THE UNIVERSITY OF MINDANAO

Matina, Davao City

A Written Report on Process Dynamics and Control:

Process Design Procedure

Submitted to:

Engr. Ramiro Emerson C. Amon

Submitted by:

MENDOZA, George Michael M.

Date:

August 2018
Introduction

Tens to hundreds of decisions are made during the control design of an industrial

process. The sequence in which these decisions are made can influence the time required to

complete the design and, perhaps, the quality of the control performance provided by the final

design. Thus, the engineer is faced with the challenge of managing a large quantity of

information and a large set of possible design decisions during the design procedure. With a

seemingly endless sequencing of decisions it can then be said that there is no single, correct

way to manage the design procedure. Different skilled engineers perform tasks in different

sequences to reach equally good solutions, and different problems can be solved more easily

by different sequences.

Among the established procedures, the procedure which provides a structured problem-

solving approach that is tailored to the control design task is used by many practitioners. This

procedure offers the following advantages when selected: (a) a prescribed procedure provides

a way to begin the design task; (b) the procedure provides a step-by-step approach that ensures

that many important issues are addressed; (c) the procedure decomposes the problem in a

manner that determines whether control is possible before continuing to detailed decisions on

control strategies; and (d) the procedure provides some guidance on managing the interactions

among the numerous design decisions.

Sequence of Design Steps (Step 1 – Step 3)

As stated on the preceding paragraphs, there is almost an infinite number of ways in

which the numerous design decisions can be reached. There is no one best sequence for all

control designs; in fact, various skilled practitioners use different sequences to arrive at equally
good designs. This report presents the control design procedure through the following

sequence:

STEP 1. Definition

The procedure begins with the definition of the problem provided in the control design

form (CDF) because of the crucial importance of this step to the quality of the design. The CDF

provides a useful checklist of the information needed in designing control systems and gives

an organized manner for documenting the information. This step involves the collection of

information appearing in the control design form and, for especially complex problems, the

formal preparation of the form. At this step, the objectives are translated to specific variables,

either directly measured or calculated using measurements, which are to be controlled.

To stimulate the thought process, abbreviated tables of sample questions are presented

for the various control objectives. The first three objectives—safety, environmental protection,
and equipment protection—are combined in one table because they all address major

deviations from normal operation, many of which could have common causes that influence

all three objectives. Smooth operation, product quality, efficiency and optimization, and

monitoring and diagnosis are also addressed.

stream or important location (e.g., the volume of a reactor or flash drum) in the process.
 STEP 2. Feasibility

The second step determines the feasibility of the control objectives for the equipment

design, operating conditions, and disturbances given in the problem definition. An analysis of

degrees of freedom and controllability determines whether it is possible to control the proposed

controlled variables with the proposed manipulated variables. This step ensures that the system

has sufficient capacity as well as degrees of freedom and controllability. If any of the results

of these steps indicate that control is not possible, the design procedure must include an

iteration in which an engineer alters the process so that the control objectives can be achieved.

STEP 3: Overview

The third step establishes an integrated view of the plant operation, concentrating on

the most important variables. The goal of this step is to obtain an overview of the feedback

process dynamics, the disturbance dynamics, the interaction in the process, and the types of

measurements and manipulated variables available for control. This overview is essential

because the design engineer makes one decision at a time and needs this overview to be able

to "look ahead" so that all decisions form a compatible design. Objectives that are easily

achieved or likely to be difficult to achieve are noted. Also, potential changes to the

instrumentation and process are identified for future use, if needed. However, no control

designs are decided at this step.

The control design procedure will be applied to a simple chemical process in this

section. The process, shown in the following figure, involves feed of a raw material from

storage to a chemical reactor.

The reaction is A → B with negligible heat of reaction. The products of the reactor are

heated and sent to a flash drum, from which the product is taken as a vapor flow which is

predominantly component B, but contains some A. A liquid stream consisting of unreacted

feed, along with some product B, is recycled to mix with the fresh feed and flows to the reactor.

STEP 1. Definition

The first step in the sequence involves a complete definition of the problem, which is

summarized in the control design form in the following tables.

The second step determines whether the control objectives are possible with the

equipment available. We assume that an analytical model of the process is not available; thus,

the design is based on qualitative analysis from the process structure and on linear models

identified empirically. There are eight manipulated external variables, so at most eight

dependent variables can be controlled. A preliminary selection of controlled variables is made

based on the CDF: (a) feed or production rate (1); (b) liquid and vapor inventories (3); and (c)

product quality (1). Thus, at least five controlled variables exist. The number of external

manipulated variables is greater than this minimum value. Therefore, it is concluded that the

degrees of freedom do not preclude a possible design, and the design procedure can continue.

STEP 3. Overview

The purpose of the third step is to gather observations about the entire system that can

be used when making sequential design decisions. The observations at this step are presented

below by hierarchy level.

LEVEL 1: Flow and Inventory

1. The feed tank has periodic deliveries of material and continuous outflow to the process.

Therefore, it is not possible or necessary to control the level. The tank must be large

enough so that it neither overflows nor goes empty for expected delivery and outflow

policies

2. The feed to the reactor is a combination of fresh feed and recycle. There also seem to

be several possible ways to control the flow to the reactor, because there are valves in

the fresh feed, recycle flow, and combined flow.

3. The liquid phase chemical reactor operation is influenced by several dominant

variables, temperature, volume, flow rate, and compositions. Since the plant has a

recycle, we should be sure that the total material and all component compositions are

self-regulating. Three categories of components are considered: (a) volatile inerts, (b)

products, and (c) reactant A.

LEVEL 3: Product Quality

4. There appear to be several manipulated variables that affect the flash product quality,

A2.

LEVEL 4: Profit

5. There are no objectives specified to increase profit beyond controlling product flow rate

and quality. However, there appear to be extra manipulated variables, or at least extra

valves in the process. This inconsistency must be resolved.

References:

Coughanowr, D. R. and LeBlanc, S. E. (2009). Process systems analysis and control (3rd ed.).

New York, NY: McGraw-Hill Companies, Inc.

Marlin, T. (2000). Process control, designing processes and control systems for dynamic

performance (2nd ed.). New York, NY: McGraw-Hill Companies, Inc.