The main elements consist of upper and lower chambers that enclose a section of the continuous roller guided cloth. The upper chamber has a system of seals around the periphery and is held down by pressure cylinders with the cloth stationary during filtration and cake wash. Vacuum applied to the lower chamber draws the suspension into the chambers and the filtrate through the cloth. In the case of the pressure filter, pressure can be applied to the feed system and/or the upper chamber. When the filtration and dewatering/drying phases are complete the chamber is raised and the band driven to discharge the solids. All filtration elements can be contained within a sealed enclosure with all actuating devices located external to the enclosure. This is a band filter that can be applied to many areas previously reserved for other types of filter while retaining the inherent advantages of the simple band concept.
Based on rapid thin cake filtration techniques for high filtration rates and optimum efficiency, the ideal cake thickness is determined by the characteristics of the product being processed. Washing of the cake is by displacement methods and high levels of dryness can be achieved before discharge. Typically the cake thickness would be between 15 to 40 mm. A vacuum of about 0.7 bar is applied to the bottom chamber and a pressure of up to 3 bar applied to the upper chamber.
The complete filtration process is contained within a sealed enclosure generally designed for positive/negative pressures of 0.2 bar to accommodate pressure purging or under-pressure operation. Penetrations thorough the enclosure are kept to a minimum and are provided with double seals with a facility for purging, evacuating or sampling between the seals.
A simple and reliable method of chamber sealing is employed. By establishing the first deposits of solids onto the cloth with vacuum and thereafter maintaining a vacuum in the lower chamber, the joint/seal area is never subjected to internal pressure. This is regardless of the pressure applied at the surface of the suspension or cake due to the pressure gradient through the layer of solids.
A number of ‘cleaning in place’ (CIP) options are available including deluge systems, total flooding and built-in ultrasonic transducers, essential when changing over from highly-actives to less-actives. Major internal components are easily removed as a unit for immersion in an ultrasonic bath or hand jetting. Various bearing materials are employed depending on application and these can be provided with flushing facilities. The units are perfectly suited for multi-product or multi-purpose.
Internal surfaces are electro-polished as required to prevent adhesion of solid residues and to facilitate bright light inspection. Hand access to all internal parts can be provided for inspection, sampling and swabbing. Samples can be drawn from the bearing housing to ensure absence of contamination.
Proven continuous operation without supervision or intervention is provided by simple control logic based on timers and pressure/flow signals. Independent control can be by PLC or pneumatic logic, or easily interfaced with a central control system.
Batch times typically vary from 5 to 20 minutes which coupled with the ability to discharge solids from either end of the filter provides flexibility on the selection of final drying equipment. Throughput is determined by Filter Cloth Area x Cake Thickness x Number of Cycles.
The formation of the initial layer of cake on the filter cloth is extremely important as it this which really acts as the filter medium. The manner in which this inaugural coating is established also influences the blocking/bridging of the pores in the cloth that in turn dictates the efficiency of the whole filtration/washing/drying process. Due to the Miller Filter having a horizontal filtration area allowing the first phase of cake building to be by settlement or gravity filtration when desirable, combined with control of the pressure across the cloth/cake and the charge rates, the characteristics of the initial cake can be optimized. This is particularly important when compressible cakes or particles of mixed sizes and shapes are involved.
Cake washing is performed by displacement washing which is much more efficient and effective than spray washing or re-slurrying.
The collected solids can be dried to a high degree within the filter and as part of the normal process which can give significant savings in both the capital and operational cost of dedicated drying facilities.
If required the filters can be supplied so that the cake can be discharged at both ends in whatever sequence required simply by controlling the direction of travel of the cloth during cake discharge. Diverters or conveyors are unnecessary when feeding two driers and thus the filter and driers can be close coupled for direct transfer of solids.
FILTER MEDIUM REGENERATION
The cloth washing and cloth drying facilities maintain the medium in pristine condition and prevents blocking of the pores (healing) which preserves the efficiency of the cloth and significantly prolongs the life of the cloth. In consequence each portion of product, from aliquot to aliquot and over complete production campaigns, is subjected to identical conditions resulting in complete consistency and compliance with common quality assurance requirements.
Operation of the CIP system and purging of the enclosure ensure safe opening and access to the enclosure. All internal parts are earthed to prevent static sparking. The operation and process can be observed through viewing panels in the enclosure. Manual intervention is not required at any time during operation.
The process is precisely controlled by simple time, flow and pressure signals and can therefore be easily changed over to process different products.
Simplicity of design and careful selection of components result in minimum downtime. The cloth can be replaced in less than 30 minutes by hand from outside the enclosure. Major internal elements and the roof are removable as a unit for ease of maintenance. On the pressure filter the hydraulic system is isolated from all other maintenance.
The compact standard design is engineered to meet the most stringent regulations and specification. It is adaptable to a wide range of installation requirements and to accommodate a range of materials of construction from exotic alloys to plastics as may be dictated by the intended application or service. There are no moving parts within the chambers.
Economic use of utilities and power together with re-circulation of first runnings and wash liquors are allied to long cloth life and minimum maintenance.
FIELDS OF APPLICATION
The Miller Filtration Systems find application in the fields of fully automated liquid processing for the separation of solids from liquids where the most stringent specifications for the final products may be required.
A key application is in the chemical and pharmaceutical industries where valuable solids are recovered from slurries often containing acids and/or solvents. Multiple displacement cake washing in fully sealed systems, with or without inert gas blanketing, are a feature of these systems.
Fully automated sealed systems find application in the chemical, pharmaceutical, food and beverage industries for the recovery and clarification of valuable liquids from suspensions where pre-coating techniques are commonly employed for achieving the specified quality requirements.
In these types of operation the unique capabilities of Miller Filtration Systems allow the ‘rest’ of liquid product (mother liquor) to be washed out of the solids residue (filter cake) at the end of the filtration thereby recovering valuable product.
MATERIALS OF CONSTRUCTION
Completed plants constructed from standard grades of stainless steel are commonly applied for many applications though units with the wetted surfaces from more exotic alloys are readily catered for in more corrosive environments. The use of plastics such as polypropylene and various perfluoropolymers has also found successful application.