UNIT RATIO MATERIAL BALANCE

After the processing steps have been selected and ordered, the amount and composition of each process stream entering and leaving each unit must be specified. This is an accounting procedure. It assumes a steady-state operation. That is, at any point in space there is no change occurring with respect to time.

If we neglect the case of nuclear reactions, this means a balance must be run over every chemical element that is present. However, when no chemical reactions are occurring in a given unit, a component rather than an element balance is run. This is then checked by running an over-all balance to determine if the total amount of material that enters each unit also leaves each unit.

The unit ratio material balance is based on the production of one pound of salable product. This basis is used because it is independent of the plant size and because the use of numbers near one minimizes the possibility of future calculation errors.

The material balance is presented on a block flow sheet so that the reader can graphically visualize what is happening. An example is given in Figure 4E- 1. Each major operation appears as a block. No attempt is made to identify the specific pieces of equipment or to size them. The blocks are interconnected with flow lines, which indicate for each substance where it enters the process, what path it follows, and where it is eventually discharged. These flow lines are keyed to a chart that gives the composition and amount of each stream in the form of a unit ratio material balance. A material balance should be given for each product made by a multipur- pose plant.

Whenever there are not enough data available to complete the material balance, the engineer should determine whether his company has any currently operating plants that have similar processing steps. If so, by assuming his plant will be similar

Unit Ratio Material Balance 85

he can at least obtain a ballpark estimate. This should be checked with the engineers who operate the existing plant.

When the object of the process engineer’s study is to estimate the processing costs of a competitor’s process, he may find it especially difficult to find quantita- tive data. Under these circumstances he should assume high yields and low losses. Then if the results show his company’s proposed process superior to its com- petitor’s, he knows his company is in a good economic position.

Even for older operating processes data are often lacking. It may be well known that the over-all material losses are 5%, but how these are distributed between the various operations may still involve a large amount of guesswork. This is usually further complicated by proposed innovations which do not appear in the older plant.

DETAILED FLOW SHEET

After completion of the unit ratio material balance, a detailed flow sheet is constructed. This is a sketch of the system that shows all the equipment that is necessary to operate the plant, all process lines, and indications of where utilities are needed. It is not drawn to scale nor does it show spatial relationships. It includes all pumps, agitators, air filters, heat exchangers, hoists, elevators, lift trucks, blowers, and mixers as well as distillation columns, reactors, storage tanks, unload- ing docks, and steam boilers. Generally anything as large or as expensive as a pump is included. Piping and electrical details are excluded.

Figure 4-l gives the symbols that should be used. If the equipment is not given in the symbol list, it is drawn to look like itself.

The items on the flow sheet are coded by letter and number, so that the equipment on the flow sheet can be identified with a specific item in the equipment list. Table 4-4 gives the code letter associated with a specific type of equipment. The numbers following the letter may be just a sequential listing in no specific order or the first number can refer to a given area of the plant. For instance, the feed storage areas might be designated 0, the feed preparation area 1, the reactor area 2, and so on. The advantage of the latter method is that it permits the engineer to determine more quickly the specific location on the flow sheet of a given item in the equipment list. This can be important, since plants contain thousands of items. The coding in Figure 4E-4 follows this method.

The flow sheet allows the engineer to visualize what is occurring -to follow the incoming material from the time it enters the plant area through storage, purifica- tion, reaction, separation, and packaging until finally it leaves as a finished product. At each step the process engineer must mentally place himself in the plant to be certain that nothing is omitted.

For instance, if the material arrives via tank car he must visualize how the material is going to get to the storage tank. First he must realize that most fluids are removed through openings in the top of a tank car. The bottom openings are used mainly for washing out the cars prior to their being refilled. Acid tank cars by law do

Plate column Packed column

Vessel (internal coils Bucket elevator a n d agitation)

Reciprocating pump Atmospheric storage

Rotary feeder Conveyor

xx Spray column Cyclone Oil-fired heater Solids Batch centrifuge

Cooler-condenser Heat exchanger

Rotary compressor Belt conveyor

Figure 4-1 Typical flowsheet symbols.

Courtesy Backhurst, J.R., Barker, J.H.: Process D e s i g n , Heinemann Educational Books, Ltd., London, 1973.

not even have a bottom opening.* When a pump is used to transfer the material, the engineer must determine how the pump can be primed. If air pressure is to be used to push the material out, he must decide if the air is to be treated before it can be released to the atmosphere. It probably contains some vapors that would contami- nate the atmosphere. Does this require a scrubbing system? This close scrutiniza- tion is necessary for each step in the process.

A system for unloading tank cars of a low-boiling compound such as methyl chloride is given in Figure 4-2. The forwarding pump shown in the figure is not necessary, but its presence is desirable, since it speeds up the unloading process. A feasible alternate system would be to use an inert gas instead of compressed methyl chloride. Air cannot be used, because it forms an explosive mixture with methyl chloride if the percentage of air is between 8.25 and Methyl chloride is also highly flammable (the flash point is below 32°F (0°C)). After unloading, the tank car

Detailed Flow Sheet 87

Table 4-4

Some Symbols Used in Equipment Lists

B C CT c v D DR E F Fi M P R RV S Agitator Blower Compressor Cooling Tower Conveyor Cyclone Drum or Tank Dryer Heat Exchanger F a n Filter Motor Pump Reactor Rotary Valve Separator

must be left under positive methyl chloride or inert gas pressure. This is to prevent air from leaking into the tank car and forming an explosive mixture. For handling such flammable compressed gases the Interstate Commerce Commission (ICC) has set up specific regulations (ICC Sec. 174-560 to 174-563). For other flammable, explosive, or toxic compounds the ICC has regulations also.

When the engineer is designing storage facilities he must consider the climatic conditions. For storage systems, the designer must determine whether a heater is necessary for high-boiling compounds to prevent freezing in winter. For low-boiling compounds he must decide whether a condenser should be installed on the tank to lower the pressure on hot summer days.

In some cases the decision whether storage vessels will be equipped with a vapor recovery system has been determined by the United States Environmental Protec- tion Agency (EPA). In 1973 it set the standards for all petroleum liquids that are stored in vessels of more than 65,000 gal (245 It states that if the vapor pressure is greater than 11.1 psia (570 mm Hg) a vapor recovery system or its equivalent must be installed on any new tanks. If the vapor pressure is between 1.52 psia (78 mm Hg) and 11.1 psia (570 mm Hg), a floating head tank may be used or a vapor recovery system may be installed. Since the former is cheaper it will usually be selected. Below 1.52 psia (78 mm Hg) only a conservation vent or its equivalent is required.

The EPA is developing a whole series of standards and will be updating the present ones. To keep up with these changes the engineer must yearly obtain a copy of the Code of Federal Regulations, Title 40 (Environmental Protection Agency), Chapter 1, part He can keep abreast of interim changes by checking a supple- ment to the Federal Register’ that lists all the changes in the Code of Federal