AMMONIA SYNTHESIS CONVERTER

Abstract: A horizontal, cold wall ammonia converter having a plurality of transverse-flow catalyst beds with a first vertical heat exchanger (19) located between the first (11) and second (12) catalyst beds and a second vertical heat exchanger (22) located between the second (12) and third (13) catalyst beds.

Description: This invention relates to horizontal converters for exothermic, catalytic synthesis of ammonia from hydrogen and nitrogen. In particular, the invention relates to a horizontal ammonia converter having a first and a second end which comprises a cylindrical pressure shell having outer heads contiguous therewith at the first and second ends; a cylindrical inner shell disposed within substantially the entire length of the pressure shell and forming a shell annulus therewith, the inner shell having first and second end closures contiguous with the cylindrical inner shell, the first end outer head forming a first gas plenum with the first end closure and the second end outer head forming a second gas plenum with the second end closure; synthesis gas inlet means disposed in the second end outer head in serial fluid communication with the second gas plenum, the shell annulus, and the first gas plenum; product gas outlet means; first, second, and third slab-flow catalyst beds defined by transverse bulkheads within the inner shell and opposed side portions of the inner shell, the first catalyst bed being proximate the first end of the converter and the second catalyst bed being spaced between the first and third catalyst beds, each of the catalyst beds having a top inlet portion and a bottom outlet portion; a first heat exchanger disposed between the first and second catalyst beds; longitudinal conduit and baffle means for conducting synthesis gas successively from the tubes of the first heat exchanger downwardly through the first catalyst bed, across tube exteriors of the first heat exchanger, downwardly through the second catalyst bed, and downwardly through the third catalyst bed; and means for conducting gas from the bottom outlet portion of the third catalyst bed to the product gas outlet means. The synthesis is well known and is typically conducted at pressures within the range from about 70 to about 325 bars and temperatures within the range from about 340°C to about 525°C. A single converter is generally employed in modern, large capacity ammonia plants. In a 1000 metric ton per day plant, the catalyst volume will range from about 40 to about 90 m³ and be contained in a converter having a diameter from about 2 to about 4 m and length or height from about 10 to about 35 m. Catalyst beds within the converter may be arranged for transverse flow, radial flow, or axial flow of gas. Transverse flow converters are quite common and usually employ a cold wall, double shell design which provides a shell annulus for passage of cooling gas adjacent the outside pressure shell. The converter of the present invention is a cold wall, transverse or "slab" flow converter. It is not feasible to contain the entire catalyst volume in a single catalyst bed because of reaction equilibrium considerations and the possibility of catalyst overheating and damage. It has, therefore, been common practice to arrange the catalyst in multiple beds with provision for interbed or intrabed cooling. Customarily, this cooling is provided by interbed injection of cool synthesis gas for direct heat exchange with partially converted gas (i.e. - a direct quench converter or some combination of direct gas quench with interbed heat exchangers of the shell and tube type). Converter designs which emphasize direct quench tend to be less costly than combination designs since fewer, smaller, shell and tube exchangers are employed according to the volume of quench gas introduced. Plants employing these designs do, however, suffer the penalty of higher synthesis gas compression costs since the totality of synthesis gas to the converter does not contact the totality of the catalyst therein. Therefore, more gas must be circulated to obtain a given amount of ammonia product. From the foregoing, it may be appreciated that ammonia converters are large, complex items of equipment and that steps toward more efficient, less costly design are needed. US-A-4 452 760 discloses a horizontal ammonia converter having one single heat exchanger. This one heat exchanger is located at the start of the synthesis gas flow through the catalyst beds. The tubes of this U-tube heat exchanger run parallel to the longitudinal axis of the converter. Apart from the fact that this preknown converter only comprises one heat exchanger, resulting in a relatively complex gas conduit, the horizontal array of the tubes thereof leads to a relatively large axial length of this converter. Therefore, comparatively high constructional expenditure is requested, influencing the height of the production costs. Further, US-A-3 440 021 discloses a converter which shows a very complex gas flow, meaning high constructional expenditure. Moreover, several concentric tubes extend along its central axis, which tubes also extend through the catalyst beds, so that the efficiency of these catalyst beds is low. It is the object of the invention to provide a horizontal ammonia converter of simple and less costly construction, having a particularly small axial length. This object is solved in that a second heat exchanger is disposed between the second and third catalyst beds, and the first and second heat exchangers have vertical tubes with respect to the horizontal converter axis and the longitudinal conduit means are disposed within the cylindrical inner shell externally from the catalyst beds. The use of two heat exchangers having vertically extending heat exchanger tubes, on the one hand achieves that the heat exchanger productivity adapts to the requirements of the process, and on the other, that the converter is of relatively small constructional length. Furthermore, the gas flow is simple, leading to economy of equipment cost. Gas flows in series through a gas inlet at one end of the converter, the shell annulus, the first gas plenum, first longitudinal conduit means disposed within the cylindrical inner shell below and externally proximate to the first and second catalyst beds providing fluid communication between the first gas plenum and the inlet portion of the second heat exchanger, the cold tube sides of the second heat exchanger, second longitudinal conduit means disposed within the cylindrical inner shell above and externally proximate to only the second catalyst bed providing fluid communication between the outlet portion of the second heat exchanger and the inlet portion of the first heat exchanger, then the cold tube sides of the first heat exchanger, the first catalyst bed located at the opposite end of the converter, across tube exteriors of the first heat exchanger that is the hot side of the first heat exchanger, through the second catalyst bed disposed between the two heat exchangers, across tube exteriors of the second heat exchanger that is the hot side of the second heat exchanger, through the third and, usually, fourth catalyst bed, and finally through a gas outlet proximate the final catalyst bed and, if necessary, located at the same end of the converter as the gas inlet. Moreover, the first heat exchanger and the second heat exchanger could have vertical tubes with inlet and outlet portions, the first heat exchanger having vertical U-tubes with inlet and outlet portions at the upper end of said heat exchanger. The converter of the invention employs no external quench gas between or within the catalyst beds. That is to say, the hot, shell sides of the heat exchangers are in exclusive flow communication with their respective upstream and downstream catalyst beds. Accordingly, all of the converter outlet gas passes initially through the first catalyst bed. Figure 1 is a preferred embodiment of the invention in which the horizontal converter employs a single-pass heat exchanger between the second and third catalyst beds. Figure 2 is another embodiment of the invention wherein a. U-tube exchanger is employed between the second and third catalyst beds. Figure 3 shows cross-sections of the converter illustrated in Figure 1 at A-A and BB. Referring to Figures 1 and 2, the horizontal converter is contained within cylindrical pressure shell 1 which is attached to first end outer head 2 shown at the right-hand side of the drawing and second end outer head 3 shown at the left-hand side of the drawing. A cylindrical inner shell 4 is disposed within and parallel to the pressure shell and extends substantially through the entire length thereof to form shell annulus 5. First end closure 6 and second end closure 7 are attached to ends of the inner shell and form first gas plenum 8 and second gas plenum 9, respectively. Gas inlet 10 is mounted in second end outer head 3 so that relatively cool ammonia synthesis gas entering the inlet flows freely into the second gas plenum 9, shell annulus 5, and then to the first gas plenum 8. First, second, third, and fourth adiabatic catalyst beds 11, 12, 13, and 14 are contained within the inner shell in such manner that opposed side portions of the inner shell define the lateral extremities of the beds. The beds are longitudinally defined by transverse partitions 15 which, as indicated in the drawings, having openings for passage of gas. End closures 6 and 7 are conveniently employed as end partitions for catalyst beds 11 and 14. Each of the catalyst beds has a top inlet portion 16 and a bottom outlet portion 17, the catalyst particles being supported by grids 18 and screens not shown. The catalyst beds are, therefore, arranged for downward flow of gas in a path substantially normal to the axis of the converter in what is commonly referred to as "slab-flow". A first tubular heat exchanger 19 having tube inlet portion 20 and tube outlet portion 21 is located between first catalyst bed 11 and second catalyst bed 12. Since synthesis gas entering the first catalyst bed contains little ammonia, conversion to ammonia with accompanying gas temperature rise is greater here than in the downstream catalyst beds. In order to provide the large heat exchange duty required for cooling gases leaving the first bed prior to their entry to the second bed, the first exchanger 19 employs vertical U-tubes which provide high internal tube gas velocity and, therefore, high heat transfer rates which thereby minimize the size of this exchanger. A second tubular heat exchanger 22 having tube inlet portion 23 and tube outlet portion 24 is located between second catalyst bed 12 and third catalyst bed 13. This exchanger may also be a vertical U-tube exchanger as shown in Figure 2 but, since less heat transfer duty is required, is preferably a single-pass, vertical tube exchanger as shown in Figure 1. Use of a single-pass exchanger minimizes the pressure drop. The flow path through the first and second catalyst beds and first and second exchangers is provided by longitudinal conduits within the cylindrical inner shell 4 above and below the catalyst beds. First longitudinal conduit 25 is located below both beds 11 and 12 and connects first gas plenum 8 with tube inlet portion 23 of second heat exchanger 22 where initial preheating of synthesis gas takes place in the tube side of the exchanger. Initially preheated gas then flows through second longitudinal conduit 26 located above bed 12 to tube inlet portion 20 of first heat exchanger 19 for final preheating. Fully preheated gas then flows from tube outlet portion 21 of the first heat exchanger to the top inlet portion of the first catalyst bed 11. As may be seen from the drawings, the alternating arrangement of catalyst beds and interchangers is quite easily adapted to gas flow characterized by low pressure drop through suitable baffling indicated generally by 27. These baffles as well as the partitions of the catalyst beds provide means for conducting gas successively from the tube outlet portion 21 of the first heat exchanger downwardly through first catalyst bed 11, across tube exteriors of the first heat exchanger, downwardly through second catalyst bed 12, and then across tube exteriors of second heat exchanger 22. In figure 1, gas flowing across tube exteriors of heat exchanger 22 passes through an opening in central partition 28 and then flows upwardly through the channel formed by adjacent transverse partition 15 and into top inlet portion 16 of catalyst bed 13. Since the converter of Figure 2 employs vertical, U-tube exchangers, first longitudinal conduit 25 is then extended by vertical conduit 29 to convey gas to the tube inlet portion 23 of the second heat exchanger which, in this instance, is a gas plenum formed by chord-shapped baffle 30 and adjacent baffle 27. In Figure 2, all other reference numerals are the same as for in Figure 1. Since gas flowing between catalyst beds 13 and 14 is not cooled, it is apparent from a process viewpoint that these constitute a single adiabatic conversion zone which may be physically embodied in one, two, or possibly three beds according to relevant process design parameters such as overall converter pressure drop and extent of conversion. In the embodiment of the invention shown in the drawings, the means for conducting gas from the bottom outlet portion of third catalyst bed 13 to gas outlet 31 located in second outer head 3 includes fourth catalyst bed 14 at the second end of the converter. The third and fourth beds are spaced apart by gas channel 32 formed by adjacent transverse partitions 15 of the third and fourth beds. As previously indicated, the converter of the invention does not employ interbed quenching to cool partially converted gas. Cool synthesis gas is, however, optionally introduced to tube inlet and outlet portions 20 and 21 by respective by-pass pipes 33 and 34 in order to provide close temperature control over the gas entering the first and second catalyst beds.

Claims: A horizontal ammonia converter having a first and a second end which comprises a cylindrical pressure shell (1) having outer heads (2, 3) contiguous therewith at the first and second ends; a cylindrical inner shell (4) disposed within substantially the entire length of the pressure shell (1) and forming a shell annulus (5) therewith, the inner shell (4) having first and second end closures (6, 7) contiguous with the cylindrical inner shell (4), the first end outer head (2) forming a first gas plenum (8) with the first end closure (6) and the second end outer head (3) forming a second gas plenum (9) with the second end closure (7); synthesis gas inlet means (10) disposed in the second end outer head (3) in serial fluid communication with the second gas plenum (9), the shell annulus (5), and the first gas plenum (8); product gas outlet means (31); first, second, and third slab-flow catalyst beds (11, 12, 13) defined by transverse bulkheads (15) within the inner shell (4) and opposed side portions of the inner shell (4), the first catalyst bed (11) being proximate the first end of the converter and the second catalyst bed (12) being spaced between the first (11) and third (13) catalyst beds, each of the catalyst beds (11, 12, 13) having a top inlet portion (16) and a bottom outlet portion (17); a first heat exchanger (19) disposed between the first (11) and second (12) catalyst beds; longitudinal conduit and baffle means (25, 26) for conducting synthesis gas successively from the tubes of the first heat exchanger (19) downwardly through the first catalyst bed (11), across tube exteriors of the first heat exchanger (11), downwardly through the second catalyst bed (12), and downwardly through the third catalyst bed; and means for conducting gas from the bottom outlet portion (17) of the third catalyst bed (13) to the product gas outlet means (31), characterized in that a second heat exchanger (22) is disposed between the second (12) and third (13) catalyst beds, and the first and second heat exchangers (19, 22) have vertical tubes with respect to the horizontal converter axis and the longitudinal conduit means (25, 26) are disposed within the cylindrical inner shell (4) externally from the catalyst beds (11, 12, 13). The converter of claim 1, characterized in that the means for conducting gas from the bottom outlet portion (17) of the third catalyst bed (13) includes a fourth slab-flow catalyst bed (14). The converter of either claim 1 or claim 2, characterized in that the second heat exchanger (12) has single-pass tubes.

Inventors: NOE, STEPHEN A.
Applicants: THE M. W. KELLOGG COMPANY
Application year: 1987
Main classification: B01J8
IPC codes: B01J8, C01C1