Method of Liquid Introduction

Some venturi scrubber designs use open pipes to deliver the scrubbing liquid, some use spray nozzles and some use a mixture of both. If the scrubbing liquid is essentially "clear" (say just a fraction of a percent solids) then quite often spray nozzles are reliable and plugging resistant. If the solids level is higher either in normal operation or in upset conditions, nozzles can plug. If chronic nozzle plugging occurs the various nozzle suppliers can be consulted to see if a more plugging resistant design can be applied. If the venturi water circuit can be isolated, various solids reduction techniques may be able to be applied to that circuit. Remedies such as increased settling capacity, liquid cyclones; polymer addition, etc. could be candidates for enhanced particle removal. If available and economical, sometimes merely increasing the blow down rate combined with adding any fresh liquid immediately in advance of the spray nozzles can help reduce plugging and serve to optimize that circuit. If the plugging is severe, it may be possible to replace a spray- nozzle based venturi design with one that uses open pipes. To do that either drawings or accurate field measurements are needed to allow the fabrication of a replacement. Often the existing recycle pump is more than adequate (particularly with respect to pressure) since an open pipe liquid circuit usually operates at only a fraction of a spray nozzle type. If the venturi design is already an open pipe type yet plugging occurs, a subtle cause may be solids build up in the feed header (or headers) used to administer the liquid. Some headers feed from the bottom that can lead to a poor distribution of solids. By feeding the liquid to above the injection point and requiring the liquid flow to be down into the scrubber tends to flush solids continually into the scrubber. Doing so often reduces local solids build up that could lead to plugging. If plugging is chronic (such as with processes that inherently must deal with high solids loadings), installing access ports and/or removable pipe sections may be the only way to optimize the liquid circuit. Some facilities use flexible hoses in the liquid recycle circuit to reduce plugging. The hoses are flexed periodically to dislodge built-up solids.

Effects of Wear

Wear is usually a result of a combination of erosion and corrosion in venturi type scrubbers. The wet environment inside the device coupled with the suspended solids that are often present conspire to cause wear particularly in any high gas velocity or direction change zones. Venturi scrubbers often operate at high gas velocities, so a tradeoff occurs between particulate removal efficiency and wear. Efficiency usually controls that debate however the wear becomes an operational "given". As part of the design wear plates (higher hardness metal, elastomeric linings, or refractory type material) can sometimes be applied. On a practical basis, however, usually maintenancescheduling measurements are taken, and repair panels or components are replaced as part of scheduled maintenance procedures if wear is chronic. To optimize rather than degrade the performance, however, the repairs should be made to duplicate the original interior surfaces as much as possible. Poor welding or abrupt interior surfaces will quickly wear perhaps faster than the original. Regarding corrosion, if chronic corrosion occurs then a change in material of construction (MOC) may ultimately be required. Test coupons could be installed in the existing scrubber to best determine the replacement MOC rather than guessing. Published corrosion resistance data may be helpful but not for the specific process at hand. Rarely is the corrosive environment exactly as described in corrosion resistance charts or literature. Sometimes the liquid is aerated or otherwise modified during contact in the scrubber that changes the overall corrosion characteristics. For example, an infeed neutralization chemical may eventually become a corrosive salt in the scrubber. If the products of reaction are not sufficiently bled away, local concentrations can exceed those described in the MOC corrosion resistance literature. For those reasons and others, wet scrubber vendors do not typically offer a guarantee against corrosion. To optimize the MOC to suit the application local testing with coupons or bench scale evaluation is recommended rather than a literature search.

Improving Gas Absorption

Venturi scrubbers provide modest gas absorption along with superior particulate removal. The modest absorption is a result of the concurrent motion of the liquid and gas coupled with the relatively short contact time. The gas stream in a venturi scrubber may be moving at over five to ten times the speed of the gas in a packed tower. In a packed or tray tower, the gas and liquid pass counterflow so that the cleanest gas "sees" the cleanest liquid. In a venturi scrubber, the gas and liquid share the same highway in the same direction. Unless the contaminant gas is reduced in partial pressure through chemical conversion to a low vapor pressure compound, the gas can leave the highway without being absorbed. Given those inherent limitations, however, gas absorption in venturi scrubbers can be optimized by controlling the recycle liquid pH and temperature much as described in the section on packed towers. In addition, sprays situated well in advance of the venturi scrubber can provide additional mass transfer surface for gas absorption. Fresh chemical added into that spray circuit rather than into the sump recycle circuit could add to the gas absorption capability of the venturi scrubber. If the scrubber uses a cyclonic type separator for droplet control, a spray header can sometimes be added at the inlet to the separator to provide additional liquid to gas surface area for mass transfer of the gas into the liquid. The spray droplet size is chosen to be the same or larger than the droplets that are expected to be produced by the venturi. If smaller droplets are formed by the spray(s) an additional droplet separation is placed on the cyclonic separator. Also, the drain in the separator needs to be checked to make certain that the increased liquid rate provided by the spray does not overwhelm the drain. If the drain capacity is marginal, the drain can be enlarged, or a supplemental drain can be added. The drain line size external to the scrubber must be adequately sized. Typically, an additional spray volume of 5%-10% of the initial recycle volume can be accommodated by an original drain size (given that drains are usually oversized). If the added spray is predicted to be greater, a thorough analysis of the drain piping should be entertained.

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