CONTINUOUS EVAPORATION PROCESS FOR ACIDIC WASTE

STREAMS

CROSS-REFERENCE TO RELATED APPLICATIONS AND PRIORITY The present application claims priority from Indian patent application number (202121024850) filed on (04/06/2021), incorporated herein by a reference.

FIELD OF THE INVENTION

The invention relates to a method for the evaporation of an acidic waste streams. More particularly to ensure continuous operation of evaporation plant without stopping it for doing CIP (Cleaning in place). Particularly it relates to the use of evaporator condensate as a CIP liquid for cleaning in place of evaporator. More particularly it relates to the simultaneous operation of more than one evaporator for cleaning as well as for evaporation process to increase the process efficiency of plant by its continuous operation.

BACKGROUND

Evaporation units (or evaporators) use the evaporation principle for the treatment of process water, wastewater and water-based waste. The typical liquid treated in evaporator is an aqueous waste having organic and inorganic pollutants in it. Different types of evaporators can face different operational issues which impact their performance. All these evaporators can concentrate the wastewater or process water as a pumpable fluid, up to 30% to 65 % w/w of total solids. They use heat pump, hot water or steam, or Mechanical Vapor Recompression (MVR), or Thermal vapor recompression (TVR) with natural or Falling Film or forced circulation. One of the major important applications of evaporator is the treatment of industrial waste like Distillery waste, Paper & pulp industries waste etc. Direct disposal of such waste to environment causes severe pollution problems. Recently, regulators have also tightened the norms with respect to discharge of industrial waste. So, for industries and distilleries employing ZLD (Zero liquid discharge) process has become an imperative.

ZLD is achieved by concentrating liquid waste and reusing concentrated waste as a fertilizer, fuel supplement or for any other value-added application. Evaporation unit is especially significant in industrial waste treatment where the waste is evaporated and concentrated to about 60 brix and fed to boiler and burnt along with a supplement fuel like coal to produce steam. The process condensate generated from this evaporation process generally contains volatile organic compounds. It is further neutralized and treated in biological or membrane system. Cost of treatment of the liquid is additional Opex for production. During evaporation process, presence of solids in waste causes scale formation inside the tube resulting in its fouling. During initial stages of scale formation, the scales are soft and as time passes, they become hard and difficult to remove. Scale formation results in reduced efficiency of heat exchange and restrict flow rate of the feed liquid in the tubes. This results in excess consumption of heat and electrical energy. To overcome this problem, the evaporators are cleaned periodically employing CIP process or mostly mechanical cleaning by using high pressure water jet for removal of hard scale formed inside the tubes.

The conventional CIP cleaning processes either make use of hydro-jets operating at high pressures around 800 to 1200 Bar or make use of chemicals. Many a times use of high pressure hydro jets leads to hammering inside the equipment causing damage to equipment in the form of leakages and breakages in tubes. CIP (cleaning in place) chemicals used for cleaning generally are acids and caustic which cause corrosion of the equipment thereby reducing its service life. This leads to significant expenditures on cleaning of the evaporator, decrease in working efficiency, and often raises a concern of safety of equipments and personal too. A major disadvantage of conventional cleaning processes is that they need to be carried out by shutting down the evaporation unit resulting in loss of production time. Conventional processes are energy consuming and expensive too. Since, evaporation is most energy intensive process in many industries it is essential that evaporation be approached from viewpoint of economical energy utilization as well process effectiveness. The present invention herein discloses a method of the uninterrupted evaporation of acidic waste stream by providing auto-swing CIP technique which prevents hard scale formation inside the tube surface of heat exchanger; hence eliminates the need for use of expensive and unsafe hydro jet process for cleaning. Spent lees generated from distillery or process condensate (Evaporation condensate) is used in place of caustic or other acids thus preventing equipment corrosion and increasing its service life apart from savings in the cost of CIP chemicals normally used. The most striking feature of this invention is that it enables continuous operation of evaporation process without need for a preventive or curative shut down.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with the accompanying drawings, wherein: FIGURE 1 depicts an exemplary plan of the invention showing several features that control the process of continuous operation of evaporator for concentrating the acidic waste stream without stoppage for cleaning in place. Next, condensate stream of evaporator is used as a CIP liquid for simultaneous cleaning of evaporator during evaporation process to increase the process efficiency of plant.

DETAILED DESCRIPTION OF THE INVENTION

In one embodiment of the present invention, as illustrated in FIGURE 1 the disclosed process includes following steps: a] acidic waste stream [stream (8)] fed to evaporatorl , b] evaporation in first evaporator(a) c] vapour liquid separation in separator to produce condensate steam (11) d] collection of condensate (CIP liquid) in storage tank (c) e] discontinuation of operation of first evaporator for CIP and shifting the acidic waste stream to second evaporator (B) f] Cleaning in place for first evaporator using CIP liquid (2) g] discontinuation of second evaporator operation for CIP and shifting the acidic waste stream to first or stand by evaporator for concentrating the remaining acidic waste stream.

The acidic waste stream comprises of spent wash stream, Stillage Stream, distillery stream, waste stream of textile industries, waste stream of paper and pulp industries, wastewater stream or a combination thereof. Said acidic waste streams containing about 10 % to about 20 % w/w solids is concentrated using two identical evaporation systems wherein one system is in operation mode and the other is in standby mode to facilitate continuous cyclic operation without any need of discontinuation for CIP operation.

The first evaporation is in operation mode wherein steam/vapor is supplied to evaporator (13) for heating purpose. The acidic waste stream (8) is introduced in first evaporator (A). The first evaporator concentrates the stream (4) to about 20 % to 65 % w/w solids, which is then collected in collection tank for further use or for disposal. The generated vapor stream from evaporator (A) is sent to vapour liquid separator (D) where liquid is separated from vapours. Next, generated vapour (12) is condensed (11) using condenser and sent to storage tank (C). Said condensate (11) comprises volatile acids and the pH of said condensate is between 2.5 to 5.5. Said condensate stream is further used as a CIP liquid (1).

During the operation of the evaporators the shell side temperature (TT 1 ) and the tube side temperature (TT2) are continuously monitored. Said evaporation is continued in operation for desired time period, particularly for about 1 to about 3 days in the first evaporator (A) until temperature difference between shell side (TT1) and tube side (TT2) of the evaporator is between 1 and 6 °C. The temperature difference criteria for switchover depends on the solid concentration of the waste stream. For Lower concentration (20%-35%) the temp difference is between 1 °C and 3 °C. For higher concentration (36% to 65%) the temp difference is between 3 °C to 6 °C.

As soon as the temperature difference exceeds the specified limit, acidic waste stream (8) is automatically shifted to second evaporator (B) while the first evaporator which is in the operation mode automatically shifted under cleaning in place mode. The acidic waste stream (8) is introduced in second evaporator (B). Said second evaporator concentrates the stream (5) to about 20 % to 65 % w/w solids which is then collected in collection tank for further use or for disposal. The generated vapor stream from evaporator (B) is sent to vapour liquid separator (D) where liquid is separated from vapours. Next, generated vapour (12) is condensed (11) using condenser and sent to storage tank (C). Said condensate stream is further used as a CIP liquid (1 ). During operation of second evaporator (B) the CIP of first evaporator (A) is simultaneously carried out using CIP liquid (2) form the Storage tank (C). Said CIP liquid is circulated in first evaporator for cleaning in place forms a used CIP stream.

Once cleaning is complete, the evaporator is kept in standby mode till the time when second evaporator (B) comes under cleaning in place (CIP) mode. The second evaporator is continued in operation until temperature difference between shell side (TT1) and tube side (TT2) of the evaporator is between 1 and 6 °C with respect to waste stream concentration and then shifted to CIP automatically as the temperature difference exceeds specified limit and the earlier first evaporator which is in standby mode is automatically switched into operation mode for evaporation of remaining acidic waste stream. These evaporators are worked in auto swing cycle mode for continuous operation, without any stoppages for CIP. The automatic shifting from first evaporator to second or vice versa is carried out using solenoid valve which is controlled by process logic controller. The generated CIP liquid is recycled back in CIP liquid feed tank and reused to optimize the concentration of solid of acidic waste stream or fed to evaporator system as feed to meet ZLD requirement.

In another embodiment of the invention, said evaporation is performed in continuous manner by using more than one evaporator in alternate mode like rising film, falling film, forced falling film, forced circulation evaporation unit or combination thereof.

In another embodiment of the invention, more than two evaporators are used to perform the continuous evaporation wherein one evaporator is working for evaporation and whereas other evaporators are in standby mode after cleaning in place.

Said process has several advantages over the known methods as listed below:

1. Process disclosed in prior art is generally discontinued for CIP after every 8 to 15 days whereas disclosed invention does not require to be stopped or discontinued for the CIP operation.

2. The process does not require harmful chemicals for cleaning, such as caustic or acid streams.

3. The process does not allow hard scale formation inside the tubes like those which are formed in conventional evaporation process which helps to eradicate the expensive and insecure hydro jet process for cleaning. 4. Due to non fouling in the evaporator, the evaporators operate at their maximum operational capacity of thereby leading to overall decreased operating cost. 5. Mechanical strength of the equipment is kept intact due to avoidance high pressure hydro jet process.

6. This process helps to evaporation plants to run continuously round the year without any stoppages for CIP operations.

7. This process helps in reduction of fouling factor for said evaporator leading to increase of heat transfer coefficients. Hence there is reduction in heat transfer area of heat exchanger and foot print area.

8. This process helps in reuse of process condensate without treatment for cleaning of evaporator. Said used process condensate are also further for cleaning purpose based on pH of the stream (<5.5)

9. The process reduced the effluent load of cleaning by 90 %.

Examples:

Example provided below gives wider utility of the invention without any limitations as to the variations that may be appreciated by a person skilled in the art. A non-limiting summary of various embodiments is given in the examples and tables, which demonstrate the advantageous and novel aspects of the process is disclosed herein by using spent wash stream with typical composition of spent wash, process condensate which is acidic in nature and its use as a CIP liquid.

Evaporation of spent wash - The sugar factories in India produces about 15 million tons of sugar by crushing about 150 million tons of sugar cane. About 7 million tons of press mud and about 8 million tons of molasses are produced as a by-product from sugar industries. Molasses is utilized in distilleries to produce ethanol. Along with ethanol, distilleries produce billion litres of spent wash (SW) as waste. Direct disposal of such spent wash to environment causes severe pollution problems. Recently, regulators have also tightened their norms with respect to discharge of spent wash. So for sugar industries and distilleries now employing ZLD (Zero liquid discharge) process has become an imperative. SW is made up of solid materials left unused in molasses fermentation process; these solids are mostly in the form of organic and inorganic complexes having very high BOD and COD demands for its degradation. A typical compositional analysis of a sample of SW is provided in Table 1.

Table 1 : Composition analysis of SW sample

ZLD achieved by concentrating liquid waste and using concentrated waste as a fertilizer or fuel supplement or any other value-added application. Evaporation unit plays a significant role in achieve ZLD especially in distilleries and other industries. Thus, in the case of distilleries, spent wash remaining after distillation of ethanol, is evaporated and concentrated to about 60 brix and burnt along with a supplement fuel like coal, Bagasse, rice husk etc in a boiler to produce steam. The process condensate generated from this evaporation process having high volatile solids is neutralized with caustic and sent for membrane treatment. Treated condensate from membrane will be used in process and reject sent to Evaporation system.

The composition for process condensate is given in table 2. Table 2: Typical composition of process condensate stream Examples 1 :

A batch containing about 224 metric (MT) tons of cane molasses (having about 45% w/w fermentable sugar and about 80% w/w total solids (TS)) and about 533 MT of process water was used for ethanol production in distillery plant. Molasses fermentation was carried out in fermentation unit producing about 614 MT of fermented wash and about 44.5 MT carbon dioxide gas. The fermented wash was distilled in distillation column to recover about 60 KLPD ethanol and about 65 MT of pre- rectifier distillation column spent lees which was further sent for treatment. Said fermentation batch also produced about 517 MT raw spent wash stream with 15% w/w total solid (TS). Next, spent wash was further concentrated by using evaporation system. Spent wash stream was introduced first into a falling film evaporator to concentrate it up to 30 to 33% w/w TS. Next, it was sent to force circulation evaporator to produce 131 MT of concentrated stream containing 55 to 65% w/w TS along with condensate of 386MT. This concentrated stream product was used as boiler fuel along with supporting fuel such as coal or bagasse to generate steam.

Condensate generated herein evaporation system was further treated and used in process application. Due to continuous evaporation process, scales were formed inside the tubes of falling film and forced circulation evaporator. In the initial 2 to 3 days, said scales were soft but later due to continuous heating process, they became hard. Said hard scale was removed by using either hydro blasting jet machine at high pressure (1000 to 1200 bars) or using clean in place (CIP) chemicals such as 1000 Kg of 5% caustic and 1000 kg of 5% acid for removal of hard scale. Effluent generated from hydro blasting treatment was about 144 MT per day or from CIP chemicals treatment was about 2 MT per day, which was further treated in effluent treatment unit.

Example 2:

A batch of about 224 metric (MT) tons of cane molasses (having about 45% w/w fermentable sugar and about 80% w/w total solids (TS)) and about 533 MT of process water was used for ethanol production in distillery plant. Molasses fermentation was carried out in fermentation unit which produced about 614 MT of fermented wash and about 44.5 MT carbon dioxide gas. The fermented wash was then distilled in distillation column to recover about 60 KLPD ethanol and about 65 MT of pre- rectifier distillation column spent lees which further sent to treatment unit. Said fermentation batch produced about 517 MT raw spent wash stream with 15% w/w total solid (TS). The spent wash stream was further concentrated using evaporation system. The evaporation system comprised of two units of falling film evaporator, two units of forced circulation evaporator and two units of vapor liquid separator. When one falling film, evaporator was in working condition, the second falling film evaporator was on standby mode. The same logic was also followed for operation of the two forced circulation evaporators. Both pair of these evaporators were arranged in auto swing cycle mode for continuous evaporation process. Spent wash was introduced in first falling film evaporator, where spent wash stream was concentrated between 38 to 50 % w/w of Total solids. Said concentrated stream was further sent to first forced circulation evaporator, where the stream was further concentrated to form a concentrated stream having total solids between 58 to 65 % w/w. Condensate from both these evaporators was collected in a storage tank. The pH of said condensate was between 2.5 to 5.5 . This condensate stream was further used as a CIP liquid.

During the operation of the evaporators the shell side temperature and the tube side temperature was continuously monitored. Said evaporation was continued in the first falling film evaporator and the first forced circulation until temperature difference between shell side and tube side of the evaporator was not exceeding 2 °C for Falling Film and 5-6°C for Forced circulation. As soon as the temperature difference exceeded specified above, the spent wash was automatically shifted to second falling film evaporator and the second forced circulation evaporator while the first set of evaporators which were operational were automatically shifted under cleaning in place mode. Said cleaning of first falling film evaporator and first forced circulation was carried out using process condensate form the collection tank. Once cleaning is complete the evaporators are kept in standby mode till the time when second falling film evaporator and second forced circulation come under cleaning in place (CIP) mode. The evaporators were worked in auto swing mode for continuous operation, without any stoppages for CIP to produce 131 MT of spent wash having about 55 to 65% w/w TS.

While the invention has been particularly shown and described with reference to embodiments listed using examples, it will be appreciated that several of the above disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also that various presently unforeseen and unanticipated alternatives, modifications, variations, or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims. Although the invention has been described with reference to specific preferred embodiments, it is not intended to be limited thereto, rather those having ordinary skill in the art will recognize that variations and modifications may be made therein which are within the scope of the claims.