Ammonia & Syngas

Ammonia & Syngas

Synthesis gas (syngas) streams require molecular sieves, often downstream of other treatment units, to remove water and other contaminants to very low levels. This is required for protection against pressure drop and catalyst poisons that can cause plant performance to decline and catalyst deactivation. Contaminants can condense at lower temperatures and form solids in downstream equipment, leading to high pressure drop, loss of flow rate and ultimately lower plant efficiency. Thus, the molecular sieve ensures consistent, efficient and predictable performance over many years in between plant shutdowns and turnarounds.

The molecular sieve is typically being used to remove water, carbon dioxide, ammonia and sometimes oxygenates such as methanol, and various sulfur compounds. Synthesis gas is most often used to produce ammonia, methanol, higher hydrocarbons (synthetic fuels), hydrogen and dimethylether. In ammonia plants, the molecular sieve is primarily used to remove water in addition to trace ppmv-level carbon dioxide and ammonia from the syngas stream consisting mainly of nitrogen and hydrogen, with a small amount of methane.

Zeochem 4A and 13X molecular sieves are used in this service. Older-generation plants use 4A as it adequately removes water, carbon dioxide and ammonia. For older-generation plants seeking enhanced performance, newer-generation plants with reduced firing in the primary reforming furnace and cryogenic inert impurity removal, our 13X sieve is used.


Applications

Our Z4-01 molecular sieve is the standard grade of the 4A type zeolite that is used for dehydration of synthesis gas for the production of ammonia in plants currently using this product or in plants not utilizing cryogenic impurity removal. It effectively removes water, ammonia and carbon dioxide (CO2) from the feed stream. Z4-01 is normally regenerated at a temperature of 550 degrees Fahrenheit (288°C) with a typical range of 500-600°F (260-315°C).

The 13X molecular sieves provide enhanced capacity and performance for ammonia plant service and are the recommended products for all newer-generation plants. The 13X molecular sieve has significantly higher capacity for water, ammonia and CO2. These products have an open pore structure with 10 angstrom crystal pore openings, giving them optimum kinetics and dynamic performance ideally suited for ammonia plant service. Our 13X products normally regenerate at a temperature of 500-550°F (260-288°C) in this application, but the acceptable range is 450-600°F (232-315°C).

Custom Solutions

Zeochem employs technical service engineers and scientists who have years of experience in adsorber design, operation and maintenance.

From the beginning of each project, our technical service engineers can provide conceptual advice and design support. As the project moves forward, we can review the detailed designs and procedures. We offer consultation on last-minute change orders and start-up assistance when the unit is to be commissioned. Follow-up service is available for troubleshooting and performance optimization.

Related Products

Zeochem offers a broad range of molecular sieve products ideally suited to each natural gas, LNG and fractionation plant adsorbent application. From dehydration of natural gas to protect downstream cryogenic equipment to final sulfur and oxygenate removal from liquid hydrocarbon streams to meet product specifications, Zeochem products are specialized for reliable operation in a wide range of applications.

13X

Type 13X offers enhanced adsorption properties and the ability to remove impurities too large to be adsorbed by the type A zeolites.

4A

4A is the sodium form of the type A zeolite molecular sieve and is widely used as a general purpose drying agent. Under certain conditions, it can also be used for removal of ammonia, alcohols, carbon dioxide, H2S and other specific molecules.


Frequently Asked Questions

It is recommended to discuss with Zeochem theproposed changes so that a system review and design simulations can be run to determine if any operating changes are necessary or recommended. Zeochem can further check to make sure that all changes fall within recommended operating guidelines, and propose alternatives ifchanges may be problematic.

Beads are round and smooth, strong and durable, exhibiting low dusting characteristics and potential breakage. The spherical shape results in only compressive forces, while pellets (extrusions) undergo compression as well as tension, making breakage more likely. The ends of the pellets also have angled edges, making them subject to chipping and breakage. In addition, beads naturally dense load for optimum loading density without the use of dense phase loading equipment.

A liquid slug can slam into the bed at high velocity, moving, displacing, and even crushing sieve beads. The liquid coats the sieve, slowing mass transfer which leads to poor adsorption of water and other contaminants, and adds more load to the regeneration step. Also, the liquid can cause accelerated coking during heating. To minimize coking, it is recommended to ramp heat at 100°F/hr (55.5°C/hr) when there is time to do so and in some cases an additional cool purge step for 30-60 minutes prior to heating is also recommended to help remove and strip out liquids prior to heating.

In general, higher sieve adsorption capacity is favored by lower temperature and higher pressure. This also help to lower the feed water concentration for water saturated applications. There is a balance to be maintained though in order to avoid approaching the hydrocarbon dewpoint in vapor phase systems. It is recommended to maintain operation at 10-20°F (5.5-11°C) above the dewpoint in order to avoid potential two phase flow. Mixed phase is always to be avoided given that adsorption and working capacity are negatively affected and can be unpredictable when this occurs. As a result, operation should always be 100% vapor phase or 100% liquid phase. For regeneration, lower pressure is favored for minimized flow rate and better turbulence, and lower temperature is favored for optimum sieve life. There are practical limitations and the heating temperature typically falls within a given range depending upon the type of sieve being used and the application details. Temperatures that are too low are too inefficient and may not remove enough contaminant; temperatures that are too high will cause accelerated coking and can cause decomposition of stream components. Pressure typically cannot be too low due to excessive velocity in the up flow direction that will cause bead movement; pressures that are too high require additional flow or time, and higher risk of regeneration refluxing and laminar flow.

Ordinarily the largest temperature swing occurs when a freshly regenerated sieve bed is placed back online. Although the bed has been cooled, it is often several degrees above the inlet feed temperature. As a result, a temperature bump of 15-20°F(8.3-11°C) often occurs, and lastsfor approximately 15-30 minutes after feed has been reintroduced to the bed. In addition, the adsorption process is exothermic, giving off heat. Normally the amount of contaminant being adsorbed is small enough to generate an increase in the product stream temperature of only2-4°F (1.1-2.2°C). Should anupset occur where a water spike or slug of water hits the bed, a much more pronounced temperature rise can result.

When beds are adsorbing in parallel and there is more flow restriction in one vessel than the others, the flow will automatically balance between the beds to achieve an equivalent pressure drop. If it is minor, then it is ordinarily not an issue. If the flow imbalance is large enough, early breakthrough on the vessel with the high flow rate (lowest pressure drop restriction) can occur. In order to better balance the flow rate between the vessels, feeds must be adjusted. This is normally done by adjusting (partially closing) valves, either manual shutoff valves or the valve travel of automatic valves. It can be a trial and error process to achieve balance so it should occur as a series of small adjustments until the operation noticeably improves. In cases where this is not possible, adjustment of the cycles and/or conditions may be possible to prevent breakthrough. When all else fails, the inlet feed rate must be reduced until breakthrough no longer occurs.

Schedule the change out well ahead of time, preferably during an already scheduled shutdown or turn around. Order and have all needed products and supplies on site well ahead of time to avoid any delays. Make sure all contractors, plant personnel, and equipment will be ready to begin the morning of the scheduled start, with any necessary orientation, training, etc. completed in advance. Follow the Zeochem guidelines and recommendations for unloading and reloading the sieve to streamline the process and avoid delays. Have contingency plans in place should there be weather or unexpected delays that occur.

When possible, first regenerate the sieve beds to ensure dryness. Normally a short 70-80% heating cycle is sufficient for this purpose given the sieve will not be as wet as during normal operation. If initial regeneration is not possible or if the sieve is loaded under sufficiently dryconditions, theunit can be started up on regular feed at 50% flow rate whilesimultaneously startinga regeneration cycle of 1 of the beds. As soon as the regeneration is completed, switch beds and start regeneration of another bed. Once all the beds have been regenerated, ramp up the feed rate to full design rate and adjust the cycles times to the technical recommendations.

The water capacity is the percent by weight that the sieve adsorbs. At equilibrium, the adsorption is basically driven to completion to determine the absolute maximum amount of potential adsorption. This is most often used as a general baseline measure of the sieve’s quality and ability to adsorb water. Dynamic capacity is the working capacity that is expected from the sieve to avoid breakthrough of water in an actual process. The design simulation determines this capacity based upon the feed and regeneration stream compositions and conditions, and the water concentration, as well as the concentration of other contaminants. It involves not only the equilibrium capacity, but calculation of the mass transfer zone, effects of other contaminants, andaging of the sieve over time. All of these factors contribute to the difference between achievable water adsorption in service and the theoretical maximum water loading for the sieve.

Typically, there are some options that allow continued operation in the plant until a change out can occur. Adjustments to the cycle times, regeneration and feed conditions, and flowrates are sometimes available and can be made. The last thing considered is a reduction in the feed flow rate once all else has been done and further adjustment may be necessary. Zeochem can help with recommendations and a prioritized plan of action.