EP0283584B1 - Process for reducing the water and ash content in crude tar - Google Patents

Description

The invention relates to a mechanical method for reducing the content of water and ash formers in hard coal tars by means of centrifuges.

In addition to heating gases, ammonia and benzene, the coking of hard coal also produces tars, which are characterized by the following analysis values:

According to Franck / Collin "Coal Tar", pages 25 to 27, the water content of raw tars can be reduced by mechanical methods.

Mechanical dewatering is preferable to distillation, since on the one hand many tar constituents pass with the water during the distillation and therefore a redistillation of the oil phase would be necessary, on the other hand the water-soluble salts remain in the distillation residue and thus reduce the pitch quality.

Water is most often separated by decanting under gravity at 60 _° C. The water content can be reduced to about 3%. Pressure drainage at temperatures up to 200 _° C and pressures of approximately 15 bar under gravity leads to a reduction in the water content. about 1 to 1.5%. However, pressure-resistant retorts are required for this. This limits the applicability to small amounts of tar.

Drainage with centrifuges is particularly effective, but also costly. Here water contents of 0.5 to 1% can be achieved. As an example, plate separators with nozzle discharge and a centrifugal number corresponding to 6000 to 7000 times the acceleration due to gravity (g) are mentioned, in which a sediment-rich concentrate is obtained at the same time.

The common dewatering and ash removal from raw tars is made more difficult by the fact that at least some of the ash formers act as emulsifiers. This creates a stable emulsion between solids, tar and water, and the degrees of separation and clarification that can be achieved are different for different tars.

They also depend on the particle size distribution of the solids, the density and the viscosity of the tars. The lowering of the viscosity due to higher processing temperatures is limited due to the tar substances boiling below 100 _° C and the azeotropes that form with water and the increasing solubility of the tar acids and bases in water. When using solvents to lower the viscosity, on the other hand, an additional distillation effort must be accepted.

The plate separators proposed for cleaning the raw tars with continuous solids discharge via nozzles are highly solids-oriented. They are therefore designed for the maximum solids concentration that occurs. If the solids content in the inlet decreases, the concentration in the discharge must also decrease accordingly. Since the solids must be flowable, solids concentrations of only about 60% can be achieved in the discharge. If the concentration drops from 5 to 0.5% in the inlet, it drops to around 6% in the outlet.

The plate packs of the separators are provided with risers that should lie in the level of the separating layer between the light (water) and heavy phase (tar), so that an optimal separation is guaranteed. Since the position of the separating layer depends on the density ratio between tar and water, it shifts when the density of the tar changes.

The difficulties caused by changing solids contents and densities of the raw tea are the reason why plate separators with nozzle discharge have not become established. Instead, raw tars are still dewatered in tanks by letting them settle and then freed from the insoluble substances in self-cleaning plate separators with batch-wise solids discharge. Ash and soot-like particles (ash-free 01) are separated to the same extent. The ash-free 01 is very unfortunate for many areas of application, such as for the manufacture of electrode binders or hard pitches for coking. For this reason, only the particularly ash-rich tars are freed of solids.

In FR-A 2 336 471 it is proposed to heat up the tar flowing out of the gravity separator with water contents up to 20% via a steam-heated heat exchanger during the coke oven gas processing and to feed it to a three-phase decanter centrifuge in order to separate it into the three phases water, tar and thick tar (see also: Petroleum and Coal, 1977, pp. 558 to 564). Residual water contents of generally less than 5% are achieved. The raw tar obtained in this way is stable in storage and ready for sale. Information on the degrees of clarification achieved is given in the Publication as little as indications of the tar content of the aqueous phase. However, since the water is returned to the gravity separator for clarification according to the process scheme, it can be assumed that the water contains not only soluble tar substances but also considerable amounts of undissolved substances.

With regard to the degree of clarification, the only goal could have been to prevent sedimentation during storage. In general, only the relatively coarse solids consisting of coke and ash dust are removed. The aim of the proposed method using a three-phase decanter centrifuge is thus a raw tar pre-cleaning, in which fluctuations in the composition are compensated for by the storage in the downstream storage containers.

Mishin et al. report in detail on the cleaning of pre-cleaned raw tars in full-shell centrifuges (Koks i Khimiya, 1978, pp. 47 to 49). The tests were carried out at temperatures between 67 and 76 _° C in centrifuges with a centrifugal number of 445. g performed. In each cycle, after the centrifuge had been filled and after a sedimentation time of 8 to 12 minutes, the water phase and then the tar phase were removed. After several cycles, the heavy tar phase containing solids was also removed. Depending on the sedimentation time, the water content decreased from 7.1 to 7.5% to 1.8 to 5.5% and the ash content from 0.14 to 0.17% to 0.06 to 0.11% . It was found that the sedimentation time should not be less than 10 minutes if sufficient degrees of separation and clarification were to be achieved. Since the process is carried out discontinuously, fluctuations in the composition of the raw tars play no role. There is also no mixing of the phases caused by an additional flow, as in continuous processes. The results are therefore not transferable to continuous processes, the results of which, as expected, must be significantly worse under comparable conditions. However, the long sedimentation times show that the proposed method is not very suitable for industrial implementation.

The task was therefore to develop a continuous process for reducing the content of water and ash formers in pre-cleaned hard coal tars, in which, despite sufficient separation and clarification levels, the demands for low sedimentation time and low losses of ash-free QI are met.

The object is achieved in that the optionally pretreated raw tar in a three-phase decanter centrifuge with a centrifugal number between 1000 and 3000 times gravitational acceleration (g), which is provided with an open single or double-start, preferably armored screw and the usual weirs and discharge devices a temperature between 60 and 105 _° C, an average residence time of the tar between 30 and 80 s and a differential speed between the screw and rotor of 10 to 50 min- ^{1 is} continuously divided into an aqueous, a tar and a solid phase, during which continuous operation, the feed is periodically interrupted briefly.

The raw tars can be treated with demulsifiers, flocculants and / or diluents before decanting, as is common in separation technology to improve the degree of clarification or separation. Decanting can also be preceded by water washing, as is customary in the tar industry to reduce the salt content.

In order to simplify the start-up process, it is advantageous to first present the solid phase and then to increase the raw tar throughput to the setpoint. The solid phase should never be whirled up by suddenly introducing a large amount of raw tar.

Decanters are generally used for the clarification of liquids whose solids content is so high that they can no longer be processed in plate separators. The usual solids content is between 20 and 60%. Such solids contents can be achieved in coke oven gas processing in coke oven plants with filling gas extraction. Surprisingly, it has now been found that even with raw tars with a QI content of less than 5 wt.

The effectiveness of the process steps according to the invention is illustrated by the following examples

In example 1, the dewatering and ash removal of the tar is carried out in a conventional three-phase decanter centrifuge, as suggested by Ullrich and Loss. The results of this comparison test thus characterize the state of the art. In example 2, the decanter modified in the manner according to the invention is used without the method being changed compared to example 1. Examples 3 and 4 show two variants of the claimed method.

Example (Veraleich)

A raw tar stored at 60 _° C is heated to 80 _° C by means of a temperature-controlled steam heating and fed in a quantity of 3.8 m3 / h to a cylindrical three-phase decanter centrifuge with conical discharge. The centrifuge is characterized by the following parameters:

Because of the simpler analysis method, only the water content in use, in the tar and water phase according to DIN 51 582, is determined as a measure of the separation effect. In order to exclude the possible start-up effects, the first sample is taken 2 hours after the start of the test. The results are shown in Table 1.

As the analysis data show, sufficient phase separation is not achieved and the degree of separation deteriorates with increasing test duration.

Example 2 (Veraleich)

A raw tar stored at 60 _° C is heated to 85 _° C and fed in an amount of 4.0 m ³ / h to the same three-phase decanter as in Example 1. The centrifuge is equipped with a single-start open helical screw (pitch 114 mm, helix height 30 mm, corresponding to approximately 3/5 of the total thickness of all phases).

The water contents of the samples are shown in Table 2.

Similar results are achieved if, instead of the one-part helical screw, one from separate individual segments is used.

The separation of the water has been improved by the installation of the open screw compared to example 1, but here, too, there is a reduction in the separation performance over the test period.

Example 3

Example 2 is repeated with a raw tar temperature of 82 ° C., the feed into the three-phase decanter centrifuge being interrupted every 4 hours for a period of 1 min.

Surprisingly, this measure prevents the drop in separation performance described in Example 2. In addition to the good and uniform water removal, the ash content is also reduced by more than 40% without a corresponding reduction in the quinoline-insoluble constituents (Ql) taking place.

The ratio of the layer thicknesses of the light to the heavy phase of about 5, which is set via the height of the discharge nozzle and the weir height, leads to the formation of a clean phase separation.

The analysis results are shown in Table 3.

Example 4

This example comprises 5 test series with different pretreated raw tars. The raw tar temperatures and throughput are also varied.

The raw tars are in a tank at 60 _° C 5 vol .-% of a tar fraction boiling between 200 and 230 _° C, 0.1 vol .-% of a commercially available demulsifier and 5 vol .-% water (test series a to c) or 10 Vol .-% of a 50% tar / water emulsion added. The contents are mixed by pumping over the entire duration of the experiment. The insert for the decanter centrifuge is removed approximately in the middle of the tank, at the same time a corresponding raw tar, tar oil, demulsifier and water or emulsion quantity are fed into the tank.

• Der Teerölzusatz soll die Viskosität des Rohteers herabsetzen und der Demulgator die Teer/Asche/Wasser-Emulsion brechen, um eine bessere Trennung zu erzielen. Das zusätzliche Wasser soll Salze aus dem Rohteer herauslösen, um den Chlorgehalt zu senken.

With this pretreatment the following should be achieved:

• The tar oil additive is said to reduce the viscosity of the raw tar and the demulsifier to break the tar / ash / water emulsion in order to achieve a better separation. The additional water is said to dissolve salts from the raw tar in order to reduce the chlorine content.

The water and the solids are separated off in the same way with the same decanter centrifuge as in Example 3. The sample is taken 22 hours after the start of the test.

The complete analysis data are summarized in Table 4. They show that the addition of oils and demulsifiers only slightly improve the degree of separation. It is surprising, however, that tars with such different water and ash content without adapting the procedure or centrifuge geometry, such as. B. the height of the extraction nozzle for the water phase or the weir height for the tar phase, so good separation results can be achieved.

The solid phase is completely free of water, so that it is e.g. B. in a two-phase decanter centrifuge can be concentrated in a simple manner to the extent that a free-flowing carbon concentrate remains, which can be used as a mixture component for hard coal.

• 2,4 m3/h ₌ 71 s
• 3,8 m³/h ₌ 45 s
• 4,0 m³/h = 42 s

The throughputs given in the examples correspond approximately to the following mean residence times of the tar, calculated on the basis of the total liquid content of the centrifuge:

• 2.4 m3 / h ₌ 71 s
• 3.8 m ³ / h ₌ 45 s
• 4.0 m ³ / h = 42 s

The residence times are thus considerably below that of Mischin et al. values of at least 10 min found for solid bowl centrifuges without the clarification or separation performance deteriorating. This result is surprising since it was suspected that the method according to the invention would lead to poorer results due to the mixing of the phases caused by the flow. In addition, it was expected that fluctuations in the raw tar composition would have a greater impact on the water and ash content of the tar phase.

Claims (5)

1. A process for the reduction of the content of water and ash formers in crude coal tars by means of a three-phase decanting centrifuge with the usual barriers and discharge devices at elevated temperature, characterized in that the crude tar - where appropriate pre-treated - is continously separated into an aqueous phase, a tar phase and a solid phase in the three-phase decanting centrifuge at a centrifuging number of between 1000 and 3000 g, provided with an open one- or two-threaded, preferably armoured worm, at e temperature of between 60 and 105_°C, an average dwell period of the tar of between 30 and 80 sec and a differential rotational speed between the worm and the rotor of from 10 to 50 min-1, the feed being periodically briefly interrupted during the continous operation.

2. A process according to claim 1, characterized in that water, demulsifiers, flocculants and diluents are added alone or in combination to the crude tar before introduction into the three-phase decanting centrifuge.

3. A process according to claim 1, characterized in that during the starting procedure the solid phase is first introduced, and the feed of the crude tar is then increased to the nominal throughput.

4. A process according to claim 1, characterized in that the height of the removal socket for the light phase and the barrier level for the heavy phase are selected to be such that the ratio of the layer thickness of the aqueous phase to the tar phase amounts to approximately 5.

5. A process according to claim 1, characterized in that the open worm is constructed in the form of a helix, the height of the helix corresponding to approximately 3/5 of the total thickness of all phases in the cylindrical part of the centrifuge, and the helix being in one piece or comprising a plurality of individual segments separate from one another.