GB2048298A - Filtration of Hot Pressurized Coal Liquefaction Slurry - Google Patents

Abstract

Hot pressurized slurry 13 from a coal liquefaction and hydrogenation reactor is fed to a continuous pressure filter 18 sealed against pressure loss, and the filter cake is repulped with cooler solvent in a pressurized repulper 23 at substantially the same pressure as the filter 18 to yield a slurry, the pressure of which is utilized to force the slurry into and through a second pressure filter 24 which also acts as a valve for final reduction of the pressure on the repulped slurry. <IMAGE>

Description

SPECIFICATION A Filter as a Valve Field of the Invention This invention relates generally to the liquifaction and/or hydrogenation of coal to recover hydrocarbons, fuels and other liquid products therefrom. More particularly, the invention relates to improved ways and means for more efficient treatment of the flashed-down residue from such liquifaction.

In a typical coal liquifaction and/or hydrogenation process as heretofore practiced, ground coal is dissolved in a solvent, usually derived from the coal itself, then hydrogenated at very high temperatures and pressures. Following this, the mass passes through several stages during which products are recovered while the temperature and pressure are reduced. The resulting flashed-down residue contains 5% to 10% solids, some of which have heat value and the residual liquid still contains valuable dissolved materials. The flashed-down fraction is still at elevated temperature (f.i. above about 7000F.) and at a pressure in the range of 150-200 psi.

Recovery of values from this fraction is achieved by filtering to separate the solid residue from the solvent. The filtrate, which contains dissolved products, may be sent to further process while the solid residue is either further treated or otherwise conserved to recover heat value.

Although such prior methods for handling the flashed-down residue are workable, they still present several distinct disadvantages. This is primarily because, from a practical standpoint, it is impossible to maintain a seal on a continuous pressure filter while discharging dry cake therefrom. If the cake is repulped under pressure then discarged through conventional metering valves, there is extreme wear on the valves thus increasing costs of operation. Moreover, there is still the cost of final separation of residual solids and solvent in order to clean the solids and recover additional products.

Summary of the Invention It is the primary object of this invention to provide improved ways and means for processing of the flashed-down residual fraction from coal by liquifaction and/or hydrogenation to separate and recover the valuable dissolved constituents as well as valuable residual solids without pressure or heat.

A further object is the processing of the residual fraction coal as aforementioned under conditions whereby the processing includes two pressure filtration steps, the second of which acts as a valve through which repulped cake from the first pressure filter is discharged.

An important related object is provision of a system in which temperature on the first stage filter cake is reduced by repulping said cake in a fresh relatively cooler solvent to yield the filterable slurry then utilizing the pressure to force said slurry into and through a second filter wherefrom cake and filtrate are separately discharged to substantially atmospheric pressure.

Since the temperature has been reduced prior to pressure release, flashing is also reduced. Also, the solvent in which the cake is repulped is selected to minimize flashing.

Briefly, the foregoing, and possibly other objects, are achieved by the steps of maintaining a slurry of flashed-down residue from a coal liquifaction and/or hydrogenation process in a pressure vessel at an elevated temperature, say above 7000F., and a pressure in the range from 1 50-200 psig; filtering the slurry on a continuous pressure filter to separate the solid and liquid fractions to yield a filtrate that is discharged to further processing and a filter cake, maintaining the pressure on the cake while cooling it to about 3000 F. by repulping it with a cool solvent in a repulper maintained at said pressure of 1 50-200 psig, thereafter filtering the resulting slurry on a second pressure filter by utilizing the existing pressure of the first filter both to effect transfer of the slurry to the second filter and expression of filtrate therethrough whereby to yield a filter cake and a clear filtrate. The filtrate is continuously discharged, via suitable metering valves, to atmospheric pressure. Since the second filter is a batch type, it must be opened periodically for cake removal.

Suitable valves and conduits are provided for isolating the first and second filters from each other and to direct pressured slurry from the first to second filters as needed. Typically, a flow control valve operation at a 5-1 0 psi pressure drop will be used to regulate flow from the repulper to the second filter. Also, suitable metering valves are employed, as needed, for reducing the filtrate from both filters to atmospheric pressure.

So far as the filter cake is concerned, there is no significant pressure drop in the first filter between the filter inlet and outlet from the repulper. Hence, a continuous filter, such as a rotary drum pressure precoat filter may be employed. (Obviously, there must be some pressure drop across the filter to effect cake formation and filtrate disharge, but this will not affect the pressure on the cake either on the drum or in the repulper.) The cake repulper is located in the same confined zone or tank as the filter, but is somewhat isolated, as by a chute, from the filter feed tank to avoid cooling of the filter feed and cake prior to discharge. In the repulper, the cake thoroughly mixed with a relatively cooler solvent to extract additional values while cooling the solids to about 3000 F. to minimize flashing following the second filter.The resulting slurry is then forced by the existing pressure into and through the second filter, which, as noted is a batch type, such as a plate and frame, from which filtrate is continuously discharged and the cake periodically discharged to atmospheric pressure.

By keeping the mixture at an elevated temperature and pressure until completion of the first filtration stage, maximum recovery of the lower boiling point products is achieved. Use of a relatively cooler, lighter solvent in the repulping stage effects recovery of other valuable products such as asphaltines and heavy oils while reducing the temperature to minimize flashing upon completion of the second filtration step when the pressure on the cake and filtrate is reduced to atmospheric.

In summary, the first filter is maintained at elevated pressure and temperature, the cake repulper effects temperature reduction and the second filter, which is located to accept the slurry from the repulper acts as a valve enabling final pressure reduction with minimum flashing concomitantly with separation of the residual solids and filtrate, thus making maximum use of the existing pressure and temperature in the residual fraction.

In order that the invention may be more readily understood and carried into effect, reference is made to the accompanying drawings and the description thereof which are to be taken as illustrative and not in limitation of the invention, the scope of which is defined only by the appended claims; including equivalents, rather than by the drawings or other description thereof.

Brief Description of the Drawings Fig. 1 is a flowsheet illustrating the various steps of the process. The actual equipment is not described in detail because it is well known in the field.

Storage tank 1 0 contains a supply of flasheddown residue from a coal liquifaction process.

Typically, this is a slurry containing up to 10% solids part of which still contain heat value. The liquid fraction is solvent and contains valuable dissolved constituents. The residue slurry is at an elevated pressure in the range from 1 50-200 psig and a temperature above about 7000 F. and this is maintained in the storage vessel.

Slurry from the storage vessel 13, still maintained at elevated pressure and temperature, is flowed to a first filter 18, which is a sealed continuous precoat filter, via conduit 19 at a rate controlled by a suitable metering valve 20. Filtrate is discharged via a suitable conduit 21 and valves 22 while the cake is discharged into a repulper 23. In the repulper, the pressure remains substantially that of the filter shell, however, the cake temperature is reduced by the introduction of relatively cooler wash solvent, some of which might be used as wash on the filter, but the majority of which is added to the cake in the repulper. The temperature reduction in this step is important because this will lessen flashing on the solvent upon discharge from the subsequent filtration step either as filtrate or residual solvent in the cake.

The second filtration step is carried out in one or more batch type pressure filters 24 to which the slurry is transferred from the repulper under the influence of pressure therein, via conduit 25 at a rate controlled by three-way valve 26. The valve 26 will, in accordance with known practice, be associated with controllers to regulate flow from the repulper so as to maintain a relatively constant pressure in the first filter. This valve may also be made responsive to changing pressure drop across the batch filter to regulate feed in response thereto. Filters 24 may, as indicated, be batch filters from which the filtrate is continuously discharged via conduits 28 and three-way valve 29 at reduced (atmospheric) pressure while the cake, as is customary in batch filters, is periodically removed when the entire filter is depressurized to atmospheric pressure.By using a pair of second filters, the process may be conducted continuously simply by alternating the filters on-stream.

As indicated, filtrate from both filters as well as the final cake, may be recycled or sent to other process as desired.

The critical feature of the invention is the use in combination of a pressured repulper in which temperature reduction of the cake is obtained by adding cool solvent and a second filter which functions as a valve for final reduction of the pressure on the cooler repulped slurry concomitantly with separation of the solvent with valuable components from the residual solids. Use of the filter as a valve eliminates the need for a metering valve. At the same time, final product separation is achieved.

The continuous pressure filter is preferably a rotary drum precoat type as that which gives a clear filtrate under conditions described while the batch filters may be the well known plate and frame filters.

Claims (6)

Claims

1. In a method for treating a flashed-down residue from a coal liquification process which residue is a slurry of residual solids in a liquor containing dissolved hydrocarbons and is at an elevated temperature of about 7000 F. and a pressure in the range from 1 50-200 psi, the improvement comprising feeding such slurry to a first continuous pressure filter zone containing both a filter and a repulper maintaining said slurry at substantially the same pressure and temperature as in said reaction zone, filtering said slurry to yield a cake of residual solids and a filtrate, discharging said cake and filtrate from said filter, discharging filtrate from said filter zone, reducing the temperature of said cake by repulping it with relatively cooler solvent in a repulper subjected to the pressure of said filter zone to yield a slurry, transferring the slurry from said filter zone under the influence of pressure therein through a closed conduit to a second pressure filter wherein said pressure effects filtration of said slurry to yield a cake and a filtrate; and separately withdrawing said cake and said filtrate from said second filter to zones of substantially reduced pressure.

2. Apparatus for separating a residual solids fraction from a liquid solvent fraction of a slurry that is at an elevated temperature and pressure comprising in combination a first pressure filter sealed to maintain pressure therein, means to feed a slurry to said filter at elevated temperature and pressure, means including valve means, to discharge filtrate from said filter, repulper means in communication with said filter and maintained at the same pressure as said filter and arranged to accept filter cake discharged therefrom, means to supply relatively cooler solvent to said repulper to form a slurry with filter cake therein, a second closed pressure filter, closed conduit means providing communication between said repulper and said second filter for conveying slurry under pressure from said repulper to said second filter whereby to effect separation therein of the solids and liquid fractions of said slurry; and means for separately discharging both said filtrate and said cake from said second pressure filter.

3. Apparatus according to Claim 2 in which said first filter is a rotary drum continuous pressure precoat filter and said second pressure filter is a batch-type filter.

4. Apparatus according to Claim 3 in which said second filter comprises a pair of plate and frame filters connected in parallel with each other and valve means are provided to alternately connect said plate and frame filters to said repulper.

5. A method for treating a flashed-down residue from a coal liquification process substantially as herein described with reference to the accompanying drawing.

6. Apparatus for separating a residual solids fraction from a liquid solvent fraction substantially as herein described and shown in the accompanying drawing.