ARNOLD, K. (1999). Design of Gas-Handling Systems and Facilities (2nd ed.) Episode 1 Part 9 pdf

DEA systems do not require this scrubber because the vaporpressure of DEA is very low.Amine Circulation Rates The circulation rates for amine systems can be determined from theacid gas f

Alternately, a separate scrubber vessel can be provided so that the towerheight can be decreased This vessel should be designed in accordancewith the procedures in Volume 1 for design of two-phase separators,For ME A systems with a large gas flow rate, a scrubber should be con-sidered for the outlet sweet gas The vapor pressure of MEA is such thatthe separator may be helpful in reducing MEA losses in the overheadsweet gas DEA systems do not require this scrubber because the vaporpressure of DEA is very low.

Amine Circulation Rates

The circulation rates for amine systems can be determined from theacid gas flow rates by selecting a solution concentration and an acid gasloading

The following equations can be used:

where LMEA = MEA circulation rate, gpm

LDEA — DEA circulation rate, gpm

Qg = gas flow rate, MMscfd

ME = total acid-gas fraction in inlet gas, moles acid gas/moleinlet gas

c = amine weight fraction, Ib amine/lb solution

p = solution density, Ib/gal at 60°F

AL = acid-gas loading, mole acid-gas/mole amine

For design, the following solution strengths and loadings are mended to provide an effective system without an excess of corrosion;MEA: c = 20 wt %

recom-AL = 0.33 mole acid gas/mole MEA

DEA: c = 35 wt %

AL = 0.5 mole acid gas/mole DEA

Acid Gas Treating 187

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:

The circulation rate determined with these equations should be increased

by 10-15% to supply an excess of amine

Flash Drum

The rich arnine solution from the absorber is flashed to a separator toremove any hydrocarbons A small percentage of acid gases will alsoflash when the pressure is reduced The dissolved hydrocarbons shouldflash to the vapor phase and be removed However, a small amount ofhydrocarbon liquid may begin to collect in this separator Therefore, aprovision should be made to remove these liquid hydrocarbons

Typically the flash tanks are designed for 2 to 3 minutes of retentiontime for the amine solution while operating half full

Amine Reboiler

The reboiler provides the heat input to an amine stripper, which

revers-es the chemical reactions and drivrevers-es off the acid gasrevers-es Amine reboilersmay be either a kettle reboiler (see Chapter 3) or an indirect fired heater(see Chapter5)

The heat duty of amine reboilers varies with the system design Thehigher the reboiler duty, the higher the overhead condenser duty, thehigher the reflux ratio, and thus the lower the number of trays required.The lower the reboiler duty, the lower the reflux ratio will be and themore trays the tower must have

Typically for a stripper with 20 trays, the reboiler duties will be asfollows:

ME A system—1,000 to 1,200 Btu/gal lean solution

DBA system—900 to 1,000 Btu/gal lean solution

For design, reboiler temperatures in a stripper operating at 10 psig can beassumed to be 245°F for 20% MEA and 250°F for 35% DBA.

Amirie Stripper

Amine strippers use heat and steam to reverse the chemical reactionswith CO2 and H2S, The steam acts as a stripping gas to remove the CO2and H2S from the liquid solution and to carry these gases to the overhead

To promote mixing of the solution and the steam, the stripper is a trayed

or packed tower with packing normally used for small diameter columns.The typical stripper consists of a tower operating at 10-20 psig with 20trays, a reboiler, and an overhead condenser The rich amine feed is intro-duced on the third or fourth tray from the top The lean amine is removed

at the bottom of the stripper and acid gases are removed from the top,Liquid flow rates are greatest near the bottom tray of the tower wherethe liquid from the bottom tray must provide the lean-amine flow ratefrom the tower plus enough water to provide the steam generated by thereboiler The lean-amine circulation rate is known, and from the reboilerduty, pressure, and temperature, the amount of steam generated and thusthe amount of water can be calculated

The vapor flow rate within the tower must be studied at both ends ofthe stripper The higher of these vapor rates should be used to size thetower for vapor At the bottom of the tower the vapor rate equals theamount of steam generated in the reboiler Near the top of the tower thevapor rate equals the steam rate overhead plus the acid-gas rate Thesteam overhead can be calculated from the steam generated in the reboil-

er by subtracting the amount of steam condensed by raising the leanamine from its inlet temperature to the reboiler temperature and theamount of steam condensed by vaporizing the acid gases

For most field gas units it is not necessary to specify a stripper size.Vendors have standard design amine circulation packages for a givenamine circulation rate, acid-gas loading, and reboiler These concepts can

be used in a preliminary check of the vendor's design However, fordetailed design and specification of large units, a process simulationcomputer model should be used

Overhead Condenser and Reflux Accumulator

Amine-stripper overhead condensers are typically air-cooled, fin-fanexchangers Their duty can be determined from the concepts in Chapter 3

as required to cool the overhead gases and condense the overhead steam

Acid Gas Treating 189

to water The inlet temperature to the cooler can be found using the tial pressure of the overhead steam to determine the temperature fromsteam tables The cooler outlet temperature is typically 130 to 145°Fdepending on the ambient temperature

par-The reflux accumulator is a separator used to separate the acid gasesfrom the condensed water The water is accumulated and pumped back tothe top of the stripper as reflux With the vapor and liquid rates known,the accumulator can be sized using the procedures in Volume 1 for two-phase separators

Rich/lean Amine Exchanger

Rich/lean amine exchangers are usually shell-and-tube exchangerswith the corrosive rich amine flowing through the tubes The purpose ofthese exchangers is to reduce the reboiler duty by recovering some of thesensible heat from the lean amine

The flow rates and inlet temperatures are typically known Therefore,the outlet temperatures and duty can be determined by assuming anapproach temperature for one outlet The closer the approach temperatureselected, the greater the duty and heat recovered, but the larger and morecostly the exchanger For design, an approach temperature of about 30°Fprovides an economic design balancing the cost of the rich/lean exchang-

er and the reboiler The reboiler duties recommended above assume a30°F approach

Amine Cooler

The amine cooler is typically an air-cooled, fin-fan cooler, which ers the lean amine temperature before it enters the absorber The leanamine entering the absorber should be approximately 10°F warmer thanthe sour gas entering the absorber Lower amine temperatures may causethe gas to cool in the absorber and thus condense hydrocarbon liquids.Higher temperatures would increase the amine vapor pressure and thusincrease amine losses to the gas The duty for the cooler can be calculat-

low-ed from the lean-amine flow rate, the lean-amine temperature leaving therich/lean exchanger and the sour-gas inlet temperature

Amine Solution Purification

Due to side reactions and/or degradation, a variety of contaminantswill begin to accumulate in an amine system The method of removingthese depends on the amine involved

When MEA is used in the presence of COS and CS2, they react to form

heat-stable salts Therefore, MEA systems usually include a reclaimer,The reclaimer is a kettle-type reboiler operating on a small side stream oflean solution The temperature in the reclaimer is maintained such that thewater and MEA boil to the overhead and are piped back to the stripper,The heat-stable salts remain in the reclaimer until the reclaimer is full,Then the reclaimer is shut-in and dumped to a waste disposal Thus, theimpurities are removed but the MEA bonded to the salts is also lost.For DEA systems a reclaimer is not required because the reactionswith COS and CS2 are reversed in the stripper The small amount ofdegradation products from COo can be removed by a carbon filter on aside stream of lean solution

Materials of Construction

Amine systems are extremely corrosive due to the acid-gas tions and the high temperatures It is important that all carbon steelexposed to the amine be stress-relieved after the completion of welding

concentra-on the particular piece A system fabricated from stress-relieved carbconcentra-onsteel for DEA solutions, as recommended, will not suffer excessive cor-rosion For MEA systems, corrosion-resistant metals (304 SS) should beused in the following areas:

1 Absorber trays or packing

2 Stripper trays or packing

3 Rich/Lean amine exchanger tubes

4 Any part of the reboiler tube bundle that may be exposed to thevapor phase

5 Reclaimer tube bundle

6 Pressure-reduction valve and pipe leading to the flash tank

7 Pipe from the rich/lean exchange to the stripper inlet

EXAMPLE PROBLEMS

Example No 7-1: Iron-Sponge Unit

Acid Gas Treating 191

Problem: Design an Iron-Sponge Unit

Solution:

I Minimum diameter for gas velocity:

2 Minimum diameter for deposition:

3 Minimum diameter to prevent channeling:

Therefore, any diameter from 16.8 in to 37.6 in is acceptable

4 Choose a cycle time for one month:

Assume Fe = 9 Ib/Bu and rearrange:

5 Calculate volume of iron sponge to purchase:

Example No 7-2: Specify Major Parameters for DEA

Required:

1 Show that a DEA unit is an acceptable process selection

2 Determine DEA circulation rate using 35 wt % DEA and an

acid-gas loading of 0.50 mole acid acid-gas/mole DEA

3 Determine preliminary diameter and height for DEA contact tower

4 Calculate approximate reboiler duty with 250°F reboiler temperature

Acid Gas Treating 193

Total acid gas outlet = 2.0%

Pa«d out = 1,015 x (2.0/100) = 20.3 psia

From Figure 7-12 for removing CO2 and H2S, possible processesare amines, Sulfinol, or carbonates

The most common selection for this application is a DEA unit

2 DEA circulation rate

Note: In order to meet the H2S outlet, virtually all the CO2 must

be removed, as DEA is not selective for H2S

3 Tower size

From Volume 1:

Use 72-in ID tower w/24 trays,

4 Determine reboiler duty:

Using 1,000 Btu/gal lean solution

q=l,000(508)(60 min/hr)

q = 30.5 MMBtu/hr

4 Ib/MMscf in the Northern U.S and 2 to 4 Ib/MMscf in Canada Thesevalues correspond to dew points of approximately 32°F for 7 lb/ MMscf,20°F for 4 lb MMscf, and 0°F for 2 Ib/MMscf in a 1,000 psi gas line.Dehydration to dew points below the temperature to which the gas will

be subjected will prevent hydrate formation and corrosion from densed water The latter consideration is especially important in gasstreams containing CO2 or H2S where the acid gas components will form

con-an acid with the condensed water

The capacity of a gas stream for holding water vapor is reduced as thestream is compressed or cooled Thus, water can be removed from thegas stream by compressing or cooling the stream However, the gasstream is still saturated with water so that further reduction in tempera-ture or increase in pressure can result in water condensation

This chapter discusses the design of liquid glycol and solid bed dration systems that are the most common methods of dehydration used

dehy-*Reviewed for the 1999 edition by Lindsey S Stinson of Paragon Engineering Services, Inc.

195

for natural gas In producing operations gas is most often dehydrated bycontact with triethylene glycol Solid bed adsorption units are used wheievery low dew points are required, such as on the inlet stream to a crvo-genic gas plant where water contents of less than 0.05 Ib/MMscf may btneiessat)

WATER CONTENT DETERMINATION

The first step in evaluating and/or designing a gas dehydration system

is to determine the water content of the gas The water content of a gas isdependent upon gas composition, temperature, and pressure For sweetnatural gases containing over 70% methane and small amounts of "heavyends," the McKetta-Wehe pressure-temperature correlation, as shown inFigure 8-1, may be used As an example, assume it is desired to deter-mine the water content for a natural gas with a molecular weight of 26that is in equilibrium with a 3% brine at a pressure of 3,000 psia and atemperature of 150°R From Figure 8-1 at a temperature of 150°F andpressure of 3,000 psia there is 104 Ib of water per MMscf of wet gas Thecorrection for salinity is 0.93 and for molecular weight is 0.98 There-fore, the total water content is 104 x 0.93 x 0.98 = 94.8 Ib/MMscf

A correction for acid gas should be made when the gas stream containsmore than 5% CO2 and/or H2S Figures 8-2 and 8-3 may be used todetermine the water content of a gas containing less than 40% total con-centration of acid gas As an example, assume the example gas from theprevious paragraph contains 15% H2S The water content of the hydro-carbon gas is 94.8 Ib/MMscf From Figure 8-3, the water content of H2S

is 400 Ib/MMscf The effective water content of the stream is equal to(0.85X94.8)+ (0.15)(400) or 141 Ib/MMscf

of the glycol by the addition of heat This step is called "regeneration" or

"reconcentration" and enables the glycol to be recovered for reuse inabsorbing additional water with minimal loss of glycol

Gas Dehydration 197

Water Content of Hydrocarbon Gas

Figure 8-1 McKetta-Wehe pressure-temperature correlation (From Gas Processors

Suppliers Association, Engineering Data Book, 10th Edition.)

Figure 8*2 Effective water content of CO 2 (From Gas Processors Suppliers

Association, engineering Data Book, 10th Edition.)

"reconcentrator" or "regenerator"1) where the water is removed or

"stripped" from the glycol and is then pumped back to the contactor tocomplete the cycle

Figure 8-4 shows a typical trayed contactor in which the gas and liquidare in counter-current flow The wet gas enters the bottom of the contac-tor and contacts the "richest" glycol (glycol containing water in solution)

Gas Dehydration 199

Figure 8-3 Effective water content of H 2 S (From Gas Processors Suppliers

Association, Engineering Data Book, I Oth Edition.)

just before the glycol leaves the column The gas encounters leaner andleaner glycol (that is, giycol containing less and less water in solution),

as it rises through the contactor At each successive tray the leaner glycol

is able to absorb additional amounts of water vapor from the gas Thecounter-current flow in the contactor makes it possible for the gas totransfer a significant amount of water to the glycol and still approachequilibrium with the leanest glycol concentration

The contactor works in the same manner as a condensate stabilizertower described in Chapter 6 As the glycol falls from tray to tray itbecomes richer and richer in water As the gas rises it becomes leaner andleaner in water vapor Glycol contactors will typically have between 6and 12 trays, depending upon the water dew point required To obtain a 7Ib/MMscf specification, 6 to 8 trays are common