Distillation: processes – separatory – And returning product of distillation step to distillation zone
Reexamination Certificate
1999-05-27
2002-04-02
Manoharan, Virginia (Department: 1764)
Distillation: processes, separatory
And returning product of distillation step to distillation zone
C196S111000, C202S153000, C202S158000, C202S182000, C202S197000, C203S040000, C203S078000, C203S100000, C203SDIG009, C261S148000
Reexamination Certificate
active
06365006
ABSTRACT:
AREA OF TECHNOLOGY
The invention relates to the field of distillation of mixtures of substances in the liquid or liquefied state, for example in the alcohol industry for purifiying alcohol from admixtures, for preparation of purified liquids in microbiology, medicine, agriculture, in processing liquid wastes and effluent, and also in petrochemistry, etc.
STATE OF THE ART
In industrial columns the distillation process takes place in special contact devices, for example of the plate type, packing type, or film type, in which the mass exchange takes place of the substances being separated, i.e. the interaction between the phases of steam and boiling liquid which coexist in them. The initial mixture of substances is heated and delivered as a feed mixture into one of the contact devices of the distillation apparatus. Vapors rise upward through the apparatus, and the liquid flows down into its lower part.
As the vapors pass through these contact devices in sequence, they become enriched in volatile substances and depleted in the ones that are heavy (in volatility), and the liquid does the reverse. In connection with this it is necessary to organize two fundamental processes. First, the successive (as a rule, counter current vertically) movement of the necessary quantity of flows, and second to ensure effective mass exchange of substances in the contact devices. If the phases coexisting in the contact devices are in contact long enough, the maximum possible enrichment and depletion of substances occurs, which is characterized as “thermodynamic phase equilibrium”, and equality is established of the chemical potentials of the substances in the vapor and liquid. Under practical conditions the contact time of the coexisting phases is limited, and therefore the flows leaving the contact devices differ from the equilibrium flows. The use of most actual contact devices especially limits how long the vapor phase stays, which is determined by the surfacing time of vapor bubbles. Therefore, single contact of the phases coexisting in the contact devices does not provide an adequate degree of mass transfer. To increase the contact time of the phases, returns (recycles) are made of the streams of liquid and vapor leaving the contact devices, which prolongs he mass exchange between the phases. The refining section of the column, located above the feed input, has a recycle performed of part of the condensate of the vapors leaving the contact device which is uppermost along their course, and delivers this part of the condensate back into the uppermost contact device in the form of reflux liquid which is enriched in volatile substances. The column's stripping section, located below the feed, has vapors of liquid which has left the contact device which is bottommost along its course returned into this same part of the column. So, countercurrent movement of the streams of liquid and vapor occurs in the apparatus.
Thus, the indicated recycling of vapor and liquid solves the problem of increasing to the maximum extent the contact time of the liquid and vapor phase coexisting in the contact device, which provides effective mass exchange between the substances in the contact device.
The stripping section is intended to isolate the bottoms, which are enriched in heavy substances and vapors enriched in the latter, which are returned with evaporation of part of the bottom liquid.
Passing successively through the contact devices, the vapors become depleted in heavy substances and enriched in light ones. Thus, at the top of the refining section part of the product is recycled which is enriched in light substances, and at the bottom of the stripping section part of the product is recycled which is enriched in heavy substances. Mass exchange of substances between phases takes place through two parallel processes—partial evaporation of liquid and partial condensation of vapors.
The path of movement of the liquid is much longer than the path of movement of the vapor. In connection with this it is necessary to increase the time which the vapors spend in the contact devices, which is usually accomplished by reevaporating the reflux liquid (liquid which appears in all contact devices).
The quantity of the flows of vapor and liquid necessary for countercurrent movement is defined by the ratio of volatile and heavy components in the initial mixture and the technical conditions of separation of the distillation products.
In certain methods the initial feed mixture is heated up in a heat exchanger to the boiling point (usually a few degrees lower), and the heated liquid is fed onto the top plate of the stripping section.
The necessary quantity of liquid in the refining section corresponds to the quantity of reflux liquid which is fed onto its top plate, and in the stripping section the feed liquid is added to this. The processes of mass exchange of phases take place adiabatically, which is also dictated by the conditions of thermodynamic phase equilibrium. The quantity of vapor in both sections is practically constant (the small reduction in it is connected with a deviation from adiabaticity, and in the stripping section it is connected with the feed not being adequately heated). The constancy of liquid and vapor flows in the column sections limits the search for their optimal recycles in the individual contact devices, since it becomes necessary to establish the greatest value of recycle caused by the more complex mass exchange conditions in at least one of all the section's contact devices.
Thus, the top contact device in the section has fed to it the maximum quantity of reflux liquid of all those possible for the sections contact devices, and correspondingly the column's bottom contact device has fed to it the maximum quantity of vapor necessary to reevaporate the volatile substances from the reflux liquid.
Separating one column into several refining and stripping sections by creating additional feed inputs or outputs of additional separation products is known in the state of the art. The top part of each of these sections has fed to it its own constant reflux for all contact devices of each section.
It follows from what has been said above, that the indicated problems in certain methods are solved by a single means: recycling part of the distillation product in the form of reflux liquid and recycling part of the bottoms in the form of vapor.
The inadequacy of known methods is the great energy consumption of distillation connected with the phase transformations of the substances being separated when they make the transition from liquid to vapor and vice versa, and the necessity of organizing the indicated recycles: reflux liquid of the top product and vapors of the bottom product of the distillation process in quantities defined by its greatest value from among all the contact devices of the column section.
Moreover, the strict connection of the sections with streams of vapor and liquid overloads the stripping section with liquid and the refining section with vapor. This leads to additional expenditures for preparing equipment and complicates control of the process.
A method and a system are known for distilling volatile substances from an aqueous medium in a column equipped with contact devices (U.S. Pat. No. 4,783,242, B01D, 1988).
Liquid leaving the bottom contact device is directed to a separator. The separator is connected with a thermocompressor which has heating vapor fed into it. The reduction in pressure in the separator produced by the thermocompression of the heating vapor causes the liquid to evaporate. The vapors formed, which are enriched in volatile components, are united in the thermocompressor with the heating vapor and returned to the bottom contact device. The remaining liquid is divided into two parts. One part is directed to the condenser and, after being warmed by vapors which are leaving the top contact device, are returned to the separator, i.e. this liquid is used as a cooling agent in the condenser, utilizing the heat of evaporation. The other part of the liquid, which represen
Aristovich Jury V.
Aristovich Valery J.
Sokolov Andrey J.
Sokolova Elena V.
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