NZ788625A - Axial/radial flow converter - Google Patents

Abstract

a cooled axial/radial flow converter, in which process gas passes from an outer annulus via a catalyst bed to an inner centre tube, the catalyst bed is divided into identical modules stacked on top of each other. The process gas reaches the catalyst through openings facing the outer annulus, passes axially down the catalyst bed of each module, leaves the module through collectors in the bottom thereof, and flows to the centre tube. The catalyst bed is cooled by cooling panels, in which the process gas is pre-heated to the reaction temperature, while at the same time the heat of reaction is partly removed from the catalyst bed. The converter is especially suitable as ammonia converter. es axially down the catalyst bed of each module, leaves the module through collectors in the bottom thereof, and flows to the centre tube. The catalyst bed is cooled by cooling panels, in which the process gas is pre-heated to the reaction temperature, while at the same time the heat of reaction is partly removed from the catalyst bed. The converter is especially suitable as ammonia converter.

Description

Title: Axial/radial flow converter The t invention relates to a novel axial/radial flow converter, preferably – but not exclusively – for use as an ammonia converter.

The entire disclosure in the te specification of New Zealand Patent Application No. 748600 is by this cross-reference incorporated into the present specification.

Ammonia converters are complicated due to the fact that the synthesis of ammonia from nitrogen and hydrogen gas (in an approximate ratio of 1:3) is exothermic, and the reactions take place at high atures and pressures. Thus, interstage cooling is generally used between a series of catalyst zones to in kinetic and equilibrium conditions appropriate for optimum conversion efficiency. There must also be provisions made for ing the catalyst zones, e.g. periodically removing and replacing st when it loses its effectiveness.

Because ammonia converters are complicated, but also very important pieces of equipment, many efforts are made to improve their efficiency. Thus, US 2004/0096370 discloses a split-flow vertical ammonia converter, in which a fixed-bed catalyst zone is configured into two mechanically ted catalyst volumes and two gas streams operating in parallel. This design maintains the ratio of gas flow to st volume so that there is no catalyst effectiveness loss. The catalyst beds and gas flow paths are configured so that the gas flow is downwards through each st volume. 18693555_1 (GHMatters) P44612NZ01 ing to US 014137, ammonia is produced in a converter in which pseudo-isothermal conditions can be approached by convection cooling of a reaction zone by positioning at least a portion of said zone in ct contact with a flow of hot gas, such as exhaust gas or pre-heated air.

The use of axial-radial flow reactors in synthesis processes is not novel in . It is e.g. disclosed in US 5.427.760, which describes axial-radial reactors in the Braun synloop with external heat sink. In US 4.372.920, an axial-radial reactor for use in heterogeneous synthesis is described, and US .352.428 deals with high-conversion ammonia synthesis. Fig. 4 of the latter US patent is an illustration of an axial-radial flow reactor suitable for use in the apparatus and s de- scribed.

US 2002/0102192 A1 describes a catalytic reactor wherein ax- ial-radial flow may be achieved with the consequent advantages of a reduced pressure ential, but without any “complex reactor internals”. The reactor has inlet and outlet ports and a bed of particulate catalyst disposed round a central region communicating with one of the ports and presenting less resistance to flow than the catalyst particles. The central region within the catalyst bed has a height equal to at least a major part of the height of the catalyst bed, and the exterior surface of the catalyst bed less than that of the reactor, thus leaving a space n the or surface of the catalyst bed and the interior walls of the reactor, said space being filled with a particulate material with less resistance to flow than the catalyst particles. 18693555_1 (GHMatters) P44612NZ01 In EP 2 167 226 B1, a wall system for catalytic beds of re- actors for heterogeneous synthesis 0: chemical compounds is disclosed. The reactors are equipped with fixed catalyst beds crossed by a gaseous ‘low 0" synthesis gas, particu- larly with axial—radial flow. The design may resemble that o: the present invention, but the er concept is not A multi-bed catalytic ter with inter-bed heat ex- rs, comprising a plurality o: superimposed catalytic beds and a common heat exchanger, is disclosed in EP 2 759 338 A1. The design of this converter does not have much in common with the design 0: the axial/radial flow converter o: the present invention.

Finally, US 2004/0204507 bes a cooled axial/radial ow converter comprising an annular catalyst bed and a p'urality o cooling panels arranged in a radial pattern inside the st bed and surrounding a central pipe. The 2O catalyst bed and the shel' o: the converter forms an outer annulus through which a s gas is passed to the cata- lyst bed. The process gas ‘lows in axial—radial direction through the catalyst bed and is subsequently colleCted in the central pipe. The axia'/radial ‘low converter 0: ,he present invention di ers "rom that ol the US application in that the catalyst bed is divided into a number 0: iden- tical modules stacked on top 0: each Other and also in that the process gas is passed through the cooling panels to pre-heat the gas.

When low pressure drop is required in a ‘ixed bed catalytic converter, a radial ‘low type converter is often selected.

However, in special cases, such as cooled catalyst bed, st shrinkage or catalyst particles having low strength combined with a high catalyst bed, this solution is not practical, and instead inter-bed cooling or parallel reactors must be selected.

A solution could consist in replacing the radial flow bed with a stack 0" identical axial ‘low canisters. Although the flow through each individual canister is axia', the as- lO sembly can have a flow n as a radial flow reaCtor, for instance taking "eed "low "rom an outer annulus and disposing the r e""'uen, ,0 an inner tube. The bed height can be adjusoed ,o m o oh r quir m nt for pressure drop and catalyst oh wiohout changing the principal layout 0: the reactor.

Thus, the present invention relates to a cooled axial/ra- dial flow converter, in which process gas passes from an outer annulus via a catalyst bed to an inner centre tube, wherein - the catalyst bed is divided into a number 0: identical modules stacked on top 0: each other, - the "eed "'ow 0" s gas reaches the catalyst through gs facing the outer annulus, passes axially down the catalyst bed 0: each module, leaves the module through collectors in the bottom thereo:, and flows to the centre tube, and - the catalyst bed is cooled by g panels, in which the process gas is pre-heated to the reaction temperature, while at th sam tim th h at 0‘ reaction is partly re— moved from the catalyst bed.

Cooling of the catalyst is possible by insertion O" verti— cal cooling plates installed radially in each canister: Zn— side the panel, incoming gas is heated up to reaction tem- re, at the same time removing heat 0" reaction ‘rom the catalyst. This principle is shown in the attached Fig— The Figure is a side view and a top view 0" a preterred em- bodiment o: the er. In the side view, a represents the total height, b is the height from the bottom to the ,op or the exchanger plates, and c is the catalyst height.

Further, d represents the outlet pro‘iles O" the canister.

In the top view of the canister, it is seen that this em- bodimen, or the canister comprises both a number 0: small exchanger plates (1) and a number 0: main exchanger plates 2O (2). In this embodiment, the number 0: small plates is larger than that 0: main .

Alternatively, the flow pattern can be simple adiabatic, and inter-bed cooling can be provided by ling a heat exchanger in the inner chamber. In this way, several reac- tion steps within th sam pr ssur sh ll will be possible.

The selection depends on the intended servicing.

One possible application 0: the novel axial/radial Slow converter 0: the ion is to use it as an ammonia con- verter, as will be described in more detail below. The Ha- ber-Bosch ammonia synthesis s to the “autothermic process” category, meaning that it is an exothermic chemi— cal reaction ‘or which the temperature is maintained by the heat 0: reaction alone. In order to achieve this ion, gas flow and heat exchange are arranged to reduce the in- crease in ature associated with the exothermic reac- tion and to suppress the need for an external source or heat once the reaction is started.

Cooling of the catalyst in ammonia production is well—known from the classical TVA converter. In the catalyst section 0: the TVA converter, the pre-heated gas flows up inside a large number 0: small tubes. There it absorbs par, 0: ,he heat generated by the chemical reaction on the caualysu. At the ,op or ,he converter the synthesis gas, now brought to a su "icient temperature, reverses its direCtion and flows down the catalyst bed where the reaCtion occars. r, the TVA converter had tull axial Slow, resulting in a high 2O pressure drop and the need for parallel converters to ob- tain high production capacities.

The axia'/radial ‘low converter according to the invention, preterab'y ‘or use as an ammonia ter, has a main flow pattern similar to ,ha, 0" a radial ‘low converter, which means that gas passes from an outer annulus via a catalyst bed to an inner centre ,ube. However, ,he jlow pattern in the catalyst bed is di "erent in that ,he st bed is divided into a number 0: modules stacked on ,op or each other. The "eed "low to each module reaches the catalyst h porus jacing the outer annulus. Then it flows axi— ally down the catalyst bed and exits the module through collectors in the bottom and flows to th c ntr tub wh I all reactants are collected.

The catalyst bed is cooled by cooling panels in which the feed gas is pre-heated to the reac :ion temperature while, at the same time, removing the hea, o reaction from the catalyst bed.

For an ammonia converter, a number 0: advantages can be ob- :ained, such as a lower pressure drop in the converter i; that is desired. Further it may be le to utilize smaller catalyst sizes, i.e. below 1.5 mm.

A higher conversion in the first bed(s) can also be ob- tained due to intra-bed cooling. Further there is a possi- bility o: obtaining the same conversion in one bed as pre- viously was obtained over the first two beds.

The bed can be loaded outside the ter. "n fact, it 2O may be loaded in the catalyst plant, :h n r duc d, s al d and shipped and finally d direc:ly in the converter shell.

The axial/radial converter 0: the inven':ion is suitable "01” revamp es as well as for grassroo:s projects. In the latter case, however, a "ul' er converter cover is required.

The idea underlying the present invention is to combine the virtues o: the TVA converter and the radial flow converter, but keeping the flow pattern outside the new bed exactly the 1 same as in ant’s radial flow converters, also :or the convenience o: revamp.

The flow concept may also be used for other types 0:I COH— verters, for which a low pressure drop is desired or in case the catalyst shrinks during the reduction process.

Claims (4)

Claims:

1. A cooled axial/radial flow converter, in which pro- cess gas passes from an outer s via a catalyst bed to an inn r c ntr tub wh r in - the catalyst bed is divided into a number 0: identical modules stacked on top 0: each other, 10 - the eed "low 0" process gas reaches the catalyst through openings facing the outer annul_us, passes axially down the catalyst bed 0: each module, l_eaves the module h collectors in the bottom thereo:, and flows to the centre tube, and - the catalyst bed is cooled by cooling panels, in which the process gas is pre-heated to the reaction temperature, while at th sam tim th h a, 0 reaction is partly re— moved from the catalyst bed.

2. Converter according to claim 1, which is used as an a converter.

3. Converter according to claim 1 or 2, wherein the 25 module is an axia' ‘low er comprising a number 0; small exchanger plates and a number 0: main exchanger plates.

4. Converter according to claim 3, wherein cooling I) 30 the catalyst is achieved by insertion 0" vertical cooling plates installed radially in each canister.