Process sequencing for amine regeneration

An improved method for amine regeneration wherein a rich amine out of a first heat exchanger is temperature controlled prior to transfer to a second heat exchanger. At least a portion of the hot overhead gasses exiting from a stripping still are transferred to the second heat exchanger. Temperature controlled heated rich amine liquid passes through the second exchanger and contacts the hot overhead gasses. The rich amine liquid is increased in higher temperature thereby and then is transferred to yet a third exchanger and finally to the stripping still for regeneration of lean amine. The reduced temperature overhead gasses are transferred to the reflux condensor for final cooling.

This invention relates to a process for an improved regeneration of amine 
in an absorption-type gas treatment process. 
Many natural gases contain hydrogen sulfide (H.sub.2 S) and carbon dioxide 
(CO.sub.2), commonly called acid gases, that must be removed prior to 
sales or processing. The removal of sulfur compounds from these acid 
gasses or "sour gasses" is oftentimes calle "sweetening." 
There are generally two types of gas treating processes: (a) absorption and 
(b) adsorption. In absorption processes, the gas stream contacts a liquid 
that selectively removes acid gases. The most common absorption process is 
the amine process. The liquid absorbent is a mixture of water and a 
chemical amine, usually monoethanol-amine (MEA) or diethanolamine (DEA). 
Sometimes triethanol-amine (TEA), diglycolamine (DGA), and 
methyl-diethanolamine (MDEA), diisoprophylamine, sulfanol and solutions of 
these, with special additives to improve efficiencies, are utilized. 
Hydrogen sulfide is a toxic gas that must be removed to extreme low 
concentrations (less than 0.25 grains of H.sub.2 S per 100 standard cubic 
feet) prior to pipeline delivery. When mixed with free water it forms a 
weak acid that can cause corrosion. 
Carbon dioxide is a non-toxic inert gas. Carbon dioxide, as such, is 
harmless in dry natural gas but when mixed wtih free water will form a 
weak acid and also cause corrosion. Inlet gas to cryogenic plants that 
contain concentrations of CO.sub.2 in excess of 0.75 to 1.0 percent 
CO.sub.2 may cause freezing problems. The CO.sub.2 will freeze to a solid 
ice in a turbo expander plant demethanizer where it may plug lines and 
even plug the tower itself. Often flooding of the demethanizer results 
from carbon dioxide freezing within the tower. 
When the plant inlet gas contains concentrations of carbon dioxide too high 
to process, all of the gas may be treated or part of the gas may be 
separated into a side stream and treated by an amine plant. Principally 
all the carbon dioxide is removed in the amine plant. When the side stream 
is processed, and sufficient gas is treated, it is blended back with the 
untreated gas, thus yielding a carbon dioxide content of the blended 
stream which is low enough for processing. 
Diethanolamine (DEA) is the most common amine used in plants operating at 
pressures above 300 psig. Monoethanolamine (MEA), a stronger base 
chemical, is commonly used for pressures below 300 psig. When pressures 
are sufficiently high, DEA is preferred and will require lower circulation 
rates, less heat input, and fewer corrosion problems than experienced with 
MEA. 
Amines remove carbon dioxide and hydrogen sulfide by a chemical reaction 
that changes the chemical form of both the amine and the acid gases. The 
new chemical changes the acid gases to a liquid form which is separated 
from the acid-free gas or sweetened gas. The chemical reaction between 
amine (called lean amine at the start of the process) and acid gases gives 
off heat when the reaction takes place. The sweet residue gas flows out 
the top of a contactor or absorber and the reacted amine (also called rich 
amine) flows out the bottom and is generally higher in temperature than 
the inlets. Lean amine is regenerated by reducing the pressure and adding 
heat to the rich amine. 
The present invention focuses on the lean amine regeneration process. The 
solution regeneration generally takes place in a low pressure still with a 
reboiler at the bottom to furnish heat to the solution. The still is 
generally a bubble tower containing either trays or packing. The rich 
amine liquid containing the sour gasses (CO.sub.2 and H.sub.2 S) is 
injected into the still near the top and flows down the tower while steam 
generated in the reboiler flows up the tower countercurrent to the 
descending rich amine. The steam aids in "stripping" the sour gasses from 
the rich amine liquid and sends them back up the tower and out the top of 
the tower. 
The heat added to the still reboiler increases the temperature of the amine 
somewhat, but most of the heat goes into generating steam which, in turn, 
flows into and up the still. This heat added or inputted into the reboiler 
must be furnished from an outside source such as steam from another 
process, hot oil or hot glycol circulated through the reboiler, or fuel 
directly fired into the reboiler. 
When sour gasses pass out the top of the still, a large amount of steam 
also goes out with the gas. This overhead steam and gas stream (called 
overhead) is generally higher in temperture than the feed to the top of 
the still. It has not been recognized in operating industry heretofore 
that this overhead has enormous amounts of available and recoverable 
thermal energy. In the normal amine unit the gasses and steam out the top 
of the still flow to condenser (called a reflux condenser) where the sour 
gasses are cooled to near ambient temperatures and most of the steam 
condenses into water. This condensing step requires considerable amounts 
of energy, and the heat removed is generally wasted to air. 
U.S. Pat. No. 3,362,891 discloses that the overhead may be used to heat 
relatively cool rich amine (90.degree. F.), thereby giving up some heat 
and condensing as reflux. However, nothing in U.S. Pat. No. 3,362,891 
teaches the enormous energy savings available by proper temperature 
control of the overhead in relation to the temperature of the feed to the 
stripper still. Further, U.S. Pat. No. 3,362,891 does not disclose the hea 
exchange sequencing of the instant invention. 
The present invention is a process for maximizing heat savings in the 
conventional amine regeneration process. By controlling outlet temperature 
of still feed from the conventionally utilized heat exchanger, immediately 
downstream of the contactor; exchanging heat from the still overhead 
gasses to the rich amine still feed; and, subsequent, further heat 
exchange of the feed with the hot lean amine discharged from the bottom of 
the still, enormous heat savings may be realized. The unique exchanger 
sequencing and temperature control features of the instant process 
distinguish this invention. The amount of heat saved in the instant 
processing reduces the amount of outside heat that must be added to the 
reboiler. This reduction of reboiler heat input is a direct savings in 
fuel costs. With some plants savings as much as thirty percent of the 
total reboiler duty should b experienced. 
SUMMARY OF THE INVENTION 
The present invention is a significant modification to the conventional or 
typical amine regeneration process. Additional steps to the amine 
regeneration process include: first, controlling the outlet temperature 
range of the rich amine still feed from the conventionally utilized heat 
exchanger immediately downstream of the contactor; secondly, the 
controlled temperature transferring of all or a portion of the hot 
overhead gasses exiting from a stripping still through a conduit to a heat 
exchange unit while flowing into the heat exchange unit temperature 
controlled rich amine liquid; and, thirdly, controlling a heat exchange 
subsequent to the still overhead heat exchange, whereby heat is 
transferred from the hot lean amine out of the bottom of the still to the 
higher temperature rich amine feed immediately prior to the feed entering 
the still. There is a transfer of thermal energy (either directly or 
indirectly) in the instant process to the rich amine liquid resulting in a 
higher temperature rich amine liquid flowing into the stripper still. 
Reduced temperature overhead gasses are then transferred to a condenser 
for final cooling of the acid gasses and water and reduced temperature 
lean amine is eventually returned to the contractor. This invention 
reduces the heat input necessary to the reboiler for developing steam in 
the stripper still. It has been found that when the rich amine liquid 
temperature immediately prior to entry into the still is raised from the 
normal operating range of 170.degree. F. to a temperature range of 
190.degree. F. to 210.degree. F., reboiler duty is reduced by as much as 
30 percent. Not only is the reboiler duty reduced, but the still 
condensing duty is decreased as well as is the lean amine cooler duty.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
While there will be shown and described and pointed out fundamental novel 
features of the invention as applied to a preferred embodiment thereof, it 
will be understood that various omissions and substitutions and changes in 
the form and details of the process illustrated may be made by those 
skilled in the art without departing from the spirit of the invention. It 
is the intention, therefore, to be limited only as indicated by the scope 
of the claims appended hereto. 
The basic amine flow diagram is shown in FIG. 1. Sour gas, i.e., natural 
gas having high concentrations of hydrogen sulfide and carbon dioxide, 
flows through a conduit 10 and then through a scrubber 12 to remove 
particular matter with the discharge conduit 14 of the scrubber 12 
entering the bottom 16 of the contactor or absorber 18 and contacts the 
lean amine solution, which enters at the top 20 of the absorber 18 via 
conduit 22. The gasses rise through the vertical contactor 18 and then 
react with the lean amine in a countercurrent direction. The gas bubbles 
through the amine whereby the chemical reaction takes place, removing the 
acid gasses and allowing the sweet natural gas to leave the top of the 
absorber through conduit 24. These sweet gasses again pass through a 
scrubber 26 and out conduit 28 on to the main gas pipeline. The 
countercurrent flow is important as it allows the leanest (purest) amine 
to contact the leanest (sweetest) gas at the top and the richest 
(contaminated) amine to contact the richest (contaminated) sour gas at the 
bottom. 
The rich amine leaves the bottom 16 of the tower 18 in the temperature 
range of 120.degree. F.-160.degree. F. and flows through conduit 30 to a 
flash tank 32. Flash tank vapors are sent via pipeline 34 to the fuel 
system and are usually sour. From the flash tank 32, the amine stream goes 
through conduit 36 to a heat exchanger 38 where the stream temperature is 
raised to approximately 170.degree.-190.degree. F. The stream then flows 
through feed line 40 to the top section 42 of the regeneration stripper 44 
(the still). The still bottoms pass through conduit 46 and are heated in 
the reboiler 48. The still reboiler 48 heats the amine and water solution 
in the bottom of the still 44 to about 240.degree. F. at about 5 psig. 
pressure. The heat converts some of the water in the amine solution to 
steam. The steam rises through the still and heats the counterflowing rich 
amine solution. This heat releases the acid gasses, and the excess steam 
and acid gasses pass out of the top of the still 44 through conduit 50. 
The steam and acid gasses then enter the reflux condenser 52 where the 
steam is condensed, separated in accumulator 54 and pumped back to the 
still 44. The acid gasses leave the separator or accumulator 54 and go 
either to a flare stack or a sulfur recover plant (not shown) via conduit 
56. 
The hot lean amine leaves the bottom 45 of the still 44 via line 58, is 
filtered in filter 60, and flows back through pipeline 62 to heat 
exchanger 38 and then is pumped by pump 63 through conduit 64 to lean 
amine cooler 66. From cooler 66, the cooled lean amine solution is pumped 
through conduit 22 to the top 20 of the contactor 18. 
The primary purpose of the hot, lean amine passing through exchanger 38 is 
to reduce the lean amine temperature while raising the rich amine 
temperature. The lean amine heat exchange at exchanger 38 is to ensure 
cool amine to the contactor 18 since the chemical reaction of removing the 
acid gasses proceeds best at inlet gas and lean amine temperatures between 
85.degree. F. and 120.degree. F. However, this transfer of heat only 
raises the rich amine temperature to a range of approxiately 
170.degree.-190.degree. F. There simply is not enough eneryy in the hot 
amine stream at this particular point in its flow path to raise the rich 
amine temperature any higher. 
The purpose of the still overhead condensor 52 is to cool the stripped acid 
gasses and condense steam from the still 44 to conserve water. The 
overhead gasses could be sent to the flare while hot; however, a 
considerable amount of water would be lost from the system, requiring a 
large make-up supply. By cooling the overhead to 120.degree. F. or less, a 
large part of the steam condenses to water and is separated in the reflux 
accumulator 54. Because of the enormous amount of thermal energy available 
in the steam, the condensor 52 has a significant work load. The separated 
water, called reflux, is pumped via pump 68 back into the amine system 
either into the lean amine stream, into the still feed, or to the top 42 
of the still 44 (as shown in FIG. 1). 
Stripping of the amine in the still 44 is of critical importance. Lean 
amine improperly stripped will produce off specification gas from the 
contactor 18. If the lean amine is not stripped sufficiently in the still 
44, acid gas left in the amine solution will be stripped out by the main 
gas stream at the top 20 of the contactor 18, thereby causing the gas 
leaving the contactor 18 to be off specification. The most common way to 
solve a condition of high residual acid gas in the lean amine is to 
increase the heat being supplied via conduit 70 to the still reboiler 48. 
Unfortunately, this results in an increased consumption of reboiler fuel, 
or in the cases where steam is used within the reboiler 48, additional 
steam must be supplied to generate higher temperatures within the stripper 
still 44. 
FIG. 2 illustrates a modified amine process of the present invention. 
Instead of the rich amine flow discharged from the exchanger 3 flowing 
directly from conduit 40 into the top 42 of the stripper still 44, the 
flow is diverted through heat exchanger 39. Heat exchanger 39 may be an 
indirect contact type exchanger or the direct contact type. FIG. 2 
illustrates heat exchanger 39 to be an indirect contact type wherein the 
rich amine flow would indirectly contact the hot overhead gasses as 
discussed below. 
Thermal energy is transferred from hot overhead gasses exiting from the top 
42 of the stripping still 44 through conduit 50 to heat exchanger 39. 
Within heat exchanger 39 thermal energy is transferred from hot overhead 
gasses to the rich amine liquid. Therefore, while the temperature T2 of 
the output of a conventionally utilized exchanger 38 is generally in the 
approximate range of 170.degree. F. to 190.degree. F., T5, the temperature 
of the input into the still 44 in the modified amine process of the 
present invention of FIG. 2 is in the approximate temperature range of 
190.degree. F. to 210.degree. F. 
The overhead gasses from the still 44 via conduit 50 are in the approximate 
temperature range of 210.degree. F. to 220.degree. F. as shown on 
temperature indicator T3 but contain an enormous amount of thermal energy 
in the form of latent heat of condensation. When these hot overhead gasses 
are transferred to heat exchanger 39, this enormous amount of latent heat 
is transferred to the rich amine flow. 
Steam condensing from the vapor phase to the liquid phase at constant 
temperture gives up considerably more heat than can be removed from either 
the vapor steam or liquid water. At atmospheric pressure, one pound of 
steam condensing from a vapor to a liquid will give up 970.3 BTU's. One 
pound of liquid water requires one BTU to raise the temperature of the 
water 1.degree. F. Thus, the amount of heat transferred from one pound of 
condensing steam at a constant temperature is sufficient heat to raise the 
temperature of one pound of water 970.3.degree. F. 
FIG. 4 is a chart showing an amine still overhead condensing curve. 
T.degree. F. represents temperature in degrees and % HR represents percent 
of heat removed. From FIG. 4 it can be seen that pure steam (curve A) 
gives up 100% of its latent heat in condensing to a liquid without a 
temperature change. The still overhead (curve B), made up of acid gas and 
steam, gives up approximately 55% of its heat in going from 220.degree. F. 
to 200.degree. F. and gives approximately 77% of its heat in going from 
220.degree. F. to 180.degree. F. Curve C shows that the cooling of acid 
gas only gives up 20% of its heat in going from 220.degree. F. to 
200.degree. F. and less than half (40%) of its heat in going from 
220.degree. F. to 180.degree. F. This tremendous difference in available 
heat has not been recognized in the industry. 
In the industry the still overhead is recognized as mainly acid gas and is 
commonly called "acid gas"; and even though the overhead has been used to 
heat the still feed as shown in U.S. Pat. No. 3,362,891, it has not been 
readily evident in the industry there is such a large amount of steam 
carrying overhead of the still with the acid gas and that this steam can 
be condensed at sufficiently high enough temperature for the heat to be 
added to the still feed 
The reduced temperature T4 overhead gas exiting exchange 39 via conduit 51, 
having an approxiamte temperature range of 160.degree. F.-200.degree. F., 
is then transferred to the conventional condensor unit 52 and separator 54 
for separation into reflux and acid gas. Because the overhead gas has 
given up much of its energy in the heat exchange with the rich amine 
liquid, less energy is utilized in the condensor unit 52 of this inventive 
process than in the conventional amine process. Further, since the inlet 
temperature T2 of the rich amine into the still 4 is higher than with the 
conventional methods, less outside heat has to be supplied to the reboiler 
48 in order to generate the proper operating temperatures within the 
stripper still 44. 
FIG. 3 illustrates an embodiment of the instant process which further 
increases the heat and energy savings over the conventional amine 
regeneration process. As with the processes of FIGS. 1 and 2, rich amine 
from contactor 18 flows through conduit 36 to exchanger 38. However, as 
shown in FIG. 3, a temperature controlled bypass line 80 branches off 
conduit 36 upstream of exchanger 38. 
A temperature controller 82, as it is commonly known in the art, senses and 
compares with a predetermined temperature the temperature of rich amine in 
conduit 40 upstream of exchanger 39. Controller 82 opens and closes bypass 
control valve 84 in response to the temperature of rich amine sensed in 
conduit 40. In the process of FIG. 3, it is critical to control the rich 
amine temperature to exchanger 39, so that maximum heat savings are 
achieved throughout the entire process, as will be discussed below. The 
temperature of rich amine into exchanger 39 will effect the transfer of 
heat from the overhead gasses out of the top 42 of still 44 to the rich 
amine. Further, the temperature of the rich amine to exchanger 39 effects 
the outlet temperature of the overhead passing through conduit 51 to 
condenser 52. 
The temperature of rich amine in conduit 41 out of exchanger 39 is sensed 
at temperature indicator T5. The rich amine out of exchanger 39 flows 
through conduit 41 to a final exchanger 90. In exchanger 90 heat is 
transferred from the hot lean amine from the bottom 45 of still 44 to the 
rich amine coming out of exchanger 39. Hot lean amine from still 44 flows 
through output conduit 58, pump 59, filter 60, and conduit 62 to exchanger 
90. The temperature of hot lean amine in conduit 62 is sensed by 
temperature indicator T7. 
The hot lean amine in conduit 62 is generally in the temperature range of 
240.degree. F. As previously stated the temperature of the rich amine out 
of exchanger 39 is in the range of 210.degree. F., always less than the 
240.degree. F. temperature of rich amine from still 44 in conduit 62. 
Therefore, by the heat exchange sequencing of the present invention, the 
best exchange from the hot lean amine to the rich amine feed at exchangers 
90 and 38 is much more efficient than has been previously experienced in 
the conventional amine process utilizing an exchanger 38. Rich amine out 
of exchanger 90 passes through conduit 43, is temperature sensed by 
temperature indicator T6, and passes into still 44 at a temperature in the 
range of 200.degree. F. to 220.degree. F. or more. 
The hot lean amine flows out of exchanger 9 through conduit 92 to exchanger 
38. Temperture indicator T8 senses the lean amine temperature in conduit 
92. While the inlet temperature of lean amine entering exchanger 38 is 
lower than is normally found in the conventional amine process 
(215.degree. F. versus 240.degree. F.), it is still sufficiently high to 
raise the temperature of rich amine from an inlet temperature of 
approximately 130.degree. F. to an outlet (from exchanger 38) to 
approximately 185.degree. F. The amount of heat recovered from the lean 
amine in the two exchangers 90 and 38 will be equal to or greater than 
that recovered from exchanger 38 in the standard process of FIG. 1. As has 
been previously stated it is critical to the instant invention process 
that the rich amine temperature out of exchanger 38 be controlled; thus, 
the purpose of the temperature bypass system including conduit 80, 
controller 82, and valve 84. 
Further, the temperature bypass system allows for controlling the lean 
amine temperature to cooler 66 thereby effecting its duty. Temperature 
indicator T9 senses the lean amine temperature in conduit 64. Because lean 
amine to cooler 66 is lower in the instant invention process than in the 
conventional process further heat and energy savings are achieved; also 
the efficiency of the chemical reaction in the absorber 18 improved. 
As temperature controlled rich amine flows through conduit 40 to exchanger 
39, the instant process provides for controls in the heat exchange between 
the rich amine and the overhead gasses unlike any known amine regeneration 
process. In addition, controlling the temperature rich amine out of 
exchanger 39 into exchanger 90, results in completing the control loop 
back to exchanger 38. 
To illustrate the significance of th reduction in reboiler 48 duty and the 
savings on the amount of heat wasted in air achievable in the instant 
inventive process, various examples follow in Table 1 which compare the 
standard amine regeneration process (FIG. 1 process), the process of U.S. 
Pat. No. 3,362,891, the modified process of FIG. 2, and the temperature 
controlled process of FIG. 3. The process of FIG. 3 is calculated several 
times to show the savings which are achievable by varying the temperature 
controlled bypass system. The following assumptions or conditions are made 
within all of the examples illustrated: 
(1) Amine unit removing 100,000 std. cu. ft. per hour acid gases 
(2) Amine: 
30 wt. percent diethanolamine 
70 wt. percent water 
(3) Amine circulation rate: 20,000 gal per hour 
(4) Amine net acid gas pick-up: 
(a) 0.54 mols acid gas per mol amine 
(b) 5.0 cu. ft. acid gas per gal amine solution 
(5) Amine still reflux rate=2.5 mols reflux per mol acid gas removed. 
Numbers and amounts for the process of U.S. Pat. No. 3,362,891 are based 
upon extracting the same 100,000 std. cu. ft. per hour acid gas with all 
other conditions the same as given in the '891 patent. Further, in the 
'891 example T1 is the temperature of rich amine out of contactor; T2 is 
temperature of rich amine entering stripper still; T3 is temperature 
overhead gasses to condenser; T4 is temperature of overhead gasses out of 
condenser; and T7 is temperature of hot lean amine out of stripper. 
TABLE 1 
__________________________________________________________________________ 
Standard Process of 
Modified 
Process U.S. Pat. No. 
Process 
Temperature controlled process of FIG. 3 
.degree.F. 
of FIG. 1 
3,362,891 
of FIG. 2 
Case 1 
Case 2 
Case 3 
Case 4 
Case 5 
Case 6 
__________________________________________________________________________ 
T1 130 90 130 130 130 130 130 130 130 
T2 185 199 185 155 165 175 185 195 200 
T3 215 210 215 215 215 215 215 215 215 
T4 -- 140 190 160 170 180 190 200 205 
T5 -- -- 210 210 210 210 210 210 210 
T6 -- -- -- 220+ 
220+ 
220+ 
220+ 
220+ 
220+ 
T7 240 235 240 240 240 240 240 240 240 
T8 -- -- -- 215 215 215 215 215 215 
T9 155 -- 182 189 178 168 157 146 141 
T10 120 unk 120 120 120 120 120 120 120 
Reboiler 
24,072 
46,859 19,582 
18,126 
16,929 
16,112 
15,669 
15,919 
16,620 
Duty 
MBTU/Hr 
Heat 18,265 
41,106 13,659 
12,131 
11,035 
10,297 
9,797 
10,076 
10,772 
wasted 
to air 
MBTU/Hr 
__________________________________________________________________________