8.3.1. Oliver Campbell vacuum filter is the most common filter in the sulphitation plants of India. Essential components of this filter are—
(i) bagacillo mixer (ii) mud trough (iii) rotary filter
(iv) filtrate receiving vessels and pumps (v) vacuum pump.
The mud to be filtered flows by gravity from the clarifier to a mixer fitted with stirrer and bagacillo from mill bagasse carrier is fed continuously for mixing with the mud. From the mixer mud mixed with bagacillo is continuously led to the mud trough provided with an agitator oscillating to and fro to prevent settling of mud in the trough.
The filter drum rotating around horizontal axis is always partially submerged in the mud of the trough. It is a horizontal cylinder with corrugated surface which is covered by 24 fine copper or stainless steel screens, each one forming a segment connected to vacuum system through tubes. The segments are equally spaced and each one occupies an area corresponding to 15° of the circumference, along the entire length of the drum. Small
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metal tubes connect the revolving segments individually to the stationary distribution valve, through which filtrate withdrawal pipes are drawn and led to filtrate connecting vessels under vacuum. The screens have 625 perforations per sq. inch i.e. 95/cm2 each with an opening of 0.5 mm. dia. The drum drive is provided by a constant speed motor through reduction gear and V. belt, with arrangement to vary speed from one revolution in 3 to 10 minutes. The vacuum generating system consists of either—
(i) barometric condenser coupled with air pump or
(ii) liquid ring vacuum pump most common being 'Nash' type.
The bagacillo supplied to the mud mixer is conveyed by a blower from underneath a screen fitted to the bagasse carrier. In some factories a vibrating screen is installed at the exit of the last mill, for recovering bagacillo. The screen can be 8 to 14 mesh depending on the bagasse moisture and fineness. But the bagacillo required for proper filtration varying from 6 to 10 kg. per tonne cone is to be separated and supplied to the filter station.
8.3.2. Operation—The mud tank receives the mixture of mud and bagacillo which acts as filter aid, and the drum as it starts revolving in the mud tank, under low vacuum (250-300 mm) sucks the mud through the lower sections which are immersed in the mud. This surface is thus coated with suspended mud and bagacillo. The juice thus drawn known as cloudy filtrate contains fine mud in suspension and is turbid in appearance. This portion of the drum with its mud coating, passes to higher vacuum zone viz. 300-450 mm. where the filtered liquor is less cloudy. The juice from this zone is known as 'clear filtrate' though it is not clear but less cloudy. This stage is followed by washing of the cake as the sections coated with cake move upwards and meet the atomised spray of hot water.
This cake washing is continued further as the cake travels further and receives water dripping from the sheet distributor. All this washing effectively desugarises the cake and the light filtrates are sucked by vacuum. After washing, the sections, with cake formed, pass further towards the scraper and enter the no-vacuum zone. The stationary scraper easily detaches the cake along the entire length of the drum. After the scraping the drum enters the mud trough again and the cycles of mud pick up, high vacuum sucking and washing go on continuously. The filter cake leaving the drum contains about 1.5 - 2%
sugar and 75-80% moisture. The quantity of filtercake is around 3.5% on cane and is much higher than the press cake from filterpresses on account of the presence of added bagacillo and high moisture. The thickness of cake under normal conditions is about 6-10 mm.
The filtrates are of lower purity than the clear juice by 1.5-2 units on account of dissolution of non-sugars during washing. The water should be of 70°-80°C and is used to the extent of 3-5% on cane, the normal practice being to use hot condensates.
Mud retention is the dry solid retained by filtercake per hundred dry insoluble solid contained in the mud fed to the filter. Mud retention varies from 50-75% under different operating conditions and high mud retention can be obtained by—
(i) good settling characteristic of mud—5-7% mud solids in feed,
(ii) correct proportion of fine bagacillo-mostly between 8 mesh and 20 mesh, (iii) good vacuum conditions.
(iv) lowest possible speed of filter drum 1 revolution in 7-10 minutes.
Hugot favours bagacillo of fibres of 3 cm. length and 1.0 mm in diameter.11 Too fine bagacillo containing pith or too coarse bagacillo with long and thick fibres is not conducive for obtaining good results with respect to mud retention and purities of filtrates. It would be good practice to fit a series of 6; 8 and 10 mesh screens on the mill bagasse elevator, the area depending on the bagasse fineness as well as actual requirement of bagacillo. Mud retention at Yeshwant factory was found to be 65% with Oliver filter, while handling sulphitation muds.12 Addition of flocculants to bagacillo mixer is recommended for improving mud retention and filter efficiency.
For sulphitation plants in India the filtering area of 0.5-0.7 m2/tch is considered adequate for normal operation.
8.4. Other filters—
8.41. Eimco continuous vacuum filter operates more or less on the same principle as the Oliver Campbell filter, but the design incorporates some special features such as—
(i) Hy-flow valves for filtrates, large ports provide streamlined passage for flow of filtrates from the drum. _
(ii) Polypropylene grid supporting fine stainless steel screens offers greater area for filtrates.
(iii) The drum submergence is adjustable from 10-30% in the trough.
(iv) Steam pipe is located under the filter cake scraper and the steam blown on the revolving screen prevents wax build up on screen perforations.
8.4.2. Eimco belt filter—This rotary vacuum filter has the drum covered with polypropylene cloth which passes around a roller where cake is discharged. The cloth is cleaned by jet spray of water before it returns to the trough but after the discharge of the cake the wash is collected in a trough underneath the wash roller, and sent back to process for use as imbibition water. The flocculant separan AP-30 is added to the mud bagacillo mixture prior to its entry into the mud trough. The cloth needs to be changed after 4-5 months operation. For maintaining cloth in good condition, the wash water applied has to be of 3kg/cm2and 82°C.
Conversion of ordinary Eimco filter to Eimco belt filter is reported to double the capacity.
8.43. Rapifloc system—The Rapifloc continuous rotary filter13 developed by Dorr Oliver permits filtration through highly porous cloth fixed over the existing perforated screens, using bagacillo as filter aid. The mud is limed to 7.5 pH and special flocculant like Separan AP 30 added in a special vessel to improve compactness and floc characteristics.
From this coagulator the mud flows by gravity into the filter mud trough. The thick cake is detached by scraper and cloth washed by special high pressure water jets under a
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special cover. Substantial increase in capacity with this system over the old established system is claimed.
8.5. Miscellaneous—
(a) The filtrates are usually returned to raw juice receiving tank which constitutes an extra load on the clarification system which is certainly undesirable, but is unavoidable since the filtrates cannot be sent to evaporator on account of the presence of insoluble impurities (1-2%). An ideal method would be to treat separately these filtrates by—
(i) chemical treatment followed by
(ii) heating to boilling and settling in separate subsider.
Tate and Lyle have developed system of separate clarification of dirty filtrates employing phosfloatation principle which is described separately.
(b) The filtrates from the mud filtrations, are collected in two vessels connected to pumps and the system upto the pump discharge is under vacuum. Any vacuum leak in this system has been found to result in entrainment i.e. to carry over of filtrates to the condenser. The filtrate pipes should preferably be of PVC or S.S. or copper and the height of filter from ground floor nearly 4-5 meters.
(c) During shut down or stoppages the filter must be thoroughly cleaned and surface steamed to clean and disinfect the filtrate passage pipes and drum surface.