CH632908A5 - Method of expanding tobacco - Google Patents

Description

The invention is described in detail below, for example.

In general, the invention relates to improvements in the method of expanding tobacco using liquid carbon dioxide as an expanding agent. In general, the invention relates to a method of expanding tobacco using a single agent that is easily accessible, relatively inexpensive, non-flammable, and extremely effective, and more particularly to the manufacture of an expanded tobacco product having a substantially reduced density, made by impregnating tobacco under pressure with liquid carbon dioxide, conversion of liquid

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Carbon dioxide in situ in solid carbon dioxide, which can be achieved by releasing the pressure quickly, and then evaporating the solid carbon dioxide and expanding the tobacco, which can be achieved by applying heat, radiation energy or similar energy-generating conditions to the impregnated tobacco, thereby creating a rapid evaporation of the solid carbon dioxide contained in the tobacco is effected.

To carry out the process according to the invention, either a whole dried tobacco leaf, tobacco in cut or chopped form or selected parts of tobacco, e.g. Treat tobacco ribs or reconstituted tobacco. In shredded form, the tobacco to be impregnated can have a particle size of 0.84-0.149 mm, but preferably not less than about 0.59 mm. The material to be treated can be in a relatively dry form or can contain the natural moisture content of the tobacco or even more.

Generally, the tobacco to be treated has at least about 8% moisture (by weight) and less than about 50% moisture. According to a special improved embodiment of the method according to the invention, the moisture content of the tobacco used in the first stage of the method should have a value of about 17 to about 25%, depending on the pressure specifically used. In the present description, the moisture content of the tobacco can be referred to as "loading moisture" or "coating OV" and is approximately equivalent to the parts that can be vaporized in the oven (OV), since generally not more than about 0.9% by weight of the tobacco weight consist of volatile components that differ from water. Surprisingly, it has now been found that when a loading OV is used in this area, the first stage of the method can be carried out more advantageously at pressures which are lower than those previously regarded as optimal. In the further description, a more detailed explanation of the pressures used in the first stage of the process is given; With regard to the specific embodiment described here, it was found that the following loading OV are preferred for the following pressures:

(The terms "loading OV" and "OV content" used here are always in% by weight, based on the dry weight of the tobacco.)

at an overpressure of 20.7 bar a loading OV

from 18 to 25%

at an overpressure of 27.6 bar a loading OV

from 17 to 25%

at an overpressure of 31 bar a loading OV of 17 to 25%

at an overpressure of 34.5 bar a loading OV

from 16 to 23%

at an overpressure of 43.1 bar a loading OV

from 14 to 19%

at an overpressure of 51.7 bar a loading OV

from 13 to 18%

at an overpressure of 58.6 bar, a loading OV of 12 to 18%.

In order to bring the moisture content of the tobacco to the desired feed moisture level,

the tobacco can be sprayed with water, brought into contact with steam or the like until the desired degree is reached, preferably by spraying with water. In order to reduce the moisture content in the tobacco to a desired value, the tobacco can be heated, preferably by indirect steam heating, until the desired content is reached.

It has been found that this improvement in the basic process makes it possible to achieve desired expansion dimensions at lower pressures than were previously found to be necessary for such an expansion. Thus, before the present invention, a wide range of pressures were found to be applicable, but it was considered necessary to carry out the process at an absolute pressure of about 35.5 bar or above to achieve a cylinder volume (CV) product , which is approaching a value of around 70 (which is generally viewed as a degree of expansion aimed at commercial products). 15 It has now surprisingly been found that a cylinder volume of 68 with an absolute pressure of 21.7 bar can be achieved if one works with a loading OV of about 20% and a yield OV of less than about 6%.

20 The terms "cylinder volume" and "corrected cylinder volume" represent units for measuring the degree of expansion of tobacco. The expression "content of parts that can be evaporated in the oven" or "volatile parts of the oven" represents a unit for measuring the moisture content (or percentage) Moisture) in tobacco. The values used in the present description with regard to these terms are determined as follows:

30 cylinder volume (CV)

10,000 g of tobacco are introduced into a cylinder of 3.358 cm in diameter and pressed together with a stamp of 1875 g and a diameter of 3.335 cm for 5 minutes. The resulting volume 35 of the filler is referred to as the cylinder volume. This test is carried out under standard environmental conditions of 23.9 ° C and 60% RH (relative humidity). Unless otherwise specified, the sample was preconditioned in this environment for 18 hours.

Corrected Cylinder Volume (CCV) The CV value can be adjusted to a specific volatile content to make comparisons easier. 45 CCV = CV + F (OV-OVs) where OVs is the special OV and F is a correction factor (volume per%), predetermined for the specific type of tobacco being treated.

so content of volatile components (OV)

The tobacco sample is weighed before and after 3 hours of treatment in a forced air oven controlled at 100 ° C. The weight loss as a percentage of the original weight represents the content of oven-volatile components.

For the expanded light tobacco used in the present description, the value of F for the calculation of CCV is 7.4.

The tobacco with the desired loading moisture content is generally placed in a pressure vessel such that it can be suitably immersed in or brought into contact with liquid carbon dioxide. For example, a wire cage or a loading platform can be used. 65 The pressure vessel containing the tobacco can be flushed with carbon dioxide gas by vacuum or another gas, the flushing process generally taking 1 to 4 minutes. The rinse level can

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be omitted without this having a detrimental effect on the end product. The advantages of purging are the removal of gases that could interfere with a carbon dioxide recovery process and the removal of any foreign gases from the tobacco that could interfere with the full penetration of the liquid carbon dioxide.

The liquid carbon dioxide used in the method according to the invention can be obtained from a storage vessel in which it is generally kept at an overpressure of 17.2 to 21.0 bar. The liquid carbon dioxide can be introduced into the pressure vessel with an overpressure from 17.2 to 27.6 bar or in a range from about 17.2 to 36.2 bar. When liquid carbon dioxide is introduced into the pressure vessel, the interior of the vessel, including the tobacco to be treated, must have a pressure which is at least sufficient to keep the carbon dioxide in a liquid state.

The liquid carbon dioxide is expediently introduced into the vessel in such a way that it is possible to bring the tobacco completely into contact and it should preferably be used with sufficient liquid carbon dioxide in order to fully saturate the tobacco. Generally this involves the use of 1 to 10 parts by weight of liquid carbon dioxide per part by weight of tobacco. Excess liquid C02 is wasteful, but applicable. The temperature of the liquid carbon dioxide should preferably not exceed about 31 ° C during the impregnation step.

The pressure during the contact stage, which is the first embodiment of the invention, is kept at an excess pressure of 17.2 to 36.2 bar, for example by heating the vessel using heating spirals or the like if necessary.

The tobacco and liquid carbon dioxide can be kept in contact under these conditions for a period of 0.1 to 30 minutes.

After the liquid carbon dioxide has been able to saturate the tobacco, generally for a total of 0.1 to 30 minutes, preferably 0.2 to 1 minute, any excess liquid carbon dioxide that may be present is withdrawn from the vessel, preferably while maintaining it the temperature and pressure conditions at the same values as during the contact step.

A particular improvement to the process, which is a second embodiment of the invention, involves the use of a drain residence time. This embodiment can be used in combination with the first embodiment described above in the pressure range during the contact stage from 17.2 to 36.2 bar overpressure; however, this embodiment can be used over a wider pressure range from 14.8 to 65.5 bar gauge pressure, and its beneficial effects are greater in the lower portion of this pressure range. According to the method, after a swelling time of, for example, at least 0.1 minute to 30 minutes to achieve impregnation of the liquid carbon dioxide in the tobacco, the liquid can be removed from the tobacco mass, for example through an outlet opening at the bottom of the chamber, the pressure in the vessel is maintained. It has been found that there are surprising advantages when using a drip dwell time of at least 2 minutes after the point where the continuous outflow of liquid carbon dioxide from the vessel has ceased. Up until the present invention, one method of controlling the process at this point was to observe the level of liquid carbon dioxide through a sight glass attached to the impregnation vessel when the liquid CO 2 was withdrawn and to interrupt the withdrawal at a time when none further fluid was observed, ie if there was no further continuous flow of liquid C02. Sometimes a brief check for additional fluid was made immediately after the withdrawal was stopped. In the method according to the invention, a drip-off residence time can be used, which consists in waiting at least about 2 minutes and preferably at least about 3 minutes and then withdrawing the liquid a second time until the renewed continuous flow of liquid stops a second time. The second liquid flow can generally last less than 1 minute. Such a drip dwell time serves to ensure that excess liquid is removed from the tobacco surface. The drip dwell time can be assisted by purging C02 or another gas down through the mass. It has been shown that the drip dwell time results in an essentially complete drain and also results in: (1) a more complete recovery of CO 2 as a liquid and less than a gas (which requires condensation at the recovery stages), (2) the removal of liquid from the tobacco surface, which has been shown to otherwise partially solidify on the tobacco surface when the pressure is reduced, thereby binding the tobacco into lumps, which in turn requires difficult handling and breaking, which the resulting formation of tobacco "fines" and (3) a reduction in the thermal load on the expansion device by eliminating the need to evaporate ineffective solid CO 2 from the surface.

The draining time should be at least about 2 minutes. However, it is preferably 2.5 to 6 minutes, with a longer duration giving a slight additional advantage, but can be used satisfactorily; the draining time depends to a large extent on the construction and the size of the vessels.

After impregnation, stripping and the draining time, the gas pressure can be reduced at a sufficiently high rate that at least part of the carbon dioxide in the tobacco is converted into the solid form.

The pressure in the vessel is released by venting the gases to bring the contents of the vessel to atmospheric pressure.

Venting generally takes 0.75 to 15 minutes, depending on the size of the vessel, but should preferably not last longer than 3 minutes, after which the temperature in the vessel is generally -85 to -95 ° C and the liquid carbon dioxide in which tobacco is partially converted to solid carbon dioxide. The pressure does not have to be at atmospheric pressure, but is expediently reduced to an excess pressure of approximately 4.1 bar. It can be seen that this is not as economical for economic reasons as reducing the pressure to atmospheric pressure.

The stripping and the draining time should preferably be carried out in such a way that the tobacco contains 6 to 25% by weight, particularly preferably 10 to 20% by weight, of solid CO 2.

After the conversion of the carbon dioxide in the tobacco into its solid form, the tobacco containing solid carbon dioxide is then subjected to expansion conditions, the treated product being subjected, for example, to an Er5

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undergoes heating or equivalent treatment to evaporate and remove the solid carbon dioxide from the tobacco. This can be done using hot surfaces or a hot air stream, a mixture of gas and steam or the action of other energy sources such as microwave radiation energy or infrared radiation. A convenient means of expanding the solid carbon dioxide-containing tobacco is to place it in a stream of heated gas, such as superheated steam, or to carry it with it, or to use it in a turbulent air flow or vortex air flow, for example at a temperature of 100 to 370 ° C, and preferably at a temperature of 150 to 260 ° C, which takes 0.2 to 10 seconds. The impregnated tobacco can also be heated by being placed on a conveyor belt and exposed to infrared heating, by treatment in a cyclone dryer, by contact in a dispersion dryer with superheated steam or a mixture of steam and air or the like. In each of these contact levels, the temperature of the atmosphere with which the tobacco is in contact should not be raised above about 370 ° C, and should preferably be 100 to 300 ° C, more preferably 150 to 260 ° C when operating at atmospheric pressure becomes.

As is known in the processing of any organic material, overheating leads to destruction, first to color formation, such as an undesirable darkness and finally to charring. The necessary and sufficient temperature as well as the duration of treatment for the expansion without such damage represents a function of these two variables and the state of the distribution of the tobacco. Thus, in order to avoid undesirable damage at the heating stage, the impregnated tobacco should have higher temperatures, e.g. 370 ° C, no longer than a few tenths of a second.

One method of effecting tobacco cell expansion is in the radiation methods described in U.S. Patents 3,409,022 and 3,409,027. Another method is to use a heat gun such as the "day-ton-heat-gun" or a similar device that works with an air outlet temperature of 190-344 ° C for a period of 0.2 seconds to 4 minutes, during which time of course the shorter times refer to the higher temperatures. With this mode of operation, the tobacco never reaches a temperature above approx. 140 ° C because it is cooled by the rapid formation of gases. The presence of steam during heating helps to achieve optimal results.

Another system is to use a dispersion dryer, for example one that is fed either with steam alone or in combination with air. An example of such a dryer is the dispersion dryer PB from Proctor and Schwartz. The temperature in the dryer can range from 121 to 371 ° C, with a contact time in the dryer from about 4 minutes at the lowest temperature to 0.1 to 0.2 seconds at the highest temperature. In general, a contact time of 0.1 to 0.2 seconds is used when the temperature of the hot gas is 260-315 ° C or slightly higher. As noted above, other types of heaters can be used

if they are capable of causing the impregnated tobacco to expand without causing excessive darkening. It should be noted that if there is a high percentage of oxygen in the hot gases, this contributes to the darkening, so that when using a hot steam mixture, a high proportion (e.g. above 80% by volume) of steam is preferred. The presence of a vapor atmosphere of 20 vol.% Or more of the total hot gas composition helps to achieve the best expansion.

A third embodiment of the present invention is to control the discharge or exit moisture of the product at the heating stage for the expansion of the tobacco, so that the exit moisture is preferably not more than 6% and more preferably not more than 3.0%, determined as OV is.

As stated above, it is important that the product from the heating / expansion stage have an exit OV content of not more than 6%. The result of this relatively dry condition is an optimal permanent expansion after restoring the standard humidity conditions. Such OV can be achieved by appropriate equilibrium between the rate of loading of the impregnated tobacco into the dryer and the temperature of the dryer gas, if a fixed gas flow is required; flow rate is another variable to be considered. The exit OV can also be reduced to a desired level by further treating the product, such as in a dryer.

After the tobacco has been obtained from the heating / expansion stage with the desired exit OV, it is then generally equilibrated (re-adjusted) to standard conditions. The readjustment is preferably carried out under standard conditions which generally consist of keeping the tobacco at a temperature of 23.9 ° C and a relative humidity of 60% for at least 18 hours.

The present method can be carried out in various forms of devices, for example those as described in BE-PS 821 568 and 825 133.

It is important that the device in which the liquid carbon dioxide-containing tobacco is converted 40 into tobacco containing solid carbon dioxide is suitable for

Absorb gases at the applicable elevated pressures, in some cases as high as 65.5 bar gauge. This vessel is preferably used for the first contact of the liquid carbon dioxide with the tobacco. The pressure vessel can exist in numerous arrangements or can be constructed in different ways. However, there should preferably be a valved inlet from a source of liquid carbon dioxide and a valved outlet at the bottom of the vessel through which the liquid can be drawn off; a second valved outlet near the top can be added for ventilation and can be inserted as part of the inlet conduit, optionally between the vessel and the inlet valve. A device for heating 55 the vessel and / or the delivery vessel, such as external heating coils, can be used. If the vessel is placed on a measuring cell, the measurement of the carbon dioxide loading is greatly simplified. An additional vessel, which is similarly equipped with weighing devices and heating coils, is advantageous, although it is not necessary, since it preheats a batch of liquid carbon dioxide from its usually low storage temperature of -20 ° C (which means an overpressure of about 14 , 8 bar can correspond). In operation 65, the fill can be placed in the main pressure vessel in a holding device, such as a wire basket, which is held above the bottom of the vessel. The closed vessel can then be flushed with carbon dioxide gas

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and the outlets can be closed, whereupon liquid carbon dioxide is introduced from the storage container, for example at a pressure of 17.2 bar in an amount sufficient to cover all the tobacco present in the vessel. The temperature can be raised by the heaters, for example by the heating coils, in order to bring the tobacco to the desired temperature, which should suitably be below 31 ° C (the critical temperature of the carbon dioxide) and this state is preferably maintained for up to 20 minutes upright while the impregnation is taking place. Excess liquid carbon dioxide is then drawn off, for example by opening the lower outlet on the vessel, into a storage container or a similar discharge system, and when all the excess liquid has been removed from the vessel, the vessel can be vented to atmospheric pressure. The tobacco can then be subjected to an operation to volatilize the solid carbon dioxide, preferably by removing the solid carbon dioxide-containing tobacco from the vessel and passing it through one of the various rapid heating systems to achieve expansion. As indicated above, systems that provide rapid turbulent contact with the hot gas or steam are particularly satisfactory for this expansion process. With a suitable control of the temperature and the duration of the treatment, the product can be obtained in the expanded state with the desired moisture content.

Apart from the improvements according to the invention, the overall process can generally be carried out in accordance with the teaching of BE-PS 821 568.

example 1

45.4 kg of bright tobacco particles the size of cigarette fillings with an OV content of 20% were placed in a wire basket in a pressure vessel. The vessel was closed except for an opening for the inlet of COa gas. The gas was brought to a pressure of 27.6 bar and the opening was closed. The vessel was then filled with liquid CO2 with a pressure of 27.6 bar and at - 7 ° C with sufficient liquid CO2 to cover the tobacco mass. The system was held for 30 seconds under these conditions and then an opening in the bottom was opened to pump out the liquid. The opening was closed and after 3 minutes it was opened again to remove the accumulated liquid. The opening was closed and the vessel opened to the atmosphere through a valve at the top so that the pressure was released over a period of 180 seconds. When the lid was removed from the container and the basket was weighed, it was found that approximately 8.2 kg of C02 was retained by the tobacco. It was not difficult to separate the product and it could be fed into the next stage at a controlled speed. A similar approach without the drip dwell time retained about 14.3 kg of CO 2 and was clogged in an uncomfortable manner.

A vertical dryer approximately 20.3 cm in diameter was used to expand at 31.1 m / s of 85% superheated steam in air at 249 ° C. The product from the CO 2 treatment was fed into the dryer at about 1.7 kg / min where the contact time with the steam was estimated to be 3 seconds. The product was recovered in a cyclone separator and had an OV content on cooling of 1.8%. His restored cylinder volume, corrected to 11% OV was 74.0 cm2 /

10 g compared to the original CV of 34.0 cm2 / 10 g for the light untreated filling material with the standard moisture content of 12.5% OV.

Example 2

45.4 kg batches of light tobacco, similar to that used in Example 1 and with a similar feed moisture content as in Example 1, were processed with 177-190 seconds of gassing, but omitting the 3 minute drain residence time and changes in expansion procedures and -conditions were carried out. The result was a varying OV content at the outlet and a varying fill power of the product, measured by CCV. The results and conditions for these approaches are listed in Table 1 in duplicate. The expansion was carried out by treatment in the tower for about 3 seconds with 81 to 88% steam. It can be seen that an optimal filling power is achieved with a low OV for the product emerging from the expansion stage and that an OV of less than 6.0% is favorable and of less than 2.5% is particularly favorable.

Table 1

Loading tower OV at CCV, cm2 / 10g into the tower temperature outlet kg / min ° C%

2.49 149 • 12.0 12.2 59.4 65.6

2.27 177 8.5 7.8 65.0 67.8

1.93 204 4.5 4.8 67.8 69.2

1.42 249 1.8 2.5 69.5 75.4

Example 3

Light tobacco with a weight of 45.4 kg and an OV of 20% was prepared for immersion as in Example 1. Liquid carbon dioxide was introduced into the chamber, which had previously been brought to an overpressure of approximately 20.7 bar; at the same pressure, the tobacco was covered and held for 30 seconds. The temperature was about - 16.7 ° C. The liquid was pumped out through a bottom valve, it was flushed with CO 2 gas from above to the bottom of the vessel for 5 minutes to aid in the removal of undrawn liquid CO 2. The vial was then vented to the atmosphere for 152 seconds and the cooled product was removed.

This product was not clumped; it was placed in an expansion tower with a steam flow of 85% and at a speed of 38.1 m / s at 249 ° C in an amount of about 1.95 kg / min total weight. The treatment time was estimated to be 3 seconds. The product contained 2.1% OV and, when readjusted to 12.0% OV, showed a cylinder volume, corrected to 11.0% OV of 68.0 cm2 / 10 g. The initial filling had a CCV of 37.3 cm2 / 10 g at 12.5% OV.

Example 4

Batches of 45.4 kg of light tobacco with different OV content were treated according to the procedure of Example 1 (impregnation, liquid removal, draining, releasing the pressure and expanding by treatment in the tower), with only the temperature of the tower gas being 232 to 254 ° C varied to compensate for increasing OV contents in the feed. The exit OV of all samples was 1.3% to 2.8% OV. After the re-

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Conditioning all samples corrected for CV, aiming for the best expansion needed, were examined in this series for 11.0% OV. The listed in Table 2 at least 68 cm2 / 10 g CCV, minimum 17%. Results show that a moisture (OV) that results in ErTable 2

Cylinder volume, based on the inlet humidity, with an impregnation at an overpressure of 27.6 bar

Inlet OV filling capacity

% restored CCV, 11% OV

C V, cm2 / 10 g cm2 / 10g

10.1

44.5

50.6

10.6

52.5

55.0

10.9

10.9

47.9

48.5

53.1

52.6

11.7

'59.6

62.3

15.5

60.7

66.5

16.0

16.0

59.6

60.4

64.6

67.0

17.1

17.0

61.0

62.2

69.1

69.0

17.4

17.5

62.2

58.6

68.7

67.9

17.7

17.9

68.7

64.6

72.9

68.2

18.0

18.4 -

62.0

67.1

70.9

72.5

19.0

19.1

60.9

63.2

68.3

70.1

18.0

18.3

69.2

60.8

74.4

68.1

19.4

19.5

60.9

56.9

72.6

69.4

19.7

20.6

63.0

68.9

73.8

73.4

21.9.

21.9

68.2

71.4

73.0

76.5

23.0

59.1

68.6

23.5

23.5

63.9

66.3

70.7

71.0

23.9

24.2

58.6

58.9

68.4

68.6

25.7

58.6

68.4

Example 5

Batches of light tobacco were impregnated and then expanded exactly as described in Example 1, but the drainage residence time (from closing to reopening the outlet) was varied as shown in the tables. The first batches (Table 3) weighed 45.4 kg and had a depth of 91.4 cm in the impregnation device; the second group (Table 4) weighed 84 kg and was 152.4 cm deep. The OV at the exit of the expansion tower was around 2% in each batch. The cylinder volume of the products was measured and sieved to compare the reduction in size. The

A 3 minute purge was equal to a 3 minute drip hold time, except that CO 2 gas was purged downward through the impregnated tobacco as the purge gas.

Tables 3 and 4 contain the results. Since a distribution of approximately that of the control test is desired, 45 and in particular a total of “small” and “fine” (S + F) proportions of not more than 2% is aimed for, it can be seen that the draining residence times are 2 minutes or gave the best results longer. The effect on the corrected CV appeared to be little or nonexistent.

Table 3

Effect of the draining time on sieve fractions with batches of 45.4 kg

Draining time sieve fractions%

Minutes Long Medium Short Small Fine L + M S + F CCVcm2 / 10g at 11% OB

Not expanded

control

55.5

35.9

7.0

0.9

0.6

91.4

1.5

-

0

38.2

50.0

9.2

1.2

1.2

88.2

2.4

74.6

I.

41.9

47.7

8.1

1.2

1.2

89.6

2.4

73.7

2nd

43.7

47.0

7.3

1.0

0.9

90.7

1.9

76.3

3rd

42.2

49.2

6.8

0.7

1.0

91.4

1.7

74.2

3 minute rinse

42.7

48.5

6.9

1.0

1.0

91.2

2.0

75.0

9 632 908

Table 4

Effect of the draining time on sieve fractions with batches of 84 kg

Draining time sieve fractions%

Minutes

Long

medium

Short

Small

Fine

L + M

S + F

CCV cm2 / 10 g

control

52.0

38.9

7.5

1.1

0.5

90.9

1.6

_

0

38.3

48.5

10.0

2.0

1.3

86.8

3.3

74.4

3rd

44.5

45.7

7.7

1.2

0.9

90.2

2.1

76.6

6

44.1

46.1

7.7

1.2

0.9

90.2

2.1

76.8

The various sieve fractions used in the above examples are defined as follows: long> 1.68 mm, (the particles are retained by a sieve with a clear mesh size of 1.68 mm); Mean <1.68 to> 0.84 mm; Short <0.84 mm to> 0.59 mm; is small <0.59 to> 0.297 mm and fine <0.297 mm.

Claims (6)

632 908 2nd PATENT CLAIMS 1. A method of expanding tobacco by impregnating the tobacco in a first stage with liquid carbon dioxide under conditions such that the carbon dioxide is kept in liquid form; Treating the tobacco impregnated with liquid carbon dioxide in a second stage under conditions by which the liquid carbon dioxide is converted into solid carbon dioxide, and then treating the tobacco containing solid carbon dioxide in a third stage under conditions for vaporization of the solid carbon dioxide and effecting the expansion of the tobacco , characterized in that a tobacco with an original OV content of 17-30% is used in the first stage and the first stage is carried out under an overpressure in the range of 17.2-36.2 bar. 2. The method according to claim 1, characterized in that withdrawing excess liquid carbon dioxide from the impregnated tobacco obtained in the first stage, wherein the carbon dioxide is kept in liquid form, the withdrawal of the liquid carbon dioxide continues to a point at which the continuous Liquid flow ceases and has a drainage time of at least 2 minutes to allow further removal of liquid carbon dioxide from the tobacco before the start of the second stage. 3. The method according to claim 1 or 2, characterized in that in the first stage a tobacco with an original OV content of 17-25% is used and the first stage is carried out at an overpressure of 24.1-31.0 bar. 4. The method according to claim 3, characterized in that one carries out the third stage so that the moisture content of the expanded tobacco obtained is 0.5-6% and preferably does not exceed about 3%. 5. A method of expanding tobacco by impregnating the tobacco in a first stage with liquid carbon dioxide in a vessel under conditions such that substantially all of the carbon dioxide is kept in liquid form; Treating the tobacco impregnated with liquid carbon dioxide in a second stage under conditions such that the liquid carbon dioxide is converted to solid carbon dioxide, and then treating the tobacco containing solid carbon dioxide in a third stage under conditions by which the solid carbon dioxide evaporates with expansion of the tobacco is characterized in that the first stage is carried out under an overpressure in the range of 14.8-65.5 bar and excess liquid carbon dioxide is withdrawn from the vessel in the first stage, the carbon dioxide being kept in liquid form and the liquid continuing Withdraw carbon dioxide from the vessel to a point where the continuous flow of liquid ceases, then maintain a drip-off time of at least 2 minutes in order to allow further removal of liquid carbon dioxide from the impregnated tobacco and then again draw off liquid carbon dioxide from the vessel , until the stop the flow of liquid before starting the second stage. 6. The method according to claim 5, characterized in that one carries out the third stage so that the moisture content of the expanded tobacco obtained is 0.5-6%. Various methods of expanding tobacco have been recommended. For example, tobacco with a Gas under pressure slightly above atmospheric pressure was contacted and the pressure was released, thereby expanding the tobacco cells increasing the volume of the tobacco being treated. Other methods that have been used or recommended have included treating the tobacco with various liquids, such as water or relatively volatile organic liquids, to impregnate the tobacco, whereupon the liquids have been driven off to expand the tobacco. Other methods that have been recommended have been to treat the tobacco with solid materials that decompose on heating to form gases that serve to expand the tobacco. Other methods include treating tobacco with gas-containing liquids, such as carbon dioxide-containing water, under pressure to incorporate the gas into and after impregnating the tobacco, heating, or reducing the pressure to expand the tobacco. Other techniques have been developed for expanding tobacco, which are to treat tobacco with gases that react with the tobacco to form solid chemical reaction products, whereupon these solid reaction products can be decomposed by heat, with the formation of gases in the tobacco that their release causes the tobacco to expand. The following should be mentioned in particular: U.S. Patent 1,789,435, 1931, describes a method and apparatus for expanding the volume of tobacco to compensate for the volume loss caused by drying the tobacco leaf. To achieve this, the dried and conditioned tobacco is brought together with a gas, which can be air, carbon dioxide or steam, under a pressure of about 9 kg and the pressure is then released, causing the tobacco to have a tendency to expand shows. The patent states that the volume of the tobacco can be increased by 5 to 15% by this method. The "Alien Property Custodian" publication 304 214 from 1943 shows that tobacco can be expanded using a high-frequency generator, but that there are limits to the degree of expansion that can be achieved without impairing the quality of the tobacco. According to U.S. Patent No. 2,596,183, 1952, the volume of cut tobacco is increased by adding more water to the tobacco to cause the tobacco to swell, whereupon the tobacco containing moisture is heated, thereby evaporating the moisture and the resultant Moisture vapor causes the tobacco to expand. U.S. Patent Nos. 3,409,022, 3,409,023.3,409,027 and 3,409,028, 1968, relate to various methods of increasing the usefulness of tobacco stems for use in smokers' articles, wherein the stems are subjected to expanding treatments using various types of heat treatment or microwave energy. U.S. Patent No. 3,425,425, 1969, relates to the use of carbohydrates to improve the bloating of tobacco ribs. In this process, the tobacco ribs are soaked in an aqueous solution of carbohydrates and then heated to bloat the ribs. The carbohydrate solution may also contain organic acids and / or certain salts which are used to improve the taste and smoking properties of the system. A publication by P. S. Meyer in "Tobacco Reporter", November 1969, describes and provides an overview 5 10th 15 20th 25th 30th 35 40 45 50 55 60 65 3rd 632 908 about tobacco bloating or expansion processes or research into expanding and handling tobacco in order to reduce costs, and also as a means of reducing the "tar" content by reducing the smoke emitted. In this publication, tobacco bloating is mentioned by various methods, including the use of halogenated hydrocarbons, the operation under low pressure or in vacuo, or the treatment with high pressure steam, which leads to leaf expansion from the inside of the cell when the outside pressure is suddenly drained. Freeze-drying of tobacco is also mentioned there, which can be used to increase the volume. A number of tobacco expanding techniques, including some of the techniques discussed in this article, have been described in the form of patents and / or published patent applications since the publication of the above publication in "Tobacco Reporter". For example, U.S. Patent Nos. 3,524,452 and 3,524,451, both from 1970, relate to expanding tobacco using a volatile organic liquid, such as a halogenated hydrocarbon. US Pat. No. 3,734,104 from 1973 relates to a special method for expanding tobacco ribs. U.S. Patent No. 3,710,802, 1973 and GB Patent Publication No. 1,293,735, 1972, both relate to freeze drying methods for expanding tobacco. ZA patent applications 70/8291 u. 1970/8282 relates to the expansion of tobacco using chemical compounds that decompose to form a gas or with inert solutions of a gas under pressure to keep the gas in solution until it impregnates the tobacco. U.S. Patent 3,771,533, 1973, teaches treating tobacco with carbon dioxide and ammonia gases, thereby saturating the tobacco with these gases and forming ammonium carbonate in situ. The ammonium carbonate is then decomposed by heat, releasing the gases in the tobacco cells and causing the tobacco to expand. Despite the advances in this field described above, no fully satisfactory process has been found. The difficulty of the previous recommendations for expanding tobacco is that in many cases the volume increases only slightly or at best only moderately. For example, the freeze-drying processes have the disadvantage that they require complex and expensive equipment and cause substantial operating costs. The difficulty with regard to the teaching of the use of thermal energy, infrared or microwave radiation energy to expand tobacco ribs is that the ribs do respond to these heating processes, but it has been found that the tobacco leaf generally does not respond effectively to this type of process. The use of special expanding agents, such as halogenated hydrocarbons, as mentioned in the Meyer publication on expanding tobacco, is also not entirely satisfactory, since these substances generally have to be volatilized or otherwise removed after the tobacco has been expanded. In addition, the introduction of foreign materials for tobacco in considerable concentrations is problematic with regard to the removal of the expanding agent after the end of the treatment, in order to avoid the influence of aroma and other properties of the smoke by substances which are continuously supplied or which are formed by burning the treated tobacco. The use of carbonated water has also not proven to be effective. While the process using ammonia and carbon dioxide gas is an improvement over previous methods, it is not entirely satisfactory in certain circumstances because undesirable salt deposition may occur during the process. Carbon dioxide has been used or recommended in the food industry as a coolant and more recently as an extractant for food flavors. It was also described in DE-OS 2 142 205 for use either in gaseous form or in liquid form for the extraction of aromatic materials from tobacco. However, there was no evidence in connection with these uses of the use of liquid carbon dioxide to expand these materials. It has been found that a process using liquid carbon dioxide can overcome many disadvantages of the known processes discussed above. The expansion of tobacco using liquid carbon dioxide is described in BE-PS 821 568 and in BE-PS 825 133. This process can be described as a process for expanding tobacco, which comprises the following stages. (1) contacting the tobacco with liquid carbon dioxide to impregnate the tobacco with the liquid carbon dioxide, (2) applying conditions to the tobacco impregnated with liquid carbon dioxide to convert the liquid carbon dioxide to solid carbon dioxide, and (3) the application conditions on the solid carbon dioxide-containing tobacco, by which the solid carbon dioxide is evaporated, causing the tobacco to expand. The present invention relates to modifications to the basic process that result in significant improvements. Generally speaking, the invention is directed to an improved method of expanding tobacco and includes certain modifications to the basic method described above. According to one embodiment, the invention relates to an improvement of the grand process, which is to control the moisture content of the tobacco used in the first stage of the basic process. A second embodiment of the invention consists in withdrawing excess liquid carbon dioxide under controlled conditions after the first stage of the basic process and before the second stage of the basic process. According to a third embodiment, the invention relates to an improvement which is to control the moisture yield of the product obtained from the third stage of the basic process. The invention further relates to various combinations of the above three embodiments and to an overall method which uses all three embodiments in such a way that extraordinarily advantageous results can be achieved.