Investigation of different equipment setups for ammonia wash and desulfurization

Dr. Frank Sowa, Michael Wolf

DMT GmbH & Co. KG,

Cokemaking Technology Division,

Am Technologiepark 1,

45307 Essen, Germany

Phone: +49 201 172 1207

Fax: +49 201 172 1241

Email: frank.sowa@dmt.de

Key words: coke oven gas, desulphurisation, ammonia wash, gas scrubber, gas cleaning

INTRODUCTION The coke oven gas (COG) scrubbers and the subsequent scrubbing liquor recovery is the centrepiece of a by-product plant for removal of ammonia, hydrogen sulfide and hydrogen cyanide using the ammonia wash system. There are several different options to design the scrubbing and wash water treatment plants of a by-product plant, applying different types of columns and internals as well as and modes of operating the interacting units. Based on a given COG amount, different equipment designs which are used in various by-product plants world-wide are investigated with a focus to optimize the process performance and the investment costs without influencing the process reliability. The different designs are assessed and compared to each other highlighting their specific pro's and con's. Thus an interesting comparison of the next generation technologies for ammonia and hydrogen sulfide removal is provided. For the investigation some basic assumptions are fixed (Table I) with data about the gas composition, ratio of CO2 and H2S and the gas velocity.

Table I: Basic assumptions for the investigation

Amount of crude COG 80.000 Nm3/h

H2S Content 5 g/Nm3

NH3 Content 8 g/Nm3

HCN Content 1.5 g/Nm3

Ratio CO2 / H2S 1 : 7

Gas velocity H2S Scrubber 2.0 m/s

Gas velocity NH3 Scrubber 1.7 m/s

STATE-OF-THE-ART-BY-PRODUCT PLANT

The coke oven gas (COG) produced in the coke oven batteries is treated in a by-product plant (BPP). In a state of the art BPP (Figure 1) the first separation between COG and a liquid consisting of ammonia flushing liquor (AFL) and tar takes place at the downcomer, where the liquid phase flows to the tar separation plant whereas the COG is cooled down in the primary gas coolers, cleaned from remaining tar in the electrostatic tar precipitators and compressed by the gas exhausters and finally discharged to the scrubbing plant.

Figure 1: State-of-the-art by-product plant The Scrubbing Plant In the H2S-/NH3-scrubbing system COG enters the H2S-scrubber first (Figure 2). The COG is cooled by sprinkling with scrubbing liquor in order to remove the heat from gas exhauster compression and then flows upward in counter flow to the wash water. Discharging the wash water at the bottom of the H2S-scrubber, a part of the wash water is led through a cooling loop, comprising of pumps and heat exchangers. Thus the wash water is indirectly cooled with chilled water and then recycled into the lower scrubbing section of the scrubber. The upper area of the H2S-scrubber is used to remove the major H2S content of the COG. The absorption of H2S is performed by NH3, which is on the one hand contained in the COG and otherwise additionally fed by deacidified water entering in the upper part of the H2S-scrubber. The deacidified water is provided by the deacidifier column (DS). Furthermore, the scrubbing process is completed by spraying the COG with pre-enriched NH3-water at the top of H2S-scrubber coming from the ammonia scrubber.

Figure 2: H2S-/NH3-Scrubbing Unit Due to the exothermic absorption of NH3 and H2S within the H2S-scrubber, the absorption heat has to be removed by an internal cooling loop. Hence, a part of the wash water is discharged from the upper area of the scrubber and indirectly cooled through intermediate heat exchangers with chilled water. Leaving these coolers, the wash water is returned into the upper part of the H2S-scrubber. The further removal of H2S and the removal of the major portion of ammonia are performed in the NH3-scrubbing system, represented by the NH3 scrubber. The NH3 scrubbing process is carried out by means of stripped ammonia water coming from the ammonia stripper (AS). The stripped water flows top-down through the NH3-scrubber whereas the COG enters the scrubber at the bottom and flows bottom-up to the scrubber outlet at the top. For removing the energy of the exothermic absorption reaction, the pre-enriched water, leaving the NH3-scrubber, is indirectly cooled by chilled water and led to the top of the H2S-scrubber. The COG is directed to the BTX-/Naphthalene scrubbing system. The enriched water is released from the bottom of the H2S scrubber to the enriched water buffer tank by gravity before the water is discharged to the DS/AS-columns of the distillation plant. The Distillation Plant The distillation plant consists of an ammonia stripper (AS) and a deacidifier column (DS). They are deemed to remove the gaseous components H2S, NH3, HCN contained in the enriched wash water leaving the scrubbing area (Figure 3) at the enriched water tank. Moreover, in the distillation plant the wash waters for the scrubbing plant are prepared, such as the stripped water for the NH3 scrubber and the deacidified water for the H2S scrubber. The enriched water from the scrubbing area is indirectly preheated in heat exchangers with hot stripped and hot deacidified water, sequentially, coming from the ammonia stripper (AS) and the deacidifier column (DS). Then the preheated enriched water is fed to the top of the deacidifier column (DS) and flows top-down in counter flow to the rising stripping steam containing NH3 (vapours) coming from the ammonia stripper (AS). The vapours leaving the DS, comprising the main part of stripped H2S, HCN as well as NH3, are directed to a vapour condenser with a downstream condensate strainer. While the condensate is returned to the DS, the vapours are led at a controlled temperature level to e.g. an ammonia cracking/elementary sulphur plant. Discharging the sump of the deacidifier, a part of the deacidified water is pumped to the ammonia stripper (AS), while the other part is fed via heat exchanger groups into the deacidified water tank, dedicated to serve as NH3 containing wash water in the H2S-scrubber.

Figure 3: Distillation Plant

Ammonia Stripper (AS) / Deacidifier Column (DS) The smaller part of the deacidified water that is pumped to the AS (Stripper) flows from top to bottom in counter flow to low-pressure steam, which is added via steam showers at the sump of the AS. The NH3-comprising vapours, streaming out at the top of the AS, are directed to the midsection of the DS, while the middle part vapours of the AS are introduced directly above the bottom section of the DS. Leaving the sump of the AS, the stripped water is split. One part is discharged to the waste water tank and then pumped to the Biological Waste Water Treatment Plant after being cooled down by various heat exchangers. The other part is pumped through cooling stages into the stripped water tank which provides wash water to the NH3 scrubber. The cleaning of coal water from the gravel filter plant is also carried out in the AS. For the decomposition of fix ammonia salts contained in the coal water, preheated caustic soda is added into the lower part of the AS (so called “NH3-fix stripper part”). The above technical process descriptions of the scrubbing and the distillation plants are supposed to be crucial for understanding the following investigations. Further gas and vapour treatment plants will be not described here. In the following various redundancy concepts and different equipment setups will be presented for both, the scrubbing plant as well as the distillation plant and with a special focus on the scrubber and distillation columns.

H2S-/NH3-SCRUBBING AREA To obtain an overview about different equipment setups it is necessary to take a look first on various possible redundancy concepts (Figure 4).

Figure 4: Redundancy concepts for the scrubbing area In Case 1 only one H2S- and one NH3-scrubber are designed which gives the operator of a BPP a redundancy of 0%. It is, however, the cheapest solution. To achieve a minimum redundancy of 50%, it is necessary to install one additional scrubber (Case 2), called FLEX scrubber. The FLEX-scrubber is designed to operate either as NH3-scrubber or as H2S scrubber. For 100% redundancy it is also possible to install an additional H2S- and NH3-scrubber, which in turn has a big impact on costs (Case 3). In addition to the number of columns (i.e. redundancy concept chosen), a significant factor for investment costs is the selection of the column internals. Different options of scrubber internals are described in the following. Generally, all scrubbers are manufactured from carbon steel.

EXPANDED METAL PACKING Expanded metal packing (Figure 5) are common internals in scrubbers, well-known, proven and cheap. DMT’s experience on European coke plants shows that scrubbers can be operated for more than 25 years without major shutdowns for cleaning/maintenance reasons. Even after this time the pressure difference is not higher than the original design (80 – 100 mm WC per scrubber). The exchange surface available for the scrubbing process can vary between 50 m2/m3 up to 150 m2/m3 depending on the spacing between the plates. Expanded metal packing give high operational reliability and reduce the probability of blockings with e.g. naphthalene, tar or other particles in case of operation failures. Depending on the redundancy concept required by the environmental authorities, spare scrubbers may be skipped for normal operation. In case of operation failures and blockings simple cleaning with steam is possible, efficient and not time-consuming. After cleaning 100% scrubbing efficiency can be regained. However, lower exchange surface of the internals results in a design of bigger column diameters.

Figure 5: View on typical expanded metal packing for the scrubbing area

STRUCTURED PACKING To increase the exchange surface of the internals it is possible to use structured packing (Figure 6). In structured packing the space between the plates is about only some millimetres. The high exchange surface is one of the most important advantages of structured packing which can go high up to more than 250 m2/m3. This allows a compact design of the columns and also the possibility of a combined scrubber design, i.e. NH3-scrubber and H2S -scrubber on top of each other. But structured packing are prone to blockings with naphthalene, tar and other solids, due to the small free space between the plates. The cleaning of structured packing with steam is less sufficient, this means that redundancy scrubbers are definitely required and that the saving potential is destroyed by additional investment costs. Operating experience in Europe shows that scrubbers, designed with internals from structured packing, more often have operating problems due to deposits, pressure difference rising and decreasing efficiency.

Figure 6: View on structured packing for the scrubbing area The average lifetime of this internal type is supposed to be about 5-7 years, which is achievable with periodic cleaning, however showing continuously deteriorating scrubbing efficiency. One additional disadvantage is that structured packing are only available

from special suppliers, this means usually that this type of internals are 3-5 times more expansive in comparison with expanded metal packing.

RESULTS OF THE INVESTIGATIONS IN THE SCRUBBER AREA The following Table II shows a comparison of the relative capital expenditure (CAPEX) of different NH3- and H2S-scrubbers based on different redundancy concepts (Case 1, 2, 3 ; see also Figure 4) and different internals (A, B, C). For a more detailed comparison the individual scrubber columns are specified in diameter and height reflecting the impact of the internals. Common basis for the design are the parameters given in Table I. Here, a typical state-of-the-art scrubbing plant has been identified with Case 2-A with expanded metal packing and a spacing of 30 mm between the plates, with one H2S-, one NH3- and one FLEX-scrubber (i.e. in total 3 columns). Case 2-A was set as 100% base case with regard to CAPEX (capital expenditure).

Table II: Comparison of the various options with expanded metal and structured packing

Assuming the same redundancy concept (Case 2-B), but using expanded metal packing with a smaller spacing between the plates of about 15 mm, reduces the relative CAPEX down to about 70 % due to smaller sizes of the columns and less internals needed (see comparison of the various options, Table II). Reducing the redundancy requirements from 50% down to 0% (only 2 columns in total), the relative CAPEX will of course decrease further. The cheapest scrubbing plants are definitely Cases 1-C (CAPEX 43%), Case 1-B (CAPEX 49%) and Case 1-C (CAPEX 64%). But even if the redundancy requirement is set at 100% (full redundancy with 4 columns in total), the scrubbing plants Case 3-C and Case 3-B reveal a lower relative CAPEX as the base case. Only Case 3-A is distinctly more expensive than the base case as it has one column of same size more as the base case. The use of structured packing with a high exchange surface reduces the design of the columns so much that even a design of a combined NH3-/H2S-scrubber column becomes feasible. The respective redundancy concept is shown in Figure 7. In this combined scrubber design the NH3- and H2S-scrubbers are installed on top of each other, the H2S scrubber in the bottom part and the NH3 scrubber in the upper part of the column. The height of the column may rise up to 50 meters. As the structured packing are prone for blockings, this COMBI-scrubber design is only feasible with a 100% redundancy requirement. As can be seen from Figure 7 Case 4 shows 2 COMBI-scrubber columns. Also utilisation of expanded metal packing with smaller spacing allows the design of a COMBI-scrubber with similar dimensions compared to the COMBI-scrubber with structured packing (see Table III and Figure 8). However, the design of a COMBI-scrubber with expanded metal packing with wider spacing between the plates is not feasible from the construction point of view. Due to the bigger diameter this COMBI-scrubber would be very massive.

Figure 7: Redundancy concepts for the scrubbing area with additional combined scrubbers Compared to the base Case 2-A, the CAPEX for Case 4-B (CAPEX 76%) and Case 4-A (CAPEX 85%) are less than the base case (see Table III). Each of the Cases 4-A and 4-B are a little less in CAPEX than their comparable counter Cases 3-B and 3-C which also have 100% redundancy, but require much more space due to the installation of 4 columns.

Table III: Comparison of the various options including the combined scrubber

Internal

Expanded Metal

Packing

30 mm spacing

Expanded Metal

Packing

15 mm spacing

StructuredPackings

200 m2/m3

Expanded Metal

Packing

30 mm spacing

Expanded Metal

Packing

15 mm spacing

Expanded Metal

Packing

30 mm spacing

Expanded Metal

Packing

15 mm spacing

StructuredPackings

200 m2/m3

Expanded Metal

Packing

15 mm spacing

StructuredPackings

200 m2/m3

Redundancy 0% 0% 0% 50% 50% 100% 100% 100% 100% 100%

[Q.ty] 2 2Diameter upper

part [m]3.6 3.6

Diameter bottompart [m]

3.3 3.3

Height [m] 48.8 49.6

CAPEX 64% 49% 43% 100% 70% 130% 92% 80% 85% 76%

Case 1-B

1 H

2S

-Scr

ubb

er

and

1 N

H3

-Scr

ubb

er

Case 3-B

2 H

2S

-Scr

ubb

er

and

2 N

H3

-Scr

ubb

er

Case 1-A Case 1-C

1 H

2S

-Scr

ubb

er

and

1 N

H3

-Scr

ubb

er

1 H

2S

-Scr

ubb

er

and

1 N

H3

-Scr

ubb

er

1 H

2S-S

cru

bb

er

1 N

H3-

Sc

rub

ber

an

d

1 F

LE

X-S

cru

bb

er

Case 2-B Case 3-A Case 3-C Case 4-B

COMBI-Scrubber

1 H

2S

-Scr

ubb

er

1 N

H3

-Scr

ubb

er a

nd

1 F

LEX

-Scr

ubb

er

2 H

2S

-Scr

ubb

er

and

2 N

H3

-Scr

ubb

er

2 H

2S

-Scr

ubb

er

and

2 N

H3

-Scr

ubb

er

Case 4-CCase 2-A

0%

20%

40%

60%

80%

100%

120%

140%

0%Case 1-A

50%Case 2-A

100%Case 3-A

0%Case 1-B

50%Case 2-B

100%Case 3-B

100%Case 4-B

0%Case 1-C

100%Case 3-C

100%Case 4-C

64%

100%

130%

49%

70%

92% 85%

43%

80% 76%

Redundancy [%]

CAPEX [%]

A = Expanded Metal Packing, 30 mm spacing, 55 m2/m3 B = Expanded Metal Packing, 15 mm spacing, 110 m2/m3 C = Structured Packing, 200 m2/m3

Figure 8: Redundancy concepts for the scrubbing area with additional combined scrubbers

CONCLUSIONS FOR THE SCRUBBER AREA Depending on the required redundancy concept, the selected internals and the design of the scrubber columns, 10 different scrubber set-up cases have been investigated and assessed regarding their required capital expenditures (CAPEX). A typical state-of-the-art scrubbing plant has been identified and set as 100% base case regarding CAPEX. The comparison of the cases has been done on a relative CAPEX basis. The cases with lowest CAPEX are scrubbing plants with no (0%) redundancy, i.e. one NH3 scrubber and one H2S scrubber. Lowest CAPEX shows a scrubbing plant equipped with structured packing (Case 1-C). However, structured packing are prone to blocking with naphthalene and tar in case of operation failures, are difficult to clean and show a deterioration of performance with increasing number of cleaning cycles. For this reason it is not recommended to use this 0% redundancy concept for scrubbers with structured packing. Second lowest CAPEX shows the scrubber plant equipped with expanded metal packing with small spacing between the plates (Case 1-B). Also in this case a quicker blocking with naphthalene and tar in case of operation failures can be expected due to the narrower space between the plates. However, expanded metal packing are easier to clean and show 100% performance after the cleaning cycles. So this case may be an option for a low CAPEX scrubbing plant with adequate operational reliability. The highest robustness against fouling shows the scrubbing plant equipped with expanded metal packing with wide spacing. It shows the third lowest CAPEX and experience shows that this scrubber configuration can be operated without redundancy. The above statements regarding blocking/fouling, cleaning and performance after cleaning can be applied to all other cases as they are related to the internals also applied in these cases. Having this in mind attention is now given to a different design of the columns. Cases 4-A and 4-B consider tall COMBI-scrubbers which are saving space in congested BPP areas. Under such restrictions COMBI-scrubbers are definitely a good option, especially as they are also saving some piping, pumps, valves, etc. In terms of CAPEX they are competing properly with the other 100% redundancy Cases 3-B and 3-C which definitely require more space for installation. Another good option in terms of CAPEX is the Case 2-B using three scrubbers (incl. a FLEX-scrubber) with expanded metal packing with smaller spacing. This 50% redundancy case shows a good operational reliability and is a little bit cheaper than the cases with 100% redundancy. Moreover, due to the installation of only three columns it is consuming less space than Cases 3-B- and 3-C.

As the requirements of each by-product plant is different in terms of redundancy and available space, the respective scrubbing plant concept has to be tailored individually through intense technical discussions with the client.

H2S-/NH3-DISTILLATION AREA Due to the commonly limited storage capacity for the highly contaminated wash water coming from the scrubbing area, a redundancy of 100 % shall be provided in the distillation plant (Figure 9). This can be realized by two ammonia strippers (AS) and two deacidifiers (DS). While one AS and one DS is in operation, the other line is on standby. Beside the type of internals also the column material allows some alternative solutions and cost savings. The material for the AS can vary between Titanium, Hastelloy and also Alloy 904L in sections (bottom part) or also for the whole column. However, due to the very corrosive media handled in the DS, the DS should always be designed from Titanium or Hastelloy. The Hastelloy material is not part of this investigation, all calculations were done only with the material of construction Titanium.

Figure 9: Redundancy concept for the distillation area

In the following paragraphs different internals are presented which can be used in AS and DS.

FIX VALVE TRAYS

From an operational point of view fix valve trays (see Figure 10) offer a very wide operational window and nearly unlimited lifetime. Additionally they reveal a self-cleaning effect in case fouling occurs due to operation failures. Another advantage is the easy access for cleaning purposes with steam without the necessity to dismantle the trays from the column. The easy access into the column is ensured by the manholes and removable tray panels which give the opportunity of short cleaning / maintenance shut-down times (< 2 days). Fix valve trays can be manufactured from Hastelloy, Titanium and also Alloy 904L. A disadvantage is the high investment cost for trays from such expansive material like from Hastelloy and Titanium.

Figure 10: View on fix valve trays for the distillation area

STRUCTURED PACKING An alternative to fix valve trays is the installation of structured packing in the AS / DS. These provide a big exchange surface which is positive for the distillation process. Structured packing may be manufactured from either metals (Titanium, Hastelloy, 904L, Figure 6), which makes the packing quite heavy, or from plastics to save weight (Figure 11). Structured packing made from Titanium may suffer from hydrogen embrittlement which may lead to a collapse of the packing with operation time. PP plastic structured packing show a softening temperatures of around 110°C. Applying so called temperature stabilizers as additive to the plastics would increase the temperature stability slightly up to 150 °C. Nevertheless, it must be noted that the use of plastic structured packing becomes delicate in the bottom area of the column where hot steam is injected for the stripping process. It is recommended to use either metal structured packing or bulk material packing from ceramics or metal (Figure 12) as temperature protection in this area. Although using plastic packing the distribution and collecting trays as well as supports have to be manufactured from metal such as the column shell is made from. Depending on the packing material the investment costs will vary distinctly. Similar to structured packing for the scrubber columns these internals are also prone to blockings, especially when also excess coal water from the gravel filter plant is treated in the AS. Cleaning with steam is difficult in case of metal structured packing and hardly possible in case of plastic structured packing. The performance will deteriorate with each cleaning cycle and finally the structured packing will have to be exchanged (limited lifetime). Operation experience in Europe shows that DS / AS-columns, equipped with structured packing (MOC plastics), are supposed to have problems with deposits.

Figure 11: View on structured packing for the distillation area made from plastics

BULK MATERIAL

Instead of structured packing also bulk material packing is an alternative and cheap internal. They can be manufactured from different materials such as metal, ceramics or plastics (Figure 12). Applying plastic bulk material will, however, be associated with the same temperature problems as identified with the plastic structured packing. In the bottom part of the columns where steam is injected the bulk material shall be of either metal or ceramic in order to protect the plastic bulk material from softening / melting in case of operation failures. Disadvantages of the bulk material are the tendency to blockings due to small free space between in the filling, the undefined distillation capacity depending on filling density and possible wall effects (slip streams) at high fillings.

Figure 12: View on bulk material packing for the distillation area (different types and MOC)

RESULTS OF THE INVESTIGATIONS IN THE DISTILLATION AREA The following Table IV and Figure 13 shows a comparison of the relative capital expenditure (CAPEX) of different AS / DS set-ups based on different internals (Case 1, 2, 3) and different MOC’s (A, B, C). The redundancy concept for all cases is always 100% redundancy. For a more detailed comparison the individual columns are specified in diameter and height reflecting the impact of the internals. Common basis for the design are the parameters given in Table I. Here, a typical state-of-the-art distillation plant has been identified with Case 1-A with fix valve trays, MOC Titanium, two AS and two DS (i.e. in total 4 columns). Case 1-A was set as 100% base case with regard to CAPEX (capital expenditure).

Table IV: Comparison of distillation columns (AS / DS) with various materials of construction (MOC) and different internals

Internal Trays Trays TraysStructured Packings

Structured Packings

Structured Packings

Bulk Material Bulk Material Bulk Material

Redundancy 100% 100% 100% 100% 100% 100% 100% 100% 100%Ammonia Stripper

[Q.ty]2 2 2 2 2 2 2 2 2

Shell Ti Ti / 904L 904L Ti Ti / 904L 904L Ti Ti / 904L 904L

Internals Ti Ti / 904L 904L HC HC / 904L 904LCeramic/

PPHCeramic/

PPHCeramic/

PPHDiameter [m] 2.6 2.6 2.6 2.6 2.6 2.6 2.6 2.6 2.6

Height [m] 12.1 12.1 12.1 11.9 11.9 11.9 12.5 12.5 12.5

Deacidifier[Q.ty]

2 2 2 2 2 2 2 2 2

Shell Ti Ti Ti Ti Ti Ti Ti Ti TiInternals Ti Ti Ti HC HC HC PPH PPH PPH

Diameter [m] 2.6 2.6 2.6 2.6 2.6 2.6 2.6 2.6 2.6Height [m] 10.3 10.3 10.3 13.5 13.5 13.5 12.7 12.7 12.7

CAPEX 100% 93% 84% 165% 148% 129% 93% 84% 73%

Case 3-CCase 1-A Case 1-B Case 1-C Case 2-B Case 3-A Case 3-BCase 2-A Case 2-C

0%

20%

40%

60%

80%

100%

120%

140%

160%

180%

TraysCase 1-AAS = TiDS = Ti

TraysCase 1-BTi/904L

Ti

TraysCase 1-C

904LTi

Struct.P.Case 2-A

HCHC

Struct.P.Case 2-BHC/904L

HC

Struct.P.Case 2-C

904LHC

BulkCase 3-A

Ceram/PPHPPH

BulkCase 3-B

Ceram/PPHPPH

BulkCase 3-C

Ceram/PPHPPH

100% 93%84%

165%

148%

129%

93% 84%73%

Internals

CAPEX [%]

MOC Shell Cases A AS = Ti and DS = Ti MOC Shell Cases B AS = Ti/904L and DS = Ti MOC Shell Cases C AS = 904L and DS = Ti

Figure 13: Comparison of distillation columns (AS / DS) with various materials of construction (MOC) and different internals Referring to the base Case 1-A (CAPEX 100%) and replacing the MOC of the bottom part of the AS-columns by Alloy 904L (Case 1-B, CAPEX 93%) reduces the CAPEX by about 7 %. Designing both AS completely from Alloy 904L (Case 1-C, CAPEX 84%) gives

a CAPEX reduction of about 16 % in comparison with the base case. In both cases the fix valve trays were adapted to the MOC of the column shell. The deacidifier columns remain unchanged in MOC. Changing the internals from fix valve trays to metal structured packing requires MOC Hastelloy (HC) and Alloy 904L for the packing. Structured packing from Titanium have not been considered due the hydrogen embrittlement issues. As well structured packing from plastics have not been considered due to the softening temperature issues. As can be seen from Case 2-A (CAPEX 165%), Case 2-B (CAPEX 148%), and Case 2-C (CAPEX 129%) the use of metal structured packing in the distillation area is not a cheap solution. These cases are by far more expensive compared to the base case (CAPEX 100%). Moreover, the height of the deacidifier columns (made from Titanium) is larger compared to the base case. Switching to bulk material packing as internals the selection of plastics bulk material is much cheaper compared to metal bulk material such as Titanium, Hastelloy or Alloy 904L. In order to protect the plastics bulk material (PPH) against softening in the bottom part of the AS, a protective layer of ceramic bulk material is considered (Ceramic / PPH). Although the column heights are larger than in Cases 1-A, B and C, the CAPEX is lowest of all cases (Case 3-A (CAPEX 93%), Case 3-B (CAPEX 84%), Case 3-C (CAPEX 73%)) at comparable MOC of the column shells due to the low cost of the internals. Compared with the CAPEX for fix valve trays as internals it means cost savings of about 9 or 11 % with a comparable MOC of the shells. Also 100% ceramic bulk material is a cheap and robust option as internals. However, it is much heavier compared to the selected option with plastics, but is absolutely temperature resistant.

CONCLUSIONS FOR THE DISTILLATION AREA Depending on the material of construction (MOC) of the column and the type and MOC of internals, 9 different column set-up cases have been investigated and assessed regarding their required capital expenditures (CAPEX). The redundancy concept assumed is in all cases a 100% redundancy, i.e. two AS and two DS. A typical state-of-the-art distillation plant has been identified and set as 100% base case regarding CAPEX. The comparison of the cases has been done on a relative CAPEX basis. The cases with lowest CAPEX are associated with low cost plastics bulk materials as internals, although the columns heights are distinctly higher than the base Case 1-A. However, similar to the structured packing, the bulk material has a similar tendency to blockings due to small free space between in the filling. The undefined distillation capacity depending on filling density and possible wall effects (slip streams) at high fillings have to be mentioned as a process disadvantage. The highest CAPEX is revealed with structured packing made from Hastelloy and/or Alloy 904L as internals. Again it has to be mentioned that structured packing is prone to blocking in case of operation failures, they are difficult to clean and show a deterioration of performance with increasing number of cleaning cycles. The highest CAPEX and the operational constraints make this set-up least attractive. The fix valve tray internals offer the best cost-benefit-ratio. CAPEX-wise the Cases 1-A, B and C rank between the Cases 2-A, B and C and 3-A, B and C. In terms of operational and maintenance benefits fix valve trays offer a very wide operational window and nearly unlimited lifetime, they reveal a self-cleaning effect in case fouling occurs due to operation failures. Another advantage is the easy access for cleaning purposes with steam without the necessity to dismantle the trays from the column. The easy access into the column is ensured by the manholes and removable tray panels which give the opportunity of short cleaning / maintenance shut-down times (< 2 days).

OVERALL CONCLUSIONS Based on a given COG amount, different equipment designs for the COG scrubbing and wash water distillation areas are investigated with a focus to optimize the process performance and the investment costs without influencing the process reliability. The different designs are assessed and compared to each other highlighting their specific pro's and con's. Resuming the results of the investigations in the scrubbing area Case 2-B (H2S-scrubber, NH3-scrubber and FLEX-scrubber, all with expanded metal packing with 15 mm spacing) turns out to be a very attractive solution providing low costs paired with a sufficient redundancy of 50% and a high operational reliability as well as good maintenance performance. In the distillation area Case 1-B (fix valve tray internals and ammonia stripper lower part made of Alloy 904L) offers the best cost-benefit-ratio. CAPEX-wise acceptable, this case offers a very wide operational window and nearly unlimited lifetime of the fix valve trays. The trays reveal a self-cleaning effect in case fouling occurs due to operation failures, they provide easy access for cleaning purposes without the necessity to dismantle the trays from the column giving short cleaning / maintenance shut-down times. Nevertheless, due to the client’s requirements related to layout space constraints, environmental demands and redundancy / maintenance philosophies the optimal solution for a scrubbing & distillation plant set-up has to be intensely discussed before any decision is taken.