % composite from % composite from Scale composition bottom bottom outside and top of the of the plate pack of the plate pack

-.plate pack Not detected Not detected Not detected CaC20 2*H2O 4.1 7.5 21.2CaC03

traces 20.4CaS03 Not detected Not detected CaS0 4

0.9(Ca3(P04)2)3*Ca(OHh 92.5 85.4 53.5Organic *H2O

1.4 0.40.4Si02 <0.1 <0.1 <0.1A1203 0.2 0.9 0.1M20

1.61.0 1.2Na20 0.30.2 0.3K20

0.50.6 1.0FeO <0.1 0.4 0.4Fe20 3

Table 1 scale composition of a number evaporator

Scale composition Fourth effect; % Fifth effect; % composition Composition

CaC20 2*H2O 47.1 36.5 CaC03 5.8 4.3 Or2anic *H2O 44.5 51.9 Other 2.6 7.5

The results of high organic debris is not surprising as very small amounts of sugar juice are in contact with the heat-transfer plates allowing rapid concentration to occur at sufficient temperature to subsequently promote degradation and precipitation reactions_ It also raises the question from experiences with other types of evaporators; does the thin film design with a low wetting number promote more scale formation? The risk appears to be-during foaming events when the juice film may be disrupted and dry out points are created. This may be a risk of this style of evaporator. However operating results do not suggest higher scaling has been a problem in everyday operation. The higher organic level treated with caustic soon after formation should not be a limitation to cleaning. The organic substances were less soluble and in higher concentration in the first and second effect. They were more difficult to remove than organic compounds in the later effects_

More so, if the foaming condition described is left unchecked, the formation of carbon is possible and a fouling product described by many as "sugar coal" will be present in varying amounts. A simple definition of sugar coal is a matrix of sucrose breakdown

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Page 7: Cleaning regimes used in falling film plate evaporators Mark ......The falling film plate evaporator combines a compact, thin, rigid plate design of high density heating surface area

products from carbon to high molecular weight polysaccharides to intermediate products of sucrose destruction. Again, strength of the design for the many process gains may also be a limitation as juice traveling down a channel formed by the plates has multiple pathways to continue on if it encounters a partially plugged channel. These multiple pathways potentially promote the growth of the plug and ifnot cleaned out may grow in size to limit the overall performance of the evaporator. Several pictures are shown of an evaporator with heavy organic scale in figure 7 and 8.

Figure 7 heavy organic scaling with heavy Figure 8 plugged juice paths layers of inorganic scaling

To effectively clean a falling film evaporator there are some new considerations that are not typical of the more common Roberts style evaporator. The amount of surface area in the same outside dimensioned vessel is significantly different. A falling film evaporator potentially will have 2 to 3 times more heat-transfer area in the same size vessel. For example: A 15 foot Robert evaporator will have approximately 25,000 ft2 of heating surface and a falling film evaporator in the same vessel may have 60,000 ft2. 1132 of an inch scale results in 16,000 pounds of scale to be removed in the Robert evaporator but over 39,000 pounds in the falling film evaporator.-These levels are generally 5 times higher than you would achieve before chemically cleaning. The gap between heat­transfer surfaces is significantly smaller in the falling film evaporator. This smaller gap is not a concern in normal operation provided the large diameter solids are being removed from the juice before it enters the evaporator. These evaporators are clearly not recommended by the original equipment manufacturers (OEM) for unfiltered juice operation. However in cleaning operations, the size of the scale released from the heating

. surfaces is important and cannot be controlled externally. Large pieces of released scale can continue creating more performance issues by plugging channels and creating poor wetting areas in the evaporator. Therefore consideration to cleaning chemicals' potential for facilitating large scale particles release is important. The use of EDTA and chemicals that dissol ve and prevent re-deposition may have more value in this style evaporator.

The wetting rate of this type of evaporator is the lowest by orders of magni tude . This suggests that the juice velocity is also lower. Studies on scale formation all other types of evaporators confirm an increase in scaling with low juice velocity.

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Page 8: Cleaning regimes used in falling film plate evaporators Mark ......The falling film plate evaporator combines a compact, thin, rigid plate design of high density heating surface area

The plate pack construction is light and flexible. Its membrane type construction allows movement and flexing that should aid in releasing scale adhering to the plate.

Figure 9: partly blocked juice channel

Chemical cleaning The reduction of chemical cleaning cycles per operating season is desired. Reducing most of the typical scales can be effectively done with ion exchange on the juice before it enters the evaporators. However over 70% of the falling film evaporator applications are in non-softened thin juice applications. The use of scale inhibitors has improved and with proper dosing is quite effective.5

, 6

Chemical cleaning can be done in the nonnal falling film mode or a rising film mode. Rising film cleaning is accomplished by increasing the recirculation liquid level sufficiently to ensure that the liquid completely seals the lower level of the plate pack. This forces the flow of cleaning chemicals upward along with energetic bubble formation to assist in scale removal. In general, chemical cleaning during the process season is considered t,o be done hot with

evaporation occurring at reduced or full rate during the cleaning cycle. There are advantages and disadvantages to both modes of cleaning. The volume of cleaning solution needed in rising film mode is significantly more. There may only be 7000 gallons of liquid in a 60,000 fe evaporator in falling film mode and over 35,000 gallons in rising film mode for the same evaporator. This requires at least five times more chemical addition to maintain the desired cleaning solution strength. More frequent addition of chemicals may be necessary to maintain the cleaning rate in the falling film mode but evaluation on the effectiveness is more easily monitored. Monitoring includes verification of the neutralization rate of the acid and visual observation of the scale

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Page 9: Cleaning regimes used in falling film plate evaporators Mark ......The falling film plate evaporator combines a compact, thin, rigid plate design of high density heating surface area

--- ---

concentration in the blow down, as it represents more closely the vessel concentration. In addition, the flow pattern of the cleaning solution entering and leaving the plate packs as well as the pressure differential across the vessel should be monitored. In general falling film cleaning should be effective and the preferred method. The exception is in severe fouling where wetting of the scale is difficult. In rising film mode, all the parameters for monitoring are more difficult as well as properly using any visual observation because the evaporator is functioning differently from design and normal operation.

Monitoring and confirmation of the effectiveness of chemical cleaning is more easily accomplished in the falling film mode. However, a disadvantage of the falling film cleaning mode is the accumulation of scale in the juice outlet tray. This requires a filtration of those solids from the outlet connection before it can reach the next effect. If not filtered out properly, the solids may create additional fouling (by plugging) in the downstream evaporators. Additionally, in the falling film mode the recirculation pumps are generally in use and if the scale is not filtered out from the overflow of the juice trays then cleaning effectiveness will be lost again by depositing the solids back on top of the plate pack. If not sufficiently rinsed, dissolved solids may precipitate on the heating surfaces again. Thus additional filtration is essential for cleaning in the falling film mode. Evaluation of the rinse and blow down cycle found scale concentrations generally in the O.2g/1 range. We did not evaluate this scale.

The rising film mode takes the solids out the top of the plate pack and down the vapor outlet ducts to the bottom of the vessel. This mode can use the recirculation pump to remove the concentration of solids as well as the high volume of liquid with dissolved solids that must be disposed of. The rising film mode clearly creates more agitation and velocity in the liquid phase. The rising film has better opportunity to ensure the cleaning chemicals contact the scale. The disadvantage is the separation of vapor from liquid is completed near the top of the plate pack and through the inlet distributor. This increases the pressure differential and creates a violent release that can create mechanical damage to the small tubes in the distribution trays. Also solid may carryover into the vapor side of the next effect. The cleaning and rinsing cycles are again longer in the rising film mode. It is recommended to be used only in severely fouled units.

Attention to the interval between cleanings and routinely calculating the heat-transfer coefficients are important to establishing the time to chemically clean. Ad Hoc is not recommended. The typical small temperature differences in these evaporators require more rigorous attention to details and measurements. The desire to schedule cleaning around other events of the sugar factory can result in lower overall evaporation requirements in the short term that will lead to the uneven flow patterns, low evaporation coefficients and subsequent higher scaling potential. These upsets to the evaporation station can result in the partial rising film mode or excessive foaming. This mixed foamed rheology must be avoided or the creation of a more difficult cleaning regiment becomes necessary to return the evaporator back to full capacity.

The standard cleaning method of alternate cycles of sodium hydroxide (caustic soda) and sodium carbonate (soda ash) addition followed by rinsing and then acid addition and subsequent rinsing and neutralizing are effective with these evaporators. The length of time has varied from several hours to 24 hours at elevated pH. The results oflonger

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Page 10: Cleaning regimes used in falling film plate evaporators Mark ......The falling film plate evaporator combines a compact, thin, rigid plate design of high density heating surface area

cycles at the elevated pH did not appear to improve the cleaning. The more important criteria appear to be the wetting and effective contact time with the chemicals. This is generally more readily accomplished in the rising film mode. Therefore using the more standard falling film mode, as high as possible recirculation and as Iowa pressure differential as possible across the evaporator helps the cleaning cycle.

The acid selection should not be based on past success with Robert evaporators. The plates are extremely thin compared to conventional tubes. They are two to four times thinner and they are shaped, reversed, superposed and continuously. welded at the seams. This eliminates the use of hydrochloric acid (muriatic acid). The use of sulfamic acid with heat reduces its effectiveness due to rapid hydrolysis. Table 1 show shows a substantial amount of CaS03. Other analysis has confirmed calcium sulfate formation. This is the result of the breakdown of the sulfamic acid and it complicates removal and re-establishing good heat transfer. Formic or other organic acids need to be considered for the acid cleaning step. In falling film mode the cost impact is relatively minor, however in rising film mode these more costly chemicals may make a high heat cleaning difficult to justify. For example the price ratio between formic acid and sulfarnic acid is 7: 1. An inhibitor is always recommended. In all cases, surfactants with dispersing and complexing abilities should be used because of the metal surface structure and multiple pathways the liquid can move through the plate packs.

If the ability to effectively dry the scale out between the caustic cycle and the acid cycle as well as before placing it back into service exists, it is highly recommended as positive results are generally yielded. The cleaning with just condensate rinses and drying cycles has proven as effective as the standard caustic and acid cleaning steps. The reasoning is the structure of the scale is porous and the wetting and drying takes the scale through a swelling and shrinking condition reducing the integrity of the scales structure and causing it to release itself from the metal as well as scattering into small free floating particles. This cleaning regime has been effective in multiple locations. The ability to dry the scale is essential and may take up to 8 hours, however applying steam reduces that time and aids in the thermal shocking of the scale. It is important the scale is completely dried. This is best accomplished by venting the vapor space to atmosphere or to a vacuum condenser. The addition of the water after drying washes loose scale but also has the additional affect of thermal shocking that results in releasing large amounts of the scale without chemical treatment. The long term integrity of the welds and thinned surfaces at stamped points is not known from this procedure. As in any cleaning operation, high washing and rinsing velocities aid in scouring the surfaces.

Again, the use of cleaning with scale inhibitor additives or EDTA should be considered. The antiscalants being used by your facility may also remove most of this scale. The mechanism of these chemicals to remove conventional scale is adequate. Estimates of the necessary reagents can be calculated and residuals can be monitored for completion of the cleaning cycle. It has been more effective than the standard cleaning regime and generally is cost effective against rising film mode especially if considerations to production losses are included. There is a limit to the amount of scale inhibitors allowed in juice. A determination should be made if the out-of-service usage of the scale inhibitor allows for an increase in the allowance without compliance problems.

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Page 11: Cleaning regimes used in falling film plate evaporators Mark ......The falling film plate evaporator combines a compact, thin, rigid plate design of high density heating surface area

-- -----

The cleaning of carbon and "sugar coal" is a challenge that was not commonly encountered in evaporators. The high efficiency and low contact volume creates more situations that can lead to rapid degradation of sucrose. A number of procedures and different chemistries and chemicals are available from suppliers. The most common are supplied by Keller& Bohacek; product names containing KEBO, and Lubrizol; product names containing KABO. Again the success of the chemistry is dependent upon the sucrose breakdown products and the number of layers of carbon-like scales and mineral scales. Long wetting times, proper temperature control and drying in between cleaning cycles will generally result in successful removal of these deposits. Attempts to use falling;film mode with large amounts of carbon-like materials did not prove successful.

Figure 10: sugar coal

A combination of gluconic acid and other components has also been used and the chemistry appeared suitable for the organic scales. It appears to break the structure into a softer and more friable form allowing removal to proceed when washing velocities are high.

If the scale is layered and mixed then multiple cleaning steps are used. The caustic step, followed by an acid step, followed by another corpplete caustic step and then the more exotic chemistries for the "sugar coal" step is preformed before finally drying and thermal shocking with a water boil step. In between each step sufficient rinsing is necessary. This cycle may have to be repeated multiple times or with scale analysis to confirm the remaining scale make-up. Then only certain steps may be needed to remove the remaining scale. Besides an analysis of the scale, the use of a boroscope can give a clear understanding of the severity of scaling from section to section as well as locations of the heaviest deposits. This is more effective than the visual inspection from the top and bottom of the plate packs. If severe fouling is found in the middle section of the plate packs, small openings may be cut between groups of plate pack sections to allow the loosened scale to flow out to the vapor duct rather than having to make it through the remaining plate pack sections. This will improve cleaning times and reduce secondary plugging from large scale particles.

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Page 12: Cleaning regimes used in falling film plate evaporators Mark ......The falling film plate evaporator combines a compact, thin, rigid plate design of high density heating surface area

Mechanical cleaning

The mechanical cleaning of the plates in situ is difficult at best. Both drilling and high pressure water cleaning have been tried. The main difficulty resides in the construction details. The almost elliptic shaped juice paths are not necessaril y aligned from one section or group of sections to the next. The largest openings are only 9 rnm and slight misalignments create interferences that the drill or high pressure wand can not always maneuver around. Remember the need to resort to mechanical cleaning means something operationally went wrong for too long a time period.

The gap between each section creates a relief that appears to minimize the consolidation and packing effects in the channels but this space may also hold particles that will plug open channels if not proper removed in the cleaning cycle. The flutes of the drill bit do become clogged regularly and if a flexible shaft driver is used the ability to direct it from group of plates into the next group often fails. The use of rigid extensions from drill bit to drill motor is not easy because the need to couple sections will limit the shaft thicknesses to generally unmanageable proportions for the length of drilling needed. The solution can be successful if blockage is limited to only one or two groups of plate packs at the top and/or the bottom.

The high pressure water cleaning solution holds promise. A high pressure jet nozzle is available that will fit the opening. The pressure of 15,000 psig is sufficient to clean a group of five plate sections, allowing solids to move in both directions. The safety aspect of the feed hose is unresolved. A thin wall tube was tried, however it was easily bent and distorted and the size is limited to the elliptical 9 rnm diameter opening, thus failures of hose or pipe limits the relieving capacity and puts the operator at high risk of injury or death. The solution of an even smaller feed supply line inside an outer shield less than 9mm must be found for in situ cleaning to become viable.

The removal of plate pack sections is a viable solution if fouling cannot be removed in situ. The removal steps and work is comparable in cost and time to removing existing tubes and installing new tubes from a typical Roberts's type evaporator. The major difference is in the falling film evaporator case most of the work is preformed by licensed code workers.

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Page 13: Cleaning regimes used in falling film plate evaporators Mark ......The falling film plate evaporator combines a compact, thin, rigid plate design of high density heating surface area

Figure 9: Water leaving back side of plate pack Figure 10: High pressure washer set up group from water blasting

Once a group of plate sections is removed, high pressure cleaning is the quicker and more economical solution to allow it to be returned to its duty service. Care must be exercised if the methodology is to clean by high pressure water jets without entering through each passage. The alignment jig to match the channel and the jet stream must be precise and rigid or damage to the end weld seams is possible. The cleaning of 20,000 ft2can be accomplished in eight to twelve hours after set up. If the methodology was to clean each

1- - ~ .

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Page 14: Cleaning regimes used in falling film plate evaporators Mark ......The falling film plate evaporator combines a compact, thin, rigid plate design of high density heating surface area

channel individually by either high pressure water jet or drilling the cost of replacement could be matched.

Several other cleaning regimes were explored including ultrasonic cleaning and complete oxidation of the "sugar coal" to carbon dioxide (C02). Neither were attempted or tested to date.

The cleaning regime employed must be successful and cost effective to allow full advantage of this technology. Differences exist between these evaporators in cleaning methodology. The best method may be the least expensive. Water boiling with a drying cycle followed by the shocking. Accepted cleaning procedure for the different scales does exist. Falling film cleaning should be the most commonly selected type with the EDTA as the chemical for calcium scale removal. The worst case of high pressure cleaning out of the vessel would be limited to excessive amounts of "sugar coal" and is marginally practical. If concerns exist of fouling with high levels of this "sugar coal," then the removable section design should be purchased. This removes or significantly reduces the risk factor in evaluating and selecting this highly efficient design.

Table 2 is a list of all the methods tried by the author with a comment about the method. Cleaning method Time Effecti veness Comments description Caustic at 5% 1-48 Organic scales were No noticeable improvements with 2 to 4% is most hours often present, long caustic boils, diminishing likely a high caustic always return beyond 4 hrs. Both rising enough turned black in first and falling film cleaning modes concentration for hour were tried. In area where there two hours were complete plugs existed,

caustic was able to concentrate "increasing embrittlement and leaching effects. I

Caustic at >10% 1-48 No increase in No advantage to high strength hours effectiveness from unless recovery of the caustic is

lower strength planned Caustic at 5% 1-48 Improves the Longer retention time may with dissolution of improve dissolution, however as soda ash Added CaC20 4*H2O scale becomes more porous acid

will remove the rest I Definite Increased effectiveness against

KEBO at 2.5 to I

Caustic with improvements in the organics including small

5% organic debris amounts of "sugar coal" more loosing and being pieces of scale were present in the removed blow down

Caustic with Definite Generally same results as the KABO at 2.5 to improvements in KEBO 5% organic debris

loosing and being removed

148

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Page 15: Cleaning regimes used in falling film plate evaporators Mark ......The falling film plate evaporator combines a compact, thin, rigid plate design of high density heating surface area

Formic acid 3 to 8% OK Effective as an organic acid; compared to sulfamic is equal, compared to HCL is less favorable for scale removal but less aggressive to the heat transfer surfaces _.

Sulfamic acid By acid Varied but an acid The layering of the scale may 5% strength: cleaning is have masked the effectiveness.

titration necessary for the Acid was sometimes consumed in method, carbonates and minutes and other times we only Never sulphites, not as broke it down as we hurried to more than effective as HCI remove it from the vessel before it I hour converted to H2S04 and create the

risk of Calcium Sulfate Sulfamic acid No advantage, 15% increase risk Nitric acid Very effective Aggressiveness makes it

cleaner on most of dangerous, no mild steel can be in the scales including the system without problems the organics

Gluconic acid Similar to EDT A, appears to be good at loosing up organic compounds

Water boil 20 Effective for high Requires rising film mode which minutes, organic loads can damage internal components, on/off large forces were generated and cycles scale was removed

Mechanical Difficult with limit Nearing last resort over replacing drilling success plate packs In place high Difficult with slight If a flexible 8 mm hose can be pressure water improvements fabricated this could proved blasting effective for the severely fouled

units. There is progress being made in this.

Out of vessel Effective Expensive and can be near the high cost of replacement. pressure water blasting

In conclusion, the cleaning is only difficult in rare cases. Falling film cleaning should generally be effective and reduce actual cleaning times over Robert evaporators. Scaling does not appear to be any greater in this style evaporator. There are many factories with lower chemical cleaning rates than when they had Robert evaporators in that evaporator service. Foam is the risk. Highly fouled evaporators should switch to rising film mode for cleaning, utilizing the forces created by bubble generation and collapsing against the

149

Page 16: Cleaning regimes used in falling film plate evaporators Mark ......The falling film plate evaporator combines a compact, thin, rigid plate design of high density heating surface area

scale. Both the juice and outlet design should be reviewed for acceptable perfonnance in operation and in cleaning modes. Modifications should be implemented to resolve problems. Filtration and recirculation flow alanns need to be part ofthe standard design. They should not be compromised.

I. Jayatilaka, D. and Punter G., Morgenroth. B. (1997): Development of Plate Evaporator technology, the market place and the choice for the sugar engineer, EuroTechLink 97

2. LQlWa/thew. D. C (1994): Evaporator fouling literature review, SMRl, Durban

3. Epstein, N (1983) : Thinking about Heat Transfer Fouling. Heat Transf. Eng., 4, 43 - 56

4. (o]Schu/ze, B.-C (2001): Untersuchungen zur Fluiddynamik von Saccharidlosungen bei der Fallfilmverdampfung an senkrechten strukturierten Platten, TU Berlin, Dissertation

5. Burnett, D.R., Scale Inhibitors - Optimization of Results

6. Wa!thew, D. C (1996): Aspects of evaporator scale formation and control in the South African sugar Industry, SPRl conference 1996

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