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:

1. Chemicals not consumed are either required to operate a piece of equipment or are inert material.
2. 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.
3. 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)

• 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 ∼ 750^oC) 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.