Importance of dehydration
- Gas gathering
: Water needs to be removed to reduce pipeline corrosion and eliminate line blockage caused by hydrate formation. The water dewpoint should be below the lowest pipeline temperature to prevent free water formation.
- Product dehydration
: Both gas and liquid products have speciﬁcations on water content. Most product speciﬁcations, except for propane, require that no free water be present . This requirement puts the maximum water in sales gas at 4 to 7 lb/MMscf (60 to110 mg/Sm3). For liquids, the water content is 10 to 20 ppmw.
- Hydrocarbon recovery.
: Most plants use cryogenic processes to recover the C2+ fraction from inlet gas. If acid gases are removed by use of amine processes, the exit gas leaves water saturated. To prevent hydrate formation in the cryogenic section of hydrocarbon recovery, the water concentration should be 0.1 ppmv or less.
Water content of hydrocarbons
- Determining the saturation water content of a gas (the dew point) is a standard but complex problem in thermodynamics.
- Reasonably good estimates of the concentration of water in the vapor phase in equilibrium with liquid water can be made at pressures below 500 psia (35 bar).
- If we make the good assumption of negligible hydrocarbon in the liquid water phase, we obtain
- Eq (6.2) provides reasonably good values for gas-phase water content, provided that the gas contains less than a few mol% of either CO2 or H2S.
- A more accurate way to determine the water content is to use charts.
- Example 1. Calculate the water content of the sweet natural gas shown in Table 6.1 at 300 psia (20.7 bar) and 80°F (26.7°C) by use of Equation 6.2 and Figure 6.1.
[table id=33 /]
- Water levels in natural gas can be reduced to the 10 pmmv range in a physical absorption process in which the gas is contacted with a liquid that preferentially absorbs the water vapor.
- The solvent used for the absorption should have the following properties:
- A high afﬁnity for water and a low afﬁnity for hydrocarbons
- A low volatility at the absorption temperature to reduce vaporization losses
- A low viscosity for ease of pumping and good contact between the gas and liquid phases
- A good thermal stability to prevent decomposition during regeneration
- A low potential for corrosions
- In practice, the glycols, ethylene glycol (EG), diethylene glycol (DEG), triethylene glycol (TEG), tetraethylene glycol (TREG) and propylene glycol are the most commonly used absorbents; triethylene glycol (TEG) is the glycol of choice in most instances.
- A typical, simpliﬁed ﬂow sheet for a glycol absorption unit
- The wet gas passes through an inlet scrubber to remove solids and free liquids, and then enters the bottom of the glycol contactor.
- Gas ﬂows upward in the contactor, while lean glycol solution (glycol with little or no water) ﬂows down over the trays. Rich glycol absorbs water and leaves at the bottom of the column while dry gas exits at the top.
- The rich glycol ﬂows through a heat exchanger at the top of the still where it is heated and provides the coolant for the still condenser.
- Then the warm solution goes to a ﬂash tank, where dissolved gas is removed. The rich glycol from the ﬂash tank is further heated by heat exchange with the still bottoms, and then becomes the feed to the still.
- The still produces water at the top and a lean glycol at the bottom, which goes to a surge tank before being returned to the contactor.
- If an equilibrium dew point of −15°F (−26°C) is required and the contactor operates at 80 °F (27°C), then a TEG solution of 99.5 wt% is required.
- However, the assumption is that the dry gas exiting is in equilibrium with the incoming lean glycol (i.e., inﬁnite number of contactor trays).
- To account for non-equilibrium concentrations, the Engineering Data Book (2004b) suggests use of an equilibrium temperature that is 10 to 20 °F (5 to 10°C) below the desired dew point temperature.
- Therefore, to obtain the −15°F (−26°C) exit dew-point temperature, the lean glycol concentration should be 99.8 to 99.9 wt%.
- Adsorption processes use solids with extremely high surface-to-volume ratios. Commercially used synthetic zeolites (i.e, molecular sieves) have surface-to-volume ratios in the range of 750 cm2/cm3, with most of the surface for adsorption inside of the adsorbent.
- In the case of molecular sieves, the adsorbent consists of extremely ﬁne zeolite particles held together by a binder. Therefore, adsorbing species travel through the macropores of the binder into the micropores of the zeolite.
- Adsorbents such as silica gel and alumina are formed in larger particles and require no binder.
- Pore openings that lead to the inside of commercial adsorbents are of molecular size; they normally range from approximately 4 Å (1 Å=10−8cm) to 100 Å. Molecular sieves have an extremely narrow pore distribution, whereas silica gel and alumina have wide distributions
- Two steps are involved in adsorbing a trace gas component. The ﬁrst step is to have the component contact the surface and the second step is to have it travel through the pathways inside the adsorbent.
- In commercial practice, adsorption is carried out in a vertical, ﬁxed bed of adsorbent, with the feed gas ﬂowing down through the bed.
- The process is not instantaneous, which leads to the formation of a mass transfer zone (MTZ) in the bed.
1. The equilibrium zone, where the adsorbate on the adsorbent is in equilibrium with the adsorbate in the inlet gas phase and no additional adsorption occurs
2. The mass transfer zone (MTZ), the volume where mass transfer and adsorption take place
3. The active zone, where no adsorption has yet taken place
- The MTZ is usually assumed to form quickly in the adsorption bed and to have a constant length as it moves through the bed, unless particle size or shape is changed.
- The value of yin is dictated by upstream processes; the yout value is determined by the regeneration gas adsorbate content.
- The length of the MTZ is usually 0.5 to 6 ft (0.2 to 1.8 m), and the gas is inthe zone for 0.5 to 2 seconds (Trent, 2004).
- To maximize bed capacity, the MTZ needs to be as small as possible because the zone nominally holds only 50% of the adsorbate held by a comparable length of adsorbent at equilibrium.
- In principle, beds can be run until the ﬁrst sign of breakthrough. This practice maximizes cycle time, which extends bed life because temperature cycling is a major source of bed degeneration, and minimizes regeneration costs. However most plants operate on a set time cycle to ensure no adsorbate breakthrough.
Process flow diagram
- One bed, adsorber #1 in Figure 6.8, dries gas while the other bed, adsorber #2, goes through a regeneration cycle.
- The wet feed goes through an inlet separator that will catch any entrained liquids before the gas enters the top of the active bed.
- Flow is top-down to avoid bed ﬂuidization. The dried gas then goes through a dust ﬁlter that will catch ﬁnes before the gas exits the unit.
- Regeneration involves heating the bed, removing the water, and cooling. For the ﬁrst two steps, the regeneration gas is heated to about 600 °F (315 °C) to both heat the bed and remove adsorbed water from the adsorbent.
- Regeneration gas enters at the bottom of the bed (countercurrent to ﬂow during adsorption) to ensure that the lower part of the bed is the driest and that any contaminants trapped in the upper section of the bed stay out of the lower section.
- The combination of feed rate, pressure drop, and adsorbent crush strength dictates the adsorption bed geometry.
- The bed diameter should be kept small. This feature also reduces the wall thickness of the high-pressure vessels and increases the superﬁcial velocity, which improves mass transfer in the gas phase.
- Example 2. An existing 4A molecular sieve bed has been processing 80MMscfd on a 12-hour cycle with two beds. Exit gas goes to a cryogenic turbo expander section. Gas ﬂow is increased to 100 Mscfd. Estimate the increased pressure drop and determine whether the bed capacity allows continued operation on a 12-hour cycle or the cycle time should be changed.
- The gas enters the bed at 120°F and 950 psig. Water content is 60% of saturation at120 °F. The molar mass of the gas is 18.5, with a viscosity of 0.014 cP and a compressibility factor of 0.84.
- The adsorption bed contains 41,000 lbs of 1/8-inch diameter beads with a bulk density of 44 lb/ft3. The inside wall diameter of the bed is 7.5 ft. The absorbent was installed 2 years ago.
Case study: ICHTHYS Project
- GEP integrity/sour service issues.
- Dew point vs weight/operability assessment.
- ICHTHYS GAS WATER SATURATED AT DEHYDRATION INLET SCRUBBER
- P=80 BARG & T=40°C (~40 kg H2O/MMSCF gas).
- For low dew points beyond -5°C need high concentrations of TEG or Adsorption units (MOL Sieve)
- Based on water-TEG equilibrium in contactor and regeneration process
- Equilibrium Achieved (TEG circulation rate std TEG=3gal/lb H2O, contact time, contactor surface area)
- Temperature (higher contactor temperature more concentration required)
- Pressure (pressure increase in reboiler will reduce lean MEG concentration, contactor pressure not significant 80 barg).
- Concentration (higher TEG concentrations lower dew point).
- pH (hygroscopic nature of TEG affect by pH-balancing with basic solution can produce salt precipitation).
Standard TEG unit not suitable at 50°C
TEG W/ STRIPPING GAS
- DEW POINTS BELOW -15°C
POSSIBLE W/ STRIPPING GAS
- REQUIRES MORE STRUCTURD PACKING WITHIN TEG REGEN SYSTEM
- REQUIRES MORE STRIPPING GAS
- Water adsorbed into chemical medium.
- Bed becomes saturated over time and requires regeneration.
- Bed saturation time 15-20 hours
- Regen. time typically 8-12 hours.
- Bed change out every 3 years.
- Superficial gas velocity and contact time requires large vessels.