Upstream unit

A vapor feed is favored when the stream leaves the upstream unit as a vapor or when most of the column feed leaves the tower as overhead product. The use of a vapor feed was a key component in the high efficiency cited previously for the spHtter, where most of the feed goes overhead. Low Column Pressure Drop. The penalty for column pressure drop is an increase in temperature differential ... 

Control philosophies applied to continuous countercurrent decantation (CCD) thick eners are similar to those used for thickeners in other applications, but have emphasis on maintaining the CCD circuit in balance. It is important to prevent any one of the thickeners from pumping out too fast, otherwise an upstream unit could be stai ved of wash liquor while at the same too much underflow could be placed in a downstream unit too quickly, disrupting the operation of both units as well as reducing the circuit washing efficiency. Several control configurations have Been attempted, and the more successful schemes... 

Schedule Complex tests should be done over a period of days. This provides the opportunity for the unit to be nearly steady. The advantages are that confirming measurements can be made. Schediil-ing a multiday test should be done when there is a likelihood that the feed stock supply and conditions will be nearly constant. The cooperation of upstream units will be required. The multiday test also requires that the downstream units can take the unit products. 

Upstream and Downstream Units Upstream and downstream units should be notified of the impending test. If the unit test will last over a period of days, analysts should discuss this with the upstream unit to ensure that they are not scheduling activities that could disrupt feed to the unit under study. Analysts should seek the cooperation of the upstream units by requesting as consistent feed as possible. The downstream units shoiild also be notified to ensure that they will be able to absorb the product from the unit under study. For both units, measurements from their instruments will be useflil to confirm those for the unit under study. If this is the case, analysts must work with those operators and supervisors to ensure that the measurements are made. 

After filter A filter located in the flow stream after another filter to remove any particulate matter that may have passed the upstream unit. 

Therefore the moments M, T2 and T3 of the RTD of the whole reactor system are simply the sums of the moments of the RTDs of its constituent parts. This, of course, can be generalised to any series combination of reactors or other flow vessels provided that they are linked together in a statistically independent and non-interactive manner this latter phrase implies that changes in operating conditions of a downstream unit are unable to influence the way in which an upstream unit behaves. 

In a coal gasification plant, the "Claus unit is nearly the last in a chain of process steps, and the foregoing steps can have a major effect on the Claus in a number of ways. Most of this paper will be examining the kind of Claus feed, but at the outset re should note that a Claus plant is sensitive to variations in feed rate, and that relatively minor ups and downs in upstream units will tend to amplify into wild fluctuations in the Claus plant. Accordingly our Claus-type plant should be inherently stable, if possible, able to ride through large changes in feed rate as well as composition. 

Disturbances. These are changes in flow rates, compositions, temperatures, levels or pressures in the process which we can not control because they are either given by the feed conditions (assumed controlled by an upstream unit), ambient conditions, such as the weather, or utilities, such as steam or cooling water. 

An exothermic reaction A -> B is taking place in a CSTR which has a cooling jacket with cooling water. The input stream is coming from an upstream unit. The main disturbances, controlled variables and manipulated variables are ... 

The reactor effluent usually contains a mixture of reactants and products. It is fed into a separation section where the products are separated by some means from the reactants. Because of their economic value, reactants are recycled back to upstream units toward the reactor. The products are transported directly to customers, are fed into storage tanks, or are sent to other units for further processing. The separation section uses one or more of the fundamental unit operations distillation, evaporation, filtration, crystallization, liquid-liquid extraction, adsorption, absorption, pressure-swing adsorption, etc. In this book we typically use distillation as the separation method because of its widespread use and our considerable experience with it. Everyone is a victim of his or her experience. Our backgrounds are in petroleum processing... 

In addition to recycle streams returned back to upstream units, thermal integration is also frequently done. Energy integration can link units together in locations anywhere in the flowsheet where the temperature levels permit heat transfer to occur. The reaction and separation sections are thus often intimately connected. If conditions are altered in the reaction section, the resulting changes in flowrates, compositions, and temperatures affect the separation section and vice versa. 

When the units are arranged in series with no recycles, the plantwide control problem can be effectively broken up into the control of each individual unit operation. There is no recycle effect, no coupling, and no feedback of material from downstream to upstream units. The plant s dynamic behavior is governed by the individual unit operations and the only path for disturbance propagation is linear along the process. 

From a dynamic viewpoint, whenever all flows in a recycle loop are set by level controllers, wide dynamic excursions can occur in these flows because the total system inventory is not regulated. The control system is attempting to control the inventory in each individual vessel by changing the flowrate to its downstream neighbor. In a recycle loop, all level controllers see load disturbances coming from the upstream unit. This causes the flowrate disturbances to propagate around the recycle loop. Thus any disturbance that tends to increase the total inventory in the process (such as an increase in the fresh feed flowrate) will produce large increases in all flowrates around the recycle loop. 

Figure 2.2 contrasts a typical column with a variable feed rate as set by an upstream unit to an on-demand column with bottoms flowrate set by a downstream operation. Further examples are given in later chapters when specific plants are considered (Chap. 8 for the Eastman process and Chap. 11 for the vinyl acetate process). 

The product gases are cooled and compressed (4) to facilitate separation of products and by-products. The suction-side of the compressor ensures that upstream units operate at a low pressure. The product gases are first dried (5) and the cooled product passed to a cryogenic separator (6) which removes hydrogen from the system. Some is recycled with the other portion passed-on for other uses. A selective hydrogenation unit (7) removes dienes and acetylenes. A final distillation train removes light hydrocarbon (C2-), propylene product, propane, which is recycled, and a C4 by-product. 

When all the storage tanks are occupied between two units and the downstream batch unit is busy, the discharge time of a batch from the upstream unit will be delayed. The duration of this delay will depend on the filling and discharge times of previous batches to and from the storage units. Now the complete algorithm (Hasebe et al. 1992) is as follows. 

A distillation column with an attached side stripper is used to separate a feed stream F into products A, B, and C, as shown in the diagram. The main column has a total condenser and a partial reboiler, and the side stripper has a partial reboiler. The columns are existing units with fixed number of trays and feed and draw locations. The external feed, a product of an upstream unit, is of fixed flow rate, composition, and thermal conditions. The pressure in the columns is determined independently and is not available as a variable. Using basic modules representation, determine the degrees of freedom of this system. What variables would you specify to define the performance of these columns ... 

The physical properties Kji, hj, Hj, and Sji, are determined by property models and are not considered as main model variables. The stage pressures Pj are determined by tray hydraulics, and so on, and are also not considered as main model variables. The external feed rates and compositions, Fj, Fj, Z., are determined by upstream units and are considered fixed in the column model. The liquid and vapor flows, Lj and Vj, are not independent variables since they can be replaced by the summations of and Vji. The stage temperatures Tj do not appear explicitly in the model equations but are implied in the property computations and are, therefore, main model variables. 

Canceling the effects of disturbance changes in upstream units,... 

Suppose that the inlet stream in the CSTR system (Figure 1.7) comes from an upstream unit over which we have no control. Then cAi, F and Ti are disturbances. If the coolant flow rate is controlled by a control valve, then Fc is a manipulated variable, while TCi is a disturbance. Also, if the flow rate of the effluent stream is controlled by a valve, F is a manipulated variable otherwise, it is an output variable. 

The solution of the above equations requires both initial and boundary conditions. Usually the transient begins from the stationary state, so that at t=0 the profiles Ci., i0,z) and T O.z) are available. Inlet concentrations c, and temperature Tm coming from an upstream unit may be constant or fluctuate in time. 

Secondly, candidates for manipulated inputs can be found. When they are flow rates connecting BFS s, the choice affect the control of both upstream and downstream. As an example, it was proposed to keep reactant recycle on flow control and to change the setpoint of this loop when production changes are required. This implies that the inventory control of the upstream unit is in direction opposite to flow, while the inventory control of the downstream unit is in the direction of flow. Manipulated flows should be chosen with care to avoid over-specification with respect to plant mass balance. The set-points of the BFS s form another category of plantwide manipulated variables. Examples are reaction conversion, separation performance, etc. 

The process sketched in Fig. 1.6 consists of two vertical cylindrical tanks with a level controller on each tank. The feed stream to the first tank comes from an upstream unit. The liquid level in each of the tanks is controlled by manipulating the flow rate of liquid pumped from the corresponding tank. The level signal from the level transmitter on each tank is sent to a level controller. The output signal from each controller goes to a control valve that sets the outflow rate. 

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