Gas Processing, Acid Gas removal and Sulfur Recovery

Question 1

/All mixtures in natural gas

Answer 1

Methane (>70%), Ethane, Propane, Butane, Condensates (heavier carbons)

Small amounts of N, CO2, H2S, water

Question 2

Natural gas is about ___% lighter than air.

Answer 2

40%

Question 3

Methane API gravity

Answer 3

340

Question 4

Natural Gas Usage Pie Chart

Answer 4

Industrial (30.3%)
Electric Power (26.4%)
Residential (21.6%)
Commercial (13.9%)
O%G Industry Operations (5%)
Pipeline Fuel (2.6%)
Vehicle Fuel (0.1%)

Question 5

Main Focus of gas processing units (2)

Answer 5

Recovery of valuable C3 to C6 components from the gas streams.

Production of desulfurized dry gas consisting mostly of methane and ethane.

Question 6

Describe the absorber-deethanizer section of gas processing (6)

Gases are compressed to___
This column contains__
Lean absorption__
The lean__
Sufficient reboil___
On the top__

Answer 6

Gases are compressed to approximately 200 psig and fed to an absorber-deethanizer (a column).

This column contains 20 to 24 trays in the absorption section (top) and 16 to 20 trays in the stripping section (bottom).

Lean absorption oil is fed to the top tray to absorb 85 to 90% of the C3 and almost all of the C and heavier components.

The lean absorption oil is usually a dehexanized naphtha (with an end point of 350 to 380°F).

Sufficient reboil heat is added to the bottom of the stripping section of the absorber-deethanizer to eliminate any absorbed ethane and methane from the bottom liquid product.

On the top tray, some amount of hydrocarbons (other than C1 and C2 ) are vaporized from the lean oil and leave the top of the column with the residue gas (C1 and C2)

Question 7

Explain Sponge Absorber Section (3)

Answer 7

This vaporized material is recovered in the sponge absorber. (8 to 12 trays)

A heavy molecular weight, relatively non-volatile material, such as kerosine or No. 2 fuel oil, is used as sponge oil.

Overhead gas from the sponge oil absorber is treated for removal of carbon dioxide and hydrogen sulfide

Question 8

Residual water is removed by___

Answer 8

chilling (HEXs), adsorption (on solid), or glycol absorption.

Chilling is most common in Western Canada. Some units have nitrogen extractor as well (via absorption).

Question 9

Debutanizer and Depropanizer section (3):

hint: feed, overhead, and bottom

Answer 9

The deethanized rich oil then flows to a debutanizer column where all the recovered C3 and C4 are fractionated and taken off as overhead product.

The overhead C3 and C4 product from the debutanizer is condensed, desulfurized, and fed to a depropanizer for separation into propane and butane.

The bottom product from the debutanizer contains pentanes and heavier hydrocarbons.

Question 10

What is removed in acid gas?

Answer 10

Hydrogen sulfide and carbon dioxide.

Question 11

5 different processes for acid gas removal

Answer 11

chemical solvent, physical solvent, direct conversion, dry (solid) bed, gas permeation

Question 11

Which type of removal process can have the highest H2S loading

Answer 11

Physical Solvent

Question 12

Which type of removal process can remove large amounts of H2S? small amounts?

Answer 12

Physical Solvent (large)

Dry bed (small)

Question 13

Most common gas treating processes. What type of unit it is. Which process does it fall under?

Answer 13

Diethanolamine (DEA) process. Amine treating unit. Chemical Solvent Processes.

It uses an aqueous solution of DEA (15 - 30 wt%). Methyl-diethanolamine (MDEA) can also be used.

Question 14

Treated gas contains less than ___ H2S and less than ___ CO2

Answer 14

4 ppm

1.5 vol%

Question 15

Describe the amine treating unit. (8)

Answer 15

The solution is pumped to the top of an absorber ( about 20 to 24 trays or an equivalent amount of packing).

Hydrogen sulfide and carbon dioxide are removed from the gas by absorption into the solution.

Rich solution from the absorber flows into a flash tank (at a lower pressure than the absorber) that permits any dissolved or entrained methane and ethane to be vented from the system.

The rich solution is then preheated and flows to a still (distillator, also 20-24 trays or equivalent packing) where the acid gases are stripped from the solution by steam generated in a reboiler.

The acid gases are taken from the top of the still through a condenser where most of the steam is condensed.

The condensate is separated from the acid gases and returned to the top of the still as reflux.

The acid gases are then sent to sulfur recovery unit where the hydrogen sulfide is converted to elemental sulfur.

The lean solution is cooled and returned to the top of the absorber.

Question 16

Regulations require recovery of about ____ of the sulfur in the gas streams.

Answer 16

99% or more

Question 17

The most practical method for converting hydrogen sulfide to elemental sulfur is the____ and it is dependent on___

Answer 17

modified Claus Process

H2S concentration in the gas

Question 18

H2S % in acid gas is

Answer 18

over 50%

Question 19

Another name for modified Claus process

Answer 19

partial combustion (“once-through”) process

Question 20

In the partial combustion process, the hydrogen sulfide–rich gas stream is burned with ____ the stoichiometric quantity of air.

Answer 20

1/3rd

Question 21

Then the hot gases are passed over an ____ to react sulfur dioxide with unburned hydrogen sulfide to produce free sulfur.

Answer 21

alumina catalyst

Question 22

Formula for the burner and Reactor

Answer 22

Burner:
2H2S + 3O2 → 2H2O + 2SO2
Reactor:
2H2S + SO2 → 2H2O + 3S

Question 23

In S.R the burner is located in a ___

Answer 23

Reaction chamber

Question 24

Purpose of reaction chamber in S.R

Answer 24

To allow for sufficient time for combustion reaction to be completed.

Question 25

Purpose of the waste heat boiler in S.R

Answer 25

To remove most of the exothermic rxn heat from gases by steam generation Also, some elemental sulfur is often condense and remove from the gas in the w.h.b

Question 26

Gas temp entering the first catalytic converter. What is its purpose

Answer 26

425 to 475 F To maintain the catalyst bed above sulfur dewpoint in order to avoid saturating the catalyst with sulfur (leading to deactivation)

Question 27

Is the reaction b/w H2S and SO2 in the converter endothermic or exothermic

Answer 27

exothermic

Question 28

What happens in the 1st condenser? What is the outlet temp and why? What is the typical amount of converter in series for most plants?

Answer 28

Gases from the 1st converter are cooled to remove the elemental sulfur as liquid. Outlet temp must be above 275 F to avoid solidifying the sulfur Most plants have 3 converters in series

Question 29

When sulfur content in the feed is higher than design, an economical method for increasing the capacity of Claus units is: Why?

Answer 29

Substitution of oxygen for a potion of the combustion air needed in the reaction furnace. This increases the capacity of sulfur production by 50% or more at a low capital cost

Question 30

Undesired products in the tail-gas of Claus units (4)

Answer 30

Carbonyl sulfide (COS) Carbon disulfide (CS2) Hydrogen Sulfide (H2S) Sulfur Dioxide (SO2)

Question 31

How is COS and CS2 formed?

Answer 31

CH4 + SO2 → COS + H2O + H2 CO2 + H2S → COS + H2O CH4 + 2S2 → CS2 + 2H2S

Question 32

Problems caused by COS and CS2

Answer 32

They cannot be completely converted to elemental sulfur and CO2. If unconverted it is a loss of recoverable sulfur and an increase in sulfur emission

Question 33

Process and catalyst used to reduce COS and CS2 to less than 20ppm

Answer 33

Process is called Shell Clause off-gas treatment (SCOT) Catalyst is Titanium

Question 34

What happens in the SCOT process (4)

Answer 34

The Claus tail-gas is combined with small amounts of hydrogen or a mixture of CO & H. Gas is heated to about 480-570 F This hot gas flows through a fixed catalyst bed where various sulfur compounds are converted to H2S The reactor effluent is cooled to ambient temperature and the H2S is absorbed with an aqueous amine solvent The absorbed H2S is regenerated from the solvent in a typical amine still and recycled to the Claus unit feed

Question 35

The H2S content in the gas at the top of the amine absorber is typical between ______. What happens to this gas?

Answer 35

50 to 400 ppm It is incinerated to convert H2S to SO2 before venting.

Question 36

Recovery of Claus units w/ and w/o SCOT

Answer 36

<=96 % w/o SCOT >99% w/ SCOT

Question 37

Uses of Sulfur (5)

Answer 37

Sulfuric Acid, Fertilizers, Pesticides, Food Preservatives, Pharmaceuticals.