The present invention relates to a method and an

apparatus for vaporizing a cold liquid by a combination of flash evaporation after heating the liquid in a heated container and further vaporizing the resulting vapor- liquid phase by indirect heat exchange with the

pressurized and heated liquid in the heated container. Flash evaporation is a process of lowering the pressure of a heated and preferably a saturated liquid which causes partial evaporation of liquid and is

conventionally employed in various applications in the industry including those where water is evaporated. In the flash evaporation process the pressure of a saturated or near saturated liquid is decreased and a new

thermodynamic equilibrium is established. In this new equilibrium point, temperature drops to the corresponding new pressure. Consequently, a part of the liquid has to be evaporated to remove heat from the fluid.

In a number of applications a vapor of the liquid with essentially reduced amounts of liquid or a complete liquid free vapor or a superheated vapor is required.

A problem prevalent in the flash operation is the amount of liquid present in the vapor-liquid phase from the flash evaporation process. It is practically impossible to have a full vaporization of a liquid in a flash evaporation process. To obtain a liquid free vapor phase, the vapor-liquid phase from the flash evaporation process must be demisted or further be vaporized in subsequent heating equipment.

This invention provides a method for vaporizing a cold liquid to a liquid free vapor or a superheated vapor in a reliable and unsupervised operation and a compact

apparatus. Furthermore, the invention is related to a method for stable and fast reacting vaporizer system to liquid feed flow change. It also provides a reliable method for vaporizing liquids which are prone to thermal cracking over heated surfaces such as electric heaters surfaces .

In a general feature of the method according to the invention for vaporizing a cold liquid, the method comprises the steps of:

(a) selecting a cold liquid to be vaporized; (b) continuously feeding the selected cold liquid at a predetermined flow rate into a pressurized heating chamber, completely filled with the same liquid and heated by a heating means to a temperature between the bubble point temperature of the liquid in the pressurized heating chamber pressure and the bubble point temperature of the liquid in subsequent vaporization zone;

(c) continuously withdrawing the heated and pressurized liquid from the heating chamber at the same flow rate as the cold liquid is fed into the heating chamber; (d) passing the heated and pressurized liquid into the vaporization zone arranged within the heating chamber, depressurizing and flash evaporating the liquid to vapor- liquid phase at inlet of the vaporization zone; and

(e) vaporization remaining liquid in the vapor-liquid phase in the vaporization zone by indirect heat exchange with the heated and pressurized liquid in the heating chamber .

The vaporization zone can be of film evaporation type when the temperature difference between the pressurized chamber and the vaporization zone is below the critical limit, a limit, after which film evaporation occurs, or of packed bed evaporation type in which two phases are passed through a preferably metal packing system when the aforementioned temperature gap is above the critical limit . In the film vaporization, the liquid of the feed vapor- liquid phase is introduced on surfaces of the

vaporization zone and forms a thin film from which the liquid evaporates. The surfaces are heated by indirect heat exchange with the heated and pressurized liquid in the heating chamber.

In the packed bed vaporization, the liquid and vapor mixture from the flash evaporation are introduced into a packing system where the liquid is slowed down by the packing and is evaporated by both indirect superheated vapor and direct contact with the packing or the wall of the vaporization zone, indirectly heated by the

pressurized chamber liquid.

As mentioned hereinbefore, some applications require a complete liquid free vapor of the selected liquid.

The complete liquid free vapor is obtained by means of the third feature of the invention, wherein the vaporated liquid in step (e) is further superheated by indirect heat exchange with heated and pressurized liquid in a superheating zone.

According to a fourth feature of the invention, the cold liquid is heated in step (b) by means of electrical heating.

In the above features of the invention heat from the pressurized liquid in the heating chamber is effectively transferred to the vaporization zone due to a lower pressure and consequently a lower vaporization

temperature in the vaporization zone.

The invention provides additionally a compact apparatus for vaporating a cold liquid.

The apparatus comprises in a general feature of the invention

(a) an elongated shell defining a heating chamber for heating and pressurizing the cold liquid and formed with an inlet for feeding the cold liquid into the heating chamber and with an outlet for withdrawing the liquid after having been heated in the heating chamber;

(b) a depressurizing device mounted in the outlet of the heating chamber for depressurizing the heated liquid;

(c) vaporization tubes arranged within the heating chamber in indirect heat exchange with the heated and pressurized liquid and formed with an inlet and an outlet; and

(d) passageways for conducting the depressurized heated liquid from the outlet of the heating chamber and to the inlet of the vaporization tubes.

If the temperature gap between the pressurized liquid and the vaporization is below the critical limit, a falling film evaporation can be devised for the vaporization tubes. The vaporization tubes are preferably in form of U-tubes, allowing both film evaporation in the downwards leg and superheating in the upward leg. Film evaporation and superheating takes place by indirect heat exchange with the heat supplied from the hot and pressurized liquid on shell side of tubes. The tube layout is however not limited to U-tube, it can be coil, single pass layout .

If the above mentioned temperature gap is above the critical limit, tubes are to be filled with packing, preferably metal packing to enhance heat transfer. The packing is used to slow down the liquid phase and allow enough time for liquid to stay in the tubes and gets vaporized. Tube internals, random and/or structure packing can be used in this design.

In the above features of the invention, the cold liquid is preferably heated by means of electrical heaters arranged within the heating chamber.

The invention is disclosed in more detail in the

following description by reference to the drawings in which

Fig. 1 is a schematic of an apparatus for vaporizing of a cold liquid according to a specific feature of the invention .

A heating chamber 1 is filled with cold liquid via inlet 6. The liquid is heated up to a temperature between the bubble point temperature of the liquid in the pressurized heating chamber pressure and the bubble point temperature of the liquid in the vaporization zone by an electrical heater 2. The same amount of hot liquid as the amount of cold liquid introduced into chamber 1 has to leave chamber 1 via outlet 5 in order to keep the chamber pressure at a constant value. The amount of hot liquid withdrawn from chamber 1 is controlled by valve 4 arranged in outlet line 5. The hot liquid is

depressurized through valve 4 and fed into vaporization zone 3. The pressure in vaporization zone 3 is lower than in heating chamber 1 and part of the hot liquid is evaporated after the depressurizing valve 4. The

remaining of liquid is vaporized in vaporization zone 3 via an indirect heat exchange with the hot liquid in chamber 1. The completely vaporized feed can be further superheated in a superheating zone (not shown) , basically a continuation of the vaporization zone. The liquid free vapor is withdrawn from outlet line 7.

Example

Methanol with flow rate of 60 kg/h is fed to a heating chamber (0 150* 700 mm) pressurized and filled with hot methanol at 205°C and 50 barg (about 8 ° C below the bubble point of methanol) . An immersed electrical heater with nominal operating power of about 35 KW _e supplies heat to methanol in chamber to control the high pressure liquid temperature at 205 ± 1°C.

When the hot liquid methanol is flashed into vaporization tubes, about 30% of liquid methanol is evaporated. The remaining liquid is vaporized in 56 U tubes with

approximate diameter of 9 mm and filled with twisted metal internals.

The pressure in the vaporization tubes is controlled at 10 barg corresponding to about 141°C boiling temperature of methanol.