Deactivation of spent mole sieve for unloading

Preparation of spent adsorbent for unloading Adsorbents exposed to a gas or liquid stream will adsorb various compounds from the process stream. Trace compounds can be concentrated on the adsorbent during its use. They may be toxic, flammable or explosive. Exposure of spent unregenerated or inadequately regenerated adsorbent to ambient air may generate heat and desorb toxic material creating a hazardous environment. Failure to remove flammable regeneration gases such as natural gas from the adsorbent following regeneration with an inert gas such as nitrogen can also result in a fire hazard. Adsorbed species remaining after inadequate regeneration and inert gas purging can be displaced by atmospheric moisture or liquid water resulting in release. If reactive species such as olefins or partially polymerized material are present on the adsorbent when exposed to air, combustion resulting in fire or release of toxic materials could occur. Therefore it is critical to remove or deactivate any potentially hazardous adsorbed materials or take special precautions prior to and during removal of spent adsorbent from the vessel. When the adsorbent is properly regenerated by a clean, nonreactive, weakly adsorbed gas such as natural gas, toxic or reactive materials will normally be removed. However, occasionally inadequate removal of hazardous materials can occur due to restricted heating gas and inert purge flow in portions of the bed due to such things as agglomerated or damaged adsorbent or a bed support failure. Therefore, when dealing with toxic materials such as hydrogen sulfide in an adsorbent bed special precautions need to be taken. Since less toxic compounds such as carbonyl sulfide (COS) and other sulfur compounds can react under certain conditions to form the more toxic hydrogen sulfide (H2S), care needs to be exercised with all sulfur compounds in adsorbent beds. Three methods can be used for preparation of a spent bed for unloading depending on the potential hazards present and availability of materials for the preparation process. If you encounter any difficulties or have questions about using the following procedures, contact your UOP representative. Procedure A – Gas regeneration followed by nitrogen purge For adsorption systems processing and regenerating with clean (free of toxic or reactive materials) and weakly adsorbed gases such as natural gas, regeneration followed by nitrogen purging is normally used. The advantages of using this technique are:
1. Avoids an extended effort to dry out the vessel internals and associated piping following use of liquid water using Procedure B.
2. Avoids potential damage to and degradation of the insulation properties of internal liners that would be exposed to liquid water using Procedure B. 3. May allow possible recovery and reuse of a portion of some used adsorbents. The disadvantages include: 1. May require a significant quantity of nitrogen to achieve satisfactory LEL especially where heating gas flow is restricted due to such things as damaged adsorbent, bed support failures or the presence of liquid hydrocarbons in the gas stream. 2. After regeneration, inert gas purging and removal from the vessel, any remaining flammable material on the adsorbent can be displaced by moisture picked up from the air or due to exposure to liquid water. Sources of ignition (flame or spark) must to be kept away from the removed adsorbent to prevent ignition of any flammable material displaced by moisture from the adsorbent. Procedure for regeneration with a clean, weakly adsorbed gas followed by Nitrogen purge. This procedure is not to be used for Adsorption systems processing or regenerating with streams containing toxic or reactive materials. For these situations, use Procedure B or C or the special unloading procedure discussed in Section V.
1. Regenerate the bed (heat & cool) with an appropriate gas. If the gas normally used to regenerate the bed is unavailable or contains toxic compounds use nitrogen or an alternate gas. Your UOP Adsorbent representative will help select an alternate gas and determine the proper operating conditions. Test the bed outlet heating gas for the presence of any unexpected toxic material such as Hydrogen Sulfide. 2. Completely cool with the clean gas used for heating, then isolate the vessel and depressure to atmospheric pressure. Install blind flanges on all inlet and outlet lines except the flare line. Connect a nitrogen purge source at the opposite end of the vessel. 3. The amount of available nitrogen is typically inadequate to provide the gas at velocities needed for effective flow distribution within the adsorbent bed. The repetitive pressuring and depressuring of the bed with nitrogen can improve the effectiveness of the purge step. It is best to pressure up and depressure down flow through the bed to avoid lifting the adsorbent or support balls. If downward flow is not possible for the purge step, contact your UOP representative for specific recommendations. 4. Continue to pressure and depressure the vessel to flare with nitrogen until the outlet mixture contains less than 10% of the lower explosive limit (LEL) for any flammables. The lower explosive limit in air of several common fluids is shown in Table 1.
 Table 1 Lower Explosive Limit* of Some Common Fluids
 Fluid Volume % in Air
Methane 5.0
Ethane 2.9
Propane 2.1
n-Butane 1.6
Hydrogen 4.0
Benzene 1.4
*Lowest concentration of vapor in air that is capable of propagating a flame through a mixture.

Procedure B – Water flood For adsorption systems processing streams containing toxic or reactive compounds, the water flooding technique is the most effective method for removing toxic materials and removing or deactivating reactive compounds. The advantages of the water flooding technique include: 1. Most effective removal of adsorbed compounds other than water. 2. Helps overcome problems of poor removal of potentially hazardous materials due to heating gas channeling and deactivate reactive polymer accumulations that otherwise might react or combust when exposed to air. 3. Helps deactivate adsorbent so that no temperature rise occurs due to water pickup from the atmospheric air or liquid water.
 Disadvantages include: 1. Requires an adequate source of safe and clean water that will not damage vessel internal surfaces. 2. Adsorbent will not be recoverable and will be more difficult to handle wet. 3. Internal liners may be damaged and are very difficult to dry out to recover insulation properties. 4. Used water needs to be disposed of in an environmentally acceptable manner. Procedure for regeneration followed by water flooding 1. Regenerate the bed (heat & cool) with an appropriate gas. If the gas normally used to regenerate the bed is unavailable or contains toxic compounds at hazardous levels, use nitrogen or an alternate gas. Your UOP Adsorbent representative will help select an alternate gas and determine the proper operating conditions. 2. Completely cool with the clean gas, then isolate the vessel and depressure to atmospheric pressure. Install blind flanges on all inlet and outlet lines except the flare line at the top of the vessel. Attach a water line to the bottom of the vessel. Be sure that the water used is from a safe source and compatible with the vessels materials of construction. 2. Deactivates the adsorbent so that no temperature rise occurs due to adsorption of water from atmospheric air. 3. When done properly (to avoid condensation), damage to the internal liner may be minimized and excessive time to dry out after dumping may not be required. Disadvantages for steam purging are: 1. Requires a sufficient quantity of steam availability to achieve good flow distribution across the bed. 2. Requires a sufficient quantity of inert cooling gas. 3. May not effectively remove toxic or flammable materials where resistance to flow exists due to problems such as adsorbent agglomeration, damage or a bed support problem. Procedure for steam purging 1. Heat the bed, but do not cool it to avoid steam condensation in the vessel. If the gas normally used to regenerate the bed is unavailable or contains toxic compounds at hazardous levels, use nitrogen or an alternate gas. Your UOP Adsorbent representative will help select an alternate gas and determine the proper operating conditions. 2. Isolate the vessel and depressure to atmospheric pressure. Install blind flanges on all inlet and outlet lines except the flare line at the bottom of the vessel. Connect the steam source to the top of the vessel.