19th AFA Int’l Fertilizer Technical Conference & Exhibition
18- 20 April 2006 Four Seasons Hotel
Doha- Qatar
Case Study : Potassium carbonate carryover in carbon dioxide gas going to urea plant
Mr. Bashar Al-Aradi Yield Consumption Engineer- GPIC
Bahrain
Gulf Petrochemical Industries Company(GPIC)
Kingdom of Bahrain
Overcoming Potassium Carbonate carryover Problem from Benfield Section
(Case Study)
19th AFA International Annual Technical Conference
Prepared By:
1. Mr. Khalid Al-Bin Ali/ Senior Process Engineer 2. Mr. Bashar Al-Aradi/ Yield & Consumption Engineer
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CONTENT PAGE ABSTRACT 2
1.0 Introduction 4
2.0 CO2 Removal System Performance in different periods
2.1 Prior 1989 Debottlenecking 5
2.2 Post 1989 Debottlenecking 6
2.3 Post commissioning of Urea Plant 7
3.0 Where was the problem then? 8
4.0 Proposals to minimize potassium carbonate carryover:
4.1 Installation of high efficiency demister in the knock-out 10 drum (D-8101) and washing CO2 gas with demin water. 4.2 Installation of a steam heater at the suction of the CO2 11 Compressor (K-8101) and hot gas recycle through K-8101. 4.3 Installation of three typical Thormann trays. 12
4.4 Installation of Direct Cooling Contact. 13
4.5 Installation of a new modified Thormann Tray in CO2 15 Desorber (C-0502). 5.0 Conclusion. 17
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ABSTRACT
In the Ammonia process, Carbon Dioxide (CO2) gas is generated as a by-
product and removed by absorption in a Potassium Carbonate solution in
the Benfield Absorption Column. The Potassium Carbonate solution is
regenerated in the Benfield Desorber where the CO2 gas is removed and
vented.
Prior to the commissioning of the GPIC Urea plant in 1998, part of CO2 gas
from the Ammonia process was used in the Methanol plant to increase
production. After commissioning Urea plant, all the CO2 gas is consumed in
the production of Urea.
At the end of 1998, increased Potassium Carbonate carryover in the CO2
gas to the Urea plant was experienced. This resulted in the deposition of
solid Potassium Carbonate on the internals of the Urea plant CO2
compressor, especially on the impellers. These deposits caused an increase
in vibration in the compressor resulting in the Urea plant being shut down
to wash the compressor internals with water. The deposits were observed
to be severe in the low pressure (LP) casing (first and second stages) with
fewer deposits found in the third and fourth stages of the high-pressure
(HP) casing.
This paper highlights the causes of the problem and discusses a number of
suggested remedial solutions considered to minimize the carryover problem
as summarized below:
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1. Replacement of the first separator demister in the CO2 Compressor.
2. Washing CO2 gas with demin water.
3. Installation of steam heaters on the compressor suction.
4. Hot gas recycle in the CO2 compressor.
5. Installation of three Thormann trays.
6. Installation of Direct Contact Cooling System.
7. Installation of new modified washing tray (Thormann tray) in the
Benfield Desorber.
Some of the proposals were implemented, some were rejected. The
reasons for implementation and rejection will be discussed in this paper.
Subsequent to the implementation of some of the above proposals, a
significant reduction with respect to Potassium Carbonate carryover was
noticed with minimum deposits in the CO2 compressor. The proposal that
contributed most substantially to solve the carryover problem was the
installation of the new modified Thormann tray in the Benfield Desorber.
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1.0 Introduction
Gulf Petrochemical Industries Company (GPIC) was formed in 1979 as an
equal partnership between the government of Bahrain, the government of
Saudi Arabia representative by (SABIC) and the government of Kuwait
representative by (PIC) with the objective of utilizing Bahrain’s natural gas
resources for the production of petrochemicals.
GPIC’s first project was a grassroots petrochemical complex at Sitra Island
on the North East coast of Bahrain. A site of 60 hectares was successfully
reclaimed from the sea to construct the Ammonia and Methanol plants,
along with related Utilities and offsite facilities. The original plant capacity
was 1000 MPTD of each product. In 1989 the Ammonia and Methanol
plants were debottlenecked to increase production to 1200 MTPD of each
product.
Part of the debottlenecking of the Methanol plant involved the diversion of
CO2 gas generated in the Ammonia process. The CO2 pressure was boosted
by a reciprocating compressor (K-1403) prior to entering the Methanol
synthesis loop.
In January 1998, GPIC commissioned a 1700 MTPD granular Urea plant
downstream of the Ammonia plant. All the CO2 gas vented from the
Ammonia process was channeled to Urea plant and none was available for
the production of Methanol.
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2.0 CO2 Removal System Performance in different periods
2.1 Prior 1989 Debottlenecking
Prior to 1989 debottleneck project, small quantity of CO2 gas from
Ammonia process, Benfield section, was channeled to the Methanol plant
(See figure #1). The pressure of the CO2 gas stream was boosted by a
reciprocating CO2 compressor (K-1403). The addition of CO2 gas boosted
Methanol production without any trouble and resulted in very steady
operation of the Methanol synthesis loop. However, a decision was taken
by GPIC management to increase the capacity of Ammonia Plant as well as
for Methanol Plant. After the debottleneck project, all units in both
Ammonia and Methanol Plants handled 20% higher load to achieve the
targeted output 1200 MTPD for each product.
In an assessment study prior to the debottlenecking project, the feasibility
of high load was investigated. Equipments were provided in parallel where
necessary, to cater for the 20% increases in flow rate demanded by the
debottlenecked conditions.
During this investigation the Benfield units were studied to check the
adequacy of both the CO2 Absorber (C-0501) and CO2 Desorber (C-0502)
to handle the additional flow rates.
The study revealed that both units would operate close to transition or
borderline conditions due to increase in gas velocities inside the columns. It
was also noted that minor Potassium Carbonate carryover with the CO2 gas
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may occur under these conditions. The magnitude of this carryover, in view
of the borderline operating conditions, was not determined.
Moreover, there was no concern for the potential adverse effects on the
positive displacement compressor (K-1403) as it was well known that this
type of machine is less sensitive to minor deposits of solids, compared to
high-speed centrifugal compressors.
2.2 Post 1989 Debottlenecking
Subsequent to the debottlenecking the target production of 1200 MTPD of
both Ammonia and Methanol was achieved and all units, including the
Benfield section and the CO2 reciprocating compressor, ran steadily and
reliably.
It was noticed during routine maintenance of the CO2 compressor (K-1403)
that some Potassium Carbonate had deposited in the compressor inlet
chambers and around the pistons. It was assumed that this was the result
of operating at borderline conditions in the Benfield section. As this did not
cause any limitation in the operation of the compressor (K-1403), it was
disregarded.
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2.3 Post commissioning of Urea Plant
Urea Plant commissioned in January 1998. The incoming CO2 gas from
Ammonia Plant is cooled in a CO2 Cooler exchanger (E-8101) and any
condensate is removed in the knock-out drum (D-8101). The CO2 gas is
then compressed in a four-stage centrifugal compressor (K-8101). After
that, the CO2 gas goes to the Urea reactor where it is reacted with
Ammonia to form Urea Carbamate.
After Urea plant commissioning, the requirement for CO2 gas rose to a level
which demanded nearly all the CO2 gas stripped from the Potassium
Carbonate solution in the Benfield section. This led to a decision to divert
all the stripped CO2 gas from the Benfield Desorber directly to the Urea
plant. Thus no CO2 gas was fed to the Methanol plant any longer and the
CO2 compressor (K1403) was taken out of service (See figure 2).
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3.0 Where was the problem then?
After several months of operation following the initial start-up, the CO2
compressor (K-8101) started showing abnormal vibrations and high
displacement levels. The plant was taken off line and the compressor
inspected internally.
Very heavy deposits of Potassium Carbonate were found in the first and
second stages of the compressor, with smaller deposits in the third and
fourth stages. It was obvious that for high-speed centrifugal compressors
the carryover and deposition of solid material on impellers is critically
detrimental, leading to vibration levels that could affect the safety and
integrity of the machine. It also resulted in serious production loss as the
only remedy was to take the plant off line and clean the compressor.
This issue became crucial to the reliable and economic operation of the
Urea plant. A task force was formed to explore various options and to
evaluate the feasibility of implementing the solutions proposed prior to the
complex turnaround in September 2000.
The following remedial solutions were considered to minimize the carryover
problem:
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1. Replacement of the first separator demister in the CO2
Compressor.
2. Washing CO2 gas with demin water.
3. Installation of steam heaters on the compressor suction.
4. Hot gas recycles in the CO2 compressor.
5. Installation of three typical Thormann trays.
6. Installation of Direct Contact Cooling System.
7. Installation of new modified washing tray (Thormann tray) in the
Benfield Desorber.
Some of the above proposals were implemented, some were rejected. The
reasons for implementation and rejection are discussed below.
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4.0 Proposals to minimize potassium carbonate carryover
4.1 Replacement of the first separator demister with high
efficiency demister in the knock-out drum (D-8101) and Washing
CO2 gas with demin water
The proposal was to wash out the entrained potassium carbonate by
installing a demineralized water sparger upstream the CO2 Cooler
exchanger (E-8101) and then separate the condensate in CO2 Knock out
drum (D-8101) using high efficiency demister(See figure#3).
4.1.1 Expected Benefits:
The expected benefits were:
1. Dissolving potassium carbonate in demin water.
2. Improving the separation efficiency in CO2 Knock out drum.
The existing demister type (York 421) had been designed to remove 99.9%
of particles up to 10 micron in size, while the new demister type (York 709)
was designed to remove 96.4% of particles up to 5 micron in size and
62.6% of particles up to 3 micron in size.
4.1.2 Post-Implementation Results
The new demister was installed in June 2000 while the demineralised water
sparger was commissioned during July 2000. The combined effect was a
marginal improvement. The Urea plant could be run for approximately 3
months continuously, compared to 2 months prior to the modification.
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4.2 Installation of a steam heater at the suction of the CO2
compressor (K-8101) and hot gas recycle through K-81O1
It was proposed to install steam heaters in the first and second stage
suction lines of the CO2 compressor (K-8101) to heat the CO2 gas stream,
which is 100% saturated with water, through the compressor (See figure#
4).
4.2.1 Expected Benefits
By using steam as indirect heating medium for the above exchanger, the
temperature of CO2 gas would be raised by 1 or 2 °C above saturation
temperature, hence maintaining superheated conditions and preventing
condensation inside the compressor.
This proposal was discarded as the risk of steam entering the CO2 gas
stream in the case of any severe leakage took place. As an alternative to
steam heating, it was proposed to recycle hot CO2 gas from the second
stage discharge to the first and second stage suction lines, effectively
achieving the same temperature increase of 1 to 2 °C (See figure# 5).
4.2.2 Post-Implementation Results
This proposal was implemented during 2000 Turnaround. Subsequent to
plant start-up, the temperature increase of about 2°C in the suction of both
the first and second stages was achieved, which contributed to the
reduction of Potassium Carbonate deposition to some extent.
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4.3 Installation of three typical Thormann trays:
It was proposed to install three bubble cap trays in series at the top section
of CO2 desorber.
4.3.1 Expected Benefits:
By increasing the number of bubble cap trays from one to three, the
contact between the two phases will increase i.e. CO2 gas washing contact
will increase and hence the potassium carbonate will decrease due to
sufficient washing.
4.3.2 Analysis:
The proposal was rejected based on the following facts:
1. The vertical space in the desorber is not sufficient.
2. Involve a major modification such as cutting and welding of the
column, which would require long period of shutdown.
3. The additional height will increase the load on the structure whose
design should be reviewed and modified.
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4.4 Installation of Direct Cooling Contact:
Its purpose is to scrub K2CO3 with process condensate and fresh make-up demin water in a packed tower to get clean CO2 gas. It will also cool the gas from 85 oC to 40 oC (See figure# 6). The CO2 gas exit of the scrubbing column will follow the normal path going to CO2 compressor of Urea Plant via existing CO2 cooler (E-8101) and separator (D-8101). The condensate from the bottom of the tower is cooled in an indirect cooler exchanging heat with sea water and fed back to scrubber as a scrubbing media. About 20 m3/hr of condensate will be sent to the stripping unit of Utility section for further polishing and recycling. The stripping unit in Utilities Section is adequate to handle the additional 20m3/hr of condensate. However, the liquid inlet distributors to the stripping column will have to be replaced as they will reach their capacity limits. One of the major concerns of this project would be the provision of about 1000 m3/hr of sea cooling water which can not be met during summer months with 5 sea water pumps running. If the 6th sea water pump is also operated then it would mean no stand-by pump will be available which is not acceptable. Hence to meet this additional sea water requirement, a 7th spare pump will have to be installed.
4.4.1 Expected benefits:
1. Eliminate K2CO3 carryover to CO2 Compressor and hence prevent
Urea Shutdowns.
2. Cooling the CO2 gas to around 40oC as against design temperature
of 50oC.
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4.4.2 Analysis:
Based on the above facts, the payback period was worked out assuming the worst operating scenario of frequent Urea Plant shutdowns and low load operation due to excessive carryover of K2CO3 in CO2 gas. During year 2000, total of 25,402 MT of Urea product was lost due to K2CO3 carryover problem, at the rate of 1700 MTPD Urea production. This is equivalent to 15 days production loss. Based on the above 15 days production loss per year, and the high budgetary cost of the project, the payback for the project worked out to be 5.7 years. The proposal was not attractive from economic point of view; therefore, it was frozen until considering the last proposal of installing new modified Thormann tray in CO2 desorber and evaluating its results.
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4.5 Installation of a new modified Thormann Tray in CO2 Desorber
(C-0502)
This proposal addressed the root cause of the problem by considering an
improved design of the Thormann bubble cap tray utilized for washing the
CO2 gas stream leaving the Benfield Desorber (C-0502).
4.5.1 Expected Benefits
The elimination, or at least minimization, of the Potassium Carbonate
carryover from the Benfield Desorber (C-0502) by the reduction of the
velocity of the desorbed CO2 gas through the bubble caps of the Thormann
tray.
The following comparison table shows the difference between the old
bubble cap and the new one:
Description Original Tray New Modified Tray
Active area (m2) 10.07 12.04
Number of bubble caps 155 250
Downcomer shape Envelope Pipe
Jet Flood Percentage 101.25 85
Material of Construction SS 304 SS 410
Pressure drop (mmHg) 5.86 3.57
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4.5.2 Post-Implementation Results
This modification was implemented during Turnaround 2000 and since then
a noticeable reduction in the carryover of Potassium Carbonate has been
observed. No forced shutdown due to deposits of Potassium Carbonate in
the CO2 compressor has been experienced since that time. It is worth
mentioning in addition, that when the Benfield Desorber (C-0502) was
opened to replace the old Thormann tray, the tray was found slightly loose
with some sections displaced. This could very well have been the result of
operating the column at borderline conditions before. It also justifies the
replacement of this tray with a similar one of higher capacity. This
replacement has finally cured the Potassium Carbonate carryover problem.
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5.0 Conclusion
Of the various proposals to overcome the problem of Potassium Carbonate
carry-over from the Ammonia plant’s Benfield Desorber (C-0502) which
was affecting the operational stability of the CO2 compressor in the Urea
plant, the replacement of the Thormann Tray with one of improved design
and higher capacity proved successful in solving the root cause of the
problem. In addition, all the other modifications resulting from the other
proposals benefited the situation marginally.
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Figure #1: Benfield flow diagram before de-bottlenecking project
Air Coolers
F-0503Separator
C-0502Desorber
F-0506Separator
E-0507CO2 Cooler
F-0505Separator
K-1403CO2 compressor for
Methanol Plant
Lean Solution outlet
CO2 Rich Solution inlet
CO2 Gas
Demister
Thormann Tray
E-0504Deionat
Preheater
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Figure #2: Benfield flow diagram after commissioning of Urea Plant
Air Coolers
F-0503Separator C-0502
Desorber
F-0506Separator
E-0507CO2 Cooler
F-0505Separator
K-1403CO2 compressor for
Methanol Plant
Lean Solution outlet
CO2 Rich Solution inlet
CO2 Gas
Demister
Thormann Tray
E-0504Deionat
Preheater
K-8101CO2 compressor in Urea
PlantD-8101
Knock Out Drum
E-8101CO2 Cooler
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Figure #3: Proposal of installing a modified demister and a water sparger.
D-8101Knock Out Drum
E-0507CO2 Cooler
K-8101CO2 compressor in Urea
Plant
E-8101CO2 Cooler
Installation of water sparger
Replacing the Demister with Improved type
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Figure #4: Proposal to install Steam Heaters
1st2nd
3rd4th
K- 8101
CO2 GAS IN
CO2 GAS IN
Proposed Steam
Heaters
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Figure #5: Proposal to recycle hot CO2 gas
1st2nd
3rd4th
K- 8101
CO2 GAS IN
CO2 GAS IN
Proposed routing
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Figure #6: Proposal of Direct Cooling Contact
Demin Water
CO2 Gas Inlet
CO2 Gas outlet
Sea Water
