Shell-and-tube apparatus for heat recovery from a hot process stream

Abstract

A shell-and-tube apparatus (1), suitable for use as a waste heat boiler, comprising a vessel with an exchanging section (2) and a separating section (3), wherein: said exchanging section (2) contains a bundle of U-tubes (4) fed with an evaporable liquid medium such as water (W) and exposed to a hot gas (G) flowing in a hot chamber around said tubes, so that said medium is partially evaporated in the tubes while recovering heat from hot gas flowing in the hot chamber (7); said separating section (3) comprises a collection chamber (16) in communication with outlet of the tubes (4) to receive the partially evaporated medium leaving the tubes; said separating section (3) is arranged to provide separation of vapour fraction and liquid fraction from the partially evaporated medium at least partially by gravity; the apparatus also comprises means for controlling the liquid level in the collection chamber and for a partial recycle of the non-evaporated liquid.

Claims

1. A shell-and-tube apparatus comprising a vessel with an exchanging section and a separating section within said vessel, wherein said exchanging section contains a bundle of U-tubes having respective tube inlet ends and tube outlet ends, and a chamber around said tubes, said chamber being in communication with an input for a process stream, said separating section comprises a collection chamber in communication with said outlet ends of the tubes; wherein said apparatus has an input for an evaporable liquid medium and a chamber within said vessel for receiving said evaporable liquid medium through said input, which is in communication with said tube inlet ends; wherein said evaporable liquid medium has a temperature, which is lower than the temperature of said process stream so that, during operation, said tubes are exposed to said process stream while the process stream traverses said chamber around said tubes, and the evaporable medium is heated and at least partially evaporated by flowing inside said tubes, and the at least partially evaporated medium is admitted to said collection chamber after leaving said tubes; wherein in said separating section, vapour fraction and liquid fraction separate from said at least partially evaporated medium; wherein said chamber for receiving said evaporable liquid medium is not part of said separating section; wherein no separation of vapour takes place in said chamber for receiving said evaporable liquid medium; wherein each tube has a first straight portion starting from the inlet end, where the evaporable medium flows downward, a second straight portion where said medium flows upwards until it reaches the outlet end of the tube, and a U-shaped portion that connects said straight portions; wherein the vessel comprises an outlet for non-evaporated liquid medium; and wherein the apparatus is configured to mix, externally to the vessel, a portion of non-evaporated liquid medium extracted from the outlet for non-evaporated liquid medium with the evaporable liquid medium fed to said input for evaporable liquid medium.

2. The apparatus according to claim 1, wherein in said separating section separation of vapour takes place at least partially by gravity.

3. The apparatus according to claim 1, comprising control means for maintenance of a controlled liquid level in said collection chamber.

4. The apparatus according to claim 3, wherein said control means is operable in such a way to maintain a volume inside the collection chamber and above a liquid level which is sufficient to allow separation of a vapour fraction by gravity.

5. The apparatus according to claim 1, wherein said separating section of the vessel also comprises means for separation of vapour fraction from the liquid fraction.

6. The apparatus according to claim 5, wherein said means includes a demister or a cyclone.

7. The apparatus according to claim 1, wherein the apparatus is horizontally arranged.

8. The apparatus according to claim 7, wherein the bundle of U-tubes is horizontal and each tube has an inlet straight portion which is on a lower part of the bundle, and an outlet straight portion which is in an upper part of the bundle.

9. The apparatus according to claim 1, wherein said evaporable medium is water.

10. The apparatus according to claim 3, wherein said separating section produces steam having purity of at least 98% in weight.

11. The apparatus according to claim 10, wherein said steam has a purity of at least 99.5% in weight.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 is a schematic section of a vertical shell-and-tube apparatus according to an embodiment of the invention.

(2) FIG. 2 is a schematic section of a horizontal shell-and-tube apparatus according to another embodiment of the invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

(3) FIG. 1 shows a vertical shell-and-tube waste heat boiler 1 according to a preferred embodiment of the invention.

(4) The boiler 1 is designed to recover heat from a hot gas G by heating and evaporating a water feed W, thus producing steam S at a suitable pressure.

(5) Said boiler 1 basically comprises a lower exchanging section 2 embodying a shell-and-tube heat exchanger, and an upper separating section 3 to receive a mixed steam water effluent from the tubes, and designed to separate steam from non-evaporated water.

(6) More in detail, the lower section 2 contains a bundle of tubes 4 having respective tube inlet ends 5 and tube outlet ends 6, and a hot chamber 7 around said tubes 4. This lower section 2 operates substantially as a shell-and-tube heat exchanger, where tubes are fed with the water W and the shell side, namely the hot chamber 7, is traversed by the hot gas G.

(7) The bundle of tubes is shown in a schematic manner. Each tube 4 is a U-tube having: a first straight portion 4a, a second straight portion 4b, and a U-shaped portion 4c to connect said straight portions. The tubes are supported by a tubesheet 32.

(8) According to a preferred embodiment of the invention, in the vertical arrangement (FIG. 1) the tubes face downward in the vertical boiler, i.e. the U-shaped connection 4c is located at the bottom of the vertical bundle.

(9) The hot chamber 7 is in communication with an inlet 8 for the hot gas G. Said gas G may be for example the product of a combustion, reforming, or exothermal chemical reaction.

(10) A gas outlet 9 for the cooled gas Gc is also in communication with the hot chamber 7. The cooled gas leaves the chamber 7 via an annular region 10 around said chamber 7. FIG. 1 also shows a distributor 11 and an impingement plate 12 for the hot gas G, and a duct 13 for admission of the hot gas G into the chamber 7.

(11) The inlet ends 5 of tubes 4 are in communication with an inlet 14 for the fresh water feed W, via a feeding chamber 15. The fresh water W, in some embodiments, may be mixed with a suitable amount of non-evaporated water recycled from the separating section 3, before it enters the tubes 4.

(12) The separating section 3 of the boiler 1 comprises a collection chamber 16 connected to the bundle of tubes 4, and said chamber 16 is in communication with the outlet ends 6 of the tubes 4, to receive the mixed water/steam effluent from said tubes. Hence, the collection chamber 16 normally contains a certain amount of water during operation. The liquid level inside said chamber 16 is denoted by reference 17. Reference 29 denotes the free space over the liquid level 17.

(13) The liquid level 17 is controlled by means of a controller 18. A suitable liquid level in the chamber 16 is maintained to facilitate steam separation by gravity, thus leaving a sufficient free space 29 for the disengagement of steam from water.

(14) The separating section 3 of the boiler 1 may be further equipped with a suitable vapour/liquid separator. In the shown embodiment, the boiler 1 comprises a steam drier 19 which is located in the top part of the upper section 3, thus defining a steam chamber 20 above the collection chamber 16 and in communication with a steam outlet 21.

(15) Non-evaporated water leaves the collection chamber 16 via a main outlet 22 and further outlets 23, 24 which are used to withdraw suitable amounts of water (water blow-down), in order to avoid accumulation of water-suspended solids in the collection chamber 16. In particular, the outlet 23 is connected to a pipe 23a and is used for continuous blow-down while the outlet 24 is preferably used, when necessary, for a discontinuous blow-down.

(16) The level regulator 18 essentially comprises two pressure gauges 25, 26 and a control unit 27 to determine the liquid level 17 as a function of the differential pressure between said gauges. Then, the level 17 is preferably regulated by controlling the flow rate of the fresh water W admitted to the tubes 4 and the amount of recycled water taken from the chamber 16.

(17) Recycle of non-evaporated water may be internal or external to the boiler 1. For example, internal recycle may be effected by feeding an amount of non-evaporated water to the water chamber 15; external recycle may be effected by mixing a portion of the water from outlet 22 with the fresh water feed W before admission to the inlet 14 of the boiler 1, The boiler 1 may comprise means such as pumps or ejectors for recirculation of water, which are not shown in FIG. 1 for the sake of simplicity.

(18) The shown embodiment provides also that the collection chamber 16 has a first portion delimited by an internal wall 30, and a second portion delimited by a dome 28 of a greater diameter compared to the rest of the shell.

(19) FIG. 2 shows an example of horizontal embodiment. The items corresponding to those of FIG. 1 are denoted in FIG. 2 with the same reference numbers, for simplicity. Hence, they are not described in a full detail and reference can be made to the above description of FIG. 1.

(20) It can be seen that the horizontal exchanger of FIG. 2 comprises an exchanging section 2 and a separating section 3 arranged side by side.

(21) The exchanging section 2 comprises a horizontal bundle of U-tubes 4. The figure shows an embodiment where the inlet straight potion 4a of the tubes 4 is on the lower part of the bundle, while the outlet straight portion 4b is in the upper part of the bundle.

(22) The separating section 3 comprises basically a collection chamber 16 to receive the partially evaporated effluent from tubes 4, a steam drier 19, a level regulator 18 to control the water level 17, a steam outlet 21 in communication with a steam chamber 20, a main water outlet 22, blow-down water outlets 23, 24. In the shown embodiment, also the outlet 22 has a water collector 22a.

(23) The collection chamber 16 has a first portion delimited by internal walls 30, 31, and a second portion delimited by a larger portion of shell 28.

(24) The operation is as follows. The exchanging section 2 operates as a shell-and-tube evaporator, where water is heated and partially evaporated in the tubes 4 by means of the heat exchanged with the hot gas G traversing the hot chamber 7 in contact with the outside surface of tubes 4.

(25) The mixed steam/water flow leaves the tubes 4 and enters the collection chamber 16 in the separating section 3 of the boiler. In the space 29 above the liquid level 17, steam separates by gravity and is further purified by passage through the steam drier 19, so that a dry steam, substantially free of water, is obtained at the steam outlet 21.

(26) Non-evaporated water is discharged by means of outlet 22. A portion of said non-evaporated water may be recycled and directed again to the tubes 4 together with the fresh water W, as explained before.

(27) It can be appreciated that the waste heat boiler meets the aims of the invention. Compared with a prior-art boiler with integrated steam drum and water evaporation on the shell side, the advantages of the proposed design is that the water is on the tube side and, therefore, there are no dead spots where deposit of suspended solids is likely to occur. All tubes 4 are homogeneously fed and heated therefore there are not areas where dry out may occur. Recirculation water to feed the tubes can be taken at a high level as in a separate steam drum, avoiding solids which concentrate near the bottom. Fresh feed water can be mixed with the recirculating water feeding the tubes effectively assuring that boiling water does not carry an excessive concentration of solids. For these reasons, corrosion is avoided and also the deterioration of the heat transfer capabilities and the overheating due to solid deposit on the heat transfer surface are greatly reduced. Moreover the portion of tubes inside the tubesheet 32 is not heated by the hot gas and therefore all the portions of tubes exposed to the hot gas are cooled by the boiling water inside the tubes.

(28) Compared with a conventional boiler with evaporation in the tube side, the advantage of this system is that the steam is separated inside the boiler without the need for external separation equipment and related piping.