Continuous Systems

Horizontal Continuous Extractor

 

Horizontal continuous extractor is a typical workhorse of solvent extraction type vegetable oil processing units. It can be adopted for steviol glycoside extraction with very little modification. In this system, the leaves are to be loaded in chain of baskets with perforated bottom, which moves in continuous circle carried by a chain drive. As the baskets move through the system, progressively dilute extractant is sprinkled on the baskets. The extractant percolates through the leaves in the basket and collects in different sumps. Extract from one sump is used as extractant for another stage. After repeated extraction with progressively dilute extracts and ultimately with fresh water, the baskets are tipped mechanically and the leaves are discharged through a chute. In this system the leaves and the extractant flows in opposite (counter-current) direction.

Advantage:

 

  1. Can be integrated into continuous process

  2. Extraction efficiency is high

  3. Final extract is fairly concentrated

 

Disadvantages:

 

  1. Cost of equipment is high

  2. Large equipment, so maintaining stable optimal thermal profile is difficult

  3. Hydraulic conductivity of soaked leaves is low and it impairs percolation   

This process has the following advantages and disadvantages :

Other Potential Options

I started working on process improvement for the aqueous extraction and considered several options. It’s a classical case of solid liquid extraction and there are several systems which can be adopted for the process. Some of the process has already been tried and ultimately I came up with the optimum solution. First I have tried several batch processes, which I am describing below.

Batch Systems

Rotating Extractor 

 

Rotating extractor is actually is a horizontally revolving cylindrical vessel. It is provided with a large hatch for loading and unloading the solids to be extracted and a drain line for loading the extractant and draining off the extract. The vessel is provided with baffle plates perpendicular on the internal surface. Sometimes, the vessel is provided with a steam jacket for heating the material in it. For extraction, the solid is loaded first in the vessel and then extractant is pumped in to completely submerge the solids. Then the ports of the vessel is closed and it is rotated at a gentle speed. The internal baffle plates facilitates through mixing of the solid and liquid. The benefits of this system is as follows

  1. It offers better process hygiene since the extraction is done in closed vessel

  2. Thermal efficiency is better since the heating is done with insulated steam jacket

  3. Mixing is gentle, so there is less pulping of the leaves. This helps in downstream filtration process.

  1. Batch process…..can not be integrated into a continuous processing system

  2. Loading and unloading is difficult. This increases the time between two batches

  3. Extraction efficiency is low and repeated extraction is often necessary. This results in diluted extract.

Percolation Extractor

 

I have used percolation extractor in some of the steviol glycoside unit designed by me. This is also a batch type extraction system. This system consists of a cylindrical vertical vessel with a perforated false bottom. The vessel is provided with two hatches – one for loading the leaves near the top flange of the vessel and another for unloading spent leaves just above the false bottom. A bed of dried leaves are made over the false bottom and hot water is sprinkled over the leaf bed, which percolates through the leaf bed and gets accumulated in the space below the false bottom. The water extract is then drawn from a bottom port of the vessel, pumped through a heat exchanger to heat it again and sprinkled over the leaf bed. This recirculation of the extract is continued until the concentration of the steviol glycoside becomes at equilibrium with that of the extract. Then the extractant is drawn off for processing and some quantity of fresh water is introduced in the system for repeat extraction.

But there are several cons for the process also

The benefits of the system are as follows

  1. Less water : leaves ratio can be used, which ensures less liquid material handling and processing.

  2. Lower amount of water is to be removed from the extract for purification, hence the process is more efficient in terms of energy use.

  3. The entire system is thermally insulated, which ensures high thermal efficiency.

  4. Good process hygiene.

  5. The extract is relatively free from suspended material dut to the filtration of the leaf bed.

The disadvantages of the process are as below

 

  1. Batch process…cannot be integrated into a continuous process system.

  2. Repeat extraction is necessary to achieve highest extraction efficiency.

  3. After a certain time during extraction, when the leaves becomes soggy, the hydraulic conductivity of the leaf bed decreases, which impairs percolation. This effect increases the extraction time.

  4. Leaf loading and unloading through hatches is time consuming and that results into longer time between the batches.

Later I have made several changes in the design to address most of the issues described above. But, the process is still a batch process requiring repeat extraction.

The Classical Chinese System

When I was in China in 2014 and visited some large stevioside extraction facility, I was surprised to see a very technically challenged water extraction system. There, the extraction system generally comprises of a long trough with parabolic bottom fitted with longitudinal helical agitator. Pneumatically operated trapdoors are provided at the bottom of the troughs.

 

For extraction, the dried leaves are loaded manually in the troughs. No mechanical pre-treatment of the leaves like grinding is necessary. Then hot water is pumped in the trough and the leaves are agitated in the water for extraction. After the completion of extraction, the water extract is drained off by a discharge tube and then the spent leaves are discharged on a conveyor belt. The conveyor belt carries the spent leaves to the solid waste processing system.

To my opinion, this process is very crude and wasteful. The reasons are below :

 

  1. It is a batch process and thus, slow.

  2. There is no thermal insulation on the trough and the entire process is carried out in an open vessel. Thus the thermal efficiency of the process is low.

  3. The water : leaves ratio is high…..otherwise the agitator can not turn. This results into diluted extract.

  4. For complete extraction, the leaves are to be repeatedly extracted with water, and the extracts generated due this repeated extraction are to be pooled with the first extract. This again results into further dilution of the extract.

  5. After the leaves become soggy, the agitator actually pulps the leaves…..which increases the suspended solid content of the extract and downstream filtration becomes very tedious.

  6. Low process hygiene since the extraction is carried out in an open vessel.

If you are interested in more details, just drop an email to me.

  1. High extraction efficiency

  2. Highest concentration of glycosides in the extract

  3. Highest water use efficiency

  4. Lowest amount of total suspended solids in the extract.

  5. Minimal extraction of fats, oils and waxes

  6. Minimal extraction of mucilages and highly polymerized substances

  7. Compatibility with GMP guidelines

  8. Compact size with minimal footprint

  9. Low cost and low payback period

I have considered all the available potential extraction systems and have done several trials for developing the optimum extraction system for Stevia leaves. Now I am finalizing design of a multistage extractor which employs both immersion and percolation method at different stages and have very efficient process control system. The system is also aimed for :

Efficient Extraction Methods for Stevia - Towards better production economics

In almost all the industrial technologies for extraction and purification of steviol glycosides, leaves are extracted with water as the first step. Generally, the process involves seeping whole dried leaves with hot water, sometime with agitation, and then separation of the liquid extract from the spent leaves.

 

As dried stevia leaves are bulky, this process involves lot of material handling. Due to the bulkiness of the leaves, often a very high water to leaves ratio is to be used….which results in low concentration of steviol glycosides in the first crude extract. Heating and handling of a large volume of water also necessitates use of a lot of energy. If a large quantity of water used in the extraction process, the extract also becomes very dilute. Removal of all water from this dilute solution also becomes very much energy extensive. Enormously large vessels are also required to store and process that dilute extract.

 

Thus, most of the time, the economics of the whole extraction and purification process depends on the efficiency of this water extraction process.

Hildebrandt Extractor

 

This type of extractor is being used nowadays in a wide scale for extraction of different natural products. In this system the solid is immersed in the extractant. The system comprises of two long sections of tubes fitted with screw conveyors inside. A feed hopper is provided in one end of the horizontal section and the solid is loaded into the tube through this hopper. Then the solid is transported to the other end be the slow moving screw conveyor. At the other end of the tube there is another section of tube which forms an angle with the first tube. There is a solvent entry port at around middle of the second tube, through which the extractant is pumped in. The solid meets the extractant in countercurrent manner when it is transported through the horizontal tube and in first part of the upward angled tube. The solid is then carried upward in the second half of the upward angle tube, where it is drained and the drained solid is ultimately discharged from the extreme end of the upward angled tube. The extract flows out through an outlet port at the extreme end of the horizontal section. The entire unit can be steam jackated for precision temperature control.  

Advantages

 

  1. Precision process control

  2. Extraction is through immersion method, so hydraulic conductivity is not an issue in extraction stage

  3. High thermal efficiency

  4. High concentration of the product in the extract due to countercurrent extraction

 

Disadvantages

 

  1. Hydraulic conductivity may be an issue in the draining stage

  2. Precision mechanical parts need high maintenance

 

Bonotto Extractor :

 

It consists of a vertical tower divided into sections by horizontal perforated plates. Each plate has an opening through which the solid can pass downwards from plate to plate. Each plate is wiped by wipers fitted with a central shaft. The wiper spreads the solids over the plates and pushes the solid through the opening. The holes on the two successive perforated plates are positioned at 180o angle. The solid to be extracted is fed at the top of the tower. The wipers on the plates gently rake the solids on the perforated plates and facilitate the movement of the solid from a plate to the next plate below, and thus ultimately reach the bottom of the tower. Fresh solvent is introduced at the bottom of the tower and it rises towards the top of the tower moving counter-currently with the solids. The rich extract leaves through a port near the top of the tower. The spent solid is discharged by a screw conveyor through a draining section at the side of the tower. It is also an immersion type of extractor.

Stevia leaves, even after soaking water, actuaslly floats over the water. This creats a serious problem for adoption of Bonotto type of extractor in steviol glycoside extraction. Gravity driven mass transfer of the solid from the top lavel towards the bottom does not happen as the leaves float in the extractant and remains accumulated at the top plates. 

Bollman Extractor

 

This type of extractor, widely used for the extraction of vegetable oils from seeds, consists of a number of baskets fixed to an endless chain having a descending and an ascending leg, enclosed in a vapour tight chamber. Each basket has a perforated bottom (wire-mesh). There are two sumps hold the extract streams. Liquids percolating through the baskets along the two legs flow down to these sumps. The solid is fed through a hopper into the basket at the top of the descending leg and partially enriched extract (50% extract) is sprayed on the solid. The liquid percolates through the slowly moving basket and collects at one of the bottom sumps of the unit. Fresh extractant is sprayed on the top basket in the ascending leg and percolates through the rising basket and collects in the other sump at the bottom in the form of 50% extract, which is sprayed on the top basket of the ascending leg. Percolation of the liquid occurs counter-current in the descending leg and co-current in the ascending leg. An extractor of this type may be 15 – 20 meters high. At the top of the tower, the baskets are tipped into a hopper, which is provided with a screw conveyor at its bottom to discharge the spent solids.

This process has the following advantages and disadvantages :

Advantage:

 

  1. Can be integrated into continuous process

  2. Extraction efficiency is high

  3. Final extract is fairly concentrated

 

Disadvantages:

 

  1. Cost of equipment is high

  2. Large equipment, so maintaining stable optimal thermal profile is difficult

  3. Hydraulic conductivity of soaked leaves is low and it impairs percolation. Sometimes chanelling through leaf matrix also occurs which also have adverse effect on extraction efficiency. 

Rotocel Extractor

 

A rotocel extractor has a rotating basket construction. It consists of a number of cells (usually 15 to 18) fixed to a central rotor. Each cell or basket is fitted with a hinged perforated or screen bottom so that the solvent can percolate through the solid feed. The fresh solid is filled in the baskets or cells, one by one, through a feed hopper. The entire carousel of the basket moves forward and brings another empty basket under the feed hopper.  There is series of pumps, called stage pumps, which pump the solvent out of the compartment at one position and discharge it into compartment at the previous position. The extractant percolates through the solid, and gets accumulated in a sump below the carrousel. The sump is divided into several radial compartments. One compartment receives permeated extract from one or two cells. The extract from a compartment is pumped and sprayed on the next basket in the counter-current direction. After the extraction is complete, the cell passes over a discharge chute and the exhausted solid containing some entrained extract is removed by opening the hinged bottom of the cell. This emptied compartment continues to rotate and then receives an amount of fresh feed, thus completing a cycle.

Rotocell extractors have all the advantages and disadvantages associated with basket type extractors. They have an added advantage of compact size, high throughput and better energy eficiency.  

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