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In distillation columns, why are trays more widely used than packings for vapour-liquid contact? What are the advantages of using trays over packings?
 
Answers
08/10/2018 A: Rajkumar Chate, Sulzer, rajkumar.chate@sulzer.com
Selection of mass transfer components trays/packing depneds on several factos. The choice is decided based on following parameters
1.High pressure distillation: Structured packings are very good for low pressure application but separation efficiency drastically drops for high pressure application hence trays are prefered over structured packing. There are few high pressure applications where vapor and liquid density difference is bigger or liquid to vapor density is higher like TEG contactor or Amine absorber where structured packing could be used.
2. Low pressure application: Often pressure drop is very important in vacuum columns, structured packing gives much lower pressure drop compared with trays for same number of theoretical stages. Structured packings are always preferred for low pressure application.
3. Capacity: Structured packings are good for gas capacity while trays are preferred when liquid load is high.
4. corrosion/Fouling: Structured packing is made from small thick material while trays have higher thickness material. Corrosion/fouling can significantly reduce the capacity or separation efficiency of packing but trays are more robust if you have corrosion/fouling issues hence trays are preferred.
5. Trays requires separate are for the liquid flow (DC) while with packing liquid and vapor are flowing through whole cross section of the column. If liquid load is not high then structured packing will requires smaller column ID (fixed cost) compared to trays hence structured packings are preferred.
Motio columns: structured packing will have a less impact of motion for column subject to motion (floating platform/on vessel) compared to trays hence structured packings are preferred.
6. Column size: if it is pilot column or very small column then it is much easy and cost effective to install packing compared to trays hence packings are preferred.
7. Cost: Packing will cost more than trays but this cost is not the only parameter for selection of mass transfer component.
19/06/2018 A: keith bowers, B and B Consulting, kebowers47@gmail.com
PROPER selection of the size, physical shape characteristics, material of construction, and installation method as well as redistribution devices are all important in achieving good V/L contact. 'Experts' in this field are not as frequent. Incorrect material selection will cause poor liquid distribution, poor contact, and poor efficiency.
Proper tray design and installation details are important and more widely known. Trays can give lower HETP which reduces cost of the vessel. Trays generally will better respond to fluctuating liquid and vapor rates and maintain efficiency over a wider range.
02/06/2018 A: Sridhar Balakrishnan, Bharat Oman Refineries Limited , laksrid@yahoo.com
Advantages of tray over packings , they are cost efficient , can handle higher liquid flow rates, better at handling any source of fouling and trays allows upward moving vapors and downward moving liquids to have good contact.
01/06/2018 A: Sudhakara Babu Marpudi, Oman Refineries and Petroleum Industries Company, m_sudhakarababu@yahoo.com
in Simple terms, Trays are used where finer separation of products are the requirement. One can produce finished products (pretreatment level products) from tray towers. Packings are used where bulk separation (intermediate products that need further processing to produce finished products) are the requirement.
31/05/2018 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
Trays were more reliable in predicting fractionation capability. It is easier to inspect a trayed column, often requiring dumping packing for inspection.