AN APPARATUS AND METHOD FOR TREATMENT OF A SOUR STREAM
FIELD OF THE INVENTION
The present invention relates to an Apparatus for treatment of a Sour stream. The invention in
particular relates to a process for treating an NH3 rich stream in said apparatus.
BACKGROUND OF THE INVENTION
Sour streams containing significant concentration of ammonia, acid gases and water vapour
are difficult to treat for separating ammonia in separation units. Such sour streams create
following problems in especially Claus Sulphur Recovery Unit (SRU):
• Requirement of higher furnace temperature in main burner for destruction of NH3
• Formation of NOx upon incineration
• Formation of ammonium salt in process lines
• Formation of hard complex material with sulphur which causes chocking in
downstream equipments
• Increased air demand and reduced unit capacity
Conventionally NH3 rich streams were treated with H2SO4 so that ammonium sulphate
((NH4)2SO4) can be obtained, which can be further used as fertilizer/chemical. However, the
recovery and separation units are constrained by capital and operating cost as well as
feasibility and effectiveness of operation. The cost of acid used and price of ammonium
sulphate make the treatment process challenging. Further, cost of crystallization of
ammonium sulphate from the solution pose another challenge.
Reference is drawn to prior art documents US 6,902,713, EP085709 A1, US 5,672,326, US
3,985,863, US 4,032,618 which disclose various method of removing NH3.
US 2,424,205 discloses apparatus for recovering ammonia from distillation gas by passing
the gas through a so called saturator tank holding in its lower portion a weak sulphuric acid
liquor bath with which the gas is brought into scrubbing contact with the resultant production
of sulphate of ammonia which crystallizes out of the bath liquor and accumulates in the lower
portion of the saturator and from which the sulphate crystals are removed in a carrying stream
of liquor. In practice, the scrubbing contact of the gas with the bath liquor is affected by
passing the gas into the saturator through a so called cracker pipe which has a discharge
mouth submerged in the bath liquor.
US 4,250,160 discloses process for the production of ammonium sulfate in a multistage
contactor (or column) inclusive of liquid cyclone recycles. A gas, or gases, which contain
sulfur dioxide, e.g., a flue gas, is contacted with ammonia in an oxygen and water
environment in a column comprised of three sections (zones). Sulfur dioxide is contacted in
vapor phase in a central section of the contactor with a stoichiometric excess of ammonia in
the presence of oxygen and water vapor to produce ammonium sulfate. A scrubbing section
for the removal of ammonia from the effluent gas by countercurrent contact with water or
acid solution is provided in the upper section of the contactor. Liquid cyclones are utilized in
the lower section of the contactor where product ammonium sulfate is removed as slurry,
crystalline ammonium sulfate is removed from the slurry, and liquid is returned as recycle to
the contactor.
The Inventors of the present invention have found that sour stream can be effectively and
economically treated in an apparatus with different sections using aqueous solution of acid.
OBJECTS OF THE INVENTION
The principal object of the present invention is to device an apparatus and process for
effectively and economically treating sour stream using aqueous solution of acid.
Another object of the invention is to improve operation of Sulphur recovery unit (SRU) and
increase capacity thereof.
SUMMARY OF THE INVENTION
The present invention provides an apparatus and process for effectively and economically
treating sour stream using aqueous solution of acid. Accordingly, the apparatus of the present
invention has three different sections, i.e. (a) a rectification section; (b) a reaction section;
and (c) a settling section.
In an embodiment of the invention, the sour stream comprises NH3, water vapor, hydrogen
sulphide and hydrocarbons.
In another embodiment of the invention, the sour stream comprises NH3 up to 70% by
volume.
In another embodiment of the invention, the aqueous solution of acid is selected from HCl,
H2SO4 and H3PO4 respectively.
In another embodiment of the invention, the rectification and the reaction sections comprise
packing selected from structured packing and random packing.
In another embodiment of the invention, the structured and random packing are metallic
packing of surface area ranging from 65 m2/m3 to 340m2/m3.
In yet another embodiment of the invention, the sections are convergent from bottom with
varying cross sectional area of each section.
In still another object of the invention, the cross sectional area of settling section is higher
than reaction section and rectification section and the cross sectional area of rectification
section is lower than the reaction section and the settling section.
Yet another object of the invention is to prepare (NH4)2SO4 by counter currently contacting
the NH3 rich stream with H2SO4, forming (NH4)2SO4 on packing in the reaction section of the
said apparatus, and collecting (NH4)2SO4 from unreacted H2SO4 in the settling section.
Further object of the invention is to prepare (NH4)3PO4 by counter currently contacting the
NH3 rich stream with H3PO4, forming (NH4)3PO4 on packing in the reaction section of the
said apparatus, and collecting (NH4)3PO4 from unreacted H3PO4 in the settling section.
BRIEF DESCRIPTION OF DRAWINGS
The above and other aspects and advantages of the present invention will become apparent
from the following detailed description of the preferred embodiments, taken in conjunction
with the drawing, wherein:
Figure 1 is a Schematic representation of the Apparatus
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides an apparatus and process for effectively and economically
treating sour stream using aqueous solution of acid.
The apparatus according to the present invention will now be described in more detail with
reference to the accompanying Figure 1. The apparatus has three different sections, i.e. (a) a
rectification section; (b) a reaction section; and (c) a settling section.
The top section (1) has diameter of 50 mm and packing support on a grid. The packing
material is metallic and random packing of diameter 25 mm. The height of the packing is 100
mm in this section.
The middle section is reaction section (2) and has diameter of 75 mm. This section has
metallic and random packing of diameter 25 mm. The height of the packing is 150 mm in this
section.
The bottom section (3) is settling section and has diameter of 360 mm. The height of this
section is 900 mm. The apparatus has one feed gas inlet line (4) , one gas outlet line (5) , one
fresh acid solution inlet line (6) over the packing of the rectification section , one liquid
circulation line(7) over the packing in the reaction section and one liquid outlet line (8) at the
bottom of the apparatus.
The rectification and the reaction section comprise packing selected from structured packing
and random packing. The structured and random packing are metallic packing of surface area
ranging from 65 m2/m3 to 340m2/m3.
The sections are convergent from bottom with varying cross sectional area of each section.
Particularly, the cross sectional area of settling section is higher than packing section and
rectification section and the cross sectional area of rectification section is lower than the
reaction section and the settling section. The cross sectional area of rectification section is 10-
40% of the reaction section.
The cross section of the settling section is designed with residence time of 15-20 min such
that reaction solution can be circulated back to reaction section for further reaction between
NH3 and unreacted acid. The circulation flow rate is kept 5 -15 times of the acid solution flow
rate. The feed gas velocity in the reaction section varies from 0.5 cm/s to 3.5 cm/s whereas in
the rectification section varies from 0.2 cm/s to 1.6 cm/s.
The liquid velocity in the reaction section varies from 0 cm/s to1.83 cm/s, whereas in the
rectification section varies from 0.3 cm/s to1.03 cm/s.
The reaction section is the main section of the apparatus, which is designed to handle
maximum gas and liquid flow with enhanced mass transfer and reaction. The packings for
this section are able to handle maximum flooding and minimum pressure drop. Since the
liquid and gas flow rate in the rectification section are less, the cross sectional area of this
section is designed higher than that of the rectification section.
Different diminutions of all the three sections of the apparatus help in improving the
operation with respect to pressure drop, flooding and controllability besides the benefits with
respect to capital cost of the equipment.
The process using the apparatus of the present invention will now be described in more detail
with reference to the accompanying Figure 1.
Synthetic gas mixture is prepared in feed gas line by using ammonia and nitrogen. Pure
ammonia is supplied from the ammonia cylinder (9) by using pressure regulator fitted in the
cylinder. The flow rate of ammonia is measured by the rotameter (10). Pure nitrogen is
supplied from the nitrogen cylinder (11) by using pressure regulator fitted in the cylinder.
The flow rate of nitrogen is measured by the rotameter (12). The pressure of the feed gas is
measured by the pressure gauge(13).
Fresh solution of H2SO4 is prepared at different concentration in a vessel (14) and the
solution is pumped by a pump (15) to the top of the packing of rectification section. The
solution flow is measured by using a rotameter (16).
The apparatus bottom solution is pumped by a pump (17) to the top of the packing of
reaction section. The solution flow is measured by using a rotameter (18).
The flow of the gas leaving the rectification section is measure by a rotameter(19). The
pressure of the gas is measured by the pressure gauge(20).A vessel (21) containing dilute HCl
is provided in the discharge line (22).
The ammonia and nitrogen are supplied from the respective cylinders. The gas flow rates are
recorded to estimate the concentration of ammonia in the feed gas entering into the apparatus.
All the gas flow meter and liquid flow meters are calibrated before use.
Fresh acid solution is pumped to the rectification section and the flow rate is measured. The
flow is kept minimum at the start-up. After some time the pump (17) is run and the bottom
solution is circulated.
The feed gas flow rate is gradually increased. The composition of ammonia in the feed gas is
kept constant. The outlet gas flow rate is recorded by the rotameter. The presence of
ammonia in the outlet gas is detected by using concentrated HCl solution. Presence of
ammonia in the vent gas makes fume of ammonium chloride. No fume is observed if
ammonia is not present in the outlet gas.
The gradual increase of the feed gas results flooding either in the rectification section or in
the reaction section. The flow rate of the gas is recorded and the presence of ammonia at the
flooding condition is checked. The above step is repeated at different ammonia concentration
to check the flooding condition and the ammonia slippage. The experiment is carried out at
different concentrations of fresh acid to check the flooding of rectification and reaction
sections at different flow rates of feed gas and the condition of zero discharge of ammonia.
(NH4)2SO4 is formed by counter current contact and reaction between NH3 rich stream and
H2SO4 on packing in the reaction section of the said apparatus, and said (NH4)2SO4 is
collected from unreacted H2SO4 from the settling section of the said apparatus to product
vessel (23).
Similarly, (NH4)3PO4 is formed by counter current contact and reaction between NH3 rich
stream and H3PO4 to form (NH4)3PO4 on packing in the reaction section of the said apparatus,
and (NH4)3PO4 can be collected from unreacted H3PO4 in the settling section of the said
apparatus.
The apparatus of the present invention is able to obtain 100% conversion of the NH3 rich
stream comprising up to 70% NH3 by volume using aqueous acid solution having 0.5-40 wt%
of acid.
The invention is now described by way of the following non-limiting examples.
EXAMPLE-I
The process was demonstrated in laboratory. The results of demonstration run were
illustrated in Table 1 and 2. In two different studies, the concentration of H2SO4 was kept 1%
and 5% by weight. The concentration of ammonia in the feed gas was maintained from 5% to
70% by volume and the concentration of nitrogen in the feed gas was maintained 95% to 30%
by volume. The fresh acid solution flow to the rectification section was varied from 25 to 75
LPH and the solution circulation flow in the rectification section was maintained from 0 to
300 LPH.
The pressure of feed gas was maintained 0.02-0.2 Kg/cm2g. The presence of ammonia in the
outlet gas was checked by using concentrated HCl. The lifting of liquid phase in the packing
section either of rectification section or reacting section confirmed flooding.
Table 1: Results for determination of maximum ammonia concentration in feed with 1 %
(wt) H2SO4 solution
Feed
NH3
flow
Feed N2
flow
NH3
in
feed
Vent
gas flow
Pressure
drop
Fresh
H2SO4
Flow
Recycle
Flow Remarks
Cm3/Sec Cm3/Sec %v Cm3/Sec Kg/Cm2 Cm3/Sec Cm3/Sec
19.7 59.0 25.0 61.4 0.02 9.7 62.5
No Ammonia in
vent
19.7 51.1 27.8 53.2 0.01 9.7 75.0
No Ammonia in
vent
15.7 47.2 25.0 49.1 0.02 9.7 75.0
No Ammonia in
vent
39.3 47.2 45.5 49.1 0.02 9.7 66.7
No Ammonia in
vent
61.4 47.2 56.5 49.1 0.02 9.7 68.3
No Ammonia in
vent
78.7 47.2 62.5 49.1 0.02 9.7 75.0
No Ammonia in
vent
114.1 47.2 70.7 60.5 0.03 9.7 75.0
Presence of
Ammonia in vent
Table 2: Results for determination of flooding with around 60% ammonia concentration in
feed and 1 %( wt) H2SO4
Feed NH3
flow
Feed N2
flow
NH3 in
feed
Vent gas
flow
Pressure
drop
Fresh
H2SO4
Flow
Recycle
Flow Remarks
Cm3/Sec Cm3/Sec %v Cm3/Sec Kg/Cm2 Cm3/Sec Cm3/Sec
66.9 43.3 60.7 45.0 0.02 8.3 50.0
No
Ammonia
in vent &
No
flooding
62.9 51.1 55.2 53.2 0.02 9.7 68.3
No
Ammonia
in vent &
No
flooding
66.9 43.3 60.7 45.0 0.012 11.1 58.3
No
Ammonia
in vent &
No
flooding
55.1 39.3 58.3 40.9 0.01 13.2 75.0
No
Ammonia
in vent &
No
flooding
62.9 43.3 59.3 45.0 0.012 16.7 66.7
No
Ammonia
in vent &
No
flooding
59.0 35.4 62.5 48.6 0.04 23.6 75.0
Ammonia
in vent &
flooding
Table 1 shows that maximum ammonia concentration in feed can be treated by 1 %( wt)
H2SO4 solution in the apparatus of the present invention. Pressure drop was varied from 0.01
to 0.03 Kg/cm2. Maximum gas flow rate was 125.9 cm3/sec and is correspond to 2.76 cm/sec.
No flooding was observed. Ammonia was not detected in the vent. But at the same condition
ammonia was noticed at ammonia concentration of 70.7 %( v) in feed gas.
Table 2 shows that gradual increase in fresh H2SO4 solution flow to rectification section and
no flooding or ammonia slippage was observed. Ammonia slippage and flooding started at
fresh H2SO4 solution flow of 23.6 cm3/sec which correspond to 1.165 cm/sec.
Example 2
Example is illustrated considering data from a commercial unit handling NH3 rich sour gas
stream in SRU.
SRU Plant was operated under two operating conditions.
Case A: NH3 rich sour gas stream along with Acid Gas was treated in SRU
Case B: Acid Gas s treated in SRU and NH3 rich sour gas stream is treated in the apparatus of
the present invention
Steam summary & recovery efficiency for this example are listed below
Description Acid Gas
Flow
(Kg/hr)
Air
Flow
(Kg/hr)
Process
Gas from
Reaction
Furnace
(Kg/hr)
Temperat
ure in
Reaction
Furnace
(Deg C)
Pressure in
Reaction
Furnace
(kg/cm2g)
Sulphur
recovery
in
Reaction
Furnace
(%)
Case A
17723
33011
50734
1451.8
0.65
68.8
Case B
16678
28720
45398
1289.2
0.52
69.6
Results indicate that as NH3 rich sour gas stream was treated in the apparatus of the present
invention, Air requirement in SRU was decreased by 15 %(wt) which minimized blower
operating cost. Also overall process gas flow was decreased by 15 % (wt) which leads to
decrease in backpressure of the system and ensured smooth operation of SRU. Results also
indicated that there was increase in sulphur recovery in reaction furnace.
The apparatus and process disclosed in the present invention provides following advantages:
• Effective and economical treatment of sour streams
• Trouble free operation of Sulphur recovery unit and capacity enhancement
• Complete conversion of ammonia
We claim:
1. An apparatus for counter current treatment of a sour stream with an aqueous solution
of acid comprising: (a) a rectification section; (b) a reaction section; and (c) a settling
section.
2. The apparatus as claimed in claim 1, for treatment of the sour stream comprising NH3,
water vapor, hydrogen sulphide and hydrocarbons with an with an aqueous solution of
acid selected from HCl, H2SO4 or H3PO4.
3. The apparatus as claimed in claim 1, wherein the rectification and the reaction section
comprise packing.
4. The apparatus as claimed in claim 3, wherein the packing is selected from class of
random packing and structured metallic packing having surface area from 65 m2/m3 to
340 m2/m3.
5. The apparatus as claimed in claim 1, wherein the sections are convergent from bottom
with varying cross sectional area of each section.
6. The apparatus as claimed in claim 5, wherein the cross sectional area of settling
section is higher than reaction section and rectification section.
7. The apparatus as claimed in claim 5, wherein the cross sectional area of rectification
section is lower than the reaction section and the settling section.
8. The apparatus as claimed in claim 7 , wherein the cross sectional area of rectification
section is 10-40% of the reaction section.
9. A Process for treating a sour stream with an aqueous solution of mono, di and tri basic
acid in apparatus of claim 1 comprising the steps of:
(a) counter currently contacting the sour stream comprising NH3, water vapor,
hydrogen sulphide and hydrocarbons with an aqueous solution of acid selected
from HCl, H2SO4 or H3PO4.
(b) forming (NH4)2SO4 or (NH4)3PO4 or NH4Cl from reaction of NH3 with H2SO4
or H3PO4 or HCl on packing in the reaction section;
(c) collecting (NH4)2SO4 or (NH4)3PO4 or NH4Cl from unreacted H2SO4 or H3PO4
or HCl in the settling section;
(d) recycling (NH4)2SO4 or (NH4)3PO4 or NH4Cl and unreacted H2SO4 or H3PO4
or HCl to the reaction section.
10. The Process as claimed in claim 9, wherein the sour stream comprises up to 70% NH3
by volume.
11. The Process as claimed in claim 9, wherein concentration of HCl, H2SO4 or H3PO4
varies from 0.5 to 40% by weight.
12. The Process as claimed in claim 9, wherein recycling flow rate is 5-15 times higher
than flow rate of aqueous solution of acid.