Heat Exchanger

Description

The present invention relates to a heat exchanger.

More specifically, the present invention relates to a heat exchanger for the rapid cooling of high temperature gas.

Even more specifically, the present invention relates to a heat exchanger for the cooling of synthesis gas (syngas) coming from the catalytic partial oxidation of light hydrocarbons, for example methane, known as Catalytic Partial Oxidation (CPO).

It is known that the preparation of syngas, a gaseous blend containing H₂ and CO in various proportions, can be effected by the catalytic partial oxidation of natural gas, methane or gaseous/liquid hydrocarbon mixtures, from refinery or petrochemical facilities, in fixed bed tubular reactors which can operate, depending on the catalyst used, at a pressure ranging from 1 to 150 atm and temperatures higher than 500° C., which, in some cases, can reach and exceed 1,000° C.

The rapid cooling of syngas at the outlet of the reaction unit is a necessity not to be neglected, as if this gas is maintained at those temperatures, even for short periods of time, it can give undesired by-products, such as alcohols or olefins (substantially ethylene and propylene) or even regenerate the starting methane. Systems for rapidly cooling a gas which is at a high temperature, are mentioned in literature, for example in the patents U.S. Pat. No. 2,896,927 and U.S. Pat. No. 4,377,132 or in “Syngas Cooler Systems for Gasification Plants”, an ALSTOM brochure. Some methods and relevant equipment include the direct cooling of gas by means of water (quenching). This solution however has the drawback of having to separate the cooled syngas from the aqueous vapour formed.

Other industrial systems consist of equipment for indirect cooling, which allow the recovery of heat contained in syngas, for the production of high pressure steam.

The object of the present invention relates to a device for the effective and rapid indirect cooling of syngas in applications in which the thermal recovery of the sensible heat of the gas is not required, for engineering simplicity or for economical reasons. For example in the production of hydrogen in medium-small-scale systems.

The Applicants have therefore found a heat exchanger, particularly suitable for the rapid cooling of gases which are at a temperature higher than 500° C., for example between 750 and 1100° C., and which allows to avoid any contact between the hot gas and the cooling liquid, normally water.

The object of the present invention therefore relates to a heat exchanger for the rapid cooling of a gas at high temperature, leaving a reaction unit/device, which comprises a coupling element to the reaction unit/device, a gas cooling and transportation pipe and a covering shell, in which:

• a) the coupling element, substantially cylindrical, is situated between the reaction unit and the shell of the exchanger, it is internally cooled by means of a cooling fluid, normally water, and is axially connected, by means of a pass-through duct, to the feeding line of the hot gas coming from the reaction unit;
• b) the transportation and cooling pipe of the hot gas is fixed to the base of the coupling element, connected to the reaction unit, at the pass-through duct, said hot gas transportation and cooling pipe consisting of two sections:
  • the first section, substantially linear, is inserted in a coaxial position in a second pipe having a larger diameter which covers it, so as to form an annular hollow space in which the cooling fluid, generally water, flows, an end of said first section forming the pass-through duct of the coupling element;
  • the second section, fixed in a continuous manner to the first section, at the other end, substantially curved in a semicircle, spirally envelopes, without touching it, at least part of said first jacketed section;
• c) the covering shell is substantially cylindrical, closed at one end and open at the other end, it is connected to said coupling element, and comprises at least the openings for discharging the cooling liquid and the cooled gas.

According to the present invention, the coupling element is axially crossed by the pass-through duct, connected to the reaction device, for example a CPO reactor for the production of syngas at a temperature ranging from 500 to 1100° C.

The external pipe which covers the first section of the transportation pipe of the hot gas is connected, at one end, to one or more specific feeding ducts of the cooling liquid, which pass through the coupling element. The coupling element, moreover, is independently cooled by means of a duct which feeds the cooling liquid in correspondence with its axis. Said fluid is discharged from the element, after following a spiral path from the inside outwards, by means of an opening connected to the side surface of the element itself.

In an alternative embodiment of the present invention, the cooling fluid circulating inside the coupling element (for independent cooling) can be discharged inside the volume contained in the shell of the exchanger.

The other end of the second pipe, which covers the first section, is free and ends with the curved section, substantially in a semicircle, so that the cooling liquid can debouch freely, but in the opposite direction, into the closed space of the shell, after flowing in the jacket between the two pipes.

The path of the liquid inside the shell volume is guided by baffles, orthogonal to the axis, which also act as a support for both sections of the gas transportation pipe.

The second section of the transportation and cooling pipe is substantially continuous to the first one, without interruption, and develops in a spiral. In order to save space, the spirals preferably envelop, without touching it, the first section of the covered pipe. It is possible however for the spirals to develop downstream of the first section.

The other end of the transportation pipe, i.e. the end of the spiral section, is connected to an opening present on the shell for the discharge of the cooled gas outside the heat exchanger, object of the present invention.

The shell has a substantially cylindrical form with the diameter of the base substantially identical to that of the coupling element and larger than the diameter of the spirals. In this way, the shell includes in its inner space the pipe system of the first and second section. The space of the shell is filled with the cooling fluid, which is discharged by the exchanger through a proper discharge opening. In an alternative embodiment of the present invention, the circulating liquid, destined for the cooling of the coupling element, also converges into the shell space. The total liquid is discharged from the exchanger, object of the present invention, through the proper opening situated on the shell. In any case, whether operating with the first or second alternative embodiment, the pipe system of the first and second section is completely immersed in the cooling liquid.

The heat exchanger object of the present invention can be better understood by referring to the schemes of the enclosed figures which represent an illustrative but nonlimiting, embodiment, and wherein:

FIG. 1 represents a longitudinal, flat sectional view of the overall exchanger;

FIG. 2 represents a front flat view of the scheme of FIG. 1, produced according to the section ZZ.

With reference to the figures, the heat exchanger, object of the present invention, comprises the coupling element A, the pipe system for the gas transportation and cooling B and the shell C.

The coupling element A also includes the ducts 1 and 2 for the feeding of the cooling fluid (water), which converge into the coaxial duct 7, and the cooling duct 4, for the independent cooling of the coupling element, which feeds the water to the center of the spiral 4′, from which it exits through 5.

The gas transportation and cooling pipe system B comprises the first pipe section 6, the coaxial pipe which jackets it 7 and the second section of the spiral pipe 8. The first section of the pipe 6 includes, in turn, the first end 3, coinciding with the axial pass-through duct of the element A, and the second curvilinear end 3′. The coaxial pipe 7 jackets the first section starting from the end 3 until the curved end 3′. At this end (3′) the coaxial pipe is not closed, to allow the water to be discharged inside the shell, as will be described further on.

The shell C includes the discharge opening 9 of the cooled gas, the discharge opening 10 of the water and the supporting baffles 11 of the two sections of the gas transportation pipe.

The operation of the heat exchanger, object of the present invention, will appear evident on the basis of the drawings and what is described above. In particular, the hot gas 12, leaving the reaction unit (not shown), is introduced into the heat exchanger by means of the pass-through duct 3 of the coupling element A. The gas flows into the first section 6 of the cooling and transportation pipe B and subsequently into the second section 8, to be then discharged at a low temperature through the discharge opening of the gas 9. As the gas flows through the first section 6, it undergoes a first rapid cooling by means of the water, fed through 1 and 2, which circulates inside the annular hollow space between the pipes 6 and 7, up to the end 3′. Here, the water flows freely in the closed space of the shell, filling it, it further cools the gas flowing through the section 8 of the cooling pipe and is discharged from the opening 10.

During operation, in order to prevent overheating of the coupling element, the latter is cooled by means of the specific system consisting of the duct 4, which feeds water to the system 4′ developing in a spiral, and of the discharge duct 5.