Hydrocarbon Compression

Archive for the ‘Acid Gas Treating’ tag

Amine Circulation Rates

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The circulation rates for amine systems can be determined from the acid gas flow rates by selecting a solution concentration and an acid gas loading.
The following equations can be used:

 Amine Circulation Rates

For design, the following solution strengths and loadings are recommended to provide an effective system without an excess of corrosion:

 Amine Circulation Rates

For the recommended concentrations the densities at 60°F are:
20% MEA= 8.41 Ib/gal = 0.028 mole MEA/gal
35 % DBA = 8.71 Ib/gal = 0.029 mole DEA/gal
Using these design limits, Equations 7-24 and 7-25 can be simplified to:

 Amine Circulation Rates

The circulation rate determined with these equations should be increased by 10-15% to supply an excess of amine.

Written by Jack

September 19th, 2009 at 10:30 pm

Iron Sponge Unit Design Procedures

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The iron-sponge process generally uses a single vessel to contain the hydrated ferric oxide wood shavings. A drawing of an iron-sponge unit showing typical provisions for internal and external design requirements was presented in Figure 7-3.

 Iron Sponge Unit Design Procedures

The inlet gas line should have taps for gas sampling, temperature measurement, pressure measurement, and for an injection nozzle for methanol, water, or inhibitors. The gas is carried into the top section of the vessel in a distributor and discharged upward. This causes the gas to reverse flow downward and provides for more uniform flow through the bed, minimizing the potential for channeling.

Supporting the hydrated ferric oxide chips is a combination of a perforated, heavy metal support plate and a coarse support packing material. This material may consist of scrap pipe thread protectors and 2-3-in. sections of small diameter pipe. This provides support for the bed, while offering some protection against detrimental pressure surges.

Gas exits the vessel at the bottom through the vessel sidewall. This arrangement minimizes entrainment of fines. Additionally, a cone strainer should be included in the exit line. This line should also have a pressure tap and sample test tap.

The vessel is generally constructed of carbon steel that has been heat treated. Control of metal hardness is required because of the potential of sulfide-stress cracking. The iron-sponge vessel is either internally coated or clad with stainless steel.

The superficial gas velocity (that is, gas flow rate divided by vessel cross-sectional area) through the iron-sponge bed is normally limited to a maximum of 10 ft/min at actual flow conditions to promote proper contact with the bed and to guard against excessive pressure drop. Thus, the vessel minimum diameter is given by:

 Iron Sponge Unit Design Procedures

A maximum rate of deposition of 15 grains of H2S/min/ft2 of bed cross-sectional area is also recommended to allow for the dissipation of the heat of reaction. This requirement also establishes a minimum required diameter, which is given by:

 Iron Sponge Unit Design Procedures

The larger of the diameters calculated by Equation 7-18 or 7-19 will set the minimum vessel diameter. Any choice of diameter equal to or larger than this diameter will be an acceptable choice.

At very low superficial gas velocities (less than 2 ft/min) channeling of the gas through the bed may occur. Thus, it is preferred to limit the vessel diameter to:

 Iron Sponge Unit Design Procedures

where dmax = maximum recommended vessel diameter, in.

A contact time of 60 seconds is considered a minimum in choosing a bed volume. A larger volume may be considered, as it will extend the bed life and thus extend the cycle time between bed change outs. Assuming a minimum contact time of 60 seconds, any combination of vessel diameter and bed height that satisfies the following is acceptable:

 Iron Sponge Unit Design Procedures

In selecting acceptable combination, the bed height should be at least 10 ft for H2S removal and 20 ft for mercaptan removal. This height will produce sufficient pressure drop to assure proper flow distribution over the entire cross-section. Thus, the correct vessel size will be one that has a bed height of at least 10 ft (20 ft if mercaptans must be removed) and a vessel diameter between dmin and dmax.

Iron sponge is normally sold in the U.S. by the bushel. The volume in bushels can be determined from the following equation once the bed dimensions of diameter and height are known:

 Iron Sponge Unit Design Procedures

where Bu = volume, bushels
The amount of iron oxide that is impregnated on the wood chips is normally specified in units of pounds of iron oxide (Fe2O3) per bushel. Common grades are 9, 15 or 20 Ib Fe2O3/bushel.

Bed life for the iron sponge between change outs is determined from:

 Iron Sponge Unit Design Procedures

The iron-sponge material is normally specified to have a size distribution with 0% retained on 16 mesh, 80% between 30 and 60 mesh, and 100% retained on 325 mesh. It is purchased with a moisture content of 20% by weight and buffering to meet a flood pH of 10. Because it is necessary to maintain a moist alkaline condition, provisions should be included in the design to add water and caustic.

Written by Jack

September 19th, 2009 at 9:36 am

Acid Gas Treating Process Selection

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Each of the previous treating processes has advantages relative to the others for certain applications; therefore, in selection of the appropriate process, the following facts should be considered:
1. The type of acid contaminants present in the gas stream.
2. The concentrations of each contaminant and degree of removal desired.
3. The volume of gas to be treated and temperature and pressure at which the gas is available.
4. The feasibility of recovering sulfur.
5. The desirability of selectively removing one or more of the contaminants without removing the others.
6. The presence and amount of heavy hydrocarbons and aromatics in the gas.

Figures 7-11 to 7-14 can be used as screening tools to make an initial selection of potential process choices. These graphs are not meant to supplant engineering judgment. New processes are continuously being developed. Modifications to existing proprietary products will change their range of applicability and relative cost. The graphs do enable a first choice of several potential candidates that could be investigated to determine which is the most economical for a given set of conditions.

To select a process, determine flow rate, temperature, pressure, concentration of the acid gases in the inlet gas, and allowed concentration of acid gases in the outlet stream. With this information, calculate the partial pressure of the acid gas components.

 Acid Gas Treating Process Selection

Next, determine if one of the four following situations is required and use the appropriate guide:
Removal of H2S with no CO2 present (Figure 7-11)
Removal of H2S and CO2 (Figure 7-12)
Removal of CO2 with no H2S present (Figure 7-13)
Selective removal of H2S with CO2 present (Figure 7-14)

 Acid Gas Treating Process Selection

 Acid Gas Treating Process Selection

 Acid Gas Treating Process Selection

 Acid Gas Treating Process Selection

Written by Jack

September 19th, 2009 at 8:43 am

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Gas Permeation

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Gas permeation is based on the mass transfer principles of gas diffusion through a permeable membrane. In its most basic form, a membrane separation system consists of a vessel divided by a single flat membrane into a high- and a low-pressure section. Feed entering the high-pressure side selectively loses the fast-permeating components to the low-pressure side. Flat plate designs are not used commercially, as they do not have enough surface area. In the hollow-fiber design, the separation modules contain anywhere from 10,000 to 100,000 capillaries, each less than 1 mm diameter, bound to a tube sheet surrounded by a metal shell. Feed gas is introduced into either the shell or the tube side. Where gas permeability rates of the components are close, or where high product purity is required, the membrane modules can be arranged in series or streams recycled.

The driving force for the separation is differential pressure. CO2 tends to diffuse quickly through membranes and thus can be removed from the bulk gas stream. The low pressure side of the membrane that is rich in CO2 is normally operated at 10 to 20% of the feed pressure.

It is difficult to remove H2S to pipeline quality with a membrane system. Membrane systems have effectively been used as a first step to remove the CO2 and most of the H2S. An iron sponge or other H2S treating process is then used to remove the remainder of the H2S.

Membranes will also remove some of the water vapor. Depending upon the stream properties, a membrane designed to treat CO2 to pipeline specifications may also reduce water vapor to less than 7 Ib/MMscf. Often, however, it is necessary to dehydrate the gas downstream of the membrane to attain final pipeline water vapor requirements.

Membranes are a relatively new technology. They are an attractive economic alternative for treating CO2 from small streams (up to 10 MMscfd). With time they may become common on even larger streams.

Written by Jack

September 19th, 2009 at 8:05 am

Distillation Process

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The Ryan-Holmes distillation process uses cryogenic distillation to remove acid gases from a gas stream. This process is applied to remove CO2 for LPG separation or where it is desired to produce CO2 at high pressure for reservoir injection.

Written by Jack

September 19th, 2009 at 8:03 am

Sulfide Scavengers

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Sour gas sweetening may also be carried out continuously in the flowline by continuous injection of H2S scavengers, such as amine-aldehyde condensates. Contact time between the scavenger and the sour gas is the most critical factor in the design of the scavenger treatment process. Contact times shorter than 30 sec can be accommodated with faster reacting and higher volatility formulations. The arnine-aldehyde condensates process is best suited for wet gas streams of 0.5-15 MMscfd containing less than 100 ppm H2S.

The advantages of amine-aldehyde condensates are water (or oil) soluble reaction products, lower operating temperatures, low corrosiveness to steel, and no reactivity with hydrocarbons.

Written by Jack

September 19th, 2009 at 8:03 am

Molecular Sieves for Acid Gas Treatment

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The molecular sieve process uses synthetically manufactured solid crystalline zeolite in a dry bed to remove gas impurities. The crystalline structure of the solids provides a very porous solid material with all the pores exactly the same size. Within the pores the crystal structure creates a large number of localized polar charges called active sites. Polar gas molecules, such as H2S and water, that enter the pores form weak ionic bonds at the active sites. Nonpolar molecules such as paraffin hydrocarbons will not bond to the active sites. Thus, molecular sieve units will “dehydrate” the gas (remove water vapor) as well as sweeten it.

Molecular sieves are available with a variety of pore sizes. A molecular sieve should be selected with a pore size that will admit H2S and water while preventing heavy hydrocarbons and aromatic compounds from entering the pores. However, carbon dioxide molecules are about the same size as H2S molecules and present problems. Even though the CO2 is non-polar and will not bond to the active sites, the CO2 will enter the pores. Small quantities of CO2 will become trapped in the pores. In this way small portions of CO2 are removed. More importantly, CO2 will obstruct the access of H2S and water to active sites and decrease the effectiveness of the pores. Beds must be sized to remove all water and to provide for interference from other molecules in order to remove all H2S.

The absorption process usually occurs at moderate pressure. Ionic bonds tend to achieve an optimum performance near 450 psig, but the process can be used for a wide range of pressures. The molecular sieve bed is regenerated by flowing hot sweet gas through the bed. Typical regeneration temperatures are in the range of 300-40()°F.

Molecular sieve beds do not suffer any chemical degradation and can be regenerated indefinitely. Care should be taken to minimize mechanical damage to the solid crystals as this may decrease the bed’s effectiveness. The main causes of mechanical damage are sudden pressure and/or temperature changes when switching from absorption to regeneration cycles.

Molecular sieves for acid gas treatment are generally limited to small gas streams operating at moderate pressures. Due to these operating limitations, molecular sieve units have seen limited use for gas sweetening operations. They are generally used for polishing applications following one of the other processes and for dehydration of sweet gas streams where very low water vapor concentrations are required.

Written by Jack

September 17th, 2009 at 3:16 pm

Zinc Oxide Reaction

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The zinc oxide process is similar to the iron sponge process. It uses a solid bed of granular zinc oxide to react with the H2S to form water and zinc sulfide:

 Zinc Oxide Reaction

The rate of reaction is controlled by the diffusion process, as the sulfide ion must first diffuse to the surface of the zinc oxide to react. High temperature (>250°F) increases the diffusion rate and is normally used to promote the reaction rate.

Zinc oxide is usually contained in long, thin beds to lessen the chances of channeling. Pressure drop through the beds is low. Bed life is a function of gas H2S content and can vary from 6 months to in excess of 10 years. The spent catalyst is discharged by gravity flow and contains up to 20 weight percent of sulfur.

The process has seen decreasing use due to increasing disposal problems with the spent catalyst, which is classified as a heavy metal salt.

Written by Jack

September 17th, 2009 at 3:08 pm