Delayed flow diagram

Figure 3-3. Flow diagram of a delayed coking unit (1) coker fractionator, (2) coker heater, (3) coke drum, (4) vapor recovery column.

Fig. IS. Schematic flow diagram showing residuum hydroprocessing in combination with delayed coking to produce cracking feeds and light products (Howell el al., 1985).

Examine the process in detail with the aid of appropriate flow diagrams, looking at upset conditions, delays which can occur, modes of failure, worst case situations, and other ways of abusing the chemical, with particular attention to redundancy and critical controls. 

The presence of recycle streams increases the complexity. A recycle stream in this context is a stream that is returned to a unit. It appears earlier in the simulation flow diagram with no change in physical properties and chemical composition from its process unit of origin. The recycle streams introduce a time delay in the mathematical equations and change the mathematical description from a finite dimensional system of equations to an infinite dimensional system of equations. Discussion of these issues is beyond the scope of this entry. 

FIGURE 7.2 Didactic representation (upper) and flow diagram (lower) of a typical single-line flow system with merging zones. S = sample R — reagent C — carrier stream LS/ Lr = sampling loops IC = injector-commutator RD = delay coiled reactor Rc = main coiled reactor D = detector shaded area = alternative IC position solid arrows = sites where pumping is applied. 

A delayed coker consists of a furnace to heat the feedstock to around 500 °C, two pressure vessels (coke drums) with a diameter of 4 to 7 m and 20 to 30 m high, together with a distillation system to separate the volatile components. Figure 13.8 shows the flow diagram for the delayed coking process. 

FIGURE 2-1 A simplified process flow diagram of a delayed coker... 

Figure 2-1 illustrates a simplified flow diagram of a delayed coker. Hot resid feed flows to the bottom of the combination tower. The combination tower bottom section acts as surge drum from which the coking heater is charged. 

Stock and flow diagrams are used for exploring system interrelationships. The tool underscores the difference between "stock" variables and their rates of change, or "flow." This type of diagramming is the basis for computer simulation model feedback, accumulation of flows into stocks, and time delays. These elements help describe how even seemingly simple systems display baffling nonlinearity. 

In 1920, the Austrian scientist Otto Loewi discovered the first neurotransmitter. In his classic experiment (which came to him in a dream), he used two frog hearts. One heart (heart 1) was placed in a chamber that was filled with saline. This chamber was connected to a second chamber that contained heart 2. Fluid from chamber 1 was allowed to flow into chamber 2. Electrical stimulation of the vagus nerve (which was attached to heart 1) caused the heart rate of heart 1 to slow down. Loewi also observed that after a delay, heart 2 also slowed down. From this experiment Loewi hypothesized that electrical stimulation of the vagus nerve released a chemical into the fluid of chamber 1 that flowed into chamber 2. He called this chemical Vagusstoff. We now know this chemical as the neurotransmitter acetylcholine. A diagram of Loewi s classic experiment is shown in Figure 11.1. 

In a Voigt (or Kelvin) element tlie spring and dashpot are parallel. If a stress is suddenly applied the spring cannot respond immediately because of the resistance caused by the viscous flow (delayed elasticity). Monolayers with a two-dimensional network and viscous material between the cross-links will display such behaviour. So, the increase of the strain is retarded. Eventually the maximum strain / K° is attained, see fig. 3.52a. After cessation of the strain the energy stored in the spring relaxes, again with a rate determined by the parallel viscosity, till AA— 0. Behaviour like this is semi-solid. In the limit of r] - 0 the block diagram of fig. 3.49b is retrieved. 

Transformation of the process diagram into a corresponding simulation flowsheet is illustrated in the lower part of Fig. 6.5. In principle all plant elements may be represented by a separate model. For practical applications, though, it is sufficient to take into account only a time delay as well as the dispersion of the peak until it enters the column. This can be achieved by a pipe-flow model that includes axial dispersion. The detector (including some connecting pipes) can be represented by a stirred-tank model. 

HPLC causes the same problem with a time delay between analysis and taking a specimen out of the photoreaction solution. However, in HPLC automation is easier to realise [109]. In Fig. 4.29 a combined irradiation and HPLC apparatus is given as a block diagram. It is controlled by a microprocessor and contains a cycle for the photoreaction including a mercury arc, a photoshutter with an interference filter, and a flow pump at normal pressure. The reaction solution circulates through a flow cell with 1 cm optical thickness, in which it is irradiated. The irradiation time is controlled by the microprocessor via the shutter. The volume of the circulating solution amounts to approximately 8 ml of solution, of which 3 ml are irradiated at a time in the cell. 

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