Advanced steam saving in cane sugar mills by introducing falling film evaporators

Energy economy in the beet sugar industry was continuously improved and refined over decades to reduce the steam consumption as far as possible, whereas the cane sugar industry for a long time attached only minor importance to this subject. However, this attitude is no longer relevant. More and more cane sugar factories are narrowly monitoring their energy consumption figures in an effort to reduce their steam consumption to a minimum by investing in equipment and also process optimisation. The new global tendency to install cogeneration plants is the main reason for the cane sugar factories to monitor not only the daily number of bags of sugar produced, but also the bagasse quantity that is saved daily. The bagasse saved serves as an energy source after the campaign to produce electric power.

In order to receive additional revenue from cogeneration, steam saving for the sugar process is mandatory. Now, plants with clearly less than 30 % steam on cane for the plantation white process as well as approximately 40 % steam on cane for the cane sugar mill including a full refinery has been operated successfully.

This paper presents the concepts as well as the operating results from cane sugar mills in India and Pakistan, where processing performance and steam consumption was clearly optimised by the installation of falling film evaporators and modification of the heating system.

1. Introduction
In the cane sugar industry, the Roberts evaporator is the most common type of evaporator to the present day. This is not only a question of costs, but can also be explained by the fact that heating tube cleaning is a simple process if performed mechanically or by high-pressure water jet. However, Roberts evaporators need a higher temperature difference in the evaporator effects compared with falling film evaporators, which is due to the hydrostatic pressure that is produced by the high juice level. This restricts the possibility of using vapour from the last effects for heating purposes, and consequently increases the factory’s steam consumption.

A concept has been developed that allows falling film evaporators to be used in the cane sugar industry, so the benefits of this technology, which has been for decades applied in the beet sugar industry with great success, can also be put to use for the production of cane sugar.

Falling film evaporators allow advanced steam-saving concepts to be implemented for sugar production. This publication shares the experience and results of evaporator plants including falling film evaporators in cane sugar factories with a particular view to the operating conditions of cane sugar mills with and without refineries.

2 Plantation White Cane Sugar Mill without Refinery

2.1 Development of ICPL cane sugar mill
In 2011, a five-effect evaporation plant with falling film evaporators was installed and commissioned for the Indian Cane Power Limited (ICPL), which was originally a 6,000 ton cane per day (TCD) plantation-white cane sugar factory that does not have a refinery. In 2012, two additional falling film evaporators were installed as standby units for effects 3 through 5 for further capacity expansion up to 9,000 TCD. It consists of five plus two falling film evaporators with a specific heating area of 2.4 m² per ton of cane per day.

Additional equipment complements the plant. This equipment includes a central condensate tank for gradual condensate expansion. Pumps for clear juice, juice circulation and thick juice are installed, which ensure that the evaporators are supplied with the required amount of juice for concentration. For better heat utilisation, the clear juice is gradually heated in heat exchangers before it enters the first evaporator. Storage tanks and feed tanks, as well as separate pumps are available for chemical cleaning. Measuring and control equipment and a central process control system provide for easy control of the complete system.

2.2 Operating results from enhanced heating system concept at ICPL
The heat system concept for ICPL is based on making maximum use of the vapour from the last effects for heating purposes. Decisive is, in particular, VP3 and VP4 utilisation for crystallisation:

• Heating of primary juice (PJ) with VP5, condensate and VP4
• Heating of limed juice (LJ) with VP3, VP2 and VP1
• Heating of clear juice (CJ) with VP2 and VP1
• Crystallisation with VP2, VP3 and VP4.

The operation with extended capacity was started from December 2013 on. Figure 1 is an overview of the crushing capacity and specific steam consumption values that were recorded during the entire campaign. This allowed raising the factory’s processing capacity to 9,000 TCD, and steam consumption was reduced to 28 % on cane (without accounting for downtimes).

2.3 Scaling of evaporators and chemical cleaning at ICPL

Since each evaporator effect is affected by scaling, the temperature gradient that is required for operation of the evaporation plant is increasing. Scale on evaporator tubes is recognised as hampering the heat flow by a solid layer with low heat conductivity.

The scale that formed in the different evaporator effects at ICPL was analysed. The main inorganic components in the scale are calcium carbonate, calcium phosphate, calcium sulphate, calcium sulphite and silicon compounds (Lehnberger et al. 2014).

At ICPL, the evaporation plant was chemically cleaned not later than after a 30-day operation period. Alkaline cleaning (a combination of a 6 % caustic soda solution with 70 % on solids of caustic soda NaOH and 30 % on solids of sodium carbonate Na2CO3 as well as an EDTA-based scale softener) was followed by acid cleaning (formic acid with corrosion inhibitor), always at temperatures near 100 °C. The exposure time was always eight hours, the concentration of the cleaning solution being adapted to the scale thickness.

3 Cane Sugar Mill with Refinery

3.1 Development of GSM cane sugar mill
In 2014, two falling film evaporators in the first effect were installed and commissioned at Ghotki Sugar Mill (GSM) in Pakistan. GSM is a 12,000 TCD cane sugar factory with a backend refinery (table 1). The existing evaporation plant at GSM before modification combined falling film evaporators in 1st effect with Roberts evaporators in 2nd through 5th effects, with a specific heating area of 1.5 m² per ton cane per day. For upgrading the evaporation plant, two new falling film evaporators were installed in the 1st effect with a heating area of 5,000 m² each (figure 2). The existing evaporators were then distributed to the remaining effects. The two new evaporators increased the specific heating area to 2.0 m² per ton cane per day. Since each evaporation effect now has its own standby evaporator, evaporator switching for cleaning purposes is much easier than before, and there is also more flexibility.
Table 1: Operating features of Ghotki Sugar Mill (GSM)

Four new tube-bundle heat exchangers for raw juice, with a heating area of 450 m² each, were installed in addition to the two falling film evaporators. As a further and final step, four additional new tube-bundle heat exchangers for limed juice and clear juice, with a heating area of 450 m² each, were installed, too.

Operation at GSM differs from ICPL in that GSM has an attached refinery, with an additional steam consumption of approx.10 % on cane, divided on VP1 (2 % on cane) and VP2 (8 % on cane). For the crystallisation at mill’s sugar house, only VP2 and VP3 are used.
Finally, juice heating and vapour bleeding show following scheme (fig. 3):

• Heating of primary juice (PJ) with VP5, condensate and VP4/VP3
• Heating of limed juice (LJ) with condensate*, VP3*, VP2 and VP1
• Heating of clear juice (CJ) with VP2* and VP1
• Crystallisation with VP2 and VP3
• Refinery with VP1 and VP2

*Heat exchangers installed before season 2015/16 only

3.2 Operational results from updated heating system concept at GSM
Ghotki Sugar Mill (GSM) in Pakistan is a 12,000 TCD cane sugar factory with an attached refinery. The evaporation plant with the two additional falling film evaporators was commissioned in December 2014. Figure 4 shows the operating data (crushing capacity and specific heat consumption) for the 2014/15 campaign as well as the 2015/16 campaign.

The targeted crushing rate of 13,000 TCD was not only achieved, but was exceeded by far. During campaign 2014/15 the average crushing rate was 13,700 TCD, which is higher by 5 % than the rated value. For the following campaign an average crushing rate of 14,700 TCD was recorded.

The relatively high steam consumption during the first two weeks in campaign 2014/15 can be explained by the fact that the new installations had not yet been insulated (new falling film evaporators, new heat exchangers, juice and steam piping). When the insulation for the equipment had been completed, the specific heat consumption dropped to values between 40 and 43 % on cane.

After installation of four new heat exchangers for limed juice and clear juice before the 2015/16 campaign the specific steam consumption dropped down to less than 40 % steam on cane as the campaign average.

With view to scale formation on the heating tubes of the falling film evaporators, the new falling film evaporators were operated for 20 days on average between chemical cleaning intervals.

4 Conclusions

Falling film evaporators have been proved to be excellent performers in the beet sugar industry. Thanks to a carefully conceived cleaning process and adequate discharge of non-condensable gases, they can also be used with great success in the cane sugar industry.

Since these evaporators require a lower temperature gradient than Roberts evaporators, and since the residence time is low, the temperature level in the last effects of the evaporation plant can be increased. Because of this, vapour from effects 3 and even 4 can be used for crystallisation, and this allows the heating system in cane sugar factories to be optimised.

When combining falling film evaporators with well-organised sugar house operation (continuous boiling, batch pans with stirrers, controlled water application), specific steam consumption values of 30 % on cane can be continuously achieved in cane sugar factories without a refinery, and 40 % on cane in cane sugar factories with a refinery.