GB2038647A - Improvement in separation by condensation - Google Patents

Abstract

A process for separating a mixture of fluids into its components or into groups of its components comprises (a) forming a gas-vapour or vapour phase of the mixture if not already in this form, (b) conveying the said phase to a chamber, (c) cooling the phase in said chamber to cause the less volatile components of the phase to condense, and (d) drawing off the more volatile components of the phase from said chamber while (e) supplying heat to the condensate in said chamber from heating means which transfer heat to the condensate in said chamber. Treatment of natural gas containing heavy hydrocarbons is specified.

Description

SPECIFICATION Improvement in vapour-gas separation The present invention relates to processes and processing plants for the separation of fluids mixtures into components or group of components. The fluid mixture can be either in liquid, gas or liquid-gas form.

In case of the fluid mixtures being in the liquid form, the liquid has to be preheated firstto the point when more volatile product is evaporated. After which the same vapour is cooled in order to condense and loose less volatile fraction from its body, thus concentrating vapour in more volatile products.

This process is repeated until desired purity of either remaining liquid or generated vapour is achieved.

In the case of the fluid mixtures being in the gaseous state, the mixture of vapours is cooled to the point when it starts condensing. The condensate is heated to evaporate more volatile fractions, while the mixture of vapours can be cooled to loose less volatile part of the mixture. By this process a transfer of mass is achieved by which more volatile fractions are transferred from the liquid state to the gaseous state. While the less volatile fractions are transfered from the gaseous state to the liquid state. The process is carried in a number of stages until desired purity of either liquid or gaseous state, or both is achieved.

Fig. I, shows the working of a commonly used partial condenser. Where vapour mixed with the gas(es), or mixture of vapours, flows over cold surface (usually tubes) hence gets cooled until it reaches temperature T2 and starts condensing. The liquid droplets are at the same temperature T2, hence in equilibrium with the gas-vapour mixture. The liquid pool is also at approximately the same temperature ie T2. Under this conditions the only way to reduce the vapour content of the gas-vapour mixture is to reduce the mixture temperature T2. In case of two, or more products in the vapour from the ratio of these products is again fixed by temperature T2 and the relative volatility of these products. In most cases it is not possible to get high purity condensate of a particular component because all the vapour components condense at the same time, although at different rate.

The present invention and its advantages over a commonly used partial condenser, described above, will now be more fully described by way of example only, with reference to the accompanying fig 2. It can readily be seen, from the above description, that although it is possible to separatevapoursfrom gases by condensing the same, however it is difficult to get high purity of these vapours in cases of more than one vapour product being present in the origi nal gas-vapour mixture when using partial con denser shown in fig. I and fig. 3.

Fig. 2 shows heaters immersed in the pool of con densate. The liquid while being heated will evapo rate the more volatile components which will join the gas-vapour phase. On the other hand the gasvapour mixture is being cooled thus condensing less volatile products, which in thurn join the liquid phase. Hence on balance there is mass transfer of more volatile products to gas-vapour phase from the liquid phase and less volatile products from the gas-vapour phase to the liquid phase. By repeating the above process a high purity of either phases can be achieved. The working of the equipment, shown in fig. 2, is as follows: Vapour(s) mixed with gas(es) is cooled by the cold surface, usually tubes, until less volatile vapour(s) starts to condense at temperature T2. Liquid droplets, at fall into the liquid pool at temperature, T3.

The liquid pool is maintained at higher temperature than the gas-vapour mixture. This means that more volatile product(s) is evaporated from the liquid thus making the pool liquid more concentrated in less volatile products. It should be added that the two phases ie gas-vapour and liquid in the pool are not in equilibrium since both of them are at different temperature.

Departure from equilibrium can be as wide as difference between T2 and T3 temperature will permit.

This facility of being able to have both phases at different temperatures permits high purity of any of the phases to be achieved.

Fig. 3, shows sketch of a modified partial condenser. The difference from commonly used partial condensers is that it also acts as a separator and in case of many vapours, being present in the mixture, this modified partial condenser can also have a number of liquid outlets ie 01, 02, 03 etc. Each outlet carries liquid with different composition. If this equipment is to act as an effective separator it is essential to have demister, which can be located in the enlarged gas outlet, or else at the discharge end of the condenser.

The most important part of the present invention is represented in fig. 4, fig. 5, or similar configuration, where cooling of the gas-vapour mixtures takes place in basically the same equipment as liquid cooling. The most convenient form of the processing equipment is in fact commonly used partial condenser with the liquid heaters at the bottom of the condenser, coolers in the top part and demister at the discharge end of this equipment, as shown in Fig. 4. The working of this equipment is the same as that of fig. 2, except that fig. 4 has many compartments, each equivalent to that of fig. 2. And in case of many vapours mixed together equipment, fig. 4., can have a number of product outlets, each with different product, or indeed the same product but of different purity ie different quantity of other products mixed which happens to condense together with the main product.In other words, the above equipment, fulfills the function of a distillation column except that it is very much simpler in operation. The liquid heating can be done with immersed coils, outside jackets, or part of the tube bundle can be used to pass heating medium, or any other form can be used, provided it fulfils the function as described above. As to the cooling, of the gas-vapour mixtures, once again it can take many forms, just the same as the heating elements.

The equipmentthatfulfills the above process description can take many shapes and forms, some are shown in fig 4, 5 and 6.

Fig 5 and 6 shows how, commonly used knock out pot, can be modified to fulfil the same function as the equipment shown in fig. 4.

It is advisable for equipment shown in fig. 3, 4, 5 and 6 to have level controls at the product outlets.

While equipment in fig. 4, should have weir plate to keep the heating coils immersed in the liquid. It is also important for the equipment in fig. 3 and 4 to have bottom buffles with cut outs at the bottom edge to allow free flow of the liquid along the bottom wall of the partial condenser.

Equipment shown in fig. 3 is suitable for the applications when gas is mixed with single vapour. It can also be used, when the mixture containes more than one vapour product, provided high purity product recovery is not required.

Equipment, shown in fig. 4, 5 and 6 is particularly suitableforthe applications with the multicomponent vapour mixtures, where it is desired to have high purity products. This is particularly true of applications in heavy hydrocarbon recovery from natural gas.

Many other shapes and arrangements are possible for the processing equipment. For instance the equipment can be similar to fig. 4 but without baffles. Or anather variation can be used with, or, without tube bundle baffles but with longitudinal shell baffle(s), otherwise the same as fig. 4. In some cases it might be advantageous to use vertical positioned equipment, or even inclined. But in that case the nozzles and heater(s) might have to be located in different position than in fig. 4. Yet another variation on fig. 4 might be equipment without tubebundle or shell baffles, otherwise similar to fig. 4.

Claims (10)

1. A process for separating by distillation a mixture of fluids into its components or into groups of its components, which method comprises (a) forming a gas-vapour phase of the mixture, (b) conveying the gas-vapour phase to a chamber, (c) cooling the gas-vapour phase in said chamber to cause the less volatile components of the gas-vapour phase to condense, and (d) drawing off the more volatile components of the gas-vapour phase from said chamber while (e) supplying heat to the condensate in said chamber from heating means which transfer heat directly to the condensate in said chamber.

2. A process according to Claim 1, wherein steps (c), (d) and (e) are repeated a number of times in a number of successive chambers, the gas-vapour phase which is cooled in each subsequent step (c) being constituted by the more volatile components drawn off in preceding step (d) from a preceding chamber.

3. A process according to Claim 1 or 2, wherein the fluid mixture comprises natural gas containing heavy hydrocarbons.

4. A process according to Claim 1, wherein the mixture of fluids is a mixture of vapours only or of liquids only, without gases.

5. A process for separating by distillation a mixture of fluids into its components or into groups of its components, substantially as hereinbefore described with reference to Figures 2 to 7 of the accompanying drawings.

6. Apparatus for separating by distillation a mixture of fluids into its components or into groups of its components, which apparatus comprises a chamber having an in let for admitting a stream of the said mixture in the gaseous state, means for cooling the gaseous mixture in the chamber thereby to cause the less volatile components of the mixture to condensate and collect in said chamber, heating means for supplying heat to the condensate in said chamber, and an outlet on the chamber for withdrawing the non-condensed fractions of the fluid mixture from the chamber.

7. Apparatus according to Claim 6, comprising a number of said chambers connected in sequence, the outlet of the first and each subsequent chamber except the last being connected respectively to the inlet of the second and each subsequent chamber.

8. Apparatus according to Claim 6 or 7, where the heating means is a heating coil extending into the bottom of the chamber, a heating jacket surrounding the bottom of the chamber or a conduit for conveying heating medium through the liquid condensate.

9. Apparatus according to Claims 6, 7 or8, wherein the cooling means is a cooling coil inside or outside the chamber, a cooling jack surrounding a part or all of the chamber in contact with the fluid to be cooled, or wherein cooling is achieved by expanding the fluid mixture or by adding other fluids which are at a lowertemperature.

10. Apparatus for separating by distillation a mixture of fluids into its components or into groups of its components, substantially as hereinbefore described with reference to, and as illustrated in, Figures 2 to 7 of the accompanying drawings.