Design of a composite polymer plant using a simulation system

Design of a Composite Polymer Plant Using a Simulation

LINDA LIN and JUDE T. SOMMERFELDI

School of Chemical Engineering Georgia Institute of Technology Atlanta, Georgia 30332-0100

and

MARGARET L. MCKENDRY

Hoechst-Celanese Corporation Charlotte, North Carolina 28323

This paper describes the use of the General Purpose Simulation System (GPSS) to simulate the manufacture of a typical line of thermotropic liquid crystal polymer resins. Specifically considered were composite polymers based on parahydroxybenzoic acid with glass, graphite, or talc added as a filler or reinforcing agent. The process examined consists of a combination of batch and semicontinuous operations. Product campaigning and equipment cleaning requirements were also incorporated into the GPSS model. Different equipment configurations and man- power distributions were simulated to aid in the design of such a polymer plant. The simulation results were analyzed, and the calculated return on investment was used to select the final plant design.

INTRODUCTION

hermotropic liquid crystal polymers (TLCPs), pos- T sessing high-performance engineering properties, represent a new class of resins that may be used as the matrix phase in composites (1). Incorporation of finely divided fillers enhances the machine pro- cessability and dimensional stability of such products. Although more than 400 TLCPs have been described in the literature [2), only a few have been made in commercial quantities. Similarly, most engineering and manufacturing experiences have been dominated by isotropic rather than composite materials.

A family of aromatic copolyester polymers with liquid-crystal properties has been produced since 1984 (3). These thermoplastic polymers are notable for high-temperature performance, self-reinforce- ment, and an ability to be processed by conventional injection-molding equipment. Made from para-oxy- benzoyl and oxybiphenylene terephthaloyl mono- mers, the polymeric structure of these resins consists of all para-para aromatic groups, accounting in part for high melting points, good solvent resistance, and high performance.

The manufacture of polymer composites typically consists of a set of batch and/or semicontinuous operations. Such processes can be simulated as a

* To whom correspondence should be directed

416

sequence of discrete events. In this work, the manufacture of a composite resin based on parahydroxybenzoic acid, with glass, graphite, or talc added as a filler, was modeled with the General Purpose Simu- lation System (GPSS) (4). The simulation results were then used to identify bottlenecks, size the equipment, and assess the number of operators.

PROCESS DESCRIPTION

The commercial manufacturing process (see Fig. 1 ) for thermotropic liquid crystal polymers based on parahydroxybenzoic acid consists of a combination of batch and semicontinuous operations. The first step occurs in a jacketed, pressurized, agitated batch reactor. The reactants are loaded into the reactor using vacuum hoses from their respective storage silos. Acetic anhydride is then loaded into the reactor, and the system is closed, heated, and allowed to react. After a predetermined amount of time, a nitrogen vacuum purge is commenced. The gaseous products of the purge consist of acetic acid and traces of acetic anhydride. The composition of this stream is used as a measure of the completion of the acetylation reaction. Upon completion, the reactants are heated, causing melt-polymerization to begin. After approxi- mately 7 h, the reaction is 99% complete. A valve at the base of the reactor is opened and, using a gear pump, the polymerized product is pumped to a twin-

POLYMER ENGINEERING AND SCIENCE, MID-APRIL 1990, Yo/. 30, NO. 7

Design of a Composite Polymer Plant

TWIN SCREW EXTRUDER

m i I I i I I I I I

I

COMPOUNDING EXTRUDER

STRAND PEUETIZER DRYING OVEN STORAGE SILO

COOUNG

Fig. 1 . Commercial manufacturing process for thermotropic liquid crystal polymers.

screw extruder. The purpose of this extruder is two- fold. On the one hand, it further devolatilizes the product and, secondly, it pelletizes the polymer.

The polymer pellets are then conveyed to a rack oven where they receive a postpolymerization heat treatment while under a nitrogen purge. This increases the molecular weight and augments certain mechanical properties, such as tensile strength. Next, the trays are manually removed from the oven, and the pellets are tested for certain quality control specifications. A determination is made as to whether each batch is within viscosity limits, using a spiral-flow viscometer. Based on the results of these tests, separate intermediate storage silos are then filled.

The proposed plant will make four distinct product lines. Each product is based on the above described resin pellets: three of the products contain a compounding substance inserted to reinforce mechanical properties or to serve as a filler. Specifically, the four products are a neat (pure) resin, a 30 wt% glass- reinforced product, a 30 wt% graphite-reinforced product, and 30 wt% talc-filled product. The compounding is accomplished using a second, slightly modified twin-screw extruder.

To minimize the downtime experienced when changing out the screw between the various product lines. a program of campaigning is employed and incorporated in the GPSS model. The final products are placed in bags, stored on pallets, and placed in a warehouse until customer demand requires shipping. A detailed flow diagram (Fig. 2) shows the appropri- ate processing times (in minutes) required for liquid crystal polymer manufacturing.

STRAND PELLETIZER

Fig. 2. Processflow diagram including processing times.

MODEL DESCRIPTION

This GPSS model begins with storage definitions that specify either capacities of holding tanks, numbers of operators, or numbers of pieces of equipment. SAVEVALUE initializations and variable definitions necessary for future calculations immediately follow. The process model blocks then begin with the GEN- ERATE block in Model Segment 1. The entire process is divided into five Model Segments:

Model Segment 1: raw material handling and polymerization, Model Segment 2: blending of out-of-spec material, Model Segment 3: finishing (compounding and packaging), Model Segment 4: campaign timer (compounding schedule), Model Segment 5 : simulation timer.

417 POLYMER ENGINEERING AND SCIENCE, MID-APRIL 1990, VOI. 30, NO. 7

L. Lin, M. L. McKendry and J . T. Sommerfeld

The GPSS block diagram for this simulation begins in Fig. 3.

Model Segment 1 consists of the arrival of raw materials, plus the entire processing model from raw material to the polymer product before compounding. Raw materials are specified to arrive quarterly, as represented by the first GENERATE block: every three months, a quarter of the raw materials needed for the entire year arrive. The first delivery arrives at the beginning of the simulation. An operator is required during raw material arrival for unloading. QUEUE and DEPART block pairs are used to collect statistics such as waiting times for an operator or for a piece of equipment to become available.

Once the availability of raw materials, as determined by a TEST block, is established, the reactor is loaded with the reactants, and polymerization begins (Fig. 4). After the reactor is captured, a test is performed to decide whether it is time for reactor cleaning. If a cleaning cycle is not necessary, the batch (transaction) passes through the reactor and another batch is started for polymerization while the first batch goes through the extruder and the oven (Fig. 5). However, if it is determined that it is time for cleaning, the reactor cleaning cycle is performed (Fig. 6). The extrusion step also includes a cleaning cycle and a similar test (Fig. 7). The base case of these simulations specifies the cleaning cycle to commence after every three uses of the reactor (and the extruder).

Both extrusion and drying require manual assistance, which is modeled by capturing an operator.

180 36

ADVANCE $ ADVANCE

iXCLN! @ ADVANCE

PI@

K I T LEAVE

1

1 LEAVE

ADVANCE 15 5

Fig. 4. Model Segment 1: polymerization and extrusion.

bDVANCE Kl i17" LtAVE

DtPARl CCRV P ADVANCE

6 0 , 12

vscN1+:! I 0 LiAVc

(OUTSP)

DEPART I ADVANCE c1 RELEASE

8= HBA

a NEAT ADVANCE

DEPARl 0 (HERE1

SPLll (HERE) fa+ s.4 TEST TLRMINATE

(THERE]

c

Fig. 3. Model Segment 1 : raw material deliveries. Fig. 5. Model Segment I : drying and quality control.

POLYMER ENGINEERING AND SCIENCE, MID-APRIL 1990, Vol. 30, No. 7 418

Des ign of a Compos i t e Polymer Plant

The pure polymer from the oven, which is assumed to be in-spec 70% of the time, can be sold as the "neat product." The remaining 30% which is out-of-spec is sent to intermediate storage, as either above- or below-specification material, with equal amounts assumed over time.

A specified amount of the in-spec material is sent to a storage tank for use in compounding composite polymers. The remaining in-spec material passes through the model to be packaged and shelved for later shipment as the neat product (Fig. 5). Again, QUEUE blocks are used extensively to collect statistical data in order to identify bottlenecks.

Out-of-spec materials are blended in Model Seg- ment 2 (Figs. 8 and 9). Assuming that blending equal amounts of above- and below-spec materials will re- sult in a suitable resin for compounding, the model starts a blending cycle as soon as one batch (transaction) of each type of material is present. Blending takes place in a mixing vessel and with some operator help. Because of the number of conditions that must be met before blending, SPLIT and MATCH blocks are employed to ascertain that those conditions are indeed met before proceeding. A small chance of 2% is included in the model for occasional blending that still yields a n out-of-spec material. The in-spec blended materials are stored for use in the compounding process where glass, graphite, and talc composite polymers are produced.

The finishing segment (Figs. 10-1 5) begins with either blended polymer or neat material set aside for

TRANSFtR '6 & TERMINATE

Fig. 6. Model Segment 1: reactor cleaning cycle.

(BACK)

SSSTOLO 6- a STOBL TERMINATE

f Al ADVbNCE r k STCT . h Y h L TRANSFER + (HERE! TERMINATE b

TERMINATE (i.il ADVANCE

(LOWB) LOOf'1 +.XSFACTI

1 f

Fig. 7. Model Segment 1: extruder cleaning cycle a n d quality control. Fig. 8. Model Segment 2: blending out-of-spec materials.

POLYMER ENGINEERING AND SCIENCE, MID-APRIL 1990, YO/. 30, No. 7 419

L. Lin, M . L. McKendry and J. T . Sommerfeld

compounding of composite materials. Independent GENERATE blocks are utilized in this segment, not because the composites are manufactured in parallel, but for model clarity. In fact, the compounding schedule follows the GATE block, controlled by the campaign timer in the next model segment. The schedule calls for each type of the composite material to be compounded for a predetermined fraction of the

i-;.k (GLCHKI

0 (CHKGL)

& (COMPlI

(OUT21 (HIGHPI Q-

(HlGHB] cd MATCH

I-p + (BACK1 TERMINATE @ 1 0- [KILL) h TERMWATE

W

Fig. 9. Model Segment 2: conclusion of blending operation. Fig. 1 1 . Model Segment 3: completion of glass-reinforced

polymer segment.

i--k (COMP1I (COMPZI

L+J ADVANCE

SBSTOBL gp (GLCHKI

I

ADVANCE * GllAl

LEAVE P S5STOBL

r - 5

(CPD7)

I

LtAVE

(COMP21

TERYINATC

LEAVE

I A

Fig. 12. Model Segment 3: compounding forced polymer.

of graphite-rein- Fig. 10. Model Segment 3: compounding of glass-reinforced polymer.

420 POLYMER ENGINEERING AND SCIENCE, MID-APRIL 1990, Vol. 30, No. 7

Design of a Composi te Polymer Plant

: S T 0 K 2 )

SSSTCUE

(CHKTC) +y LEAVE (CHKGR,

i-k (COMPPl iCOMP3) W S F E F

LEAVt 0 111"

Fig. 15. Model Segment 3: completion of talcIfilled polymer segment.

Fig. 13. Model Segment 3: completion of graphite-reinforced polymer segment.

month. The finished product after compounding is packaged and shelved for shipping of customer or- ders. Operator assistance is specified where appro- priate.

To accompany the finishing segment, the campaign timer in Model Segment 4 (Fig. 16) includes the LOGIC blocks to control the actions of the GATE blocks. Initially, all switches of the LOGIC blocks are in the reset mode (off or open). As soon as the material set aside for compounding is detected to be present, the logic switch is toggled to the set mode (on or closed). At this time, the glass campaign cycle begins and continues for the indicated amount of time specified in the variable definition section of the GPSS program input file. The switches for the glass, graphite, and talc campaigns toggle between set and reset modes to allow the compounding of each composite material. The switch called SCREW accounts for extruder cleaning and screw changing times built in between the campaign times.

The purpose of Model Segment 5 (also in Fig. 16) is to specify the simulation clock for the model. The model incorporates a start-up period of six months and is then reset for a simulation time of one year. The start-up period is specified in the GPSS program input file.

SIMULATION RESULTS

Various combinations of personnel and equipment were simulated to determine the most desirable plant

(COMP3)

DEPART

KNANCE

6 0 , 12

SSSTOBL 4 E C t l K )

Fig. 14. Model Segment 3: compounding of talczfilled polymer.

POLYMER ENGINEERING AND SCIENCE, MID-APRIL 1990, VOI. 30, NO. 7 421

L. Lin. M . L. McKendry and J. T. Sommerfeld

setup. Table 1 summarizes the simulation results in terms of the before-tax return on investment (ROI).

The return-on-investment calculations were performed with a spreadsheet, based on the standard estimated total capital investment tables found in Peters and Timmerhaus (5). Equipment costs were estimated by contacting various vendors and solicit- ing crude estimates. Included in the raw material

iMONTHJ I LOGlCS

MVANCE

iiDVANCE

PDVANCE 60 .12

ADVANCE

TERMINATk b TERMINATE

Fig. 16. Model Segments 4 (campaign timer) and 5 (simulation timer).

costs are any associated tax or additional freight charges. Operating labor costs are based on standard industry pay scales and the number of operators as specified in the GPSS simulation. The annual profit figures are based on 100% sale of the products. The sales prices are based on current industry averages for similar products. A complete economic analysis was performed for each case in the preparation of Table 1.

DISCUSSION

As expected, the ROI is greatest for Case 1 because of the lowest equipment and operator costs. However, running the plant with only one operator, as in Case 1, is both unsafe and unfeasible. Case 3 depicts the opposite extreme, wherein three operators serve only to lower the profit. Thus, operation with two people seems the most reasonable.

The effects of varying the number of pieces of equipment were also easily assessed from the GPSS model. Although only minimal waiting times were experienced in single-reactor simulations, multireac- tor arrangements were also examined. The results confirmed expectations; that is, the waiting time di- minished to zero. However, the increase in equipment costs was not justified by the substantial decrease in the ROI.

Another parameter studied was the frequency of washings in the reactor and extruder. A s anticipated, the frequency of washing increases costs and de- creases the ROI. However, the relationship between wash cycles and quality of the final product needs to be addressed. Since the polymer cannot be com- pletely removed from the reactor after every batch, buildup occurs. This residual amount hinders heat transfer and contaminates the next batch, causing a potential for serious quality problems. Based on dis- cussions with producers, it was concluded that wash-

Table 1. Summary of Case Studies.

Case Number of Number of Number of Number of Number of ROI (BFT),

1 1 1 1 1 1 63.7 2 1 1 1 1 2 48.6 3 1 1 1 1 3 37.4 4 2 1 1 1 1 58.6 5 2 1 1 1 2 46.0 6 2 1 1 1 3 36.6 7 2 1 2 1 1 53.8 8 2 1 2 1 2 48.2 9 2 1 2 1 3 33.0

10 2 2 2 1 1 50.9 11 2 2 2 1 2 38.3 12 2 2 2 1 3 29.3 13 2 2 2 1 1 45.0 14 2 2 2 1 2 34.8 15 2 2 2 1 3 25.2 16 2 2 2 1 3 26.7 17 2 1 1 1 1 32.7 18 2 1 1 1 1 32.9 19 2 1 1 1 1 41.5

Number Re actors Extruders Ovens Compounders Operators %

422

~~~~

POLYMER ENGINEERING AND SCIENCE, MID-APRIL 1990, Vol. 30, No. 7

Design of a Composite Polymer Plant

ing after every three batches (as described under the model description section) is an acceptable compro- mise between cost and quality.

The built-in randomness of the GPSS simulation highlights the importance of quality control. There exists a potential for stockpiling out-of-spec material

equipment requirements for a liquid crystal polymer plant. Once the basic model is developed, operating parameters are easily varied to obtain statistical data on operator and machine usage, thereby deducing proper investment strategy.

that can be neither blended nor stored Therefore, extreme care must be taken by operators during the reaction step, including tight control of temperature

1. 2.

3. 4.

5.

and quantity of raw materials.

CONCLUSION

This process simulation using GPSS has allowed effective evaluation in the selection of personnel and

POLYMER ENGINEERING AND SCIENCE, MID-APRIL 1990, Vol. 30, NO. 7

REFERENCES

P. D. Frayer, Polym. Compos., 8, 379 (1987). V. V. Tsukruk, V. V. Shilov, and Y. S. Lipatov, Acta Polym., 36, 403 (1985). J. J. Duska, Plastics Eng.. 42 (121, 39 (1986). T. J. Schriber, Simulation Using GPSS, Wiley, New York (1974). M. S. Peters and K. D. Timmerhaus, Plant Design and Economicsfor Chemical Engineers, McGraw-Hill, New York (1980).

423

Documents

Design of a composite polymer plant using a simulation system