Pre-coat filter

Alluvial filtration

In continuous alluvial filtration, the filtering material is continuously supplied mixed with the product to be filtered. The filter medium forms the filter, in one or more layers, which retains the undesirable particles.

Technical

presentation of

a continuous trickling filter

Bell
Filtering plate||
Cake discharge opening
Motor for plate rotation
Filtering plate washing ramp
External residual filtration element (depending on model)
Dosing pump for filtering aid
Mixing tank for filtering aid, with agitator
Main feed pump
Flow indicator
By-pass

Main elements

The bell

There are two types of bell: vertical and horizontal.

The filter plates

In vertical bells, the trays are either horizontal or vertical.

Tray construction

The trays are made up of

:
a wide-mesh metal frame to support the filtration fabric and drain the filtrate.
a stainless steel wire or reps fabric. The fabric pore diameters currently encountered are generally between 50 and 120 µm.

Horizontal platforms

Maillaige

reps |

The mixing tank

It is fitted with an agitator.

The feed pump

It determines the flow of wine to be filtered.

The dosing pump

It ensures that the wine is alluviated.

The by-pass

It ensures a partial return of the filtered wine with the wine to be filtered.

The different alluvial filters

Horizontal bell and vertical plate filters

The horizontal bell and vertical plate filters are still widely used, but their manufacture has almost been abandoned. | | | |

ADVANTAGES	DISADVANTAGES
- Both sides of the plates are used	- Significant consumption of water when cleaning
- Low space requirement in height	- Significant loss of wine
- -

Automatic

cleaning of

the | | | | | | | - | | | | || |||

filter
\| \| \| \|\|
Automatic filter cleaning	- Residual filtration impossible
- Easy dismantling of filter elements	- Water gluing impossible
- Price often attractive	- No dry extraction of filter cake (pollutant discharge)
\|
No water gluing
possible
- No dry extraction of filter cake (pollutant discharge)
- Impossibility of changing wine quality
\|	- Watertightness problem with the filtration column
	- Very delicate maintenance of the filter cake \|

Vertical bell and horizontal plate filters

These are the most common alluvial filters. | | | |

ADVANTAGES	DISADVANTAGES
- Very good stability of the cake; filtration can be stopped without destroying the precoat	- Only one side of the tray is used

| |

- Impractical removal of wire elements

|||| | | | | ||||

- Ability to change wine quality without mixing (maximum use of the precoat) \| - Ability to change wine quality without mixing (maximum use of the precoat) - Ability to change wine quality without mixing (maximum use of the
precoat
)
precoat)	- Depending on the brand, sealing problems on the column \|
- Dry extraction of the cake	- More costly investment
- Possibility of gluing to the ea	\|
- Residual filtration	possible \|
- Low water consumption for washing
\|
- High cycle	length
\| - Automation capability
\|
- Small footprint	\| \|

Alluvial plate filters with hollow frames

They are derived from the classic plate filter.
Hollow frames are inserted between the filter elements and fitted with suitable support plates. | | | | | | |

ADVANTAGES	DISADVANTAGES
- Very large filtering surfaces	- Delicate maintenance of the cake
- No mechanical movements subject to wear	- Impossibility of gluing with water
\| -
Versatility (use of plates or additives)	- No residual filtration\|\|
- Easy maintenance (solid material)	- Long, tedious cleaning (high labour input)
- Long cycle times	- Time-consuming set-up\|\|
\|	- Very high filtration cost
\|	- High filtration residue

Candle filters

Candle filters are rarely used in oenology. These are vertical bell filters with vertical filter supports. These supports are like "pipes" made either of stainless steel washers stacked on a spindle, or of a stainless steel coil. | | | | | | -

ADVANTAGES	DISADVANTAGES
- Very large filtering surfaces	- Very delicate holding of the cake
- No moving filter elements	- No sticking to the water
\|
No residual

- No residual filtration
\|	- High water consumption when washing
\|	- Problem maintaining filter media
\|
- Impossibility of changing wine quality
\|	- Very high price

Precoat

This is an essential phase which conditions all subsequent filtration.
Theobjectives of the precoat are

:
To ensure instant clarification
To facilitate the debatizing of the filter cake

Realisation:
It can be done in one, two or even three parts depending on the objectives set and, above all, on the state of the product to be filtered.

Thebasic rules are

:
In all cases, the adjuvant, deposited directly on the substrate, must have a permeability greater than or equal to 1 darcie.
Never use fine kieselguhr (pink diatomaceous earth) directly, as it is difficult to attach and some of its very fine particles could pass through the support during filtration.
Use a clear product (water or already filtered wine) to avoid clogging too quickly and to ensure a good cycle length.
The flow rate through the filter should always be as high as possible (1.5 to 2 times the filtration flow rate) to ensure that the additive is evenly distributed over all the plates.
Generally speaking, the minimum quantity of filtering agent for the precoat should be 1 kg/m² (up to 2 kg/m² may be used).
In the case of a precoat in several parts, avoid mixing the filtering agents. In addition, never place a high-permeability diatomaceous earth after a finer diatomaceous earth, as this will form a layer of icing inside the cake, leading to rapid, totally irreversible clogging.

When filling the filter, leave the air vent open to prevent degradation of the filter cake, as the CO2 will interfere with the installation of the precoat.
Under normal conditions, when the precoat is in place, the pressure drop is 0.3 to 0.5 bar.

Filtration additive

Filtration by alluviation requires the choice of one or more filtering additives adapted to the type of product to be clarified and to the objectives sought.

Generally speaking, diatoms can be classified into 3 categories

:
Rapid or roughing diatoms Destined

for pre-filtration, they are used to clarify young wines or wines with poor filterability. Their permeability ranges from 4 to 12 darcies (see definition of darcies in the chapter on filtration aids)

.
Medium or clarifying These

white diatoms are used to obtain medium to good levels of clarity. Their permeabilities range from 1 to 4 darcies

.
Fine or finishing These

are pink diatoms with permeabilities ranging from 0.02 to 1 darcies. They are generally used for very tight filtration to ensure perfect clarity and, under certain conditions, almost total retention of yeast.
Note: perlites are categorised in the same way. Cellulose and by-products are only technical complements to the constitution of the precoat.

Note: perlites are categorised in the same way. Cellulose and by-products are only technical complements to the pre-layer.

Alluvium

Throughout

the filtration process, the regular addition of kieselguhr makes it possible to

:
increase the filtration cycles by delaying clogging
reduce the differential pressure
maintain the porosity of the cake as high as possible

Implementation

:
Alluviation is carried out with the admixture used to form the last part of the precoat. Never use a diatomaceous earth that is "faster" than the one used to finish the precoat. A layer of icing will form on the inside of the cake, as in the case of a poor precoat.
The optimum dosage depends on:

- the nature of the particles to be filtered
- the quantity of particles to be filtered (charge)

The type and quantity of additive to be used must be chosen to ensure a slow, steady increase in pressure at the filter inlet. Experience makes these choices easier

Plateaux horizontaux chargés du gateau filtrant

Horizontal

plates

loaded with filter cake

We can obtain as a general rule :

Type of wines\|\|	Average alluvial doses
New wines

Early filtration on products with a high load and very poor filterability	150 to 170 g/hl
Raw wines

| | |

Unfined, racked	110 to 130 g/hl
Fined wines	80 to 100 g/hl\|\|
Pre-filtered wines + fining	60 to 80 g/hl\|\|

Filtration line

In this type of filtration, the filtering medium is the filter itself, in a continuous process.
The filtration cycle consists of 2 stages:

Constitution of

the precoat (sizing)

|On

the filtering support, a filtration layer is constituted with a filtration aid which can be kieselguhr (diatomaceous earth), perlite, or even various fibres (cellulose, polyethylene, etc.)

Alluvionage

mixture of wine to be filtered and filter aid is added throughout the filtration process. This superimposes a series of filter layers comprising the filter aid and the retained impurities

Residual

filtration

Horizontal plate filters are equipped with residual filtration devices, the aim of which is to clarify and evacuate all the wine contained in the filter at the end of the cycle.
There are 2 types of system

:
The external auxiliary filter
The plates integrated into the base of the filtering "pack"

Removing the filter cake at the end of filtration or after clogging.

Depending

on the type of filter, the cake is removed as follows

:
by cleaning with water for models with vertical plates
in paste form or not for models with horizontal plates

In the

latter case, 2 configurations are possible

:
automatic by moving the filter elements
manual by tilting the plates

Cleaning
Washing is carried out using water pressurised by the general supply pump. The appliance can be fitted with an internal washing ramp, which may or may not be mobile.

The

cleaning discharge is pasty.

Note: Periodic chemical cleaning is necessary to prevent clogging and scaling of the filter elements

Choosing the right equipment

Defining the most suitable filtering surface for a given production unit requires a whole series of technical, economic and practical parameters to be taken into account. To do this, a case-by-case study should be carried out with a professional.

Resolving malfunctions

Pressure ΔP increases too quickly

The additive is unsuitable

- Use a less fine diatomaceous earth
The alluvium is insufficient

- Increase the concentration of additive in the mixing tank
Increase the alluvium flow rate
The flow rate of the main pump is too high

Reduce feed pump flow

- Increase bypass volume

Pressure ΔP is rising too low

Admixture unsuitable

Use a finer diatomaceous earth
Alluviation is greater than required

- Reduce the concentration of adjuvant

- Reduce the flow rate of alluviation
The flow rate of the main pump is too low

- Increase the flow rate of the feed

pump Pressure ΔP rises sharply

Alluviation adjuvant is finer than that of the last precoat,
resulting in
clogging The concentration of solids increases (e.g. at the bottom of the vat)
The

wine load is too high

Reduce the impurity load by dilution: Some of the filtered wine is returned to the main pump suction via the bypass.
This is often necessary when filtering new wines.
Reduce the inlet flow rate to the bell.
Increase alluviation by accelerating the flow rate of the dosing pump.
Increase the concentration of adjuvant in the alluviation tank.
The

filter cake deteriorates

Activate the air vent frequently, especially in the case of young wines with a high CO2 content
Avoid pressure surges by handling the valves carefully.

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