Simdean
Envirotec Ltd 
Pollution solutions through Engineering Excellence
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Simdean
Envirotec Ltd Pollution solutions through Engineering Excellence
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Simdean Multivent Scrubber
For an exploded view of the scrubber and the elements involved in its manufacture follow the hyperlink Scrubber blow up For a dimensioned drawing of the range of scrubber sizes from 1000 m3/hr to 100,000 m3/hr gas volume flowrate follow the hyperlink Scrubber sizing The Multivent scrubber is a new gas liquid contactor comprising a matrix of elementary modules referred to as cell, in which gas and liquid circulate vertically under counter-current flow. For details of our pilot plant program follow the hyperlink [Pilot Unit] Liquid is introduced at the top of the cell, gas flows vertically from the bottom between the cell walls. In each cell, placed in series, gases are successively washed by the mixing of the two phases. A cell is formed by two rectangular sections, one convergent and one divergent, the intersection between the two forming the CELL NECK. Several cells placed in series form a FLOW CHANNEL. The cell is characterised by its height, width, length and the width and the shape of the cell neck.
Multivent Scrubber Hydrodynamics The hydrodynamic operation of the MULTIVENT SCRUBBER. can be divided into three flow ranges: The first being "dripping flow". This occurs when the gas and liquid flowrates are very low. No interaction occurs between the fluids. Each flows as if it were independent of the other . The liquid flows like a film over the surface of the cells. The second phase is known as "wave flow". This occurs when either the gas or liquid flowrate is increased to a particular value which causes a small increase in pressure drop through the cell. An unsteady state temporarily develops exhibiting wave flow phenomenon. The final phase is known as "hanging flow" and occurs as the flowrates increase and the waves of the previous flow regime meet at the cell neck. The phenomenon produces a sudden increase in pressure drop and liquid retention in the cell. Hanging flow occurs in all cells arranged in series. This is characterised by liquid accumulation at the top of a cell. The phenomenon occurs at virtually constant gas side pressure drop, independent of the gas or liquid flow rates in the cell. Operating Regime The operating regime of the Simdean Multivent scrubber is defined between the minimum flowrate required for hanging flow and the onset of flooding of the cells i.e. when the liquid flowrate is greater than the critical value. In practice, for a mass ratio of (liquid to gas flow) L/G = 1, the regime occurs between 80 and 120% of the nominal flow rate of the gas treated by MULTIVENT SCRUBBER. Pressure Drop Pressure drop in the operating regime is between 20 and 35mm WC per cell for a L/G ratio between 0.8 and 1.2 for the nominal flow rate of the gas treated by the Multivent scrubber. The pressure drop of one cell is additive, which means that for five cells placed in series the total pressure drop will be between 100 and 170 mm WG depending on the L/G ratio adopted. In practice units are normally fitted with either three, five or eight cells placed in series depending upon the application
Multivent Scrubber Efficiency Absorption efficiency The absorption efficiency of a cell depends on the scrubbing conditions and on the type of chemical reaction required. In the case of an irreversible and almost instantaneous chemical reaction like sulphur dioxide absorption in aqueous sodium hydroxide, the absorption efficiency of a cell in the scrubber operating regime is from 55 to 65%. For five cells placed in series, the efficiency can reach 99.5% Particulate removal In a survey carried out by Bayer in Germany on more than 25 scrubbers results showed that the scrubbing efficiency depends on the energy used. For a given energy consumption, the particulate removal has an efficiency limit, whatever the scrubbing system. MULTIVENT SCRUBBER is situated in the range of average energy consumption and its efficiency limit is situated around the micron. Experiments have been carried out with silica J + K type density 1.7, whose particle size distribution extends from 1.3 to 13.1 microns with a maximum of 5 microns. The efficiency obtained with five cells arranged in series in about 98.7% for a dust concentration of 1950 mg/Nm3 at the scrubber inlet. Figures quoted above are, in simple cases, quite definite. Problems met in practice are seldom so simple, and their physical and chemical parameters are not so definite. Notably, with regard to odours, the numbers of odoriferous substances is often important, particularly if they do not have similar chemical characteristics. Unit Sizing Volumetric capacity is obtained by placing the cells in PARALLEL Absorption or Removal efficiency is obtained by placing the cells in SERIES The drawing below shows the general arrangement of the scrubber together with a key for the major dimensions
The table below shows the dimensions of the scrubber based on the volumetric flowrate through the scrubber.
Construction The Multivent scrubber is constructed as a tower with a rectangular base. Inside the body are inserted SIMDEAN scrubbing elements forming the MULTIVENT UNIT and the demister with frame. The bottom of the scrubber is used as a collector for the scrubbing liquid, and as a storage for the recycle pump. The liquid is distributed by gravity. An equal quantity of scrubbing liquid is fed to each flow channel from a perforated pipe at low pressure. The different parts are:
For each application and part of the scrubber, the materials used for manufacture are carefully selected to avoid corrosion and wear, The factors determining the choice are:
The materials commonly used are:-
In general, the main body of the scrubbing unit which houses the transfer beds and liquor holding and distribution system would be fabricated from 4.5 mm thick uPVC extruded sheet material complying to BS3757-78 part II type CI. The whole of the fabrication would be spark tested on completion using equipment giving a proof range of 20 to 25 kVA. All flange areas would be at least 9 mm thick uPVC and large panel areas would be ribbed as is usual. The whole external area of the housing would be GRP reinforced as the design calculations call for. The base of the unit which forms a liquor storage area would also have suitable body bands encapsulated into the GRP laminations. The Multivent bed unit located in the housing would be a three, five or eight neck in series unit assembled from an extruded chemically resistant uPVC rigid material. Special frames would secure the bed panels to provide a volumetric capacity sufficient for the duty. Above the bed module would be positioned the liquid reagent distribution pipework complete with entry channels for liquor direction towards the first neck in series. A manifold pipe would exit the housing via a double puddle flange so that the distribution system is removable for periodic inspection. It should be noted that due to the unique system of liquor introduction and the bed design no maintenance would be required on the bed unit at anytime as it has completely non blocking characteristics and hence its high absorption efficiency. Distribution of the liquor from the holding sump to the bed modules is achieved using a centrifugal pump set constructed from suitable corrosion resistant materials, pipework for distribution is manufactured from uPVC. The system is provided with all necessary control valves and fittings for normal operation, dumping, overflow and draining of the liquor sump. Above the bed module the scrubber is furnished with a mist eliminator to prevent any carryover of scrubbing liquor. The eliminator used is a Munters Ltd type DV270 droplet separator constructed from polypropylene.
Control and Logic of Operation In general the Multivent scrubber is supplied with a standard control panel comprising the following:- 1 - off sheet steel, wall mounted enclosure protected to IP55 or for external use IP65 The enclosure will house the following controls:
Control Panel Logic The control panel is fitted with a PLC for control of the correct starting and shut down procedures for the scrubber. In addition the PLC supervises the alarm function of the scrubber and ensures that on systems fitted with duplex circulating pumps, if one of the circulation pumps "trips" then the stand-by pump is brought into operation automatically. The standard control logic for the Multivent Scrubber is described below:- As the system start button is pressed the circulation pump is started, an alarm relay on the flow monitor is made. The alarm relay is set to prove minimum scrubber liquor flow through the scrubber. A timer is then energized which gives a pre-set delay on start up for the system fan. After the delay period the system fan starts automatically. This ensures that the scrubbing column is supplied with sufficient scrubbing liquor to ensure correct operation. Shut down of the system is the opposite of start up with the system fan being shut down first and then after a suitable delay the circulation pump being shut down. The delay in circulation pump shut down ensures that no contaminated exhaust is allowed to pass through the scrubber without being treated by the scrubbing liquor. A number of safety systems are built into the panel logic. If the alarm relay on the flow monitor is at any time not satisfied (apart from at start up), the system fan and circulation pump will shut down (to protect pump motor). If two circulation pumps trip when running then the system fan will automatically shut down. If the circulation pump should fail for any other reason, the system fan is shut down. Should you wish to add any further alarm/safety circuits into the operation of the control panel these can be accommodated through reprogramming of the PLC. We would ask that any alterations to the standard logic are brought to our attention at the earliest possible time. Due to the physics of the scrubbing process it is necessary to periodically top up the scrubbing liquor to account for evaporation losses. The scrubber sump is fitted with a level control system comprising a panel mounted level relay which is externally wired to a conductivity type sensor with two low level probes. Logic of control as follows:-
Exhaust Stack The exhaust stack will be fabricated from GRP clad PVC sheet and will be designed in accordance with the recommendations made in Secretary of State’s Guidance Note PG2/4 of the Environmental Protection Act 1990 i.e. the main body of the stack will be designed to provide a gas velocity of less than 9 m/s and the final outlet velocity will be in excess of 15 m/s. The stack will discharge vertically upwards and will not be fitted with any restriction at the final opening. Pilot Unit Testing For finding a solution to a particular pollution problem a pilot plant trial provides a simple, but positive, method of determining the treatment required. For more details follow the hyperlink [Pilot Unit]
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keithwhale@simdean-environmental.co.uk with questions or comments about
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