Process Input – Output Structure
// Process Concept Diagram
A diagram uses the stoichiometry of the main reaction pathway to identify the feed and product chemicals.
The toluene hydrodealkylation process
Important factors to consider in analyzing the overall input output structure of a PFD:
- Chemicals not consumed are either required to operate a piece of equipment or are inert material.
- Any chemical leaving a process must have either entered in one of the feed streams or have been produced by a chemical reaction within the process.
- Utility streams are treated differently from process streams. Utility streams rarely directly contact the process streams. They usually provide or remove thermal energy or work.
// Generic Block Flow Process Diagram
This chemical process broken down into six basic areas or blocks Each block provides a function necessary for the operation of the process.
GBFPD for toluene hydrodealkylation process
- Each of the process blocks may contain several unit operation.
- Several process blocks may be required in a given process, e.g, multiple process blocks in a single process.
- Each unit operation can be placed into one of these blocks.
// Process Flow Diagram
The PFD, by convention, shows the process feed stream(s) entering from the left and the process product stream(s) leaving to the right.
// Utility Streams
Utility streams, such as cooling water, steam, fuel, and electricity, rarely directly contact the process streams. They usually provide or remove thermal energy or work.
Utility streams for Toluene hydrodealkylation process
// Feed Purity and Trace Components
- In general, the feed streams entering a process do not contain pure chemicals. The option always exists to purify further the feed to the process. (Purification before feeding ?)
- The question of whether this purification step should be performed can only be answered by a detailed economic analysis.
- However, some common-sense heuristics may be used to choose a good base case or starting point. The following heuristics are modified from Douglas.
// Heuristics: Feed Purity and Trace Components
- If the impurities are not present in large quantities (say < 10 ∼ 20%) and these impurities do not react to form by-products, then do not separate them prior to feeding to the process.
- For example, the hydrogen fed to the toluene HDA process contains a small amount of methane (5 mol%–see “Stream 3 in Table 1.5”). Since the methane does not react (it is inert) and it is present as a small quantity, it is probably not worth considering separating it from the hydrogen.
- If the separation of the impurities is difficult (for example, an impurity forms an azeotrope with the feed or the feed is a gas at the feed conditions), then do not separate them prior to feeding to the process.
- For example, again consider the methane in Stream 3. The separation of methane and hydrogen is relatively expensive (see “Example 2.3”) because it involves low temperature and/or high pressure. This fact, coupled with the reasons given above, means that separation of the feed would not normally be attempted.
- If the impurities foul or poison the catalyst, then purify the feed.
- For example, one of the most common catalyst poisons is sulfur. This is especially true for catalysts containing Group VIII metals such as iron, cobalt, nickel, palladium, and platinum [7]. In the steam reformation of natural gas (methane) to produce hydrogen, the catalyst is rapidly poisoned by the small amounts of sulfur in the feed. A guard bed of activated carbon (or zinc oxide) is placed upstream of the reactor to reduce the sulfur level in the natural gas to the low ppm level.
- If the impurity reacts to form difficult-to-separate or hazardous products, then purify the feed.
- For example, in the manufacture of isocyanates for use in the production of polyurethanes, the most common synthesis path involves the reaction of phosgene with the appropriate amine. Because phosgene is a highly toxic chemical, all phosgene is manufactured on-site via the reaction of chlorine and carbon monoxide.
- CO + Cl2 −→ COCl2 (phosgene)
- For example, in the manufacture of isocyanates for use in the production of polyurethanes, the most common synthesis path involves the reaction of phosgene with the appropriate amine. Because phosgene is a highly toxic chemical, all phosgene is manufactured on-site via the reaction of chlorine and carbon monoxide.
- If carbon monoxide is not readily available (by pipeline), then it must be manufactured via the steam reformation of natural gas. The following equation shows the overall main reaction (carbon dioxide may also be formed in the process but it is not considered here):
- CH4+ H2O→ CO+ 3H2
The question to ask is, at what purity must the carbon monoxide be fed to the phosgene unit? The answer depends on what happens to the impurities in the CO. The main impurity is hydrogen. The hydrogen reacts with the chlorine to form hydrogen chloride that is difficult to remove from the phosgene, is highly corrosive, and is detrimental to the isocyanate product. With this information, it makes more sense to remove the hydrogen to the desired level in the carbon monoxide stream rather than send it through with the CO and cause more separation problems in the phosgene unit and further downstream. Acceptable hydrogen levels in carbon monoxide feeds to phosgene units are less than 1%.
- If the impurity is present in large quantities, then purify the feed.
- This heuristic is fairly obvious as significant additional work and heating/cooling duties are required to process the large amount of impurity. Nevertheless, if the separation is difficult and the impurity acts as an inert, then separation may still not be warranted. An obvious example is the use of air, rather than pure oxygen, as a reactant. Because nitrogen often acts as an inert compound, the extra cost of purifying the air is not justified compared with the lesser expense of processing the nitrogen through the process. An added advantage of using air, as opposed to pure oxygen, is the heat absorbing capacity of nitrogen, which helps moderate the temperature rise of many highly exothermic oxidation reactions.
// Addition of Feeds Required to Stabilize Products or Enable Separations
- Generally, product specifications are given as a series of characteristics that the product stream must meet or exceed. Clearly, the purity of the main chemical in the product is the major concern. However, other specifications such as color, density or specific gravity, turbidity, and so on, may also be specified.
- Often many of these specifications can be met in a single piece or train of separation equipment. However, if the product stream is, for example, reactive or unstable, then additional stabilizing chemicals may need to be added to the product prior to it going to storage. These stabilizing chemicals are additional feed streams to the process.
- The same argument can be made for other chemicals such as solvent or catalyst that are effectively consumed in the process. If a solvent such as water or an organic chemical is required to make a separation take place, for example, absorption of a solvent-soluble chemical from a gas stream, then this solvent is an additional feed to the process. Accounting for these chemicals both in feed costs and in the overall material balance (in the product streams) is very important.
// Inert Feed Material to Control Exothermic Reactions
- In some cases, it may be necessary to add additional inert feed streams to the process in order to control the reactions taking place. Common examples of this are partial oxidation reactions of hydrocarbons.
- For example, consider the partial oxidation of propylene to give acrylic acid, an important chemical in the production of acrylic polymers. The feeds consist of nearly pure propylene, air, and steam. The basic reactions that take place are:
C3H6 + 3/2 O2 → C3H4O2 + H2O Reaction 1
C3H6 + 5/2 O2 → C2H4O2 + H2O + CO2 Reaction 2
C3H6 + 9/2 O2 → 3H2O + CO2 Reaction 3
All these reactions are highly exothermic, not limited by equilibrium, and potentially explosive. In order to eliminate or reduce the potential for explosion, steam is fed to the reactor to dilute the feed and provide thermal ballast to absorb the heat of reaction and make control easier.
// Inert Feed Material to Control Equilibrium Reactions
- Sometimes it is necessary to add an inert material to shift the equilibrium of the desired reaction.
- Consider the production of styrene via the catalytic dehydrogenation of ethyl benzene:
C6H5CH2CH3 ↔ C6H5CH=CH2 + H2
ethyl benzene ↔ styrene + hydrogen
This reaction takes place at high temperature (600 ∼ 750oC) and low pressure (<1 bar) and is limited by equilibrium. The ethyl benzene is co-fed to the reactor with superheated steam. The steam acts as an inert in the reaction and both provides the thermal energy required to preheat the ethyl benzene and dilutes the feed. As the steam to ethyl benzene ratio increases, the equilibrium shifts to the right (LeChatelier’s principle) and the singlepass conversion increases. The optimum steam-to-ethyl benzene feed ratio is based on the overall process economics.