IV Distillation Sequencing

Transcription

1 IV Distillation Sequencing Outline 1. Basic Concepts of Distillation Sequence Design 2. Choice of Sequence and its Operating Pressure. 3. Performance of Distillation Column (Sieve tray and packed tower) 4. Separation and Recycle for continues process Department of Chemical Engineering - UPN Veteran Yogyakarta Page 1 of 18

2 IV.1. BASIC CONCEPT OF DISTILLATION SEQUENCING IV.1.1. Introduction Consider: Separation of a homogeneous multi-component fluid mixture into a number of products, whereas all separations are carried out using distillation only. If this is the case, how to choice the distillation sequence? For example: two simple column sequences (direct and indirect) could be employed in separation of a three-component mixture into three relatively pure products. Department of Chemical Engineering - UPN Veteran Yogyakarta Page 2 of 18

3 Direct Sequence vs Indirect Sequence direct sequence the lightest component is taken overhead in each column requires less energy for both reboiling and condensation supplied by utilities component A (light material) is only vaporized once indirect sequence the heaviest component is taken as bottom product in each column requires more energy for both reboiling and condensation supplied by utilities Component A (light material) is vaporized twice can be more energy-efficient if the feed to the sequence has a low flowrate of the light material (A) and a high flowrate of heavy material (C) Alternative sequences for the separation of a four-product mixture. Department of Chemical Engineering - UPN Veteran Yogyakarta Page 3 of 18

4 Number of possible distillation sequences using simple columns The problem is that there may be significant differences in the capital and operating costs between different distillation sequences that can produce the same products. In addition, heat integration may have a significant effect on operating costs (would be discussed next). IV.1.2. Practical Constraints 1. Remove as early as possible: a. A particularly hazardous component safety consideration b. Reactive or heat-sensitive component to avoid problems of product degradation c. Corrosive component to minimize the use expensive material of construction 2. The main component that difficult to be condensed should be removed as early as possible using refrigeration system or high pressure system 3. Don t take the final product from the bottom of column if: a. The component is decomposed in the reboilers (it can contaminates the product) b. Polymerization inhibitors are used to inhibit polymerization of some components when distilled. These polymerization inhibitors tend to be nonvolatile, ending up in the column bottoms Department of Chemical Engineering - UPN Veteran Yogyakarta Page 4 of 18

5 IV.2. CHOICE OF SEQUENCE AND ITS OPERATING PRESSURE IV.2.1. Heuristics of Choice of Sequence (Smith, R., 2005) 1. Component with its relative volatility close to unity or that exhibit azeotropic behavior should be removed last. 2. The lightest components should be removed alone one by one in column overheads (use direct sequence). 3. A component composing a large fraction of the feed should be removed first. 4. Favor splits in which the molar flow between top and bottom products in individual columns is as near equal as possible. Department of Chemical Engineering - UPN Veteran Yogyakarta Page 5 of 18

6 Example 4.2.1: Data for a mixture of alkanes to be separated by distillation are as follows: The relative volatilities have been calculated on the basis of the feed composition to the sequence, assuming a pressure of 6 barg using the Peng Robinson Equation of State with interaction parameters set to zero. Different pressures can, in practice, be used for different columns in the sequence Use the heuristics to identify potentially good sequences that are candidates for further evaluation! Solution: Alternative-1 Heuristic 1: Do D/E split last since this separation has the smallest relative volatility. Heuristic 2: Favor the direct sequence: Heuristic 3: Remove the most plentiful component first: Heuristic 4: Favor near-equimolar splits between top and bottom products: All four heuristics are in conflict here: Heuristic 1 suggests doing the D/E split last, and Heuristic 3 suggests it should be done first. Heuristic 2 suggests the A/B split first and Heuristic 4 the C/D split first. Department of Chemical Engineering - UPN Veteran Yogyakarta Page 6 of 18

7 Solution: Alternative-2: Take one of the candidates and accept, say, the A/B split first. Heuristic 1: Do D/E split last. Heuristic 2: Heuristic 3: Heuristic 4: Again the heuristics are in conflict Heuristic 1 again suggests doing the D/E split last, whereas again Heuristic 3 suggests it should be done first. Heuristic 2 suggests the B/C split first and Heuristic 4 the C/D split first. There are 14 posible sequence This process could be continued and possible sequences identified for further consideration. Some possible sequences would be eliminated Quantitative measure as other consideration Since heuristics (qualitative procedure) can be in conflict, a quantitative measure of the relative performance of different sequences would be preferred The vapor flow up the column as a physical measure can be readily calculated. This provides an indication of both capital and operating cost. More vapor flow up the column, more heat duty required for reboiler and condenser, increase the operating cost of hot utility (steam) and cold utility (water or refrigerant) A high vapor rate leads to a large diameter column, and also requires large reboilers and condensers, therefore the capital cost increases Consequently, sequences with lower total vapor load would be preferred to those with a high total vapor load. But how is the total vapor load predicted? Department of Chemical Engineering - UPN Veteran Yogyakarta Page 7 of 18

8 Prediction of the total vapor load Underwood Vmin D 1 R min (4.2.1) Eq. (4.2.1) can also be written at finite reflux. Defining R F to be the ratio R/R min (typicaly R/R min = 1.1): V D 1 RF R min (4.2.2) R min can be calculated: 1 x DLK xdhk Rmin 1 xflk x (4.2.3) FHK where: = relative volatility between the key components x DLK = mole fraction of light key in the distillate x FLK = mole fraction of light key in the feed x DHK = mole fraction of heavy key in the distillate x FHK = mole fraction of heavy key in the feed Assuming a sharp separation: only the light key and lighter than LK components in the overhead only the heavy key and heavier than HK components in the bottoms where: R min 1 x DLK 1 F 1 xflk 1 D (4.2.4) F = feed flow rate D = distillate flow rate Combining (4.2.4) and (4.2.2): thus: V RF F R V D1 D F F (4.2.5) 1 D 1 R 1 F F F... F F F... F F... F A B LK A B LK HK NC (4.2.6) Department of Chemical Engineering - UPN Veteran Yogyakarta Page 8 of 18

9 Example 4.2.2: Using the data (below) for a ternary separation of benzene, toluene, and ethyl benzene. Based on the vapor flow-up, determine whether the direct or indirect sequence should be used! Symbol Component Flowrate (kmole/h) Relative Volatility A Benzene B Toluene C Ethyl Benzene Relative Volatility betwwen adjacent components Solution of Example 4.2.2: Direct Sequence: A/BC and B/C V kmole/h Hence we should use the direct sequence Indirect Sequence: AB/C and A/B V kmole/h Department of Chemical Engineering - UPN Veteran Yogyakarta Page 9 of 18

10 Direct and Indirect Sequence (example 4.2.2) 269 kmol/h 269 kmol/h 282 kmol/h 269 kmol/h 282 kmol/h 282 kmol/h 57 kmol/h 57 kmol/h V = kmol/h V = kmol/h Example 4.2.3: Using the Underwood Equations, determine the best distillation sequence, in terms of overall vapor load, to separate the mixture of alkanes in Example into relatively pure products. The recoveries are to be assumed to be 100%. Assume the ratio of actual to minimum reflux ratio to be 1.1 and all columns are fed with a saturated liquid. Neglect pressure drop across each column. Relative volatilities can be calculated from the Peng Robinson Equation of State with interaction parameters assumed to be zero (see Chapter 4). Determine the rank order of the distillation sequences on the basis of total vapor load for all column pressures fixed to 6 barg with relative volatility calculated from the feed to the sequence. Department of Chemical Engineering - UPN Veteran Yogyakarta Page 10 of 18

11 Solution of Example 4.2.3: IV.3. PERFORMANCE OF DISTILLATION COLUMN (Plate/Tray Column and Packed Column) Department of Chemical Engineering - UPN Veteran Yogyakarta Page 11 of 18

12 Distillation Tray and Packing Distillation Tray Distillation Packing Plate/Tray Column vs Packed Column Plate/Tray Column Packed Column Contact of vapor-liquid relatively chanelling and backmixing could be good happen More liquid hold-up --- Easy to be cleaned Small Pressure drop, prefer to vacuum operation --- Cheaper for corrosive fluid --- Prefer to small diameter Can be used for liquid that contains Solid particle plugs the packed solid particles --- Foaming liquid --- Lighter Products can be taken from the --- side-stream Department of Chemical Engineering - UPN Veteran Yogyakarta Page 12 of 18

13 IV.4. SEPARATION AND RECYCLE SYSTEM FOR CONTINUES PROCESS IV.4.1. Introduction Do separation for some reasons: 1. to achieve product specification 2. to meet environment law Material to be separated: 1. reactants 2. main product 3. byproduct (could be sold 4. waste (could not be sold) Department of Chemical Engineering - UPN Veteran Yogyakarta Page 13 of 18

14 IV.4.2. Function of Process Recycles 1. Reactor conversion: Consider FEED PRODUCT with conversion of about 95% Incomplete conversion in the reactor requires a recycle for unconverted feed material. 2. Byproduct formation Consider: 1 st : FEED PRODUCT + BYPRODUCT or 1 st : FEED PRODUCT 2 nd : PRODUCT BYPRODUCT Using a purge saves the cost of a separator but incurs raw material losses, and possibly waste treatment and disposal costs. This might be worthwhile if the FEED- BYPRODUCT separation is expensive. Department of Chemical Engineering - UPN Veteran Yogyakarta Page 14 of 18

15 3. Recycling byproducts for improved selectivity Consider: If a byproduct is formed via a reversible secondary reaction then recycling the byproduct can inhibit its formation at source. 4. Recycling byproducts or contaminants that damage the reactor When recycling unconverted feed material, it is possible that some byproducts or contaminants, such as products of corrosion, can poison the catalyst in the reactor. It is clearly desirable to remove such damaging components from the recycle stream. Department of Chemical Engineering - UPN Veteran Yogyakarta Page 15 of 18

16 5. Feed impurities If the impurity has an adverse effect on the reaction or poisons the catalyst 5. Feed impurities (continued) I if the impurity does not have a significant effect on the reaction, then it could perhaps be passed through the reactor and be separated Department of Chemical Engineering - UPN Veteran Yogyakarta Page 16 of 18

17 5. Feed impurities (continued) As with its use to separate byproducts, the purge saves the cost of a separation, but incurs raw material losses. This might be worthwhile if the FEED-IMPURITY separation is expensive. Care should be taken to ensure that the resulting increase in concentration of IMPURITY in the reactor does not have an adverse effect on reactor performance. 6. Reactor diluents and solvents. An inert diluent such as steam is sometimes needed in the reactor to lower the partial pressure of reactants in the vapor phase. Diluents and solvents are normally recycled. Department of Chemical Engineering - UPN Veteran Yogyakarta Page 17 of 18

18 7. Reactor heat carrier. The introduction of an extraneous component as a heat carrier effects the recycle structure of the flowsheet. 7. Reactor heat carrier (continued) This figure illustrates the use of the product as the heat carrier. This simplifies the recycle structure of the flowsheet and removes the need for one of the separators. The use of the product as heat carrier is obviously restricted to situations where the product does not undergo secondary reactions to unwanted byproducts. Department of Chemical Engineering - UPN Veteran Yogyakarta Page 18 of 18