It can be seen from Table 1 that it is advisable to use high-efficiency flow-guided sieve tray in the PVA production. The economic analysis is made as follows:
The structure of high-efficiency flow-guided sieve tray is simple, and can be manufactured easily and cheaply. So the technology investment is small, about RMB 0.5 million.
(2) Energy saving
Because the return ratio decreases from 1.3 to 0.85, the technology can save energy 2164 MJ h"1 (i.e. 8000 t water vapor per year), amounting to RMB 0.48 million annually.
(3) Water saving
This modification can save energy 240000 t water vapor per year, amounting to RMB 96000 annually.
(4) Increasing the VAC yield
After modification, the VAC concentration at the bottom of the column decreases to 0.02-0.03%, amounting to RMB 0.638 million annually.
(5) Improving the product quality
As mentioned above, due to the decreasing VAC concentration at the bottom of column, the product quality is improved, amounting to RMB0.8 million annually.
(6) The return period on investment
The return period on investment is about 2.7 months.
Thermally coupled distillation (TCD) is only related with multi-component separation process. Among all possible new schemes for multi-component distillation process, the thermally coupled distillation (TCD) schemes are very promising for both energy and capital cost saving [15-18]. Before introducing different TCD processes, we provide the following definitions to help our understanding.
(1) Single source:
Most often the problem to be considered has only a single source which is to be split into all the desired products.
(2) Sharp separation:
If each column separates the feed into products with no overlap in the components between them, it is performing a "sharp" separation. An example is to split a mixture containing components A, B, C and D into the pure-component product A and the mixture of B, C and D. It is a sharp separation solution only if one column is used for this separation.
(3) Simple columns:
Distillation columns having one feed and producing two products are denoted as "simple" columns. However, if the top and/or bottom of the column are connected by liquid and vapor streams, this column can also be taken on as simple column. As shown in Fig. 5, suppose that at the top a total condenser is added afterwards, and then
where D (kmol s~1) is the flowrate of the top product, and R is reflux ratio; at the bottom, suppose that a reboiler is added afterwards, and then
where B (kmol s"1) is the flowrate of the bottom product; at the feed,
where F (kmol s"1) is the flowrate of the feeding mixture. In the shortcut design of simple distillation, the thermal quality of the feeding mixture, q, is needed and defined by the heat required for transferring the present state into saturated vapor state per kmol ^ the latent heat of vaporization of the feeding mixture per kmol
Apparently, q = 1 for saturated liquid, q = 0 for saturated vapor, 0 < q < 1 for the mixture of liquid and vapor, q > 1 for subcooled liquid and q < 1 for superheated liquid. Sometimes q can be regarded as the liquid fraction of the feeding mixture, provided that the molar latent heat of vaporization is equivalent among the components to be separated, of course. Under this condition, q = (10)
Fig. 5. One form of simple distillation column.
(4) Column section:
For instance, for a simple distillation column as shown in Fig.5, there are two column sections, i.e. rectifying section and stripping section, indicated by the number "1" and number "2" respectively.
(5) Separation sequence
Thompson and King  provide the following formula to determine the number of simple distillation sequences for separating an A-component mixture into N pure-component products:
Number of simple distillation sequences =--(11)
For instance, for a ternary separation, the tree structure and flowsheet of separation sequences are illustrated in Fig. 6, where A is the lightest component, B the middle and C the heaviest component.
Example: Please draw the tree structure of a quaternary separation.
Solution: In terms of Eq. (11), in total there are six kinds of separation sequences, the tree structure of which is drawn in Fig. 7.
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