Field of the Invention

This invention relates to a process for obtaining dust-free and dried tar as well as a tar-free and dried filter cake product from the dust-laden tar phase which by sedimentation has been separated from the gas condensate obtained during the pressure gasification of carbonaceous fuels.

Prior art

By means of fixed-bed pressure gasification reactors solid fuel, such as coal, coke or other carbonaceous fuel, is gasified with steam and oxygen as gasification medium at elevated temperature and, in most cases, under overpressure to obtain a synthesis gas containing carbon monoxide and hydrogen, wherein a solid ash is obtained, which is discharged from the reactor via an ash discharge grate which in many cases is formed as rotary grate. This type of reactor frequently also is referred to as FBDB (= Fixed Bed Dry Bottom) pressure gasifier, cf. Ullmanns Encyklopadie der technischen Chemie, 4th Edition, Vol. 14, p. 383, Chapter 2.3.3.

Together with the raw synthesis gas produced in the fixed-bed pressure gasification reactor, non-decomposed steam, dust particles and tar are discharged from the gasifier. In a condensation means downstream of the gasifier, these constituents are separated from the gas and as gas condensate transferred into a settling tank, a so-called tar separator. In the tar separator, a phase rich in water, a phase rich in oil, and a phase rich in tar and solids is separated from the gas condensate by sedimentation.

The International Patent Application WO 2012/104686 A1 describes a process in which the gas condensate separated from the synthesis gas is supplied to a filter device, without first being cooled and depressurized. In an example disclosed there, a temperature of 200 °C and a pressure of 26 bar are mentioned as typical values for the gas condensate supplied to the filter device. In this filter device, the solid constituents, i.e. the dust entrained from the fixed fuel bed of the gasification reactor, are mechanically separated from the gas condensate. It is indicated that the solid constituents are obtained in a substantially tar-free state. The filtrate then is cooled, depressurized to atmospheric pressure and supplied to a gravitational separator for separating the tar. In this way, the dust entrained from the fixed bed and the tar contained in the gas condensate are separated from the gas condensate and obtained as process product.

In this process it is disadvantageous that the gas condensate is filtered in its entirety and at very high temperature and very high pressure. This requires a very large and technically complex filter device.

Therefore, it is the object of the present invention to provide a process in which the filter device puts through a smaller volume and is operated at lower pressure and lower temperature and thus is less expensive in terms of production and safety.

Description of the Invention

This object is solved by a process for obtaining dust-free and dried tar and a tar- free, dried filter cake product from dust-laden gas condensate separated from a raw synthesis gas produced by fixed-bed pressure gasification of coal or coke by means of a gas cooling and washing device, comprising the following process steps carried out one after the other: a) cooling of the gas condensate to a temperature in the range from 60 to 100 °C and depressurization to atmospheric pressure,

b) separation of a dust-laden tar phase from the gas condensate by

sedimentation,

c) comminution of the dust particles in the tar phase

d) separation of the dust particles from the tar phase by filtration in a filter

device which comprises at least one, preferably a plurality of filter units connected in parallel,

e) discharging the tar phase from the filter device and introducing the tar phase into a stripping plant which comprises at least one, preferably a plurality of stripping tanks connected in parallel,

f) separating the moisture from the tar phase by stripping with dry inert gas, g) discharging the dust-free and dried tar phase from the respective stripping tank for further treatment,

h) decoupling a filter unit of process step c) from the inflows and outflows of the tar phase, then charging the filter cake left in the filter unit with tar-free gas condensate in the same flow direction as the tar phase for rinsing the filter free of tar, then charging the filter cake left in the filter unit with dry inert gas in the same flow direction as the tar phase for rinsing free of moisture, wherein a tar-free, dried filter cake product is obtained.

Further advantageous aspects of the process according to the invention can be found in sub-claims 2 to 8. By applying the process according to the invention, not only a dust-free and dried tar product, but also a tar-free, dried filter cake product surprisingly is obtained, which in contrast to the processing methods known from the prior art is obtained completely dry and tack-free. The solid matter obtained as filter cake product therefore can easily be stored and conveyed and can be processed further advantageously. Due to its high carbon content it is suitable for example as fuel. Preferred Aspects of the Invention

A preferred aspect of the invention consists in that inprocess step 1 c) the dust particles contained in the tar phase are comminuted to a particle size of below 2 mm, preferably below 1 mm. As a result, the pipe conduits and pipe conduit fittings and conveying means, by which the tar phase is conveyed, are subjected to less abrasion. For comminution, grinding or crushing devices known per se can be used, in which the comminution proceeds discontinuously or preferably continuously. Preferably, the comminution device used also has a conveying effect, so that additional conveying devices can be saved wholly or in part.

Another advantageous aspect of the invention consists in that for conveying the tar phase through the at least one filter unit a displacement pump, such as e.g. a hose pump, piston pump, plunger pump or membrane pump, is used. These pumps are quite useful for conveying highly viscous liquids loaded with solid particles.

Another advantageous aspect of the invention consists in that before the filtration according to process step 1 d), the tar phase is heated to a temperature in the range from 80 to 130 °C. The dynamic viscosity of the tar phase thereby is lowered, whereby it can be conveyed through the pipe conduits and the filter with less expenditure of energy. At the same time, the abrasive load of the conveying means, pipe conduits and fittings is lowered by this measure. Another advantageous aspect of the invention consists in that a surface filter is used in process step 1 d). In this type of filter, the dust separated from the tar phase, the filtrate, forms a filter cake on the filter means. After the filter unit has been shut down and rinsed, this filter cake can easily be detached from the filter means and mechanically be removed from the filter. Surface filters therefore are quite useful as separating filters, i.e. for obtaining the solid matter suspended in a liquid phase. The handling of the separating filters is particularly easy, when surface filters of the leaf or candle filter type are used.

Another advantageous aspect of the invention consists in carrying out the filtration of the tar phase in several steps. In this way, particles of different sizes can be deposited separately, wherein the deposition of the coarse particles is effected in the first step and that of the finer particles is effected in the one or more succeeding steps. In this way, the filter cake product is fractionated corresponding to the particle size.

Another advantageous aspect of the invention consists in using nitrogen as inert gas in process steps 1 f) and/or 1 h). Nitrogen is inexpensive and unproblematic for the environment. By using nitrogen in process step 1 h), the filter cake product obtained, which has a high carbon content, also is rendered inert, so that during the subsequent storage or handling the risk of self-ignition is reduced.

Exemplary Embodiments

Further developments, advantages and possible applications of the invention can also be taken from the following description of non-limiting exemplary embodiments and numerical examples as well as the drawing. All features described and/or illustrated form the invention per se or in any combination, independent of their inclusion in the claims or their back-reference.

With reference to the drawing, Fig. 1 , the invention will now be explained in detail.

Fig. 1 shows a process diagram of the invention.

The gas condensate 1 is passed via heat exchanger 21 , by which it is cooled down to a temperature in the range from 60 to 100 °C and via an expansion device 22, by which it is expanded to atmospheric pressure, into a so-called tar separator 2, in which the gas condensate is liberated from oil, tar and dust by sedimentation. The cleaned gas condensate leaves the tar separator through conduit 3. Through conduit 4, the oil phase is withdrawn from the tar separator. At the bottom of the tar separator, the tar phase 5 loaded with dust particles is withdrawn and passed into an apparatus 6 in which the solid particles are comminuted. For comminution, grinding or crushing devices known per se can be used, in which the comminution proceeds discontinuously or preferably continuously.

By means of a displacement pump 7, the tar phase subsequently is conveyed through a heater 8 and then through the filters 9a or 9b into the stripping tank 10a or 10b. Nitrogen 1 1 is heated in the heater 12 and passed into the stripping tanks 10a, 10b, in order to there expel moisture from the tar phase. Tar phase 13 liberated from dust and moisture is drained from the stripping tanks for the further treatment. When the loading of the filter has reached a desired extent, the filter is shut down, in that it is decoupled from the tar supply. The tar phase then is passed over the parallel filter.

From the shut-down filter and the filter cake settled on the filter means, all tar residues are rinsed out with tar-free gas condensate 14, which is heated to 90 °C by using the heater 15. The gas condensate used for rinsing is recirculated into the tar separator 2 via conduit 16.

Subsequently, the filter is rinsed free of moisture with nitrogen via conduits 17a, 17b. In a first phase, rinsing is effected with nitrogen heated to 95 °C, in order to expel the moisture as effectively as possible. In a second phase the filter, and hence the filter cake, is cooled with nitrogen having ambient temperature. Via conduit 19, the nitrogen is passed into the tar separator 2, in order to there settle possibly entrained substances.

Cooling the filter cake by rinsing with cold nitrogen is effected, in order to facilitate the subsequent mechanical detachment of the filter cake from the filter means. By vibrating the filter candles (not shown) used as filter means, the filter cake is detached and for the further treatment discharged from the filter as dry, tar-free filter cake product 18a, 18b.

Subsequently, the filter is again put into operation, in that it is again traversed by dust-laden gas condensate. Since the full filtering effect only is obtained when a filter cake with sufficient layer thickness has formed on the filter means, the tar phase is recirculated into the tar separator 2 via conduit 20, until the desired filtering effect is achieved.

The filtration can be carried out in one or more steps.

The tar exiting from the filter means subsequently is drained into tanks for separating the moisture from the tar. In the tanks, the tar is stored at atmospheric pressure. The moisture from the tar bath, which has a temperature of preferably between 80 and 130 °C, thereby can ascend and be discharged from the tank.

In a preferred aspect of the invention, the tar bath is rinsed with dry inert gas, e.g. nitrogen, so that moisture is stripped out of the bath. To prevent the tar bath from cooling down due to stripping, the purge gas or stripping gas is heated to the bath temperature before entering into the bath.

The cleaned and dried tar subsequently is transferred into storage or transport containers.

Industrial Applicability

The present invention provides a process with which tar and dust can be obtained as valuable substances from gas condensate, as it is obtained as by-product e.g. during the gasification of coal. The dust is obtained as tar-free, dried filter cake product, which in contrast to the processing methods known from the prior art is obtained completely dry and tack-free. The solid matter obtained as filter cake product therefore can easily be stored and conveyed and can be processed further advantageously. Due to its high carbon content it is suitable for example as fuel.

List of Reference Numerals

1 gas condensate

2 tar separator

3 gas condensate, cleaned

4 oil phase

5 tar phase

6 apparatus for particle comminution

7 displacement pump

8 heater

9 filter a, b

10 stripping tank a, b

1 1 nitrogen

12 heater

13 tar phase, dust-free and dry

14 gas condensate, tar-free

15 heater

16 gas condensate, recirculation

17 nitrogen a, b

18 filter cake product a, b

19 nitrogen

20 tar phase, recirculation

21 heat exchanger

22 expansion device