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What If the Containment Is Easily Stripped?

In some processes, such as odor control or the absorption of alcohols, the contaminant can easily strip from the absorption liquid. If the contaminant vapor pressure is not neutralized through chemical reaction and/or cooling, recycling that liquid to the absorption zone can cause the contaminant to strip from solution causing a reduction in the performance of the hardware.

For odor control, often pH control and oxidation are typically used to reduce that vapor pressure by converting the absorbed odor to an oxide or salt. The odor may be relatively soluble compounds such as hydrogen sulfide or perhaps lower solubility such as reduced sulfur species. The odor may be amines or other soluble odorous organics. For the absorption of alcohols, high bleed rates (water in then directly out) are sometimes used. For the latter, additional cooling and/or multiple stages are incorporated so that the partial vapor pressure of the liquid at the point of discharge (the device outlet) is kept at a minimum.

Possible Problems Given the Chemical Addition Odor Control Example

With odor control, for example, the emissions control can reveal various problems:

  • 1. The addition of chemical may cause an odor. Some oxidant chemical such as sodium hypochlorite is often received "stabilized" at an elevated pH. For use, the pH of the stabilized chemical is reduced prior to injection into the odor control device. Reducing the pH activates the oxidation potential however reducing the pH too fast or at the wrong location can free undesirable "bleach odor" and waste chemical.
  • 2. If an oxidant such as hydrogen peroxide is used, the peroxide may react slower than desired thus reducing the odor reduction of the hardware.
  • 3. If the chemical is not mixed efficiently, erratic odor reduction can result.
  • 4. The residual oxidation/reduction potential (ORP) of the post reaction residual (usually measured in the sump) may be insufficient for the application. Often an excess ORP is required to move the reaction in the desired direction rather than simple equilibrium.

Problems If the Absorbed Containment Easily Strips

With the control of alcohols and similar compounds that easily strip, some problems can be as follows:

  • 1. Excessive water consumption.
  • 2. High liquid discharge temperature that raises the partial pressure of the contaminant causing an unwanted release of the contaminant as vapor.
  • 3. Buildup of slime or scale in the absorption zone.
  • 4. Higher than optimal (low) liquid concentration at the point of cleaned gas discharge of the control device.

Optimizing Odor Control Applications

To optimize an odor control system, some of the following techniques may be considered:

  • 1. Premix and pH control the additive chemical (if that chemical can impart its own odor) prior to injection of the chemical into the recycle liquid stream. Much like the general recommendation (above) for an acid-duty packed tower, premixing and establishing better control of the chemical as that chemical enters the absorption zone can improve performance and reduce chemical costs. If adding the chemical to the sump can be avoided, consideration should be given to premixing to optimize the performance.
  • 2. To achieve the required odor reduction using a chemical such as hydrogen peroxide, adequate retention time of the oxidant in solution may be required. Peroxide suppliers publish data that indicates typical retention times to complete the oxidation. Some soluble odors can take many minutes for the oxidation to go to completion. As a result, often the device sump (whether internal to the device or remote) is inadequate for the current conditions. The device retention sump or tank may at one time have been adequate but given process changes no longer is. Some systems started with only a sump large enough to meet the hydraulic demands of the recycle pump (volume and NPSH). In that case, an additional sump may be required to achieve the required residence time. If a tank exists but is too small and additional tank (space permitting) may need to be "sistered" with the existing tank. For systems that already use an external retention sump, adding the peroxide to the device drain can sometimes be adequate. Another technique to increase the residence time in an existing tank is to add a center baffle ("mid-feather") that forces the oxidant to take a longer path than if the oxidant was simply added to the sump. Assuming proper liquid coverage in the absorption zone, sometimes decreasing the liquid recycle rate to the device serves to increase the in-sump residence time for oxidation.
  • 3. Improved mixing of the sump or external recycle tank can improve performance. Paddle mixers are possible candidates to provide the mixing. Another method involves "turning the tank over" by drawing liquid from the bottom of the sump or tank and returning some of that flow back to the tank. If the main recycle tank is of adequate capacity, a portion of that flow can be sent back. An additional small pump might be needed. Typically, these additional pumps only "turn over" about 10% of the sump volume.

4. Often an excessive amount of oxidant must remain in the sump to maintain the oxidation reaction. Though some of this oxidant will be wasted by way of the blowdown, the excess is required to drive the oxidation reactions. Thus, an ORP probe mounted in the sump or blowdown header is often used to provide the control signal. Based upon experience learned for that specific application or device, the ORP probe and controller provides the signal that administers the oxidant chemical. On hypochlorite type systems, the reaction is rapid and a more neutral ORP signal may be adequate. If that signal is not reliable, a specific ion electrode could possibly be a substitute. For peroxide-based systems, the ORP may be set at a higher potential. The goal, of course, to conserve chemical is to only set the ORP set point at a level required to achieve the desired odor reduction.

Optimizing Applications That Easily Strip

For applications that are designed to control contaminants that easily strip from solution, there are some techniques that may be applied:

  • 1. Excessive water consumption is common on systems that are configured for "water once through". The facility may have available to it copious amounts of water or "used to have" copious amounts of water. The simplest operating method was to send the water into the scrubber inlet header then directly out. With strippable type compounds, the liquid rate (concentration of contaminant in the liquid), the liquid temperature, and the location of application in the control device are the functioning items that define the optimization possibilities. If excessive water is being observed (or measured) then reduction in temperature of that liquid to reduce the vapor pressure, reducing the contaminant concentration and the location of application of the cleanest liquid is paramount.
  • 2. Sometimes reducing the infeed liquid temperature can help achieve optimization. Even if the operation is "water once through" that infeed water may be excessive. Removing some of the heat using heat exchangers on the gas stream, liquid stream or both can improve performance. Many areas are limited by the ambient wet bulb temperature (if cooling towers are used) for defining the temperature to which the liquid stream can be reduced. Chillers can be added if the cost justifies their use. If the gas stream source contains water vapor, sometimes the condensation of that water vapor to liquid water can serve to dilute the discharge stream thus reducing, at least to a small extent, the partial vapor pressure of the contaminant.
  • 3. Some of these strippable contaminant applications are "organic" related. They may include fermentation processes or baking processes that could evolve alcohols for example. These type sources can lead to slime or scaling buildup that can coat transfer media (such as packing) in the absorption zone of the device. The coating can reduce mass transfer thus reducing performance. A switch to larger free-pass type packed bed media run at a higher liquid to gas ratio may improve performance at modest additional cost. Packed tower media vendors may suggest larger dumped type packing or even structured type packing as an alternate.
  • 4. The interface conditions at the junction of the liquid and contaminant gas at the point of exit from the control device determines the ultimate gas outlet conditions. If strippable contaminants still exist at the gas outlet, some of those contaminants may still strip off. To reduce the tendency to strip, the most dilute, coolest, cleanest liquid must be applied at the point of gas discharge. Some devices applied to this type process use mesh pads for droplet control. The mesh pad inherently builds up liquid until that liquid accumulates enough to be heavy enough to drain. That means that some contaminants also build up in the mesh pad. As they concentrate, they can reach equilibrium with the gas stream thus setting a "floor" below which the contaminant concentration cannot be reduced. It is sometimes possible to change the mesh pad to one that retains less liquid (larger, more open mesh) so that less liquid is retained. In addition, that pad can be sometimes continually face sprayed with cooled, clean water. One must make certain however that the mesh pad does not flood so consultation with the mesh pad supplier is recommended. An alternate is to add a separate cross flow droplet separator using clean, cooled water applied to an inclined chevron type droplet eliminator. The droplet eliminator holds up a minimal amount of liquid therefore the resulting liquid film is of lowest contaminant concentration thus the vapor pressure is lowest. The inclination assists in the draining of the chevron blades thus minimizing the liquid hold up and keeping those surfaces cool and dilute. These type units have been successfully used on ethanol-type processes.
 
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