Generation of Synthesis Gas Off-Shore : Oxygen Supply and Opportunities for Integration with GTL Technologies

NATURAL GAS CONVERSION V Studies in Surface Science and Catalysis, Vol. 119 A. Parmaliana et al. (Editors) o 1998 Elsevier Science B.V. All rights reserved.

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Generation of Synthesis Gas Off-Shore 9 Oxygen Supply and Opportunities for Integration with GTL Technologies Dennis M. Brown*, D e r e k Miller, Rodney J. Allam

Air Products PLC, Walton-on-Thames, Engl and Peter J. A. Tijm Air Products and Chemicals, Inc., Allentown, USA SUMMARY

Oxygen is frequently used in generating the synthesis gas required for converting natural gas to liquid products (GTL). With the increasing need to convert gas associated with deposits of crude oil, and as a large part of such deposits are in remote areas and frequently off-shore, there is a need to design and build oxygen plants capable of operating at sea. This paper gives an overview of Air Products' proposals for oxygen generation suitable for Floating Production, Storage and Off-Loading (FPSO) applications. It pays particular attention to safety, size and weight issues, and integration of the various unit operations. Air Products' considerable experience in methanol, DME and Fischer-Tropsch available from its syngas-to-liquids pilot plant, makes it particularly well placed to address integration of the exothermic downstream GTL unit. ASSOCIATED GAS - THE GTL OPTION The conversion of natural gas to synthetic fuels such as Fischer-Tropsch "white crude" (1,2,3) offers an alternative to flaring, re-injection, or LNG production. New Fischer-Tropsch technology developments have reduced the costs of producing the sulphur-free white crude, and the increasing environmental constraints in the developed world are leading to close study of this application. In particular, the operation of a Fischer-Tropsch unit on board a ship (FPSO) is a very interesting and productive way of converting usually-flared gas into a clean, synthetic crude which can be mixed with the regular crude oil in the ships' tanks. This recent application has become classed as the gas-toliquids (GTL) option. Although primarily aimed at Fischer-Tropsch, other liquid products include methanol (4,5,6), for either chemical or fuel use, and possibly, Dimethyl Ether (DME) which is proposed as a clean diesel alternative (7). SYNGAS GENERATION AND GTL TECHNOLOGIES In each case of a GTL technology, the natural gas is first converted to a synthesis gas (syngas) prior to conversion of that syngas to the ultimate product. Conventional Steam-Methane Reforming (SMR) is now often combined with oxygen blown secondary reforming - usually an autothermal reformer (ATR). Recent developments in reformer technology include the Advanced Gas Heated Reformer offered by ICI and the proposed BP-Kvaerner reformer. Partial oxidation processes (POX) offer an alternative to endothermic reforming by reacting methane directly with oxygen. This reaction is highly exothermic and the heat can be used for steam/power generation.The choice of syngas generating technology can be complicated. The H2:CO ratio required for Fischer-Tropsch and methanol synthesis is essentially 2:1 which can be conveniently provided directly by oxygen-blown ATR units. However, conventional SMR technology can be preferred in add-on situations or where the "extra" H2 offered from SMR's can be utilised in hydro-treating the Fischer-Tropsch white crude to clean jet or diesel fuels. Grass-roots applications usually favour POX, particularly as the heat generated can be used for site power requirements. From a CO2 emission perspective, and this is becoming increasingly important

950 in those parts of the world where such emissions are taxed, a combination of conventional SMR with an ATR secondary unit may be preferred. Off-shore, a whole new set of additional rules apply. On a ship, the size - particularly the height - of the units is important as are weight and footprint considerations. Efficiency may be of secondary importance to convenience of size, ease of operation and safety issues. The oxygen plant takes on a wholly new perspective on a ship that is rocking, rolling, heaving and yawing in heavy seas. These operating and safety considerations have a major influence on the choice of syngas generation offshore. O X Y G E N S U P P L Y AT SEA - ADDRESSING THE ISSUES Oxygen at the tonnage scale can be supplied by several technologies. The traditional process for high purity (99%) large (>300TPD) plants uses cryogenic distillation of air. Vacuum Swing Adsorption (VSA) technology is also offered commercially generally in modules up to 150 TPD and where high 02 purity is not required (90-98% purity). Polymeric membranes can also be harnessed to produce oxygen, although many multi-modular units are usually required for any significant tonnage. The issue of weight, size or footprint area, is important at sea and for GTL processes currently being considered, which use upwards of 500 TPD oxygen, regular membrane production of 02 is probably not viable. VSA units are also large, although stackable, and may offer an option where purity is not required. However, use of VSA will require compression of the gaseous oxygen product to serve in the syngas/GTL trains. Compressors do leak, and gaseous oxygen compression is not recommended for ship-board applications. The traditional cryogenic ASU can avoid 02 compression by producing liquid oxygen at low pressure and pumping this liquid to the pressure required for downstream service. This cycle (with vaporisation) is the recommended route for providing large quantities of oxygen at sea. The main challenges for large ASU's on ships relate to the design and operation of such plants to maintain production under the swaying motion experienced on board a ship located in open sea, especially the special safety features required for an application involving fuel, oxidant, ignition sources and personnel in close proximity. A. P r o c e s s D e s i g n

In a cryogenic ASU, air is compressed, cleaned and cooled to cryogenic temperatures, where it is distilled in a double column arrangement to produce a low pressure 02 product. There are then fundamentally two ways in which pressurised 02 can be produced : compressor or pump. The compressor route takes cold gas 02 from the column, warms it to ambient and compresses it. The pump route, takes liquid 02 (LOX) from the column, pumps it to delivery pressure and vaporises it against a high pressure air stream. For a shipboard application, a pumped LOX design is preferred. Oxygen compressors always have some risk of leaks and fire, which is handled on-shore by the use of safety barriers. This is inappropriate for a ship application, where an 'inherently safe' option is preferred.In order to prevent operational and safety problems on an ASU, it is important that all contaminants in the air are adequately dealt with. Most impurities are removed by the front end molecular sieve units- taking out CO2, water, and most hydrocarbons. Some hydrocarbons get through the front end and naturally accumulate in the LOX. With a pumped LOX cycle there is effectively a very large draw-off or purge, which minimises the chances for impurity build up. Another 'inherently safe' feature recommended for ship applications is the use of a copper-wound coil heat exchanger for the vaporisation of the pumped LOX. For on-shore application, the LOX is typically vaporised in an aluminium plate fin exchanger. However, due to the potential for a fire (aluminium will burn in pressurised 02) a blast shield is provided. Again, this is inappropriate on a ship. Since copper will not burn as readily in 02, a copper exchanger is preferred off-shore. This choice has the added advantage that it can vaporise LOX at any pressure (aluminium plate fin exchangers have limited design pressures) and is therefore equally viable for Fischer-Tropsch or methanol production.

951 Distillation can be performed using either trays or packing. On-shore, structured packing is the technology of choice due to its lower power requirements resulting from low pressure drop. In offshore GTL applications, power saving is probably not a significant issue, but packing is still the preferred choice - the reason being the potential difficulties associated with column movement on a ship (8). Trays have free liquids which can be severely maldistributed by the swaying motion of the ship. Packing only has thin films of liquid, which will be affected much less - this subject will be discussed in more detail later on. Another safety feature is the selection of copper packing in sections of the plant where 0 2 purities are sufficiently high that aluminium or stainless steel packing could burn. In practice this means the bottom section of the LP column. The reboiler/condenser which provides reflux for the HP column and boil-up for the LP column needs careful consideration. Two technology choices exist : the traditional thermosyphon reboiler and the more efficient downflow reboiler. The main issue of importance in the equipment selection is safety. If the liquid 0 2 is allowed to dry boil, build up of potentially hazardous impurities can occur. On land, both types can operate successfully. Currently, however, the thermosyphon is the preferred choice for off-shore applications since it is easier to maintain good liquid flow distribution and avoid dry-boiling. One of the major technical challenges of building an ASU on a ship is in designing distillation column systems which are tolerant of wave motion. The most significant motions in terms of impact on the distillation column are swaying and permanent tilt. Typical values for North Sea / West of Shetland locations are :- Swaying : up to 16 degrees from vertical; Swaying Period : 10 - 20 seconds, with lower movements (say 2 degrees) up to 40 seconds; Permanent Tilts : up to 4 degrees. To put these numbers in perspective, a shipboard ASU making 0 2 might have a cold box height of 35m and be located 15m above the centre of rotation of the ship - with a 16 degree sway, the top of the cold box will move 14m away from the vertical position. A typical land-based, high efficiency cold box might be 50m high and will be installed with an allowable deviation away from vertical of less than 25mm. B. Paeked Column Performanee under Swaying and Offset Conditions Performance of packed columns, particularly on ASU's, can be very sensitive to the ratio of liquid to vapour (L/V). Column tilt will adversely affect performance in two ways. Part of the column will be over-irrigated with liquid, with some liquid running down the wall and bypassing the packing. Other parts of the packing will have too little vapour, perhaps with some vapour not seeing any liquid at all and also bypassing. In addition, high liquid loading provides more resistance to vapour flow, which will tend to force vapour to drier areas of the packing. There may also be problems with initial distribution of liquid into the packing, when the distributors are not horizontal. C. Effect of Bypassing on Oxygen Plant Performance In practice, the distillation columns will be designed to avoid complete liquid/vapour bypassing. However, in order to understand the consequences of these phenomena taking place, a sensitivity study indicates the effect of either gas or liquid bypassing. The study is based on an ASU designed to make 99.6% purity 0 2 at a recovery of 95% of the oxygen in the feed air. Bypassing is assumed to take place equally and simultaneously in all distillation sections in the HP and LP columns. At a product purity of 99.6% it can be seen that even relatively small amounts of liquid bypassing will cause a dramatic reduction in the 0 2 recovery. Above 3% bypassing, purity cannot be maintained at any recovery. Vapour bypassing is not as severe, for example 10% bypassing causes a loss in recovery of about 20%. It is interesting to note that if we allow the 02 purity to drop a little, the sensitivity to bypassing is greatly reduced (e.g. even 10% liquid bypassing only reduces recovery of 0 2 by 10%).

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Bypass % The following can be concluded from this exercise: (i) distillation performance is potentially very sensitive to column motion, if this results in vapour or, in particular, liquid bypassing. The column internals will need to be designed to minimise the effects of column motion; (ii) impact on the downstream GTL process of varying 02 purity needs to be understood. For example in particularly bad weather with a lot of column movement, if column performance starts to drop off, a lower 02 purity will permit significantly higher 0 2 production;(iii) another way of maintaining O2 production, even under adverse conditions, is to provide an oversized Main Air Compressor, such that even if 02 recovery starts to fall, 02 production can be maintained - this might well be more economical than installing extra packing to keep 02 recovery up. In other words there is an optimisation between investment in the distillation column and investment in the Main Air Compressor.

D. Dynamic Study of Swaying Column Performance Under ship movement, it is quite likely that column performance will vary as a function of time. Liquid inventories in the columns will tend to buffer out this effect. A study was carried out to understand the impact of varying distillation performance by creating a full dynamic simulation model of the plant (complete with liquid/vapour inventories process control loops, etc.). The simulation modelled the plant running at steady state and then oscillated the HETP. The simulation assumed a constant air feed to the plant and constant 0 2 product, and the 02 purity was varied. The HETP was varied between 70% and 100% over a 20 second period to simulate performance as the column passes through the vertical position (good performance) through to the tilted condition (poor performance). The purities in the packing were seen to start oscillating and that 0 2 purities near the bottom of the LP column tend to get worse, before stabilising at a lower purity. The 02 product, which is based on the purity of the liquid in the sump of the LP column drops as expected, but does not oscillate. It also takes about 2 hours for the purity to drop from 99.6% to about 99.4%. There is a lot of dampening of the product purity due to the liquid inventory in the sump and the purities in the plant as a whole only move slowly due to the total liquid hold up in the columns. One of the important design considerations relates to the amounts of liquid in the plant. More liquid helps column stability, but could adversely affect safety. The dynamic simulation can be used to investigate this in order to determine the optimum capacities. The largest liquid inventory is in the sump of the LP column - for a thermosyphon reboiler between 30-50% of the total plant inventory is located here. If we reduce this inventory to 10% of its normal value and rerun the simulator, we find that the 02 purity changes more quickly (it now takes about 1 hour to change) but is still quite stable.

E. Packed Column Design for Shipboard Applications There are fundamentally two problems associated with designing packed columns for operation under swaying conditions : how to minimise liquid maldistribution feeding into the packing, and how to

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minimise liquid maldistribution within the packing (9,10). A novel design developed by Air Products, uses a mildly pressurised distributor to give a coarse distribution, feeding into a second segmental "egg box" distributor. This distributor consists of individual irrigation cells which do not connect with each other - each cell will typically have one distribution hole. In between each cell is a space for the vapour to pass through. The purpose of the egg box distributor is to prevent liquid slopping from one side of the column to the other, and to provide a liquid buffer which dampens down any variations in the feed to each cell. The combination of the two distributors can cope with all expected oscillating movements (typically 8-12 degrees) and also permanent tilts (typically 2-3 degrees) without any significant additional complexity or cost. Structured packing consists of vertical corrugated perforated sheets which are arranged in layers. The sheets in each layer are aligned. Usually, each layer is rotated in relation to the one above and below, in order to promote mixing, which helps the distillation process. In a column which is tilting, there will be little tendency for liquid to maldistribute across the sheets, but there will be a tendency for sideways movement along the sheets. In order to minimise this tendency as the column moves, the basic structure of the packing can be used to advantage. By having the sheets arranged in individual cells within a layer, with some sheets being orientated perpendicularly to others, the degree to which liquid can move is severely restricted to the size of each cell. In this way, although there will still be some local maldistribution, there will be no gross effects such as large amounts of liquid running down the walls of the column. COLD BOX/PLANT LAYOUTS A variety of different cold box layouts have been investigated. A number of parameters have been considered such as cost/height/plot area/safety. Fundamentally height is an issue for several reasons: taller columns will sway more and will be more likely to be affected by ship motion; there are height limitations (typically 35m) on ships due to helicopters, etc. Conventionally, the LP column is located on top of the HP column - this can be done within the 35m limit. Different arrangements for the columns and heat exchangers have been investigated. It is believed that the best option will be to have all of the cryogenic equipment within one large box or can. Primarily for safety reasons, it will also give a smaller plot area at similar cost to other options. Options to reduce cold box height include installing the LP and HP columns side by side, with transfer pumps. These arrangements can also be accommodated within a single cold box, within a larger plot area. Overall, a conventional stacked column arrangement is simplest, cheapest and will perform adequately under all required operating conditions. There are many permutations for overall ship layouts - the turret can be at the front, middle or rear of the ship. This study has taken a typical layout and, in discussion with shipping experts W.S. Atkins, has come up with a conceptual layout based on the principle of keeping all of the equipment containing hydrocarbons in a separate area to the ASU/control room (incorporate the philosophy of an "invisible line" across the ship - fuel on one side, non-fuel on the other). SAFETY : COLD BOX On land, cold boxes are designed to provide a containment for the cryogenic equipment and insulation. It is an accepted risk that very occasionally there will be process leaks within the box. The risk is manageable because personnel are generally well away from the box and there is plenty of space for dispersion of leaking fluids. On a ship such incidents could lead to major problems both to personnel and to surrounding equipment, and even the ship itself. Such incidents are totally unacceptable for off-shore application. Air Products has developed a contained system which addresses all of the common potential hazards associated with ASU cold boxes. The process equipment is located within a container having a central void surrounded by perimeter insulation. In on-shore plants, the whole free space in the box is packed with insulation. If a gas leak should occur, there may be a restriction to gas flow that panel buckling can occur. On the perimeter insulated box recommended for off-shore, there is free passage

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to the top of the cold box, where a relief system will direct the cold vapours to a safe location without over pressuring the cold box structure. This containment will hold all of the process equipment containing cryogenic fluids, including the LOX pumps, such that any leak will be contained. The bottom of the box contains a sealed membrane, sized to contain the full liquid inventory held within the process. The sides and top of the perimeter are internally clad with metal sheeting designed to deflect any spraying liquid down into the sump. If there is a leak, any spilt liquid can remain in the sump indefinitely. In practice, in order to effect repairs, the liquid will have to be removed and a special "dip tube" arrangement for removal of any spilt liquid is incorporated using a venturi run on compressed dry air. The concept of perimeter insulation and liquid sump, whilst being unusual in the ASU industry, is somewhat similar to the methods already employed in LNG transportation - a proven technique for bulk cryogenic liquid containment on ships. S A F E T Y : AIR IMPURITIES There are a variety of different air contaminants of concern when designing an ASU. A shipboard ASU will need particular care due to the close proximity of hydrocarbon sources and the potential for poor air circulation patterns due to confined equipment layouts. Most contaminants are removed in the front end clean-up adsorbers, which are particularly designed for this purpose. However, if design limits are exceeded, breakthrough into the plant can occur. The main area of concern is the build-up of hydrocarbons in the liquid oxygen. Unfortunately, most contaminants which are not totally adsorbed on the molecular sieves end up in the LOX. The build-up of these contaminants will be limited by the LOX draw-off or purge. With a pumped LOX cycle, this purge is effectively about 20% of the air so contaminants can only build up by a factor of 5. CONCLUSIONS We can conclude by saying that today's cryogenic ASU's, with certain modifications, are technically viable for off-shore production of 02 and that the safety considerations, whilst critical, can be adequately dealt with. It is clear that there are a number of unique features required for a shipboard ASU which will make the design different than a landbased plant. These key features are covered by technology developments specifically tailored to this application. ...

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References:

1 M.J.Gradassi; N.W.Greene," Economics of Natural Gas Conversion Processes", Fuel Processing Technology, 42, 65-83, 1995. 2 Choi et.al, " Design/Economics of Natural Gas Based Fischer-Tropsch Plant", AIChE Spring General Meeting, Houston, TX, 2-3 March 1997. 3 B.Jager, R.Espinoza, "Adv. in Low Temperature Fischer-Tropsch Synthesis", Catal..Today, 23, 17-28, 1995 4 R.Hymas, "The Impact of New Methanol Technology on Global Technology", World Methanol Conference, Tampa, FL, 89-101, 8-10 Dec 1997. 5 J.P.Lange, "Perspective for Manufacturing Methanol at Fuel Value",Ind.Eng.Chem.Res., Vol.36, No.10, 1997. 6 H.Gohna, "Concepts for Modern Methanol Plants", World Methanol Confer., Tampa, FL, 139, 8 Dec 1997. 7 Fleisch et.al, "A New Clean Diesel Technol:, SAE, Intern. Congress & Exposition, Detroit, MC, Feb, 1995. 8 D.Dunne, "Distillation Performance Improvement", Proceedings of ICI Methanol Technology Operators Forum, San Diego, California, 1997. 9 Tanner et.al, "Modelling the Performance of Packed Column Subjected to Tilt", Trans ICHemE, 74, partA ,March 1996. 10 Tanner et.al, " Effects of Tilt and Motion on a Packed Deaeration Column", IChemE Sym Ser No 128, Blll-Bll9