Sichter, N.J., Lee, R.L., Tyson, R.B., Broadfoot, R. and Dunn, K.G. Proc. Aust. Soc. Sugar Cane Technol., Vol. 25, 2003

Transcription

1 EXPERIENCES WITH THE BALCKE-DÜRR PLATE EVAPORATOR AT TABLELAND FACTORY By N.J. SICHTER 1, R.L. LEE 2, R.B. TYSON 2, R. BROADFOOT 3 and K.G. DUNN 3 1 Bundaberg Sugar Ltd., Bundaberg, Qld 2 Bundaberg Sugar Ltd., Tableland Mill, Mareeba, Qld 3 Sugar Research Institute, Mackay, Qld KEYWORDS: Falling Film Plate Evaporator, Juice Evaporation, Heat Transfer Coefficient, Scaling. Abstract A 1401 m 2 Balcke-Durr falling film plate evaporator was installed at Bundaberg Sugar s Tableland factory for the 2001 season. Testing of this evaporator was carried out during the 2001 and 2002 seasons. Initially the scaling rate was rapid and heat transfer performance was disappointing. However, following modifications to the juice distributor the heat transfer performance was improved and the scaling rate was reduced. With the new juice distributor heat transfer coefficients typically ranged from W.m 2.K 1 immediately after a clean to about W.m 2.K 1 after 14 days of operation. The paper describes experiences with operation of the evaporator and the heat transfer performance. Introduction Bundaberg Sugar Ltd. (BSL) investigated the feasibility of electricity co-generation and paper pulp production during the period These studies identified the need for energy-efficient evaporator configurations which necessitate the operation of evaporators at a low temperature difference. The falling film plate evaporator (FFPE) technology (Grant et al., 2000) was seen as having potential to achieve evaporator configurations that would allow low steam consumption. During 2000, Bundaberg Sugar staff visited installations of Balcke-Dürr Thermal (BDT) falling film evaporators at the cane factory Ingenio La Gloria in Mexico, and at four beet sugar factories in Europe. A decision was taken to install a BDT falling film plate evaporator at Tableland Mill for the 2001 season. This was one year before increased evaporator capacity was required for a planned factory expansion and allowed testing of the unit without risk of disrupting crushing operations. A 1401 m 2 BDT falling film evaporator was installed. This was the first installation of a FFPE into an Australian sugar factory, and only the third such installation in a cane factory in the world. BDT supplied the plate evaporator modules through their Australian agents Senior Thermal Engineering Australia Pty Ltd. The plate modules were delivered to Bundaberg Foundry Engineers Ltd (BFEL) who constructed and installed the evaporator vessel. The unit, which has juice channels of 9 mm diameter, was installed as the first effect of a quadruple evaporator set. The features of the BDT FFPE are described by Grant et al. (2000).

2 Throughout the 2001 and 2002 crushing seasons, Bundaberg Sugar and Sugar Research Institute (SRI) staff conducted performance assessments of the evaporator and the other vessels of the set. Calibration checks were made on the critical transducers of the evaporator set at regular periods during both seasons. This paper reviews the performance of the BDT evaporator during the 2001 and 2002 seasons. Installation Foundations for the vessel and the pump sets were completed at the end of 2000 in readiness for erection of the vessel after the end-of-year shutdown period. The vessel was constructed by BFEL in two sections. The top section had the plate pack and juice distribution system installed and the bottom section contained the juice collection tray and ESJ feed pipe. When the two sections arrived on site, they were lifted onto the supporting steelwork and the joining of the sections was completed. Service pipework, connections to the condensate pump and juice recirculation pump, lagging, and instrumentation were then completed. Representatives from BDT were on site during the pre-2001 season steam trials to commission the vessel with water. Pre-steam commissioning involved checking the level of water above the juice distributor plates with the recirculation pump running, and testing interlocks for low recirculation flow, low juice level in the vessel, and emergency water addition. Figure 1 shows the schematic layout of the quadruple effect evaporator station at Tableland Mill. Evaporator supply juice 1A 1B Syrup FFPE Roberts Roberts Roberts Roberts Fig. 1 Schematic layout of the juice circuit of the Tableland Mill evaporator station. Overview of the performance during the 2001 season Figures 2, 3 and 4 present the results of the heat transfer performance data for the 2001 season.

3 Fig. 2 Heat transfer coefficient (HTC) values for the BDT evaporator during the 2001 season. Fig. 3 Effective temperature difference for the BDT evaporator during the 2001 season. Fig. 4 Vapour condensation coefficient values for the BDT evaporator during the 2001 season.

4 During the first half of the 2001 season (to late August), the performance of the evaporator was very disappointing with respect to low heat transfer efficiency, poor effectiveness of chemical cleaning, and rapid scaling of the plate surfaces. During this period, the heat transfer coefficient (htc) typically ranged from around 4000 W.m 2.K 1 following a clean to 1800 W.m 2.K 1 after two weeks of operation. The prime concern was the rapid rate of decline of htc due to scaling and the low htc value after two weeks operation. The distribution of the scale on the plate packs (particularly that of a soft black scale on the underside of the bottom plate pack) convinced BSL and SRI staff that the poor heat transfer performance and propensity for scale development were initiated, or at least substantially worsened, by poor distribution of juice onto the plate surfaces. The assessments showed that there was minimal transverse flow of juice within the juice distributor and within the plate pack. Excellent juice distribution in falling film evaporators is critical because high rates of heat transfer rely on uniform wetting of the total heating surface. At the end of August 2001, BSL modified the juice distribution sheet in two stages in accordance with modifications designed by BSL and SRI staff. In the first stage, two thirds of the top layer of the distributor were changed and the performance of the vessel monitored over the next two weeks. When this showed an improvement in the distribution of scale on the plate pack, the remaining one third was then also modified. These modifications approximately doubled the number of tubes on the top juice distribution sheet. At the same time, an extensive cleaning program was undertaken to try to restore the surfaces of the plate packs to the original condition. The modifications to the juice distribution system made a major improvement to the heat transfer performance for the last six weeks of the season. Following the modifications, the heat transfer coefficient typically ranged from about 5000 W.m 2.K 1 immediately after a clean to between 2000 and 2500 W.m 2.K 1 after 14 days of operation. This is an excellent htc value when clean. However, the BDT evaporator still experienced a rapid decline in htc value due to scaling. Analysis of the data showed that the rate of decline in htc for the BDT evaporator was considerably faster (estimated to be 25% faster) than the htc decline for the Roberts evaporator operating in parallel. [Note: the rate of decline is defined by the time decay parameter in an exponential decay function of htc versus time.] During the 2001 season, the vapour condensation coefficient (VCC) (vapour rate supplied to the evaporator divided by the heat transfer area) was typically between 38 and 40 kg.h 1.m 2 after a clean, declining to about 30 kg.h 1.m 2 after 14 days of operation. The effective temperature difference T eff (vapour temperature at the entry to the plate pack minus the juice temperature in the vessel) ranged between 4 and 6 o C when clean and 10 and 13 o C when scaled (at the end of the operating period and just prior to cleaning). This increase in T eff occurred despite the reduction in the vapour condensation coefficient during the operating period. It appeared that the modifications to the juice distributor, and the subsequently improved wetting of the plate surfaces reduced the T eff values at the end of the 14 days of operation to 9 to 10 o C. The juice recirculation rate (supply rate to the distributor) was maintained in the range of 400 to 600 m 3.h 1 through the season, with the majority of operating time at 500 m 3.h 1.

5 Overview of the performance during the 2002 season Prior to the 2002 season, the following changes were implemented: A new juice distribution system was installed. This system was manufactured by BDT and included a top distribution sheet with a slightly greater number of tube risers than in the modifications made by BSL in August The number of tube risers in the new distributor sheet was double the number in the original sheet. The perforated sheets below the top distribution sheet also included additional holes; additional holes were also provided in the juice feed pipe. A chemical preparation and storage tank was installed to facilitate sulfamic acid cleaning of the evaporator set. Any blocked channels in the plate pack were cleaned out. Throughout the 2002 season, logged data were forwarded regularly to SRI. Different processing circumstances were experienced at different periods of the 2002 season and the operation of the evaporator is best assessed in terms of performance during these periods. Early in the season (up to late June) the cleans were not very effective because sulfamic acid was not available at the mill site due to problems with shipping of the material. Mid-season period (from late June to early September) when normal processing conditions and reasonably effective cleans were experienced. During this period on/off trials of two weeks duration were undertaken using scale inhibitors to determine if reduced scaling could be achieved. Scale inhibitor was added to the ESJ as well as to the juice entering the evaporator vessels. The scale inhibitor was added for periods ending 2/8/02 and 23/8/02. Late in the season following ineffective cleans on 6/9/02 and 20/9/02 due to neutralisation of the acid with caustic soda retained in the recirculation pipework. This circumstance arose during attempts to automate the cleaning procedure. The latter part of this period was characterised by interruptions to crushing as the cane supply became intermittent. Figures 5, 6 and 7 show the results of the heat transfer performance data for the 2002 season. It is apparent from the plots that the poor cleaning performance observed during the early and late periods of the season detracted from the heat transfer performance of the evaporator. During these periods, at the end of two weeks of operation, heat transfer coefficients reduced to low values (about W.m 2.K 1 ) and the effective temperature difference increased to 11 o C (for a vapour condensation coefficient of ~35 kg.h 1.m 2 ).

6 Fig. 5 Heat transfer coefficient values for the BDT evaporator during the 2002 season. Fig. 6 Effective temperature difference for the BDT evaporator during the 2002 season.

7 Fig. 7 Vapour condensation coefficient values for the BDT evaporator during the 2002 season. In order to establish the performance of the evaporator under normal processing circumstances, the data for the mid-season period are assessed. With the new juice distributor installed the heat transfer coefficient of the evaporator typically ranged from 5000 W.m 2.K 1 immediately after a clean to about 2600 W.m 2.K 1 after 14 days of operation. These htc values are similar to the results obtained in the last six weeks of the 2001 season, following the modifications to the juice distributor being implemented. When clean, the BDT evaporator achieved an excellent htc value. However, the evaporator still experienced a rapid decline in htc due to scaling; although, for the 2002 season, the rate of decline in htc was similar to the Roberts evaporator operating in parallel. Observations by BSL Tableland staff indicate that during the mid-2002 season, less scale was formed in the BDT evaporator than for the 2001 season and the scale was light grey and crystalline. It was also observed that the crystalline scale flaked off as the vessel dried out. There was little evidence of the troublesome soft black scale that was present for most of the 2001 season. The scaling rates for the BDT evaporator in the 2002 season were about 25% slower than for the 2001 season. The average values for the scaling rate constant (a) (as defined in the equation below) for the FFPE and the Roberts evaporator operating in parallel are compared in Table 1. htc(t) = htc clean e at/100 where a = the time decay parameter in the exponential decay function of htc versus time (h 1 ). Table 1 Scaling rate constants of the BDT and Roberts #1 evaporators. Season Average value of a FFPE Roberts

8 The recirculation rate of juice for the 2002 season was consistently at about 550 m 3.h 1. This compares with an ESJ feed rate to the BDT evaporator of about 95 to 100 m 3.h 1. The juice supply rate onto the plate pack corresponds to a wetting number (litres of juice per centimetre of circumferential length of heating surface in a horizontal plane per hour) of 5.0 L.cm 1.h 1 at the top of the pack and about 4.6 L.cm 1.h 1 at the bottom of the pack (after allowing for evaporation of water from the juice). The effective temperature difference ranged between 4 o C (and slightly less than 4 o C) when clean, and about 8.0 to 8.5 o C when scaled, and just prior to the clean after two weeks of operation. The T eff values for the clean condition are slightly lower than the T eff values measured during the latter part of the 2001 season, after the modifications to the juice distributor had been made, and this is attributed to operation at a lower vapour loading in the 2002 season. The T eff values for the scaled condition in 2002 are lower than for the latter part of the 2001 season (8.0 to 8.5 o C compared with 9 to 10 o C) even though the VCC values at the end of the two week period are similar (both ~30 kg.h 1.m 2 ). This is attributed to the reduced scaling rates of the BDT evaporator during the 2002 season. Cleaning procedures used in the 2001 and 2002 seasons For the 2001 season, caustic soda (~15%) was initially used to clean the vessel (boiled for approximately 3 hours), followed by sulfuric acid (400 litres boiled for approximately 1.5 hours). After a number cleans it was found that this system was ineffective. During the first part of the 2001 season, the final vessel was also establishing a scale that was difficult to remove and other caustic formulations were trialed. These chemicals did not improve the cleaning of the BDT evaporator. In the latter part of the season a number of cleaning protocols were trialed that showed promise in developing a better cleaning program. These included: Maintaining caustic strength at 13%; Using rising film boil with caustic; Using sulfamic acid instead of sulfuric acid. Rising film boiling with caustic involved filling the vessel to above the distributor with caustic and then boiling with a small amount of steam (~10 t.h 1 ). The benefit of this system was attributed to the improved wetting of the scale with caustic and the increased agitation caused by bubbles rising up through the plate pack. The disadvantages of this system are the large quantities of caustic soda and the time required to transfer the cleaning solutions and water into and out of the vessel. When the vessel is cleaned in this manner there is not sufficient dilute caustic available to boil the rest of the set at the same time. Rising film boiling adds approximately 3 to 5 hours to the cleaning time. Sulfamic acid proved to be effective in removing hydroxyapatite scale from the plate pack. A chemical stock system which was capable of mixing sulfamic acid that was supplied in 1000 kg bags was constructed and installed for the 2002 season.

9 At the start of the 2002 season, a modified method of cleaning the BDT evaporator was trialed. This involved using approximately 30 m 3 of caustic and then filling the rest of the vessel with water. With the recirculation pump running, steam is applied. The level of caustic in the vessel slowly reduces via evaporation and make-up to No.2 evaporator. This method provides rising film boiling with dilute caustic at the start of the clean and falling film boiling with the higher strength caustic towards the end of the clean. Concentrated caustic from the last evaporator is recirculated back into the first vessel. This method allows the other evaporators to be boiled with caustic at the same time. This cleaning procedure proved to be very effective and is preferred. The total cleaning time is approximately eight hours. Conclusions Based on the evaluation program, with emphasis on results obtained following the modifications to the juice distributor and during periods of normal operation, the characteristics of the BDT evaporator are summarised as: An excellent htc is obtained when the plate pack is clean (~5 000 W.m 2.K 1 ) but the htc reduces to ~2 600 W.m 2.K 1 after 14 days of operation. An effective cleaning program for the evaporator at the first effect position uses caustic soda followed by sulfamic acid. This program can be conducted in about eight hours if fast transfer of water and chemicals is undertaken. The cleaning procedure developed by the BSL Tableland staff has proven to be reliable and effective. Well-regulated procedures are required to avoid problems such as intermixing of chemicals. For VCC values of 30 kg.h 1.m 2, the T eff values for the evaporator are as follows: Status of the evaporator Immediately after cleaning After one week s operation After two weeks operation T eff, o C to 8.5 The juice recirculation rate (supply rate) to the evaporator has been held at about 550 m 3.h 1 which corresponds to a wetting number of approximately 4.6 L.cm 1.h 1 at the base of the plate pack. It is critical to achieve a uniform wetting of the plate surfaces at all times (during juice evaporation and chemical cleaning). No problems have been experienced at Tableland Mill due to sediment on the top distribution sheet (e.g., due to mud carryover with the juice or recirculation of scale). The problems with the initial distribution system in the 2001 season highlighted the need for an effective design of juice distributor. Based on the results of the 2002 season, including visual inspection after two weeks of operation, the presently installed juice distributor appears to be working effectively and providing adequate wetting to the total cross-section of the plate pack.

10 Acknowledgments The authors acknowledge the assistance provided by the staff of Tableland Sugar Mill and by Mr Gerard Grant from Senior Thermal Engineering during the course of this project, and thank Bundaberg Sugar Ltd for permission to publish this paper. Commonwealth Government funding through the Australian Greenhouse Office supported the project to evaluate the FFPE. Funding was also provided by Bundaberg Sugar Ltd and Sugar Research Institute. REFERENCE Grant, G., Morgenroth, B. and Neipoth, K. (2000). The Balcke-Duerr falling film plate evaporator An innovative concept for evaporation in the cane industry. Proc. Aust. Soc. Sugar Cane Technol., 22: