Gas and liquid contact apparatus – Contact devices – Porous mass
Reexamination Certificate
1999-03-04
2001-09-11
Bushey, C. Scott (Department: 1724)
Gas and liquid contact apparatus
Contact devices
Porous mass
C261S110000, C261S112200
Reexamination Certificate
active
06286818
ABSTRACT:
TECHNICAL AREA
The present invention concerns internal members or baffles for mass transfer columns, also called column baffles. The invention relates in particular to packing that serve the purpose of mass transfer between a liquid and a gaseous or vapor phase.
STATE OF THE ART
Internal members, also described column baffles, are used in various applications both in thermal separation technology and in mixing technology for mass transfers and heat exchanges between two fluid-phases. They can be installed as separating or mixing members in columns and similar devices, which are used, among other things, for distillation, rectification, sorption, reaction or extraction. In the case of thermal and mass transfer/exchange between a liquid and a gaseous or vapor phase, the members' purpose consists of distributing the gas or vapor and the fluid, which can either flow through the column in the opposite direction of the gas or vapor flow or in the same direction, evenly across the column cross-section and to create as large a liquid surface as possible for mass and thermal transfer/exchange. Plates and/or packings are used as column members. Over the classic plate column, packed columns, i.e. columns whose members consist of packings, offer particular benefits with regard to pressure drop throughout the column, the flow rate and thus also the low thermal stress for sensitive substances. As far as investment costs are concerned, packed columns are usually considerably less expensive than plate columns due to the smaller building volume that is required.
Apart from so-called random packings, structured packings are also being used increasingly as packings for mass transfer columns. When used in distillation columns, structured packings offer especially great benefits in the areas of separation efficiency and increased scaling towards columns with larger diameters as compared to random packings.
Due to their structure, structured packings can be divided further into a “corrugated surface” type and a “grid packing” type. Packings of the corrugated surface type have plates that are crimped in a criss-crossing pattern, which are installed as paraxial members in the columns, and they create only one preferred direction that is recognizable in the cross-section projection of the column due to their structure of the gas or vapor flow as well as the fluid flow for one packing element each. A typically structured packing of the corrugated surface type is shown in DE 26 01 890.
Contrary to this, structured packings of the grid packing type have a grid-shaped, open structure, and the grid elements (lamellae or partial areas) are arranged in a net, grid or fan shape. The grid elements are connected on cross or knot areas, with arrangements possible in two-dimensional or three-dimensional form. Structured packings of the grid packing type can create more than one preferred direction that is recognizable in the cross-section projection of the column due to their structure of the gas or vapor flow and/or the fluid flow for one packing element each. For a suitable grid packing's design or structure even crimped sheets can be used to set up this packing type. A typical structured packing of the grid packing type is the one described in EP 069 241, which is sold under the “Rombopak” brand name.
Structured packings of the corrugated surface type have the advantage of a simpler production procedure over those of the grid packing type. However, due to connected surfaces (closed structure), the disadvantage arises that for structured packings of the corrugated surface type, lower wetting of the packing surface is achieved and thus also a smaller fluid surface for the mass transfer and heat exchange than is the case with grid packings. Therefore, higher efficiency in the separation can be achieved with separation columns equipped with structured packings of the grid packing type than can be achieved with separation columns equipped with packings of the corrugated surface type.
In applications with high purity requirements, however, the utilization of packed rectification columns still meets with resistance in the industry because the maldistribution problem, especially in the fluid phase, occurs both with random packings and structured packings. Maldistribution manifests itself through the fact that a fluid flow, which initially is evenly distributed over the cross-section of the column, displays uneven final fluid distribution after having passed through the packing. The separation efficiency's quality, however, depends on the evenness of the gas or vapor and fluid over the entire cross-section of the column's interior along the flow path and the intensity of its mixing process. The maldistributions occurring in the fluid and vapor flows can be divided into two different maldistribution types. Stochastic maldistribution describes a (random) small-scale maldistribution. Over large surface areas, distribution evens out. Systematic maldistribution, on the other hand, characterizes a large-scale maldistribution in relation to the column cross-section. In industrial reality, stochastic maldistribution is always superimposed on systematic maldistribution.
A special type of stochastic maldistribution is the insufficient, rough pre-distribution of the fluid above a packing bed. Over a large surface the fluid flow is evenly distributed, i.e., into a certain number of evenly distributed partial fluid flows of the same kind, generated by the fluid distributor that is utilized. Over a small surface, however, maldistribution occurs, i.e., surface areas without fluid supply and areas where there is a fluid flow of the distributor. This type of stochastic maldistribution due to insufficient pre-distribution of the fluid can be improved through sophisticated distributors or additional distribution aids.
From DE 44 18 488, we know that an increased flow of fluid and vapor occurs in the area of the column wall—the so-called wall effect, a special type of systematic maldistribution —since the random packing's or packing's wall layer offers lower resistance. In these areas, the fluid participates insufficiently in mass transfer. According to the article “Problems and Experiences with Large Technical Packing Columns, Chem.-Eng.-Tech. 64 (1992), No. 1, pages 6-16” by Martin Gann et al., it is true that, due to their structure, structured packings run a lower risk of systematic maldistribution than random packings, however they are not—as previously assumed—self-distributing.
In particular systematic maldistribution in the fluid and vapor flow causes negative effects on separation efficiency because balanced mixing over the column cross-section is not ensured. In the case of stochastic maldistribution on the other hand, more or less distinct cross-mixing can continuously balance and compensate for the negative influence.
As can be seen, among other things, from DE 44 18 488, packing manufacturers recommend multiple collection of the fluid and its redistribution over intermediate distributors as a common remedy for maldistribution. To achieve this, the packing is divided into several beds between which the fluid collector and the intermediate distributor are arranged. This measure is beneficial for both types of maldistribution mentioned above; mainly, however, to eliminate systematic maldistribution. In the utilization of fluid collectors and distributors, however, their cost and the increased building height of the columns proved to be disadvantageous.
To avoid the sophisticated collector and intermediate distributor mentioned above, attempts have been made to utilize distribution elements which include the function of both collecting and redistributing the fluid. U.S. Pat. No. 5,523,062A (Chemical Research & Licensing Company) deals with such a combined distribution element for even distribution of gas and liquid flow, which is suited for a catalytic distillation column. The zigzag-shaped distribution element is arranged crosswise between the packing elements, with flow openings for the gas and fl
Birch, Stewart Kolasch & Birch, LL
Bushey C. Scott
Kuhni AG
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