Authors:
W. EL-MOUDIR, M. EL-BOUSIFFI, Libyan Petroleum Institute, Tripoli, Libya
S. BSEBSU, M. IBRAHIM, Sirte Oil Co., Tripoli, Libya
This article was first published at Nitrogen and Syngas Conference, 25-28 February 2007, Bahrain
A case study was conducted to investigate the reasons behind a decline in the handling capacity of a CO2 compressor in a 1,000-tonnes/day urea plant. The suspected sources and the problem route were investigated. Preliminary investigations revealed that the compressor internals were fouled with deposits of white powder. Laboratory analysis showed the presence of potassium salts. These deposits are believed to be responsible for this decline in the compressor capacity and, consequently, reduction of the urea plant productivity.
The compressor is cleaned periodically, either during turn-around or when the productivity falls sufficiently to force a shut-down. Deionised water (during turn- around, every two years) or on-line LP steam (during normal operation period) is used for washing and cleaning. A forced shut-down for one day results in a direct loss of production worth thousands of US dollars (urea price > $200/tonne).
The CO2 feed to the urea plant is supplied from a Benfield acid gas removal section in the ammonia plant, which uses hot potassium carbonate solution as the absorbent. The study methodology to identify the main causes of problem is presented along with brief details on process equipment inspection and evaluation, laboratory analysis and process simulation. Main findings and a plan of future work are presented.
Keywords: Urea plant, Benfield process, Carry-over phenomenon, Operational analysis
INTRODUCTION
The CO2 removal unit was operating satisfactorily in respect of the CO2 removal capability but was performing unsatisfactorily with respect to the purity of the CO2 gas being sent to the urea plant.
Ammonia process
Ammonia is produced from water, oxygen and natural gas (Fig. 1). Ammonia plants are commonly integrated with other plants, particularly with nitrogen fertilizer and, especially, urea plants, which make use of the by-product CO2 produced in the ammonia process. In ammonia plants, there are several purification stages and reactions in catalytic reactors that are the key to successful and economic operation. CO2 has to be removed as it acts as poison to ammonia catalyst.
FIG. 1 CHEMISTRY OF THE AMMONIA PROCESS [1,17]
Benfield process (CO2 removal system)
The potassium carbonate to CO2 absorption has been known for many years. Benson and Field owned a patent on a hot potassium carbonate process that was called the Benfield process [2]. The absorption and desorption are based on the following reaction:
K2CO3 + CO2 + H2O = 2KHCO3 + Heat (1)
The existing Benfield process consists of a packed absorption column to absorb CO2 in the lean Benfield solution (Benfield solution contains aqueous potassium carbonate/bicarbonate, vanadium and diethanolamine; see Fig. 2). The CO2-rich solution from the absorber is then stripped of its dissolved CO2 with steam in another packed column (regenerator column) to produce lean solution. The outlet process gas from Benfield section absorber has a very low CO2/CO concentration (0.1-0.3%).
The desorber has one washing tray at the top, situated above the packed bed, and utilizes part of returned condensate to wash the outlet CO2 gas from any entrained solution. A mist eliminator is also provided to remove entrained liquids with gas. The CO2 gas is then passed through heat exchange coolers, then through the condensate separator unit. The CO2 now has high purity and is ready to deliver to urea plant.
FIG. 2: BENFIELD SECTION SCHEMATIC WITH MAJOR EQUIPMENTS
Operational history of the ammonia plant
The plant was commissioned in 1978 with design capacity of 1,000 t/d and revamped in 1991 to increases the production capacity to 1,200 t/d. In the early 1990s the company which carried out the upgrading claimed in its feasibility study of upgrading that the Benfield section was capable of handling the upgrade without need for any major modifications. The quality of CO2 gas was not changed significantly but the total volumetric flow rate was increased by 3.4% (wet basis) of the original flow rate.
In view of the long-term successful operation of the ammonia plant, there was nothing to suggest a problem related to potash carry-over. The carry-over phenomenon was not noticed either in the CO2 stream from the desorber or in the process gas stream from the absorber. On the other hand, the problem was noticed in the urea plant, which receives the CO2 from ammonia plant.
UREA PROCESS
Urea is produced by combining ammonia and carbon dioxide at high pressure (140 bar) and high temperature (180-190°C) to form ammonium carbamate, which is then dehydrated by heat to form urea and water, according to the following reaction:
2NH3 + CO2 → NH2COONH4 → CO(NH2)2 + H2O (2)
Pressurised CO2 is a vital feed for the urea operation.
Operational history of urea plant
The urea plant was commissioned in 1981. Since start-up, the plant has rarely been shut down for internal reasons. However, during turn-arounds small quantities of potash deposit were noticed in the CO2 compressor, but they were not causing much concern as they had no discernible effect on the operation load. But ever since the upgraded ammonia plant came back into operation in 1991, there has been a problem with heavy potash deposits which have even necessitated unplanned shut- downs in some cases.
A number of modifications have been implemented in an attempt to overcome the problem, but without sign of success. These measures include modification of the CO2 supply pipeline, such as modified drains and installation of traps to provide better drainage of condensate. Furthermore, an impingement plate was installed in the gas inlet nozzle in the upstream knock-out drum of the CO2 compressor. Yet, there was no improvement in the situation. During turn-around (planning shut-downs) since 1996, and whenever the CO2 compressor is shut down; the low-pressure section was cleaned with LP steam as a temporary solution, as per in-house procedure. This washing avoids the need to open the compressor for cleaning and assures restoring the compressor performance. This temporary solution only improves the performance for a while, as the performance is gradually lost within the three months after cleaning. Moreover, during every available opportunity, the compressor is cleaned in order to maintain its performance as steady as possible.
POTASH CARRY-OVER: PROBLEM STATEMENT
The potash transfers from CO2 desorber in the ammonia plant up to CO2 compressor in the urea plant. It has been reported that the CO2 compressor experiences deviation from design values with compressor vibration. Its efficiency habitually deteriorates over a relatively short period of operation after cleaning and maintenance jobs, roughly three months. This leads to load limitation for the compressor, which results in a drop in urea production. Ultimately, it leads to a forced shut- down for cleaning purposes. Repetition of this cycle of shut-down, cleaning and start-up is burdening the plant equipment and reducing production. At the urea plant turn-around, during maintenance on the compressor, smooth deposits of white scale layer inside the compressor in the low pressure section (first and second stages); see Photo 1.
Photo1: Deposits on the compressor internals
In order to maintain stable operating conditions, reduction of the CO2 flow rate to the compressor becomes essential. Moreover, increase of compressor energy load to overcome capacity limitations is applied. As a result, the urea plant productivity is reduced gradually. Chemical analysis of the deposits confirmed the presence of potassium carbonate (K2CO3) and bicarbonate (KHCO3) salts which are defined as potash (Table 1). The chemical composition is similar to the absorption solution used in Benfield section.
POTASH PRESENCE ALONG CO2 PATH: INDICATION AND MONITORING
The monitoring program is applied for tracing and tracking potassium ion transfer. It has been found that potash is available in profile along the CO2 pipeline and scrubbing facilities from the ammonia plant up to the urea plant. The analysis of potash presence in CO2 path defined, as presence of potassium ions (K+) in water or condensate, has been carried out on a regular basis to follow up the problem. Some analyses may be done on an hourly basis when the problem starts to affect the compressor. The illustration in Fig. 3 confirms the presence of a potash profile in different locations. It covers the area of the CO2 transfer line from the desorber in the Benfield section up to the compressor in the urea plant. Clearly, CO2 passes many scrubbing, separation and washing facilities before it enters the compressor. However, potash is present before the compressor in unexpected concentration.
FIG. 3: PRESENCE OF POTASH IN CONDENSATES AND CIRCULATED WATER
Under normal circumstances, and as designed, the potash concentration should be 6 ppm or less in the first scrubbing unit downstream the CO2 desorber (in Fig. 3, it is 2.5 ppm) and should be nil in the last scrubbing unit before the CO2 compressor directly (in Fig. 3, it is 36 ppm). Fig. 3 shows average values and they are usually found in neighbourhood of these values.
ANALYSIS OF PROCESS AND SCRUBBING UNITS ALONG CO2 PATH
Evaluation of the process and scrubbing units along the CO2 path are essential to check whether they are involved in the potash carry-over problem or not and to what extent they are. These units handle CO2 at different stages and conditions from the ammonia plant to the urea plant. The evaluation was done separately and will be illustrated as follows: CO2 compressor, knock-out drum, CO2 pipeline, CO2 cooler and finally to the CO2 desorber.
CO2 compressor
The carbon dioxide compressor is one of the most critical equipment items in the urea plant, as urea production based on a chemical reaction between NH3 and CO2 under high pressure. A centrifugal compressor with four stages and intercoolers is used to compress the incoming CO2 from the ammonia plant, which is relatively close to atmospheric pressure (1.10 kg/cm2a) and at 39ºC, up-to high pressure (143 kg/cm2a) and 125ºC). The output conditions are the feed conditions of the urea production process. Because of cooling, a very small amount of water would be condensed as a result of variations in operation conditions between stages. Obviously, the CO2 feed stream is saturated with water. The compressor consumes power through its steam turbine. To develop the required gas velocities and head, impellers must rotate at very high speeds, which make design of the compressor components (e.g. driver, gear) and its operating conditions extremely critical. The operating data were used to calculate the efficiencies of the individual stages (Table 2).
The problem can be noticed from the presence of potassium ions in condensates, increase in com- pressor vibrations, and variation of operating conditions, as reported (Table 3).
No recommendation can be given, as the compressor is not the sources of problem but it suffers from it.
Knock-out drum
A knock-out drum is provided to eliminate mist entrainment in the CO2 gas fed to the CO2 compressor (see Fig. 3). This drum is a vertical vessel housing a 150 mm thickness mist eliminator. Checking the design is demanded to know whether this unit functions correctly or not.
Drum size
Typical dimensions for standard knock-out drums cannot always be generalized for all drums and in some cases specific design parameters make the drum unique. However, a good way is to compare the knock-out drum with typical drum dimensions. Four sizing methods were used in this study (the first and second are called the sizing of separator [2,4], the third is called the selecting gas/liquid separator method [5] and the fourth is the knock-out drum design method [6]).
They produced good demission values and specially the first and second methods. However, some deference values were also found for the other two methods regarding the disengagement. The disengagement height is less than typical values of both methods. The minimum height should be at least half the tower diameter and this is important because it reduces non-uniform gas flow through the mesh pad. However, this comment is disregarded as the four methods prove the drum design satisfactory.
Operating vapour load
The mist eliminator is an important device for holding back entrained liquid droplets. The superficial velocity of gas should be lower than 4 m/sec so as to prevent any re-entrainment from this device. The pressure drop generated across the wire mesh is usually very small; however, it should be monitored but in the existing drum there is no means to do that. Wire mesh may become partially clogged with time, partially or fully flooded. Therefore, it can malfunction due to one of the following suspicious effects:
- Damage to the mist eliminator
- Blockage of mist eliminator,
- Entrainment at high loads (high gas velocity),
- Unsuitability of mist eliminator.
The mist eliminator pad is accommodated with a 150 mm thick bed, which is known as a prefect typical size [5]. The first and second points were declined as inspection confirmed not only that that there was no damage or blockage but even that it was clean.
Entrainment can be very high and this depends on the droplet size that can escape from the mist eliminator. Drum height enhances elimination of small droplets in vapour space. The mist eliminator provides the finishing function for the elimination of mists of very small droplets. The size of minimum droplet can be determined as follows:
Determination of minimum droplet size settle down by gravity can be found by using Stoke’s law [3, 5]. At normal operating vapour velocities, the minimum droplet size that cannot settle is 200µm, and anything smaller than that size will be carried out with vapour. The mist eliminator provides a last barrier for those smaller-than-200 µm droplets.
The key design variable for entrainment in separation vessels is the vapour load factor “K value” which was derived by Souders and Brown [4, 6, 7]. Their derivation is based on force balance calculation on a droplet falling through the vapour space.
K = V.(ρV / (ρL- ρV))0.5
where;
K = Load factor (m/sec)
V = Vapour velocity (m/sec)
ρV and ρL = density of vapour and liquid (kg/m3)
For normal operation, the K loading factor is 0.061 m/sec. The K-value for a vertical knock-out drum is typically designed to operate between 0.061 and 0.106 m/sec which is the optimum range [5].
The mist eliminator might flood during operation if the velocity goes beyond the designed value (1.41 m/sec), leading to high loads mentioned in third point of suspicious phenomena. In our case, the velocity is maintained below this value (i.e. 1.41 m/sec), which confirms that the tower design and operation is acceptable. Vapour velocity as a cause of flooding was checked with recent correlations on mist eliminator performance [8]. The results show the operation is far from flooding. The operating pressure drop is estimated to be 0.001089 kg/cm2. Therefore, the third suspicious point is declined.
It is possible to estimate the droplet size that cannot be captured by the mist eliminator by using Fig. 4. Typically, a 150-mm mist eliminator cannot capture the average drop size of 10 µm and below [5, 6]. From Fig. 4, the droplets sized in the range 50-4 µm cannot be eliminated from going to the CO2 compressor. The entrainment quantity estimated from Fig. 4 would be somewhere between 0.028 and 0.28 mg/m3 (540.8-5,408.7 mg/h).
Assuming that this amount of entrainment would contain some dissolved potash in various concen- trations, as indicated in Table 4 (in wt-% potash), the concentration range is assumed from very low and up to the salt concentration of potassium carbonate absorption solution in the Benfield process.
FIG. 4: ENTRAINED CONCENTRATION OF WATER AT ATMOSPHERIC CONDITIONS [5]
Generally speaking, entrained liquid droplets have potash concentrations varying from very small up to 30 wt-% (Table 4); although the CO2 gas passes through many scrubbing units. The calculation result was in agreement with estimated values of potash flow rate (1,440 mg/h potash) reported by the urea plant. This shows that the fourth subspecies point is most likely the case. The mist eliminator is not suitable for eliminating potash from going to the CO2 compressor; therefore it should be changed or upgrade to better type.
CO2 pipeline
A 1,500-m long pipeline carries the CO2 gas (at a relative humidity of 100%) from the ammonia plant to the urea plant. This pipeline is protected from direct sun and rain, and it is thermally insulated to minimize temperature losses and the effects of draughts on the CO2 temperature (the inlet temperature is around 45ºC and at outlet is 39ºC). There are condensate drains (traps) at ten points along the pipeline to remove condensate. Inspections confirmed that all the drains were working properly. The prediction results have shown that the water condensate should eliminate any potash at the first traps in the line leaving nothing to be further carried out with the gas stream, even in the case of a large amount of potash associated with CO2 gas stream. There was an assumption that CO2 pipeline might be fouled with potash or potash precipitated in large amounts somewhere along the pipe. This possibility was dropped as the internal inspection showed no sign of any accumulation in the pipeline.
CO2 cooler
The function of the CO2 cooler is to cool and wash the CO2 gas. This tower is responsible for eliminating any potash that might be available in the CO2 stream. The CO2 cooler is a packed tower with polypropylene Raschig rings as the packing material. The CO2 gas outlet should be around 45ºC (see Fig. 5).
FIG. 5: CO2 COOLER AT NORMAL OPERATING CONDITIONS
Performance evaluation of CO2 cooler
1- Packing pad
Raschig rings are widely used packing as they are usually cheaper per unit of cost but sometimes less efficient than many other types. This lower efficiency is due to the occurrence of considerable channelling, which directs more liquid to the walls of the tower rather than distributing it equally. Dumping of Raschig rings is flexible for in loading, operation and handling of dirty fluids. For better design, the column diameter divided by packing size ratio should be greater than 30. In existing system it is 54 and fully satisfactory [9].
2- Trough liquid distributor
This type of distributor has capability of handling large vapour loads and it allows for good liquid distribution. This type of distributor is usually used with structured packing rather than random dumped packing. This is due to the desire to have an overflow sheet of liquid on the packing. It has been mentioned [7, 9] that this type of distributor cannot produce uniform distribution over the packing and especially in the area near or next to tower walls. Poor distribution reduces the effective wetted packing area and promotes liquid channelling. In the dumped conditions most packing follows a conical distribution down the tower, with the apex of the cone at the liquid impingement point. After about 3.5 m of vertical height [5], the liquid tends to flow vertically downward unless redistributed. To overcome this flow behaviour a better and more efficient packing is recommended.
3- Pressure drop and flooding condition
The pressure drop and flooding condition can be determined based on the generalized pressure drop correlation (GPDC) for packed beds. The check of operability at different loads to improve the performance of separation was investigated. The flooding factor and pressure drop are
within safe values even if the water flow rate is increased from 120 m³/h up to 220 m³/h. Entrainment would be enhanced by flooding conditions, and as the flooding factors predicted within normal range, entrainment would be negligible. There is agreement between pressure drop measured and predicted.
4- Elimination of potash
The active area available for contact between water and CO₂ is small (it may be due to the amount of gas that escapes without washing or to liquid channelling). This could cause the escape of potash in the gas outlet and increases the gas outlet temperature, which is what was noticed on site. Calculations were carried out to test a number of possibilities. Fig. 6 shows the tower system with water make-up flow rate.
FIG. 6: PREDICTION RESULTS WITH DEMINERALISED WATER MAKE-UP
Clearly, no potash can escape from the tower, however much comes in with the CO2 gas. The possibility of potash escape from this tower is very small (all potash will be dissolved in water). Therefore, potash carryover should be eliminated even if large amount of potash is associated with CO2. This is not the actual situation on site.
5- Mist eliminator and liquid entrainment
As a recommended rule of thumb and to avoid liquid entrainment with the vapour out of the tower, the K load factor value is used. If the tower operates at a vapour velocity beyond normal operation velocity of 1 m/sec, it will overload, leading the mist eliminator to be flooded and enhancing heavy re-entrainment.
The droplet size that can be captured by mist eliminator can be determined by using Fig. 4 illustrated previously in the evaluation of the knock-out drum. As mentioned, the 150-mm mist eliminator cannot capture drop sizes below 10 µm [5, 6]. From Fig. 4, the droplets sized in the range 50-4 µm cannot be prevented from going to the CO2 compressor.
6- Disengagement height
This can be checked by using the sizing method given by Amistco [4]. The current height is 0.75 m and in agreement with the Amistco sizing method (≥ 0.6 m).
How to improve CO2 cooler performance
The water flow rate can easily be increased up to 200 m3/h to improve washing efficiency without any modification. The pump and current sea water cooler can handle the increase of circulating water flow. The design and rating tool from Alfa Laval (webCAlc) was used to simulate this unit [10]. There would be an increase in heat load on this unit. That means there is a necessity for extra heat transfer area (perhaps by installing extra plates) or by installing another plate-and-frame heat exchanger. Normally, this type of heat exchanger can be increased in its capacity by 16% of original designed [10].
Benfield CO2 desorber evaluation
An evaluation of the Benfield section at the ammonia plant was carried out. This tower must be the original source of the potash carry-over problem. Investigations concentrated on finding out why potash solution is being carried out in the CO2 stream. Process simulation provides the answer. Evaluations of both the absorber and the desorber were done and compared with current operating data mentioned in the plant data records.
A number of operating scenarios were investigated to help in identifying any unseen reasons in the process behind the problem. Results have been obtained after many trials. It was concluded that this tower is very sensitive to any small change in the operating conditions such as heat loads from the reboiler or temperature variations or even the pressure profile across the column.
The GPDC sizing method was used to check sizing of design and operation of this tower. Sizing results were within acceptable ranges with respect to flooding and pressure drop for both original and revamp designs. The tower is fully capable of handling liquid and gas loads in the original design and in the revamp case. The foaming tendency of the solution was monitored and found to be within acceptable level, although there is a small amount of degradation of DEA.
Maloperation checking
After revamp the desorption tower seems to have been operating on the borderline of its design capacity. The increase of vapour load and circulation rate was applied. However, the desorber ought to have been able to handle the revamping case easily. It was therefore checked for maloperation as follows.
1- Mechanical configuration of outlet nozzle and mist eliminator
The outlet nozzle location plays a great role in the vapour flow behaviour. In our case, the side outlet can enhance disturbances of the flow which might cause local re-entrainment on the mist eliminator (due to a non-uniform velocity profile on the top section).
To overcome this, two options are possible:
- Move the outlet nozzle to a central position in the top section; or
- Improve the mist eliminator to prevent velocity variation and local entrainment.
However, the first option requires investment and many modifications which are not possible in the current situation. The second option is worthy of further investigation. Special design of mist eliminator at different thickness should be applied but no information was found on such a type of mist eliminator.
FIG. 7: AMMONIA DESORBER TOP SECTION
In general, mist eliminators are typically capable of eliminating very small liquid droplets (i.e. 10 microns). The pressure drop across a wire mesh pad is sufficiently low to be considered negligible. The mist eliminator is efficient only when the gas velocity is low enough that re-entrainment of the coalesced droplets does not occur. If the flow becomes high enough above normal value of 2.14 m/sec (overloading), the re-entrainment would occur as the mist eliminator would be flooding.
2 – Disengagement height
The disengagement height from the top surface of the mist eliminator up to the central gas outlet nozzle is 0.95 m, which is far short of the values recommended elsewhere: 3.1 m [4], 2 m [5], 2.5 m [6], and 1.82 m [11]. This cannot be improved as it would required radical mechanical modification to the tower.
3- Desorber operation
Operating conditions and equipment malfunction might be a reason behind the carry-over problem. The conditions that can give an indication are an increase in the process gas flow rate and/or a change in tower temperatures. These changes might be a consequential result of upsets elsewhere in the ammonia plant as a whole (e.g. a change of process gas flow rate).
Temperature and pressure at the tower top section have a significant influence on the stability of operation. Top section pressure is usually well controlled through controlling gas outlet flow. However, pressure build-up can be experienced at very high flow rates and in very short periods of time. This increase will create back pressure to the desorber. The pressure controllers will respond immediately to reduce any change in pressure to the normal value. Even a slight increase in pressure aggravates an increase in temperature, and this can generate huge load differences in operation. Furthermore, the expected sequence of changes that might lead to the overload could be assumed as follows:
- Change of heat load supplied to the reboiler due to changes in temperature or flow rate of process gas coming from the shift reaction section;
- Change of temperature profile in the tower leading to extra water evaporation (high quantity of flow could be generated);
- High load build-up as well as the pressure in the downstream Pressure increases and control system responses to reduce this increase in pressure.
- Operating at high pressure leads to extra water to be evaporated with respect to that condition. Sudden carry over of solution and washing water due to flashing occurs,
- Overloading of the downstream equipments occur due to overflow rate (high-velocity gas flow) moving as plug flow.
Alternatively, it could be the other way around. Pressure change may occur first, rather than change in the heat load supply to the tower. Table 5 summarises results of a number of scenarios on desorber performance. Depending on both top pressure and heat load supplied to desorber, the operating performance of the tower can be predicted (i.e. outlet conditions obtained). In all cases, CO2 flow rate is constant and only water causes changes.
As can be seen, over heat load supply can increase the outlet flow rate considerably as well as the outlet temperature with top section pressure. Water evaporation is always the result of this increase, as it evaporates significantly with temperature change corresponding to the pressure operating at that situation. The optimum value for removal of CO2 and evaporation of water should be determined at each operation scenario.
* This is the original reported operating condition according to the design.
Flooding conditions might be experienced even for very short periods of time in the top section. It can be concluded that change in operation is the preliminary reason for malfunction and potash carry-over. The higher load will affect handling capacity of all downstream equipments as they will be under overloading situation.
4- Washing section
The washing section has one sieve tray. The liquid flow for this tray, which is returned condensate, is very small in comparison with the vapour load. However, it is known that this type of tray cannot provide good liquid hold-up equivalently for good contact between liquid and vapour. As mentioned in the previous point, washing water in the top section might evaporate completely with CO₂ gas if the operating conditions change significantly. Under current circumstances, it cannot be modified due to mechanical difficulties. The water flow has already been doubled an it is not recommended to increase it much further as it will affect the absorption solution strength.
SUGGESTIONS TO OVERCOME THE PROBLEM
Suggested solutions are constructed on the basis of avoiding any major mechanical change to the existing system to reduce any risk as well as to minimize any investment that might be needed. These suggested solutions will be implemented at the first opportunity in a turn-around.
CO2 desorber
1. Better monitoring of the operation parameters (i.e. heat loads, flow rates, temperature and
pressures) to ensure good control on operating conditions,
2. The existing mist eliminator should be modified to reduce local entrainment. Consultation
with mist eliminator vendors should be made to investigate applying one of the following
suggestions to minimize pressure drop:
o Install a new mist eliminator with higher separation efficiency.
o If a unique mist eliminator can be provided which can handle non-uniform flow
based on non-uniform mist eliminator thickness (see Fig 8).
FIG. 8: UNIQUE MIST ELIMINATOR TO REDUCE NON-UNIFORM GAS FLOW
3. Possibility of replacing the DEA activator in the Benfield section with ACT-1 [12] should be investigated. DEA is not thermally stable and can easily suffer from degradation, in which case it will produce other chemicals that enhance the foaming tendency and reduce solution activation
4. Gamma scanning [16] should be utilized for checking the desorber and CO₂ cooler during operation to identify if flow patterns and columns internals function correctly.
CO₂ cooler & knock-out drum
1. Upgrading mist eliminators of the CO₂ cooler and knock-out drum to a better-performance type. Measuring pressure drop should be applied to monitor their operation.
2. The existing CO₂ cooler system is capable of handling an increase in water washing flow rate (from current load of 120 up to 200 m³/h). Higher scrubbing efficiency is expected at higher flow rates of water.
3. Installation of a spray distributor above the mist eliminator to wash it and the CO₂ gas exiting from the column.
4. Packing material can be changed to enhance the scrubbing efficiency.
New knock-out drum
Installation of another knock out drum before the compressor and after the existing KO drum is highly desirable. This will minimize the entrainment of very small liquid droplets. The proposal is given with preliminary design [4] and economic estimation [9]. We selected a slightly smaller diameter than that of the existing knock-out drum with 2.15m, to consider the slight change in pressure drop might form in the new KO drum. The pay-back period of investment will be within a short period of time (3 months).
POTASH DEPOSITS PROBLEM MENTIONED ELSEWHERE
The problem of carry-over associated with Benfield process is well known all over the world. Many comprehensive studies (see Table 6) and reports have been found that mentioned this kind of problem; although, they were not given lots of information.
FINAL COMMENT
Process simulation was used to simulate and identify the performance of each unit involved from Desorber to CO2 compressor. The aim of simulation was to find out whether each unit has contributed to the carry-over problem or not and how far it is involved. However, results indicated that all scrubbing units are capable of removing any potash associated with CO2. These scrubbing units will only lose some of their performance, simply, if they operate at overloading conditions.
It is concluded that potash solution could be carried over with CO2 due to overloading and mechanical configuration in the desorber. Overloading could be due to pressure and temperature variation, which could lead to evaporation of large quantities of water, especially during certain events such as start-up, shut-down and feed load change. The mechanical configuration of the top section of the desorber could contribute to carry-over problem.
Potash carry-over phenomena in desorber, CO2 cooler, CO2 pipeline and knock-out drum are complex phenomena, as it relates to macro sizing of liquid droplets, which is difficult to describe in empirical calculations or steady-state simulation. It is much better to check it with the use of Computational Fluid Dynamic (CFD) modelling and dynamic simulation on the basis of a real-time case for a whole process under equipment specifications and conditions. However, the simple approach illustrated in this work proved to be enough, with less efforts for computation and information.
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Acknowledgement
The authors would like to express their gratitude to Sirte Oil Company (SOC) and Libyan Petroleum Institute (LPI) for their support of this work and permission to publish the data and results. Moreover, the authors would like to extend their thanks to all management and operating staffs at SOC Mersa ElBrega Chemical Complex for assistant in completing this study.
