Absorber Optimization

27.1 Define Complete Absorber Operation Including Ancillary Equipment

In a packed tower, the packed bed design and operation provides the basis upon which the desired mass transfer of the contaminant gas into the liquid can be achieved. The packed section (or "absorption" or "contact" section) is essentially of mechanical design (the surface of the absorbing or cooling liquid being mechanically increased, or "extended" as that liquid passes over the media) but that often exhibits a "chemical" function. As the gases are absorbed, for example, there may be an exotherm that could reduce the absorption of subsequent gases through the reduction in gas solubility. The absorbed gases may react with chemicals in the liquid thus producing reaction products that may reduce the rate of absorption. Those reaction products may also produce solids and/or scale that can mechanically affect the flow of the liquid over the packed bed media. As a result, various types of packing may be harnessed by the designer to maximize the mass transfer while minimizing solids build up and scaling. The options range from dumped type to structural.

A more subtle but no less important condition can occur wherein the pH and temperature of the liquid in the contact bed is not optimized.

For example, with the absorption of gaseous ammonia into an acidic solution, many packed towers are applied using the "conventional" arrangement of using a counterflow configuration with the recycle sump at the packed tower base. Sometimes, the recycle sump is even external to the absorption section. Make up acid under pH control and make up water under level control are often added to this sump. From the sump a pump recycles the liquid back to the top of the packed tower and the liquid is distributed within the tower using sprays or distribution troughs. The liquid passes through the absorption zone and drains back to the sump. Some of the liquid is typically bled away to remove the reaction products (in this example, salts). This mode of operation is often adequate however the procedure is too often not optimized.

FIGURE 27.1

Structured Packing (Brentwood Industries, Inc.)

Minimize Potential Problems

Some problems are the following using ammonia absorption as an example:

Unless the Gas Is Absorbed and Reacted, Minimize Recycle

The recycling of the liquid returns some of the products of reaction back to the absorption zone could inhibit the subsequent absorption of the ammonia. The intent is to keep individual stages focused on their specific function rather than needlessly recycling liquid back to the gas/ liquid contact area.

Dont Waste the Make Up Water

Since make-up water is usually added to the sump then a short time later is bled away, water can be wasted. Arrange the piping so the makeup water goes through the absorber at least once. This often involves adding the makeup water directly to the recycle header.

Control Temperatures

In an exothermic reaction, much of the cooling effect of the make-up water is lost since a portion of that water is bled away before the water can control the exotherm in the absorption zone. Conversely, if the reaction is endothermic, temperature control using heated infeed liquid may improve the absorption process.

Careful pH Control, If Used, Is Critical

The pH in the absorption zone is not optimized. Ammonia scrubbers using acid are often operated at a pH of 3 or less. Higher pH levels may result in the gas phase reaction of the ammonia with the acid causing the gas phase formation of particulate. That particulate may cause a visible emission. If the pH is measured in the scrubber sump, the pH is post reaction. The liquid surrounding the pH sensor contains both the unreacted acid, products of the reaction (salts), and other compounds that may affect the pH reading.

Make Chemical, If Used, Pass at Least Once Through

If the make-up acid, for example, is added to the sump and that sump is bled some of the unreacted acid will be bled needlessly (and perhaps expensively) away prior than passing through the absorption zone where the acid is useful.

Locate Sensors for Most Rapid Response

Given the typical high liquid volume of the sump (often sized to maintain the proper pump suction pressures), a pH change response time delay is inherent. A large volume of liquid surrounds the probe. The absorbed ammonia tends to raise the pH of the sump, but a large sump requires large amounts of ammonia to significantly alter the pH. If in addition the pH is controlled near pH 7 (often required to meet liquid discharge permit limits), the response parameters of the probe may be at or near the non-linear near vertical pH response curve range under which the system response creates reduced control. If make up acid is added to this large tank volume, the acid must be thoroughly mixed to obtain a representative and stable pH. These conditions conspire, though subtly, reduce the overall system response time if the ammonia infeed rate changes.

Adjustments to Consider

To optimize the absorption, certain adjustments can be made:

Use Dilute Chemical Make Up If Possible

To reduce the negative influence of the products of reaction, the liquid entering the liquid header(s) at the discharge of the absorption zone must be kept as dilute as practical. Adding the make-up water just prior to the liquid distribution header can achieve this.

To conserve water, the liquid circuit can be configured to only blow down reaction products. This can be achieved by configuring the recycle loop by making the water at least go through the absorption zone once. Adding the make-up water (as in "a" above) also conserves water.

If Exotherms Is Present, Add Cold Make Up Water to Header

As a bonus, adding the (typically colder) fresh water make up at the header rather than at the sump can also help remove some of the reaction exotherm. Adding the colder liquid at the infeed header rather than at the sump provides the desired heat transfer at a location (at the discharge of the cleaned gas(es) where that cooling can be most effective. Doing so can thereby stabilize the absorption.

Premix Additive Chemicals

To optimize the pH in the absorption zone, the make-up acid can be added and mixed using an in-line static mixer prior to injection into the scrubber liquid distribution stage and therefore into the absorption zone. Adding any make up chemical to the liquid header rather that into the sump makes that chemical go through the absorption stage at least once. If the products of reaction are then bled from the sump, the flow of those reaction products back to the absorption zone where those products may inhibit absorption is reduced.

Historically, premixing is accomplished by injecting the chemical into the pump suction. The pump acts as a mixer. Some of the downside effects are that the chemical is typically strong at that point and localized corrosion and/or erosion may occur. If the chemical reaction creates a gaseous reaction product, cavitation may also occur though it is difficult to detect. Plus, the uniform distribution of chemical is properly needed at the liquid distribution section of the packed tower (spray headers, troughs, etc.) and may not be uniform at the pump.

Alternate pre-mixing involves purposely creating changes of direction in the piping prior to the liquid distribution stage of the tower. Multiple 90-degree elbows, though adding to the head losses, are often effective. Inline static mixers such as shown in the following figures can also be effective and take up less space.

In these devices, fixed internal baffles cause the chemical and carrying liquid to mix though the liquid stream travels in essentially one direction.

FIGURE 27.2

Kenics Static Mixer photo.

A typical configuration is shown in Figure 27.2. The internal baffles that cause the mixing are shown in Figure 27.3. These devices can be used singly or in series depending upon the mixing difficulty.

Another mixing method to provide uniformity to the scrubbing liquid infeed stream is to use a mix tank equipped with an agitator. Sometimes these tanks are equipped with a dedicated pump that draws a liquid stream from the tank sump and returns it to the top of the tank on a continuous basis. A portion of that recycle is bled on demand to the scrubber liquid recycle circuit.

FIGURE 27.3

Kenics Photo showing internal baffles.

Choose Effective Probe/Sensor Locations

A pH probe mounted in the liquid inlet header (or in a service by-pass whichever is most convenient) can produce superior pH control at the top of the absorption zone. The signal from the pH probe is used to control the makeup acid amount.

The relatively low volume of liquid in the inlet header provides for a faster pH response than if the pH probe is mounted in the higher volume sump.

To reduce operational complexity, the existing device level control and blowdown hardware (often using a flow meter and control valve on the pump discharge) can be retained. The sump pH probe can be used to monitor and control the blowdown since a representative blowdown pH is often needed (or required by the operating permit) if that flow is to be sent to sewer or other discharge.

Seek Uniform Liquid Distribution

As mentioned in previous chapters, the liquid needs to be properly distributed across the absorption zone. Sometimes adjustments to the liquid header(s) or liquid distribution device are needed.

The added hardware for optimization is usually minimal. How the hardware is used is the key to effective optimization. The make-up acid and make up water are pre-mixed, and pH controlled. The make-up stream is then blended with the recycle from the sump. The liquid distribution stage is the same. The liquid still drains from the absorption zone into the sump or sump tank. The sump pH probe can still be used but its chore is to monitor the post reaction pH (same as the blow down pH).

An added effect is that the system will now more rapidly respond to any changes in the ammonia input rate. The infeed pH is measured in a much lower volume, flowing, premixed condition away from the less stable zone near pH 7.

If the above optimization is implemented, the gases moving through the absorption zone will be exposed to liquid that is the most dilute, of the optimum pH and is the coolest. Every volume of make-up water will have gone through the device at least once prior to discharge. Every weight of acid will have at least gone through the absorption zone once.