Technological Engineering: The most efficient Way to raise your Plant to State of the Art

Transcription

1 Technological Engineering: The most efficient Way to raise your Plant to State of the Art SERGIO PANZA Ammonia Casale S.A. Lugano, Switzerland Of all the companies that specialize in the development of advanced technologies for the fertilizer industry, Casale Group has a portfolio and reference list that are second to none. At the forefront of ammonia plant technology development for over ninety years, and with engineering skills and experience to match, Ammonia Casale, the original company out of which the rest of the Casale Group evolved, is uniquely able to handle every stage of complete ammonia plant revamp projects and has plenty of references to show for it. Ammonia Casale adopts the same integrated approach to the implementation of all such ammonia revamp projects. It is referred to as Technological Engineering, signifying the most detailed and intimate collaboration of Technology and Engineering teams that is a hallmark of the Casale Group. The paper also provides some examples of projects currently under execution. FOREWORD Ammonia Casale is well known for its know-how, design and in-depth knowledge of technologies in the field of ammonia plant upgrading and complete plant revamping projects. Amongst others, these include key modifications to ammonia converters, reformers, CO2 removal sections, machinery and other equipment. Casale s unique policy has, for many years, been based upon the development and application of the most advanced technologies for ammonia plant revamping, designed to attain the best improvements in plant performance at the minimum cost. Performance may be enhanced by reducing energy consumption, increasing capacity, improving reliability, reducing the environmental impact of the plant, or any combination of these, according to the specific circumstances at the individual site. There are so many variables, starting with the scope of the project required by the plant owner, that no two projects are ever exactly alike

2 S. Panza It is, therefore, essential to identify the most critical items to be improved for the best return. Ammonia Casale has done exactly that in many projects over the years and has developed and applied a number of specific proprietary technologies, which are presented below. Projects undertaken have ranged from simply replacing the internals of ammonia synthesis converters and the supply or revamping of other individual equipment items such as secondary reformers, pre-reformers and shift converters, to comprehensive revamping of complete ammonia plants to maximize their capacity and minimize their energy consumption. In these projects Casale not only acted as process designer but also undertook the detailed engineering, procured the equipment and bulk materials, and supervised the site works. In other words, Casale assumed total responsibility as Technological Engineer for the entire project. REVAMPING PROJECT PHILOSOPHY Ullmann s Encyclopaedia of Industrial Chemistry defines revamping as the modernization of an existing old plant, but Casale has another definition: the upgrading of an existing ammonia plant through modification of existing items and process sections to make them more efficient and productive. The difference is not trivial because, for instance, simply replacing an existing item or section with a new and bigger one to attain a specific technical target is likely to be significantly more costly than modifying the existing hardware so that it can be reused. This has been demonstrated in a number of revamping projects that Casale has performed in which, through scrupulous analysis of the plant and careful selection of the revamping plan, the plant s capacity has been increased and/or its energy consumption has been reduced at the most modest possible cost without in any way compromising safety or operating efficiency under all foreseeable conditions of temperature, pressure or process stream composition. To illustrate that, the following paragraphs provide some specific examples of how proper process design can contribute to a successful ammonia plant revamp. CASALE APPROACH TO REVAMPING PROJECTS Scope of activities Revamping projects are substantially different from new plant projects and require a specific strategy to make the result a success. First of all, Casale provides an experienced and structured project organization to manage and execute revamping projects. This organization is attuned to the specific needs of the project and it is well acquainted with local companies, institutes and workshops with which it can work to cover all aspects of a project. One of the most important phases of a revamping project is the initial site survey and collection of data on the candidate plant. During this phase the co-operation of the client is absolutely crucial to the ultimate success of the project. With the best will in the world, the engineering company in charge of the revamp cannot possibly match the client s specific experience and in-house know-how about the plant. So it is essential for the client s team to be very much involved at this stage. All too often the plant documentation is not up to date, and that can give rise to errors in the detail engineering, materials specification and construction, adding to the cost of the project and delaying its execution. Casale has solved this problem with the introduction of laser scanning: the existing plant is scanned with a high-resolution laser scanner and an accurate 3-D model of the plant in its as-built state is made. This model is then elaborated to include the modifications required in the revamping scheme. The final 3-D model is used to produce detail engineering documents, as well as in planning the dismantling, installation and maintenance activities with the client. Throughout the design process, particular care is taken to minimize the amount of time the plant will have to be shut down to carry out modifications that cannot be made while it is in operation. As regards equipment procurement, Casale maintains a list of high-quality manufacturers who have proven experience in revamping. Detailed inspection schedules help ensure timely delivery of a highquality finished article. 110 Nitrogen + Syngas 2014 International Conference & Exhibition (Paris February 2014)

3 Technological Engineering: The most efficient Way to raise your Plant to State of the Art Casale s supervisory activities on site include checking on quality and compliance with design and generally ensuring smooth progress of the construction work. Last but not least, Casale attaches great importance to training the plant operators, involving them as early as possible in the project and providing specific training sessions on running the modified plant to the best effect. Identifying bottlenecks The first step in analysing a plant for a revamping project is to identify the bottlenecks in other words, the capacity-limiting items of the plant. There are various reasons why these items might be operating at their maximum load: for example, they are worn out and no longer able to operate at their original efficiency, or their technology is obsolete, or they were quite simply underdesigned in the first place. However, in some cases an apparent bottleneck in an equipment item may be alleviated by making modifications somewhere else in the plant which changes its inlet or outlet conditions sufficiently to allow that item to operate more efficiently. A good example of that is the additional capacity that can be created in the ammonia refrigeration section when modifications are made elsewhere in the ammonia synthesis loop. Ammonia Casale has demonstrated this in several successful revamping projects that it has undertaken in Russia and the CIS, mainly on GIAP, TEC and Chemico plants. In most of them extra capacity in the refrigeration section was created by installing a new cartridge in the ammonia converter with three axialradial flow catalyst beds (Fig. 1) in place of the axial-flow internals originally provided. Thanks to the significantly greater efficiency of the Casale internals and the lower pressure drop they impose, both the operating pressure and the gas circulation rate could be reduced. That, in turn, lowered the duty on the synthesis loop chillers to the extent that a small to moderate capacity increase was possible without modifying the refrigerant compressor in the TEC and Chemico plants or the ammonia absorption refrigeration units, the so called AXY units, in the GIAP plant. Existing converter Casale revamp Table 1: Synloop Main Operating Data After Ammonia Converter Revamping Production [MTD] NH 3 at conv. outlet [%mol] Circulation [Nm/h] Pressure at conv. inlet [kg/cm 2 g] , , Fig. 1: Revamped ammonia converter Debottlenecking solutions Once the bottlenecks have been identified there are various options for overcoming them. If, for example, the limitation is in the primary reforming furnace, some of the available debottlenecking solutions are: Replace the catalyst tubes with thinner-walled tubes in a higherspecification alloy Install a pre-reformer Extend the radiant box to accommodate additional catalyst tubes Shift some of the reforming duty from the primary to the secondary reformer. Casale has used all these options at various times in complete ammonia plant revamping projects. Each has its own merits and demerits, which may or may not be significant under the individual conditions of a project. Thus, the extremely expensive option of retubing the furnace with thinner-walled catalyst Nitrogen + Syngas 2014 International Conference & Exhibition (Paris February 2014) 111

4 S. Panza tubes is only really appropriate if the existing tubes are close to End-of-Life conditions anyway. Installing a pre-reformer (Fig. 2) is attractive not only because most of the construction can be done while the plant is running but also because it necessitates only minor modifications to the rest of the plant. So the plant does not have to be down for very long. The drawback, however, is that the plant acquires an additional catalytic reactor, and that increases the maintenance requirement. Extending the primary reformer radiant box and shifting the reforming capability between the primary and the secondary reformer are suitable where a large capacity increase is required but they entail a potentially serious economic penalty on account of the extensive downtime. In general the choice of debottlenecking solution, for this section of the plant as for any other, depends on several factors such as: Allowable down time Targeted final capacity of the ammonia (and urea) plant Plant reliability Remaining lifetime of the existing item Environmental impact Return on investment References. Each client accords different weight to each of these considerations, so the solution ultimately chosen can vary considerably between projects. M.P.Steam Feed gas S/C 3.2 T 970 F T 1150 F T 1470 F D-901 B-901 B-701 Fig. 2: Example of prereformer arrangement Laser scanning To expedite plant analysis and the subsequent steps of the project, in all of its revamping projects since 2007 Casale has employed laser scanning. That provides a precise picture of the plant layout and allows an extremely accurate three-dimensional model of the plant to be worked up. And from now on a new scanner is available which can reproduce the colors of the real plant in the scanned model. Using point clouds from the laser scan, it is possible to study specific sections of the 3D model to check the safety, accessibility, maintainability and constructability of proposed modifications in minute detail. This safeguards against untoward interferences between new and existing parts of the plant. Thus the technique drastically limits the likelihood of mistakes resulting in unforeseen problems that commonly affect revamping projects, and it makes it much easier to control the schedule and cost of the project. 112 Nitrogen + Syngas 2014 International Conference & Exhibition (Paris February 2014)

5 Technological Engineering: The most efficient Way to raise your Plant to State of the Art Instrumentation and electrical system For Casale s instrumentation engineers the most important responsibilities in a revamping project are to verify the suitability of existing control and safety valves for new process conditions in the revamped plant, and Safety Integrity Level (SIL) analysis for safety loops for the revamped section of the plant, including the integration of new and modified interlocks and advanced controls into existing systems. Satisfactory operation and reliability of a plant depends on (amongst many other things!) safe and efficient operation of the electrical equipment. One of most important issues in a plant s electrical system is the need to accommodate the start-up load of some very large electric motors. If this is increased after revamping, it may necessitate careful revision of the electrical system as a whole. COMPLETE PLANT REVAMPS FOR RUSSIAN/CIS PLANTS Besides ammonia converter internals, Casale has supplied the Russian/CIS countries with other proprietary technologies such as Casale s secondary reformer burner, ammonia washing unit and shift converter internals, and has undertaken the engineering work to upgrade reforming and CO2 removal sections. Most of these modifications were done on an individual basis rather than as part of a complete plant revamp. In recent years Casale decided to use the know-how, technologies and experience it has developed in complete plant revamps in other parts of the world to develop tailor-made revamping schemes for the rather complicated GIAP and TEC plants in Russia and the CIS. Superficially, the flow scheme of these plants (Fig. 3) appears very similar to that of ammonia plants more or less anywhere in the world. Only after each single area has been deeply analysed do some fairly significant differences between the Russian-type ammonia plants and their western counterparts become evident. Fig. 3: Usual ammonia plant block scheme for a Russian type plant Technological differences GIAP and TEC plants in that part of the world are rather elderly technology; most were built during the 1970s and are therefore years old. The reforming furnaces in both the GIAP and TEC plants have larger radiant boxes than is usual in ammonia plants built outside the USSR at the construction time, with over 500 catalyst tubes The CO2 removal sections in these plants are also notably different from those of standard western ammonia plants of the day. The TEC plants are usually (but not always) equipped with two absorbers and Nitrogen + Syngas 2014 International Conference & Exhibition (Paris February 2014) 113

6 S. Panza two strippers (regenerators). In contrast, the CO2 removal units in the earlier (AM70) GIAP plants were originally designed with one absorber and one regenerator, but the absorber is of rather complex internal design, being equipped with trays interconnected by several cylindrical downcomers. The revised CO2 removal process used in later (AM76) GIAP plants is equipped with one absorber (similar to that of the AM70 process) and two regenerators. The top part of each regenerator is equipped with 20 trays between which are two solution/solution heat exchangers, making a total of 38 exchanger bundles in each column. The synthesis compressors in both TEC and GIAP plants were designed with four compression stages to supply a maximum synloop operating pressure close to 300 kg/cm2g. Interstage cooling was provided by air coolers while, in most plants, there was a water removal device (ammonia washing unit or molecular sieve dryer) between the second and third stages. The synthesis loop in the GIAP plants is usually equipped Fig. 4: GIAP AM76 CO2 regenerator top, with a single ammonia converter, while in the cold section of with detail of the heat exchangers these plants a cold gas/gas exchanger is integrated with the high-pressure loop separator. The TEC plants were originally designed with a single ammonia converter, but most have been revamped in the past by adding a parallel supplementary converter. Another big difference between GIAP plants and most western ammonia plants is the use of absorption refrigeration instead of the more usual compression refrigeration system. The GIAP plants are designed with several of these packages, capable of supplying ammonia at +1 C, -10 C and -30 C. Fig. 5: 114 Ammonia absorption refrigeration system Nitrogen + Syngas 2014 International Conference & Exhibition (Paris February 2014)

7 Technological Engineering: The most efficient Way to raise your Plant to State of the Art Casale revamp for TEC plant Casale has undertaken a comprehensive revamp of a TEC plant in Russia, raising its capacity from 1,600-1,650 MTD to 2,000 MTD and reducing its energy consumption by almost 1 Gcal/MT. The Casale engineering study revealed that the following major changes were needed: Revamp of the primary reformer radiant box Replacement of the secondary reformer burner Revamp of the HTS internals Upgrading of the CO 2 removal section Revamp of the air compressor Revamp of the synthesis gas compressor Revamp of the parallel smaller ammonia converter (so called baby reactor) Installation of a chilled water circuit Installation of an additional BFW pump Replacement of the turbine drives for the ID fans Installation or replacement of various heat exchangers. Primary reformer and secondary reformer The primary reformer radiant box is designed with more than 500 catalytic tubes with an internal diameter of 85 mm. Since the existing tubes were already at their end of life, the primary reformer revamp was accomplished by retubing with thinner-walled microalloy tubes of the same outside diameter (115 mm). The new tubes guaranteed a heat flux within the specification for this section. Flue gas is withdrawn from the primary reformer by two parallel ID fans. As the existing back-pressure turbine drives of these fans were already operating at their maximum capability, they were replaced with new ones. The secondary reformer burner was replaced by a Casale Advanced Secondary Reformer Burner. The new burner imposes lower pressure drop than the original burner on both air and gas sides. It also produces a shorter flame, which prevents flame impingement on the surface of the catalyst bed, and more even process gas distribution, which allows a closer approach to the equilibrium. That in turn means that the methane slip is reduced and, consequently, the inerts content in the synthesis loop is lower. Fig. 6: Casale advanced design burner and gas distribution Nitrogen + Syngas 2014 International Conference & Exhibition (Paris February 2014) 115

8 S. Panza HTS internals To guarantee a reduction in operating pressure drop across the HTS converter, the axial-flow internals were replaced with the Casale axial-radial type. In an axial-radial catalyst bed most (about 90%) of the gas passes through the catalyst bed in a radial direction, resulting in much lower pressure drop than in a purely axial-flow catalyst bed. And not only is the pressure drop lower to start with: it rises significantly less as the catalyst ages. That means that the suction pressure at the synthesis gas compressor is always higher than in the unmodified plant and so compression costs are reduced. Mechanically the bed is very simple, comprising just two vertical perforated walls and one closure plate. Fig. 7: Axial-radial concept Fig. 7: Axial-radial concept CO2 removal section One of the targets of the revamp was to reduce the steam:carbon ratio substantially, from 3.8 down to To reach this target it would be necessary to revamp the CO2 removal section so as to keep the CO2 slip below 500 ppm mol. To that end Casale has co-operated with Giammarco Vetrocoke to adapt the CO2 removal system to the GV Low-Energy scheme. As mentioned, the CO2 removal unit in the TEC plant was designed with two absorbers and two regenerators. The only hardware modifications needed in this section were to reroute some pipework and install a process ejector to allow the two regenerators to run at different pressures in series instead of in parallel (Fig. 8). The final outcome of this modification is to increase the number of vapor/liquid equilibrium stages, which has the effect of reducing the regeneration heat requirement and, thus, steam consumption in the reboilers. 116 Nitrogen + Syngas 2014 International Conference & Exhibition (Paris February 2014)

9 Technological Engineering: The most efficient Way to raise your Plant to State of the Art Fig. 8: CO 2 removal section revamped to run on the GV Low-Energy scheme Machinery The air compressor is one of the main bottlenecks of the plant and, therefore, a complete revamp of the machine, including its steam turbine drive, is required to make the plant suitable for the increased plant load. The synthesis gas compressor train was theoretically adequate for the new operating conditions with the exception of the circulator wheel. However, in order to optimize the operation of this machine and reduce its energy consumption under the new conditions, it was decided jointly with the client to revamp this compression train as well. Under the new conditions the existing BFW pumps would be working too close to their rated conditions and, considering their age and that of their turbine drives, it was decided to install a smaller, electrically driven BFW pump in parallel with the existing ones. Nitrogen + Syngas 2014 International Conference & Exhibition (Paris February 2014) 117

10 S. Panza Baby synthesis converter revamping As indicated above, the original TEC plant synthesis loop was revamped several years ago through the installation of an additional ammonia converter in parallel with the original one. This additional converter is smaller (it is known as a baby converter) than the original main one and is based on three axial beds with quench cooling. In view of the fact that the main converter was going to be revamped by Ammonia Casale according to the axial-radial concept, Casale decided to revamp the baby converter as well so as to decrease the pressure drop across this revamped reactor and to increase its conversion efficiency. The selected layout had two beds and an indirect interchanger. Chilled water In spite of the significant load increase, the refrigeration section did not require any modification, thanks to the installation of new chilled water exchangers in the synthesis loop and in the syngas compressor interstage, just upstream of the existing chillers. The chilled water is obtained from a lithium bromide unit, which supplies water at a Fig. 9: Two-bed interchanger design temperature of 5-10 C, using as utilities LP steam (2 4 kg/cm2g) to regenerate the system and cooling water to remove the heat exchanged. The Li-Br units are particularly useful because they use low-level waste heat, avoiding the revamping of costly items such as the refrigerant compressor, and avoiding an increase in the consumption of valuable energy such as MP steam (35-40 kg/cm2g). Plant performances On the basis of the outcome of previous revamp projects the expected improvement in plant performance after revamping is shown in Table 2. Fig. 10: Li Br unit 118 Nitrogen + Syngas 2014 International Conference & Exhibition (Paris February 2014)

11 Technological Engineering: The most efficient Way to raise your Plant to State of the Art Table 2: Plant Performance Before and After Revamp Prerevamp Post revamp (expected) Production, MTPD Energy saving, Gcal/MT 1.04 Specific energy consumption, Gcal/MT New ammonia absorption refrigeration unit for GIAP AM70 plant GIAP ammonia plants are different from all the other ammonia plant types in the world in that they use ammonia absorption refrigeration to provide the chilling needed to condense the ammonia produced. As they are such a rarity not much effort has been made in the past to make improvements to these units. Ammonia absorption refrigeration systems have, in fact, been in use for more than 70 years. They are sealed systems and are known to be very reliable; they are virtually maintenance-free. The only moving parts in the circuit are a number of pumps. The instrumentation is simple and the control of the plant can be highly automated. The power consumption of an absorption refrigeration unit is very low, but it does require a source of low-grade heat. This heat can be supplied either as low-grade steam, from a gas burner, or any from other source of heat that might be available. The popularity of absorption refrigeration waned in the face of the development of more reliable process compressors, which made it possible to design and supply a refrigeration circuit at much lower cost. The disadvantage of compression-based refrigeration systems, however, is that high-grade power or steam is required to drive the compressor. Over the last decade the structure of the compressor market has changed. Larger compressor manufacturers have swallowed up the smaller manufacturers, thereby significantly reducing competition. The drive for ever larger compressors, in particular for LNG plants, has resulted in the principal suppliers concentrating their attention ever more on the larger end of the market, and this has led to a marked increase in prices at the less interesting smaller end of the market. There is also evidence that western vessel manufacturers may now be employing more advanced manufacturing technologies to remain ahead of or at least competitive with Chinese prices. All these factors have made ammonia absorption refrigeration competitive once more as a means of recovering product from the ammonia synthesis loop. Nitrogen + Syngas 2014 International Conference & Exhibition (Paris February 2014) 119

12 S. Panza cooling fluid rich solution heating fluid condenser NH 3 vapour + solution heat exchanger chiller (evaporator) lean solution cooling fluid Fig. 11: Ammonia absorption refrigeration system As stated previously, the GIAP AM 70 ammonia plant is designed with three types of ammonia absorption unit, capable of supplying ammonia at +1 C, at -10 C and at -30 C. The most critical ammonia absorption unit is the one that supplies ammonia to the synthesis loop chillers 606A and 606B. These chillers work with ammonia at -10 C and, especially during summer, are not able to guarantee the same performances as during the cold season. The main reason for this underperformance is the use of air coolers as the rectifier cooling unit in the existing ammonia absorption unit. With the aid of a selected ammonia absorption refrigeration system designer, Casale has designed a package which is able to supply ammonia at -10 C, in which the rectifier cooling unit is an evaporative condenser. Moreover Casale engineered the integration of this new unit in the existing ammonia absorption refrigeration units. All the project steps, from basic engineering up to construction, were controlled by Casale. The new AXY was the biggest ever supplied and installed in Russia (~10 Gcal/h). CONCLUSIONS The revamp schemes in the cases described above are tailored to the particular requirements of specific ammonia plants of a certain age and technological level. The age of the equipment was obviously a prime concern and a major influence on the ultimate nature of the revamp. But the broad principles set out in this paper are applicable to any ammonia plant anywhere: even plants built in recent years can benefit from revamping. All that is needed are clear objectives for the revamp, minute analysis of the plant and its performance record, sound and appropriate revamp technologies, scrupulously rigorous engineering, accurate judgment of the value of the various alternative possibilities in relation to their cost, and careful operator training to ensure that the plant is properly run when it returns to service. When all the necessary skills and experience are under one roof, the total is equal to substantially more than the sum of the parts. That is what Casale means by Technological Engineering. 120 Nitrogen + Syngas 2014 International Conference & Exhibition (Paris February 2014)