From Production of Viscose to Fully Automatic Filtration in the Sugar Industry (full
Sometimes it’s worth looking beyond existing, established processes, visible at the example of Lenzing Technik GmbH, an internationally operating engineering company with headquarters in Salzkammergut, Austria.
As a subsidiary of the Lenzing AG – the market leader in the production of cellulosic fibers with wood as a raw material – the Lenzing Technik GmbH has been focusing on the value chain from cellulose to the finished viscose fiber. The company’s portfolio ranges from engineering services to development and manufacturing of processing equipment as well as turnkey facilities. Even whole viscose production plants have been engineered and built.
A small team of experts at the Solid-Liquid Separation Division of Lenzing Technik has been engaged in the filtration of viscose, a homogenous, but contaminated high-viscosity fluid with honey-like appearance. Until the first filtration systems with automatic sludge discharge were developed, viscose was filtered with filter presses. In a typical viscose plant up to 100 such presses were installed, frequently taking up several thousand square meters of filter area.
The filter presses were equipped with filter cloths made of cotton, needle felt or pulp sheets. Filtration was usually targeted at a fineness of 20µm. Similar to bag filters, which are still in use for smaller filtration tasks; the cloths were perfused by viscose. After their dirt holding capacity had been reached, they needed to be removed manually and replaced with new cloths.
The first automatic systems were introduced in the 1970s, but did not reach market penetration. Especially worth mentioning are horizontal disc filters, which use filter aid to build/precoat a cake on a package of horizontally placed, round filter plates. After finishing the filtration cycle, the disc package was put into rotation and thereby discharged the filter cake. Although still in operation in the beverage industry, this system could not establish itself widely for several reasons such as the high complexity and costs for apparatus and installation, the discontinuous filtration as well as filter aids consumption and its regeneration.
At about the same time, the first backwash filters (mainly using metal fiber fleece as filter media) were introduced to the market. Generally considered a suitable technology, the first systems failed due to several significant disadvantages. Some used extremely complex and expensive filter elements, which needed periodic thermic regeneration, as they were not efficiently back washable. Other unfavorable ones needed up to 20% of the filtrate flow to clean the filter from impurities (referred to as “reject”). Hence, further complex filtration facilities for processing of the reject were necessary.
The development of the “Kolben-Korb-Filter” (KKF) filter by Lenzing Technik GmbH allowed inexpensive, automated filtration of high amounts of viscose at a fineness of 20µm and minimum backwash quantities. The “Kolben-Korb Filter” has been a market leader in the global viscose industry ever since, and soon achieved success in other fiber-producing industries as well.
In viscose filtration several requirements need to be strictly met, which are also reflected in sugar processing.
If the production process of viscose as a fluid is interrupted for several hours, it hardens – and coagulation takes place. Devices and tanks that are filled with coagulated viscose need to be cleaned in a miner’s fashion, as there is no way to dissolve coagulated viscose.
A similar thing happens in sugar processing where unintended crystallization of sugar at least involves efforts for re-starting the production process. Due to the timely limited manufacturing cycles, additional production downtimes are especially critical. Furthermore, the continuous downstream process of evaporation, crystallization and separation should not be discontinued due to a still stand in the filtration step.
Lowest reject quantities
In sugar processing, an efficient filtration has positive effects on the downstream vaporization process as well as helping to improve product quality of refined sugar. Details on the reached filter fineness in thick juice and standard liquor are described in the course of this report.
It is not uncommon, that 100,000 backwashes and more can be executed, before the filter material needs to be changed for the first time.
The increased viscosity of thick juice and standard-liquor, which forced other systems to backwash with external media, did not represent a challenge for the engineers of Lenzing Technik. Quite contrary to this, the KKF filter concept even needed to be adapted to the lower viscosity of the sugar solution. The result of this adaption is meanwhile patented and established in a variety of different applications and processes under the name “Lenzing OptiFil®.”
Low operating costs
Mostly, the investment costs for the integration of processing plants are multiplied by the costs for installation (e.g., set-up, control system, piping, valves, installation area, etc.). In this respect, the OptiFil has the advantage of an easy system construction with only one automatic valve per filter. While the filter is able to manage relatively high flow rates, it does not require more installation area than a parking motor scooter.
The patented Lenzing OptiFil is a fully automatic, continuous system that works according to the principle of depth, surface or cake filtration, depending on the selected type of filter material. A metal or synthetic fiber fabric or fleece (see Figures 2 and 3) is used as filter media, retaining particles of different sizes either inside or on its surface.
After the pre-determined degree of contamination has been reached, the filter material is cleaned by backwashing a small quantity of filtered medium, with continuous filtration during backwashing.
In detail, the filter material of the Lenzing OptiFil is installed outside a perforated supporting structure (“perforated drum”). In case of cake filtration, a very thin filter cake (of typically 0.5 – 2mm) is formed inside the holes of the perforated drum during the filtration from the inside (Room P1) to the outside (Room P2). During the partial backwash from “Room P2” (Filtrate) to “Room P3” (Reject), the cake is completely discharged within a few seconds, using a small amount of filtrate to force it out of the filter. New cake formation already starts during backwash and is typically finished resulting in clear filtrate within less than 10 seconds.
Traditional filtration methods
Candle precoat filtration
At a specific flow rate of only 2,9 m³/m²*h, an enormous filter area was needed to handle the total flow rate. So the 8 required candle precoat filters (each taking up 125 m²) caused considerable investment costs as well as a huge space requirement.
cleaning by rotating nozzles
Disposable bag filtration
Despite the insufficient filtrate quality, the differential pressure built quickly, so that an average of 24 bags of the two used multi-bag filter housings had to be changed during every shift. Therefore, bag filtration is no real alternative for the filtration of thick juice, although the level of investment costs would be lower.
Costs for filtration
|For more information contact:|
|Stefan Strasser, Product Manager|
Lenzing Technik GmbH
Filtration and Separation Technology