Introduction

The demand for hydrocarbon liquids treating facilities has increased in recent years as more liquefied petroleum gas (LPG) products are being produced. The most common contaminants in LPG products are hydrogen sulfide (H₂S), mercaptans (RSH), carbon dioxide (CO₂), carbonyl sulfide (COS) and carbon disulfide (CS₂) where each of these contaminants cause problems in the finished products. These contaminants not only lead to odor problems but also form sulfur oxides on combustion. They can cause corrosion of equipment unless the liquid is adequately dehydrated. The presence of significant quantities of CO₂ can increase vapor pressure and lower the heating value of the hydrocarbon liquids. COS and CS₂, although not corrosive in LPG, will hydrolyze slowly to H₂S, resulting in off-spec product.

Most gas processing plant operators treat LPG to meet the typical specification as: H₂S ≤ 1 ppmw, mercaptans ≤ 5-10 ppmw, COS ≤ 1-2 ppmw, total sulfur ≤ 30 ppmw, and water ≤ 10 ppmw.

Both the quantity of each contaminant present in the untreated hydrocarbon liquid stream and the product specifications determine the treating technology selection. Molecular sieve technology is commonly used for treating natural gas liquids (NGLs), liquefied petroleum gas (LPG), and condensate (C5+) fractions to extremely low contaminant levels.

The molecular sieve process can be attractive when the amount of sulfur contaminants is low in a feed stream – usually 500 ppmwt for H₂S, less than 100 ppmwt for mercaptans, and less than 50 ppmwt for COS (Mokhatab, 2018a). In this case, molecular sieves can remove all the sulfur components to commercial / export grade LPG. 

Molecular sieves can dry the feed stream while simultaneously removing sulfur contaminants. Their drawbacks are high capital and operating costs. The formation of COS is another concern if both H₂S and CO₂ are present. COS formation has to be minimized, as it will be converted to H₂S in the presence of water, resulting in an off-spec product and corrosion. Molecular sieve manufacturers can supply customized sieves to minimize COS formation.

Type of Molecular Sieves for Sulfur Removal

Molecular sieves 5A and 13X are commonly used for desulfurization (5A for light sulfur while 13X for heavy and branched sulfur species). Type 13X has larger pore openings and therefore has better kinetics than type 5A; however, coadsorption of BTX (benzene, toluene, and xylene) components, which can block pores and deactivate the molecular sieve, may hinder heavy mercaptans removal. It is sometimes better to use a compound bed with successive layers of molecular sieves for adsorbing the different impurities. This combination increases the useful capacity of the bed, which is typically the case when water, H₂S, mercaptans, and other sulfur species need to be removed at the same time.

Regeneration Method and Steps

Temperature swing adsorption (TSA) is used to regenerate a molecular sieve bed in both natural gas and hydrocarbon liquids’ drying and treating applications to meet stringent product specifications. In this method, the change in the adsorption equilibrium is obtained by increasing the temperature.

To regenerate the molecular sieve in liquid-phase applications such as LPG treating, below steps are generally involved (Jain, 2018):

• Depressurization
• Draining
• Purging
• Heating
• Cooling
• Repressurization and Standby

Note, typical cycle time for an LPG treating unit consisting 3 molecular sieve vessels is 24 hr including 8 hr of adsorption, 8 hr of standby and 8 hr of regeneration (draining, heating and cooling).

Design Considerations of Regeneration Phase

The major parameters involved in the design of liquid-phase regeneration cycle as well as the standard recommendations for several of these parameters will be briefly presented here.

For molecular sieves type 13X, the regeneration temperature is normally around 300 ºC.

The heating temperature is slowly increased through the application of a heating ramp-up step. The heating ramp-up is typically 5°C/min–10°C/min (Jain, 2018). The heating time should be a minimum of 2 hr. Below this value, the regeneration is often incomplete. Insufficient or incomplete regeneration of the adsorbent beds will lead to a sudden loss in adsorption capacity and premature breakthrough. Insufficient adsorbent bed regeneration is a result of one or all of the following factors (Mokhatab et al., 2018b):

• Low regeneration gas flow rate/temperature
• Insufficient regeneration time, and
• Change of regeneration gas composition

To fully regenerate the adsorbents, the inlet and outlet temperatures of the adsorber in the regeneration step should be monitored. In order to make sure the adsorbents are properly regenerated, three points have to be checked (Mokhatab et al., 2018b):

1. The inlet temperature should reach the temperature required to adequately regenerate the bed (depending on the desiccant type).
2. The outlet temperature should show an almost constant value (during the heating step) for 30 – 120 min, depending on the vessel size and cycle time (see below figure). This is necessary to make sure that the adsorbents near the vessel walls are fully regenerated.
3. The temperature difference between inlet and outlet streams at the end of the heating cycle should not be more than 50 – 60 ºF depending on the vessel insulation (see below figure).

The depressurization/pressurization rate should be 1 bar/min – 3 bar/min. In liquid phase applications, typical time allocated for draining and refilling steps is 1 hr – 2 hr. These steps are followed by a cold purge step, usually for around 1 hr (Jain, 2018).

For typical liquid-phase treating with molecular sieves, the adsorption flow tends to be upwards, and the regeneration flow downwards. Molecular sieve regeneration is similar to that for gas-phase application. However, the beds must be designed to ensure that the maximum velocities are not exceeded under the extreme flow conditions, to avoid bed lifting and attrition of molecular sieves. The fluid velocity should be in the acceptable range to prevent any channeling or bed lifting (Mokhatab et al., 2018b). For liquid streams, the turbulent flow plus maximum velocity should be approximately 0.6 m/min – 0.8 m/min for good contact between the liquid and molecular sieves. To optimize pressure drop and regeneration gas flowrate requirements, a good design ratio of height to diameter of 1.5 – 5 is recommended for liquid phase applications (Jain, 2018).

References

Jain, S., “Proper Regeneration of Molecular Sieves in TSA Processes – Parts 1 and 2”, Gas Processing & LNG (2018).

Mokhatab, S., Northrop, S., Echt, W.I., “Dealing with Sour Natural Gas Liquids in Gas Processing Plants”, paper presented at the 68^th Annual Laurance Reid Gas Conditioning Conference, Norman, OK, USA (February 25-28, 2018a).

Mokhatab, S., Poe, W.A., and Mak, J.V., “Handbook of Natural Gas Transmission and Processing”, 4^th Edition, Elsevier, USA (2018b).

Author: Mr. Saeid Mokhatab, Technical Consultor